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MARINE INTERNAL COMBUSTION DIESEL EGINE I

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                          MARINE DIESEL ENGINES.
                        NOTES FOR BE(Marine Engineering) CADETS.
                    MARINE DIESEL ENGINE PRINCIPLE AND PRACTICE.
                    Notes prepared By: Prof. K. Venkataraman. CEng, FIMarE, MIE.

1. Engine Classification:
ENGINE:
Any machine, which produces power, is called an engine.

HEAT ENGINE:
Any engine, which produces power or work from a supply of heat, is called Heat Engine.
The heat can be supplied by burning, i.e. by combustion of fuel.

EXTERNAL COMBUSTION ENGINE:
If the combustion of fuel takes place outside the engine, it is called an external combustion engine, e.g.
steam engine, steam turbine, etc.

INTERNAL COMBUSTION ENGINE:
If the combustion of fuel takes place within the engine itself, it is called an Internal Combustion Engine.
Fuel economy, simplicity, and low operational costs make it more popular than external combustion
engines.

CLASSIFICATION OF INTERNAL COMBUSTION ENGINES:
Internal combustion engines can be classified according to different criteria as follows:
1. According to ignition System.

a) Compression Ignition Engine (C. I. Engine)
In this type of engine, the heat of the compressed air itself ignites the fuel. No other means of ignition
are required, e.g. Diesel Engine.
In a Compression Ignition Engine, e.g. Diesel Engine, a piston reciprocates in a cylinder. At downward
stroke of piston, air enters the cylinder. At upward stroke of piston air is compressed. Due to
compression pressure and temperature of air becomes quite high (over 35 bar and 500*C respectively).
Finely atomised fuel oils sprayed into such compressed air ignite spontaneously and produce power.

b) Spark Ignition Engine (S. I. Engine)
In this type of engine (Otto engine), the fuel is ignited by the spark produced by a high-tension electrical
circuit. In spark ignition Engine, liquid gasoline is sprayed or drawn through a nozzle or jet into the air
stream going to the working cylinder. A combination of mild heating and reduction of pressure partially
vapourises the gasoline. Proportionate mixing of air and gasoline vapour is done in carburetor. Mixture
enters the cylinder where at a suitable time, an electric spark ignites the mixture, which burns then
quickly and produces power.

Spark Ignition Engine Versus Compression Ignition Engines Similarities.
   1. Both are Internal Combustion Engines.
   2. Both run on liquid fuels.
                                                                                                            2



Dissimilarities.

                     S. I. Engine.                                         C. I. Engine.
1. Ignition system required                            1. Not required.
2. Draws air and fuel into the system.                 2. Draws only air into the cylinder.
3. Compresses air and fuel together.                   3. Compresses air only.
4. Fuel is mixed with air at before compression        4. Fuel is mixed with air at the end of
starts.                                                compression.
5. As too much compression of air and fuel             5. Only air can be compresses without pre-ignition
mixture causes pre ignition and detonation             and detonation, so compression ratio can be high
permissible compression ratio is not high(about 7).    (about 16).
6. Efficiency, being proportional to compression,      6. Higher efficiencies can be obtained due to
is limited due to less compression ratios.             possible higher compression ratio.
7. Uses highly volatile liquid fuels so that it can    7. Uses less volatile liquid fuels
mix with air at low temperature.
8. Fuel used is costly.                                8. Cheaper fuel can be used
9. More fuel is used for same power.                   9. Less fuel consumption.
10. Lighter in weight.                                 10. Heavier and stronger engines due to higher
                                                       pressures involved.
11. Initial cost less.                                 11. Initial cost high.
12. Smooth operation.                                  12. Certain roughness in operation encountered,
                                                       especially in high-speed engines at light loads.


2. According to Operating Cycles.
(a) OTTO CYCLE (Constant Volume Combustion Cycle).
It is the ideal air standard cycle for Petrol engine, the gas engine and the high-speed oil engine. The
engines based on this cycle have high thermal efficiency but noisiness results particularly at higher
power due to higher pressures in the cylinders.

(b) DIESEL CYCLE (Constant Pressure Combustion Cycle).
It is the ideal Air standard cycle for Diesel Engine, especially suitable for low speed Diesel Engine but
not for high speed Diesel Engine. (The thermal efficiency is lower than Otto cycle engines but engines
run smoothly due to lower pressures in the cylinder.

(c) DUAL COMBUSTION CYCLE (Constant Pressure and Constant Volume Combustion Cycle).

Modern Diesel Engines do not operate purely on constant pressure combustion cycle but some part of
combustion process takes place at constant volume while the rest is completed at constant pressure.
In general, this cycle resembles Constant volume combustion Cycle more than constant pressure
combustion cycle. It is suitable for modern Medium and High Speed Diesel Engines. The thermal
efficiency is more than Diesel Cycle but less than Otto cycle. Also noise level is in between the two.
This is a more practical engine.

3. According to Strokes/Cycle.
In an engine, the following events form a cycle:
    a) Filling the engine cylinder with fresh air.
                                                                                                          3
    b) Compressing the air so much that injected fuel ignited readily by coming in contact with hot air
        and burns efficiently.
    c) Combustion of fuel.
    d) Expansion of hot gases.
    e) Emptying the products of combustion from the cylinder.
Depending on how many strokes of piston are required in completing this cycle, the engines can be
divided into two classes:
1. Four Stroke Engine
An engine, which needs 4 strokes of the piston (2 in and 2 out) to complete one cycle, is called Four-
stroke engine.
2. Two Stroke Engine
An engine that needs only 2 strokes of the piston (1 in and 1 out) to complete one cycle is called Two-
stroke engine.
4. According to Piston Action:
    (a) Single Acting Engine




One end of the cylinder and one face of the piston are used to develop power. The working face is at the
end, which is away from crankshaft. Generally, single acting vertical engines develop power on the
down stroke.
(b) Double Acting Engine
                                                                                                          4
Both ends of the cylinder and both faces of the piston are used to develop power on the upward as well
as on the downward stroke.
 c) Opposed Piston Engines.




Two pistons travel in opposite directions. The combustion space is in the middle of the cylinder between
the pistons. There are two crankshafts. The upper pistons drive one, the lower pistons the other. Each
piston is single acting.

5. According to Piston Connection.
    (a) Trunk Piston Type.




The piston is connected directly to the upper end of the connecting rod. A horizontal pin (Gudgeon Pin)
within piston is encircled by the upper end of the connecting rod. This construction is quite common,
especially in small and medium size engines.

   (b) Cross Head Type.

The piston fastens to a vertical piston rod whose lower end is attached to a ‗cross head‘, which slides up
and down in guides. The crosshead carries a crosshead pin, which is encircled by the upper end of the
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connecting rod. This more complicated construction is common in double acting engines and large slow
speed single acting engines.




                                   Crosshead type Engine arrangement.

Comparison between Trunks Piston Versus Cross Head Engine.

Most medium and small size engines use trunk pistons. Resulting side thrust causes the piston to press
against the cylinder wall, first on one side, then on the other. At the top of stroke, when the gas pressure
is greatest, side thrust is negligible (due to small connecting rod angle). So most of wear takes place at
the middle of stroke: making piston skirt increases thrust-bearing area, and hence reduces wear. In
medium and small size engines, due to lower gas pressure, units‘ side pressure is so small that neither
piston nor liner wears much.
In crosshead engines, crosshead takes the side thrust, which will be high in large engines. So, crosshead
engines have the following advantages:

   1.   Easier lubrication.
   2.   Reduced liner wear.
   3.   Uniformly distributed clearance around piston.
   4.   Simpler piston construction because the ‗Gudgeon pin‘ and its bearing are eliminated.

However these advantages of cross head engines are offset by:
  1. Greater complication.
  2. Added weight.
  3. Added height.
  4. Careful adjustments.
                                                                                                         6
   6. According to Cylinder Arrangement

      a) Cylinder-in-Line Arrangement
This is the simplest and most common arrangement, with all cylinders arranged vertically in line. This
construction is used for engines having up to 12 cylinders. The arrangement is shown in figure below.




(b) V - Arrangement:
If an engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame and
crankshaft with an inline arrangement. Also engine becomes quite long and takes up considerable space.
So V-arrangement is used for engines with more cylinders, (generally 8, 12, 16) giving about half-length
of engine, more rigid and stiff crankshaft, less manufacturing and installing cost. Angle between two
‗Banks‘ is kept from 30* to 120* (most commonly 40*, 75*), as shown in the figure.
                                                                                                            7
(c) Flat Arrangement.
It is a V-engine with angle between the banks increased to 180*. Generally, it is used in trucks, buses,
rail cars, etc. where there is little headroom. Arrangement is shown in the following figure.




(d) Radial Arrangement.
In a radial engine, all the cylinders are set in a circle and all point towards the centre of the circle. The
connecting rods of all the pistons work on a single crankpin, which rotates around the centre of the
circle. Such a radial engine occupies little floor space. By attaching the connecting rods to a master disk
surrounding the crankpin, up to 12 cylinders have been made to work on a single crankpin. The
arrangement is shown in the figure below.




7. According to Method Of Fuel Injection.

(a) Air Injection Engine
The fuel is injected into the cylinder by a blast of high compressed air. It was commonly used on early
diesel engines. Being too heavy and complicated, this system is now obsolete.

(b) Airless (or Solid or Mechanical) Injection Engine.
Fuel is injected into the cylinder, through the fuel valve, by high-pressure fuel pump. At present, it is
being used for all types and sizes of diesel engines.
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8. According to method of Charging.

(a) Natural aspirated Engine.
The vacuum is created when the piston moves away from the combustion space draws in the fresh
charge.

(b) Supercharged Engine.
The charge is admitted into the cylinder at a higher than atmospheric pressure. This high pressure is
produced by a pump or blower or exhaust gas turbocharger.

9. According to Fuel Used.
(a) Heavy Oil Engine.
It can burn fuels of high viscosities, e.g. 1500 sec. Redwood No. 1 or 350 sec. Redwood No. 1.

(b) Diesel Oil Engine.
This uses diesel oil.

(c) Gasoline Engine.
This uses gasoline as fuel. It can also use ‗kerosene‘. As the 'perfect mixture' of fuel and air is led to
cylinder for compression, compression ratio is limited to 7 to avoid self- ignition, power loss, knocking,
etc.

(d) Gas Burning Engine.
It uses gaseous fuels at higher compression. Three ways have been adopted to burn gas at higher
compression. The engines are named accordingly as follows:

i) Gas Diesel Engines
They compress air alone. At the end of compression, they inject the gas at high pressure into the cylinder
just at the moment it is to fire. With gas, a small amount of ‗pilot oil‘ is also admitted to assist the
ignition and to cause smooth and prompt ignition.

ii) Dual Fuel Engine
Admit the gas and air at the same time and compress the gas/air mixture at diesel compression ratio. At
the end of compression, they inject fuel oil, which the high temperature of the gas-air mixture ignites to
fire the mixture. Using ‗lean mixture‘ unlike to ‗perfect mixture‘ of gasoline engine prevents self-
ignition.

iii) High Compression, Spark Ignited Gas Engines.
Like dual fuel engines, they compress a mixture of gas and air to high pressure, preventing self-ignition
by using a ‗lean mixture‘ but they use spark, instead of oil, for ignition.

10. According to Speed.

   1. Slow Speed Engines: 100 to 150 r.p.m.

   2. Medium Speed Engines: 300 to 1000 r.p.m.

   3. High Speed Engines: More than 1000 r.p.m.
                                                                                                        9
   The following table compares the various aspects of Slow Speed, Medium Speed and High Speed
   Engines:
               Slow Speed.                Medium Speed.                     High Speed.
    1. No Gearing.                 Gearing Necessary. Four        Gearing Necessary. Four
    2. Four or Two Stroke          Stroke better.                 Stroke only.
        acceptable.
               Slow Speed.                Medium Speed.                     High Speed.
    3. Poor quality fuel           Better Fuel required.          Distillate Fuel only.
        acceptable.                Diesel oil/Gas oil.
    4. Crankcase can be            Trunk Piston type.             Trunk Piston Type.
        separated from
        combustion zone.
    5. Less noise and vibration. More noise.                      Most vibration and noise.
    6. Less fatigue failure.       More.                          Most.
    7. Fewer stresses due to       More.                          Most.
        heavier scantling.
    8. Heavy and Large Size.       Compact.                       Extremely Compact.
    9. More head-room
        required.                  Less.                          Least.
   10. Heavy Lifting Gear          Light parts easy to handle.    Lighter parts can be handled
        required for heavy parts.                                 manually.
   11. Engine r.p.m. is            Engine r.p.m. limited by       Engine r.p.m. limited by
        limited by propeller       piston speed.                  piston speed.
        efficiency.
   12. Can have long strokes.      Small strokes.                 Smallest stroke.
   13. Large bore cylinders.       Small bore.                    Smallest.
   14. Heavy & large pistons.      Light and small piston.        Lightest & smallest piston.
   15. Round section               ‗I‘ section connecting rod.    ‗I‘ section connecting rod.
        connecting rod.
   16. Failures less and easier to More and difficult to          Most and very difficult to
        manage.                    manage.                        manage.

11. According to Bore/Stroke Ratio:

a) Square Engine:
If bore/stroke is about one, crankshaft web dimensions become less compared to journal and crankpin.

b) Over Square Engines (Short Stroke)
If bore/stroke > 1, web dimensions (less height, more thickness) are such that webs will be weak. So
generally over square engines are not used.

(c) Long Stroke Engines.
Generally, engines have stroke/bore >1. This gives crankshafts of good strength. Most common ratio is
stroke/bore = 2. 0: 2.2.

(d) Super-long Stroke Engines.
To have better propeller efficiency and better combustion even with lower grade fuels, lower r.p.m.
engines with longer strokes are gaining popularity. These engines have stroke/bore ratio = 3.
                                                                                                          10
12. According to Use.
Engine can be named as Marine, Auto, Tractor, Locomotive, Aero-engines, and Rocket Engines
according to their use. On ships they can be called Main Engines if used for propulsion or Auxiliary
Engines, if used for generation of electricity. The Diesel Engines used in Marine power plants are
termed as ‗Marine Diesel Engines‘.

The Diesel Engines find the following application on board merchant ships.
   1. Main Propulsion.
   2. Electric Power generation.
   3. Emergency Pumps (e.g. fire pump).
   4. Life Boat.
   5. Emergency Generator.
   6. Emergency Air Compressor

REASONS FOR WIDE USE OF DIESEL ENGINES IN MARINE POWER PLANTS.

1. Small fuel consumption:
Diesel Engine is one of the most efficient heat engines. Hence it gives more power with less fuel. It is an
engine of high economy.
2. Cheap fuel:
Diesel engine uses fuel costing very less as compared to other engines.
3. Economy at light loads:
Diesel Engine is not only efficient when it is fully loaded, but also when it is partly loaded.
4. Greater Safety:
Diesel fuel is non-explosive and less flammable at normal temperatures and pressures. It requires special
effort to make it start to burn. This feature makes it very attractive in the marine trade, because it would
be much safer carrying diesel oil on board ships.
Diesel exhaust gases are less poisonous than other engines, because they contain less carbon monoxide.
5. Ignition System is not required:
Diesel engines do not require battery or magneto running them.
6. More power can be produced due to more compression allowed.
7. Diesel Engine is more robust and stronger.
8. Economy in small sizes:
As great contrast to steam power plant, a small diesel engine has nearly as good an economy as a large
one. This makes it possible to enlarge a diesel engine plant with additional units as the load grows. At all
stages of growth, the efficiency is high.
9. Sustained economy in service:
Again in contrast to a steam power plant, diesel efficiency falls off very little during thousands of hours
of use between overhauls.
10. Lightness and compactness:
Diesel engine plants have less weight and space per unit power. It is therefore well suited to portable and
mobile installations.
11. Independence of water supply:
A diesel engine requires very less water in contrast to steam plants.
12. Quick Starting.
A cold diesel engine can be started instantly and made to carry its full load in few minutes. It is therefore
ideal for supplying emergency power.
13. Easily in Maneuvering:
A diesel engine can be made to run at full power in either direction.
                                                                                                            11
14. Economy in Labour.
No fire room force is needed.
15. Freedom from nuisance:
There are no ashes to be disposed of, no noisy and dusty coal handling and pulverising equipment to
maintain, no smoke, and noise can be easily eliminated. Due to above mentioned reasons, Diesel engines
are quite popular on board ships.
These reasons can very well be regarded as the advantages of Diesel Engines over other prime movers
such as gasoline engines, gas turbines, steam engines, steam turbines and hydraulic turbines.

However, Diesel engines also have certain disadvantages, which can be listed as following:
1. Cost:
Diesel engines, because of the higher pressures at which, they work, require sturdier construction, better
materials and closer fits than gasoline engines. Therefore, they cost more to build.
2. Weight:
Because of sturdier construction, weight per power is more than gasoline engines.
3. Attendance:
A diesel engine requires more attention than an electric motor running on purchased current. It also
requires more attention per unit of power produced than a large steam turbine.
4. Fuel Cost:
Oil used in Diesel engines is costlier than coal. Hence, steam power plants using coal as fuel are cheaper
in operation.

RECENT TRENDS:
The Diesel Engines is at present acknowledged to be the best prime mover in a wider range of marine
applications than any other engine. Due to higher efficiency, lower specific fuel consumption and
capability to use cheaper fuel, Diesel engine is preferred to spark ignition Engine, gas turbine and steam
turbine for moderate power applications.
However, small pleasure boats are still powered popularly by spark Ignition engines and very large ships
e.g. Tankers and VLCCS are still powered by steam turbines. Gas turbines are popular on naval vessels.
However, Diesel engines are making in rails in heavily these fields too. Even, U.S. merchant ships
dominated heavily by steam propulsion are more and more embracing diesel engines.

In Diesel engines also, there is tough competition between medium and slow speed engines.
However, the recent trend is towards having very slow speed super-long stroke engines e.g. SULZER/
RTA: M.A.N-B & W/ LMC, due to significant improvement in propulsion efficiency and specific fuel
consumption at low speeds as well as their ability to burn very poor grade fuels which are available
now-a-days.

The worse quality of fuels available and increase in the cost of oil has led to renewed interest in coal-
fired ships. Keeping in view the limited world reserves of oil, coal fired ships seem to provide a good
alternative in 2000‘s but at present the position of Diesel engines remains unchallenged.

              ****************************Kv************************************
End of Engine Classification/ BIT/AMET/BE/Motor/KV/May 2003.
                                                                                                          12
Engine Cycles & Timing Diagrams.
Introduction.
We will discuss the full series of the separate steps or events, which follow each other while a diesel
engine is in operation. We will also discuss the timing diagrams of Two Stroke Cycle & Four Stroke
Cycle Engines.

1. Various Steps or Events of a Cycle.

First Step.
Air is introduced into the cylinder because no fuel will burn without air. Burning or combustion is a
process of uniting fuel or combustible with the oxygen in air. The process is chemical reaction which
means that fuel & oxygen, in uniting, change into new substances.

Second Step.
The air must be squeezed or compressed to a high pressure. Two reasons for compressing the air are to
get high temperature and high pressure there by higher power. In a diesel engine the air is compressed so
much that it becomes hot and in fact, it will be hot enough to ignite oil that is sprayed into it.

Third Step.
The fuel is injected into the cylinder in the form of fine spray after the air has been compressed and thus
heated to a high temperature. It must be in the form of fine spray so that a cloud of oil droplets will
spread through all the air in the cylinder.

Fourth Step.
Combustion takes place after the oil is sprayed in the cylinder. This will generate a large amount of heat.
The gaseous mixture gets hotter and grows larger or expands due higher pressure.
It pushes on the piston, which in turn transmits the force through the connecting rod to the crank of the
crankshaft. This will make the crankshaft revolve.

Fifth & Last Step.
When the piston has finished its preceding power stroke and the gases in the cylinder have lost their
pressure, the spent gases must be exhausted.

A cycle is a full series of separate steps or events, which follow each other.
For a Four Stroke Cycle Engine, a complete cycle requires four stroke of the piston.
For a Two Stroke Cycle Engine, a complete cycle requires two stroke of the piston.

―Four Stroke Cycle‖ & ―Two Stroke Cycle‖ engines are abbreviations, which do not really make any
statement other than what is stated above.

4-Stroke Cycle Engine.
In a 4-stroke engine, the engine crankshaft makes two revolutions for each working cycle.
The four corresponding piston strokes are as follows:
1. Suction Stroke, 2.Compression Stroke, 3. Power Stroke & 4. Exhaust Stroke.
The engine has air inlet and exhaust valves. By the opening and closing of these valves in proper
sequence, the piston can be made to perform its main function of transmitting power to the crank.
In addition to that the piston also performs subsidiary functions of drawing air into the cylinder,
compressing the air and subsequent expulsion of exhaust gases.
                                                                                             13

                         Four Stroke Cycle Engine:




                           A, B & C: SUCTION STROKE.
            (A): Piston at top of stroke, inlet valve open, air intake begins:
                       (B): Piston descending, air being taken in.
(C): Piston at the bottom of the stroke, all valves closed, air intake full and completed.




                           D: COMPRESSION STROKE.

             (D): Piston rising. All valves closed. Air being compressed.
                                                                                                             14




                                       E & F POWER STROKE.
(E): Piston at the top of the stroke. Inlet and exhaust valves closed. Injector spraying oil into hot air.
      (F): Piston descending. All valves closed. Hot high pressure gas forcing the piston down.




                                    G & H EXHAUST STROKE.

                    (G): Piston at the bottom of the stroke. Exhaust valve opens.
       (H): Piston rising. Exhaust valve opens. Exhaust gas being driven out of the cylinder.
                                                                                                       15

            TIMING DIAGRAM AND POWER CARD OF FOUR STROKE CYCLE:




   The timing diagram shows the closing and opening of the valves. The working cycle is illustrated as
a ‗P - V‘ diagram (pressure-volume). The line ‗l – l‘ represents atmospheric line. The piston is
considered to have just moved over the ‗top dead centre‘ and is on its way down. The air inlet is already
open and because of the partial vacuum created when the piston moves towards its bottom position,
fresh air is sucked into the cylinder. This process is represented in the 'p-v' diagram by the line ‗1-2‘
which is termed suction line. This movement of the piston is called 'Suction Stroke".
                                                                                                        16

After the piston has moved over bottom dead centre, the suction valve closes and the volume of air in
the cylinder is compressed during the course of the up stroke of the piston. This is represented by the
line ‗2-3‘ in the above diagram and termed as compression line. This movement of piston is
compression stroke.
The ignition takes place at point 3 and combustion continues for the duration of fuel injection, ending at
point 4. After this combustion products expand to point 5 when the exhaust valve opens. Power is
produced between point ‗4 – 5‘.
The pressure drops in the cylinder to the exhaust line from 5 to 6. The exhaust valve remains open till
after piston passes over the top dead center. The combustible gases are expelled. The line 6 to 1
represents this. The pressure is slightly above atmosphere, because of the resistance in the exhaust pipe.
This stroke is 'exhaust stroke'.
A 4-stroke engine requires two complete revolutions of the crankshaft to finish working cycle.
This means inlet, exhaust & fuel valve must only function once for every two revolutions of the
crankshaft.
In order to activate those valves in the correct sequence, it is necessary to operate them from a shaft,
which rotates at half the speed of the crankshaft. This is called camshaft.

Two Stroke Cycle Engine:

In this engine, two of the strokes necessary to complete working cycle in a 4-stroke engine are
eliminated.
The remaining strokes are as follows:
--- Compression Stroke.
--- Power Stroke

The working cycle is illustrated by 'p-v" diagram in the next page. The compression takes place by 1 to
2. The combustion process and expansion take place as described for a 4-stroke engine. At point 4, the
exhaust valve at top of the cylinder opens (uniflow scavenging).
At point 5, the piston exposes the ports in the cylinder wall. The result being that fresh air, known as
scavenge air, flows into the cylinder & flushes out exhaust gases.
Piston covers the port in cylinder wall at 6 and the exhaust valve closes at point 1.
The compression beings a new working cycle.
The pressure of scavenge air is little higher than the atmospheric air.
2-stroke engines carry out useful work for each revolution of the crankshaft.

This means fuel and exhaust must function each revolution. The camshaft must rotate at same speed of
the engine crankshaft.

REASONS FOR TIMING.
EXHAUST AND INLET VALVE TIMING.
These deal with the expulsion of the burnt gases and recharging the cylinders with fresh air. The overall
efficiency of an engine depends largely upon getting the exhaust gases out or scavenging them.

The exhaust value is opened after the piston has traveled about 80% of the working stroke. By that time
it has done its useful work, the energy has been spent. The opening of the valve will allow a large part of
the exhaust gases to be blown out of the cylinder. The cylinder pressure equalises with the pressure in
the exhaust line during this period. This is referred to as the ‗blown down‟ period.
When the piston moves upwards the piston movement expel exhaust gases.
                                                                17
         Two Stroke Timing Diagram (Uniflow Type).




Two Stroke Engine – „Pressure/Volume‟ Diagram (Uniflow Type).
                                                                                                         18
In a 4-stroke engine, towards the end of exhaust stroke and beginning of section stroke, both exhaust and
inlet values are open. This is called „overlap period‟. This would further help in achieving an efficient
scavenging. Exhaust valve closes after the piston has moved over the top dead centre. The inlet valve
remains and the down ward movement of the piston lowers the pressure in the cylinder and thereby
atmospheric air is drawn in. The air in the inlet passages to the inlet valve will gain a high velocity and
in turn kinetic energy. Use is made of this effect to keep the air inlet value open until the piston is past
bottom dead centre. The air then continues to flow into the cylinder until its kinetic energy is lost and
airflow ceases. The inlet value is closed now.
In two stroke engines, the events described above as taking place will have to be carried out in about
120* of the crank movement. It will require the assistance of low-pressure air. The speed of opening of
valve or part has to be rapid so that the pressure of gas falls quickly. It would be easier now for scavenge
air to rush in and get the gases out.
We have to briefly discuss the combustion process to understand fuel timing.
Combustion takes place in three distinct stages:
     1. Ignition Delay Period, during which some fuel has been admitted but has not yet been ignited.
     This is the stage during which fuel is atomised, vapourised, mixed with air and raised in
     temperature.
     2. Rapid or Uncontrolled Combustion, following ignition. The pressure rise is rapid during this
         stage.
     3. Controlled Combustion. The first stage or the delay period exerts great influence on both engine
         design and performance. The pressure reached during ‗rapid or uncontrolled combustion‘ will
         depend upon delay period.

The longer the delay, more rapid and higher is the pressure rise. It is because more fuel will be present in
the cylinder before rate of burning comes under control (in the 3rd stage).
This will cause rough running and ‗diesel knock‘. But at the same time, there must be certain amount of
delay period for proper mixing.
One of the main factors that affect delay period is ‗fuel timing‘. If it is too early, the delay period is
more because the pressure and temperature are low in the cylinder. If the injection is too late, the fuel
will burn during the expansion stroke. The pressure rise in the cylinder will drop considerably, reducing
the efficiency. The exhaust temperature will be high and may cause overheating of the engine in severe
cases. So, an optimum angle has to be introduced to get best effect. It depends on delay period. The
injection is earlier on higher speed engines. Four-stroke or Two-stroke cycle will make no difference on
the point at which injection begins.
                                      Four Stroke Timing Diagrams:




                  Four Stroke Un-supercharged.           Four Stroke Supercharged.
                                                                                                      19

                                            Two Stroke Timing Diagram:




                    Two Stroke Uniflow Engine.                   Two Stroke Loop Scavenging Engine.




                   Uniflow Scavenge Engine.    Loop Scavenge Engine.
                      (Exhaust Valve).
            ********************************Kv***********************************
End of Engine Cycles & Timing Diagrams/ BIT/AMET/BE/Motor/Kv/May 2003.
                                                                                                       20
Engine Indicator and Indicator Diagrams:
An engine indicator is used to record pressure/volume or indicator diagrams taken off engines, the areas
of these indicator diagrams represent the work done per cycle of one unit.
There are two types of engine indicators:
    1. Mechanical type: This records indicator diagrams on paper.
            a) Can record pressure within the engine cylinder at any part of the engine cycle.
            b) Not considered reliable of engine speed more than 150 rpm.
            c) Small lightweight models can be used for engines with speeds up to 350 rpm.
            d) Mean indicated pressure (m.e.p) from an indicator power diagram.

2. Pressure indicator type - this measures maximum combustion pressure only.
    a) Also known as maximum pressure indicator.
    b) Compression pressure is recorded with fuel cut off.
    c) No engine speed limitation.
    d) Often used on medium speed engines.
    e) Does not record indicator diagrams on paper.

Mean Effective Pressure and Indicated Power:




                                       Power Indicator Diagram.
Referring to the Indicated diagram (Power card), the area of the diagram divided by its length represents
the mean pressure effectively pushing the piston forward and transmitting useful energy to the crank in
one cycle. This, expressed in N/m2, is termed the indicated mean effective pressure (pm).

Power is the rate of doing work (basic unit is the Watt) or:
                                         1 Watt = 1 J/s = Nm/s
Let:
pm = mean effective pressure (N/m2).
A = area of piston (m2).
L = length of stroke (m).
N = Number of power stroke per second.
Then:
 Average force (N) on piston = pm x A newtons.
Work done (J) in one power stroke = Pm x A x L newton-metres = joules.
Work per second (J/s = W) = pm x A x L x n watts of power,
Therefore:
Indicated power = pmALn.
                                                                                                            21

This is the power indicated in one cylinder. The total power of a multi-cylinder engine is that multiplied
by the number of cylinders, if the mean effective pressure is the same for all cylinders.

                           Construction and Working Principle of Indicator:




                                             Engine Indicator.
An engine indicator consists of a small bore cylinder containing a short stroke piston which is subjected
to the same varying pressure that takes place inside the engine cylinder during one cycle of operations.
This is done by connecting the indicator cylinder to the top of the engine cylinder in the case of single-
acting engines, or through change over cocks and pipes leading to the top and bottom ends of the engine
cylinder in the case of double-acting engines. The gas pressure pushes the indicator piston up against the
resistance of a spring, a choice of specially scaled springs of different stiffness being available to suit the
operating pressures within the cylinder and a reasonable height of diagram.
A spindle connects the indicator piston to a system of small levers designed to produce a vertical
straight-line motion at the pencil on the end of the pencil lever, parallel (but magnified about six times)
to the motion of the indicator piston. The ―pencil‖ is often a brass point, or stylus, this is brought to
press lightly on specially prepared indicator paper which is scrapped around a cylindrical drum and
clipped to it. The drum, which has a built-in recoil spring, is actuated in a semi-rotary manner by a cord
wrapped around a groove in the bottom of it; a hook at its lower end to a reduction lever system from the
engine crosshead attaches the cord, passing over a guide pulley. Instead of the lever system from the
crosshead, many engines are fitted with a special cam and tappet gear to reproduce the stroke of the
engine piston to a small scale. The drum therefore turns part of a revolution when the engine piston
moves down, and turns back again when the engine piston moves up, thus the pencil or stylus on the end
of the indicator lever draws a diagram which is a record of the pressure in the engine cylinder during one
complete cycle.
                                                                                                       22




                                  Line Diagram of Engine Indicator.

Above figures show an engine indicator which is suitable for taking indicator diagrams of steam
reciprocating engines and internal combustion engines up to rotational speeds of about 300 rev/min. In
this type, the pressure scale spring is anchored at its bottom end to the framework, and the top of the
piston spindle bears upwards on the top coil of the spring, the upward motion of the indicator piston thus
stretches the spring.
Types of Indicator Diagrams:
Four types of indicator diagrams or cards can be obtained from a slow-running diesel engine:
    1. POWER CARD:




This is taken with the indicator drum in phase with piston movement. The area within this diagram
represents the work done during the cycle to scale. This may be used to calculate the power produced
after obtaining the indicated mean effective pressure of the unit.
                                                                                                        23
   2. COMPRESSION DIAGRAM:




This is taken in a similar manner to the power card but with the fuel shut off from the cylinder. The
height of this diagram shows maximum compression pressure. If compression and expansion line
coincide, it shows that the indicator is correctly synchronized with the engine.

   3. DRAW CARD or OUT-OF-PHASE DIAGRAM:




Taken in a similar manner to the power card with fuel pump engaged but with the indicator drum 90*
out of phase with piston stroke. This illustrates more clearly the pressure changes during fuel
combustion.

   4. LIGHT SPRING DIAGRAM:




Taken similar to power card and in phase with the engine stroke, but this diagram is taken with light
compression spring fitted to the indicator. This shows clearly pressure changes during exhaust and
scavenge in enlarged scale. This can be used to find any defects during those operations.
                    24




TWO-STROKE CYCLE.
                                                                                                 25




            Typical Power Card with Out Of Phase Card taken on the same Diagram.




Trace of a power card taken over a full cycle with the card „opened‟ out so that the compression
curve appears to the left of the vertical (tdc) line and the combustion and expansion occurring to
the right of the same line. This is common way for electronic monitors to record events in the
cylinder, again relevant pressures and angles may be well recorded on the print out.
                                                                                    26




                           Card taken by Electronic Device.
    Typical print taken from an electronic measuring device.
    Pressure and their relevant angles are automatically printed on to the card.
    Very useful for checking engine performance.




-------------: Early Injection.               T.D.C
-------------: Normal Injection.
-------------: Late Injection.
-------------: Late Injection with After Burning.
            *****************************Kv************************************
                                                                                                       27

3. Combustion.
This is an exothermic reaction (one in which heat is liberated by the action) between a fuel and oxygen.
Liquid fuels consist of carbon, & hydrogen, in the form of hydrocarbons, with small quantities of
sulphur & traces of other metallic Impurities such as vanadium.
A typical fuel analysis, by mass would be:
C = 5%, H2 = 12%, S = 3%, with a C.V. of 44000 KJ/Kg.
(19000 BTU/lb.)
The oxygen is obtained from the air, which can be considered to contain 77% nitrogen & 23% oxygen
by mass.
The nitrogen plays no active part in the combustion process but it is necessary as it acts as a moderator.
With pure oxygen, the combustion would be violent & difficult to control & it would produce very high
temperatures, creating cooling, metallurgical & lubrication problems.
The reactions, which occur, are:
2H2 + O2 ----------- 2H2O – liberating 142 MJ/kg. H2.
C + O2 -------------- CO2 – liberating 33 MJ/kg. C.
S + O2 --------------- SO2 – liberating 9.25 MJ/kg. S.
2C + O2 --------------2CO – liberating 10 MJ/kg. C.
Combustion will only occur within limits in the air/fuel mixture. If too much air is supplied all the fuel
will be burnt but the excess of oxygen & nitrogen will carry away heat. If too little air is supplied
incomplete combustion will occur, when all the hydrogen will be burnt but only part of the carbon, with
the remainder only burning to carbon monoxide or not burning at all. In diesel engine practice it is usual
to supply between 100 & 200% excess air by mass, though 15% is sufficient for a steady flow
combustion process (boiler).
This difference has two reasons:
    1. As the combustion proceeds in the diesel engine, the fuel finds less & less air to combine with in
        a boiler air is constantly being fed in.
    2. More air is needed in the diesel engine as it lowers the maximum temperature, allowing Cast iron
        to be used.
Combustion Process.
Fuel is injected into the clearance volume towards the end of the compression stroke, as a fine mist of
very small droplets, which have a surface area many times that of the accumulated fuel charge. These
droplets are rapidly heated by the hot compressed air, which has a temperature of between 550* to
650*C, causing vaporisation. The vapour mixes with air and when the mixture exceeds the spontaneous
ignition temperature, (S.I.T.) combustion begins.

The process can be divided into four phases :
   1. Injection delay.
   2. Ignition delay.
   3. Constant volume combustion.
   4. Direct burning.

Injection delay:
A time lag of about 0.005 seconds occurs between trapping the fuel charge in the pump barrel and
starting injection into the engine cylinder. This is due to:
            a) Elasticity of high-pressure fuel lines & system.
            b) Slight compressibility of the fuel charge.
            c) Leakage past the pump plunger & injector needle.
            d) Opening delay of the pump discharge valve & injector needle.
                                                                                                         28
In a slow speed engine the lag period accounts for up to 5* of crank movement. In a high speed engine it
may account for 20* or more and because of point (a) it is necessary to use fuel lines of similar length
for all cylinders, when the fuel pumps are grouped together.

Ignition Delay.
Ignition delay is another short period of time delay, which is sufficient to account for several degrees of
crank angle. Several factors are involved:
            a) Spreading and penetrating of the fuel in to the clearance volume space.
            b) Heating of the fuel to cause vaporization & then exceeding the fuels‘ spontaneous
                ignition temperature.
            c) Mixing of the fuel & air in the clearance volume space before detonation.

Constant Volume Combustion.
Ignition occurs at T.D.C. when the fuel charge, which has entered during the ignition delay period, burns
rapidly causing a sharp rise in cylinder pressure with little movement of the piston occurring. Modern
four stroke engines may attain 100 bar; at this point where as a two stroke engines are likely to operate
with pressures of 75 to 98 bar.

Direct Burning.
The remainder of the fuel burns as it enters the cylinder and mixes with air. The excess air and
combustion gases prevent high temperatures and rapid combustion so the pressure remains about
constant. Injection and combustion should cease simultaneously at the end of this period.

Factors Affecting Combustion.
In order to attain good combustion it is essential that:
    a) Sufficient air is supplied.
    b) Compression is high enough to give a temperature above the spontaneous ignition temperature.
    c) Good mixing of the air and fuel is obtained.
All of these give problems. The factors affecting combustion are:
            1. Atomisation.
            2. Penetration.
            3. Turbulence.

1. Atomisation.
The rate of heat absorption and burning depends upon the surface area of the fuel particles. As this must
be rapid it follows that the surface area needs to be big & this is achieved by breaking up the fuel into
small droplets. The amount of the fuel pressure, diameter of injector nozzle holes and the viscosity of
the fuel, affect the process.
2. Penetration.
To use all the air in the combustion space it is necessary to give the fuel particles sufficient energy to
enable them to penetrate to the extremes of the space. This is controlled by the fuel pressure, the size of
the particle & the length to diameter ratio of the nozzle hole (From 2:1 to 5:1). The latter also controls
the angle of spray.
3. Turbulence.
To aid mixing of fuel with air and atomisation, friction between the fuel & air is needed. Friction is a
function of the relative velocity between the fuel particle and the air, and may be obtained by either of
two methods.
    a) Fuel seeks air.
    b) Air seeks fuel.
                                                                                                          29
    a) The air is static or slow moving and the mixing energy is obtained from the fuel particles.
         Injection pressures of 200 to around 1000 bars are needed from multi-holed nozzle injectors.
         Advantages are, simplicity, economy and easier for cold starting the engine. The latter because
         little air movement means reduced heat loss to the cold liner and piston crown (also assists in the
         burning of heavy fuel). Disadvantages are in producing and sealing high fuel pressures.
    b) The air is made to swirl rapidly at the end of the compression stroke by using a pre-designed
         combustion chamber. Single holed nozzles and lower fuel pressures are used, 70-100 bars.
         Advantages are simplicity of injection, equipment and rapid combustion (useful in high speed
         engines). Disadvantages are complicated combustion chambers and high rate of heat loss to
         surroundings. Causes difficulties in cold starting, sometimes needing cylinder combustion space
         heating system.
In practice, a combination is often used minimum fuel pressures being used with a small degree of swill
produced by vaned inlet valves or tangentially cut scavenge ports. Quantity of swirl causes half the
liner circumference to be traversed during combustion.

Combustion Faults.
Detonation.
The combustion process is regarded as a controlled explosion with a flame front speed of about 25 m/s.
However if combustion conditions are not correct double ignition may occur and a ‗detonation‘ may
result. The latter occurs when the mixture is rapidly compressed by an initial ignition and the remaining
mixture is overheated and burns almost instantaneously (Flame speed 2000 m/s). The detonation can set
up very high pressures, temperatures and causes vibration of the cylinder and piston. It also reduces the
efficiency of the engine as energy is absorbed producing the vibration.
After burning.
This occurs when combustion extends into the expansion period after the injector has closed. It is caused
by poor ignition qualities or very poor atomization and produces high exhaust pressures and
temperatures.
Injection timing.
Early injection produces high firing pressures; late injection produces low firing pressures and high
exhaust pressures. In both cases the engine power is reduced.
All these faults could be seen very clearly in indicator cards of each unit.

Ideal Combustion.
To obtain maximum thermal efficiency, the combustion process should be carried out as close to the
Otto cycle as practically possible. This means, the rate of rise of pressure should be as rapid as possible,
without exceeding the designed mechanical and thermal loading. To achieve maximum mean effective
pressure the fuel remaining after the initial period of rapid rise, should be burned at a rate which will
hold the cylinder pressure constant, at the maximum design value until the fuel is burned.
Some of those factors affecting the ideal combustion can be considered as follows.

Injection timing.
Using jerk injection system, it has been found that the shortest delay period occurs when it includes
T.D.C.
            1. Early injection results in increased delay since the pressure and temperature are still
                rising, so auto injection energy has not been reached.
            2. Late injection causes increased delay since the piston is accelerating away from the
                cylinder head and temperature and pressure fall rapidly.
In each case, the rate of pressure rise is increased due to the large quantity of the fuel in the combustion
space before the chemical reaction is initiated. The reaction, which follows involves a massive amount
of fuel and approximates to detonation.
                                                                                                            30
This results in ‗Diesel knock‘, the effects of which are determined objectionable. Many engines are
timed later than that which gives maximum mean effective pressure to reduce the rate of pressure rise
and the maximum pressure. This however involves some sacrifice in efficiency and power output.
Engine R.P.M.
Since the delay period is determined mainly by the fuel characteristics, it follows that delay tends to be
independent of engine speed. The delay angle however will vary with engine speed and have
considerable influence on the pressure / crank angle diagram.
In each case – 10 deg. BTDC & 20deg. BTDC the delay angle is increased with increase in speed.




                - - - - - - -: High Speed.                 -----------: Slow Speed.
Other factors influenced by engine speed may include.
   1. Fuel spray characteristics (since fuel pumps are engine driven and pressure and temperature in
       cylinder affect secondary atomisation).
   2. Volumetric Efficiency (since the piston speed & valve opening characteristics influence the gas
       exchange process).
   3. Combustion chamber wall temperature (since rate of heat input & rate of heat conduction
       determines the wall temp).
Fuel / Air Ratio.
As fuel is being injected there will be local fuel-air ratios varying from infinity near the injector to zero
where fuel vapour has not yet reached. Provided the vapourisation is not complete before injection
commences the amount of fuel injected would have no direct effect on the delay period. However, with
reduced Fuel /Air ratio, combustion temperatures are lowered, which reduces the cylinder wall
temperature. With some engines, this may have the effect of increasing the delay period.




                                    Varying Fuel/Air Ratio Diagram.
                                                                                                        31
From the above diagram it may be seen that:
    1. The delay period is not effected.
    2. There is little reduction in rate of pressure rise.
    3. Provided that only a small proportion of the fuel is injected during the delay, it will have limited
        effect, on the maximum pressure.
Combustion can take place with extremely low Fuel/Air ratios, probably due to burning taking place
close to the injector where the local F/A is high enough for stable reactions to occur.
Turbulence.
The turbulence effect is probably associated more with the mixing process rather than with propagation
of chemical reactions.
Turbulence takes place possibly in two ways:
    1. Primary turbulence: Due to the way in which the air enters the cylinder. In large diesel engines
        this is produced by the angularity of the inlet ports, near the end of compression, when the air
        density is high, the effective swirl will be greatly reduced.
    2. Secondary turbulence: Squish, is produced, by the shape of the piston crown and cylinder head.
        The air is made to move readily inward and across the path of the automised fuel. This may help
        to secure short second and third combustion stages.
Turbulence after complete combustion, say due to detonation, can break down the cool insulating layer
of gas near the cylinder head wall, which will:
    1. Reaches cylinder wall temperature locally (Hot spot).
    2. Increases heat loss to cooling water.
    3. Breaks down the oil film on the cylinder walls. Promotes micro seizure and service wear.
Compression ratio.
The compression ratio determines the air pressure and the temperature at the moment of fuel injection
and will have a considerable influence on the degree of secondary automisation, the delay period, and
the rate of rise of maximum pressure. Increasing the compression ratio alone, in the range used for diesel
engines, has only a marginal effect on the power developed and cycle efficiency.
High compression ratio, do however increase cylinder friction loss, ring leakage, and starting torque
requirements. With highly pressure charged engines, the cylinder air charge is increased which allows
more fuel to be burnt, but if working close to the Otto cycle the maximum pressure can be high. To limit
the maximum pressure and therefore maximum stress the engine is designed to operate with lowest
compression ratio consistent with satisfactory running and starting.
Turbocharging.
This has the tendency of raising both the pressure and temperature at the point of fuel injection. This is
beneficial in reducing the delay period and the rate of pressure rise. The degree of supercharging is
limited not so much by combustion considerations but by durability and reliability of the components
concerned in stressing the high maximum cylinder pressure and high heat flow rates.
Air inlet and Jacket water Temperatures: Increasing both of above:
    1. Reduce the delay period.
    2. Reduce rate of pressure rise.
    3. Reduce heat flow to the cylinder coolant.
    4. Reduce power developed due to reduced air mass.
    5. Increases cylinder wall temperature.
    6. Increases cycle efficiency due to reduced heat loss.
Increasing the air inlet temperature has the effect of down rating the engine and lowering the smoke
thresh-hold. In each case this is due to the reduced air mass. This effect can be pronounced when
operating in the tropics, where both air and sea temperatures are high. One should keep in mind that,
while operating in cold climates, where sea and air temperatures are low, the inlet air temperature should
not be brought down too low, as humidity in the air may cause corrosive damage to cylinder liners.
***********************End of Combustion/BIT/AMET/BE/Motor/Kv/May 2003********************************
                                                                 32


4. Marine Diesel Structural Parts:




MAIN STRUCTURE OF MODERN LARGE POWER SLOW SPEED DIESEL ENGINE.

                     Type: MAN/B&W: „MC‟ Type Engine.
                                                                                                         33
Bed Plate:
The bedplate is a substantial, rigid structure which forms the base on which the engine is built. It is
supported by the ship structure through the double bottom arrangement, but this support does not reduce
the rigidity needed & in fact with some modern vessels, the hull is left flexible and the bedplate stiffened
so that a simple four-point attachment to the hull can be used. This reduces the distortions developed in
the bedplate when hull deflection occurs.

Forces applied to the bedplates:
    1. Firing load from cylinders.
    2. Side thrust from guide faces.
    3. Unbalanced inertia forces in the running gear.
    4. Weight of engine structure & running gear.
    5. Torque reaction from propeller.
    6. Hull deflections due to hogging, sagging, racking.
    7. Vibration due to torque variations, shock loading.
    8. Thermal stresses due to atmospheric and lubricating oil temperature changes.
    9. Inertia & gyroscopic forces due to ship's movement in heavy seas.
In addition to withstanding forces due to the above causes,, the bedplate should provide.
    1. An oil tight chamber to contain the oil splash & spray of the forced lubricating oil system.
    2. A drainage grid to filter out large particles before they enter the oil sump or drain tank.
    3. A housing for the thrust bearing.
Having provided for all the above the bedplate should also be small & light to keep the overall size and
mass of the engine to a minimum.

Basic Structure:

The bedplate consists of longitudinal and transverse girders as shown below:




Longitudinal Girders may be single or double plate construction.




                         Single plate type.                    Double plate type.
                                                                                                           34
Their purpose is to maintain longitudinal alignment by providing sufficient rigidity to withstand the
hogging & sagging of the hull structure & provide a stiff support for attachment of the transverse
girders.
The double plate form is stiffer but more complex than the single plate and makes access for holding
down arrangements more difficult. The single plate form is becoming more popular with the use of box
bedplates and similar construction of columns.
Transverse Girders are deep plates lying between the longitudinals and fitted with pockets to carry the
main bearings. A deep plate is needed to give sufficient stiffness (―I‖-value) to resist the firing load
without bending. Inadequate stiffness will cause distortions of the bearing pocket, which will ‗nip‘ the
main bearings, gripping the crankshaft journal and causing ‗wiping‘ of the bearing.

The girders may be of single or double plate construction with a flat plate on the top to give a landing
for the „A‟ frames or equivalent.




The double plate arrangement provides the greatest strength & stiffness but holes must be cut in the
plate to allow access for welding and inspection. These holes must be large enough to allow easy entry
by a welder and can, seriously weaken a double plate arrangement for a small engine. To restore
strength & stiffness a tube may be welded through the girder holes.
Depending upon the material used, the attachment of the transverse girders to the longitudinal girders
may differ most are welded but some may be bolted if the girder is cast as this reduces repair difficulties,
allows stress relieving of the girder only and lessens risk of distortions.

Types of Bedplates.
The two most common types are:
   1. Box type.
   2. Trestle type.

   1. Sulzer, B & W, MAN and Doxford, all use the box or flat bottom type as it can be mounted
      directly to the tank top plating (via chocks) and is suitable for fabricated construction.
   2. G.M.T. & Mitsubishi are examples of engines still using the trestle type. This type provides a
      deep and therefore stiff transverse section. To accommodate this deep suction however the
      bedplate must be seated on special built up stools in the double bottom structure or a special well
      must be left in the double bottom structure. Both complicate the double bottom structure. If the
      ‗well‘ is used an added attraction is a reduction in engine height.
                                                      35




            Flat or Box type Bedplate Construction.




Tie-bolt.




              Trestle type Bedplate Construction.
                                                                                                        36
Bedplate Materials.

   1. Fabricated mild steel. Slow speed engines Sulzer, Doxford.
   2. Cast iron. Medium speed engine (small).
   3. Composite type. Fabricated mild steel longitudinal girders and cast steel transverse girders.
                       Engines that use above are B & W, G.M.T. Mitsubishi, MAN. Sulzer.

   1. The all welded form of construction gives the lightest bedplate (about 25% less than C.I.) with
      the greatest strength against shock loads & the highest guarantee of manufacture. It is also the
      easiest to repair. However it possesses poor vibration damping characteristics & due to the
      multitude of welds is liable to cracking. To ensure freedom from distortion the welding sequence
      must be correct and after welding the bedplate requires stress relieving by heating to 600*C and
      holding for 1 hour/inch (25 mm) of plate thickness. Normal plate thickness is 1½‖ - 2‖ (35-50
      mm). The size of the bedplate is controlled by lifting equipment available and the size of the
      stress-relieving furnace. Because of these factors plates are normally made in at least two parts.
      Transverse girders are normally cut from a single plate and supporting ribs welded on below the
      bearing pockets. Pockets are usually of cast steel. Examples: M.A.N. & SUIZER Engines.


   2. Cast Iron: It is never used for large bedplates any more as the quality of generator of a defect free
      casting is not good enough. Frequently used for small engines however. The main advantage is
      the materials ability to absorb vibration (not shook), which limits vibration transmission through
      the engine & reduces the frequency of cracking in the bedplate. Any cracks are difficult to repair
      & require a ‗Metalock‘ type repair, which cannot be effected by ship's staff. The material has a
      low tensile strength and is usually supported by tie-bolts. Examples: Only small medium or high-
      speed engines use this type of bedplate.


   3. Composite construction involves fabricated mild steel for the longitudinal girders and cast steel
      for the transverse girders. This system has the advantage of a continuous transverse girder with
      the bearing pocket integral. Strengthening ribs can be cast in and the complete unit stress
      relieved before bolting or welding to the longitudinal girders. The cast steel must be of wieldable
      quality, up to 0.23% C. The steel has a higher resistance to cracking compared to fabricated mild
      steel due to the irregular grain flow and lack of welds. Examples: B & W, Doxford, G.M.T.,
      Mitsubishi engines.

The following surfaces of the bedplate must be machined:

   1.   Top face: For attachment of ‗A‘ frames.
   2.   Bottom seating face: For chocks, tie-bolt heads and oil sump pan.
   3.   End face: For thrust block housing, turning gear & end chocks.
   4.   Side face: For side chocks and Entablature cover plates.

Faults found in Bedplates:
   1.   Cracks.
   2.   Oil leaks.
   3.   Loose chocks.
   4.   Loose ‗A‘ frames.
                                                                                                     37
   1. Cracks usually occur:




           i)     Under bearing pockets on fabricated mild steel bedplates.
           ii)    Radially around tie bolt & frame boltholes.
           iii)   Between longitudinal and transverse girders.
           iv)    Around ‗lightening‘ holes.
           v)     At the base of serrated seating for main bearing keeps.

Causes may be:
                    a)   Bearing wear & therefore overloading.
                    b)   Slack tie bolts.
                    c)   Vibration.
                    d)   Poor welding or stress relieving.
                    e)   Stress risers on welds -(Coarse welds should be ground).

Repair :
 For mild steel & cast steel crack chipped out and welded, but care should be taken to ensure a minimum
distortion by determining the optimum welding sequence.

For Cast Iron the crack should be arrested by drilling a small hole, sketch or photograph the crack for
future assessment. The crack could be ―Metallocked‘ or supported by a mild steel doubling plate, bolted
on, if serious.
    2. Oil leaks:
            i)     Sump pan.
            ii)    Doors and casings.
            iii)   Crank case relief valves.
            iv)    Bedplate cracks.

   3. Chocks may fret if the holding down bolts get lack and due to the movement of bed plate chocks
      ‗bed‘ into the tank top. As a temporary measure the chock should be shimmed up and the bolt
      hardened down and as soon as possible the chock should be removed, the tank top faced up by
      grinding and a new, thicker chock prepared and re-bedded.

Bedplate inspection.
   1. Cracks.
   2. Corrosion. This may be due to moisture or acidic compounds in the oil. If the bedplate has been
      painted, remove flaking paint and cheek for pitting. After that do not repaint.
                                                                                                38
3. Cleanliness. Check for sludge and carbon building up in corners, under bearings, behind bearing-
   cover studs, etc.
4. Loose connections - bolted transverse girders, A-frames, oil pipes, sump grids, chocks and
   holding down bolts.
5. Oil leaks - through cracked welds, loose sumps, leaking seals.
6. Faulty welding - on new engines - under cutting, blowholes, slgg; etc.
7. Faulty castings - porosity, blowholes, inclusions etc.




                                   MAN Engine Bedplate.




                                   Engine Bedplate sketch.
                                                                                                          39
ENGINE CHOCKS.
These are needed between the bedplate and tank top to ensure that any variations in the surface of the
tank top does not cause misalignment. Up to 200 chocks per engine may be fitted. They also permit any
chaffing or fretting to be repaired by adjustment of individual chocks and any subsequent distortions
after fitting (due to settlement) to be corrected.
End chocks are fitted at each end of the long girder to position the engine, absorb collision loads and in
the case of the integral thrust block, absorb propeller thrust & propeller excited vibrations.
Side chocks are needed to absorb side loads due to components of unbalanced reciprocating forces and
thermal expansion. They also prevent chaffing of the supporting chocks and tank top and also help the
holding down bolts resist the lateral forces when the vessel is rolling.

Chocks are usually made of cast iron or steel. Cast Iron chocks are popular because:
    1. Easy to form.
    2. High compressive strength & low malleability.
The chock retains its shape under load reducing the chance of bolt slackening & therefore bolt fracture.
Unfortunately this also means that the chock is hard and liable to ‗bed‘ into the tank top or bedplate. It is
also brittle and therefore liable to fracture under excessive impact loads, hence minimum chock
thickness should not be less than 30 mm. Steel is used to reduce those problems and allow easier fitting.
Steel chocks should be used for clearances less than 30 mm.
Epoxy resin is increasing in popularity and now widely used for small, medium & large engines. The
compound has the following advantages:
             i)     Elimination of fitting & machining.
             ii)    Increased support as large areas of the bedplate can be used.
             iii)   Elimination of breakage, fretting and slackness.
             iv)    Improved resilience, which absorbs vibrations, reduces noise and gives greater
                    ductility.
The compound is suitable for any bedplate, which can be fitted with a sealing dam to contain the
compound while it is setting (may take up to 24 hrs with some heating, around 16*C necessary). It can
be used on new engines or as a replacement on old engines. Where the chock is deep, steel rollers are
added to the resin chock to increase strength & Durability.

Fitting of engine chocks:

Process is more or less similar for cast iron, steel or resin chocks.
   1. Bedplate is aligned on the tank top using temporary chocks, jackscrews or wedges using the
       sagging wires pilgrim or optical alignment method.
   2. Crankshaft is budded & deflections taken after the engine is fully built up and the vessel is
       floating in even keel with all transmission shafts in place.
   3. Metal chocks are machined slightly oversize and then hand filed and scrapped. It is bedded in its
       place and fitted. Minimum 70 to 80% bedding is required. For bedding purpose the chock could
       be tapered up to 1/100 from outside to inside.
   4. For Resin Chocks the surfaces are cleaned, a dam prepared around the chock area, holding
       down bolts placed in position and greased and all surfaces sprayed with a releasing agent. Resin
       is mixed and poured into position. When solid, temporary support can be removed and after 24
       hours, holding down bolts tensioned. A 1μm per mm of chock thickness is allowed for shrinkage.
   5. Crankshaft deflections are retaken to confirm alignment. The deflection reading should be the
       same at the end of fitting the chocks as it was when taken before fitting as per step 2.
                                                                                                              40
A third material, rubber is used for some installations; usually high speed diesel engines in small vessels.
These are resilient mountings and fitted to reduce vibration transmission from engine to hull or vice
versa. Very careful selection of the right size and stiffness must be made in order to obtain the optimum
operation and sufficient flexibility must be arranged in all connections to the engine to prevent any
restrictions. This includes the output shaft coupling. Generally flexible hoses connect all pipelines and
for engine exhaust pipe line a good metal exhaust bellow is fitted. 4 to 8 mountings are normal.

Some points regarding Epoxy Resin:
Resin chocking is a recent development in engine chocking arrangement. It has been widely used now
for large, medium and small engines. It has following advantages:
    1. Reliable permanent alignment without machining foundation, bedplates or chocks.
    2. Provides uniform precise mounting for superior retention of critical alignment.
    3. Resists degradation by fuels, lubricants, eliminates corrosion in chock area.
    4. Non-fretting.
    5. Reduces noise levels, maintaining the alignment and hold down bolt tension.
    6. The modulus of resin helps to maintain crankshaft deflection and machinery alignment during
        hull flexure or distortion.

MOST IMPORTANT
This is a liquid; it conforms to all irregularities in the fitting surface, providing a precise contact fit
between machinery bases and foundations (after solidification).
Properties of the chock after it has cured:
             i)      Compressive strength: 1330 kg/cm2.
             ii)     Tensile strength: 350 kg/cm2.
             iii)    Shear strength: 380 kg/cm2.
             iv)     Heat distortion temperature: 93*C.

Bedplate holding down bolts.
Holding down bolts may be fitted or clear. If collision & side chooks are used the bolts are usually clear.
If not the bolts at the flywheel end are fitted, remainder clear, to ensure the coupling to output shaft is
not strained. Bearing faces of bolt heads & nuts must be normal to the bolt shank & parallel to each
other to prevent any bending stresses. If necessary the bedplate & tank top may be machined.




                                 Traditional holding down bolt arrangement.
                                                                                                        41
Procedure for fitting the above holding down bolt.
           a) Harden the stud into screwed tank top to achieve watertight seal on the conical face.
           b) Tighten lower nut, tack weld or caulk over thread for locking.
           c) Tighten upper nut.

Modern method of holding down bolt arrangement.
The traditional method suffers the problem of fretting cast iron chocks and bolt failure particularly under
slow speed diesel machinery. In modern days to eliminate the above problem long elastic bolts with
extended collars are used.
These bolts possessed high resilience and are highly stressed when tightened. As such when strained
while in service, there will be less reversal of stress which results in reduced possibility of fatigue
failure.
When these bolts are tightened and slightly stretched, the bedplate, chock and tank top seating are under
compression. When holding down bolts come under strain while in service, the parts under compression
expand and the mating surfaces of the chocks remain in contact with the bedplate and tank top seating.
Fretting is hence avoided.
Their cost is considerable and an additional £40,000 for the bolts of a 6-cylinder engine is typical.
Epoxy Resin chock.
‗Chockfast‘ system needs only simple bolts and nuts to give permanent engine security. It is claimed
that the use of pour able resin chocks overcomes bolt stretching, slack nuts and bolt failure, while also
offering considerable economies when erecting the engine since perfect matching takes place between
engine bed-plate and the unmachined tank-top seating.




                           Line diagram for holding down bolt and chock.
                                                                            42

Modern slow speed main engine bed plate arrangement over ships structure.




   Bedplate holding down arrangement for the above engine with long bolt.
                                                                                              43
Earlier engines bedplate and holding down arrangement showing main chock, side chock, end chock
                                   and hydraulic stretching tool.




         Chocking arrangement with tall bolts and washer system of holding down bolts.
                                                                                                     44
Inspection requirement pertaining to holding down bolts and engine chocks.

Holding down bolts are strained while in service and thus required to be tightened up occasionally if
troubles with bedplates is to be prevented. Even the most imperceptible movement of the bed plate will
cause fretting to occur on the bedded mating surfaces of the bed plate, chock and foundation plates. If
fretting occurs in areas covering a number of adjacent chocks, the crankshaft may be seriously damaged
through misalignment.
New installations should have the bolts checked after a few running hours and at least every six months
after that. A record should be kept. These holding down bolts should be checked fully if the vessel had
met with an accident, such as grounding near engine room, fire in engine room or near the engine room
and collision.


Bedplate Inspection
   1. Cracks (split around the parts mentioned earlier).
   2. Faulty welding - on new engines (under cutting, blow holes, slag etc.).
   3. Faulty castings - porosity, blowholes, inclusions etc.
   4. Corrosion.
   5. Cleanliness - sludge and carbon build up in corners, under bearings, cover studs etc.
   6. Loose connections - bolted transverse girders, A-frames, oil pipes, chocks and holding down
       bolts.
   7. Oil leakage.


***************************************Kv******************************************
End of Bedplate/chocks/BIT/AMET/BE/KV/May 2003.
                                                                                                            45
Engine Frames and Cylinder Blocks:

Engine Frames.
These fit between the bedplate and cylinder block beam. They are sometimes referred to as the
entablature. They serve the following functions.
             a) Support the cylinder blocks, turbo-chargers, camshaft and driving gear, scavenge belt etc.
             b) Provide a facing for the girders & absorb the guide forces.
             c) Develop an oil tight easing, for forced lubricating oil system, & support pipes &
                walkways.
„A‟ – Frames.
In old engines the frames were of cast iron and made hollow to reduce weight without reducing rigidity.
The frames or columns were held in compression by tie-bolts. These frames were later fabricated from
mild steel tube and plate with guides of cast iron bolted onto the frames. This type of arrangement uses
individual frames at each transverse girder position of the bedplate with the longitudinal spaces between
frames filled by plates bolted to the frames. The structure is strong and rigid in the transverse plane but
relatively flexible longitudinally. This makes oil tight fixing of the side covers difficult unless very
heavy covers or longitudinal stiffness are used. It also produces a weak structure if exposed to internal
pressure from a crankcase explosion and will allow alignment of the cylinder blocks to the bedplate to
vary in relation to ship movement.
The ‗A‘-frame construction is now being abandoned in favor of longitudinal girder construction.
Improved methods of prefabrication which can be relied upon to produce large, distortion free units has
allowed longitudinal girders to be manufactured so that the longitudinal stiffness of the structure can be
increased without altering the transverse stiffness. This also contributes to the bedplate stiffness and
reduces effects of hull hogging and sagging. ‗MAN‘ engine manufactures claim that the bedplate only
contributes 17% to the overall stiffness compared to 60% for the traditional ‗A‘-frame construction.

In the ‗Sulzer‘ engine the fabricated longitudinals form a sandwich by enclosing a cast iron centerpiece
at each transverse girder spaces. The cast iron centerpiece forms the crosshead guides. The structure is
bolted together.

In the ‗B&W‘ engine the entablature retains the ‗A‘ transverse section but both longitudinals and
transverse components are fabricated into a box form. The guide faces are bolted to the transverse
components. The entablature is formed in two pieces connected at the camshaft drive position at the
middle of the engine.

In the ‗MAN‘ engine, regular box shaped fabrications are used, again with longitudinal and transverse
sections welded together to form a single unit. The layer sizes (more than 700 mm bore) have the box
divided into 2 on the horizontal plane. The upper box has openings on the back into which the cast iron
guide faces are bolted. In the ‗Doxford-J‘ engine a continuous girder is fabricated for the guide side of
the framework with the columns at each main bearing position welded to the longitudinal. The front of
the engine is left more open to allow easy access to the running gear.
Apart from increased stiffness which reduces:
            i)      Misalignment,
            ii)     Bearing distortion,
            iii)    Vibration,
The structure is more oil tight, as fewer joints are required & the structure ‗works‘ less. It is also easier
to build the engine & ensure equivalent alignment when the engine is reassembled in the ship.
                                                                        46




M.A.N. Engine Bedplate, Lower frame, Upper frame and Cylinder jacket.
                                                                                47




Transverse section of Sulzer Engine, showing all internal bolts and fittings.
                                                                                                        48




„Doxford‟ Engine Structural Arrangement.               „B&W‟ Engine Structural arrangement.

Tie Bolts.
These are fitted to relieve the frames of tensile stress.
The bolts are mounted between the transverse girder of the bedplate and the upper face of the cylinder
jacket. As this in variably makes the bolt very long it is sometimes fitted in two lengths joined at the
base of the cylinder jacket. Hydraulic tightening tensions the bolt and this pre-tensioning should be
sufficient to keep the frames in compression throughout the engine cycle. This produces a substantial
tensile stress in the bolts requiring them to be checked frequently.
Transmission of Firing Load.
In most single acting engines, apart from ‗Opposed Piston Engines‘, the long tie bolts transmit the main
gas loads from the cylinders. Two bolts are fitted to each transverse girder and they pass through the
casting through tubes constructed in the engine frames and through the entablature or cylinder jackets
where locking nuts are fitted. Tie-bolts are prestressed during assembly and carry the firing forces from
the cylinder cover to the transverse, beam and thence the ship's hull. Tie-bolts should be as close to the
                                                                                                           49
crankshaft axis as possible to minimise bending stress on the transverse girders of the bedplate and to
prevent unbalanced loads being transmitted to the welds.
The further the tie-bolts are the greater will be the bending stress. Hence any method of bringing the tie-
bolts close together will decrease the stress. Therefore ‗Sulzer‘ and ‗Fiat‘ engines have used jacking
bolts between A-frames and main bearing upper half keeps. This ensures that the tie-bolts are as close as
together as possible. Great care must be taken that the tie-bolts are correctly tensioned before tensioning
jacking bolts otherwise if the tie-bolts were tensioned after jacking bolts, the latter and main bearing
keeps could be over-stressed.




   „Sulzer‟ Engine Main Bearing Jack bolt arrangement (See page 40 drawing for full details).

CONSEQUENCES OF RUNNING AN ENGINE WITH SLACK TIE BOLTS:
Cylinder beam would flex and lift at the location of the slack bolt landing faces of the tie bolt upper and
lower nuts, landing faces of the cylinder beam on the frame would fret and machined faces would
eventually get destroyed. The fitted bracing bolts between the cylinder jackets will also slacken and the
fit of the bolts would be lost.
If fretting has occurred in an uneven pattern where the cylinder beam lands, and the tie bolts are
tightened, the alignment of cylinder to the piston stroke will be destroyed. The fitted bracing bolts
between the cylinder jackets will also slacken and fit of the bolts will be lost.
Fretting may make the nut landing face out of square and if tie bolts are tightened on the damaged face,
a bending moment will be induced in the tie bolt, this may cause an uneven stress pattern in the tie bolt
which could lead to early fatigue failure. Damage may take place in the bedplate in way of cross girder.
Rigidity of the whole structure will be destroyed. Guide force will have to be absorbed by the frame
bolts and dowels, which may stretch and slacken allowing the structure to ‗work‘. This may destroy the
piston alignment. Guide faces and bars may get slackened (these are bolted to the supporting structure).

TENSIONING OF TIE RODS AND CHECKING THE PRETENSION (‗SULZER RLA‘. ENGINE).
Bedplate, columns, cylinder jackets are greatly relieved of the gas forces set up and freed from tensile
stresses when tie rods are properly tensioned. In order to avoid vibration all tie rods are held in position
by special guide bushes located on the lower end of the cylinder jackets. These bushes are of two parts
and clamped on to the rods. Clamping bolts jam the tie rods in the bores. Tie rods are pretensioned by
                                                                                                         50
hydraulic tensioning device. Tightening is carried out in two steps to avoid to reduce additional stresses
on the jackets.
Note: For new engines it is recommended that all tie rods be checked for correct pretension after the first
year of service and if necessary pretensioned to the value specified. After that it is sufficient to make
random cheeks during major overhaul. The bolts should be checked approximately 4000 to 6000
running hours.




   1. Cylinder jacket. 2. Tie rod lower washer. 3. Tie rod main nut. 4,5,6. Hydraulic tool lower half
      with cylinder. 7. Hydraulic tool stretching piston screwed to tie rod. 94933. Hydraulic oil
      connections. 11. Oil vent screw. 12.Tommy to tighten the nut after stretching. K. Washer
      cylinder to keep the tie rod nut clear.
                     Tie rod tensioning hydraulic tool as fitted to the tie rod top.
Procedure for checking the pretension of the tie rods.
   1. Remove the thread protecting hoods from all tie rods and clean the contact face of the
      intermediate ring.
   2. Screw pretensioning jacks on the tie rods (two on opposite sides) until the lower part of the
      cylinder rests on the intermediate ring. Slightly slacken vent screws.
   3. Connect both pretensioning jacks with hoses to the high-pressure oil pump and operate pump
      until air has escaped. Retighten vent screws.
   4. Operate pump until 600 bar pressure is reached and maintain this pressure.
   5. Check with a feeler gauge through measuring point ‗S‘ for any clearance.
   6. If any clearance does exist tighten the tie rod not by a tommy bar, until it rests firmly on the
      intermediate ring. (Check with feeler gauge). If no clearance, pressure is to be released
      immediately. All the tie rods are to be checked in this manner.
   7. After checking has been completed the threads to be protected with anticorrosive grease.
Procedure for Loosening or tightening of tie rods.
   1. Before loosening or tensioning tie rods, the thrust bolts of the main bearings must be loosened.
   2. Clamping bolts must be removed.
   3. Loosening and tensioning has to be carried out in stages (three stages).
                                                                                                             51
    The order is shown in the diagram given below:




            Half full value.◄----------------------1st Strech.--------------------------►Half full value.
   Full Value. ◄----------------------------------------2nd Strech. --------------------------------►Full Value.
   Full Value (Second time).◄----------------------- 3rd Strech (Final) ---------------------► Full Value.
Tie rod loosening:
Apply pressure slightly over the engine pressure prescribed slowly. Then nut should be slackened by say
1 to 1½ turns. (If pressure is set for 600bar, then apply say 620bar).
Tensioning:
Preparation.
    1. Clamping bolts to be tightened.
    2. The lower tie rod nut is screwed on and secured with looking screw.
    3. The contact surface of intermediate ring and upper tie rod nut are to be cleaned and coated with
        ‗molykote‘ paste.
                                                                                                         52
    4. Screw eye bolt into the tie rod and carefully lift until the lower tie rod nut rests tightly against
        bearing girder. (It is best to use block and tackle between crane hook and tie rod. The tie rod can
        thus be lifted by hand until the lower nut lands on the girder. With a crane, it may not be possible
        to feel when the lower nut lands).
    5. In this Position, the upper tie rod nut is tightened with a tommy bar until it rests firmly on the
        intermediate ring and the tie rod is removed from the lifting device. (There must be no clearance
        between the bearing girder and the lower nut).
Procedure of Tensioning.
    1. Once all the preparations mentioned above have been made the engine manufacturers‘
        instruction for tensioning has to be followed.
    2. Mount pre-tensioning jacks on the two tie rods 1/1 in the middle as shown in the sketch and the
        lower part of the cylinder has to rest on the intermediate ring.
    3. Connect pre-tensioning jacks to high-pressure oil pump and vent the system.
    4. Operate pump until a pressure of about 350 bar. (1st stage, Half value) is reached. Maintain this
        pressure while the two upper nuts are tightened by tommy bar and a snug fit is obtained.
    5. After the operation and tensioning of tie rods 1/1, go over to 2/2, 3/3, 4/4 till out ward end of the
        engine.
    6. Repeat the same for full pressure from 1/1 to out ward end.
    7. For a third time do the same for full pressure once again to eliminate any residual stresses on the
        cylinder jackets.
    8. After completion of pre-tensioning procedure, coat the protruding thread with non-acidic grease
        and fit the protective hoods.
 Important points to note.
    1. Individual tie bolt must not be loosened completely without slackening the rest at least partially.
    2. Thrust bolts of bearing covers must be slackened before loosening or tightening the tie bolts.
    3. All components and contact surfaces involved in the process of tensioning the bolts must be
        checked for cleanliness, levelness, perpendicularity and parallelism.
    4. Every care should be taken to protect the threads.
    5. Tightening or loosening should be always done in stages.
    6. Before checking the bolt tightening, check that the supporting chocks are firmly fixed.
    7. Increase the pressure of the hydraulic tightening tool very slowly, while checking bolts.
    8. Always ensure that the hydraulic tool is kept in good condition along with the flexible hoses and
        the pressure gauge fully calibrated.
The following faults can be found in the structure:
    1. Cracks.
Tremendous improvement has taken place in welded structures, but still cracks can be found at junction
welds. Cracks can also occur around boltholes, or where the stress pattern is complex. So, the most
likely places for inspection to detect cracks are:
            a) Behind the guides.
            b) Around main bearing pockets.
            c) All junction welds.
            d) At any weld.
            e) Securing bolts and dowels between bedplate and frame, frame and cylinder.
            f) Around guide securing bolts.
    2. Loose Bolts.
Tie bolts keep the engine structure under compression throughout the cycle and the structure is designed
accordingly. Tie bolts may get slackened and if the slackening is considerable, the structure will not
remain under compression during combustion. The guide force will have to be taken up by the frame
bolts and dowels. This could be high enough to stretch and slacken the bolts and this would allow the
structure to "work". Bolts holding the guide faces and bars to the supporting structure may also get
                                                                                                       53
slackened. This may seriously affect the piston alignment. Fretting would take place at the landing faces
of all the parts held together.
    3. Misalignment.
Alignment of the whole structure (assemble) is of extreme importance. The initial alignment may be
carried out by a plumb line from crossed laths on the top of the liner, frame is now adjusted until the
plumb line lies evenly in a hole through lath, which is mounted between frame positions. Fitted bolts or
clear bolts and dowels now secure the frame. Now the liner and guide alignment is carried out by piano
wire, calipers/micrometers. Misalignment may occur due to:
            a) Settling of the structure.
            b) Fire.
            c) Grounding, collision.
            d) Cracking of frames.
            e) Distortion of bedplate.

Indications of misalignment.
           a) Overheated bearings.
           b) Overheated guide slippers.
           c) Uneven wear of liner.
           d) Piston slapping.
           e) Excessive vibration.
           f) Wear of stuffing box, piston rod.
    **********************************Kv****************************************
Reference: Running & Maintenance of Marine Diesel Engine By Mr. John Lamb.
          B&W, Sulzer, MAN, Doxford Engine manufacturers‘ Manual.
End of engine frames, cylinder blocks and tie rods/BIT/AMET/BE/KV/May 2003.
                                                                                                          54

Cylinder Cover:
This, in combination with the cylinder walls and piston crown provides the perimeter of the combustion
chamber. It is therefore exposed to high mechanical and thermal loads. Sufficient penetrations must be
made in the cover to house:
   1. Inlet & exhaust valves.
   2. Fuel valve or valves.
   3. Air Starting Valve.
   4. Relief valve.
   5. Indicator cock.
This makes the cover complicated and it is therefore usually cast.
Stresses in a Cylinder cover.




Valve housing - holding down studs of valve cause tensile stress in cover, which increases as the valve
expands if the valve sealing face Is at the bottom of the pocket. Thermal load can also cause tensile
stresses & distortion of inner face of cover.

4-Stroke Engine.
These are usually made of cast iron because of the number of valve penetrations and the need for large
inlet air and exhaust gas passages. To accommodate the passages and give adequate strength due to the
use of cast iron a very deep casting is needed. In order to avoid high thermal loads good cooling is
needed and this in turn demands thin metal sections. To achieve optimum strength and reduce
temperature stresses is very difficult and therefore cylinder covers are prone to failure. The biggest
problem area is between the valve and fuel injector pockets, and this is the most likely area for cracks to
occur. To overcome the problem the fuel valve may be offset from the center of the head, which
adversely affects combustion, but permits larger cooling water passages and therefore improves cooling.

Further improvements can be made if:
           a) Water-cooled valve cages are used, particularly for the exhaust valve.
           b) Separate sleeves for fuel valves are screwed & rolled into the cover.

The first gives more direct cooling as the full flow of jacket-cover coolant is directed through spaces
adjacent to the seat and stem and the ease of removal allows more frequent overhauling. The latter are
                                                                                                       55
an advantage as the sleeve can expand and contract within the cover reducing the total stresses and being
of thin section occupy little space so that an adequate water flow can be arranged between the valve
pocket and the exhaust valve.
A further improvement is the use of four valves (2 inlet valves and 2 exhaust valves). These allow more
room for the central fuel valve, provide longer areas for gas flow and reduce valve inertia. They increase
the complexity of the cover however.
The cover shown below uses a water-cooled cage for the exhaust valve but a directly mounted air inlet
valve with a renewable seat. In order to reduce the material thickness a strong back type of construction
is used with the lower face (called the flame plate) made thin and supported through heavy vertical ribs
from an integrally cast strong back plate. The cover strength is further improved by using a deep casting.




                                  Cylinder cover for 4 – Stroke Engine.
With the advancement of metallurgy and machining modern cylinder covers are integral cast with inserts
or bore cooled. A modern cylinder cover for a 4 – stroke engine is shown below along with service
temperatures of the combustion chamber. To remove heat and keep all components in the combustion
zone with in the designed thermal loading the cooling system is modernised.
Major advantages of the new bore cooling system cover design are:
    1. Extremely low valve seat temperatures and excellent temperature distribution.
    2. Extremely large stiffness resulting in valve seats with considerably improved sealing properties
        and low dynamic stresses.
    3. With the above properties, the valve overhaul intervals even with low quality fuel service will
        certainly be greatly increased.
                                                                                           56




       Sulzer Z 40/48 Engine Measured temperatures in *C in combustion chamber area.
                            500 r.p.m. b.m.e.p. 20.50 kp/cm. 687 BHP/Cyl.




AS 25/30 Sulzer engine Cylinder head temperatures between original design and new bore cooled
          design for the same engine of same power. 1000 r.p.m. – b.m.e.p. 16.29 bar.
                                                                                                           57
2 – Stroke Engine:
In a 2-s cycle more heat is liberated in the cylinder in a given time than with the 4-s engine,
consequently cooling is more important. However fewer penetrations occur in the cover because no air
inlet valves are fitted and in loop-scavenged engines no exhaust valves either.
Because the heat stresses are greater but in a simpler cover cast alloy steel can be used. To further
improve the arrangement a 2-part cover can be used.

Sulzer Engine Cylinder cover:
 Water-cooled cast steel outer cover forming the majority of the combustion space wall with a water-
cooled S. G. cast iron central insert carrying all valves. This cover holds only one fuel valve, one air
start valve, one relief valve and an indicator cook. Latest RND-M engine uses a single, forged steel
cover employing bore cooling.




      Sulzer RND – Type Engine Combustion Chamber: Cylinder Cover normal Cast Type.




         Sulzer RN 90 M type Engine Combustion Space: Cylinder cover Bore Cool Type.
                                                                                                             58




           Sulzer Engine Progressive Advancement in Cylinder Cover design and cooling.

B&W Engine Cylinder Cover:
Water-cooled cast steel cover into which piston crown penetrates at top dead center. This carries the fuel
valves (2 or 3), air start valve, relief valve etc. A large cast iron central insert carries the exhaust valve &
gas passage. The cover/liner joint is below the combustion zone, which relieves the liner of the firing
load and protects the joint. Latest B & W engine again uses a forged steel cover with bore cooling.




                   B&W :VT2BF: Engine Cylinder Cover (Old conventional Type).
                                                                       59




     B&W Engine early Bore Cooling Arrangement of Cylinder Cover.




B&W Engine: K90MC:                          B&W Engine: K90MC:
Old Type Cylinder Cover.                    New Type Cylinder Cover.
 Cast In Cooling pipes.                       Bore Cooled Type.
                                                                                                      60
MAN Engine Cylinder Cover:
Upper & lower parts with water-cooling to lower only. The lower accepts the thermal stresses while the
upper acts as a strong back to absorb the mechanical load. The last engines built by them also used bore
cooling for their cylinder covers.




                       MAN Engine: K/Z Type: Three piece Cylinder Cover.




                       MAN Engine: KSZ-B                    MAN Engine: KSZ-BL-C.
                       Old Conventional Type.               Bore cooled Piston only.
                                                                                       61




               MAN: KSZ: C/CL Engine Piston, Liner and Cylinder Cover.




MAN: KSZ 52/105 C/CL Engine Section through Piston, Liner and Cylinder Cover showing the
                    temperature gradient due using Bore Cooling.
                                                                                                         62




Sulzer Engine: Bore Cooled Piston, Liner and Cylinder Cover showing the temperature gradient.


Defects in cylinder covers:

    1. Cracking: Due to the same process as cracking in piston crowns. Generally occur around the
       fuel valve pocket or between the fuel and exhaust valve pocket. They are caused by overheating,
       casting strains or notch effects (particularly in 2 – stroke cast covers). Cracks can be repaired by
       chain studding for temporary repair, ‗Metalock‘ for a semi-permanent repair or by welding if the
       material is suitable.
    2. Burning: Due to flame impingement. Repair is by welding if the material is suitable.
    3. Distortion: Due to uneven tightening down of the cylinder cover over the liner face, overheating
       of cylinder cover (particularly if scale is present) or unrelieved casting strains. It causes liner
       joint leakage and or liner flange cracking.
    4. Deposits: Scale & silt due to poor quality water or contaminated water. Not usually found when
       distilled water is used.
    5. Corrosion: Due to inadequate or nonexistent water treatment.

************************************Kv*********************************************
End of Cylinder Covers/BIT/AMET/BE/Kv/May 2003.
                                                                                                        63
5. Engine Valve Gear and Valves:
Valve Gear:
It designates the combination of all parts, including the various valves, which control the admission of
air charge and the discharge of exhaust gases in four stroke engines, the discharge of exhaust gases in
some two stroke engines (uniflow scavenging type), the admission of fuel in air- injection and some
mechanical-injection engines, and the admission of compressed air for starting most of the larger
engines.
Valve Actuating Gear:
It designates the combination of those parts only which operate or actuate the various intake, exhaust,
fuel and air-starter valves, open and close them at the proper moment in respect to the position of the
piston and crankpin, and hold them open during the required time.
Valve Timing Gear:
It designates the combination of those parts only which affect and control the moment of opening and
closing of the valves with respect to crank and piston position. These parts include cams, camshaft and
camshaft drive. The valve gears of diesel engines vary considerably in their construction, depending on
type, speed, and size of the engines. The action of the various parts of a valve gear may be best
explained using the figure on following pages.
The crankshaft drives the camshaft by chain or gearing. A cam on camshaft lifts the push rod, which
operates the rocker arm, which in turn, changes the upward motion of the valve, thus opening it. As soon
as the closing side of the cam moves under the push rod, the valve spring starts to return the valve to its
seat and eventually closes it.




                    Arrangement of Valve Gear in a Four - Cycle Diesel Engine.
                                                                                                       64
Valve Actuating Gear:
In most engines, this gear consists of rocker arms, which actuate the valves, push rods which connect the
rocker arms and the cams on the camshaft, and a drive connecting the camshaft to the crankshaft
Rocker Arms:
The rocker arm has one end on valve stem and the other end, through a hardened steel roller, with the
cam profile; if the camshaft is located near the cylinder-head, as shown in the figure.




                                           Cam and Rocker-arm.
If the camshaft is located much lower, the other end of the rocker arm is in contact with the upper end of
the push rod and lower, with the rotating cam through a cam follower. The rocker arm is pivoted at or
near the centre and the pivot pin rest is held in brackets rigidly bolted to cylinder head. Secured to the
cylinder head, pivot pin rests in a bronze bushing or needle bearing. Rocker arm's contact to valve stem
is by means of roller or more often by means of a setscrew, which is used to adjust ‗Tappet clearance‘
(or lash).

Figure below shows the valve operating gear large marine Diesel Engine (B&W Engine K-EF Type).




The cams shrunk onto the camshaft 4 operates the exhaust valves through rocker arm 12 mounted on
pivot pin 13, push rod 10 guided by guide bush 11 and roller 2 which runs in needle bearing 3.
                                                                                                           65
The roller guide 1 is prevented from turning in the bores in the housing by the key and keyway 6. The
housing is closed at the top by a cover 7, which is provided with a scraper ring 8 to prevent oil leakage.
The housing around the cam discs serves as a lubricating oil bath.

Automatic Valve – Lash Adjusters:

Automatic valve - lash adjusters are used on some engines to avoid the necessity of a clearance
otherwise needed in the valve gear to allow for expansion due to temperature changes. They also
eliminate the need for manual adjustment in order to take care of wear at various points of the valve
gear. Automatic adjusters may be either mechanical or hydraulic. The mechanical type uses a cam
(generally located at the end of the rocker arm over the valve stem) and a spring, which turns the cam so
as to take up the clearance when the valve is on its seat.




                                        Hydraulic Lash Adjuster.

The above figure shows a hydraulic lash adjuster built into the end of the rocker arm above the push rod.
It consists of a small cylinder (called ‗lifter cylinder‘) containing a plunger, spring, and ball check valve.
The plunger rests against the upper part of the rocker arm, while the spring pushes the cylinder
downwards toward the push rod. In operation, oil under pressure from the lubricating oil system enters
the lifter cylinder, past the ball check valve and is trapped under the plunger, which has previously taken
up the clearance. When the rocker arm moves downward to open the valve, the trapped oil transmits its
force through the cylinder to the push rod. If the valve stem expands, there is sufficient leakage of oil
past the plunger to permit the lifter cylinder to rise slowly so that there is no danger of holding the valve
open. In some engines such as B&W: K-EF type, automatic valve adjusters are incorporated at the
bottom of the push rod.

Push Rods:

These are generally hollow, to obtain stiffness without unnecessary weight. Usually (in small high speed
engines), lower end of push rod carries a head or ‗follower‘ of flat or mushroom shape, which rides on
the cam; a rounded head at the upper end fits into a cup on one end of the valve rocker arm. In many
engines, side thrust on the push rod is avoided by using a hinged follower, which rests on the cam and
transmits the cam action to the push rod; the follower carries usually a roller, which runs on the cam and
thus reduces friction. A cam and follower arrangement is shown in the figure (Page 59).
                                                                                                         66




                                        Hinged Cam – Follower.

Mechanical valve actuated Exhaust valve for large uniflow early B&W Engine.
The mechanical valve actuating gear (see page 57 for operating gear) has got shortcomings. Due to
inertia of parts, there is inherent delay in opening and closing of valves. The buckling of push rods,
valve bouncing and inherent problems of mechanical linkages make hydraulic actuation of valves an
attractive option.




                  B&W. K type – Engine: Exhaust Valve. Mechanically Operated.
                                                                                                    67
Hydraulic Valve Actuating Gear:
Hydraulic Valve Actuating Gear is now more common. For Example B&W Engine K-CIF Type and
later versions use Hydraulic Valve Actuating gear as described below. The figure on the following pages
shows the construction and operation.




         Hydraulically Operated Exhaust Valve showing the arrangement of connections.
Description.

A cam on the camshaft actuates the Exhaust valve. Though a roller, the movement is transmitted through
a push rod to the plunger in a hydraulic oil cylinder, which through a high-pressure pipe, is connected
with the hydraulic cylinder on the exhaust valve.
                                                                                                        68




                   B&W: L – GFCA Type Engine Exhaust Valve Shown in Detail.

The roller guide is pressed downwards onto the cam by the action of a helical spring. The push rod rests
on a retaining washer. Hydraulic oil cylinder is attached to the push rod housing by eight studs. The
cylinder is provided with an exchangeable liner and has on the outside a skirt, which forms the outer
wall of a cooling duct round the cylinder. The skirt is sealed in an oil tight manner against the cylinder
by means of two rubber rings.
The pressure oil from the camshaft lubricating system is supplied through an elbow union at the bottom
of the cylinder. Part of oil passes through cooling duct and is drained to the oil pan of the roller guide
housing. The remainder of oil is led through two holes, one at bottom and one on top with a non-return
valve, to the pressure space of the cylinder.
The lower bore in the cylinder of exhaust valve is provided with a throttle valve used for adjusting flow
by which fine adjustment of closing and opening of exhaust valve can be made. Leaking oil from the
hydraulic cylinder on the exhaust valve is drained through bleed pipe to top of hydraulic cylinder of the
                                                                                                        69
roller guide, from where it is drained along with any oil from safety valve and the protective hose to the
roller guide housing.

Air Spring Type Exhaust Valves:
Modern engine exhaust valves now a days have eliminated the spring actuation of the valves. Springs
that held the valves to the valve seat and allowed the valve to open and close are totally eliminated due
to bouncing effects and the damage it caused to valve seats.
The springs are replaced with ―Air Spring or Air piston‖ type of valve operation, where the air pressure
holds the valve on its seat. The figures below will illustrate such modern valves, which are in use these
days.




    Combustion Space and Air Spring Operated Exhaust Valve: Sulzer: RTA 58 – 84 Engine.
                                                            70




Air Spring Exhaust Valve With Hydraulic Actuating System:




     Position Of Exhaust Valve With Out Air Pressure.
                                                                                 71




                     Air Spring Exhaust Valve In Closed Position.




                      Air Spring Exhaust Valve In Open Position.

*************************************Kv********************************************
                                                                                                          72
Inlet and Exhaust Valves:
Air inlet and exhaust valves of the mushroom type are always used in four stroke engines and sometimes
in two stroke engines.
To handle large flow rates, they are of large size. Both open into the cylinder, so that the greater the gas
pressure in the cylinder, the more firmly are the lids pressed against their seats. Therefore the springs
employed to close the valves require being strong enough to keep the lids on its seat during the low-
pressure period of the cycle of the engine.
It is not unusual to find exhaust valves having smaller diameter than inlet valves.

The reasons are:
          a) Exhaust valves open against higher pressures within cylinder.
          b) Exhaust gases assist in expelling the gases through open exhaust valve, unlike the inlet
              valve.
          c) Being smaller assists in keeping them cool which is important as exhaust valves operate
              at higher temperatures.

However, large engines can have them of same size. As inlet and exhaust valves withstand different
thermal loads, they might differ in material also. Also, exhaust valves require cooling. In large engines it
is better to duplicate inlet and exhaust valve. It gives better gas and airflow resulting in reasonable sized
valves, better volumetric efficiency, better scavenging, cooler piston and liner and better performance.
The figure on the following pages illustrate typical valves and shows common terminology. Typically
the valve seat is angled at 45* for diesel engines, although some valves use a narrower 30* angle from
the horizontal. The 30* angle allows less restriction across the seat and flow can start sooner and end
later. Valve guides, typically make of Cast iron, guide the valve stems, which tend to wear against the
valve guides due to angularity of up and down motion of the valve contribution by rocker arm action.
It is important that the clearance between the valve and the valve guide i.e. valve guide clearance, be
within the manufacturer's specification.

The valve seat must be smooth, not only to prevent leakage, but also to allow for good heat transfer. As
valve seats are prone to damage, burning and distortion, Exhaust valves have replaceable valve seat
inserts, as shown in solid black in the figure on following page 66.
As the inserts are ground away and exceed dimensional limitations desired, they are replaced with new
inserts.

If a valve had to be repaired or replaced the entire head might have to be removed because the valves are
installed solidly in the head. Heads can be removed on small engines but on large engines head removal
is certainly difficult. Valve cages are used to overcome this difficulty. An example is illustrated in the
figure on next page. A copper gasket is inserted between the combustion chamber and the cylinder and
the head in the valve cage. This copper gasket prevents both leakages from the combustion chamber and
carbon build up.
Valve Cages.

To help prevent welding or freezing up valves due to deposition of carbon particles around and to avoid
uneven wear, exhaust valves of modern large engines are given rotation by providing vanes or ‗roto-
caps‘. Each valve cage for exhaust valves may have its own water jacket. Also, to have effective
cooling, Bore cooling is incorporated in the modern exhaust valves.
                                                             73




Valve Construction.                   Caged Exhaust Valve.




                      Valve Seat Insert.
                                                                                                            74
Large Slow Speed Engine Exhaust Valves: B&W: KGF Type:

The figure on the next page shows an exhaust valve used in K-GF type B&W Engines.
The exhaust valve housing is made of pearlitic cast iron and provided with a chamber for the cooling
water. The valve housings are provided with loose valve seating 33 made of steel with stellite valve
seating surfaces. The loose seating is fixed with the screws 32 and can easily be replaced when worn or
burned through.

The exhaust valves are mounted in the centre bores in the cylinder covers and tightened against seating
at the bottom by means of studs, nuts and the sleeves 28. Seating at the assembly surfaces is achieved by
grinding.
A valve guide 8 is pressed into the valve housing are provided with bronze linings 21.

The exhaust valve spindles 18 are produced from one-piece stainless steel forgings having high strength
and corrosion Properties. The seating surface is stellite. The shield 7 is shrunk on to the valve spindle
and serves to prevent gas leakage and oil-coke deposits on the valve spindles and guides. The ends of
the spindles, at the point where they are activated by the rocker arms, are hard-faced with a material of
great wear-resistance. The steel split ring 14 is mounted in a groove around the spindle and prevents the
spindle from dropping down into the cylinder during possible replacement of a broken valve spring,
which can be carried out without removing the exhaust valve.

To prevent the escape of gas, the spindles are provided with a sealing ring 11. This ring is mounted in a
recess in the bottom of the upper spindle guide.

Exhaust valve closing is effected by means of two sets of coil springs 12 and 13 for each valve. The one
set of springs is arranged con-centrically inside the other, and each set consists of two springs mounted
end-to-end. In the middle of this spring assembly is mounted a spring guide 20 which is connected to the
rocker arm by means of studs. The lower springs rest in the spring guide 9, while the top of the upper
springs presses against the spring guide 16, which transfers the spring force to the valve spindles
through the tapered and two-part locking rings 17.

The two part tapered ring is forced into a recess in the valve spindle by the spring force and rests against
two small conical surfaces at the top and bottom. As any play at the contact surfaces would very quickly
give rise to wear, the fitting of the ring in both the spindle recess and in the spring guide 16 is carried out
very accurately. The two halves in each valve are matched together and must not be exchanged with ring
halves from other valves.

The supply of cooling water to the exhaust valve-cooling chamber takes place through short elbow joints
3 which transfer the cooling water from the cooling water chamber in the cylinder covers. The elbow
joints are fitted into holes in the top of the cylinder covers and sealing is provided by means of the
rubber rings 2. The discharge of cooling water from each valve takes place through an opening 10 from
where the water passes through a pipe provided with a thermometer and a vent cock. The cooling water
is led from these pipes to a common discharge pipe. The exhaust valve housings are fitted with cleaning
covers 31.

Sketches: Page 68 for details: Page 57 for Operating System: Page 59 for B&M-K Type Valve:
                                             75




Main Engine Exhaust Valve: B&W: K-GF Type.
                                                                                                                76
Recent Design Exhaust Valves:

Modern exhaust valves with latest technology have been explained in pages 62, 63, and 64 under the
heading ―Air spring types of exhaust valves‖.
The exhaust valve is centrally located in the cylinder. It is forged from a Nimonic heat resistant alloy
and is mounted in a cage with a bore cooled valve seat. It could rotate on its seat by the force acting on
the vanes provided on the valve stem. This valve is hydraulically actuated from camshaft and has an air
spring.
Use of air spring contributes to a very smooth dynamic behavior of the whole valve system. Thus the
valve gear failure by vibrations happening due to use of helical springs is minimised. Also, valve lift can
be increased.
                    Exhaust Valve Casing.            Exhaust Valve.          Cooling Water outlet.




                                                                                        Cooling Water in.
Cooling Water In.
      Sulzer: RTA: Engine Exhaust Valve Seat Arrangement with Cooling system.




         Exhaust Valve and Valve Seat Temperature Gradient Curve for RTA: 58 Engine.
Note: At R1 rating show an average of 360*C measured at the valve seating face with a fully symmetrical temperature
distribution well below the critical 450*C for good service independent of fuel quality.
                                                                                                        77
Hydraulic valve actuating system is well suited to long-stroke engines with mid height camshafts valve
rotation is essential for reliable heavy fuel operation. The valve impeller on the stem is a simple and
very effective means of rotating the valve. It helps to ensure a uniform seat temperature distribution and
to keep the seat clean as well as dent and impression free as much as possible.

Valve Material:
Inlet Valves:
Any good quality steel that can be heat-treated e.g. 3% Ni-steel.

Exhaust Valves:
Material Requirements.
    1. The material should retain its greatest strength at high temperatures.
    2. No tendencies to air harden.
    3. Critical temperature above 800*C.
    4. No tendency of high temperature scaling.
    5. Hot and cold corrosion resistant.
    6. Able to be forged and machined easily.
    7. Capable of consistent and reliable heat treatment.
Most diesel engines use an Austenitic heat-resisting alloy steel. The seating surface can be stellited.
Typical heat treatment: Heat up to 950*C and cool in air to give a Brinnel Hardness of 269.
Surface Treatment:
Surface treatment is frequently used to improve or modify valve steel characteristics. Chrome-cobalt-
tungsten alloy available in various grades of hardness is widely used. The hardness when deposited is in
the order of 375 to 425 Brinnel. The valve head is treated to more than 430*C to reduce contraction
stresses. The value face is now sweated by an oxyacetylene flame and the alloy deposited continually by
welding (1.02 mm to 1.52 mm).
Valve Seat Inserts:
Alloy Irons, with high percentage of molybdenum and Chromium with a Brinnel number of Approx.
500 are best. Alloy steel with stellited seating surface are also in common use. The methods employed
for fitting the inserts include screwing and shrinking.
Valve Guides:
Valve guides are mostly made of Cast Iron. To avoid scaling etc at high temperatures alloy Irons are
preferred. Phosphor Bronze and Gun metal have also been successfully used. Alloy Iron guides with
Bronze linings also are in common use.
Valve Housing:
Mostly made of pearlitic cast iron and provided with a chamber for cooling water.

Valve problems and methods of increasing valve life, reducing overhaul frequency:

Most of the problems are the consequence of operating at increased output imposing greater mechanical
and thermal stresses. The larger diesel engines often use heavy residual fuels, which contain relatively
high ash and sulphur content together with traces of metal salts capable of causing exhaust valve
corrosion or forming a brittle glass like deposit on the hot valve seat of four stroke engines. Two-stroke
engine runs much cooler, at the same time much longer service life requiring between overhauls.
Modifications done on valves to over come all these problems:
           a) Exhaust valve material have improved strength and hardness at high temperatures.
               Material used is Austenitic steel. This material avoids cupping, cracking and seat
               deformation.
           b) Valve rotors (Rotocaps) will help minimize local over heating of valve seat and valve
               body.
                                                                                                          78
           c) Valve temperature can be reduced by:
                   i)     Uniform cooling of cylinder head around exhaust valve seating.
                   ii)    Using as wide valve seat as possible.
                   iii)   Radially thin valve seat insert.
                   iv)    Using as close a valve stem to guide clearance as possible.
                   v)     Well-cooled guide should be as close to the valve as possible.
                   vi)    Using bore-cooling system.
                   vii)   In some highly rated engines some extra cooling is always obtained by
                          scavenge air and over lap valve timing.

Modern highly rated engines using poor quality residual fuel oil have the following problem.
Vanadium in the fuel forms vanadium pentoxide (V2O5) with a dew point of 690*C. Above and around
this temperature vanadium pentoxide is a corrosive liquid and apart from corrosive effects, a number of
complex sodium, vanadium salts can be formed by the combustion of residual fuel oil. These salts can
adhere to the valve seat if the temperature is high. The acidic effect corrodes the valve seat. The deposit
hinders the heat transfer from valve to the valve seat leading to high temperature condition of the valve
body. Eventually, the deposits breakaway locally leading to local blow-by and valve burning. This is
some times called ―Hot corrosion‖ by engineers.

Depositing a layer of ‗Stellite‘ or ‗Dolite‘ or similar metal combats seat ―tramping‖ on the valve head
seat and the valve seat inserts. The coating must be as thin as possible. Being brittle material thick
coating will hinder heat transfer in this area leading to valve over heating and failure.

There is a tendency in turbocharged engines, to find that the inlet valve and its seat wear excessively at a
greater rate than the exhaust valve. This because of fretting caused by little oil in the valve/seat surface.
The above problem can be solved by:
            a) Improved lubricating conditions.
            b) Decrease valve seat loading by reducing the seat angle from 45* to30*.
            c) Oil additives specially barium or calcium.
            d) Valve head rigidity.

Valve bouncing can be reduced by the following modifications:
   1. By increasing the number of springs instead of using one or two heavy springs of bigger
       diameter. In B&W: K-GF type engines six helical springs are used.
   2. By means of hydraulic valve actuating gear as described in earlier part of this notes.
   3. By improving the spring material.
   Typical spring material value may be:
                       UTS                150 Kg/mm2
                    Yield point.          120 Kg/mm2
                    Elongation.                 5%
                       BHN                      400
                                            .
                     Carbon.                  45 to .55
                                            .
                       Mn.                    30 to .60
                                            .
                        Si.                   15 to .30
                                            .
                        Cr.                   75 to 1.1
                                            .
                        V.                    15 to .25
                         S.               Not over .03
                         P.               Not over .035
                                                                                                          79
Opposed piston engines are called ‗Cover less‘ (Doxford Engine) or Semi-covered‘ (Harland & Wolff
Engine) if the top piston is smaller in diameter than the main piston. These engines have the following
disadvantages:
            a) Complex running gear.
            b) Increased maintenance and survey.
            c) Piston cooling is necessary and more complicated than exhaust valve cooling.
            d) Long bearing span to provide room for side crank or eccentrics for driving the top piston.
In view of above these engines are no more manufactured and have become obsolete.
Shrouding of Valves:
If the seat material is softer than the valve, shroud builds up in the seat and vice-versa, due to operation
of the valve. Small amount of shroud can seriously affect the air or gas flow across the valve because of
reduction in effective opening area and less streamlined flow. Shroud should be removed by seat cutter,
machining or special grinding machine.

Ceramic Coated Valves and Piston crowns:
Some engine-builders feel that the peak is being reached in diesel engine development. To develop
further, new materials may have to be developed. In this context, ceramics may well prove to be a main
link in the chain to develop the next generation ultra-efficient diesel.
Work on ceramics has been progressing in the USA by a company well known as an authorised repairer
of many engine makes. AMT in Miami has experimented with bonding a type of ceramic to steel,
aluminium and cast-iron. In particular, work has concentrated on coating piston beads and Valves.
 Results from operators using the company‘s ‗Cerro-Plasmic HB‘ coatings, have shown a number of
significant improvements in engine performance.




 For instance, results with a coated piston crown showed the metal temperature to be reduced by 194*F
and crown underside temperature reduced by 135*F. On a six-cylinder engine, the effect on two
cylinders with the pistons, cylinder heads and valves coated, was that fuel pump rack position could be
reduced by 10.5%. In another example, an engine with coated piston and valves increased its output at
1800 rev/min from 1880 to 1940 bhp. In all cases, the company reports that bonding method proved
successful and there wear no instances of coating failure.
Currently, ship owners are conducting trials with coated valves in large B&W slow-speed diesels. Also,
tests are being undertaken with coated hot gas inlets and outlets on BBC-VTR750 turbo-chargers.
Results are extremely good and found that great reductions in corrosion and heat loss in the components.
         *********************************Kv**************************************
End of Engine Valves/BIT/AMET/BE/KV/May 2003.
                                                                                                         80
Cams:
A cam is a device for transforming uniform rotary motion to intermittent reciprocating motion. An
eccentric differs in providing a continuous reciprocating motion similar to that of a crank. The cam drive
has been universally adopted because cams can be made in shapes that will give the desired rapid
opening and closing of the valves not possible with the eccentric. Another advantage of cam drive is that
it simplifies the reversing mechanism, by allowing endwise movement of camshaft.
Cams are precisely positioned on this shaft, being shrunk on, integral or keyed for positive positioning.
Some cams have provision for a limited range of adjustment, but in all cases the cam must be tightly
secured before the engine can be operated.
At present, even some of large engines have cams forged or cast integral with the camshaft and then
machined, usually ground to the required exact shape. The advantage of such an integral camshaft is that
if one valve of one cylinder is timed correctly, all the valves in all cylinders will be timed correctly. On
the other hand, any change in timing will affect all valves and cylinders.
In operation, cams are subjected to impact and are hardened in order to reduce wear. The shape of cam
determines the points of opening and closing of the valve, the velocity of opening and closing, and the
amount of the valve lifts from its seat.
The desired cam shape or profile is obtained by accurate grinding. The grinding stone repeats the shape
of a master cam and thus ensures accuracy of all cams.
Following figures show some profile of cams:




                                   Profiles of intake and exhaust cams.
The difference in profiles of various cams can be noticed.
In two stroke engines there is no cam for the intake, but if exhaust valves are used, at least two exhaust
valves per cylinder are present which may be operated either by a common or two separate cams, so that
the number of cams is about the same as in 4-stroke engine. When twin cams are fitted, in reversing the
camshaft is made to slide laterally so that the ahead cam slips out of gear and the astern cam into gear on
the same roller.
Cam profile regulates opening and closing of valve. Timing of fuel valve opening and closing can be
advanced or retarded by adjustable toe piece. Roller clearance also affects on fuel admission side, cam
profile should show a gradual rise or slope, on closing side sharp drop. Thus usually the same cam is not
equally suitable for both ahead and astern running.




                                           Fuel-injection cams.
                                                                                                          81
In case of Sulzer two cycle engines, same cam is used and the fuel timing for both ahead and astern
running is unaltered.




The figures show the cams for SULZER and M.A.N. engines, the arrangements for cam adjustments can
be noticed easily.
Some Air-starting valve cams are shown below:




Camshafts:
In most of the modern engines the cams and shaft are forged or cast in one piece. In some engines the
camshaft is a straight round shaft and the cams are separate pieces, machined and keyed to the shaft. In
some larger engines, the camshafts are made up of two or more sections bolted together by flanges with
fitted reamed holes to assure accurate timing. Most camshafts are made of forged steel, usually of
nickel-chromium alloy steel, and the larger camshafts are often bored hollow. They are heat-treated and
cams are usually surface hardened. The camshafts are carried in plain bearings.




Camshaft bearings.
To insure good support, the camshaft is usually carried by a series of camshaft bearings. One bearing
being located between each pair of cylinders. Bearings may be either plain bushings or split sleeves.
If plain bushings are used, their bores are larger than the cams, so that the camshaft may be withdrawn
endwise. If split bushings are used, the camshaft may be removed sidewise from the engine.
                                                                                                        82
                                       Sulzer Engine Camshaft.




Slow Speed Engines.
Camshaft bearings are usually of the Journal type, operating in white-metal lined or bronze bearing
shells or bushes, and lubricated by oil supplied from and returning to the engine lubricating oil System.
If there are chances of fuel oil contamination in the camshaft bearing or cam lubrication then the
lubricating oil system for camshaft will be an independent one..
Medium Speed Engines.
Similar to slow speed or roller or needle-roller bearings.
High Speed Engines.
Needle-roller bearings.

Camshaft Drive:
In four-stroke engine, the camshaft speed must be exactly one-half the crankshaft speed, so that
the camshaft makes one complete revolution while the crankshaft makes two.

In two-stroke engine, camshaft speed is exactly same as the crankshaft speed. Because these speed
relations must be exact, the connecting drive must be positive.

The drive arrangement used for particular engine-depends largely on where the camshaft is located and
on whether an auxiliary camshaft (for fuel pumps, etc.) or a ‗power take‘ off shaft is included. The
camshaft may be located on the cylinder block, using short push rods, or at the cylinder head level,
without push rods. For the sake of good appearance and cleanliness, the camshaft and push rods are
often enclosed completely.
This requires use of gears or chains. Many drive arrangements are used.
                                                                                                       83
Figures below show six typical layouts for camshaft drive.




                                         Typical Camshaft Drives.
Camshaft Gear Drive.
Gears for camshaft drive must be accurately cut and heat-treated to resist wear. Helical teeth are
preferred to spur teeth for greater quietness and more even transmission of power. A fiber or other non-
metallic gear is sometimes introduced in to the train of gears for the same reason.




                                MAN Engine: Cam Shaft Gear Train.
                                                                                                         84
Depending on the number of cylinders of the engine, the camshaft drive is located either in the middle or
at the after end of the engine (flywheel-end). The rotation of the crankshaft is transferred to the camshaft
by way of four spur gears. In order to facilitate erection, the spur wheel on the crankshaft is of two-part
design. The two intermediate gear wheels are supported on bearing pins, which are flanged on one and
on to the column and on the other end held in position with clamp flanges. The axial clearances of the
intermediate gear wheels are adjusted by machining the locating ring to the required size.
The bearings of the intermediate wheels and the gear teeth are lubricated by the low-pressure oil system.
The lubricating oil nozzles spray direct onto the gear teeth. These nozzles can be dismantled for
cleaning. The screw and pipe connections in the gear wheel space must be secured by wire respective
locking plates.
Marking of the gear wheels:
All gear wheels for camshaft drive are stamped ‗Flywheel end‘ on after side. In case the drive has to be
overhauled the gear wheels must therefore be refitted with the marks facing flywheel side.
The two intermediate gear wheels are stamped ‗Upper wheel‘ and ‗lower wheel‘ markings are to be
observed, when refitting these gear wheels. Further the intermediate gear wheel bearing pins and flanges
are marked together with the column, which again must be observed in case of an overhaul. One should
adhere to the engine manufacturer‘s instructions for that engine while adjusting, overhauling and testing.
Chain Drive:
Multiple link chain drives are used on many engine designs. Following figures shows a multiple-strand
silent chain drive used on a 4-stroke cycle, engine. Drive is from the crankshaft sprocket (12), passing
the idler sprocket (10) and the chain takes up adjustment (9). The camshaft sprocket (7) and camshaft
drive gear (8) engage the camshaft gear (4) and provide a 2 to 1 reduction in speed. Over speed
Governor (1) is driven through pinion (2) from governor drive gear (3), which is turning at camshaft
speed, as is the fuel transfer pump (5). The drive chain (16) goes to the pump drive sprocket (15) and the
pump drive gear (14) from which the water pump is driven through gear (13). Chain Drives must be kept
tight. Hence, period check up on the idler adjustment is necessary. The chain is kept tant from the
camshaft drive (7) across the pump drive (15) and on to the crankshaft sprocket. Travel is clockwise, as
viewed, with the crankshaft sprocket pulling the chain. If slack developed, timing changes would occur,
and slapping of the loose chain result in serious fluctuation in timing.




                         Chain drive system of camshaft and auxiliary drives.
                                                                                                          85




MHI: UEC85LSII: Engine. Bore: 850mm; Stroke: 3,150; Power output: 5250PS/Cyl; RPM: 76.
   Camshaft gear train, Turning gear, Crank shaft, Fuel pump with cam and all components could be seen.
                                                                                                        86
Chain Drive: B&W Engine: K-EF Type:
The figure on the following page shows a chain drive for B&W Engine (K-EF Type). The camshaft (2)
is driven from the chain wheel on crankshaft (6) by means of a chain drive consisting of two identical
roller chains (3) guided by means of two guide rails. The Tension of the chains can be adjusted by
means of a tensioning arm (4). A separate chain transmission runs from the intermediate wheel (8) to the
chain wheel (10), which drives the cylinder lubricators, generator, and starting air distributor. A further
chain transmission (9) from the intermediate wheel drives the engine governor.
Chains and guide rails are lubricated through spray nozzles mounted between the guide rails and the
chain wheels.
Cams and couplings are shrunk on to camshaft and can be adjusted or removed by means of hydraulic
tools.
After adjustment on the test bed, the engine is provided with a series of marks and corresponding. Pin
gauges enabling checks to be made for correct adjustment.




                              Chain Drive of B&W Engine (K-EF Type).
                                                                                                         87
Roller Chains:
Design:
Chains are manufactured to comply with BS 228 or ISO R606 standards. This ensures interchangeability
between chain makes and provides for minimum breaking load.
The factor of safety employed is high and for camshaft chains this is about 50.
For chains up to 64mm pitch, standard chains are used. Above this however, greater accuracy is paid to:
   a) Link pitch: By grinding and horning the bores of bushes in assembled links.
   b) Surface finish: On links, pins and rollers, to improve resistance to fatigue.




                                         Details of Roller Chain.

Provided the chain is adjusted correctly and dampers provided with a generous supply of lubricant, the
chain drive efficiency may be as high as 99% and this efficiency varies little during its operating life.
The Chain Pitch: This is determined by the crankshaft speed and the shaft diameter.
The Sprocket Size: This is governed by the space available.
Normal and Peak Loads: The maximum allowable bearing pressure between pin and bush determines
these loads, after accounting for Factor of Safety (governs the diameter and length of bearing).
The anticipated fatigue life for a two-stroke diesel engine chain may be about 80,000 hours, during
which time the chain will have worn up to 0.7% of its normal length (most of the increase in length is
due to wear). This is about 30% of the chain rated wear capacity. 2% elongation is considered the end of
useful life, but to ensure reliability replacement is required at 1.5%.

Adjustment:
Excessive tension results in overloading of the bearing surfaces, both on the chain and shaft, causing
unnecessarily rapid wear.
Insufficient tension results in excessive vibration, slap and backlash, all of which can cause higher than
normal peak loads. This may also cause damage to the Neoprene faced dampers.
Incorrect tension will result in incorrect fuel injection timing.
Too tight: Causes advancement of injection.
Too Slack: Causes retardation of injection.
                                                                                                      88
Chain adjustment is measured on the longest length between two wheels, preferably the strand
containing the adjusting jockey wheel. Turn the engine on the turning gear, so that the strand on which
the adjustment has to be measured, is on the slack side of the drive.
Using the adjusting jockey, tighten the chain until the total movement at mid point in the strand, when
moved by hand, is equal to ONE PITCH when measured normal to the strand.




              Adjusting jockey adjusting system used in B&W Engine Chain Drive:
Adjustment of chain tension:
   1. Turn engine so as to slacken the longest free length of the chain.
   2. At middle of the longest free end pull the chain away from the guide bar. In this manner it shall
      normally be possible to move the chain a distance corresponding to approximately one half of
      the chain link (pitch) away from the guide bar. If the chain is too slack or too tight, adjust the
      chain tension.
   3. Loosen nut locking. Loosen nuts A –B – C – D.
   4. Tighten nut C until load set length of the spring reduced to „x‟ mm (as per instruction manual of
      the engine).
   5. Turn nut B to sit on lock bar.
                                                                                                         89
    6. Turn nut A and lock.
    7. Now the chain is tensioned by the spring tension and held in position.
    8. Tighten nut C till the thrust washer of the spring bears against the distance pipe of the chain
        tightening bolt.
    9. Tighten nut D and lock.
    10. Now the chain is tensioned to the correct value and the spring is gagged.
    11. Repeat item 2, and check the tension is correct.
Inspect the Neoprene facing on the dampers and replace if required. After the chain is adjusted then
adjust the dampers.

On the taught side of the chain, measure accurately over the longest practical length between bearing pin
centers, and calculate percentage elongation from:
                            Percentage Elongation = 100 (M—XP)
                                                                  XP
M = Length measured in cm. X = Number of link pitches in the length M. P = Pitch of the chain in mm.
Faults that can be found in chain drive:
   a) Crack or breakage of roller or side plates.
   b) Stiff joints.
   c) Seized rollers.
   d) Wear of ends of bearing pins.
   e) Scoring on inner surface of links, due to misalignment.
   f) Damage to ends of the link plates.
Some rollers are heat treated to provide durability, rather than extreme hardness, so that some superficial
marking may be present but will not affect performance.

                                     Valve Gear Maintenance:
To keep the valve gear in good operating condition, all components of the valve gear are maintained
according to a maintenance schedule, which depends on the engine, operating conditions and fuels used.
Though the checking, dismantling and overhauling is to be done whenever necessary, yet following
schedule that is typical of a B&W KGF type engine.
Exhaust Valve:
i) Cheek and adjustment ----------------- 500 - 1000 hours.
ii) Overhauling ------------------------------ 8000 hours.
iii) Overhaul of hydraulic oil cylinder ----- 8000 hours.
iv) Inspection of roller guide ---------------- 500 - 1000 hours.

Chain Drive:
  1. Inspection of chain drive, chain wheels, Rubber guide bars, bolt connections, L.O. System, 500 -
      1000 hours for new engine, and for normal engine 8000 hours.
  2. Check of Chain Tension, 500 - 1000 hours for new engine and normal 4000 hours.
  3. Inspection of running Surface of cams around 8000 hours.
  4. Check tightening of camshaft bearing and coupling bolts 500 - 1000 hours for new engine and
      normal 4000 hours.
  5. Inspect ion of camshaft bearings done at 4 yearly surveys.
  6. Replacement of cams on camshaft only when necessary.
  7. Adjustments of camshaft due to chain wear only when necessary.
  8. Check control gear timings 500 to 1000 hours for new engine and when necessary.
                                                                                                         90
The abovementioned are figures for a typical slow speed large Marine Diesel engine. However, for
medium speed engines, the figures might vary, as given below.

Exhaust Valve
For heavy oil operation -------------------------- 500 - 1000 hours.
For Light Fuel operation ------------------------- 2000 - 8000 hours.

Inspection Procedure and Criteria.
   1. Exhaust valves should be examined for carbon deposits, cracks, scores and pitting, burning or
      abrasions of valve and valve seats.
   2. Valve and valve seats are to be examined for ‗shrouding‘ or ‗pocketing‘ which can take place as
      a result of excessive wear or frequent regrinding.
   3. Valve stem diameter and bore of valve guide should be measured.
   4. Valve stem should be tested for straightness by rolling on a surface table or on a centre lathe.
   5. Valve springs should be examined to see that they are free from pitting, cracking, scoring and
      corrosion. The free length of spring should be checked.
   6. Cams, cam followers, tappets and all the valve operating gear parts should be inspected to see
      that there are no cracks, flaking, scores or burrs, and to see that wear has not reached a stage
      which would prevent the accurate setting of clearances.
   7. Push rods should be checked for straightness.
   8. In camshaft gear drive, the teeth should be checked for pitting, abrasion, deformation and back-
      lash.
   9. Camshaft chain drives:
          a) Rollers and links should be inspected for dents, pitting, abrasion and corrosion marks.
          b) Bearing pins and links should be inspected for cracks and fracture.
          c) Chain should be inspected for 'stretch'.
          d) Chain tension should be checked.

Allowable Limits.
   a) Chain ‗Stretch‘ 2% max. Usually 1% for 10 links.
   b) Chain Tension (sag), ½ link min. 1 link max.
   c) Gear Back Lash, 0.25 mm (normal) 150% (max.).
   d) Tappet Clearance (Typical):
                   i)     Exhaust Valve -------------- 0.6 mm, 1.0 mm. max.
                   ii)    Inlet Valve ------------------ 0.5 mm, 1.0 mm. max.

Valve Bouncing.




The valves are opened by the rocker arm pushing on the valve stem tip, but they are closed by two or
sometimes three coil springs. Multiple springs are used, not only because of the additional force they
                                                                                                        91
provide but because of a characteristic of the spring known as resonance. Valve opening is done with a
series of impulses, and at a certain engine RPM, these impulses will occur at the resonant frequency of
the spring. When this happens, the spring loses its effectiveness and allows the valve to float. To prevent
this floating condition, two or more springs having different pitch, diameter, and wire size are used; and
because of their different configuration, they have different resonant frequencies, so the engine can
operate throughout its full range of RPM without valve float problems.
Valve bouncing is a phenomenon, which happens due to sudden release of compression energy of the
spring at the moment of valve closure due to tappet clearance allowance. The valve would jump up and
down on seat before closing shut finally. Spring surge (resonant vibration) at this moment also tends to
help generate valve bouncing.
Valve bouncing can be reduced by:
                            (i)     Increasing number of springs.
                            (ii)    Improving spring material.
                            (iii) Using inner and outer springs, right and left handed.
                            (iv)    Using hydraulic valve actuating gear.
                            (v)     Using air spring.
Tappet Clearance.
The tappet clearance, which is the clearance between push rod roller and cam base circle at valve closure
in cold engine, is usually measured between rocker arm and valve spindle top. Tappet clearances are
necessary to allow for thermal expansion of valve spindles. Clearances should normally be set while the
engine is cold and the cam follower is off the cam peak. Wear of valve gear tends to increase clearances.
Excessive tappet clearance will cause the valve to open late and close early and will reduce valve lift. It
will also cause noise and damage working surfaces. Insufficient clearance will cause the valve to open
early and close late with increased valve lift. It may prevent the valve from closing completely causing
burning of valve, low compression etc.

Vibration in chain drive.
Following reasons can cause vibrations:
                         (i)     Excessive stretch.
                         (ii)    Misalignment of wheels.
                         (iii) Error of pitch of chain or chain wheels.
                         (iv)    Unmatched chains.
                         (v)     Stiff links.
                         (vi)    Twined or bent chain.
                         (vii) Improper balancing of engine power.
The various reasons must be dealt with at earliest to avoid failure of chain drives.

 ***********************************Kv****************************************
References:
    1. Diesel Engine Operation and maintenance by Mr. V. L. Maleev.
    2. Diesel and High Compression Gas Engines by Mr. Edger J Kates.
    3. Instructions for K-GF Type Engines by B&W Engineering.
    4. Diesel Motor Ships Engines by Christen Khak.
    5. Instruction Manual for Sulzer RND-M Engines and B&W K-EF and K-GF Engines.
                       ***********************Kv****************************
    End of Engine Valve Gear and Valves/BIT/AMET/BE/KV/May 2003.
                                                                                                          92

   6. Cylinder Liners.
The liner is regarded as a thick cylinder under the action of a fluid pressure. The material is to be strong
to with stand the tensile loop stress. The interior surface forms the wall of the combustion chamber.
There is a considerable temperature stress on the material of the body. The two surfaces tend to expand
at differential rates for being at different temperatures. But the body prevents their free expansion this
causes a stress to be set up. The liner is secured at the top flange by cover studs. A compressive stress is
set up on this part of the liner. Besides, the surface of the liner needs to be resistant to wear and
corrosion. The choice of material must also consider such factor as its amenability to various
metalworking and forming processes such as casting, machining, surface treatment etc.
                                                Cylinder Liner.

                      Wet Type.                                            Dry Type.

          Slow and Medium Speed Engines.                                  Small Engines.

             Cooled by Cooling Water.                                     Air Cooled (Fined liners used).

Liner should withstand high mechanical load (Gas pressure) and thermal load caused by heat flow.
To achieve above olden days they used one, two or three piece liner construction. Liners are cylindrical
construction fixed at one point on the top and expand at the other point downwards.
Mechanical Stress.
Maximum pressure: Firing pressure: 45 - 50bar for non-supercharged engine and around 85 - 100bar in
supercharged engines.




This pressure produces circumferential stress (Hoop stress) and longitudinal stress.
Hoop Stress = 2 x Longitudinal stress. So Hoop stress is only considered.
Hoop Stress = P x D Where P: Cylinder pressure. D: Cylinder diameter. T: Cylinder thickness.
                2T
Hoop stress will increase if bore size and firing pressure increases.

Thermal Stress.
Resistance to heat flow through the metal of the liner produces a temperature gradient across the metal.
Due above expansion will be different at the inner and outer wall of the liner.

                                Stress δt ∞ ∆T.         ∆T = Temperature gradient.
Temperature gradient will increase as metal thickness increases.
                                                                                                     93
Increase in metal thickness is good for withstanding mechanical stress but bad for thermal stress.




So liner design was a complex issue for long time, but with the advent of bore cooling, availability of
new-alloyed material and new machining technology the above problem to great extent has been solved.




                                   Comparison of stresses in a liner.
                                                                                                           94
Material.
Cast iron is generally regarded as a suitable material for construction of diesel engine cylinder liner. In
order to improve strength and induce specific desirable properties such as strength and surface
properties, cast iron is alloyed with the inclusion of small quantities of nickel, chromium, molybdenum,
vanadium, copper etc. Such inclusions refine the grain structure of the material. The total percentages of
alloying inclusions should not exceed beyond 5%.
Good quality „Pearlitic Grey Cast Iron‟ consist of the following alloying material:
Carbon: 3 to 3.4%. Its graphite flakes assist lubrication.
Silicon: 1 to 2.0%. Improves fluidity and graphite formation.
Manganese: 0.6 - 0.8%
Phosphorous: 0.5% maximum. Reduces porosity
Vanadium: 0.15%. Refines grain structure
Titanium: 0.05%. Improves strength

Specification
Ultimate tensile strength: 200 Mn/mm2.
Ultimate bending strength: 520 Mn/mm2.
Ultimate compressive strength: 900 Mn/mm2.
Brinell Hardness: 180 - 220 HB.
Ductility: 1 to 5% Elongation.

Reasons for using Cast Iron:
   1. Can be cast in to intricate shapes.
   2. Has good wear resistance:
          a) Due to large surface of irregular shaped graphite flakes.
          b) Due to semi-porous surface holding oil pockets.
   3. Possesses good thermal conductivity.
   4. Damps out vibrations due to rapid combustion.
   5. Cheap material.

One of the advantages of cast iron is its excellent performance as a lubricated sliding surface. The
presence of graphite in its microstructure is mainly responsible for this. During the running in period,
fresh surface containing graphite is exposed, leaving minute pores on the working surface. These
cavities store oil. The graphite also acts, as a lubricant in the dry state.
The porous structure of cast iron is of particular advantage in the liner operation as a remedy against
intensive galling action. When scuffing and scoring takes place, the interaction between two rubbing
surfaces tears the metal at the surface, exposing fresh graphite, which acts as lubricant. This minimises
the use of extreme condition i.e. seizure. The porous surface of cast iron helps to retain lubricating oil in
the minute cavities, which will readily ooze out to keep the surface wetted in molecular layer of
lubricating oil.

As the liners are put to more severe conditions of working as regards temperature, pressure, with oil ash
and in corrosive environment the need for a harder wear and corrosive resistant surface was realised.
Some other methods and materials were tried. Harder surfaces of cast iron or steel were not successful as
it failed to meet the demand of the service. Austenite cast iron, although provided adequate protection
against abrasion and corrosion, failed to produce a mated gas sealed surface with the piston ring.
Hardened steel liners did not protect the surface, as surface was quickly dried up due to lack of
retentivity of oil. Low alloyed cast iron, therefore, remains the choicest material for liner construction.
The surface is lapped with a coarse hone and this together with a surface, which is naturally porous
keeps lubricating oil adhered to it.
                                                                       95
 Some liner sketches with reference to type of scavenging.




Simple loop-flow            Loop-flow            Full loop-flow port
  scavenging.         opposed-port scavenging.         scavenging.




       Uniflow valve scavenging.      Uniflow port scavenging.




       Sulzer: RD Liner.                    Sulzer: RND Liner.
                                                                                  96




         B&W: EF Type Engine Liner (See bore cooling arrangement).




Sulzer: RND Engine Liner (See bore cooling arrangement above the liner sketch).
                                                                                                   97

                            Sulzer: RN-M Engine Liner Port Openings.




For the loop-scavenging each scavenge port and exhaust port has specific design to enhance
scavenging process. 1: Exhaust ports. 2: Scavenge ports. 3: Lubricating oil quill fitting space.
                                                                                            98




Doxford Opposed Piston Engine: 3- Piece Liner top two sketch: Below is bore cooled liner.
                                                                             99
Sulzer: RND 90: Bore cooled Liner: Below RN 90 & RN90M Liner Stress shown.
                                                                                                   100




                       Latest Sulzer RTA Engine Liner with water guide ring.
1: ‗0‘ Ring. 2: Cylinder liner. 3 & 3a: ‗0‘ Ring. 4: Water guide ring. 5: SIPWA pick-up or plug.
6: ‗0‘ Ring. 7: Water guide jacket. D, D1: Leakage drain. L: Water transfer hole.
KB, KB1: Check holes for leakage water. KW: Cooling water space. LR: Empty space.
TB: Cooling drillings. SS: Scavenge Ports.
                     **********************Kv**************************
                                                                                                         101




Improvements for Sulzer RND-M engines due to Bore-cooled cylinder cover:

Its introduction was prepared by:
     1. Extensive Finite Element analysis of mechanical and thermal loading (above figure shows the
         appropriate FE- network. The cover was analysed in combination with the cylinder liner and part
         of the jacket, in order to assure proper boundary conditions).
     2. Testing of prototype cover on test engine with experimental measurements of strains and
         temperatures and investigations regarding bore holes clogging.
     3. In-service testing of a prototype cover on a marine engine with regular checking.
     4. Strain and temperature measurements on the first series covers.

Liner Cooling:
With reference to all the liner figures shown in previous pages:
The cylinder liner is of special close grain cast iron. It is fresh water-cooled and the division bars
between the exhaust parts are hollow and also water-cooled. In order to reduce the thermal stresses of
the cylinder liners, particularly for the really large bore engines, the so-called bore cooling system has
been applied in the upper part of the cylinder liner. This is a system of holes drilled tangentially at an
oblique angle into the cylinder wall so that the cooling is led as close as possible to the hot liner wall of
the liner, thus the temperature of the running surface is reduced. The cold part of the cylinder outside the
bores is relatively thick and embraces the inner portion like a shrunk-on ring and so reduces the
pulsating stresses caused by the gas pressure. The cooling water is collected in the water guide ring and
left out of the cylinder cover.

Causes of cylinder liner wear.
The causes and prevention of cylinder liner, piston ring and ring groove wear, are one of the most
controversial subjects connected with diesel engines. Cylinder wear is also one of the most important
factors governing maintenance costs and repair work on engines. The combination of very highly rated
two stroke engines and the use of residual fuels have aggravated this problem.
                                                                                                        102

The main causes of cylinder liner wear may be broadly classified as follows:
   1. Normal frictional wear: Caused by metal-to-metal contact under boundary lubrication
       conditions. This may be aggravated by oil with inadequate load carrying properties, too low a
       viscosity or an inadequate oil supply.
   2. Abrasive wear: Caused by hard foreign matter introduced with the induction air and by hard
       particles of carbon, asphalt, wear debris and ash forming constituents present either in the fuel or
       lubricating oil. Frictional and abrasive wear are often linked together under the general
       description of abrasive wear. Sources of hard particles are air borne dust, ash in the fuel and
       carbon from combustion. To minimize this type of wear one have to keep engine air filters clean,
       keep scavenge space clean, effective centrifuging fuel oil, fuel pump and injector in good
       condition, fuel timing correct and keeping the fuel temperature correct thus achieving good
       combustion.
   3. Corrosive wear: Caused by acidic products of combustion, especially condensed sulphur oxides.
       This is especially troublesome when burning high sulphur content residual fuels. Sulphur burns -
       Combines with oxygen – Produce heat – Sulphur dioxide. Hydrogen + Oxygen gives water H2O
       – Gives out heat. Sulphur dioxide + Water = Sulphurus acid. Sulphur dioxide + Oxygen (Catalyst
       - vanadium pentoxide) SO3. SO3 + H2O → H2SO4. This H2SO4 in dilute condition causes all the
       damage. The dew point of this acid is around 110*C to 180*C depending on concentration. This
       temperature condition does exist in the liner face so results in corrosion.
               To minmise this corrosive wear one should use alkaline based cylinder oil.
            Fuel with 4 to 5% sulphur should use alkaline oil with Total Basicity Number 70.
              Fuel with 1% sulphur should use alkaline oil with 20 to 30 TBN cylinder oil.
   Alkalinity in the oil neutralizes the acid thus prevents corrosion. This is only effective when the
   correct amount oil is fed in to the liner and the correct number TBN oil is used for that particular fuel
   oil sulphur content. If either the feed rate or the TBN number is not correct for that amount of
   sulphur content in the fuel, particular wear pattern will occur in the liner, generally termed as
   ‗Clover Leafing‘.
                                       Lubricating oil supply via quill.




                                         Lubricating oil supply.
                               Sketch showing the „clover Leafing‟ pattern.
                                                                                                       103
Reasons for the above type of wear:
    a) Insufficient oil supply.
    b) Inadequate TBN number oil.
    c) Failure of oil distribution in liner face.
    d) Jacket cooling water temperature too low.
    e) Turbo-charger air cooler not properly controlled. Supplying cold air to scavenge space.
    f) Jacket cooling water outlet temperature low, keeping the liner surface temperature around dew
       point of the acid. Keep the cooling water temperature as high as possible with out forming
       vapour pockets and spoil rubber seal rings on liner.
    g) Keep quantity of starting air to the engine while maneuvering as possible.
    h) To protect ‗Waste Heat System‘ always bypassed when the engine on light load.
4. Scuffing wear:
Scuffing wear occurs when lubricant oil film fails to separate the ring face on the liner face and
subsequent contact while operating, friction heat is generated to such an extent that ―Micro-welding‘ or
‗Micro-seizer‘ takes place.
The reasons for this is as follows:
    1. Liner surface too smooth therefore retains too little oil on its face.
    2. Water on surface of the liner repelling the oil film formation.
    3. Blow by of combustion gases across the ring sealing face breaking the oil film formation.
    4. Poor oil distribution on the liner surface by the quills and/or the gutter groves.
    5. Deposit on piston top landing absorbing the lubricating oil.

Liner Wear Pattern.
Maximum normal wear occurs at the top of the liner in a Port-Starboard direction and around scavenge
ports. The reasons are:
    1. High temperature prevalent at the top dead center reduces oil viscosity and therefore oil
        thickness.
    2. High gas pressure increases ring loading causing penetration of oil film.
    3. Slow movement of piston allows oil wedge to break down (Reversal of piston movement).
    4. Movement of ship is maximum in Port – Starboard side than Forward -Aft side causing excess
        wear.
    5. High temperature makes oil film less resistance to acid penetration.

Wear Rate.
For Two Strake engine a wear rate of 0.1 mm per 1000 hour is normal.
Maximum acceptable rate is 0.25 mm per 1000 hr.
Maximum total wear is acceptable is 0.75% of bore.
Useful life span: 70,000 - 80,000 hours
For Four Stroke engine the wear rate is 0.02 mm per 1000 hours.

Liner Faults (See sketch page 104).
   1. Crack across liner flange due to uneven and excessive tightening of cylinder cover studs.
   2. Hoop stress crack due to poor liner support.
   3. Circumferential crack along wear ridge due to stress concentration or more likely new rings
       hitting ridges.
   4. ‗Star‘ or ‗Craze‘ cracks caused by flame impingement.
   5. Star cracks around lubricating quill due to water leaks.
   6. Cracks across port bars due to over loading, poor cooling, scavenge fire, poor fitting of liner in
       its position and usage of wrong ‗O‘ rings on the liner.
                                                                                                     104
                                               Liner Faults.




Causes of Excessive Wear.

When excessive wear of piston rings and cylinder liner occurs, the cause is usually one or more of the
following factors:

   1. Improper running in.
   2. Misalignment of the pistons, or distortion of cylinders, preventing bedding-in of pistons and
       cylinders.
   3. Inadequate oil supply, or unsatisfactory arrangements for lubricant type and quality.
   4. Lubricating oil too low in viscosity, or too low in alkalinity (Total Base Number -TBN).
   5. Piston ring clearances incorrect.
   6. Unsuitable cylinder liner material or unsuitable piston ring material or hardness factor between
       ring material and liner material not compatible.
   7. Contamination of lubricating oil, by extraneous abrasive material.
   8. Cylinder wall temperatures too high or too low.
   9. Overloading the engine.
   10. Scavenge air temperature too low, especially in humid climates, resulting in excessive quantities
       of condensed water entering the cylinder thus aiding acidic formation.
   11. Inefficient combustion, promoting deposit formation degradation of the lubricating oil.
   12. Use of a low-sulphur fuel (containing less than say 1% sulphur) in conjunction with highly
       alkaline cylinder oil - this particular fuel/lubricant combination is not necessarily harmful but
       cylinders wear due acidic wear and/or scuffing. Low Sulphur fuel oil with high T.B.N lubricating
       oil will leave balance alkaline salts which due to heat will form in to abrasive material and
       mixing with lubricating oil will score the liner leading to abrasive wear.

Cylinder Liner Wear: (See figure above).
Liner shown is a single acting type.
   a) Travel of the top ring
   b) Travel of the bottom ring.
                                  105




Vertical Wear Profile of Liner.
                                                                                       106




Specimen Engine Cylinder Liner Gauging Chart (Chart given is for Opposed Piston Engine).
                                                                                                          107
Liner Lubrication:
The inner surface of the cylinder liner is lubricated through quills, which are equipped with non-return
valves. In order to prevent any leakages into the fresh water-cooling system, outside sleeves have been
arra1nged around the quills, which are sealed by rubber joints. Any oil leakages from the quill as well as
water leakage from the cooling spaces will pass to the outside. The quills may be inspected with out
draining off the jacket cooling water.

         Sulzer RD and RND Engines development of cylinder lubricating oil stud or quill.




Requirements of a cylinder liner/rings lubricant.
It may be stated that the essential properties of a good cylinder lubricant are:
    1. It must reduce sliding friction between rings and liner to a minimum, thereby minimising metal-
       to-metal contact and frictional wear.
    2. It must possess adequate viscosity at high working temperatures and still be sufficiently fluid to
       spread rapidly ever the entire working surface to form a good absorbed oil film.
    3. It must form an effective seal in conjunction with the piston rings, preventing as ‗blow-by‘
       burning away of the oil film and lack of compression.
    4. It must burn cleanly, leaving as little and as soft a deposit possible. This is especially true of high
       additive content oils as unsuitable types can form objectionable ash deposits.
    5. It must effectively prevent the built-up of deposits in the ring zone and in the parts of port
       exhausted 2-stroke engines.
    6. It must effectively neutralize the corrosive effects of numerous acids formed during combustion
       of the fuel.
Lubrication is difficult to achieve because:
           a) Piston direction changes every stroke.
           b) In two stroke engine no non-working stroke.
           c) In diaphragm engine no oil is returned. Therefore supply is limited and consumption is
               controlled. So no cooling effect for the liner.
                                                                                                         108
            d) Piston and rings distort due to gas pressure and temperature.
            e) All fuels contain abrasive contaminants.
            f) Liner temperature (working surface) varies causing change in viscosity.
For two stroke engine one quill fitted per 300 mm circumference or less. Total per cylinder is about 6 to
10 numbers.
Best position for the quill in line with 1st and 2nd ring position with the piston at the top dead center.




              Sight Glass Fluids: a) Water: b) Calcium Nitrate solution: c) 75% Glycerin and water .

                                Early Typical Cylinder Liner Lubricator.

The lubricator shown above is still in use in older engines. Modern engines are fitted with multi plunger
lubricator pumps as shown in the next page. These pumps are required to control the feed rate accurately
to ensure proper lubrication of the liner and also to keep the alkalinity value correct such that all acid is
nutralised and there is no alkaline salt left in the oil after it has lubricated.
                                                                                                                   109




                              1. Dust cover. 2. set screw. 3. Housing cover. 4. Upper cover.
                             5. Operating twisted disc. 6. Control twisted disc. 7. Main plunger.
                        8. Control plunger. 9. Pump element. 10. Base plate. 11. Lubricator housing.
                      12. Worm gear. 13. Pump shaft. 15. Drive shaft. 16. Pipe connection to quill.
                                           A: Suction pipe. B: Delivery pipe.
                                          I.V.O. Cylinder Lubricator.




                                Accumulator Cylinder Lubrication System.
This system could control cylinder oil feed with reference to engine load change and also could control quantity of oil
                              feed with respect to fuel sulphur content/ alkalinity value.
                                                                                                     110

    Matching of Cylinder oil Alkalinity to Fuel Sulphur Content for Two Stroke Diesel Engine.
                        B&W Engine Manufacturer‟s Recommendation.




The above chart helps operating engineers to select the correct TBN cylinder oil along with the correct
quantity of oil to be fed to the liner depending on the sulphur content of the fuel oil burnt for that
voyage. The quantity of oil has to be controlled for change in engine load and while manuvering.
Running in of new liners or running in of new piston rings should be done with great care. The engine
manufacturers use various methods, which are given in the next page.
                                                                                    111
Maximum Liner wear rate
  mm/1000 hours.
        ↑




            Maximum Cylinder Liner Wear Rate/ Engine Running Hours.


Maximum wear rate
 mm/1000 hours.
     ↑




                          →    Cylinder oil feed rate: g/ bhp /hr.   →

  Effect of Cylinder oil feed on liner wear rate in Turbo-charged Uniflow Engine.
                                                                                                            112
Feed rate increase above
  Normal Maximum
         ↑




                                     → Running Hours.→
Cylinder Liner Running in Method No: 1: Cylinder oil feed rate adjustment during running in.




  Cylinder Liner running method No: 2: Progressive Increase of Engine Speed and Cylinder
                    Lubricating Oil Alkalinity during running in Period.
         *******************************Kv********************************
             End of Cylinder Liners/BIT/AMET/BE/KV/June 2003.   End of Part One of Marine Diesel Engines.
                                                                                                         113
PISTON.
Piston Construction:
In the majority of highly rated 2 stroke engines, the piston is either of two or three part construction Cast
steel crown and cast iron skirt are combined in the piston shown below to obtain the strength and heat
resistance of steel in the upper section where these properties are important and the good wearing
properties of cast iron in the lower section, where piston bears against the cylinder. The piston ring
grooves in the piston crown are chromium-plated. Old engines used to have pistons of cast iron, then
with the switch from air blast fuel injection to direct airless injection around 1930, firing pressure rose
from some 40 kg/cm2 to 50 kg/cm2 and on towards 60 kg/cm2. This higher loading led to the use of
forged steel piston crowns.
Piston Materials:
Cast Iron:
There was a time this was the most common material for piston. Cracking of cast iron piston was much
reduced by the use of iron castings of pearlitic structure, with fewer tendencies to growth; as well as oil
cooling of the larger sizes of piston. The increasing mean pressures, piston speeds of industrial turbo-
charged Diesel engines are making it increasingly difficult to obtain a satisfactory safety factor for
single piece pistons. High tensile spheroidal graphite cast iron is also not very suitable because its creep
properties are not as good as Flake graphite cast iron. Flake graphite present in the structure gives good
wearing properties it does not stand very high thermal stress. It is now mainly used for small low rated
engines. Commonly used for skirt of composite pistons.
Forged Steel:
Chrome molybdenum steel has become quite common these days for piston crown. It is heat resisting,
strong and ductile. It has Poor wearing properties.
Aluminum Alloy:
Mostly used for smaller pistons of medium and high-speed engines. The piston saves in weight i.e. much
lighter and gives better heat flow. The drawback to aluminum-alloy pistons is high rate of expansion of
the material. This means that an appreciable clearance has to be allowed when the piston is cold to
ensure a safe running clearance when the piston reaches maximum temperature.
Piston Cooling:
Let us trace the development of the mode of cooling used by one large marine engine manufacture.
    1. At first the pistons were cooled with seawater, which was led in and out through telescopic
        ‗trombone‘ pipes whose glands never sealed perfectly. Consequently seawater used to get into
        the crankcase, causing corrosion on parts of the running gear. Corrosion on the crank and cross
        head pins gave rise to bearing troubles. Moreover corrosive attack inside the piston cooling space
        could never be ruled out entirely despite protective coatings.
    2. Improvement came with the adoption of close fresh water cooling circuit, enabling heat extracted
        from the engine heat to be rejected to the seawater via the exchanger. Though this improved
        matters, corrosion could still not be completely prevented on pistons and bearings.
    3. With the adoption of oil as standard piston coolant in the early 1940‘s, this problem was
        circumvented. But with the advent of supercharging in the mid 1950‘s and the attendant
        increased thermal loads, the piston cooling oil showed a growing tendency to carbonise at the hot
        spots in the piston cooling space. This obstructed the heat transfer leading to the overheating of
        the piston surface facing the combustion chamber and eventually to burning away of piston
        material. In view of the further increase in mean pressure and still greater cylinder boxes
        anticipated, it was concluded that oil cooling had reached its limits.
    4. Studies and measurements on a test cylinder with 76 cm bore confirmed that the return to water-
        cooling for the pistons of supercharged engines would be beneficial with regard to piston
        temperature.
    5. However the advantages of water-cooling for the pistons could be exploited only after ways and
        means were found to prevent water leaking into the crankcase. As before the water is led into and
                                                                                                      114
       out of the piston through telescopic pipes but this are now placed inside a watertight enclosure.
       In this way any leakage from the glands is collected and led off and there is no possibility of
       water getting into the crankcase and causing corrosion there.




                Early Piston of a large slow speed diesel engine (Cocktail shaker Type).




   Modified type to allow cooling water to reach the crown lower part to remove the heat of the
                   crown material and keep it with in the designed thermal stresses.
Flow Pattern Of Coolant:
Figure below show a piston rod which has a through going bore for the cooling Oil outlet pipe, which is
secured to the funnel of the cooling element. The cooling oil is supplied through a telescopic pipe
connection on the crosshead and passes through a bore in the foot of the piston rod and on through the
bore in the piston rod to the cooling element.
                                                                                                          115
Four angled bore in the cooling element give the oil a rotary movement inside the Piston crown. The oil
is passed on through a number of milled grooves in the upper edge of the cooling element to the funnel
and the outlet pipe in the piston rod. From a bore in the piston rod foot the oil is led through a discharge
spout to a slotted pipe in the engine frame and through a control device for checking of flow and
temperature.
In this way, the flow is such that piston-cooling oil enters at the lowest part of the cooling space and
leaves from the upper most part. The flow direction is arranged in this manner so that the piston is
always partially full of coolant and the underside of piston crown is always in contact with it. This is
particularly important in slow speed propulsion engines as when the piston is running at dead slow speed
the coolant in piston is not ‗shaken up‘ in the way it is done when engine is running at full speed. If the
coolant flow took place in the opposite direction, it would be possible at very slow speed for the coolant
to drain from the piston and loose contact with the crown. The piston could become overheated.

Some water-cooled pistons have the outlet for the water at approximately half the Cooling space height.
When running slow, the piston is half full of water and piston movement agitates the water in the piston
the water gets splashed on the underside of the crown. The splash method of cooling is called ‗cocktail
shaker cooling‘ (See drawing in page 2).
This piston consists of 3 main parts, which are piston crown, cooling element and piston skirt. The
cooling element is tightened to the upper end of the piston rod and transmits the combustion pressure
from the piston crown to the piston rod. Between the piston crown and skirt, which are assembled with a
number of screws, there is a heavy clamping ring, which presses the piston rod and the cooling element
against the crown.
Latest Development:
The GFCA, GB and MC pistons shown the figure below reflect the gradually increased firing pressures
which are inevitable result of the demand for ever decreasing fuel consumption figures. The material for
the crowns remains the same well proven chrome-molybdenum steel, which has excellent strength
against thermal stresses and a reasonable temperature rating.
The GF pistons were originally rated for a firing pressure of 84.3 bar. This pressure has been increased
first to 87.3 bar and finally to 95 bar justified partly by the reduced thermal load, which is a reflection of
the higher efficiency of constant pressure turbo charging, and partly by the excellent service record with
an almost non existent failure rate.
The utter simplicity of the GF (CA) piston crown gives low production cost which is important, as
pistons are components with a finite service life. The simplicity, however gives way to some minor
deficiencies, which are the result of the flexible connection to the piston rod, which in turn is necessary
because of the thermal deformation of the crown. These deficiencies do not limit the useful lifetime of
the crown.
The GB pistons, rated to 112 bars are the next step. With this design the mentioned deficiencies are
cured by the firm connection to the piston rod. The changed cross-section gives adequate strength to
limit the mechanical stress level to the same as for the GFCA pistons, giving the same very low failure
rate.
The MC piston crown, like the GB type, is rigidly bolted to the piston rod at its inner support, which has
a smaller diameter relative to the bore, than the GB type. A new structural member shaped as an inverted
truncated cone, transmits a considerable part of the gas force to the rod, and leaves a toroidal cooling
chamber outside the supporting ring thus making a reasonably thin plate possible.

A consequence of higher firing pressure is higher thermal efficiency. This is reflected in the relatively
small heat losses, which include the piston, thus oil cooling is sufficient even for the biggest bores.
                 L90GFCA.                         L90GB.                            L90MC.
                                                                                                               116




        Part sections through recent B&W Engine Pistons reflecting the gradual increase in firing pressures.
                         The crown of the MC design bears directly on the piston rod flange.




     MAN/B&W design latest Piston, Piston rod, and Stuffing box (Bore cooling system used).

Some Piston design and Piston Sketches with Cooling System:
The new piston design (Opposed piston engine ‗Doxford- J‘ Type) has the effect of improving cylinder
compression. Improved fuel consumption and starting have been achieved by fitting the firing ring
closer to the crown, thus reducing the crown volume and, piston temperature during the power cycle.
During the expansion stroke the firing ring will reach the opening edge of the exhaust port and the air
inlet port at a later crank angle then the existing design. The pressure in the cylinder will be slightly
lower giving a similar pressure ratio between the cylinder, exhaust belt and air inlet trunking. This
results in reduced gas leakage past the piston crown, thus increasing the effective power stroke and
reducing the fuel consumption. The lower pressure ratio also has the advantage of reducing the amount
of lube oil that is blown off the firing ring and crown. The firing ring has been fitted closer to the crown.
                                                                                                      117




                            Doxford Engine: 76J, 67J: Type Piston in section.
In reducing the crown volume, care has been taken to ensure that that there is no impingement from the
spray pattern of the new or existing fuel injector nozzles. The reduced volume of the piston gives a
decreased minimum volume, and therefore an increased compression ratio to aid starting, either in the
ahead or astern directions.
The insert fitted inside the cooling space of the piston has the effect of reducing the crown temperature
by increasing the flow of the cooling media at the underside of the crown. The Insert ensures that flow
pattern is directed to the critical areas and reduces the possibility of having high temperature pockets.
The specific fuel oil saving is a maximum of 1.65 g/bhp hour, which is attained at 60% load.




                                    B&W Engine Piston with skirt.
            CI. Skirt is short. This is because this engine is a „Uniflow‟ scavenging type.
                                                                           118




Sulzer RD Engine Piston with piston rod and telescopic pipe arrangement:
                                                                                                     119
                              MAN: K/Z Engine Piston and Piston rod.




                 Piston Cooling arrangement and details of telescopic pipe system.
                      As the engine is a ‗Loop Scavenger‘ the piston skirt is long.
The piston cooling water telescopic pipes are arranged outside the engine crankcase at lower level of the
       engine to avoid any contamination of engine oil system due telescopic pipe gland leakage.
                                                                                                  120




       MAN Engine KSZ/B Type Piston.                        MAN Engine KSZ/BL Type Piston.
          Cast conventional type.                              Forged Bore Cooling type.




     KSZ/BL Type piston, cylinder cover and liner material surface temperatures well below 350*C
             due to bore cooling. Thus for the same power the thermal loading is very low.
Piston material specification:
    1. Cast iron: Common in olden days. Now aluminum alloys are used. Cast iron is cheap, has good
       thermal conductivity and good wearing property due graphite flake structure. Can be used in low
       speed engines.
           a) Tensile strength: 28 kg/mm2, Bending strength: 48 kg/mm2, Compressive strength: 90
               kg/mm2, Hardness: 190 – 230 HB.
           b) C (total): 2.9 to 3.2%, C (bound): 0.7 to 0.9%, Si: 0.9 to 1.5%, MN: 0.8 to 1.0%,
               P: 0.1 to 0.3%, Ni: 0.6 to 1.2%, Cr: 0.3 to 0.5%, V: 0.1 to 0.2% or Mo: 0.4%.
    2. Forged steel: (Crowns - Heat resistant).
           a) Tensile: 65 kg/mm2 @ 20*C and 20kg/mm2 @ 800*C. Yield point: 30 kg/mm2,
               Elongation: 30%, Hardness: 163 to 241 HB.
           b) Carbon: 0.3 – 0.4%, Mn: ≤ 2.0%, Si: 2.0 – 3.0%, Cr: 16 – 20%, Mo or V: 23 – 27%.
    3. Cast steel: (Heat resistant).
           a) Tensile: 55 kg/mm2, Yield point: 28 kg/mm2, Elongation: 19%, Hardness: 190 HB.
           b) Carbon: 0.25%, Mn: 0.5%, Si: 0.15%, S: not over 0.040%, P: not over 0.040%.
                                                                                                    121
        c) Piston head special molybdenum cast steel: C: 0.13-0.18, Si: 0.3-0.5, Mn: 0.5-0.8,
            Mo: 0.3-0.5.
  4. Aluminum Alloy:
     For Casting: (Aluminum Copper alloy: Large diameter pistons for high speed engines).
        a) Tensile: 30 kg/mm2, Yield: 26 kg/mm2, Elongation: 0.3%, Hardness: 130 HB.
        b) Cu: 4.0%, Mg: 1.5%, Ni: 2%. Impurities: Si: less than 0.7%, Zn: less than 0.3%, Fe: less
            than 0.8%. Specific weight: 2.75.
     For Drop Forging: (Small high speed engines. Made stronger by drop forging).
        a) Tensile: 44 kg/mm2, Yield: 29 kg/mm2, Elongation: 10%, Hardness: 110 HB.
        b) Si: 0.85%, Cu: 2.2%, Mg: 1.6%. Specific weight: 2.8.


                        Medium Speed Four Stroke Diesel Engine Pistons:




Four stroke trunk piston (In section, one half perpendicular to the other to show Gudgeon pin fixture).
                                                                                                          122




Four Stroke Trunk Piston of Medium Speed Engine using blended fuel and developing higher power.
To withstand higher pressure load and thermal heat the piston crown is made of cast steel with special
cooling arrangement.
Pielstick engine piston:
The trunk piston is cast in one piece out of light alloy material (aluminium-silicon). The steel coil
embedded in the casting is taken behind the piston rings. Lubricating oil is supplied to the coils through
the gudgeon pin. The inside of the pin is divided for this purpose into separate passages for oil inlet to
the coils and return to the pin after circulation. Separate vertical passages drilled inside the connecting
rod convey oil for cooling the piston and bring the return to the crankcase.

Four compression rings seal the combustion space. Two oil control rings are also provided on top and
bottom of the gudgeon pin. The top compression ring is chrome plated and fitted in an insert. Other
compression rings are made of cast iron with a copper-lead inlay.

The gudgeon pin is constructed of alloy steel which is nitrided at the surface and ground finish. It is
floating in position and provided with steel plates at both ends to prevent the pin from coming out.
                                                                                                           123




Pielstick: PC2 Type: Trunk Piston:
   1. Top Ring chrome plated. 2. Piston rings. 3. Oil retainer ring. 4. Gudgeon pin. 5. Piston body.
    6.Side plate. 7. Oil scraper ring. 8. Cylinder liner. 9. Side plate. 10. Bedded steel coil for oil passage.
                                                                                                         124
Piston Stresses:
    The piston is subjected to compression and tensile stresses caused by:
    1. Gas pressure, which causes distortion of the crown. See figure below.




   2. Thermal stresses, due to temperature difference between inner and outer surface of the piston. The
crown and upper ring groove suffers distortion as a result of free expansion on the hot side being
restricted by the cooler surface of the piston. See figure below.




Inertia Effects:
The crown of a piston is subjected to very high gas pressure, which will subject the top surface of the
crown to compressive loading, and the lower surface of crown will be under tensile loading. The piston
crown will be like a uniformly loaded beam.

As the piston is moving upward, towards the end of its stroke its velocity will be reducing or in other
works retardation will occur. The inertia effect will tend to cause the piston to bow upwards, so that top
surface of the crown along with sides will be under tensile loading and lower surface of the crown will
be under compressive loading. The pressure on the top of the piston nullifies the inertia effects when the
piston approaches top center position in the upward direction.

It must be understood clearly here that the inertial forces on a 4 stroke engine at the end of exhaust
stroke will not be nullified in the same manner as in the case of compression stroke, because the gas
pressure at the end of exhaust stroke is insignificant. When the piston is retarded on its approach
downwards to bottom center the piston crown tends to bow downwards, and its upper surface and the
piston walls are in compression. The lower surface of the piston crown will be under tension.
                                                                                                    125
Design consideration of piston crown:

   1. Marine engine pistons instead of being flat are always either concave or convex at the top, the
      object being to increase their strength and allow greater freedom during expansion and
      contraction, which naturally result when an engine is started from cold and stopped after a long
      run respectively. Flat surfaces are not self-supporting if subjected to a pressure on one side, as
      occurs when an engine piston is loaded. An unsupported flat surface takes a concave form on the
      side, which is loaded and as the load on gas pressure increases, the curvature of the concave
      surface increases, the amount of curvature being dependent on the pressure applied. Thus if the
      piston top were flat it would go through a pattern of varying concavity increasing as the pressure
      in the cylinder increased and decreasing as the pressure diminished. This regular and cyclic
      change of form, which would occur in a flat piston crown, would cause it to suffer from fatigue.
      Early failure would occur at the point where the greatest variations in stress occurred, no matter
      how well the other parts of the piston were designed. Curved surfaces such as the curvature of
      convex or concave piston crowns are very nearly self- supporting when loaded on one side. As
      these curved surfaces are self-supporting they do not change then shape or move to any degree
      with the changes of pressure over a working cycle in the engine. Consequently the risk of fatigue
      failure is so reduced that it now rarely occurs. The upper part or crown of the piston also forms
      the lower part of the combustion chamber. The concave or convex curvature must thus also be
      arranged in conjunction with the fuel-injection nozzle spray pattern so that good combustion is
      encouraged with in the form of combustion chamber produced.

  2. The material and thickness of piston crown bearing in mind its pressure loading and the stresses
     induced by temperature gradient.


                  Sulzer Z 40/48 Engine. (500 r.p.m., b.m.e.p. 20.50 kp/cm. 687 BHP/Cyl).




             Measured temperature in *C in the combustion chamber area components.


                                 Four Stroke Medium Speed Engine.
                                                                                                 126
B&W: L35MC type engine measured temperatures in the combustion chamber at „mcr‟ and with
non-cooled valve seat insert. Nearly all values are close to those calculated for various components.
                      Two Stroke, Slow Speed high powered uniflow engine.




      Sulzer: RN type: Two Stroke, Slow Speed high powered Loop Scavenging type engine.




                Sulzer: RN90 Engine.                    Sulzer: RN90M Engine.
                Old conventional type.                  New design bore cooling type.

                 Isotherms around the combustion chambers of above two types.
                                                                                 127
    Sulzer: Z40/48 Type Medium Speed Engine Rotating Piston.




 Section through the piston.      Pawl and toothed rim in the piston.
 Also seen are connecting rod      Mechanism that makes the piston rotate.
and bottom-end bearing.           1. Piston body. 2. Pawls. 3. Toothed rim
                                  rotatable in piston. 4. Point of action of toothed
                                   rim onto annular spring. 5. Annular spring.
                                   6. Link between spring and piston.
                                                                                                        128




The rotating piston transforms the swinging motion of the connecting rod against the piston axis into a
slow revolving motion of the piston itself round its longitudinal axis by means of a ratchet mechanism.
Thus, a fresh-lubricated section of the piston skirt is brought to the loaded area for every-piston stroke.
Should against all probability, local scuffing start to develop, the rotation separates the two affected
areas and does not allow scuffing to aggravate to a seizure. Apart from the above, the Grey Cast iron
piston skirt and cylinder liner provide for excellent running conditions between the two surfaces.
Rotating pistons for reliable and economical service:
            a) At each stroke, a new oil-wetted portion of the piston is always in contact with the
                pressure side of the liner, this minimizes the danger of piston seizures.
            b) The piston rings rotate as well, and so local overheating of the liner resulting from blow-
                by at the ring gap can be avoided.
            c) Due to the symmetrical design the thermal and mechanical deformation is also
                symmetrical.
            d) Lubricating oil is distributed uniformly - a favorable factor in respect of the wear rate on
                cylinder liners and piston rings.
            e) Smaller clearances are possible between piston and liner.- This reduces piston slap and
                consequently lowers the danger of cavitation on the water side of the cylinder liner.
            f) The spherical top-end bearing eliminates the edge pressure problems associated with
                gudgeon-pin bearings. The specific bearing pressure is comparatively lower.
            g) The separate, load controllable cylinder lubrication system, together with the oil scraper
                placed at the lower end of the piston skirt, permit exact matching of the amount of oil
                reaching the running surface.
            h) Low total oil consumption remaining constant over long running periods: 1.3 g/kWh (1
                g/BHPh) or below.
                                                                                                          129
It is believed that the maximum safe temperatures at the three most critical zones for an aluminum-alloy
piston in a Diesel Engine are respectively: for the crown 370* to 400*C for the top ring groove 200* to
220*C for the gudgeon pin bosses 200*C to 220*C.
If the crown temperature exceeds 400*C, failure will probably occur from cracking. If the top ring
groove temperature exceeds 220*C for any length of time, trouble may be expected from:
             i) Stuck piston rings.
             j) Formation of carbon at the bottom of the ring groove, causing the ring to be packed out.
So during inspection and survey the piston crown, ring grooves must be checked for cracks. Suitable
crack detection method to be followed if in doubt.
Faults:
     1. Cracking of crown - due to thermal and mechanical stresses.
     2. Cracking through piston wall especially in way of top ring groove - due to fluctuating gas load,
         excessive thermal stresses. The crack starts from inside wall.
     3. Cracking may take place due to following reasons, apart from the reasons mentioned above:
             a) Unsuitable material for the rating of the engine or inadequate machining.
             b) Excessive scaling on the cooling side, cavitational erosion.
             c) High coolant temperature.
             d) Local impingement.
             e) Poor atomization and high penetration of fuel.
             f) High water content in fuel.
Gudgeon Pin Boss Failure:
If the boss is not well supported, fatigue failure in the vicinity of boss will occur. It may originate at two
points:
     a) Immediately above boss- extending circumferentially around piston.
     b) Longitudinally along top of the boss.
Lower ring groove with square corners close above gudgeon pin hole presents weak spot. Having drain
holes in 45• on each side of gudgeon pin center should not aggravate this.
Flexure of gudgeon pin under load is less but throws the load on inner end of boss. There have been
cases, where boss has split. To prevent this inner end is usually rounded off, to avoid load concentration
at ends.
Piston Running Hot:
     a) Inadequate circulation of cooling media and/or supply not sufficient.
     b) Excessive deposit in cooling space (this could be scale or carbon).
     c) Lubrication not sufficient.
     d) Faulty piston ring fitting, clearance inadequate. Too high temperature in top ring groove area.
         This would cause blow-by.
     e) Distorted liner.
     f) Misalignment of piston.
     g) Overloading of unit i.e. excessive fuel.
     h) Excessive water content in fuel.
     i) Insufficient air from turbocharger.
     j) Late injection of fuel.
     k) Engine running slow speed - full flow of coolant not maintained.

Piston design considerations:
           1. The shape of the top of the Piston crown which forms one part of the boundary of the
              combustion space - so while designing this it has to be considered how it is going to vary
              the shape of combustion chamber. The ‗compression ratio‘, ‗live air ratio‘ have to be
              given due considerations. In certain types, the piston crown is machined to provide
              channels and cavities for the favorable reception and redirection of jets of flame issuing
                                                                                                      130
                from the pre-combustion chamber. In direct injection engines, the piston crown may be
                convex (part spherical), part conical with central depression, hearty flat, concave (part
                spherical) etc. With four-stroke engines, it may be necessary to arrange ‗cut outs‘ in the
                surface of the crown, to clear the heads of the inlet and exhaust valves during overlap
                period at the top of the idle strokes.
           2.   The material and thickness of piston crown; bearing in mind its‘ pressure loading and the
                stresses induced by temperature gradients. The provision of tapped holes for lifting, in
                view of their weakening effect.
           3.   The material and thickness of the sidewalls of the piston, which accommodate the piston
                pressure rings and transmit the pressure load to the gudgeon pin (trunk piston engines) or
                to the bottom flange of the piston body (crosshead engines) should be adequate enough
                for withstanding the designed load inclusive of factor of safety.
           4.   The design of the gudgeon pin for strength and bearing pressure; as well as the gudgeon
                pin bosses (trunk) and closing plates if required.
           5.   The proportions and clearances of piston walls and skirts.
           6.   The accommodation of scraper rings at the mouth of the piston for controlling lubricating
                oil consumption.
           7.   The conveyance of coolant to and from the piston cooling space, which acts as a heat sink
                for the piston crown and ring belt.

Main troubles to be avoided:
  1. Piston seizure; all too often followed by an explosion of the lubricating oil/air mixture in the
      crankcase.
  2. Cracking of piston crown and sidewalls, due to cyclic variation of pressure stress superimposed
      on stress clue to temperature gradient.
  3. Burning Of Piston crowns, this occurs mainly of large slow running engines.
  4. Excessive lubricating oil consumption (trunk engines).
  5. Sticking of piston rings by viscous deposits, formed by oxidation of lubricating oil and fuels
      leading to ‗blow-past‘ which if excessive can lead to piston seizure or scavenge fire (crosshead
      engines).
  6. Breaking of piston rings.
  7. Scuffing of piston rings and cylinder liners,

Composite and Ribbed Pistons:
The early cracking of piston crowns was an inducement to make pistons with separate detachable parts:
     a) To enable the crown to be made of heat resistant material, not suitable for piston body.
     b) To give crown some freedom of expansion in the hope of preventing cracks.
     c) To enable a cracked crown to be replaced without scrapping the whole piston.
The piston is tapered from ring groove area to the top of the crown. To withstand high, mechanical
stresses it would be advisable to have a thick self-supporting convex crown. To keep the thermal stresses
in limit a thin crown is ideal. For ideal combustion it is usual to have a concave down. Having a thin
crown, with internal ribs for support, makes a compromise.
Modern engine design has over come all these problems by using alloy steel forged crown and
incorporating bore cooling system.
Maximum Safe Temperature for Aluminum alloy Piston Crown:
Crown: 370*- 400*C: Top ring groove: 200-220*C: Gudgeon pin boss: 200-220*C.

     *******************************Kv****************************************
                                                                                                        131
Piston Coolant:
The coolant used for removing and conveying the heat from a piston may be either fresh water, distilled
water or lubricating oil, Water has the ability to remove more heat than lubricating oil. This can be seen
from the fact that specific heat of water is approximately 4 whilst the specific heat of lubricating oil is
about 2 (Both in S I Units).
Further the temperature range (t2 – t1) of cooling water passing through a piston may be of the order of
14*C while for cooling oil it will be 10*C for similarly rated engine.
Let Q = Quantity of heat removed in any given time.
Q = Weight of coolant used in time T x (t2-t1) x Specific heat.
If weight of water used is unity.

QW (Heat removed by water) = 1 x 14 x 4 = 56.
If weight of oil used in the same time is WO
QO (Heat removed by oil) = WO x 10 x 2.
If same amount of heat removed:
QO = QW.           WO20 = 56.
                    WO = 56/20 = 2.8.
So it can be seen for same cooling effect amount of oil circulated is about 3 times the water. In actual
designing practice there are many other factors to be taken into account.
Fresh and Distilled Water Piston Cooling system:
Advantage:
    1. The main advantage of cooling pistons by water is the ability of water to absorb large amounts of
        heat.
    2. Relatively easy to obtain.
Disadvantage:
    1. The piston cooling water conveyance pipes and attendant gear must be kept out of the crankcase
        as far as possible, due to the danger of contamination of the crankcase lubricating oil by water
        leakage. Because of possible contamination of Jacket cooling water with oil, the jacket cooling
        water system must be made separate from the piston cooling system. This necessitates
        duplication of cooling water pumps, piping, motors, starters, coolers and control equipment.
    2. When an engine has water-cooled pistons, the piston cooling space should be drained of water
        after the engine is shut down for an extended period. A drain tank is necessary for the same
        purpose Cascade type filter is often incorporated for separation of oil and water.
    3. There is risk of scaling and corrosion if water is not properly treated and maintained.

Lubricating Oil Piston Cooling System:
Advantage:
   1. The piston cooling oil pump is combined with the lubricating oil pump and piston cooling oil
      cooler is combined with the lubricating oil cooler. This makes overall simplicity in the system.
   2. Internal stress within the material of the Piston is generally less in oil-cooled piston than in
      water-cooled piston. Good design in water-cooled piston can improve its condition of working.
   3. No risk of crankcase-system oil contamination, even when piston cooling oil conveyance piping
      is fitted inside the crankcase.
   4. Simpler arrangements for cooling-oil conveyance piping with less risk of ‗hammering‘ in piping
      and bubble impingement attack.
Disadvantage:
   1. Larger power requirements for pumping cooling oil.
   2. Larger amount of lubricating oil required giving some cooling effect.
                                                                                                       132
Flow Pattern of the Coolant:
The flow is such that piston cooling oil or water enters at the lowest part of the cooling space and leaves
from the uppermost part. It should move in such a manner that upward movement of coolant is uniform
on opposite sides of the piston to give even cooling without causing distortion due to unequal expansion.
The flow direction is arranged in this manner so that the piston is always full of coolant and the
underside of piston crown is always in contact with it. This is particularly important in slow speed
propulsion engines, as when the piston is running at dead slow speed the coolant in piston is not ‗shaken
up‘ the way it is done when the engine is running at full speed.
If the coolant flow took place in the opposite direction, it would be possible at very slow speed for the
coolant to drain from the piston and lose contact with the crown. The piston could become overheated.
Some water-cooled pistons have the outlet for the water at approximately half the cooling space height.
When running slow, the piston is half full of water and piston movement agitates this water in the piston
and the water gets splashed on the underside of crown and piston wall.
When the engine is stopped a jet action from the piston cooling pipe nozzle directs cooling water onto
the piston crown, thus removing residual heat and catering for an emergency stop at full speed. The
splash method of cooling is called "cocktail shaker cooling"
Quality Requirements for cooling water:
Engine cooling water is a consumable store, which should be carefully selected, treated and continually
watched. If this is neglected, corrosion, erosion and cavitation may occur on the watersides of the
cooling system and deposits may be formed. These deposits impair heat transfer and may cause thermal
overloading of the engine parts, which have to be cooled. Therefore, the water should be treated before
the engine is put into operation. During operation care should be taken that the specified concentration is
always used.
Corrosion and cavitation on the thrust side of the cylinder liners may occur in all water-cooled
combustion engines. This is caused by the concerted action of corrosion and cavitation. The cylinder
liner is set into vibrating motion with varying amplitudes and accelerations by the piston during working
stroke resulting in negative and excess pressures at the interface of liner wall and cooling water. When
the liquid is reaching vapour pressure, it forms vapour bubbles, which as they collapse at the subsequent
pressure rise in the course of positive stroke of the vibratory cycle of the liner wall, produce high local
pressure and temperature peaks. The impact intensity, which acts the liner into vibrating motion,
depends on engine revolutions. This explains why cavitation is less frequent occurrence in medium and
low speed engines, than in high-speed engines (revolutions 700 r.p.m).
Vibration Fissure* Corrosion is a damaging mechanism which is caused by dynamic and corrosive load
simultaneously. This can be the cause for the formation of cracks and rapid progress of cracks in water-
cooled mechanical loaded engine parts, due to a faulty water treatment. Corrosion attack is avoided,
when a cohesive protective coating or surface film is formed on the metal cooling surfaces. This
protective coating can be obtained by adding corrosion inhibiting oil or a chemical corrosion inhibitor to
the cooling water. (*Fissure - narrow opening or crack of some length and depth.)
Laboratory tests and practical experiences confirm, that certain emulsifiable corrosion inhibiting oils are
better in reducing successfully vibration fissure corrosion and cavitation than chemical inhibitors.
Corrosion preventive oil forms an oil-in-water emulsion, and the emulsifier in the oil provides for a
protective layer on the metal cooling surfaces, which prevents corrosive damage.
Characteristic of water should be Within the Following Limits:
Type of water: Fresh water, free of impurities.
Total hardness: Maximum 100 German Hardness*
PH-valve at 20*C: 8.
Chlorine ion content: Maximum 50 mg/L
(* - 1* German Hardness = 10 mg CaO in 1 L water).
                                                                                                        133
Total hardness of water combines temporary and permanent hardness. The calcium and magnesium salts
mainly define it. The hydrogen- carbonate part of the calcium and magnesium salts determines the
temporary hardness and the remaining calcium and magnesium salts (sulphates) determine the
permanent hardness. The temporary (carbonate) hardness is determining for the formation of calcium
deposits in the cooling system. Water with a total German hardness of more than 10*should be diluted
with distilled water or rainwater or can be softened by chemicals. If the water has a hardness which is
lower than specified by the manufacturer of the adding inhibitors, the water should be hardened by
mixing with hard water or by adding certain chemicals.
When distillate (i.e. from a fresh water generator) or non-saline water is available, this should be used as
engine cooling water. However a slight hardening will then be necessary, depending upon the additive
used. This water is free from calcium, and mineral salts so that there will be no formation of deposits
reducing-heat transfer and impairing the cooling effect. On the other hand it will be more corrosive than
normal hard water, because it will not develop a thin scale, which provides for a temporary protection
against corrosion. Consequently water distillates should be treated with special care and concentration
constantly watched.

Cooling water Additives:
Only these additives to be used which give adequate protection of the engine against corrosion and
cavitation, both in service and during standstill, and which do not attack the materials and seals of the
cooling system.
The conditions for the effective application of corrosion inhibitors are:
              a) A clean cooling system.
              b) Suitable water.
              c) Properly prepared cooling water for initial fill.
              d) Continual supervision of the concentration.
              e) The condition of the cooling system.
If it is additives prime task to prevent cavitations, an emulsifiable corrosion inhibiting oil should be
selected. As deposits have an adverse effect on the activity of the additive and i.e. the stability of
emulsion - it is essential that all surfaces in contact with the cooling water are free from rust and other
contaminants before the cooling system put in service. If deposits are found to be present, the entire
system should be flushed or cleaned with solvent. This is done most effectively by special firm, or
supervised by an expert from the supplier of the solvents. The cleaning agent should not attack the
material or seals in cooling system. When cooling water additives are used, the manufacturer's
instructions concerning the water quality to be used, additions, concentration and storage should be
carefully followed. For low speed engines lower concentrations are usually allowed than for high-speed
engines. When draining the treated cooling waters observe environmental protection regulations.

Anti-Corrosion Oil:
This inhibitor is an emulsifiable mineral oil containing additional agents. A thin protective oil film,
which does not affect heat transfer and prevents deposits, is formed on the metal surfaces of cooling
system. Frothing can occur with oil emulsions, but this may be corrected by maintaining the water pH
value of the solution between 8 and 9. Adding hardening powder, such as calcium sulphate and 10%
magnesium sulphate does this.
Note: Anti-corrosion oils are not recommended and not suitable when there is a possibility of cooling
water temperature dropping below 0*C or rising above 95*C.

Chemical Inhibitors:
Additions of sodium-nitride and sodium-nitride-borate basis have shown to be satisfactory. The new
regulation for waste-water disposal and the possibility of cooling water (Fresh water) leakage into the
                                                                                                      134
sea water side prohibits the use of chromate in cooling water system. Nitride and nitrate are not suitable
for galvanised pipes or in a cooling water system where cooling side is protected by Zn-anodes.
Note: Corrosion inhibiting oils mixed with chemical additions may cause deposits in the cooling system
and reduce the heat transfer. In case the cooling water treatment is changed from oil to a chemical
inhibitor or the other way round, the entire system should be carefully cleaned first.

Anti-Freeze Agents:
When the engine is operating at temperatures below the freezing point of water, an anti-freeze agent
should be added. Suitable anti-freeze agents (Glysantin 3059) protect against corrosion and are also
effective in protecting against cavitation. The additional water treatment is then not necessary. An
adequate corrosion protection is obtained when the concentration is adjusted for a low-temperature
protection level. Any type of anti-freeze medium agents in use, causing corrosion in the cooling system
it should be used only during one winter. Anti-freeze agents must not be mixed with each other.
Note: When cooling water contains a corrosion inhibiting oil, no anti-freezing agent should be added,
otherwise the emulsion will break and decompose at once. Chemical additives are normally compatible
with anti-freeze agents, but with the later added, a different concentration of chemical additive may be
required.

Damage on piston crown:
A diesel engine piston may be damaged by:
            i)     Direct oxidation at high temperature at the skin owing to flame impingement,
            ii)    Catalytic oxidation promoted by fuel ash in a corrosive environment,
            iii)   Wet corrosion by sulphuric and sulphurous acids during low temperature operation or
                   during stand-by periods.
The proper selection of material and its treatment is one measure of prevention against such
deteriorative damage. Liners are made of alloyed cast iron, but could he chrome plated. Pistons are steel
forging or castings containing small additions of chromium and molybdenum. Ring grooves may be
plain quench-hardened; chromium clad or fitted with hardened steel or cast iron inserts.

Direct Oxidation:
Present day diesel engines burn a low grade of fuel oil containing sulphur. During the combustion, the
crown of piston comes in contact with products of combustion and air containing oxygen, steam, carbon
dioxide, sulphur dioxide, sulphur trioxide etc. Metals exposed to such condition will be coated with an
oxide film. High rate of cooling or a protective coating by chrome will prevent the peeling of the layer.
If the layer is allowed to thicken it will splinter under the effect of flame impingement. A new surface
will be exposed to damaging action and thus the wastage will penetrate.

Catalytic oxidation:
Residue oils contain vanadates and compounds of sodium and sulphur. The vanadates are oxidised
forming vanadium pentoxidc. Free sulphur or sulphur compounds are oxidised forming sulphuric and
sulphurous acids. The attack of molten oil ash containing vanadates and sulphate could be severe. The
severity of the damage is associated with overheating.

Wet corrosion:
The presence of sulphur is responsible for this type of damage; it is the dew paint of sulphur trioxide
with steam that matters as regards this type of corrosive damage.
Overheated Piston:
Knocking at both ends of the piston travel associated with drop in engine revolutions, rise in cylinder
and piston cooling temperatures, rise in exhaust temperature, smoke in exhaust, will indicate a hot piston
working with high friction against the liner surface.
                                                                                                       135
A piston can he overheated owing to the following:
   1. Failure of coolant circulation.
   2. High friction on liner caused by rings seized in the groove, insufficient ring clearance, long skirt
       touching the liner body.
   3. Failure of cylinder lubrication.
   4. Improper combustion caused by sticky, leaky or broken rings loss of compression, worn liner,
       worn injector holes, incorrect fuel timings, unsuitable fuel, insufficient air,
   5. Unbalanced cylinder load.
   6. Continued overload operation.
Whenever a hot piston is detected:
           a) The engine should be slowed down without stopping.
           b) This measure will immediately reduce heat generation both frictional as well as from
              combustion of fuel.
           c) Identify the affected cylinder by observation of temperatures, noise, etc. The fuel supply
              is terminated by lifting the pump plunger.
           d) The unit is cooled down by maintaining circulation of coolant in piston and liner.
           e) Increase lubrication in affected cylinder.

                              Checking the piston crown with template.




*********************************Kv*****************************************
                                                                                                         136
PISTON RINGS:
Working condition of this simple component should be properly understood. The piston rings must
provide an effective seal of the combustion space. Under ideal condition the piston ring surfaces are in
complete contact over its entire depth and periphery with the liner surface and ring width on landing
area. The initial seal is established between the ring and the liner by a radial pressure exerted by the ring
when pressed on liner. Following combustion the ring is forced down on the grove landing surface by
the gas pressure. The gas pressure is also throttled at the back of the ring through the small clearance
space thus increasing further the radial pressure against the liner.
The sealing of combustion space by a set of rings on reciprocating piston follows the labyrinth principle.
The gas pressure leaked in behind each piston ring is successively throttled down to the pressure
prevailing at the underside of the piston. In this way its natural tendency to leak out is progressively
diminished. The number of rings, the ring section area and the contact areas are determined by
consideration of strength, pressure difference, volume of space to be scaled, etc. From the foregoing it
will be clear each ring is different, being the highest at the top ring and diminishing successively at the
bottom ring.
Action of Ring:
                                           Gas pressure in cylinder.




                            Share of gas sealing load accepted by each ring.
                                                                                                         137
Ring Clearancees:
The 3 major clearances to take note are:
1. Side or Axial clearance.




This clearance allows the ring to slide in the ring groove so that it can maintain contact with the liner
wall.
Typical values would be:
0.10- 0.15 mm for four-stroke engines.
0.2 - 0.25 mm for two-stroke engine.
This value may be doubled for the top ring to allow for groove distortion, carbon deposits and thermal
expansion. The maximum acceptable clearance is about 0.5 mm.

If the clearance is too small the ring will seize in the groove causing ring collapse and gas ‗blow-by‘. It
will-also excessively restricts the gas pressure build-up behind the ring reducing the supporting forces,
which hold the ring against the liner wall. This again may be sufficient to cause ring collapse.

If the clearance is too large then the ring will be allowed to bend excessively reducing the contact face
height against the liner and encouraging an oil scraping action. It will also allow the ring to hammer on
the groove landing, increasing wear and may generate a L.0 pumping action, particularly on a 4- stroke
engine (oil pumped around the back of the ring) leading to heavy lubricating oil consumption.

2. Butt Clearance:
            Butt Gap.                                Expansion Gap.




             Plain Gap.             45 deg. Mitre Gap.        Reduced width gas passage.   Lap Gap.

This clearance is necessary to allow circumferential thermal expansion and permit fitting of ring to a
solid piston. The three most common shapes of ring end used to reduce gas leakage are shown above.
The plain gap is popular on 2- stroke engines for the top and second rings as it is robust although the
45* ‗Mitre‘ gap is almost as popular. The mitre gap gives a reduced width gas passage for the same
circumferential clearance. The lap shape reduces the gas passage width even further and also increases
its length considerably thereby improving gas sealing but the ends are relatively fragile and the rings are
expensive to manufacture. Therefore, it is likely to be used for lower rings only.
                                                                                                          138
The gap clearance is measured with the ring clear of the cylinder
(called the ‘free gap’) and with the ring in the unworn part of the
cylinder (called the ‘closed gap’).

Typical free gap: 31/2 x Ring radial thickness.
Typical closed gap: 0.005 - 0.006 mm/mm cylinder diameter.

If the closed gap is large, poor sealing and blow by results, while too small a clearance may permit ring
end contact due to thermal expansion leading to heavy ring pressure against the liner and causing heavy
wear. The butt clearance is a useful judge for ring wear and loss of spring tension as the closed gap may
be measured through the scavenge ports. Ring wear is more commonly measured by removing the ring
and measuring the reduction in radial thickness.

3. Back Clearance:




The depth of the ring grove should always be greater than the ring radial thickness so that the piston side
loads are never transmitted to the liner wall via the rings. An excessive clearance can allow the build up
of large hot gas volumes leading to overheating and oil oxidation producing heavy deposits.
Ring Material:
Most popular material is cast iron alloyed principally with silicon, manganese, chromium, phosphorous,
copper and molybdenum. The presence of graphite in cast iron in lamellar form improves the material
property in sliding. The graphite acts as a lubricant in the dry state. The material must be resilient, strong
to withstand the pressure in the cylinder and sufficient resistant to wear. The range of hardness in such
piston ring material differs from 160 - 190 Brinell. The type of rings used in large engines is regarded as
soft. The use of these rings in engines with high maximum pressure advantageous as it takes bedding
quickly over the liner surface during the running in period.
Depending on the size of the ring a choice is made between strength and wear resistance. Strength
obtained by refined grain structure resulting in finer graphite flakes, wear resistance is due to large
graphite flakes. There is a limited use of S.G. iron for very large rings. This has improved strength but
poor wear resistance (up to 800 MN/m2).
The ring has hardness slightly higher than that of the liner material so that wear between the ring
and liner is at the optimum low rate with the easily replaceable ring suffering most wear.
Typical hardness: 200-220 HB. Typical UTS: 200 Mn/m2.
                                                                                                          139
After manufacture of rings, some of them may be coated or heated. Chromium plating or Molybdenum
spraying is done for resisting scuffing attack on 4 stroke engines. Copper plating, Silver plating,
Phosphorus treatment, Cadmium flashing and Tin flashing can prevent ring scuffing during initial
running in.
                          Alloyed piston ring iron and material specification.




                                      Perlitic structure. Flake graphite.
              Nominal modulus of elasticity: 100,500 Kg/mm2. Tensile Strength: 26 Kg/mm2.
                    Bending Strength: 45 Kg/mm2. Brinell Hardness: 200/220 Kg/mm2.
Chemical Analysis: Total Carbon: 3.2 – 3.5. Silicon: 1.30 – 1.6. Manganese: 0.6 – 0.8.
                      Phosphorous: 0.35max. Sulphur: 0.10max. Copper: 0.35 – 0.55.
                      Molybdenum: 0.40 – 0.60. Specific gravity: 7.25 grms/cc.
The above alloyed piston ring iron is for use in un-supercharged and supercharged engines using heavy
fuel oil. Alloying with copper and molybdenum will improve corrosion resistance and mechanical wear
properties. Sulzer engine manufacturers‘ for their RD/RND/RTA type engines use this above material
for piston rings.
Forces acting on a piston ring:
In Radial direction:
The force exerted through the ring tension. The gas pressure behind the ring, approximately equal to the
cylinder pressure in the case of a top piston ring. The friction force between ring and groove.
A force created by the movement of the wedge of oil between the liner and ring.
A force due to the pressure of gas on the outer surface of the ring again transmitted through the oil film.
In Axial direction:
Gas pressure on the upper side face of the ring. Gas pressure on the lower side face of the ring.
The friction force between the ring and liner surfaces. The inertia force due to the weight of the ring,
which varies according to the acceleration or deceleration of the piston.
Wall pressure:
Generally, wall pressure is calculated to be due to the single factor of the ring tension and the mean
value taken over the surface in contact with the liner. For most normal applications rings are produced
which have uniform pressure pattern. For very high-speed engines it is necessary for positive ovality to
be produced to maintain contact of ring and liner, particularly in the region of the gap. For highly
supercharged slow speed two stroke engines rings are produced by cam turning to have negative ovality
to prevent ring ends catching in the ports of the liner. This negative ovality is frequently combined with
chamfering of the ring ends to prevent port contact, with the result that the possibility of blow-by at the
joint is increased and it is sometimes advisable to fit a scat or gastight piston ring in a lower ring groove.
                                                                                                       140
A better solution is to radius the cylinder liner port edges to prevent port contact. The wall pressure is
designed to vary according to the diameter of the cylinder.
The wall pressure is proportional to the cube of the diameter /radial
thickness ratio and so it can be calculated that a ring has lost 30-
40% of its tension when the radial thickness has worn down by 10-15%.
                                        Uniform Pressure Pattern:




                                            Negative Ovality:




                                            Positive Ovality:
                                   141




Piston Ring Wall Pressure Chart:
                                                      142




Piston groove wear on lower landing control method.
143
144
                                                                                                      145
Rings will normally rotate on piston due to the variation in gas pressure.
Typical rate of movement is 1mm to 3mm per cycle.
Ring Shapes:




                             For ‗Bedding in‘ quickly rings are shaped thus:




                  Ring edges are rolled inwards to avoid striking on the port openings.




   Ordinary ring edge striking port opening.            ‗K- Ring‘ Type is rolled in to avoid this problem.
                                                                                                       146
       Diagrammatic arrangement of the method of applying the plasma coating technique:




Plasma technique of application.
With the plasma technique a jet of inert gas blown through an electric are is ionised and a temperature of
roughly 18,000*C is obtained; thus, its physical state is transformed from gas to plasma. As in the case
of other changes of state (for example, from solid to liquid in the case of smelting) heat energy may be
absorbed or lost without noticeable variation in temperature. If metal powder is added to the gas jet
blown through a nozzle in the plasma state, it can be smelted and blown at high speed against other
surfaces. Although, for the reasons given above, the gas jet loses energy as a result of the powder
smelting, its drop in temperature will be negligible such that it carries the smelted or semi-smelted
particles to the surface against which they are blown, with practically no change in temperature.
This prevents the formation of a film of ‗cold gas‘ on the surface, which acts as heat insulation,
preventing the particles from adhering to the surface. This is roughly the principle by which materials
with a high melting point are deposited on the sliding surface of piston rings with good characteristics of
adhesion, compactness, and uniformity.




Face Contouring.                Back Contouring.               Face Contouring.
The above two diagrams illustrate the shapes of piston rings with plasma-coated surfaces. The left
diagram shows that used at the top groove to result in balanced gas pressures on the ring, while the right
sketch shows second and third ring designs.
                                                                                                        147
                                 Ring Face Contoured for Treatment.




Granodising:
All cast iron piston rings are ‗Granodised‘ before leaving the works. This process is a phosphating
process, which prevents rusting and also etches the surface of the ring slightly so that the surfaces retain
oil.
Chrome Plating:
Chrome plating of the working surface has the following advantages for medium speed and fast engines.
     1. High surface hardness and wear resistance.
     2. Low coefficient of friction.
     3. Resistances to corrosive wear.
In slow speed engines we advise chrome plating of the cylinder liner as a better alternative to chrome
piston rings.
Copper Plated Rings: Copper plating of the working faces of rings is useful where bedding in problems
are encountered. A coarse turned finish is recommended or the inclusion of small grooves in the wearing
surface.
Cadmium or Tin Flash Rings: A thin layer of tin or cadmium can prevent ring scuffing and can again
be used for initial running in.

Liner wear reduced when piston rings move freely. 6 to 10% of power in large engine is lost in
friction and 2/3rd of this in cylinders between rings and liners.

Checking piston ring wear:
                                                                                                 148
        Sulzer RN68M type engine working piston and piston ring clearance checking list.




All measurements should be taken after cleaning and recorded properly. Any defects found should be
made good. Damaged or worn our rings to be renewed.
                                                                                                           149




The axial clearance of the piston rings in the two top grooves is already in new condition somewhat
larger than that of the remaining rings. This clearance is measured most suitably with a feeler gauge.
The piston rings must be fitted in the grooves for the measurement.
The radial wear of the piston rings can be established by means of an outside micrometer or
corresponding gap gauges (dimension ‗l‘). Corresponding to the wear of the piston rings at the running
surface, the ring gap ‗S‘ increases automatically. In order to measure the ring gap, the piston rings are
placed individually in a ring gauge with a bore corresponding to the bore of a new cylinder liner.
If such a ring is not available, the top part of a cylinder liner can be used for this purpose, this part being
not subjected to wear as a rule.
Piston rings, which can be used further on, are to be fitted whenever possible in the original position into
the same groove as before. Owing to the long service period between two overhauls engine makers‘
recommend, however, to fit at each piston overhaul new piston rings into the two top ring grooves.
Rings, which have sharp edges, due to wear but have still sufficient width for fitting them for further
service period must be chamfered carefully (lx45*).
The dimensions established by the measuring are to he recorded in a table (see page 35).




                Sticking piston ring in a trunk type piston will lead to ring collapse and blow back.
                                                                          150
                  Escape of gas behind rings due to:




                            Untrue groove.




Some ring collapse conditions in a large powered diesel engine pistons.
                                                                                                         151
Piston Rod Stuffing Box:

The bore for the piston rod in the bottom of the scavange air box is fitted with a Piston rod stuffing box,
which is designed to prevent lubricating oil in the crankcase from being drawn up into the scavange air
space. The stuffing box also prevents seavange air (in the scavange air space) from leaking into the
crankcase.
The stuffing box is mounted with a flange bolted to the bottom of the scavenging air box. The stuffing
box is removed together with the piston rod during piston inspection.

The stuffing box housing consists of 2 parts, which are bolted together. The housing is provided with six
machined ring grooves, the three upper most of which are fitted with sealing rings to prevent seavange
air from passing downwards along the piston rod. The upper most of these sealing rings is a combined
sealing and scraper ring, provided with an oblique edge, which serves to prevent sludge form the
scavange box from being drawn down to the other sealing rings.

The three lower most ring grooves are fitted with scraper rings which scrape the lubricating oil is
returned to the crankcases through bores in the housing.

A cofferdam is provided at the uppermost scraper ring groove. Through a bore in the housing and a pipe,
this cofferdam communicates with a control funnel on the outside of the engine. This funnel provides a
means of checking that the sealing rings and scraper rings are functioning correctly. A blow-by of air
indicates that the sealing rings are defective, whereas outflow of oil means defective scraper rings.

The sealing rings each consist of 4-piece base ring, which accommodates eight sealing segments. The
parts are held together round the piston rod by means of a helical spring fitted in a groove that is
machined out on the outside of the base ring. The oil-scraped off by the scraper rings are led away
through bores in the base ring and stuffing box housing and returned to the crankcase. Like the sealing
rings, the scraper ring segments are pressed against the piston rod by the action of helical spring fitted in
an external groove on the base ring.
The gaps of the ends of the sealing and scraper ring sections ensure that the rings will bear against the
piston rod even in worn condition.

Key for the sketch of piston rod stuffing box on page 39:

0113: 0-ring.                   1914: Lamella for scraper ring.
0202: Spring washer.             2082: Scraper ring.
0391: Nut.                       2171: Spring.
0579: Fitted bolt.               2260: Scraper ring, complete.
0668: Screw.                     2359: Stuffing box housing.
0757: Spring washer.
0846: Flange.
1192: Scraper ring.
1281: Spring
1370: Scraper ring, complete.
1469: Spring pin.
1558: Packing sealing ring.
1647: Packing sealing ring complete.
1736: Cover scaling ring.
1825: Cover sealing ring, complete.
                                                       152




B&W Engine: L35MC/MCE Type: Piston Rod Stuffing Box.
                                                        153
Excessive Cylinder Wear: Mechanical Causes.
 Process:                First Cause:         Result:
                                                                            154

          Excessive Cylinder Wear: Chemical Causes:

           Process:                   First Cause:                Result:




KEY:
       MZ: Micro seizure. CL: Clover-leafing. CO: Collapse of rings.
       BR: Breakage of rings. BL: Blow-by.
       AW: Abrasive wear. CW: Corrosive wear.
                                                                                                 155

                    Micro seizure between ring and liner due to oil film failure.




Ring in contact with liner       Frictional heat fuses metal         Fused metal in ring tears
   with out oil.                and Micro-seizure taking place.        and scores the liner.




                Trunk piston lower edge tapered to avoid micro seizure with liner.
                                                                                 156

Inspection method in port via scavenge ports the condition of rings and liner.
                                                                                                                   157
                                 Piston Rings as seen through Scavenge Ports.




The wear condition and the shine portion and scored portion will indicate the micro-seizure condition.


                             New Micro seizure ring as seen in section and on face.




                         Recovering Micro seizure ring as seen in section and on face.




Every engine manufacturer will give these methods and ships engineers should make these inspections
and record properly. Any fault found should immediately be rectified as per the manufacturers‘
recommendation.

****************************************Kv*****************************************
End of Pistons and Piston rings: Reference: Coagency between piston rings, piston and cylinder liner: CHR. Grum;
                                            ‗Motor-ship‘ supplements on Sulzer Engines and B&W Engines.
                                             Instruction manuals of Sulzer and B&W engens.
                                            Engineer‘s Handbook of Piston rings by G.Industriale.
                             ***************************Kv***************************
                                                                                                         158




Plasma Coated Piston Rings.
Plasma Coated Piston Rings and seal rings for use in applications subjected to high friction and wear
have been developed as a result of several years of research work carried out by Koppers. In anticipation
of ever increasing speeds, temperatures and pressures in industrial applications, this research work is
continuing. Some fields being covered include the basic study of the plasma process, the study of new
wear resistant formulations deposited with the plasma gun and evaluation of coatings via bench, engine
and field application tests.
The plasma gun is a device that creates temperatures up to 17700*C through ionization of gases and the
use of thermal energy which is created by re-forming the ionized molecule. Refractory type metals and
metallic carbides which have melting points normally un- attainable by conventional methods can be
melted and deposited on the wearing surfaces of rings and other parts. The plasma deposited carbides,
including tungsten carbide and chromium carbide, are thermally stable and show wear resistant
characteristics superior to that of chromium plate under the most severe operating conditions.
Diameter: Normally up to 225 mm. Larger sizes in special cases.

Surface Treated Piston Rings.

Surface Treated Piston Rings are available with various surface coatings to meet particular operating
conditions. Coatings are used for wear resistance, corrosion resistance, to facilitate rapid seating and to
retard scuffing and scoring.

Pareo Lubrite (trademark of Parker Rustproof Company): Experience has established the Pareo Lubrite
process as a widely used production method of reducing wear on bearing surfaces. The non-metallic, oil
absorptive coating permits rapid break-in of new surfaces and, by maintaining an adequate supply of oil
in the pores of the coating, aids in retarding scoring and scuffing. It is general practice for standard cast
iron rings to be Pareo Lubrized.

Dry Film Lubricants: In applications of high surface loads, a liquid lubricant may be forced out from
between two bearing surfaces causing early failure prior to wear-in. Dry film lubricants, including
molybdenum disulfide, TFE-fluorocarbon resins and others, are often employed in such applications to
overcome the tendencies toward galling and seizing. Dry film lubricants are also used in applications
where liquid lubricants must be excluded and where high surface temperatures and fretting wear occur.
In many piston ring applications it is found to be advantageous to apply dry film lubricants over a Pareo
Lubrized surface to obtain improved adhesion and longer retention of the film during wear-in.

Electrodeposited Tin and Cadmium Plate: Tin plate and cadmium plate are used in applications
where mildly corrosive conditions exist and to retard scoring and scuffing of bearing surfaces. Both
metals are quite soft and they improve surface compatibility by their ability to smear into the opposite
bearing surface. Either metal may be applied to the cylinder-contacting surface only or over the entire
ring surface as required. The standard thickness is 0.005 to 0.010 mm but heavier coatings may be
specified.



Kv/ICE/BE/03.
                                                                                                          159




“Flash” Chromium Plate: This plate, which has a thickness limited to 0.025 mm or less depending on
contour of ring section, is much thinner than the chromium plate, as used on piston rings in applications
such as internal combustion engines and compressors. The thin, "flash" plate may be applied over the
entire ring surface, to the cylinder-contacting surface only or to the flat side surfaces only. It is used on
rings in various types of reciprocating, rotary or static seals, particularly where abrasive wear or
corrosion is a problem.

Chrome Plating:
Chrome plating of the working surface has the following advantages for medium speed and fast engines.
     High surface hardness and wear resistance.
     Low coefficient of friction.
     Resistances to corrosive wear.
     In slow speed engines we advise chrome plating of the cylinder liner as a better alternative to
       chrome piston rings.
Copper Plated Rings: Copper plating of the working faces of rings is useful where bedding in problems
are encountered. A coarse turned finish is recommended or the inclusion of small grooves in the wearing
surface.

Cadmium or Tin Flash Rings: A thin layer of tin or cadmium can prevent ring scuffing and can again
be used for initial running in.

Granodising:
All cast iron piston rings are ‗Granodised‘ before leaving the works. This process is a phosphating
process, which prevents rusting and also etches the surface of the ring slightly so that the surfaces retain
oil.


Corrosion Protection: A corrosion-inhibiting medium is used on all ferrous rings for both intra-
operational protections during manufacture and as a final coating for protection of the rings from
corrosion during shipment and storage.



Liner wear reduced when piston rings move freely.


6% to 10% of power in large engine is lost in friction and 2/3rd of this in cylinders
between rings and liners.



Kv/ICE/BE/03.
                                                                                                       160

                                        Oil Scraper Ring.




                           Coil Spring Expander and Serpentine Expander.




The Coil Spring Expander and the Serpentine Expander are both used to provide tension for
Conformable Rings as illustrated above. They are high quality springs made to closely controlled
specifications. The Coil Spring Expander is made of round wire coiled into the form of the usual coil
spring and fitted with a joint pin at one end as an aid to assembly. The Serpentine Expander is made of
rectangular wire formed with axial reverse crimps. Both are of the abutment type, installed with ends
butted together. When a ring, fitted with one of the expanders, is closed, the expander is compressed,
thus providing tension for the ring. When used with a slotted oil ring, the expander is sufficiently open
to provide ample drainage for the oil.
                                                                                                       161

                               Marine Diesel Running Gear:
Crank shaft:
Materials used for crankshaft are:
   1. Forged steel.
   2. Cast steel.
   3. Nodular iron.
   4. Malleable iron.
   5. Grey iron
The order given represents roughly the order of strength and also the size.

The components forming the crankshaft are the main journal, the web and the bottom endpin.
The three components may form a solid unit.
   1. Forged:
   In which case the deformed crystal structure produces a grain flow, which can be oriented in a
   particular direction. It is arranged that the grain flow is a continuous path through the journal, web
   and pin, in the direction of the shaft polar axis. Discontinuity of the grain flow should be avoided
   (Fatigue resistance). Large forgings will be produced on a large hydraulic press, paying particular
   attention to temperature control during the forming process. Heat treatment to stress relieve and
   possibly grain relieve will be carried out after forging and before machining. Small crankshafts may
   be drop-forged or die-forced. The forging process is normally a lengthy process (operation) requiring
   great skill of the forge-master, and is costly.

   2. Cast: Will not exhibit grain flow characteristics and the strength of the section will be essentially
      the same in all directions. The heavily stressed web section will be made larger and heavier than
      a similar forged structure. Cast structures are normally less expensive and can achieve geometry
      not feasible with forced shafts, counterweights can be cast integral with the webs.

   3. Fully and semi-built shafts: In the case of the fully built shaft, the three components produced
      separately - journals and pins forged, webs forged or cast. The holes in a pair of webs are
      machined in one operation, to provide sufficient interface fit with the bottom endpin. Accurately
      aligning and shrink fitting the pair of webs to the pin produce a center throw assembly. The
      correct numbers of center-throws are than assembled and line-bored to achieve the true
      alignment with each other. The center-throw assemblies are than shrink mounted on to the
      journal in the correct sequence and angularity to each other. With semi-built shafts the center-
      throws are either forged or cast in one piece- the castings possibly being cheaper and quicker to
      produce. Reference marks are provided at each shrink fit by machining a flat rectangular surface
      across the junction. A light chisel cut is made radially across the junction of the web and pin or
      web and journal. These should be inspected regularly to ensure no slip has occurred.

The shrinkage allowance for both pin and journal vary between 1/570
and 1/660 of their respective diameters.

Stresses in crankshafts:
The major stresses are produced from four sources:
   1. The working cycle: These are imposed by the connecting rod and include gas pressure forces
       plus inertia forces from piston, connecting rod and crosshead assembly.
   2. Torsional vibration: Caused by the crankshaft winding up and unwinding in the reverse direction
       due to non-uniform applied torque from the engine and resisting torque from propeller.
                                                                                                       162
   3. Axial vibration: Drive from the crankshaft being alternatively extended and compressed along its
       axis due to bending of pins, webs and journals.
   4. Misalignment: From improperly adjusted or damaged bearings or from inadequate foundation for
       the bedplate.
Regions of high stress:
High stresses normally arise from stress concentrations at discontinuities along the shaft.
They may be found in two regions:
   a) Around oil holes in pins and journals. Caused by torsional shear stress which results in cracks at
       45* to the shaft axis. Edges need to be well rounded with a high surface finish. The position of
       the oil hole relative to the crank throw should be carefully considered.
   b) Around the fillets- due to bending or combined bending and twisting. The crack may develop
       into both the pin and web. The fillet radie should be at least 0.05%of the pin diameter. Double
       radie or re-entry radie may be used to avoid encroachment on to the bearing area. Cold rolling, of
       the fillets is sometimes employed to increase fatigue resistance by introducing residual
       compression stress in the fibers.
   c) Careful choice of pin diameter to web thickness and counter-boring of the pin in way of the
       shrink fit can improve stress distribution across the discontinuity.

Materials for forged and built-up shafts:
            1. Forged and non hardened pins: 40 carbon steels: U.T.S. 570 - 700 MN/m.sq.; are suitable
                for white metal thin walled shell bearing and copper-lead with white metal overlays or
                aluminium- tin surfaces.
            2. Forged and hardened pins: 1 % chrome - molybdenum with 3 % nickel U.T.S. 700
                MN/m.sq; B.H.N. 480- 490, after induction or flame hardening suitable for harder
                bearing materials such as copper- lead and aluminium- tin.
            3. Heat treatable medium speed: 2 ½% nickel- chrome- molybdinum ; U.T.S. 850-1000
                MN/m.sq. ; B.H.N. 300. The chrome- molybdenum steels may be nitrided to give surface
                hardness of 900- 1000 Y.P.N.
Fully built and semi-built crankshaft material:
Unalloyed carbon steel in normalized condition. Some times low alloyed chrome-molybdenum steels
have been used.
Tensile strength is around: 590-680 N/mm2.
Material analysis of cast steel is as follows:
Carbon: 0.2%, Silica: 0.32%, Manganese: 0.7%, Phosphorous: 0.01%, Sulphur: 0/015%.
Where journals or pins are surface hardened it should:
    1. Stop short of the fillet radius so that no 'discontinuity exists in the fillet itself.
                                 or
    2. The hardened surface should carry right through the fillet.
Chrome plating is some times used as a reconditioning procedure, particularly in solid shafts and may be
applied to new shafts when used with modern fatigue resistant bearing metals (lead-copper-aluminium -
tin), which provide less conformability and embed ability than white metal.
The chrome plating produces a smooth, hard surface but is more difficult to lubricate under boundary
condition ( surface oxides do not readily react with fatty acids in the oil to produce the metallic soap ).
Some materials used for journals and pins can develop fatigue cracks originating at the interface
between the parent metal and the chromium coating.
Crankshaft Design Ratios:
    1. Crank pin diameter: Should be at least 0.6 x Cylinder bore.
    2. Crank pin length: Enough to accommodate a connecting rod bearing of at least 0.3 x pin
        diameter.
    3. Journal diameter: Should be longer than the pin to account for the additional twisting moment.
                                                                                                      163
     4. Journal length: Can be as low as 0.3 x journal diameter, provided the centrifugal loads are
        counter balanced (many engines are balanced by boring the pins rather than adding weights).
             a) Overlap of pins and journals increases overall strength of crankshaft provided fillets are
                 adequate and the web is designed to distribute the stress over a large area.
             b) Careful and calculated counter boring of journals and pins can be used to reduce stresses
                 in both pins and web.
             c) With counter bored shafts, webs should be designed to distribute stress uniformly over
                 the full shrink fit area - excessive beveling to reduce mass should be avoided.
             d) Fillets should be as long as possible (at least- 0.05 x pin diameter), edges well rounded
                 off, good surface finish and possibly rolled.
             e) Surfaces should be free from hold, keyways or dowels in areas of high stress.
             f) Where size and weight is not unduly restricted, cast shafts or centre-throws are
                 satisfactory alternatives to forgings.
Problems:
If the shrink fit between journals and web slips, possible caused by abrupt stopping of shaft, say by
water or fuel in the cylinder. 2 or 3 degrees can possibly be tolerated. The shaft may be realigned by
removing running gear and main bearings, in way of the damage, to provide good access.
By circulating liquid nitrogen at controlled rate through the journal and applying uniform heat across the
web, the shrink fit can be relaxed, using two hydraulic jacks, suitably placed (one to provide turning
moment in opposite direction to slip and the other providing shaft control) the web can be re-moved
relative to the journal (movement indicated by fall in jack pressure) to true alignment. Care must be
taken to prevent cold burns to personnel and compartment must be thoroughly ventilated to remove inert
gas.
After allowing temperature to normalize, bearings and running gears can be refitted and crankshaft
deflection taken. The engine is then run on trial to prove satisfactory. Regular checks must be taken and
inspection period gradually extended until the repair is considered permanent.

Cracks in crankshaft:
Any crack suspected must first be confirmed by dye penetrant tests or magnetic particle test and if
possible ultrasonic test, to determine the depth of the crack.
As soon as crack has been confirmed the master and owner must be informed, since the vessel cannot
function normal it may be classified as a casualty. If the crack is shallow it should be removed by
grinding and surface polished, so that its effect as a stress raiser is eliminated. Shaft and bearing
alignment is checked and adjustments made, if required. A certificate must be obtained from the
classification societies surveyor to allow the vessel to proceed at reduced speed.

Balance Weights

Some engine builders fit heavy weights to the cranks in order to balance the reciprocating masses to give
even turning moment & reduce vibrations. In case of cast webs it is part of the web. In case of forged
web, the masses are attached by long studs and dovetail recesses (See sketch on page 48).
                                                                                                    164




Sketch for forces on a crankshaft.




The force that occur in a vertical diesel engine crankshaft are as follows:
   i)      Static weight of engine components (moving).
   ii)     Alternating forces produced by varying gas pressure.
   iii)    Inertia forces of the moving parts.
   iv)     Centrifugal force at crank.
   v)      The crank-web is subjected to tensile, compressive and shear stresses. Shear in way of
           journal.
                                                                                                      165
Important Factors Affecting Crankshaft Life:

           a) If webs are flame cut, this should preferably be done hot. Crack detection before and after
              shrinkage is necessary.
           b) Edges of crank-webs should be well rounded off.
           c) Great care has to be taken of discontinuities in the shaft such as fillets, oil grooves, oil
              holes etc. Fillets should have proper radius. Oil holes should be machined smooth and
              lips should be well rounded.
           d) Material should be in optimum condition both as regards soundness and fatigue strength.
           e) Surface hardening by the ‗Nitriding‘ process increases fatigue strength. Hardening will
              reduce rate of wear, but the process has to be carefully carried out.
           f) Proper care has to be taken of the bedplate as regards designing and maintenance.
              Distorted/Cracked bedplate has been the cause of crankshaft failures in several cases due
              to misalignment.
           g) Crankshaft alignment has to be maintained in the specified limit at all times.
           h) In the presence of water (form piston cooling or other system leakage) Crankshaft can
              severely and seriously corrode with consequent adverse effect on fatigue life. The
              necessary precautionary measures should he taken.
           i) Web "Breadth/Thickness" ratio, Bridge piece thickness have to be maintained in the
              specified range.

Crankshaft Problems:
The majority of failures of steel crankshafts are fatigue failures. Majority of failures originate from
following points:
    1. Lip of an oil hole in a crankpin. In severe cases of torsional vibrations, cracks may be found at
       several crankpins and journals.
    2. At the fillets between crank webs and crankpins/journals.
    3. At flange couplings of conventional design.
    4. A common type of crankshaft failure is characterised by a crack originating from crankpin and
       web fillet (at the underside of crankpin) and proceeding through the rectangular section of the
       web.
    5. Cracks can develop in a crank web, particularly in the higher stressed regions, midway between
       pin and journal.
    6. Cracks can develop at the fillet between the journal and web, particularly in the arc between the
       position corresponding to 10'o clock and 2'o clock position, when the piston is at TDC.
    7. Twisting or slipping.
Crankshaft Inspection:
         i)     Should be checked for cracks giving special attention to the points mentioned above.
         ii)    Working surface of journals and pins to be examined for signs of corrosion or pitting
                caused by water or acid contamination of lubricating oil.
         iii)   Shrink fit reference marks.
         iv)    Tightness of coupling bolts.
         v)     Tightness of oil pipes and bearing locking devices.
         vi)    Oil holes cleanliness.
         vii)   Balance securing arrangement.
         viii) Blank plugs in oil holes and oil tightness.
         ix)    Crankshaft deflections

Crankshaft Alignment:
Crankshaft alignment accuracy will depend on the following:

   1. Shaft must be resting on the lower half of the main bearing.
   2. Tie bolts should be correctly stressed.
                                                                                                        166
   3. All unit running gear in place.
   4. Main engine bedplate holding down bolts should be tight.
   5. Engine room temperature should be noted (noting engine hot or cold).
   6. Camshaft drive chain tension should be correct.
   7. Loading of the ship should be same at each time during alignment check, (Draught should be
      same) and checked.
   8. Alignment should be checked only when the ship is afloat.

Method of Testing Alignment:
Assuming hull deflections are not excessive and the bedplate is neither distorted nor bearing pockets
worn, following methods could be adopted for alignment checking:

       Bridge gauge to measure the wear down at each main bearing.
       Remove main bearing bottom half in turn and measure thickness to check for wear down
        amount.
       By using dial gauge measure deflections between crank webs of each unit.
Deflections
    All steps under ‗Crankshaft Alignment‘ to be observed.
    In addition to abovementioned points following points should be noted: Readings should be
       taken at same position between webs and Engine should be turned in the same direction ("ahead"
       direction)




Interpretation of crankshaft deflections gives an indication of high or low bearings.
―When a bearing between two cranks is higher than those on either side of it, crank webs will tend to
open out when the cranks are at bottom dead centre and close in when cranks are on top dead centers‖.
Vice versa if there is a low bearing between two cranks.




To interpret deflection readings one should refer to engine manufacturer‘s engine manual at all times.
                                                                                                     167
Crankshaft deflection effect will be dependent upon the rigidity of the foundation (Bed plate of the
engine) and the vessel. Measurements should be taken, as far as possible, under the same set of
condition – which must be recorded with the measurements.
Changes under similar conditions appearing suddenly or gradually over an extended period give
evidence of foundation subsistence.
In case of unpermissible change the plant must be realigned and main bearing inspected.
Apart from readings taken from a planed program, additional readings may be taken:
    1. After collision or grounding.
    2. After bearing or crankshaft damage.
    3. Prior to and after every dry-docking in floating condition.
Welded Crankshaft for Large Diesel Engines:
Shrink-fits have always limited design. It is now practical to produce heavy, solid crankshaft for two-
stroke diesels by welding, instead of the traditional shrink-fit. B &W Diesel engine manufacturer‘s
around end 1983, realised that, sooner or later, they would have to abandon the shrink-fit method of
assembling heavy crankshafts. It had meant that the shape and weight of the crank had to be fairly
constant because the web needed a minimum width and radial thickness in order to ensure a sufficient
grip on the pin. Because of the sizes involved, mono- block crankshafts are impracticable. Welding,
therefore, appeared to be the ideal alternative since the weld would be positioned right in the middle of
the bearing journals where the stresses on the crankshaft are lowest.
To make welding practical, the level of residual stresses and distortion must be known and low; the weld
zone must be as strong as the unheated metal and easy to test.
Preliminary and experimental studies led to the conclusion that a narrow-gap with low heat input,
combined with submerged-arc welding, would be the most suitable method.
Two ways of assembly:
A crank-arm (a half crank-throw with a half main journal cast or forged integrally to its web) is shown
on the figure below. A full crank throw is shown on the right and the solid crankshaft below.




Two assembly methods are possible: welding two crank-arms together through the pin to form a crank-
throw and then making the crankshaft by welding these. Crank-throws through the main journals; or, by
welding together forged or cast complete crank-throws. The crank-arms are closed die hammer forgings
                                                                                                        168
in unalloyed carbon-manganese steel. An example of the second method is the single, free form forged
crank-throw, welded to its neighboring throws in a lathe, during machining. The forged heat-treated and
rough machined crank-throws for this shaft have been made by Kobe Steel Works in Japan.
Research: Before welding procedures could be finalised, a great deal of research was necessary,
including a comprehensive finite-element computer analysis, which enabled us to predetermine the
stress concentration factors in the fillets between the crank-webs and the journal.
A finite-element subdivision is shown in figure below.




Stresses were also measured by strain gauges on a scaled-down steel crank-throw to check the results of
the calculations. Metallurgical investigation showed that any of the tested materials, from normalised
low-carbon manganese steel to hardened and tempered low-alloy Cr/Mb steel, could be welded with the
submerged arc welding, narrow-gap process, given a sufficiently high pre-heating and interpass
temperature. Fatigue testing of full-sized specimens showed properties superior to the base material.
Dynamic fracture testing showed that, if a fatigue fracture was already present, the growth per number
of cycles was the same as in the base material.
Owing to the rotational symmetry of the narrow-gap, submerged arc-welding process, only very small
angular distortions of the crankshaft occur. The stresses built up during welding, and partly relieved by
heating, cause insignificant changes in length.
Welding and testing: The joint has a narrow-gap of 16 to 2Omm, formed by parallel-machined surfaces
of the crank-arms, crank-throws or shaft journals. A backing ring is placed in the bore and tack-welded
to the journals to ensure correct positioning and full penetration. By tilting the journals and crank-throws
with a manipulator during welding, the deposit is laid on each side of the groove alternately, with an
overlap after each revolution. This process may also be described as a multi-pass temperbead technique,
with each layer consisting of two passes side by side.
While the process is automatic, in view of the importance of the product, the presence of a certificated
operator will always be necessary. After welding, the journal is post-heated to ensure slow and even
cooling. When cold, the backing ring and the roots of the welds are removed, by drilling out the central
bore. After machining of the outer journal surfaces, the shaft is ready for non-destructive testing.
Subsequently, the welds of the whole shaft are stress-relieved.
Ultrasonic testing is performed with angular and normal probes on the outside of the journal and around
the weld. Echo amplitudes from possible planar and volumetric defects are compared with signals from
                                                                                                                         169
ideal reflectors. Magnetic-particle inspection of the weld and its surroundings is performed both outside
and inside the central bore at the pin and/or journal. Such shaft finished will have the approval of some
of the worlds‘ leading classification societies including the Europeans', LR, IRS, ABS, NK and the
newly formed Society of Mainland China.
Advantages of welded crankshafts:
The requirements outlined for the B&W: L-MC engines have led to a revised design and reduced
principal dimensions: cylinder distance, total engine length, width of bedplate, and height of engine,
including the space necessary for piston overhaul. Welding the crankshaft assists this, also, machining
and other expensive processes, can be reduced. The solid welded crankshaft represents a considerable
reduction in weight, due to the absence of shrink-fits, and freedom to choose large journal diameters
without overlap restrictions. As it is now possible to reduce the thickness of the arms or webs, longer
journal bearing lengths can be obtained, resulting in lower specific bearing pressures. The width of the
arm can be reduced, thus providing a crank-web with less inertia. This means crankshafts with higher
natural frequencies of torsional vibrations, which is important in slow-speed engines. Designers will
now have much more freedom to offer owners an engine with great flexibility in operation, coupled with
optimum economy in weight as well as in space and cost.




                     Latest B&W: MC type Engine Semi-built Crankshaft.
        ***********************************Kv****************************************
Reference: B&W Engine manual. MER October 1983: Welded crankshafts for large engines by Mr. N.H. Metz. B&W Diesel A/S.
                                                                                                        170
Connecting Rod:

This is a highly stressed component resulting from:
    1. Gas force loads: Which is a maximum compressive load at T.D.C. (15% of maximum at 90*
        A.T.D.C)
    2. Inertia loads: Resulting from the reciprocating running gear is maximum compressive at B.D.C.
        and maximum tensile at T.D.C. (particularly in 4 stroke engines).
    3. Transverse inertia loads: Known as " whip" resulting from the mass of the connecting rod and its
        oscillating motion. This is maximum at about 80* past T.D.C. and is greatest in high speed
        engines.
For calculation purposes the component is considered as a strut subject to buckling and transverse
loading. May be circular or ‗H‘ section, usually circular for slow speed engines and ‗H‘ for medium and
high speed, where the transverse loading is greatest. In ‗V‘ engines there may be additional transverse
loading from the connecting rod.
The connecting rod may be required to transport oil between the top &, bottom end bearings - circular
sections are most suitable for this purpose.
Stress and load concentration is reduced at the ends of the rod by increasing the area through a tapered
section, having generous fillets. Solid ends provide a rigid platform for the top end-bearings and gives
good support to the bottom end bearing. This essentially used for thin shell bearings, to prevent fretting
between the back of the shell and it‘s housing.
 Accurate and uniform pre-tensioning of the bottom end bolts is necessary to:
    1. Reduce the risk of fretting between palm and housing.
    2. Eliminate bending moments on the bolts (caused by uneven tightening, resulting in stress
        concentration in the root of the thread.
    3. Reduce the range of stress fluctuation, which is a major factor in fatigue failure (the maximum
        stress way be increased but, the fluctuation range is reduced).
    4. Provide the correct ‗nip‘ to the thin shell bearings (to prevent fretting on the blocking piece and
        fatigue-crazy cracking on the bearing surface).
For lower power engines a ‗forked‘ top end arrangement has been used which allows top end bearing to
be integral with the connecting rod and provide access to the piston rod nut.
With increased power the greater flexibility of this design resulted in:
    1. Misalignment between top-end pins and bearings resulting in edge loading (due to load acting
        between the forks, producing a bending moment).
    2. Cracking at root of the fork due to repeated flexing resulting in fatigue.
The bottom end bearing is located on the palm by a spigot through which the oil passes. This helps to
relieve the bolts of shear forces imposed by the transverse loads.

Failures:
Usually due to abrupt stopping the engine or breakage of bottom end bolts.
Cracks may develop:
   1. Around the edges of the boltholes.
   2. On the underside of the foot running across the line of fillet run out (particularly if compression
       plates are fitted).

Materials for Connecting Rod:
Forged steel: Carbon: 0.30 – 0.50% (Normalised).
              U.T.S: 500 – 700 N/mm2.
Forgings should have a fine grain structure. It should be free from coarse non-metalic inclusions and
segregations especially in highly stressed areas.
                                                                                    171

           Sulzer type engine: Old conventional design connecting rod.




                          (A)                (B)
Connecting Rod large end bolts: (A) Old original design. (B) Improved new design.
                                                 172




Medium speed trunk piston type connecting rod:
                                                                                                173
          Modern MAN/B&W Engine MC Type engine Connecting rod with cross head:




Connecting Rod Bolts:
Important Designing Considerations:
    Well-formed fillet between bolt head and shank. There should be a proper chamfer at the mouth
       hole.
    There should be smooth radii wherever there is a change in diameter.
    Surface of the bolt should be given a high degree of finish.
    It would be beneficial to reduce the diameter of bolt shank less than the core diameter at the
       bottom of the thread (about 10% less).
    Bolt material should have adequate strength and high resilience.
                                                                                                        174
      It would be ideal to make the bolt of uniform cross-sectional area but it is necessary to have
       certain parts of shank enlarged in diameter for the fitting portions.
Bolt Material:
Low alloy steel (alloy content < 5%).
U.T.S: 750 to 1100 N/mm2.

Tightening of Bolts:
Tightening of important bolts such as these should not be left to chance.
 Following methods are in use:
    1. Applying the desired preload by means of hydraulic cylinder and following up nut.
    2. Measuring the extension of the bolt with a micrometer device whilst the bolt is tightened.
    3. Hand tightening lightly, and then turning up the nut through a predetermined and calculated
        angle with respect to the bolt.
    4. Using ‗torque spanner‘, e.g. a spanner which reads the torque or set to give way at a
        predetermined torque.
Methods (1) & (2) as mentioned above are most accurate. Method (3) is good if bolt stiffness is known
and calculation is accurate.
Torque spanners (method 4) are useful for small medium sized bolts, care has to be taken as regards
Lubrication.
Failure in Bolts:
Failure is essentially due to fatigue.
Factors contributing to failures are as follows:
    1. Stress concentrations at bolt heads, change of section, surface finish etc.
    2. Over stretching of bolt.
    3. Uneven tightening.
    4. Inadequate pretension.
    5. Improper seating of nut or bolt head causing bending stresses.
    6. Corrosive attack in the form of bending.

Crosshead:
This provides an articulating, junction between the connecting rod and piston rod. It is constrained in the
transverse direction by Guides, which absorb and transfer the reaction from the connecting rod the
engine frames.
Modern high power engine may be transmitting up to a 600 ton load at
peak combustion pressure, through the crosshead which produces large
guide reaction as connecting rod angularity increases, two sets of
guides are provided, since the reaction is reversed when passing
through BDC & TDC, (with single acting engine).
For high powers, crossheads are kept short and the pins increased in diameter. This provides:
    1. Very rigid structure which has little deflection under load.(improved fatigue resistance and better
       alignment with bearings).
    2. Large top bearing area to reduce specific loads.
    3. Possibility to use harder bearing material with higher fatigue resistance (in conjunction with
       chrome plated pins).
    4. Higher peripheral velocity to entrain the oil (thicker oil films can be produced).
Various methods employed to improve reliability and operating life include:
           a) Extended bearing area across the full width of the crosshead (DOXFORD. See page 60).
           b) Partial unloading of bearing area (by dividing bearing length into two steps with surfaces
               machined slightly eccentric to each other (FIAT. See page 61).
                                                                                                       175
             c) Producing a symmetrical pin block to allow it to be turned through 180* (B&W).
Lubrication of top end bearing:
The oscillating motion of the top end prevents effective lubrication by hydrodynamic fluid films alone.
Thick film lubrication however, does exist and is produced by a combination of actions:
    1. Hydrodynamic lubrication - can only exist so long as there is relative movement between pin and
        bearing to entrain the oil. The higher the entraining velocity, thicker the oil film (larger pin
        diameters produce higher entraining velocity).
    2. Squeeze film lubrication - provided a thick oil film exits, a suddenly applied load will the oil
        viscosity to increase and the bearing material to deflect. The combined effect restricts oil flow
        from the bearing, which acts as a dash- pot.
    3. Elasto-hydrodynamic lubrication- uses the combined effect of elastic deformation of the surface
        and hydrodynamics of the fluid, to trap a ‗pool‘ of oil between the surfaces. To ensure thick film
        at the end of compression, since film assisting, mechanism may be required.
Pressure boosting:
This may be achieved by:
    1. Pressure control valve and separate oil feed manifolds to top end & main bearings (Sulzer).
    2. Using a booster pump, to increase the feed pressure to about 20 bar driven by the motion of the
        connecting rod (M.A.N. See page 63).
    3. Fitting an oil restriction valve in the oil feed telescopic pipes to restrict back flow on the down
        stroke, do increasing the pressure to about 25 bar (oxford).
Partial unloading: This activated by machining two bearing areas, on both pins and bearings, slightly
eccentric to each other. The area with the greater length carries most of the load during the working
stroke, transferring load to the other surface during compression, this allows replenishment of the oil
film during the unloaded period (GMT design).
Due to inertia effect - Particularly with 4- stroke engines load reversal allows the oil film to be
replenished.
Factors governing load capacity:
    1. The type of lubrication - whether full lubrication can be obtained.
    2. Size of bearing area - determined by the length and diameter of the pin.
    3. Length of bearing - as bearing length is reduced a greater amount of oil leakage occurs, reducing
        film thickness.
    4. Entraining velocity - large diameter pin improve the oil feed to the film.
    5. Alignment - whether there is a risk of edge loading due to deflection of pin or bearing seating.
    6. Bearing material - has the material the required properties with regard to fatigue, fatigue strength
        & resistance and crushing strength.
    7. Pin surface characteristics - is the surface hard enough (3 times of that of bearing material) and
        the surface finish good enough (2-3 microns C.L.A. for lead copper &. aluminum tin, 5-7 micron
        C.L.A. for white metal).
    8. Bearing clearance - reducing the clearance increases the load capacity but also increases friction
        loss in bearing (insufficient clearance can cause self loading, too much increases dynamic
        loading and side leakage of film oil).

Failure of crosshead bearings: Types of failure may include:
   1. Plastic flow - due to over heated bearing surface.
   2. Extrusion - causing blockage to oil holes and oil grooves, white metal projecting from bearing
       sides.
   3. Crazy cracking- accross surface of bearing due to fatigue.
   4. Detachment- white metal overlay or white metal solid cracked through the backing and bond
       broken.
                                                                                                        176
    5. Corrosion - of the tin rich matrix of the white metal due to electrolytic action of impurities (sea
       water or acids) in the oil.
Failures in bearings may be caused by one or more of the following:
    1. Insufficient supply of lubricant.
    2. Contaminated lubricant.
    3. Inferior quality of bearing material or manufacture.
    4. Inadequate bond to backing.
    5. Misalignment of running gear.
    6. Deterioration of pin surface finish.
    7. Bearing over loaded- particularly during manoeuvering on light distillate fuel, if full pressure is
       maintained for a prolonged period (Engine timing).
                                                                          177
         DOXFORD ENGINE: „J‟ Type: Centre top end bearing.




‗FIAT Engine‟ exploded view of the eccentric type crosshead components.
                                                                                   178
 SULZER Engine: RD, RN Type: Crosshead bearing with guide shoe shown with out the pin.

        Sketch for Crosshead bearing and guide shoe arrangement: Sulzer Engine:




1. Crosshead pin. 2. Guide shoe. 3. Guide shoe holding cover. 4. Guide path.
                                                                                                 179
5. Guide rail for guide shoe. 6. Adjusting shims. 7. Crosshead bearing lower part. 8. Column section.
180
                                                                                                     181
Crosshead Guides:
Three constrain the movement of the crosshead (primarily in the transverse direction) and transmit the
torque reaction through the frames to the bedplate. Two approaches are used.
    1. Guide support beam between ‗A‘ frames (See page 60): The guide shoe (usually a cast steel) is
        bolted directly to the back of the crosshead pin block. The reaction force passes through the
        centre line of the pin block onto the cast guide pin bars. The guide bars are bolted to a
        longitudinal girder, which in turn is bolted to the frames. The longitudinal girder may be
        continuous (DOXFORD) and provide additional longitudinal strength for the engine structure.
        Such an arrangement has an extended ‗load path‘ to the bedplate, reducing stiffness and inducing
        bending moments in the guide bar carrier beam. There are reduced bending moments in the
        crosshead itself.
    2. Guide support on ‗A‘ frames (See page 61 & 62): The guide shoe may be a double-faced
        symmetrical casting bolted to the end of each pin (B&W) or mounted on circular section on the
        extended pin length (Sulzer- provides, self-alignment capability). The guide bars are bolted
        directly to the frames - rigid castings located on machined surfaces top & bottom where
        adjustable shims can be fitted. This arrangement reduces the ‗load path‘ and provides more rigid
        support, while at the same time allows the guide bars to expand independent of the frames and
        relieves the bars of any twisting moments (including misalignment) carried by the frames. The
        shoes have equal bearing area on "ahead & astern" faces, which allows reversibility and
        interchangeability. Bending moment in the crosshead block is increased with guide reactions
        applied at the pin ends.

Guide lubrication:
Large flat surfaces subjected to high loads and reciprocating motion
are difficult to lubricate. The problem lies in establishing and
maintaining fluid film over a long stroke, since the oil tends to be:
   1. Scraped off the stationary surface - by the leading edge of the shoe.
   2. Squeezed from between the surfaces - by the high load.

The solution is obtained by:
    1. Dividing the area up-into a number of smaller areas. The grooves acting as oil feeders to the
        small areas.
    2. Machining lead in chamfers on each side of the groove produces an oil wedge in each stroke
        direction to entrain the oil.
    3. Apply boosted oil feed - from the crosshead block, through the shoe, which carries the white
        metal surface and the oil feeder grooves.
    4. Load reversal - transfer of torque reaction from the ‗Ahead‘ to the ‗Astern‘ faces when passing
        through BDC & TDC allows oil film to be reestablished on the unloaded faces.
The surface is in fact acting as a fixed pad bearing, cooling of the guide bare is provided by:
    1. Oil circulation - through hollow guide bar.
    2. Lubricating oil between the surfaces.
    3. Metal to metal by direct transfer (conduction) through support arrangement to ‗A‘ frames.
Clearance should be minimum (to reduce ‗slap‘ on load reversal) consistent with ability to produce oil
film and allow for difference in expansion during warming up. Clearance should be taken with long
feeler gauge both when engine is cold and also when the engine is hot. Adjusting liners are provided for
clearance control and adjustment.

     **********************************KV****************************************
                                                                                                      182


Development of Engine Bearing Material:
The first bearing material, which can be considered as having been invented for the purpose, was white
metal. The invention is credited to Isaac Babbit, who patented it in USA in 1839. This was a tin base
metal containing 11% antimony and 6% copper.
Not many years ago there were over 450 white metal specifications in use. Many of these were of
dubious value and under increasing economic pressure a rationalization programme was started in 1968,
resulting in the number of alloys in general use being reduced to fewer than 20, with the bulk of the
production being confined to four or five. White metal remains the best all round bearing material within
its strength limitations. In practice whitemetal cannot be run at surface temperatures exceeding about
130*C due to its low melting point. It possesses the useful property of self-heating after a minor wipe.
The strength is very temperature dependent and the figure below illustrates how the fatigue strength of
the tin base alloy falls with rising temperature.




                        Fatigue strength whitemetal and 40% tin-aluminium.

To improve the strength and particularly the fatigue strength of whitemetal many additional alloying
elements have been tried, the most successful, developed in the early 50's containing 1% cadmium and
about 0.2% nickel. It has been found possible to include small quantities of chromium and other
structure refining elements, which has resulted in further improvement in strength, without sacrifice of
the other good properties. In search for stronger materials for internal combustion engines, the copper
lead and lead bronze alloys were introduced in the late 1920's. These alloys were lined on steel strips.
Although having the highest strength ratings, these alloys have a poor performance in terms of the
following.
            a) Compatibility.
            b) Conformability.
            c) Embeddability.
            d) Corrosion resistance
Performance can be improved considerably by applying an overlay of lead-tin or lead-tin-copper,
usually to a thickness of 0.025 mm. Once the overlay is worn through, however, then corrosion may
cause the onset of fatigue. Seizures tend to be hard, causing heavy damage to the shaft and it is normally
necessary to use hardened shaft to minimise the risk.
                                                                                                         183

In the early 50's the Glacier Metal Company Ltd pioneered a process of bonding tin-aluminium alloys
onto a steel back: alloys containing 6% and 20% tin were used. Whilst the strength is slightly lower than
that of copper lead alloys, it is quite adequate for the majority of applications, whilst the general balance
of properties is superior, corrosion resistance being particularly good. The alloys have gained wide
acceptance.
In the late 60's a third alloy containing 40% tin was developed primarily for use in slow speed diesel
engine crossheads, where it was felt that white metal was reaching the limit of performance but a soft
alloy with good surface properties was still highly desirable.
The fatigue strength of tin-aluminium is not temperature dependent compare to white metal.
The latest development in high strength aluminium alloys contains 10.5% Silicon. This alloy is as strong
as the lead bronze but has very much superior surface properties and corrosion resistance and all in all
appears to a very promising future.
Bearing Properties:
A good bearing alloy should posses following essential properties:
    1. Mechanical Strength: Here a compromise must be adopted, as too soft a bearing material, while
        possessing other desirable properties, tends to flatten under, heavy loads. On the other hand, a
        harder alloy capable of withstanding higher loads may possess high frictional characteristics and
        of greater importance it may be brittle and have poor fatigue characteristics.
    2. Softness and Melting Point: The softness and modulus of elasticity of a bearing alloy should be
        as low as possible but hard enough to withstand heaviest continuous loading or shock loading to
        which it is likely to be subjected, without plastic deformation. Low melting point constituents‘
        will under boundary conditions with high local asperities contact temperatures, enable the softer
        metal to melt and flow locally and/or deform plastically. When there is dirt, rust or other foreign
        matter entering the bearing with the oil, it is advantageous if the bearing alloy has the ability to
        absorb such contaminants, thus avoiding damage or scoring of the journal. White metals or
        babbits are good in this respect. This property of bearing alloy is commonly termed
        ‗embeddability‘.
    3. Compatibility: This is an indication of anti-weld or anti-score characteristics of a bearing with a
        given bearing and journal combination under boundary lubrication conditions. Softer alloys are
        beneficial.
    4. Corrosion Resistance: Particularly under high temperature conditions, decomposition products of
        lubricating oil, such as weak organic acids and even peroxides, attack some metals used in
        bearing alloys. In diesel engines the attack may be caused by mineral acids formed as a result of
        condensation of sulphur oxides, especially in fuel with high sulphur content.
In general, it is the steel working part, which corrodes more than bearing alloy. Alloys containing the
following are more susceptible to corrosive attack: Lead, cadmium, silver, zinc, copper & phosphorous.

Note:
       a) Provided that load and temperatures are not excessive, good tin-base or lead-base white
          metal alloys are most widely used bearing alloys for diesel engines, marine turbine and
          auxiliary machinery.
       b) Thick white metal linings (up to 12.7 mm) which was the old practice have relatively
          poor compressive strength at high operating temperature. This will cause fatigue failure
          of bearing material if load is high. This is particularly true in modern diesel engines,
          (highly rated) where bearing loads are very high. That is why the modern practice is to
          use bearing shell with thin lining of white metal.
For example, in small high speed diesel: Fatigue strength of 0.3 mm
whitemetal lining will be 141 kg/cm2 and for 0.08 mm whitemetal lining
it will be 211 kg/cm2.
                                                                                                                    184
It must also be noted that thinner the lining poorer the conformability and embeddability.


Properties of Bearing Metals:

          Material.                      Shaft Hardness: ‗Brinell‘.                  Load Carrying Capacity (kg/cm2).
     1. Tin-base WM:                          150 or less:                                      56 to 105.5
     2. Lead--base WM:                        150 maximum:                                      56 to 84.5
     3. Copper-lead:                          300                                            105.5 to 176
     4. Aluminium-Tin:                        300                                                  281 +
     --------------------------------------------------------------------------------------------------------------------
Oil Grooves:
The use of oil grooves in the pressure areas of bearings should be avoided, as the pressure built up
within the oil films tends to escape from the high to the low-pressure zones, thus reducing the possibility
of establishing hydrodynamic conditions. However, in some diesel engine bearings, a circumferential
groove is used to convey oil to other bearings. In such designs the bearing length is usually increased to
compensate for the groove. Longitudinal groove should not extend to the end of bearing to help establish
hydrodynamic lubrication.
Bearing Housing Design:
The load carrying capacity of a bearing is considerably affected by the design of the housing into which
it is assembled. Housing bore should be smooth; diameter should be somewhat less than the diameter
corresponding to the free peripheral length of the bearing liners.
              a) Bearing is held in the housing by an interference fit, or nip, sufficient to prevent relative
                   movement and to promote effective beat transfer.
              b) Cap holding down bolts should be as closely pitched as possible (or practicable) to keep
                   distortion of the housing to a minimum.
              c) If the housing is insufficiently robust, the bearing will be subjected to strains and may
                   lead to failure. On the other hand if too stiff it will not yield to crankshaft deflections. It
                   will give rise to severe edge and other local loads on the bearing.
Measurement of Clearance:
The following methods are used:
     1. Lead clearance - strips of lead wire between bearing and pin/journal.
     2. Feeler Gauge.
     3. Dial Gauge.
     4. Inside and outside micrometer
Above mentioned methods are adopted to check oil clearance.
The following methods can also be used to measure bearing wear.
     5. Bridge Gauge.
     6. Measuring thickness by micrometer.
Lead Clearance:
It is advisable not to use this method in thin shell bearings. Clearance can be measured over a wide
bearing surface. It is very important that the wire is soft enough and does not make indentation on the
bearing alloy. The wire should not get compressed less than 2/3rd of its original diameter. The bearing
bolts should be tightened to required torque or radial position should be marked on the nut, so that it can
be tightened to same position while boxing up.
Feeler Gauge:
Feeler gauges are easy and simple to use where it is accessible. It is very important to make sure that
sufficient length of feeler gauge has been inserted to give a representative measurement of the full length
of the bearing. It must not be taken in way of oil grooves; this may restrict entry of the feeler cauge. The
feeler gauge should not score the bearing surface or the shaft. In case of a very small clearance,
                                                                                                        185
sometimes the feeler gauge tends to break because of insufficient stiffness. It is ideal for checking guide
clearance.

Inside and Outside Micrometer:
Inside and outside micrometers are used for measuring internal bore of bearing halves (bolted) and pin
diameter. It is used where clearance is large compared to the diameter.
Bridge Gauge:
It is used to measure main bearing wear down. The measurement should be done without disturbing the
lower half of the bearing. The machined face of the bearing housing should be absolutely clean where
the gauge is fitted. The gauge is stamped with original bearing, comparison is made with the same to
assess the wear down. They should be stored in a protective box.
Measuring Thickness by Micrometer:
Bearing thickness can be measured by using a micrometer directly for large bearings.

Bearing Failures:
Wiping:
Very often, wiping is a slight transient phenomenon and is undetected until the machinery is opened up
for survey. In other cases, of course, complete bearing failure occurs.
It may be due to the following reasons:
                i)     Temporary lack of oil.
                ii)    Very slow start-up.
                iii)   Too small clearance.
                iv)    Misalignment.
                v)     Fabricated Bedplate Cross-girder.
                vi)    Tin oxide corrosion.
If wiping is only discovered upon opening up of the machinery for inspection, it is usually a sign that
bearing has in fact done its job correctly and has saved further damage occurring.
                                                                                                       186

          Relationship between Temperature and Maximum Stress causing whitemetal failure.
Note:
Minor wiping occurs in many soft alloy bearings without serious results. But if it involves a major area
due to say interruption of oil supply, the entire metal lining may melt involving complete bearing failure.
For instance:
            a) In the case of misalignment, it is quite common for a bearing to bed itself in by wiping
                and then to run correctly.
            b) In the case of slow start up, not enabling the fluid film to be rapidly generated, the white
                metal has safely run under boundary lubrication for a short time until hydrodynamic
                lubrication has been established.
            c) In the case of too small a clearance the bearing itself has compensated for a design or
                manufacturing error.
The temperature at which bearing surface yields is a function of load. The limiting temperature of large
white metal bearings ranges from 130* to 200*C.

Fatigue:
Repeated loading causes fatigue, which may be due to the basic characteristics of the engine. The
damage takes form of crazed cracking which penetrates the white metal and continues close to, and
parallel with, the white metal/backing interface, allowing pieces of white metal to fall out. Poor bonding
can accelerate this kind of failure. Where poor bonding has assisted failure, the bare base metal is
revealed when the failure occurs.

Tin Oxide Corrosion:
This failure is due to the corrosion of the tin phase of the white metal to form tin oxide. Tin oxide is
extremely hard and brittle and after having formed breaks off rapidly causing wear of the surfaces and
breakdown of the oil film. The appearance of the film so formed is grey in the early stages of formation
becoming progressively darker as its thickness increases and particles become detached.
Cause of the Corrosion is due to presence of water in the lubricating
oil. In marine service the water is almost certainly contaminated
with chlorides, which promotes electro-chemical action. It is
probable that this form of corrosion has always taken place in marine
bearings but, in the past, bearing loads and clearances have been
able to tolerate a thin layer of oxide and the bearing has continued
to work with a slight rise in temperature. More recently, with higher
loads, when the oxide layer becomes thick, the bearing temperature
may rise sufficiently to melt the underlying metal and failure occurs
by wiping.                          Regular or continuous removal of water from
lubricating oil should prevent tin oxide formation.

Cavitational Erosion:
This is unusual form of bearing failure but could be quite severe. It is confined mainly to shell-type
bearings fitted to high or medium speed engines and to bearing subjected to fluctuating loads, such as
crankpin bearings. Bearings exhibiting this kind of failure usually have complete areas of the bearing
metal missing. These cavities are usually around oil grooves or holes particularly in low-pressure areas.
It is caused by an implosion of gas or air bubbles released from a lubricating oil film under particular
conditions. Some of the dissolved air is released as bubbles, usually fairly large in size, which form
cavities in the oil film, unless they escape with oil through end of bearings.
                                                                                                       187
In a bearing subjected to fluctuating load, particularly if applied suddenly, the air bubbles or cavities
collapse or implode. The pressures set up locally during these implosions are very high, possibly
exceeding 221 kg/cm2, and may cause a pitting or cavitation effect in the area. It appears that cavitation
erosion is lessened with viscous oils, as these appear to dampen the implosion effect. There is a practical
limit to acceptable oil viscosity, because with high viscous oil the temperature of oil will increase.

Why are crosshead bearings difficult to lubricate as compared to main or bottom end bearing?
  1. Loading is much more severe especially with turbo-charging and highly rated engine. The full
     load from combustion is applied directly to the bearing.
  2. The movement of the bearing about the pin is oscillating rather than linier or circular at constant
     speed as such oil wedge formation is difficult. Connecting rod swing through an angle of 25-30*,
     depending on connecting rod length/crank throw ratio which is usually 4:1.
  3. The loads are invariably down wards, as such no separation between the pins and lower half
     bearing.
  4. Another problem associated with crosshead design is due to the tendency of the pin to distort
     under load. In this case, the inner edges of the lower half bearing are subjected to heavy
     fluctuating load. This leads to fatigue cracking of the inner bearing edge.

Manufacturer's Method of Overcoming such Problems:
Sulzer Engines: RD, RND, RL Type:
   1. Largest possible diameter pin will offer more load area with less distortion. There will be higher
       peripheral speed between bearing and pin, which assists in oil wedge formation.
   2. Uniform loading of bearing by elastic distortion using flexible bearing support.




                     (a) Traditional.                  (b) Conjugate
                         Defluxions in typical crossheads (exaggerated for clarity)

Under heavy loads, bearing support distorts as well as the pin and loads remains evenly distributed.
Fatigue cracking is prevented. This is termed conjugate deflexion whereby the bearing deflexion follows
that of the crosshead pin.
    3. Bearings are fine-machined, no hand scraping to improve lubrication. The lower half is thin
         walled. Thin wall bearings have high fatigue resistance and easy to renew. Bearing bolts are
         hydraulically and evenly tensioned.
    4. Crosshead made of forged steel and consists of an enlarged central section with 2 large diameter
         crosshead journal pins for X-head bearings. Aluminium-tin shell bearings with an overlay of
                                                                                                       188
       softer material (white metal) are used. Special lubricating system with fine filter and lubricating
       oil pressure of 16 bars is used. Maximum crosshead bearing pressure would be high since
       combustion pressure is 100 to 102 bar for RLA engines. Hence increasing the lubricating oil
       pressure to the X-head to a level where it caused separation of the surfaces during the low load
       part of the cycle, a squeeze film is provided for during the high load part.

MAN: Type Engine:
  1. Highly polished and stiff pin.
  2. Crosshead made of forged steel and consist of a large, stiff cylindrical pin flat machined on the
     top and at the centre of its length only, to enable piston rod and guide shoe to be secured. The
     ends of the X-head pin and the full length lower half forms the bearing surface. Load is thus
     spread over a large area, which ease the pressure and facilitate lubrication.
  3. MAN engines have pressure pumps mounted on X-head and driven by movement of connecting
     rod to supply oil to X-head bearings (See page 63 for the figure of this pump and arrangement).
     The pump operates on its delivery stroke when the piston is at the bottom part of its stroke and
     unloaded. Thus the crosshead pin is lifted by oil pressure only against the weight of the running
     gear. Pump suction stroke occurs when piston is in the top of the cylinder. Two pumps are fitted,
     each delivery to separate set of oil grooves. In the event of failure of one of the lubricating oil
     pump sufficient oil pressure is supplied by remaining pump. Both top and bottom halves are thin-
     walled bearings.

   Some sections of Engine Main Bearings and Bottom end Bearings:




                MAN. Engine: Flywheel end main bearing and bottom end bearing:

Lubricating oil passage is also shown. 1. Crankshaft flywheel end flange. 2. Bearing oil deflector.
3. Oil supply line to aftermost main bearing. 4. Main bearing upper holding cover. 5. Crankshaft web
counter weight. 6. Crankshaft. 7. Connecting rod. 8. Connecting rod oil passage. 9. Bottom end bolts.
10. Oil passage in crankshaft.
                                                                                        189




Sketch above show „Doxford‟ J- type engine bottom end bearing and main bearing (Old type).
                      Note the spherical bearing for self-alignment.
           Drawings below show Doxford „J‟ type engine new type Main bearing.
                                                                               190




SULZER Engine: RND: RTA Type: Main Bearing showing the holding down arrangement.
                                                                                                        191




             Old type. Either end support.              New type with full pin support at the bottom.
       Piston rod passes through the pin and fixed.         Piston rod lower flange bolted to pin.
                                        See notes on page 58 & 59.

                  Crosshead bearing design change to accommodate more firing load.

***************************************Kv*******************************************
End of main engine running gear.
References:
Materials for marine machinery by Institute of Marine Engineers.
Diesel Engine Design by H.F.P. Purday.
Engine Manuals: Sulzer, B&W, MAN and Pielstic.
Design aspects of Large Marine Engines by J.F. Butler and F. Orbeck.
                           ******************Kv*********************
End of part II Marine Diesel Engines.
                                                                                                       192

   Topic wise Question Bank for Marine Internal combustion Engineering: I & II.
                                For BE (Marine Engineering) Cadets.


1. ENGINE PERFORMANCE, INDICATOR DIAGRAMS AND POWER BALANCE:

   1. With reference to main engines under running conditions explain how:
          a) Efficiency is assessed.
          b) Unbalance of power between units is detected.
          c) Balance of power is restored.
          d) Valve timing is critical to combustion and efficiency.
   2. Explain how it may be determined whilst an engine is running that the:
          a) Valve settings are correct.
          b) Balance of cylinder power is being maintained.
   3. Sketch in detail an indicator, labeling the principal features. Describe how it is used to obtain the
      various performance diagrams. Identify the various diagrams and explain their value in engine
      operation.
   4. Explain with reasons to what extent engine efficiency and output is dependent upon:
          a) Barometric pressure.
          b) Ambient temperature.
          c) Sea temperature.
          d) Atmospheric humidity.
   5. Give a reason why conformity to each of the following details is good practice in taking indicator
      diagrams:
          a) Cylinder cock is left open for a period of time before attaching indicator.
          b) Indicator dismantled and examined before use.
          c) Return spring operation and length of cord checked.
          d) Engine power increased above normal full power for a brief period of time before taking
              diagrams under normal conditions.
   6. Describe how, other than by taking power diagrams, the balance of power between cylinders can
      be checked. Explain why under steady service conditions the power developed in a cylinder
      tends to alter.
   7. Explain how the following conditions effect engine performance:
          a) Burnt exhaust valves.
          b) Worn cams and followers.
          c) Scored fuel injector needles.
          d) Broken piston rings.
   8. Describe how the working condition within the cylinders is checked in the case of:
          a) Slow speed engines.
          b) Medium speed engines.
          c) Explain why the methods differ.
          d) State with reasons, which is more accurate.
   9. Describe how the following faults are detected during the operation of a diesel engine. State
      possible causes and remedies.
          a) After burning.
          b) Early firing.
          c) Choked fuel valve.
          d) Leaky piston rings.
Kv/ICE/BE/QB/1/03.
                                                                                                     193




   10. State why in large engines power balance between cylinders is desirable.
            a) Explain why it is never achieved in practice.
            b) State how it is checked and imbalance countered.
   11. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to the progressive deterioration of combustion conditions and reduction of
       developed power:
            a) Entering the tropics.
            b) Fouled hull.
            c) Worn fuel pump plungers.
            d) Stretched camshaft chain.
            e) Worn piston rings and liners.
   12. Suggest with reasons which one of the following courses of action is likely to prove most
       effective in countering the following simultaneously prevailing conditions. All cardinal points
       in cycle late, engine speed and power reduced, exhaust gas temperature high, dirty funnel
       emission, emission of noise from engine increasing.
            a) Increase engine speed.
            b) Reduce tappet clearance in head valves.
            c) Overhaul the indicator.
            d) ‗Blow through‘ indicator cock.
            e) Renew indicator cord.
            f) Tighten camshaft chain.
   13. After prolonged full power operation of a main engine it is noticed that the temperature of the
       coolant return from one jacket is appreciably higher than the remainder. Describe how the cause
       is traced and corrected.
   14. Give two good reasons why heavy distillate fuel is heated before injection. Describe the effect on
       engine condition and performance of excessive heating of fuel. Specify the precautions taken
       when maneuvering on heavy fuel.
   15. Identify with reasons four essentials for good combustion in a cylinder. State the indications of
       poor combustion. Describe the effect of poor combustion on engine operation and maintenance.
   16. Suggest with reasons the most likely cause of trouble if the following conditions are prevailing
       simultaneously in an engine unit:
            a) Rise in temperature and reduction in flow of coolant return from jacket.
            b) Reduction in exhaust temperature.
            c) Tendency of funnel haze to dirtiness.
            d) Slight reduction in engine power.
            e) Tendency of piston rings to screech.
            f) Reduction in turbocharger speed.
   17. Explain how the following conditions affect engine performance:
            a) Insufficient tappet clearance.
            b) Worn cams and followers.
            c) Carbon encrusted fuel injector nozzles.
          ********************************* Kv*********************************************

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   2: BED PLATE & HOLDING DOWN ARRANGEMENT:

   1. Describe with sketches how structural strength and rigidity is imparted to bed plates by:
           a) Holding down bolts.
           b) Main tie bolts.
           c) Frame and entablature bolts.
   2. Suggest with reasons, which one of the following references to engine structure appears to be
       most accurate:
           a) The greater flexibility resulting from fabrication avoids cracking to which bed plates are
               prone.
           b) Engines assembled from separate and complete units are stronger than those in which
               units share a common bedplate and air box.
           c) Transverse and longitudinal stiffness are of equal importance.
           d) Vertical forces are of little significance in a well-balanced engine.
   3. Describe with sketches how structural strength is imparted to bed plates by:
           a) Supporting ship's structure.
           b) Entablature.
           c) Frames.
   4. Sketch a main engine structure comprising bedplate, frames and entablature, and showing the tie
       bolts in position. Explain how rigidity is imparted to the whole assembly. Give a reason for the
       incorporation of tie bolts in many engines.
   5. Sketch a main engine ‗holding down‘ arrangement. Give reasons why engines are mounted on
       chocks rather than directly on foundation plates. State what types of bolts are commonly used in
       these applications. State how longitudinal and transverse movement is prevented?
   6. With reference to fabricated bed plates explain with sketches how:
           a) Combustion loads are transmitted from the cylinders.
           b) Longitudinal members are strengthened in way of the transverse members.
   7. With reference to large fabricated bed plates state with reasons:
           a) What defects are likely to occur in service?
           b) Where they are likely to occur.
           c) Why they occur.
           d) How they can be countered.
   8. Suggest with reasons why the following conditions can cause persistent slackening of ‗holding
       down‘ bolts:
           a) Irregular foundation plate.
           b) Cast bedplate and frames.
           c) Loose chocks.
           d) Poorly balanced reciprocating masses.
   9. Sketch an engine holding down arrangement of current design. Describe with sketches how
       longitudinal and transverse movement is prevented. Define the routine maintenance to keep these
       arrangements in good condition.
   10. With reference to main engine structure explain why:
           a) Chocks are provided to resist lateral longitudinal movement of bedplates.
           b) Engines are carried on chocks rather than directly on the foundation plate.


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   11. With reference to main engine structures:
          a) Point-out two areas of apparent weakness.
          b) State how the weaknesses commonly manifest themselves.
          c) Give a reason for the weaknesses in the areas identified.
          d) Suggest how resultant faults may be avoided or corrected.
   12. Sketch one bay of a fabricated bedplate. State with reasons which bedplate surfaces are usually
       machined. Give reasons for the reinforcements incorporated in such bedplates.
   13. With reference to large fabricated bed plates give reasons why:
          a) Transverse members are commonly of cast steel construction.
          b) Longitudinal members are often of box construction.
   14. With reference to engine holding down arrangements:
          a) Sketch an arrangement used for large engines.
          b) State what are the advantages of current practice compared with previous practice.
          c) Explain why regular systematic inspection of the arrangement is advisable.
   15. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to persistent slackening of ‗holding down‘ bolts:
          a) Fabricated bedplates and frames.
          b) Unbalanced reciprocating masses.
          c) Heavily strengthened ship structure under engine.
          d) Lightly strengthened ship structure under engine.
          e) Persistent overloading of engine.


3: VALVE TIMING & ACTUATING GEAR:

   1. Explain how the following conditions affect engine performance:
                    a) Stretch in camshaft chain.
                    b) Worn fuel pump barrels and rams.
                    c) Leaking fuel pump spill valves.
   2. Sketch the profiles of ahead and astern cams for main engine fuel injectors. Show how such
      cams are mounted on the shafts. State why two cams are fitted in some instances and not in
      others.
   3. Describe how injection timing is varied.
   4. With reference to inlet and exhaust valves state:
          a) Why spinners are sometimes carried on valve stems, how valve bounce is countered in
              some engines.
          b) How shrouding on seats affects valve effectiveness.
          c) Why more than one spring is sometimes fitted to valves.
          d) With reasons, the circumstances under which clearances between cam followers are
              adjusted.
   5. Describe with sketches how a camshaft chain is fitted.
   6. State how correct tension is determined in a large engine camshaft chain. Explain how camshaft
      timing is checked and adjusted.
   7. Describe with the aid of a timing diagram the effect of camshaft chain wear on engine operation.
      State how this fault becomes apparent and is remedied.

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   8. Give reasons for progressive slackness of camshaft chains in service. State what effect ‗stretch‘
       has on timing. Describe with sketches how correct tension and timing is restored. Describe how
       tightness is achieved when the normal method proves inadequate.
   9. Explain why ‗lost motion‘ is provided on some camshafts.
           a) Sketch in detail such an arrangement.
           b) Describe how it operates.
   10. Sketch and describe two different arrangements of controlling the timing of the exhaust gas flow
       from two stroke cycle engines. Give two advantages and two disadvantages of each method
       described.

4: STRUCTURE, FRAMES, GUIDES & TIE BOLTS:

           1. Sketch an engine structure in way of a frame. Explain how the entablature, frames and
              bedplates are formed into a rigid structure. Give a reason why engine structures are not
              built up from separate units.
           2. Sketch a transverse section through an engine in way of a pair of tie bolts showing how
              the whole structure is held together. Give reasons why the tie bolts are situated as shown.
              Explain why tie bolts are not fitted in all engines.
           3. With reference to main engine structures:
           a) Sketch an arrangement whereby the keeps of the main engine bearing are secured by jack
              bolts and give reasons for this arrangement.
           b) Explain why cast bedplates are occasionally used in preference to fabricated ones.
           4. Sketch a main engine structure comprising bedplates frames and entablature, and
              showing the tie bolts in position. Explain how rigidity is imparted to the whole assembly.
              Give a reason for the incorporation of tie bolts in many engines.
           5. Sketch a tie bolt in position in a large engine. Define the purpose of tie bolts. Describe
              how correct bolt tension is ensured. Explain why some engines have tie bolts whilst other
              engines operate satisfactorily without them.
           6. Sketch and describe a large powered diesel engine bedplate and explain how it is secured
              to the ship‘s hull.

5: CRANKSHAFT, CONNECTING ROD & CROSSHEAD:

   1. With reference to large crankshafts explain:
         a) How and where ‗interference fit‘ is utilized.
         b) Nature of and reasons for surface finish of the mating surfaces.
         c) Why dowel pins are rarely used.
   2. With reference to large crankshafts explain why:
         a) Journals and pins have generous fillets.
         b) Webs are solid.
         c) The value of deflection readings is apt to be overrated.
   3. Explain with reasons how the use of boiler grade fuel can effect the following components:
         a) Cylinder liner.
         b) Turbo charger.
         c) Crankshaft.
         d) Exhaust valves.
   4. Describe how crankshaft deflection measured. State how the measurements can be checked for
      accuracy. Specify with reasons other cheeks should be made on the crankshaft.

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    5. Describe how alignment of large crankshafts is checked. Explain how the readings would be
        affected by other factors pertaining to engine and the ship.
    6. Suggest with reasons which one or combination of the following conditions is likely to
        contribute most to crankshaft fracture:
            a) Heavily fouled lubricating oil filters.
            b) Engine overload.
            c) Excessive clearance between crosshead slippers and guide plates.
            d) ‗Wiped‘ main bearing.
            e) Slack ‗holding down‘ bolts.
    7. With reference to large crankshafts explain:
            a) Why some have forged webs and others have cast webs.
            b) Why main bearing 'wear down' is regularly checked.
            c) With sketches, how the possibility of fracture is reduced by design.
    8. With reference to large crossheads, describe with sketches how:
            a) The piston rod is secured.
            b) The connecting rod is attached.
            c) The slipper(s) are mounted.
    9. Sketch a crosshead pin designed to alleviate load concentrations. Explain why the problem does
        not arise with bottom ends. Explain why thin walled shell bearings are sometimes used for top
        ends.
    10. Describe how top end clearance is measured. State how the measurement is checked to confirm
        accuracy. Specify other cheeks and tests made whilst the bearing is ‗opened up‘.
    11. Sketch a commonly employed method of attachment of slippers to crossheads. Explain why
        guide clearance is strictly limited. Give reasons why lubricant is generally fed to the slippers and
        not the guides.
    12. With reference to large crossheads explain:
            a) How guide clearance is adjusted and piston rod alignment checked.
            b) With sketches how slipper and pins are lubricated.
            c) Why top end clearances are greater in proportion to pin diameter than bottom end
                clearances.
6. ENGINE BEARINGS & BEARING BOLTS:

   1. Suggest with reasons the most likely cause of trouble if a crankshaft main bearing 'wipes' under
      the following simultaneously prevailing conditions:
          a) Power imbalance between units.
          b) ‗All fabricated‘ bed plate.
          c) Heavy weather with ship in ballast.
          d) Tie bolts tightened by spanner and hammer.
          e) Slack holding down bolts.
   2. Explain why in an engine unit random positioning of the crankpin is unacceptable for checking
      the top and bottom end clearances. Describe the cheeks that are made to ensure the provision of
      adequate clearance of these bearings.
   3. Explain why it is not good practice to simply drop bottom halves only when examining and
      adjusting large bottom ends. State how top halves are made accessible for inspection.
   4. Describe how the defect is traced and the cause determined when white metal particles are found
      in main lubricating oil filters. State two precautions taken before putting an engine into service
      after bearing replacement.
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   5. Suggest two possible causes why the white metal of a main bearing may ‗wipe‘. Describe three
       precautionary measures to be taken before the engine is put back into service after bearing
       redress or replacement.
   6. With reference to bottom end bearings explain why:
           a) Excessive clearance is as intolerable as insufficient clearance.
           b) Regular examination of top halves is as important as for bottom halves.
           c) Pinching screws are frequently provided in top halves.
   7. With reference to large crankshaft bearings:
           a) Describe with sketches how lubricant is fed to crank pins for main journals.
           b) Explain why the condition of main bearing top halves is not representative of bottom
               halves.
           c) Apart from lubricating the bearings, what other function has the lubricating oil got?
   8. Describe with sketches how the top and bottom dead center positions of a crank are found. State
       with reasons the crank positions at which the top and bottom end bearings are adjusted. Give
       reasons why bearing clearances taken with lead wire may not be reliable.
   9. Explain why a particular reel of lead wire used for measuring bottom end clearance may not be
       suitable for measuring top end clearance. Explain why in a unit, random positioning of the
       crankpin is unacceptable for checking the top and bottom end clearances. State what cheeks are
       made to ensure the provision of adequate clearance of these bearings.
   10. Sketch a bottom end bearing for a large engine showing how lubricating oil enters and leaves the
       bearing, and how oil escape from the ends is prevented. Describe how bottom end clearance is
       checked and adjusted.
   11. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to failure of top end bearings:
           a) Ovality in pins.
           b) Excessive clearance between slipper and guide plates.
           c) Engine overload.
           d) Badly worn cylinder liner.
           e) Dirty lubricating oil filters.
   12. With reference to the replacement of bottom end bearings state with reasons:
           a) Where the crank is positioned.
           b) Why a threaded hole is machined axially into the end of large bearing bolt.
           c) Why pinch bolts are fitted in way of each bearing bolt.
   13. Describe how the following clearances are measured and adjusted:
           a) Top end bearings.
           b) Ahead and astern guides.
   14. Explain why in comparison to bottom end bearings, top end bearings generally have greater
       propensity to failure, clearance in proportion to pin diameter and diameter in proportion to
       length.
   15. Give two reasons why torque wrenches and hydraulic spanners are now used extensively in the
       overhaul of diesel engines. State why such devices need careful manipulation. Give two
       applications in large engines where the use of these devices has proved particularly useful.



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7. CYLINDER HEAD & VALVES:

   1. Explain why the following details are provided in some cylinder head valves and not in others:
           a) Twin exhaust valves.
           b) Two or more springs per valve.
           c) Stellite inserts or deposition on lids and seats.
   2. Sketch a cylinder relief valve for a large engine. Define the conditions under which it may lift.
       Explain why it needs regular attention although it may never have lifted.
   3. With reference to inlet and exhaust valves explain why:
           a) They are frequently mounted in cages separate from the cylinder head.
           b) They open into the cylinder.
           c) Clearance between cams and followers is advisable.
   4. With reference to inlet and exhaust valves state with reasons:
           a) Circumstances under which clearance between cams and followers needs adjustment.
           b) Why some valve stems carry spinners.
           c) How valve bounce is countered.
   5. Sketch a cylinder relief valve suitable for a main propulsion engine. Describe the operational
       defects to which such valves are susceptible and state how these defects can be avoided.
   6. With reference to main engine inlet and exhaust valves give reasons why:
           a) Exhaust valves are water-cooled.
           b) Slackness of valve stems in the guides is as undesirable as being too neat.
           c) Multi-spring loading is sometimes employed.
           d) Shrouding on lips and seats should be removed as soon as it appears.
   7. With reference to inlet and exhaust valves state:
           a) How overhaul frequency can be reduced.
           b) How valve lift alters in service and the consequential effects.
           c) How (b) is corrected.
   8. Sketch a main engine cylinder starting air valve. Explain how it operates. State with reasons why
       a leaking cylinder starting air valve should receive immediate attention.
   9. Explain why it is bad practice to:
           a) Gag leaking cylinder relief valves.
           b) Lap injector needles with grinding paste.
           c) Clean injector nozzles with a sharp tool.
   10. Explain the possible consequences of operating main engines with:
           a) Cylinder starting air valve leaking.
           b) Cylinder relief valve seized in closed position.
           c) Burnt exhaust valve.
   11. Suggest with reasons which one of the following references to cylinder valves appears to be the
       most accurate:
           a) The removal of carbon deposits from seats and stems can be more detrimental than
               advantageous.
           b) Carbon accumulation on injector nozzles is due to inadequate fuel preparation.
           c) Tightness of starting air and relief valves is essential during engine operation.


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8. CYLINDER LINERS AND JACKET:

   1. Give the common causes necessitating cylinder liner replacement. Describe how a large liner is
      replaced at sea. Suggest the precautions to be observed before the engine is returned to service
      and during the ‗running in‘ period of the new liner.
   2. Describe with sketches the principal steps to be taken during withdrawal of a large cylinder liner.
      Explain with sketches how upon reassembly, absolute tightness of water seals is ensured. State
      what precautions must be observed before starting the engine after liner replacement.
   3. Sketch a large cylinder liner in position in an engine, labeling the principal features and showing
      the direction of the flow of coolant through the passages around the liner. Show by sketches how
      the cooling spaces are sealed. Suggest with reasons the likely causes of overheating in cylinder
      liners.
   4. Sketch a section through one unit of a cylinder liner lubricating oil pump. Explain:
          a) How the oil supply rate is controlled.
          b) Why variations in oil temperature should be avoided.
          c) When, relative to the position of the engine piston, the oil should enter the cylinder.
          d) Why quantity and quality (TBN) of oil pumped in to the unit with respect to engine speed
              is critical?
   5. Describe with sketches a large cylinder liner. Explain:
          a) How the liner thermal expansion is accommodated.
          b) State with reasons the materials used for liners.
          c) Describe the tests applied after a liner is fitted in an engine.
   6. Describe how cylinder liners are checked for wear. Explain how these measurements are
      recorded. Explain why allowable wear is limited and governs liner replacement.
   7. What are all the improvements achieved in modern day liner design to withstand both pressure
      load and thermal load associated with high power long stroke engines.

9. PISTONS, RINGS & STUFFING BOX:

   1. With reference to piston rings:
         a) Give reasons for progressive fall off of piston ring performance in service.
         b) State in detail as to which ring clearances are critical.
         c) State what effect face contouring, beveling, ring cross section has on ring operation.
         d) Material properties of rings and liners have on ring life.
   2. Sketch a crankcase diaphragm gland.
         a) Explain its purpose and how it operates.
         b) State why it needs regular attention.
         c) State why scraper rings is carried on the piston when a crankcase seal is provided.
   3. Sketch a water-cooled piston showing the direction of the coolant flow.
         a) Show in detail the location and nature of the water seals.
         b) Describe how the coolant is conveyed to and from the piston.
         c) Explain how it is ensured that the crown receives an adequate supply of coolant.
   4. Sketch a piston for a large two-stroke cycle engine.
         a) Identify the causes of burning and cracking of piston crowns.
         b) Explain how these forms of deterioration can be largely avoided.

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   5. Give reasons for piston ring breakage. Define with reasons the likely consequences of ring
       breakage in turbocharged engines. Suggest two ways of avoiding breakage.
   6. Describe with sketches the procedure of withdrawing a large piston and rod.
           a) State why upon reassembly alignment may require checking.
           b) State how alignment is checked and corrected.
   7. Explain the various ways in which piston ring breakage may occur.
           a) State how ring breakage can be largely avoided.
           b) Describe how new rings are fitted to a piston.
   8. Explain why piston crowns occasionally crack.
           a) State how such cracking shows up in operation.
           b) Describe how it can be largely avoided.
   9. With reference to fitting piston rings:
           a) Describe how principal clearances are checked.
           b) Give three reasons for rejecting a used ring.
           c) Give three reasons for rejecting a new ring.
   10. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to breakage of piston rings:
           a) Engine overload.
           b) Engine coolant pump stalled.
           c) Turbo charger fouled on gas side.
           d) Partially choked air intake filter.
           e) Worn cylinder liner.
           f) In adequate cylinder lubrication.
   11. Sketch in detail a crankcase diaphragm gland.
           a) Define the purpose of the gland.
           b) Give a reason why a stuffing box of soft packing is not suitable for this application.
           c) Identify the indications that the gland needs attention.

10. FUEL SYSTEM:

   1. Sketch a hydraulically operated fuel injector.
          a) Explain how the valve operates and injection timing varied.
          b) State why rapid action and absolute tightness of the valve is essential.
          c) Define the significance of the shape and size of the nozzle holes.
   2. Identify with sketches those parts of fuel injectors directly contributing to spray pattern.
          a) State how atomisation deteriorates and is corrected.
          b) State how penetration deteriorates and is corrected.
   3. Sketch in detail a scroll type fuel injection pump. Explain how the quantity of fuel is metered and
      how the governor ‗cut out‘ functions. State how this pump is reset after overhaul.
   4. Sketch a fuel injection pump for a main engine. Explain how the quantity of fuel to individual
      cylinders is varied. State why it is common practice to employ a surcharge pump to feed the
      main pump.
   5. Sketch and describe a fuel injection pump of the scroll type. Compare the main advantages and
      disadvantages of the scroll and non-scroll types of fuel pump.
   6. Identify with reasons four essentials for good combustion in a cylinder. State the indications of
      poor combustion. Describe the effect of poor combustion on engine operation and maintenance.

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   7. Sketch a main engine fuel injection pump other than the scroll type. Explain how the fuel is
       metered both manually and by governor. State why fuel pumps are sensitive to fuel conditions.
   8. Sketch an arrangement for operating the fuel injectors of the main engine. Describe how it works
       and how injection timing is set. State how fuel injection is regulated to suit load.
   9. Sketch and describe a recirculating type of fuel injector used on large powered diesel engine.
       Describe how such an injector operates. Why such injectors are used?
   10. Explain with reasons how the use of boiler heavier grade fuel can affect the following:
          a) Cylinder liner.
          b) Piston rings.
          c) Fuel pump.
          d) Exhaust valves.



11. LUBRICATION SYSTEM:

   1. Draw a line diagram of a complete main engine lubricating oil system labeling the principal
      items and showing the direction of the flow in all lines and oil-ways leading to the bearings.
      Explain why a large quantity of oil is kept in reserve. Explain why filters are fitted in addition to
      purifiers.
   2. With reference to cylinder lubrication state what are the:
          a) Indications that the correct quantity of oil is being used.
          b) Consequences of both excessive and insufficient cylinder lubrication.
          c) Difficulties of achieving correctly timed injection.
          d) Desirable qualities for cylinder oil when heavy distillate fuel is used.
   3. Compare the functions and characteristics of lubricants for:
          a) Cylinder liner walls.
          b) Crankshaft and connecting rod bearings.
   4. Explain how lubrication of cylinder liner walls is achieved in:
          a) Large slow speed engines.
          b) Small high-speed engines.
          c) Medium speed aux. Engine.
   5. State with reasons which one of the following courses of action would most help to correct a
      pronounced discoloration of the lubricating oil:
          a) ‗Freshen up‘ from reserve tanks.
          b) Increase purifier throughput.
          c) Increase frequency of filter pack cleaning.
          d) Overhaul piston rod stuffing boxes.
          e) Check tank top integrity of sump tank.
   6. Define the cause of corrosive wear on liners and piston rings. Explain the part played by cylinder
      lubrication in neutralising this action. State how the timing, quantity and distribution of cylinder
      oil are shown to be correct.


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11. SPEED GOVERNING & FLYWHEEL:

   1. Sketch in detail a section through a hydraulic governor as fitted to medium speed unidirectional
      engine. Explain how it operates under frequent and wide load changes.
   2. Differentiate between the functions of flywheels and governors for auxiliary engines.
          a) Explain why load-sensing governors are usually fitted to engines driving an alternator.
          b) Sketch a governor for this duty and explain its actions.
   3. Sketch an inertia governor as fitted to some main propulsion engines.
          a) Explain how it operates.
          b) Define its limitations.
   4. Sketch and describe a hydraulic governor as fitted to an auxiliary diesel engine. Why is it fitted
      and what maintenance does it require.

12. SCAVANGING, SUPERCHARGING & TURBOCHARGING:

   1. Describe with line sketches each of the following systems of turbo-charging:
          a) Constant pressure.
          b) Pulse.
          c) Tuned exhaust.
          d) State what are the advantages claimed for each system.
   2. With reference to large turbochargers describe with sketches how:
          a) Bearings are lubricated.
          b) Turbines are sealed from atmosphere.
          c) Compressors are cooled.
   3. Describe with sketches how in large turbochargers:
          a) Blades are secured.
          b) Bearings are lubricated.
          c) Turbines are cleaned in service.
   4. Sketch a large turbocharger labeling the principal features. Show in detail two common methods
      of mounting the turbine blades. State why the blades are a loose fit in the rotor and why they
      cannot work out.
   5. With reference to turbo-charging explain:
          a) How removable element air filters are cleaned, mentioning the precautions taken during
             the operation.
          b) Why air coolers are used in conjunction with turbochargers and why appreciable amounts
             of oil might be found in the air system.
          c) Why excessive cooling of the air should be avoided.
   6. With reference to turbochargers define the cause of the following irregularities:
          a) Reduction in air temperature gradient across cooler.
          b) Rise in gas temperature in engine exhaust.
          c) Rise in air temperature in scavenge manifold.
          d) Drop in air pressure in scavenge trunking.
          e) Suggest remedies in each case.


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   7. Draw a line diagram of a complete turbo-charger labeling the principal components.
           a) State why air coolers and water separators are incorporated.
           b) Describe how sufficient air is made available under reduced power and maneuvering
               conditions.
   8. Identify the principal factors contributing to fouling on the gas side of turbochargers. State what
       are the effects of running engines with fouled turbochargers. State how turbocharger fouling
       reveals it. Describe how it can be appreciably reduced under operational conditions.
   9. With reference to main turbo-chargers explain with sketches how:
           a) Turbine glands are sealed and cooled.
           b) Rotor/impeller shaft is supported.
           c) Bearings are lubricated.
   10. Explain how an engine can continue to operate when one of the two turbochargers is out of
       action. State why the performance of turbochargers tends to deteriorate in service. State how this
       tendency is countered.
   11. Sketch and describe two different arrangements of controlling the timing of the exhaust gas flow
       from two stroke cycle engines. Give two advantages and two disadvantages of each method
       described.
   12. Give reasoned opinions as to the accuracy of the following statements:
           a) Air cooler cleanliness is critical to turbocharged engine performance.
           b) Engine operation is totally reliant upon turbocharger operation.
           c) Perfect axial balance does not exist between compressor and turbine.
   13. With reference to pressure scavenging of main engines give reasons for the following faults:
           a) Oil/air leakage into crankcase.
           b) Very dirty scavenge space with occasional fires.
           c) Persistent water accumulations in scavenge space.
   14. State with reasons which one or combination of the following courses of action would be most
       effective in correcting reduction in scavenge air pressure:
           a) Increase fuel to engine.
           b) Open exhaust gas boiler bypass.
           c) Clean air filter.
           d) Reduce coolant flow through air cooler.
   15. With reference to turbochargers give reasons for the following irregularities:
           a) Gas temperature at turbine exit abnormally high.
           b) Air pressure at compressor discharge abnormally low.
           c) Sluggish ‗run up‘ and rapid ‗run down‘.
   16. Draw a line diagram of a complete pressure charged scavenge system operating on the constant
       pressure principle, labeling the principal components and showing the direction of flow in all
       passages. State why such a system is essential for two stroke cycle engines. State with reasons
       what advantages the constant pressure scavenge system has over its constant volume counterpart.
   17. With reference to turbocharged main engines state:
           a) What effects dirty air filters have on engine performance?
           b) How dirty filters are detected.
           c) Why care is needed in engine operation after filter cleaning or renewal.


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   18. Explain why air coolers and water separators are generally associated with large turbochargers.
       Give a reasoned explanation for the position of the air cooler and water separator relative to the
       compressor. State the faults to which air coolers are susceptible and how they are dealt with.
   19. With reference to turbo-charging:
          a) Sketch and describe a turbocharger.
          b) Why air coolers are used in conjunction with turbochargers.
          c) Why excessive cooling of air should be avoided.
   20. Give reasoned opinion as to the accuracy of the following statements:
          a) Air cooler cleanliness is critical to turbocharged engine.
          b) Engine operation is totally reliant upon turbocharger operation.
          c) Perfect axial balance does not exist between compressor and turbine.

13. STARTING & REVERSING SYSTEM:

   1. Sketch a main engine cylinder starting air valve. Explain how it operates. State with reasons why
      a leaking cylinder starting air valve should receive immediate attention. State with reasons what
      normal maintenance is required.
   2. Sketch in detail a starting air distributor. Explain how a main engine can be started from any
      position. State how air is supplied to the distributor.
   3. Sketch a starting air distributor for a main engine. Explain how it sets the engine in motion for:
          a) Ahead propulsion.
          b) Astern propulsion.
   4. Draw a line diagram of a complete main engine starting air system from reservoir to cylinder
      valve labeling all the principal components and indicating the direction of flow in all lines.
      Explain how the system functions. State with reasons what protection devices are incorporated.
   5. Describe with sketches the means whereby the engine controls cannot:
          a) Countermand the bridge telegraph.
          b) Be countermanded by contrary running of the engine.
          c) Explain why without (b) some engines may continue to run ahead with the controls in the
              astern position.
   6. Describe with sketches those devices fitted to main engines as a protection against:
          a) Racing in heavy weather.
          b) Lubricant supply failure.
          c) Coolants supply failure.

14. COOLING SYSTEM:

   1. Sketch and describe an arrangement for conveying coolant to a large piston. State with reasons
      the points of weakness in the arrangement and how they are dealt with. Explain how positive
      circulation of coolant to the piston is ensured.
   2. With reference to engine coolant explain:
          a) Why the use of oil is limited.
          b) Why potassium bichromate is commonly used as an additive to distilled water.
          c) What problems arise from practice (b)?

Kv/ICE/BE/QB/14/03.
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   3. Describe how engine-cooling water is treated to maintain it in an acceptable condition.
          a) State why treatment is necessary.
          b) Explain the action of each chemical used.
   4. Sketch and describe an arrangement for conveying coolant to a large cylinder liner of modern
      design. Explain how the thermal loading is kept with in the design value even though the power
      developed per unit is high and it experiences high temperature.

15. AIR COMPRESSORS AND RECEIVER:

   1. With reference to reciprocating air compressors explain why:
          a) The suction filter should be kept clean.
          b) The correct top end, bottom end and main bearing clearances should be maintained.
          c) The suction and discharge valves need regular attention.
          d) Intercoolers should be kept clean.
   2. Sketch diagrammatically a two-stage reciprocating air compressor, labeling the principal features
      and showing the direction of airflow through the compressor.
          a) Give a reason for air compression in stages.
          b) Give a reason for inter-stage cooling.
          c) Give a reason why stages are often in tandem.
   3. With reference to air compressors state why:
          a) Clearance volume should be as small as possible.
          b) Cylinder lubrication should be minimal.
          c) Intake filters should be cleaned regularly.
          d) Suction and delivery spring loaded plate valves are used.
   4. With reference to two stage air compressors state:
          a) With reasons what parts need constant attention.
          b) Why apart from inefficiency, bottom end and main bearing wear down tolerance is
              usually restrictive.
          c) Why valve springs occasionally collapse and how this shows up in practice.
   5. With reference to air compressors explain the purpose of:
          a) Suction filters.
          b) Drain valves.
          c) Bursting discs.
          d) Relief valves.
   6. Suggest with reasons, which one of the following practices in air compressor operation should
      not be encouraged:
          a) Maintaining the finest ‗bumping‘ clearances in al1 stages.
          b) Annealing bursting discs at regular intervals.
          c) Restricting cylinder lubrication to absolute minimum.
          d) Muffling intake filter in a dust, grit or oil laden atmosphere.
   7. Sketch an air compressor intercooler showing how differential expansion is accommodated and
      where the drains and safety devices are fitted. Give two reasons why intercoolers are fitted to
      multistage compressors. Give four reasons why tubular coolers are superior to cooling coils.
      State why each drain and safety device is fitted.


Kv/ICE/BE/QB/15/03.
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   8. Explain why, when overhauling air compressors ‗bumping‘ clearances need checking. Explain
       why bottom end adjustment does not affect ‗bumping‘ clearance. Explain why in multistage,
       single crank compressors it is not sufficient to simply adjust the ‗bumping‘ clearance in one
       stage alone.
   9. Give reasons and remedies for the following faults in air compressors:
           a) Excessively hot discharge pipe.
           b) Intermediate stage relief valve lifting.
           c) Noticeable reduction in free air delivery.
   10. With reference to air compressors explain why:
           a) The first stage is sometimes rotary.
           b) Multi-tubular air coolers are preferable to cooling coils.
           c) It is necessary to lubricate the cylinder walls with oil even though water is entrained in
               the air.
   11. State with reasons which one or combination of the following courses of action would be most
       effective in reducing the temperature of an abnormally hot air compressor discharge line:
           a) Increase cylinder lubrication.
           b) Cheek bottom end and main bearing ‗wear down‘.
           c) Cheek cleanliness of intercoolers.
           d) Examine the air intake filter.
   12. Describe with sketches a two stage, single crank, air compressor. State what routine attention is
       required whilst the compressor is running. Give two reasons why performance will ‗fall off‘ in
       service and how it is countered.
   13. Describe how ‗bumping‘ clearances are measured and adjusted in multistage, single crank, air
       compressors. State with reasons what is the effect of excessive ‗bumping‘ clearance in:
           a) Low-pressure stages.
           b) High-pressure stages.
   14. With reference to main air reservoirs:
           a) Sketch in detail the manner in which mountings are attached to the shell.
           b) Explain why regular internal inspection is advisable.
           c) Differentiate between the specific functions of relief valves and fusible plugs.
   15. Describe with reasons the attention given to the following components during air compressor
       overhaul:
           a) Bottom end main bearings.
           b) Suction and delivery valves.
           c) Inter-stage coolers.
           d) Pistons and cylinders.
   16. With reference to main air receivers state:
           a) Why internal surfaces are given a protection coating.
           b) What engine design factors determine volumetric capacity?
           c) Why fusible plugs are fitted.
           d) Where relief valves should be fitted.




Kv/ICE/BE/QB/16/03.
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16. OPERATING PROBLEMS, FIRES & EXPLOSIONS:

   1. Define the cause of failure in each of the following instances:
           a) Engine fails to turn on starting air.
           b) Engine turns on starting air but fails to fire on fuel.
           c) Engine fires on fuel, relief valves lifting as engine turns.
           d) Suggest how the trouble is traced and corrected in (a) and (b).
   2. Suggest with reasons the most likely cause of trouble if the following conditions are prevailing
      simultaneously in an engine unit:
           a) Rise in temperature and reduction in flow of coolant return from jacket.
           b) Reduction in exhaust temperature.
           c) Tendency of funnel haze to dirtiness.
           d) Slight reduction in engine power.
           e) Tendency of piston rings to screech.
           f) Reduction in turbocharger speed.
   3. Explain the possible consequences of operating main engines with:
           a) Cylinder starting air valve leaking.
           b) Cylinder relief valve seized in closed position.
           c) Burnt exhaust valve.
   4. Describe with sketches those devices fitted to main engines as a protection against:
           a) Racing in heavy weather.
           b) Lubricant supply failure.
           c) Coolant supply failure.
   5. After prolonged full power operation of a main engine it is noticed that the temperature of the
      coolant return from one jacket is appreciably higher than the remainder. Describe how the cause
      is traced and corrected.
   6. Identify the common causes of scavenge fires. Identify the indications that a scavenge fire is
      imminent. Suggest with reasons, tests and precautions carried out before running an engine to
      full power after extinction of a scavenge fire.
   7. With reference to explosions in starting air lines:
           a) Suggest with reasons three factors that are likely to substantially contribute to the
              creation of hazardous conditions.
           b) Describe how detonation is initiated.
           c) Explain how primary explosions trigger secondary explosions.
           d) State what means are employed to harmlessly dissipate the appreciable energy released
              by the explosion.
   8. Explain how and why explosions occur in starting air systems. State how such explosions can be
      avoided. Give two reasons why bursting discs are fitted in airlines.
   9. Sketch a crankcase explosion relief valve. Describe how it operates. State how its effectiveness is
      maintained. With reference to such safety devices state:
           a) Their essential function.
           b) How a flame trap works.
           c) How oil affects flame trap effectiveness.


Kv/ICE/BE/QB/17/03.
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   10. With reference to crankcase explosions state what:
           a) Initiates detonation.
           b) Leads to secondary explosions.
           c) Effects the severity of an explosion.
           d) Means are employed to harmlessly dissipate the appreciable energy released by the
               explosion.
   11. Sketch a crankcase oil mist detector system showing the run of the sampling pipes. Describe how
       an oil mist detector operates and sampling is controlled. State what maintenance it requires and
       how it is tested.
   12. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to the progressive deterioration of combustion conditions and reduction of
       developed power:
           a) Entering the tropics.
           b) Fouled hull.
           c) Worn fuel pump plungers.
           d) Stretched camshaft chain.
           e) Worn piston rings and liners.
   13. Explain why it is bad practice to:
           a) Gag leaking cylinder relief valves.
           b) Lap injector needles with grinding paste.
           c) Clean injector nozzles with a sharp tool.
   14. Sketch a crankcase explosion relief valve. With reference to such safety devices state:
           a) Their essential function.
           b) How a flame trap works.
           c) How oil affects flame trap effectiveness.
   15. State with reasons which one or combination of the following courses of action is most likely to
       be effective in removing the cause of the persistent lifting of a cylinder relief valve:
           a) Reduce lift of air inlet valves.
           b) Replace injector.
           c) Cheek tension of camshaft chain.
           d) Increase coolant supply to unit.
   16. Explain the causes of scavenge fires and how they are discovered. Describe how scavenge fires
       are dealt with. State how the frequencies of scavenge fires can be reduced.
   17. Describe how the defect is traced and the cause determined when white metal particles are found
       in the main lubricating oil filters. State two precautions taken before putting an engine into
       service after bearing replacement.
   18. Suggest with reasons which one or combination of the following conditions is likely to
       contribute most to breakage of piston rings:
           a) Engine overload.
           b) Engine coolant pump stalled.
           c) Turbocharger fouled on gas side.
           d) Partially choked air intake filter.
           e) Worn cylinder liner.
           f) Inadequate cylinder lubrication.
*************************************Kv********************************************
Kv/ICE/BE/QB/18/03. End of question bank.
                                                                                                     210


         Question Bank for Marine Internal Combustion Engineering: I & II.
                               For BE (Marine Engineering) Cadets.

   1.  What is the function of main engine bedplate and frames?
   2.  What are the advantages of supercharging an internal combustion engine?
   3.  Sketch and describe the cycle of operation of a four-stroke engine with a timing diagram.
   4.  How are the main engine tie rods tightened?
   5.  Explain why tie rods are required? Which internal combustion engines do not require tie rods?
   6.  Write short notes on:
                    (a) Jackets and liners of a large powered engine.
                    (b) Pistons of four-stroke medium speed engine.
                    (c) Cross heads.
                    (d) Bedplates of large main engine.
   7. Sketch and describe the different types of scavenging and indicate their advantages and
       disadvantages.
   8. Sketch and describe a turbo charger suitable for large slow speed engine.
   9. Sketch and describe the operation of crankcase explosion relief valve.
   10. What are the differences between two and four stroke engines?
   11. Explain in detail three types of cylinder liner wear?
   12. Name all the parts fitted to a four stroke engine cylinder head and explain their function.
   13. What are the types of scavenging used in large two stroke engines?
   14. What are the types of supercharging used in diesel engines?
   15. Sketch and describe the working cycle of a two-stroke engine with timing diagram.
   16. Describe in detail the differences between slow speed, medium speed and high-speed diesel
       engines.
   17. a) Explain why the air coolers are fitted between the turbo-charger and the scavenge manifold of
       the engine.
           b) Sketch a water separator fitted after the cooler.
           c) Describe the problems that would occur if the tubes of the cooler became fouled.
           d) Describe the problems the unit will face if the scavenge air is cooled to a very low value.
   18. Sketch and describe pulse and constant pressure types of supercharging indicating advantages
       and disadvantages.
   19. Sketch and describe in detail the jacket cooling water system suitable for a large marine diesel
       engine.
   20. Discuss in detail the causes of scavenger fires. How are they detected and prevented?
   21. What method is adopted for tensioning the tie rod in Diesel engines and why?
   22. What are the reasons for an overheated crankshaft main bearing?
   23. What are the reasons for an overheated main engine crosshead bearing?
   24. Which method of scavenging gives maximum scavenging efficiency and why?
   25. What is the need for grouping the exhausts in a multi-cylinder diesel engine?
   26. What type of compressor is used for turbo supercharging?
   27. What do you understand by ‗solid‘ injection system?
   28. Why does increasing compression ratio reduce delay period in a diesel engine?
   29. What is the first step taken when oil moist detector indicates a warning signal?
   30. What amount of heat (in percentage) is lost through exhaust gas and how? In modern ship power
       plant explain how this heat lost is recovered to enhance efficiency of the plant?

Kv/ICE/BE/QB/03.                                                                     *** 2.
                                                                                                      211
                                                                                               Page: 2.




   31. Draw the timing diagram of a two stroke supercharged engine and explain the significance of
                    a) Exhaust port or valve opening early or late.
                    b) Scavenge port opening early or late.
                    c) Fuel injection early or late.
   32. Discuss the development of a modern piston of a marine diesel engine with reference to
                    a) Material of construction.
                    b) Thermal deformation.
                    c) Cooling medium.
   33. Compare the uniflow scavenging with reversed flow or loop scavenging pertaining to diesel
       engines.
   34. Explain the need and utility of after cooler in a turbo-charging system of a marine diesel engine.
   35. Discuss supercharging of a Marine Diesel engine by constant pressure and pressure pulse
       systems. Comment on their relative advantages and disadvantages.
   36. Enumerate the troubles that might arise due to improper cooling of engines.
   37. a) Discuss the cause and origins of scavenge fires.
           b) How are they detected?
           c) What precautions are taken to prevent damage?
   38. Why an engine is called an Internal Combustion Engine?
   39. Describe a Trunk Piston Diesel Engine?
   40. Name and describe two principal constructional differences between a uniflow type large slow
       speed engine and a similar loop scavenging Marine Diesel Engines.
   41. What are bedplate and frames pertaining to an internal combustion engine?
   42. Draw a valve Timing Diagram of a 4 stroke Diesel Engine.
   43. What purposes does Scavenging process serve in a 2-Stroke Diesel Engine?
   44. What is a Supercharged Engine?
   45. What is the purpose of Supercharging?
   46. Sketch and describe with the help of a line diagram a Turbo Charger suitable for a large diesel
       engine.
   47. What is the significance of turbulence during combustion process?
   48. Indicate two main requirements of a fuel Injector.
   49. What are the four phases of combustion and explain each one.
   50. Why does a piston ring break?
   51. Explain in detail why are A-frames required for Marine Diesel engines?
   52. What are the advantages of 4-stroke cycle engine over 2-stroke cycle engine with reference to
       scavenging?
   53. Draw the pressure-crank angle diagram during scavenging process of a 2-stroke cycle engine.
   54. How does a scavenge fire manifest itself and how would you avoid it?
   55. Why is oil mist detection important in a diesel engine?
   56. Discuss the valve timings of a 4-stroke cycle engine for Marine applications and the significance
       of various events.
   57. Discuss the valve timings of a large 2-stroke engine operating on uniflow scavenging and loop
       scavenging system.
   58. Discuss the constructional details of a cross head as fitted to a Marine Diesel engine.


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                                                                                                        212
                                                                                             Page: 3.



   59. Explain the requirements and the special features of a Marine Diesel Engine with long stroke
       fitted for main propulsion.
   60. Discuss the manufacturing details of connecting rods employed for Marine Engines.
   61. Discuss the types of crankshafts manufactured for use on a large slow speed diesel engine.
   62. Discuss the advantages of constant pressure type turbo charging over pulse method.
   63. Why is uniflow scavenging preferred over other methods of scavenging? Compare their relative
       performance in terms of scavenging efficiency.
   64. With the help of a neat sketch, explain the principle of working of turbo compressor.
   65. What are the requirements to be satisfied by the fuel injection system when residual fuel is
       burned in Marine Diesel engines?
   66. Discuss the causes and prevention of crank case explosions in Marine Engines.
   67. Discuss in detail combustion process in a Diesel engine.
   68. Clearly explain with the help of neat diagrams the theoretical working cycles of 4-stroke and 2-
       stroke Diesel Engines.
   69. Explain in detail the jacket water-cooling system for a Marine Diesel Engine, which uses
       telescopic pipes out side the engine casing.
   70. Explain with the help of a neat sketch the operation of a Rotary Scavenging Pump.
   71. Explain the Uniflow Scavenging in an Opposed Piston Engine.
   72. Clearly explain the different types of combustion chambers used for fuel mixing with air in a
       Diesel Engine.
   73. Explain in detail how a fuel is prepared for efficient combustion.
   74. What is the nature of the stressing of the ‗big end bearing bolts‘ of a 2 stroke cycle engine and
       how does this influence the material selection for the bolts?
   75. What is the purpose of cross head in a large marine diesel engine? State two important reasons
       why this is incorporated in such engines.
   76. Name and describe two essential safety devices fitted to the crankcase of a large marine diesel
       engine.
   77. What are the mountings fitted on a cylinder head of a diesel engine and explain their purpose.
   78. What is the safety feature and device employed in a diesel engine cylinder head and what does it
       protect the cylinders head from?
   79. A large marine diesel engine (usually of the 2 cycle type) is constructed as an assembly and not
       as a mono block. What is the single most important component in such an engine that holds all
       these together, also relieving these of the combustion loads?
   80. Sketch and describe a fuel cam, exhaust valve cam and an air-starting cam.
   81. Where leaf springs are used in a centrifugal clutch what is the preferred material and strength?
   82. Describe a centrifugal coupling employing slippers and its role in power transmission.
   83. What will happen if a solid non-ferrous bush bearing in a rigid housing begins to run dry and
       why?
   84. Why is the surface finish of a journal in a bearing very important?
   85. What conditions predetermine the type of bearing such as angular contact and spherical roller
       bearings to be employed?
   86. What is type of damage that can occur in the rolling contact bearings of a large Turbo charger?
       How is it obviated?


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                                                                                              Page: 4.




   87. The chain in the chain drive of the camshaft of a large marine diesel engine ‗stretches‘ during
       service. Describe why and how, what are the undesirable effects and methods of correction.
   88. Explain the role of the integral thrust bearing of a large marine diesel propulsion engine. How is
       it lubricated? What are the various types of surface failures due to wear when surfaces are in
       contact, stressed and in relative motion?
   89. What are the causes of crankcase explosion? Explain in detail how these could be avoided.
   90. What are the causes of scavenge fires? Explain in detail how these could be avoided.
   91. Discuss the importance of piston cooling of I.C. engines and the various cooling agents used in
       marine field and why?
   92. What are the stresses acting on a crankshaft? Outline how it is constructed to achieve the
       intended function.
   93. How is the crankshaft supported? How is the supporting structure connected to the hull of the
       ship?
   94. What is the single major constructional difference between a large marine diesel engine piston of
       a trunk type and a crosshead type engines? Illustrate with a sketch.
   95. Which is the hottest part of a marine diesel engine piston? Indicate by a sketch with approximate
       values of the prevalent temperature in the various spots of the piston.
   96. In a large marine diesel engine where do cams find their use and fulfill what functions? Discuss
       the profiles of the various cams employed and why they are chosen.
   97. In a large marine diesel engine how is efficient combustion achieved?
   98. What are the mechanical components associated with combustion process of a diesel engine and
       discuss the nature of these components.
   99. Why does explosion occur in a starting air pipe of a diesel engine? Explain in detail how this
       explosion could be avoided or eliminated.
   100.What are the methods that are adapted for tightening important large bolts and nuts in a large
       slow speed diesel engine?

**************************************Kv********************************************
Kv/ICE/BE/QB/03: End of random question bank.

				
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