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AUTO POWER PLANT




  AUTO POWER PLANT (U6) 98.604.
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MODULE- I

1. Introduction: Types of power plants, classification of engines, working of 2- stroke & 4- stroke
with relative merits & demerits

2. Constructional details of engine components: cylinders-types, cylinder liners, engine block, types
of cylinder head, Gasket materials, piston types, function, materials, piston rings, piston pins,
connecting rod, crank shaft, flywheel, camshaft, valve & valve mechanism, Inlet & Exhaust
manifold construction, Hydraulic tappets.

MODULE-II

1. Fuel supply system in petrol engines: Types of fuel feed systems, fuel Pumps & filters,
construction, air filter types & construction, carburetion, simple carburetor, Different circuits in
carburetor, types of carburetors.

2. Fuel supply system in diesel engines: cleaning system, transfer pump, injection pump, nozzles,
their functions & necessity, simple & multiple unit pump, CAV, Bosch pump. Modern distributor
type pumps, maximum & minimum speed governors, injection nozzle & types of injectors.

MODULE III

1. Cooling system: Necessity of cooling, types of cooling, Air cooling & water cooling, forced
circulation, thermostat, water pump, Radiator, pressurised cooling, antifreeze solution, liquid
cooling & Oil cooling

2. Lubrication system: Function of lubrication system, classifications of lubricants, types of
lubricants, properties of lubricants, oil filter, oil pumps, crankcase ventilation, oil additives, and
specifications of lubricants.

References:

1. Newton and Steeds - Motor vehicle, Illiffee publications, London.

2. Kripal Singh - Automobile engineering Volume I & II, standard publications, Delhi

3. Joseph Heitner - Automobile mechanics, CBS publishers, New Delhi

4. T.R.Banga and Nathu Singh - Automobile Engineering, Khanna publishers, Delhi.

5. A.W Judge - Modern petrol engine, Chapman & hall, London

6. P.M. Heldt - High speed diesel engines Chillon Co, New York.



                                     AUTO POWER PLANT
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(MODULE- I: Introduction: Types of power plants, classification of engines, working of 2- stroke
& 4- stroke with relative merits & demerits)

The distinctive feature of our civilization today, one that makes it different from all others, is the
wide use of mechanical power. At one time, the primary source of power for the work of peace or
war was chiefly man's muscles. Later, animals were trained to help and afterwards the wind and the
running stream were harnessed. But, the great step was taken in this direction when man learned the
art of energy conversion from one form to another. The machine which does this job of energy
conversion is called an engine.
Definition of Engine: An engine is a device which transforms one form of energy into another
form. However, while transforming energy from one form to another, the efficiency of conversion
plays an important role. Normally, most of the engines convert chemical energy of the fuel into
thermal energy and this into mechanical work. Therefore they are called 'heat engines'.

Power Plant: The power plant for a vehicle is an engine which provides the motive power to all
the units and the parts which need power. All the major units of the vehicle functions due to power
from the power plant only. Automobiles can be classified into many and various ways, but when
classified by types of engines, it would be as follows:

1. Gasoline automobile. This is an automobile that uses the gasoline engine as the prime mover.
Compared to the other engines, the gasoline type has numerous advantages including ease of
operation and ability to produce high output in relation to size and weight. When the advance and
development of the automobile is acclaimed today, it is only the gasoline automobile that is
receiving the acclaim,

2. Diesel automobile. This automobile uses as its prime mover the Diesel engine which was
developed sometime after the gasoline engine. The Diesel engine uses the cheaper heavy or light
petroleum oils as fuel and has found wide application in buses and trucks. It could be said today
that gasoline engines are intended for use in passenger cars and the Diesel engines for trucks and
buses and other heavy vehicles.

3. Gas turbine automobile. This automobile uses gas turbine as the prime mover and is still in the
experimental stage in the United States and other nations. Compared to the piston reciprocating
internal combustion engine, the gas turbine has less vibration, capable of producing higher speed,
and can operate on a wide range of low cost fuel. These advantages promise a bright future for gas
turbine automobiles.

4. Electric & Hybrid automobile. In the electric automobile, the electricity charged in the batteries
is used to turn the motor and run the automobile. These batteries are heavy and moreover cannot be
recharged readily and because of these defects, there are practically few electric automobiles in use
today. However, it has the advantage of not emitting exhaust gas to cause air pollution, a serious
social problem of today. If new high-energy density batteries can be developed, the electric
automobile would surely find considerable use. Hybrid vehicle is an improvement over electric car,
which uses IC engine along with electric motor powered by the batteries.
Definition of ‘Heat Engine’: Heat engine is a device which transforms the chemical energy of a
fuel into thermal energy and utilizes this thermal energy to perform useful mechanical work. Thus,
thermal energy is converted to mechanical energy in a heat engine.
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Heat engines can be broadly classified into two categories
(a) External combustion engines and (b) Internal combustion engines

External combustion engines are those in which combustion takes place outside the engine whereas
in internal combustion engines combustion takes place within the engine. For example, in a steam
engine or a steam turbine, the heat generated due to the combustion of fuel is employed to generate
high pressure steam which is used as the working fluid in a reciprocating engine or a turbine.
Another example of an external combustion engine is a closed cycle gas turbine plant in which heat
of combustion in an external furnace is transferred to gas, usually air, which is used in a gas
turbine. Stirling engine is also an external combustion engine.
In an internal combustion engine the products of combustion of fuel and air within the cylinder
form the working fluid. Petrol, gas, and diesel engines, Wankel engine, and open cycle gas turbines
are examples of internal combustion, engines. Jet engines and rockets are also internal combustion
engines. In case of gasoline or diesel engines, the products of combustion generated by the
combustion of the fuel and air within the cylinder form the working fluid.

Classification and Some Basic Details of Heat Engines

Engines whether Internal Combustion or External Combustion are of two types, viz.,

(i) Rotary engines
(ii) Reciprocating engines

Of the various types of heat engines, the most widely used ones are the reciprocating internal
combustion engine, the gas turbine and the steam turbine. The steam engine is rarely used
nowadays. The reciprocating internal combustion engine enjoys some advantages over the steam
turbine due to the absence of heat exchangers in the passage of the working fluid (boilers and
condensers in steam turbine plant). This results in a considerable mechanical simplicity and
improved efficiency of the internal combustion engine.

Another advantage of the reciprocating internal combustion engine over the other two types is that
all its components work at an average temperature which is much below the maximum temperature
of the working fluid in the cycle. This is because the high temperature of the working fluid in the
cycle persists only for a very small fraction of the cycle time. Therefore, very high working fluid
temperatures can be employed resulting in higher thermal efficiency.
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Further, in internal combustion engines, higher thermal efficiency can be obtained with moderate
maximum working pressure of the fluid in the cycle, and therefore, the weight to power ratio is less
than that of the steam turbine plant. Also, it has been possible to develop reciprocating internal
combustion engines of very small power output (power output of even a fraction of a kilowatt) with
reasonable thermal efficiency and cost.

The main disadvantage of this type of engine is the problem of vibration caused by the
reciprocating components. Also, it is not possible to use a variety of fuels in these engines. Only
liquid or gaseous fuels of given specification can be efficiently used. These fuels are relatively
more expensive.

Considering all the above factors the reciprocating internal combustion engines have been found
suitable for use in automobiles, motor-cycles and scooters, power boats, ships, slow speed aircraft,
locomotives and power units of relatively small output.

BASIC ENGINE COMPONENTS AND NOMENCLATURE

Even though reciprocating internal combustion engines look quite simple, they are highly complex
machines. There are hundreds of components which have to perform their functions satisfactorily to
produce output power. The important engine components and the nomenclature associated with an
engine are given in the following sections.

A cross section of a single cylinder engine with the major components of the engine and their
functions are briefly described below.




Cylinder: As the name implies it is a cylindrical vessel or space in which the piston makes a
reciprocating motion. The varying volume created in the cylinder during the operation of the engine
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is filled with the working fluid and subjected to different thermodynamic processes. The cylinder is
supported in the cylinder block. The nominal inner diameter of the working cylinder is called the
cylinder bore and is designated by the letter d and is usually expressed in millimeter (mm).

Piston: It is a cylindrical component fitted into the cylinder forming the moving boundary of the
combustion system. It fits perfectly (snugly) into the cylinder providing a gas-tight space with the
piston rings and the lubricant. It forms the first link in transmitting the gas forces to the output
shaft. The area of a circle of diameter equal to the cylinder bore is called the piston area and is
designated by the letter A and is usually expressed in square centimeter (cm 2).

Combustion Chamber: The space enclosed in the upper part of the cylinder, by the cylinder head
and the piston top during the combustion process is called the combustion chamber.

Dead Centre: The position of the working piston and the moving parts which are mechanically
connected to it, at the moment when the direction of the piston motion is reversed at either end of
the stroke is called the dead centre. There are two dead centres in the engine as indicated in Fig.
They are (i) Top Dead Centre (ii) Bottom Dead Centre.
Top Dead Centre (TDC) is the dead centre when the piston is farthest from the crankshaft. It is
designated as TDC. It is also called the Inner Dead Centre (IDC). Bottom Dead Centre (BDC) is the
dead centre when the piston is nearest to the crankshaft. It is designated as BDC. It is also called the
Outer Dead Centre (ODC).

Stroke (L): The nominal distance through which a working piston moves between two successive
reversals of its direction of motion is called the stroke. Ie, it is the distance traveled by the piston
between TDC and BDC. One stroke is equal to 1800 or half rotation of the crank shaft. Half of the
stroke is referred as the crank-throw (The distance between the centre line of the crank shaft and
the crank pin). Stroke is designated by the letter L and is expressed usually in millimeter (mm).

Displacement or Swept Volume (Vs): The nominal volume swept by the working piston when
traveling from one dead centre to the other is called the displacement volume. It is expressed in
terms of cubic centimeter (cc) and given by


Engine capacity is the total piston displacement of all the cylinders. So it is the product of number
of cylinders and the swept volume of one cylinder and expressed in cubic centimeter (cc).

Clearance Volume (Vc): The nominal volume of the combustion chamber above the piston when
it is at the top dead centre is the clearance volume. It is designated as Vc and expressed in cubic
centimeter (cc).

Compression Ratio (r): It is the ratio of the total cylinder volume when the piston is at the bottom
dead centre, VT, to the clearance volume, Vc. It is designated by the letter r.



Mean effective pressure: This is the average effective pressure throughout the whole power
stroke. In fact the cylinder pressure varies considerably during the power stroke. Thus it is more
helpful to refer to the mean pressure instead. It is expressed in bars or kilo Pascals.(1 bar = 100
kPa).
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Power: It is the work done in a given period of time. Doing the same amount of work in a lesser
time would require more power.

Indicated Horse Power (IHP): The power developed within the engine cylinders is called
indicated power. This is calculated from the area of the engine indicator diagram. It is usually
expressed in kilowatts (kW).

Brake Horse Power (BHP.): This is the actual power delivered at the crankshaft. It is obtained by
deducting various power losses in the engine from the indicated power. It is measured with a
dynamometer and is expressed in kilowatts (kW). It is always less than the Indicated Power, due to
frictional and pumping losses in the cylinders and the reciprocating mechanism.

Engine torque: It is the force of rotation acting about the crankshaft axis at any given instant of
time.
                       It is given by, T = F x r
Where, T is the engine torque (Nm), F is the force applied to the crank (N) and 'r' is the effective
crank-radius (m). As the value of 'r' varies during the power-stroke, the torque on the power-stroke
is continually varying. Moreover, there is no torque delivered during the three idle strokes.
Therefore, the engine manufactures always quote the average value of torque throughout the engine
cycle. Engine torque goes through the vehicle transmission system, to the road wheels and is
responsible for rotation of the latter and hence for pulling of the vehicle.

IC ENGINE CLASSIFICATION
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The IC engine can be classified on the basis of cycle operation in cylinder, type of fuel, method of
supply of fuel, type of ignition, etc.

1. Basic engine design: Reciprocating engines, rotary (Wankel) engines.

2. Working cycle: Engines working on Otto cycle (spark-ignition or S.I. engines), and engines
working on diesel cycle (compression-ignition or C.I. engines).

3. Number of strokes: Four-stroke engines and two-stroke engines (both SI and CI engines).

4. Fuel used: Gasoline (or petrol), compressed natural gas (CNG), liquefied petroleum gas (LPG),
diesel oil (light, diesel oil, LDO and high speed diesel oil, HSD), fuel oil, alcohols (methanol,
ethanol). Dual fuel and multi-fuel engines have also been developed.

5. Fuel supply and mixture preparation:
        (a) Carbureted types, fuel supplied through carburetor.
        (b) Injection type:
                (i) Fuel injected into inlet ports or inlet manifold.
                (ii) Fuel injected into the cylinder just before ignition.

6. Method of ignition: In S.I. engines battery ignition or magneto ignition.

7. Method of cooling: Water cooled liquid cooled or air cooled.

8. Cylinder arrangement: Inline, V, radial, opposed, radial, etc.

9. Valve or port design and location: Overhead (I head), side valve (L bead) valves & in two stroke
engines: cross scavenging loop scavenging, uniflow scavenging.

10. Application: Automotive engines for land transport, marine engines for propulsion of ships,
aircraft engines for aircraft propulsion, industrial engines, prime movers for electrical generators.

THE WORKING PRINCIPLE OF ENGINES

If an engine is to work successfully then it has to follow a cycle of operations in a sequential
manner. The sequence is quite rigid and cannot be changed. In the following sections the working
principle .of both SI and CI engines is described. Even though both engines have much in common
there are certain fundamental differences.
The credit of inventing the spark-ignition engine goes to Nicolaus A. Otto (1876) whereas
compression-ignition engine was invented by Rudolf Diesel (1892). Therefore, they are often
referred to as Otto engine and Diesel engine.

ENGINE CYCLES

In an internal combustion engine when combustion of fuel takes place, then in this process some
complicated chemical, thermal and physical changes occur. Friction occurs, between the working
medium and the engine and between the working parts of the engine. The account of these all the
variables create a very complex problem. Therefore, a theoretical approximate method is adopted to
account these all variables, which is called "power cycle". This cycle is based on different
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simplified assumptions thereby increasing the accuracy of the engine. The commonly used cycles
on which I.C. engines work are as given under.
(i) Otto cycle.

This is the cycle on which gas, petrol and many other types of fuel engines run. It is very common
for spark ignition engines. The theoretical as well as actual Otto cycle is shown by the graphs in
Fig. The thermodynamic process take, place during the cycle are as given under.




(a) Process 1-2: Adiabatic Compression or work done on the working medium by the piston.
(b) Process 2-3: constant volume, Heat supplied instantaneously.
(c) Process 3-4: Adiabatic Expansion or work done by working medium on the piston.
(d) Process 4-1: Constant volume, Heat rejected instantaneously. In this cycle heat is supplied at
constant volume, therefore it is often called the "constant volume cycle".




The actual Otto cycle is slightly modified when used in I.C. engines. The diagram of actual Otto
cycle is shown in Fig, which consists of four strokes as given under:

(a) Suction stroke 1-2: A charge is drawn into the cylinder by the outward movement of the piston.
(b) Compression stroke 2-3: The charge is compressed adiabatically by the return stroke of the
piston, to the clearance volume of the cylinder.

(c) Constant volume 3-4: The compressed charge is ignited at point 3 by an electric spark thereby
increasing pressure to 4.

(d) Working stroke 4-5: The burnt gas expands approximately adiabatically, thereby forcing the
piston through the working stroke 4-5. This stroke provides power to the engine and also drives the
piston through the other three strokes of the cycle. The excess energy during this process is stored
in the fly-wheel.
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(e) Exhaust stroke 5-1: During this process the burnt gases are pushed out through an exhaust port
by the inward movement of the piston, thereby completing the cycle.

(ii) Diesel cycle.
Referring to Figure, the theoretical diesel cycle consists of two adiabatics, one constant pressure
line, and one constant volume line. This cycle is very important and popular for the diesel engines.
The basic difference between the Otto and diesel cycle is, in the heat supplied system. In the Otto
cycle heat is supplied at constant volume, while in the diesel cycle the heat is supplied at constant
pressure.




The diesel cycle shown in Fig consists of the thermodynamic processes as given under:

(a) Adiabatic 1-2: Compression or work done on the working medium by the piston.
(b) Constant pressure 2-3: Heat supplied and part of work done.
(c) Adiabatic 3.4: Expansion or remaining work done by the working medium on the piston.
(d) Constant volume 4-1: Heat rejected instantaneously

In this cycle heat is supplied at constant pressure therefore it is often called "the constant pressure
cycle". The air standard efficiency of this cycle is as given under:




The actual diesel cycle is slightly modified when used in I.C. engines. Because the engine uses
heavy oil as a fuel which differs irk ignition. The engine needs, an accessory, an oil pump or air
compressor for forcing the oil fuel into the cylinder. Therefore the engine based on this cycle is
called "compression ignition engine". The diagram of actual diesel cycle is shown in Fig which
consist four strokes as given under:

(a) Suction stroke 1-2: During this stroke the pure air is drawn into cylinder by the outward
movement of the piston.
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(b) Compression stroke 2-3: During this stroke pure air is compressed to the clearance volume by
the inward movement of the piston. The air compression at this stage is 40 times the atmosphere.
The temperature at this stage is sufficient to ignite the fuel.
(c) Constant pressure 3-4: Under this condition, the fuel is forced into the cylinder by the oil pump
or air compressor, which is immediately ignited by the hot air in the cylinder. Therefore moving the
piston outward at the constant pressure up to the point of cut off.
(d) Working stroke 4-5: During this stroke the mixture of burnt fuel and air expands adiabatically
to 5. Thereby moving the piston outward and supplying the power to engine crankshaft. This power
also drives the oil pump or air compressor as well to overcome the resistance of the other three
strokes of the cycle.
(e) Exhaust stroke 5-1: During this stroke the piston moves inward thereby forcing out the burnt
gases of the cylinder, thus to complete the diesel cycle.


FOUR-STROKE SPARK IGNITION ENGINE

In a four-stroke engine, the cycle of operations is completed in four strokes of the piston or two
revolutions of the crankshaft. During the four strokes, there are five events to be completed, viz.,
suction, compression, combustion, expansion and exhaust. Each stroke consists of 1800 of
crankshaft rotation and hence a four-stroke cycle is completed through 7200 of crank rotation. The
cycle of operation for an ideal four-stroke SI engine consists of the following four strokes

(i) Suction or intake stroke (ii) Compression stroke (iii) Expansion or power stroke (iv) Exhaust
stroke
(i) Suction or Intake Stroke: Suction stroke
starts when the piston is at the top dead centre
and about to move downwards (0 to 1 in PV
diagram). The inlet valve is open at this time
and the exhaust valve is closed, Fig (a). Due to
the suction created by the motion of the piston
towards the bottom dead centre, the charge
consisting of fuel-air mixture is drawn into the
cylinder. When the piston reaches the bottom
dead centre the suction stroke ends and the
inlet valve closes.

(ii)Compression Stroke: The charge taken into the cylinder during the suction stroke is compressed
by the return stroke of the piston (1 to 2 in PV diagram). During this stroke both inlet and exhaust
valves are in the closed position, Fig (b). The mixture which fills the entire cylinder volume is now
compressed into the clearance volume. At the end of the compression stroke the mixture is ignited
with the help of an electric spark between the electrodes of a spark plug located on the cylinder
head. Burning takes place almost instantaneously when the piston is at the top dead centre and
hence the burning process can be approximated as heat addition at constant volume. During the
burning process the chemical energy of the fuel is converted into heat energy producing a
temperature rise of about 20000C (Process 2 to 3 of PV diagram). The pressure at the end of the
combustion process is considerably increased due to the heat release.

iii) Expansion or Power Stroke: The high pressure of the burnt gases forces the piston towards the
BDC, (process 3 to 4 of PV diagram), with both the inlet and exhaust valves remaining closed,
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Fig (c). Thus, power is obtained during this stroke. Both pressure and temperature decrease during
expansion process.

(iv) Exhaust Stroke: At the end of the expansion stroke the exhaust valve opens and the inlet valve
remains closed, Fig (d). The pressure falls to atmospheric level as a part of the burnt gases escape.
The piston moves from the bottom dead centre to top dead centre (process 5 to 0 of PV diagram)
and sweeps the burnt gases out from the cylinder almost at atmospheric pressure. The exhaust valve
closes at the end of the exhaust stroke and some residual gases trapped in the clearance volume
remain in the cylinder. These residual gases mix with the fresh charge coming in during the
following cycle, forming its working fluid.

 The details of various processes of a four-stroke Spark Ignition engine with side valves are shown
   in Fig. The ideal indicator diagram, showing the PV plot for the four-stroke SI engine is also




     shown.

Each cylinder of a four-stroke engine completes the above four operations in two engine
revolutions, ie, one revolution of the crankshaft occurs during the suction and compression strokes
and the second revolution during the power and exhaust strokes. Thus, there is only one power
stroke out of four strokes and to complete one cycle, the crankshaft has to turn two revolutions.

FOUR-STROKE COMPRESSION IGNITION ENGINE
The four-stroke C1 engine is similar to the four-stroke SI engine but it operates at a much higher
compression ratio. The compression ratio of an SI engine varies from 6 to 11 while for a CI engine
it is from 14 to 22.
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In the CI engine during suction stroke, air,
instead of a fuel-air mixture, is inducted. Due
to the high compression ratio employed, the
temperature at the end of the compression
stroke is sufficiently high to self ignite the
fuel which is injected into the combustion
chamber. In CI engines, a high pressure fuel
pump and an injector are provided to inject
the fuel into the combustion chamber. The
carburetor and ignition system necessary in
the SI engine are not required in the CI
engine. The ideal sequence of operations for
the four-stroke CI engine is 1lows:

Suction Stroke: Filtered air alone is inducted
during the suction stroke. During this stroke
intake valve is open and exhaust valve is
closed, Fig (a).

Compression Stroke: Air inducted during the suction stroke is compressed into the clearance
volume. Both valves remain closed during this stroke, Fig (b).

Expansion or Power Stroke: Fuel injection into the compressed air in the cylinder starts nearly at
the end of the compression stroke. The rate of injection is such that the combustion maintains the
pressure constant in spite of the piston movement on its expansion stroke, increasing the volume.
Here the heat is assumed to have been added at constant pressure. After the injection of fuel is
completed (i.e. after fuel cut-off) the products of combustion expand. Both the valves remain closed
during the expansion stroke, Fig(c).

Exhaust Stroke: The piston traveling from BDC to TDC pushes out the products of combustion.
The exhaust valve is open and the intake valve is closed during this stroke, Fig (d).
Due to higher pressures in the cycle of operations the CI engine has to be more sturdy than a spark-
ignition engine for the same output. This results in a CI engine being heavier than the SI engine.
However, it has a higher thermal efficiency on account of the high compression ratio (of about 22
as against about 11 in SI engines) used.

TWO-STROKE ENGINE

As already mentioned, if the two unproductive strokes, viz., the suction and exhaust could be
served by an alternative arrangement, especially without the movement of the piston then there will
be a power stroke for each revolution of the crankshaft. In such an arrangement, theoretically the
power output of the engine can be doubled for the same speed compared to a four-stroke engine.
Based on this concept, Dugald Clark (1878) invented the two-stroke engine.
In two-stroke engines the cycle is completed in one revolution of the crankshaft. The main
difference between two-stroke and four stroke engines is in the method of filling the fresh charge
and removing the burnt gases from the cylinder. In the four-stroke engine these operations are
performed by the engine piston during the suction and exhaust strokes respectively. In a two-stroke
engine, the filling process is accomplished by the charge compressed in crankcase or by a blower.
The induction of the compressed charge moves out the product of combustion through exhaust
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ports. Therefore, no piston strokes are required for these two operations. Two strokes are sufficient
to complete the cycle, one for compressing the fresh charge and the other for expansion or power
stroke.




The different stages of the two-stroke cycle are illustrated in Fig. Referring to the fig(a), the
exhaust port is in closed position and inlet port is in open position. The piston moves upward by the
momentum of the flywheel, thereby compressing the charge at the top of the piston and entering the
new charge through the inlet port into the crank case due to creating the vacuum by the piston.
As soon as the piston reaches just at the top dead position the spark ignition takes place and thereby
increasing the temperature and the pressure inside the cylinder. Due to explosion, the hot gases
expand and thus piston moves downward. Refer Fig (b), both the ports are in closed position and
the piston is still in downward movement which compresses charge available in the crank case.

As soon as the piston moves slightly more downward, first exhaust port becomes open and then
transfer port, thereby scavenging the burnt gases and entering the new charge into the cylinder as
shown by the directions of arrows in Fig (c). The process is completed by reaching the piston at
bottom dead position. Then piston again moves upward and closes both transfer as well exhaust
port, while the inlet port is already in closed position as shown in Fig (d). Now the engine has
completed one cycle and crank shaft has been turned through one rotation and the engine is ready
just to start the next cycle. In this way every rotation of the crankshaft of this engine consists one
useful or power stroke thereby realizing double power than four-stroke engine of the same size.

Scavenging: The burnt gases escape from the cylinder by virtue of the excess of their own pressure
over that in the silencer or atmosphere. But the final clearing is done by the forcing of the air or the
mixture of the charge into the cylinder which is known as "scavenging". There are three different
methods of scavenging in the two-stroke cycle:

(a) Mixture compressing in crank-case: In this method the mixture of air and fuel is compressed in
the crank-case by the piston during working-stroke. This method is adopted in petrol engines or
engines working on constant volume cycle.
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(b) Pure air compressing in crank-case: In this method the pure air is compressed in the crank-case
by the piston during its working stroke, which is utilized for scavenging purposes. This method is
adopted in diesel engines or engines working on constant pressure and dual combustion cycle.

(c) Pure air compressing by a pump: In this method pure air is compressed by a separate pump. The
pump is driven by the crank-shaft of the engine. This method is adopted in both the petrol as well
diesel engines.

COMPARISON OF SI AND CI ENGINES

Description            SI Engine                          CI Engine
Basic cycle            Otto cycle or constant volume heat Diesel cycle or constant pressure heat
                       addition cycle.                    addition cycle.

Fuel                   Gasoline, a highly volatile fuel. Self- Diesel oil, a non-volatile fuel. Self-
                       ignition temperature is high.           ignition temperature is comparatively
                                                               low.

Introduction of fuel   A gaseous mixture of fuel and air is      Fuel is injected directly into the
                       introduced during the suction stroke. A   combustion chamber at high pressure
                       carburettor is necessary to provide the   at the end of the compression stroke.
                       mixture.                                  A fuel pump and injector are
                                                                 necessary.
Load control           Throttle controls the     quantity   of
                       mixture introduced.                       The quantity of fuel is regulated in the
                                                                 pump. Air quantity is not controlled.
Ignition               Requires an ignition system with spark
                       plug in the combustion chamber. Self-ignition occurs due to high
                       Primary voltage is provided by a temperature of air because of the high
                       battery or a magneto.                  compression. Ignition system and
                                                              spark plug are not necessary.
Compression ratio      Up to 11. Upper limit is fixed by
                       antiknock quality of the fuel.         14 to 22. Upper limit is limited by
                                                              weight increase of the engine.
Speed                  Due to light weight and also due to
                       homogeneous combustion, they are Due to heavy weight and also due to
                       high speed engines                     heterogeneous combustion, they are
                                                              low speed engines
Thermal efficiency     Because of the lower CR, the
                       maximum value of thermal efficiency Because of higher CR, the maximum
                       that can be obtained is lower.         value of thermal efficiency that can be
                                                              obtained is higher.
Weight                 Lighter due to lower peak pressures.
                                                              Heavier due to, higher peak pressures.
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      COMPARISON BETWEEN FOUR-STROKE AND TWO-STROKE ENGINES:


          Four-stroke cycle Engine                              Two-stroke cycle Engine


1. The cycle is completed in four strokes of the   The cycle is completed in two-strokes of the
piston or in two revolutions of the crankshaft.    piston or in one revolution of the crankshaft.
Thus one power stroke is obtained in every two     Thus one power stroke is obtained in each
revolutions of the crankshaft.                     revolution of the crankshaft.

2. Because of the above, turning movement is More uniform turning movement and hence
not so uniform and hence heavier flywheel is lighter flywheel is needed.
needed.
                                              Because of one power stroke for one revolution,
3. Again, because of one power stroke for two power produced for same size of engine is more
revolutions, power produced for same size of (theoretically twice, actually about 1.3 times), or
engine is small, or for the same power the for the same power the engine is light and
engine is heavy and bulky.                    compact.

4. Because of one power stroke in two      Because of one power stroke in one revolution
revolutions lesser cooling and lubrication greater cooling and lubrication requirement.
requirements. Lesser rate of wear and tear.Greater rate of wear and tear.
                                           Two-stroke engines have no valves but only
5.The four-stroke engine contains valves and
valve mechanism.                           ports (some two-stroke engines are fitted with
                                           conventional exhaust valve or reed valve).
6. Because of the heavy weight and Because of light weight and simplicity due to
complication of valve mechanism, higher in the absence of valve mechanism, cheaper in
initial cost.                              initial cost.

7. Volumetric efficiency more due to greater       Volumetric efficiency less due to lesser time for
time of induction.                                 induction.

8. Thermal efficiency higher, part load Thermal efficiency lower, part load efficiency
efficiency better than two stroke cycle engine. lesser than four-stroke cycle engine. In two-
                                                stroke petrol engines some fuel is exhausted
                                                during scavenging.

                                                  Used were (a) low cost, and (b) compactness
9. Used where efficiency is important, in cars, and light weight important. Two-stroke (air-
buses, trucks tractors, industrial engines, aero- cooled) petrol engines used in very small sizes
planes, power generation, etc.                    only: lawn mowers, scooters, motor cycles,
                                                  mopeds etc. (Lubricating oil mixed with petrol).

                                                   Two-stroke diesel engines used in very large
                                                   sizes, more than 60 cm bore, for ship propulsion
                                                   because of low weight and compactness.
                                                                                                     17
The two-stroke engine was developed to obtain a greater output from the same size of the engine.
The engine mechanism also eliminates the valve arrangement making it mechanically simpler.
Almost all two stroke engines have no conventional valves but only ports (some have an exhaust
valve). This simplicity of the two-stroke engine makes it cheaper to produce and easy to maintain.
Theoretically a two-stroke engine develops twice the power of a comparable four-stroke engine
because of one power stroke every revolution (compared to one power stroke every two revolutions
of a four-stroke engine). This makes the two-stroke engine more compact than a comparable four-
stroke engine In actual practice power output is not exactly doubled but increased by only about
30% because of (i) reduced effective expansion stroke and (ii) increased heating caused by
increased number of power strokes which limits the maximum speed. The other advantages of the
two-stroke engine are more uniform torque on crankshaft and comparatively less exhaust gas
dilution. However when applied to the spark-ignition engine the two-stroke

IC ENGINE CLASSIFICATION BY CYLINDER ARRANGEMENT

One of the most common methods of classifying the reciprocating engines is by cylinder
arrangements. A number of cylinder arrangements popular with designers are described below.
Two terms used in connection with cylinder arrangements must first be defined.

Basic Type of Cylinder Arrangements

1. In-line engines. In-line engine is an engine with one cylinder bank, i.e., all cylinders are arranged
linearly, and transmit power to a single crankshaft. This type is very popular with automobiles
where 4 and 6 cylinder in-line engines are quite common.

2. V engines. An engine with two
cylinder banks (i.e., two in-line
engines) inclined at an angle to
each other and with one
crankshaft. Most of the bigger
automobiles use the 8-cylinder V-
engine (4-cylinder in-line on each
side of the P).
3. Opposed cylinder engine. An
engine with two cylinder banks
located in the same plane on
opposite sides of the crankshaft. It
can be visualized as two 'in-line'
arrangements 180 degrees apart. It
is inherently well-balanced and
has the advantages of a single
crankshaft. This design has been
used in small aircrafts.
4. Opposed piston engine. When a
single cylinder houses two
pistons, each of which drives a
separate crankshaft, it is called an
'opposed piston' type of engine.
The Opposed piston arrangement like opposed-cylinder arrangement is inherently well balanced.
                                                                                                   18
Further, it has the advantage of requiting no cylinder head. In addition the relative piston velocity
rate of change of volume) is doubled for a given crank and piston speed. As shown, this
arrangement lends itself to cylinder porting and straight flow-through of gases for scavenging,
where the openings of the inlet and exhaust ports are controlled, by the position of pistons. In
Junker's design both the pistons drive the same crankshaft, the farther piston being connected to the
crankshaft by two side connecting rods.
5. Radial engine. Radial engine is an engine with more than two cylinders in each row equally
spaced around the crankshaft. The radial engine is most commonly used in conventional air-cooled
aircraft engines where three, five, seven, or nine cylinders may be used in one bank and two or
three banks may be used. The radial engine presents the problem of fastening 3, 4, 7 or 9
Connecting rods to a single crank. A master rod is guided by the crank and articulated rods are
attached to the master rod. It should be noted that the master rod executes the same motion as the
connecting rod in other conventional engines, while an articulated rod follows a slightly different
path since the point of attachment is not at the centre of the crankpin. Vertical shaft radial engines
are used in large stationary power plants with vertical shaft generators mounted below. An odd
number of cylinders per bank is necessary with alternate-cylinders firing in successive revolutions
for four-stroke cycle radial engines, but any number of cylinders can be used for two-stroke
engines.
Besides the above important types of cylinder arrangements, the other types which have been used
are as follows:
6. X-type. This design is a variation of V type. It has four banks of cylinders attached to a single
crankshaft.
7. 'H-type: The 'H' type is essentially two 'opposed cylinder 'types, utilizing two separate, but
interconnected crankshafts.
8. U- type: The U type is a variation of opposed piston arrangement.
9. Delta type: The delta type is essentially three opposed pistons with three crankshafts.

SI ENGINE CLASSIFICATION BY VALVE LOCATION

The SI engines (as well
as CI engines) may also
be classified by valve
location.           The
classification of SI
engines      by   valve
location is shown in
Fig. The T-head design
shown is now obsolete.
The side valve, or L-
head design was quite
popular up to 1960. The
most popular design
today is the overhead
valve design, which is
also called I-head or
valve-in-head engine. A
combination of side
valve      design   and
overhead valve design
                                                                                                    19
is occasionally made to give an F-head. Here the intake valve is located in the head (overhead)
while the exhaust valve is located in the block (overhead).

IC ENGINE CLASSIFICATION BASED ON TYPE OF VALVE

On the basis of type of valves, the automobile engines are classified as i) Poppet valve engines, ii)
Sleeve valve engines iii) Rotary valve engines.




               Sleeve valve                                          Rotary valve
(i) Poppet valve engines: Poppet valves are used in almost all internal-combustion engines which
operate on the four-stroke cycle. These valves are very simple in construction as well as reliable in
service. The different arrangement - of these valves in I.C. engines are shown in Fig above.

(ii) Sleeve valve engines: In these engines the inlet and exhaust ports are controlled by sliding the
sleeves. These sleeves act as cylinder and surround the piston. Sleeve valves may be of double-
sleeve and single-sleeve types. Principle of the single sleeve type valve is clear from the simplified
sketch given below. The sleeve crank is driven by the main crank-shaft of the engine. The sliding
sleeve is pivoted at the one end of the crank-shaft thereby sliding the sleeve up and down. Due to
this up and down movement of the sleeve, the inlet and exhaust ports are controlled. Engines based
on these valves are very simple, silent in operation, having symmetrical shape of the combustion
chamber and give higher thermal efficiency.

(iii) Rotary valve engines: In these engines the inlet and exhaust of the charge is controlled by a
rotary valve. The principle of the disc type rotary valve is clear from the simplified sketch given. It
consists of a rotating disc driven by the main crankshaft of the engine. The disc consists of a port
which alternately comes in front of the inlet as well as the exhaust ports thereby controlling the
charge or air entering to the cylinders. These engines are noise free but pressure sealing and high
consumption of lubrication are the main problems in them.

Gas turbine plants
Now these days on experimental basis the gas turbines are being used as power plants in
automobile trucks, buses, cars, boats and airlines. There is a difficulty to reduce the high speed of
the turbine by gear system, even though has given very fruitful results for road transportation. This
power plant is a simple heat engine which utilizes the expansion from the combustion of fuel and
air in a chamber. To understand the working cycle of a turbine, refer the given Fig. It consists of
three units
                                                                                                 20




.
(a) The 'compressor', which induces air through the air cleaner and silencer, from the atmosphere.
Then due to centrifugal force it compresses and thorough the air under pressure in to the
combustion chamber. This compressor has an independent shaft or may be mounted on the
common shaft.
(b) The combustion chamber, in which compressed air is received from the compressor, then fuel is
injected under pressure into it and burnt. These hot gases are then fed to the turbine. Gasoline,
kerosene or oil can be used as a fuel in this plant.
(c) The turbine, which consists vanes through them the hot gases from combustion chamber are
passed under pressure and thereby driving the common shaft and giving up the power to
compressor as well main drive. Finally the hot gases are released into the atmosphere. The power
from the main shaft is reduced through reduction gear and further utilized for driving the road
wheels of the vehicle.

The use of a gas turbine in transport vehicles is not so popular, even though it has some advantages
over a reciprocating type petrol engine, which are given below.

Advantages of gas turbine over reciprocating engine

1. It directly produces rotative effort and thereby minimizing the noise.
2. It runs more smoothly due to the absence of reciprocating parts.
3. It has compact shape.
4. It is a lighter in weight and has simple construction.
5. Cooling system is avoided and consumes negligible lubricant.
6. Gives high mechanical efficiency even at low internal pressure.
7. It has low maintenance cost.
8. It has negligible vibrations.
9. A fuel of poor quality can be utilized to run it efficiently.
10. It gives more fuel economy and has more life as well reliability.

NUMBERING OF ENGINE CYLINDERS

In case of in-line engines, cylinders are numbered from the front to the rear, number one cylinder
being at the front. For numbering of V engines, two different approaches have been adopted and
                                                                                                   21
both are in use. In one system (adopted by Ford
Motor Co.) one bank cylinders are numbered one
to four for V-8 engines, with number one at the
front, whereas the other bank cylinders are
numbered five to eight with number five at the
front (Fig b). In the second system (used by
General Motors), however, the cylinders are
numbered in die same order in which the
connecting rods are attached to the crankshaft
(Fig c) starting with number one at the front of the
crankshaft.

FIRING ORDER

To obtain best engine performance in multi
cylinder engines, the occurrence of power strokes
in various cylinders is not kept directly one after the other. For example, in 4cylinder engines it is
not kept as 1-2-3-4, but is provided as 1-3-4-2 or 1-4-3-2. Similarly firing order of a 6-cylinder in-
line engine is 1-5-3-6- 2-4 and in a V-8 engine it is 1-5-4-2-6-3-7-8 (wherein, cylinder nos. 1 and 5
are the front ones). The
driver must know the
correct firing order of the
engine so that he is able to
connect the H.T. wires
correctly to various spark
plugs. The occurrence of
various events in the 4-
cylinder engine having
firing order as 1-3-4-2 is
shown.

SQUARE ENGINE

This is an engine having stroke equal to its bore. In the ordinary engines the stroke / bore ratio is
more than 1. For the same crankshaft speeds, the square engines have lower piston speeds than the
corresponding engines of larger stroke. They develop more power and consume less fuel as
compared to the longer stroke engines. The examples are the Vauxhall and the Velox square
engines.

R.A.C. RATING

The formula adopted by the Royal Automobile Club of England has been used for taxation
purposes. It gives the horse power of an engine as:
H.P = (Diameter of cylinder in inches)2  x   (Number of cylinders) / 2.5
In deriving this formula, the conditions prevailing at that time were considered. These were:
Piston speed = 100 ft/min, Mean effective pressure = 90 psi, Mechanical Efficiency = 75 %

Due to improved design and better materials, all these factors have changed by now and it is seen
that the actual horse power of an engine today is about five times its R.A.C. rating.
                                                                                                    22
S.A.E. RATING

The Society of Automotive Engineers, U.S.A., have specified the method of measuring the power
output of an engine, for standardization purposes. The engine is run without the generator, air
cleaner, cooling fan, etc. However, the standard water circulating pump and fuel pump are fitted to
the engine. The horse power of the engine is measured with a dynamometer and the measured
values are then corrected for the standard temperature, pressure and humidity conditions prescribed.

DIN RATING

In this DIN (Deutsch Industries Normale of Germany) method, the horse power of the engine is
measured with all the accessories, e.g. generator, air cleaner, cooling fan, etc. fitted on the engine.
The measured value is corrected to the standard conditions of temperature, pressure and humidity.

          CONSTRUCTIONAL DETAILS OF ENGINE COMPONENTS
Module-I:Q.2. (Constructional details of engine components: cylinders-types, cylinder liners,
engine block, types of cylinder head, Gasket materials, piston types, function, materials, piston
rings, piston pins, connecting rod, crank shaft, flywheel, camshaft, valve & valve mechanism, Inlet
& Exhaust manifold construction, Hydraulic tappets.)

INTRODUCTION

The number of cylinders and their arrangements in the automobile engines may be different but the
principle, the constructional details and materials of their various parts are similar. Therefore to
understand the functions of the various engine parts and the trouble shooting as well as servicing of
the engines, the thorough study of the individual part is essential. The engine parts can be classified
in two groups as given under:
(i) Stationary parts These parts are such as engine block, cylinder head, crankcase, cylinder liners,
exhaust and inlet manifolds, exhaust and inlet ports, half bearings, tappet cover, oil sump, gaskets,
mufflers, carburetor, parts of ignition system, water cooling pipes etc. ,
(ii) Moving parts Crankshaft, flywheel, camshaft, connecting rods, Pistons, gudgeon pins, timing
gears, valves, valve-tappets, piston rings etc.
The main parts of an automotive engine are

1. Cylinder block and crankcase. 2. Cylinder head. 3. Sump or oil pan. 4. Manifolds.
5. Gaskets. 6. Cylinder. 7. Pistons. 8. Piston rings. 9. Connecting rods. 10. Piston pins.
11. Crankshaft. 12. Main bearings. 13. Valves and valve actuating mechanisms.
14. Mufflers.

CYLINDER BLOCK AND CRANKCASE

The basic framework of the engine is formed by the cylinder block. It houses the engine cylinders,
which serve as bearings and guides for the pistons reciprocating in them. Around the cylinders,
there are passages for the circulation of cooling water or liquid. Cylinder block also carries
lubrication oil to various components through drilled passages. At the lower end, crankcase is cast
integral with the block. Previously, crankcase was cast separately and attached to the block, but
now the integral or the mono-block construction is preferred. Crankcase is shaped simply like a box
having no bottom, its roof being formed by the lower deck of the cylinder block. The bottom of the
                                                                                                  23
crankcase walls is flanged to strengthen the casing and to provide a machined joint face for the
sump to be attached. The Crankshaft is supported in the crankcase through a number of bearings
called main bearings. The construction of the crankcase has to be such as to provide very high
rigidity, because it must, provide reactions for the heavy forces set up due to gas pressures in the
cylinders




.
At the top of the cylinder block is attached the cylinder head. Other parts like timing gear, water
pump, ignition distributor, flywheel, fuel pump, etc. are also attached to it. Camshaft may be
mounted in the cylinder block or head. It is always placed parallel to the crankshaft. When in the
cylinder block, it is mounted to one side of the cylinders either low down, just above the crankshaft
or high, slightly below the cylinder head. The crankshaft is supported in usually three bearings
force fit in the cylinder block bore.
The cylinder block is basically a casting product. Both mono-block and individual cylinder casting
techniques have been tried but the mono-block has proved more useful due to the following reasons
and is, therefore, being employed universally.

1. The problem of having good water sealed joints is reduced, because of reduction in their
quantity; more positive water circulation is achieved.
2. The manufacturing operations, like machining the joint faces, cylinder boring etc. are simplified.
3. Closer cylinder spacing is achieved, thus reducing engine size.
4. An assembly far rigid than in the individual cylinder construction is achieved, thus reducing
tendency to vibrate.

The individual cylinder construction, on the other hand, has its own advantages:

1. Replacement of a single-cylinder casting is less costly than replacing a multi-cylinder casting.
2. Because of lesser weight of individual cylinders, handling during repairs is easy.
3. In case of air cooled cylinders having fins around them to increase the heat transfer area, it is
easier to adopt individual cylinder construction than to cast en-block.
                                                                                                  24
The materials used for cylinder block are gray cast iron and aluminium alloys. Gray cast iron is so
called because of its gray appearance when fractured (compared to white appearance of white cast
iron fracture). In this the carbon is present as flake of graphite which makes it more wear and
corrosion resistant, apart from its better machinability. The aluminium alloys used for cylinder
block are the aluminium-silicon types which have good casting properties, corrosion-resistant and
retain their strength at moderate temperatures encountered in the engine block. Out of these two
materials, cast iron is the one which is mainly used because of the following advantages:

1. It is a good foundry material.       2. It has high machinability.
3. It does not wrap under the high temperatures and pressures developed in the cylinders.
4. Due to its slightly porous nature, it retains better the lubricating oil film
5. It does not wear too much. 6. It has sound-damping properties.
7. It has a low value of coefficient of thermal expansion. 8. It is relatively cheap.

Aluminium alloys have been used as alternative cylinder block material chiefly owing to their
lightness. The density of aluminium is about one third that of cast iron. However, considering the
lesser strength of aluminium, which necessitates the use of thicker sections to carry same load, the
saving in weight in case of aluminium alloys is only about 50% compared to cast iron. Besides this,
aluminium has a higher thermal conductivity than cast iron. This helps to run the engine cooler and
thereby making it possible to use higher compression ratios. However, it has the disadvantages of
more wear, to obviate which cast iron liners have to be used. Besides this, the threads in aluminium
are damaged easily in case of any mishandling. Further, since it cannot withstand high temperatures
unlike cast iron, there are more chances of damage occurring in case of overheat or any loss of
coolant. Aluminium alloy cylinder blocks with chromium plated cylinder bores have also been used
to obtain lightweight and also less wearing surface.
The gray cast iron for the cylinder block usually has the composition
3.5% carbon, 2.5% silicon and 0.65% manganese. Carbon serves to provide graphite which
improves lubrication; silicon provides wear resistance by forming pearlitic structure, while
manganese increases the strength and toughness.
 A typical aluminium alloy for cylinder block contains 11 % silicon, 0.5% manganese and 0.4%
magnesium. Silicon reduces expansion and increases strength and wear-resistance, while
manganese and magnesium improve strength of aluminium structure.

CYLINDER HEAD

It is attached to the top of the cylinder block by means of studs fixed to the block. The gaskets are
used to provide a tight leak proof joint at the interface of the head and the block.
The cylinder head forms a part of combustion chamber above each cylinder. It also contains spark
plug or injector holes and cooling water jackets. Besides, valve openings are provided in the head,
upon which is also mounted the complete valve operating mechanism. Depending upon the valve
and port layout, the cylinder head may be classified as loop flow type, offset cross-flow type or the
in-line cross-flow type.




(a) Loop flow type              (b) Offset cross flow type                 (c) Cross flow type
                                                                                                  25
In the loop-flow type, the -inlet and the exhaust manifolds are on the same side, which facilitates
preheating of the intake air. When, however, with the same valve arrangement, the inlet and the
exhaust manifolds are placed on different sides of the cylinder head, it is called offset cross-flow.
This type gives lower exhaust valve temperatures. In the in-line cross-flow type the valves are
positioned transversely and usually inclined to each other, while the inlet and the exhaust manifolds
are on different sides of the cylinder head. This arrangement gives better performance, but is
costlier. The head is almost universally cast en-block even for individual cylinder constructions for
the cylinder block.
The cylinder heads cast integral with cylinder blocks have also been produced in very few cases
and that too in racing engines, which obviates the necessity of a gas tight joint. But this advantage
of the integral construction is not very important because use of gaskets gives a reasonably good
gas tight Joint. Further the detachable head types have many other advantages over the integral
construction:

1. From production point of view, the cylinder block casting with open bore for the detachable type
head, is much more simplified.
2. Operations like de-carbonizing and valve grinding are simplified.
3. The compression ratio can be changed slightly by changing the thickness of the gasket used
between the block and the head.

       The materials used for cylinder heads are generally cast iron and aluminium alloy. Apart
from weight reduction, more uniform temperature is maintained in case of aluminium cylinder
heads because of greater thermal conductivity of the aluminium alloy. Quite often, aluminium alloy
head is also used along with cast iron cylinder block and crankcase. Grey cast iron for cylinder
head is same as that used for cylinder-block but aluminium alloys are usually different, a typical
one containing 3% copper, 5% silicon        and 0.5% manganese in a matrix of aluminium.
Copper increases the hardness and strength of aluminium with time on account of age-hardening.
However, it decreases the corrosion resistance. An alternative alloy commonly used consists of
4.5% silicon, 0.5% manganese and 0.5% magnesium in aluminium matrix.

       However, for certain heavy duty engines, e.g., in racing cars, where the cool running of the
engines is a major consideration, the use of copper alloys as cylinder head material has achieved
much success. The thermal conductivity of these copper alloys is approximately twice that of
aluminium and six times that of cast iron. Because of this engine tends to run cooler and
consequently higher compression ratios can be employed without occurrence of detonation.
However, these are not useful for ordinary engines, because when the engine is over cooled; its
thermal efficiency is decreased thereby increasing the fuel consumption.
OIL PAN

Oil pan or sump forms the bottom half of
the crankcase. It is attached to the
crankcase through set screws and with a
gasket to make the joint leak-proof. Its
functions are
1. To store the oil for the engine
lubricating system.
2. To collect the return oil draining from
the main bearings or from the crankcase
walls.
                                                                                                   26
3. To serve as a container in which any impurities or foreign matter, e.g., liquid fuel, condensed
water, blow-by gases, sludge, metal particles etc., can settle down.
4. To enable the hot churned up lubricating oil to settle for a while before being circulated.
5. To provide cooling of the hot oil in the sump by transfer of heat to the outsider air stream.

        The sump has a shallow downward slope at one end which merges with deep, narrow
reservoir at the other end. This is done to ensure that-under all conditions of vehicle running, there
will be oil in the reservoir where oil pump is mounted. Inside the sump, baffles are usually provided
to reduce oil surging during running of the automobile. At the bottom of the oil sump a drain plug
is provided to drain out the dirty oil at the time of oil replacement. Generally, the sump is made of
pressed steel sheet since it is not expected to have much rigidity. However, in some cases, sump of
aluminium alloy casting is used, which has adequate stiffness and rigidity. This also provides better
oil cooling on account of its higher thermal conductivity. Moreover, with such a sump, particularly
when provided with outside ribs, resonant vibration noise is avoided. However, cast sump cannot
withstand shocks, which may cause cracks. On the other hand pressed steel sheet sump in such a
situation may be dented but will not crack.

CYLINDER LINERS

The problem of cylinder wear in the engines is a very acute one especially when cylinder block is
made from aluminium alloy. The solution to this has been found by the use of cylinder liners,
which can be replaced when these are worn out. Liners are also used to restore to its original size of
a cylinder block which has been rebored beyond allowable limits. They are made in the form of
barrels from special alloy iron containing silicon, manganese, nickel and chromium. These liners
are cast centrifugally. The liners may be further hardened by nitriding or chromium plating. For
nitriding, liners are exposed to ammonia vapour at about 500 degree C and then quenched.
Chromium plating improves their resistance to wear and corrosion. Aluminium alloy liners with
chromium plating on the inside have also been used especially in combination with aluminium
cylinder blocks. The use of aluminium alloy results in increased thermal efficiency due to better
heat conduction. Further, as the pistons used are invariably of aluminium alloy, the relative thermal
expansion between the liner and the piston does not take place, due to which larger cold clearance
is not needed.

The cylinder liners are of two types, the dry and the wet type. These are discussed in detail below.

Dry Liners

As shown in Fig, this type of liner is made in the
shape of a barrel with flange at the top which keeps
it into position. The entire outer surface bears
against the cylinder block casting and hence has to
be machined very accurately both from the inside
and the outside. It is put in position by shrinking the
liner. This introduces some stresses due to shrinkage
and hence the liner bore has to be machined
accurately again after the liner has been put into the
cylinder casting.
Too loose liner will result in poor heat dissipation
because of the absence of a good contact with the
                                                                                                       27
cylinder block. This will result in higher operating temperatures. If the lubrication is also deficient,
it may cause scuffing. Too tight a liner is even worse than the too loose case. It produces distortion
of cylinder block, liner cracking, hot spots and scuffing. Even if a correct liner is fitted in a cylinder
block which itself is badly distorted, it will result in poor sealing action of rings if the liner is thin
because then it will also tend to adopt the shape of the distorted block in which it is fitted. Even if
the liner is thick enough to resist change of shape, there will be some hot spots which will lead to
scuffing on the inner surface of the liner.

Wet Liners

        This type of liner is shown in Fig. It is in direct
contact with cooling water on the outside and hence the
entire outer surface does not require very accurate
machining. But the water-tight joints have to be
provided. This is done as shown in the fig. At the top,
the liner is provided with a flange, which fits into the
groove in the cylinder block. At the bottom either the
block or the liner is provided with grooves, generally
three in number. The middle groove is left empty and
in-the top and bottom ones are inserted by packing rings
(seals), made of synthetic rubber. Arrangements for
drainage are provided from the middle groove, for any water that may leak through the upper ring.
However, the umber of grooves will vary depending upon the design. The wet liners are sometimes
coated with aluminium on the outside, which makes them corrosion resistant.
The main Troubles experienced with a wet type liner are
(a) Breaking of flange. This may be caused by wrong tightening sequence of cylinder head bolts or
their excessive tightening, uneven counter bore in the block to receive the liner, or the worn out
counter bore seat at the inner edge causing the seat to tilt downward. All the factors listed here are
such as to result in excessive stressing of the liner flange.
(b) Scuffing near sealing ring area. While installing, the rubber sealing rings at the lower deck may
get locally twisted or rolled. On twisting, the rubber becomes hard and this is accelerated under
high temperature of the cylinder. Due to this localized hardening of the rings, very high pressures
are exerted against the cylinder liner resulting in its distortion so that running clearance is deceased
and the scuffing occurs near the affected area.

(c) The ineffective sealing resulting into leakage of cooling water into the crank case is caused
when the sealing rubber rings are damaged or lower deck area, apart from causing eroded or pitted
or unclean at the sealing ring surface of the bore. Excessive wear of the block at the lower deck
area, apart from causing directly, an ineffective seal, also allows the liner to swing which allows
foreign particles to become embedded in the sealing rings and decrease their effectiveness.

Comparison of Dry and Wet Liners

1. Dry type liners may be provided either in the original design or even afterwards, whereas the wet
type have to be included in the original cylinder design.

2. A leak proof joint between the cylinder casting and the liner has to be provided in case of wet
liner, whereas there is no such requirement in the case of dry liners.
                                                                                                   28
3. In case of wet-liners, the construction of cylinder block is very much simplified.

4. Better cylinder cooling is ensured in the case of wet liners because the cooling water is in direct
contact with the liner in the case.

5. Dry type cannot be finished finally before they are fitted to the cylinder block because of the
shrinkage stresses produced, whereas wet type can be finished before fitting.

6. For perfect contact between the liner and the block casting in case of dry liners, very accurate
machining of both the block and the outer liner surfaces is required, whereas no such necessity is
there for wet liners.

ENGINE PISTON

The functions of an IC engine can be listed as below:

1. To transmit the force of explosion to the crankshaft.
2. To form a seal so that the high pressure gases in the combustion chamber do not escape into the
crankcase.
3. To serve as a guide and a bearing for small end of the connecting rod.

Apart from its capability to perform the above functions efficiently, the piston must have some
other desirable characteristics:

1. It should be silent in operation both during warming up and the normal running.
2. The design should be such that the seizure does not occur.
3. It should offer sufficient resistance to corrosion due to some products of combustion, e.g. sulphur
dioxide.
4. It should have the shortest possible length so as to decrease overall engine size.
5. It should be lighter in weight so that the inertia force created by its reciprocating motion is
minimum.
6. Its material should have a high thermal conductivity for efficient heat transfer so that higher
compression ratios may be used without the occurrence of detonation.
8. It must have a long life
Constructional Features and Materials
                                                                                                          29
A typical engine piston is shown in fig. The top of
the piston is called head or crown. Generally, lost
cost, low-performance engines have flat head as
shown. In some such pistons which come quite close
to the valves, the piston head is provided valve
relief. Pistons used in some high powered engines
may have a raised dome, which is used to increase
the compression ratio as well as to control
combustion. In some other engines, the pistons may
be specially dished to form a desired shape of the
combustion chamber, jointly with cylinder head. In case of a piston containing part of the
combustion chamber in its crown, compression ratio can be controlled very accurately, but the
disadvantage is that in this case much larger amount of heat has to be dissipated through the piston
and the rings. Towards the top of the piston few grooves are cut to house the piston rings and these
are called ring grooves. The bands left between the grooves are known as 'lands'. These lands
support the rings against the gas pressure. The supporting webs transmit the force of explosion
directly from the crown to the piston pin bosses, thereby relieving the ring groove portion from of
large loads and thus preventing deformation of the ring grooves. The part of the piston below the
rings is called 'skirt'. Its function is to form a guide suitable for absorbing side- thrust due to gas
pressure. The side thrust is produced on account of the inclination of the connecting rod with the
cylinder axis. The skirt is provided with bosses on the inside to support the piston pin. It must be of
sufficient length to resist tilting of the piston under load. It is kept quite close fitting in the cylinder,
but even then it is separated from the cylinder walls by means of lubricating oil film for smooth
running.
        The material used for pistons at one time was cast iron which has good wearing qualities.
As the technology developed, aluminium alloy containing silicon replaced cast iron as piston
material because of two distinct features. Firstly, it is as much as three times lighter than cast iron,
which makes it desirable from inertia point of view. Secondly, it possesses a higher thermal
conductivity, which causes it to run cool. But the aluminium alloy has its own disadvantages too. It
is not as strong as cast iron and hence thicker sections have to be used, as a result of which the
weight of piston is increased. It is seen that an aluminium alloy piston, in actual practice, is only
about 50 per cent in weight as compared to its cast iron counterpart. Further, aluminium is
relatively soft, as a result of which fine particles in the lubricating oil become embedded in it.
Aluminium alloy piston with fine particles embedded in it causes a sort of grinding or abrasion of
the cylinder walls, thus shortening cylinder life. Another important drawback of using aluminium
alloy pistons for cast iron cylinders is their unequal coefficients of expansion which causes engine
slap; because if the cold clearance is kept just sufficient, there is danger of seizure at high operating
temperatures and if the cold clearance is kept large, the engine knocks or slaps when cold. This
difficulty has been overcome by different methods, some of which are given below.

1. Keeping the heat away from lower part of the piston as far as
possible.
This is done by
(a) Cutting horizontal slot

in the piston on the thrust and
non- thrust sides, just below the
oil control ring. Thus the skirt
does not become very hot and
                                                                                                30
consequently it does not expand quite so much. In some designs the circumferential slots are made
in the oil control ring groove and these slots end in inclined slots extending downwards. These
elongated slots provide additional heat barriers and so reduce even more the amount of heat
reaching the working faces of the skirt. Moreover, the drooping ends make the skirt flexible in the
upper region.

(b) Making a heat dam .
It consists of a groove cut near the top of the piston. This reduces
the path of heat travel from the piston crown to the skirt. The skirt,
therefore, runs cooler and does not expand much.

2. Use of vertical or T-slots.
Vertical or T-slots on the thrust side of the piston were earlier used
quite commonly. These slots allow the piston skirt to expand
without increase of diameter.
However, the mechanical
strength is decreased on
account of slot. Moreover,
with the slot the skirt tends to
collapse inwards without
elastic recovery. As a result
the diameter is reduced
permanently which increases
the piston slap, instead of
decreasing it. For these reasons, fully split skirts are no longer used. However, as a compromise,
semi-split skirts in which the slot goes only about halfway up with blunting holes at the ends to
avoid stress concentration, have been used in pistons of some light duty engines. Heavy duty
pistons never use any such slots.

3. Taper Pistons.
The pistons are sometimes turned tapered, the
crown side being smaller in diameter than the
skirt end. As higher temperatures occur
towards the crown, that side expands more
than the skirt, due to which the piston
diameter becomes uniform under running
conditions.

4. Cam ground pistons.
The pistons are cam ground such that they
have elliptical section instead of the usual
circular one. The minor diameter of the
ellipse lies in the direction of the piston pin
axis. Such pistons, after expanding at
operating temperatures become circular
automatically, being the more expansion
along minor axis caused by the metal of
piston bosses there. Generally taper and
ovality are combined in the same piston.
                                                                                                   31
5. Use of special alloys.
Special alloys having low coefficient of expansion or rather whose coefficient of expansion is
nearly equal to that for cast iron have been used in the manufacture of pistons, without split or
specially shaped skirts and giving no piston slap. One such alloy is "Lo-Ex" alloy. It is an alloy
having 12-15% silicon, 1.5-3% nickel and about 1% each of          magnesium     and       copper.
However, such pistons are costlier than the ordinary aluminium alloy piston.

6. Wire wound pistons. A band of steel wire is Put between the Piston pin and the oil control ring,
thus restricting the expansion of skirt which is of split type or cam ground type.

7. Auto-thermic Pistons.

Referring to the fig, in which the section of an autothermic
piston is shown, it is seen that the piston contains low
expansion steel insert at the piston pin bosses. These inserts are
so moulded that their ends are anchored in the piston skirt as
shown. There is no chemical or metallurgical bond between the
steel and the aluminium there is only a mechanical bond. In
this case pistons are cam ground with the major diameter
perpendicular to the piston pin axis. At higher temperatures
during running the bimetallic action of the struts causes them
to bend outward, thus causing the piston to expand along the
piston pin, whereas in the direction perpendicular to piston pin there is corresponding contraction of
the piston due to metallic action.

Another such arrangement winch provides even better expansion control enabling smaller skirt to
cylinder wall clearances to be employed is shown in Fig. 2.35. In this steel inserts are employed on
the thrust and non-thrust sides of the piston skirt. There is clearance between the inserts on the
inside and the upper piston region, whereas the outer faces of the steel inserts are cast with the
aluminium alloy. The gap between the steel inserts and the upper piston region interrupts the heat
flow to the skirt and keeps its temperature
and hence expansion lower. Moreover,
these steel inserts are having lower
coefficient of expansion than the
aluminium alloy, there by preventing the
skirt from expanding as much as it would
if made of only aluminium alloy. Besides
strengthen the skirt, these inserts still
maintain the flexibility between the upper
part of the piston and the piston pin bosses.

8. Bi-metal Piston: These pistons are made from both steel and aluminium. The skirt is formed by
steel and the aluminium alloy cast inside it forms piston head and piston pin bosses. As the
coefficient of thermal expansion for steel is quite small, the piston will not expand much and hence
smaller cold clearances can be maintained.

9. Offset Piston. Another method which is sometimes used to eliminate slapping tendency of
pistons is to offset them from the cylinder centre line towards the major thrust side. The piston pin
hole is not symmetrically placed as usual, but is slightly offset as shown in the figure.
                                                                                                    32
 As the piston approaches top dead centre during compression
stroke the offset causes it to tilt slightly so that its top skirt
surface on the minor thrust side is placed against the cylinder
wall. Simultaneously the bottom surface of the piston skirt on
the major thrust side is also placed against cylinder wall. When
the top dead centre is passed, the inclination of the connecting
rod forces the piston against the major thrust surface. However
since the lower surface of the skirt is already in contact with the
cylinder wall because of the offset, the piston skirt will move
smoothly with full wall contact and will avoid the piston slap.

PISTON FAILURES

1. Piston Scuffing. This occurs when due to excessive heat the piston expands and becomes tight in
the cylinder. As a result the lubricant is squeezed out from the cylinder walls causing metal to metal
contact. The main reasons for the piston scuffing or scoring are:

(i) Insufficient lubrication of cylinder walls. The lubrication system may be inspected and suitably
serviced.
(ii) Overloading the engine. This depends on driving habits and the conditions.
(iii) Detonation resulting in high engine temperatures. Suitable measures may be taken to prevent
detonation.
(iv) Inefficient cooling system leading to overheat, which may be inspected and rectified.
(v) Leakage of cooling water in the cylinders causing lubricant film breakdown.
(vi) Piston pin may be too tight in either the piston bosses or the connecting rod bush. This will put
a restraint on free expansion or contraction of the piston due to increase or decrease in its
temperature. Thus it will not come back to its natural shape when the engine is stopped. On
restarting, therefore, with the increase of temperature the piston will not return to circular shape,
but to some other distorted shape, which will result in scuffing and scoring in the areas of the piston
skirt located about 450 from the bosses.

2. Burnt Piston. This may be chiefly on account of detonation or pre-ignition. The burning due to
detonation is generally at a point farthest from the spark plug where the hot end gases rapidly
release their energy. In case of pre-ignition, however, the burning is usually near the centre of the
piston head. Remedial measures should be taken to avoid the recurrence of these.

3. Damage to ring land. This may be on account of the following reasons.

(i) Excessive ring groove side clearance.
(ii) Detonation or pre-ignition.
(iii) Ring not compressed properly while installing.
(iv) Attempt to take out Piston-rod assembly without first removing cylinder ridge.
(v) Leakage of water into the cylinder. The source of leakage may be identified and properly
rectified.

4. Damaged piston boss and circlip groove. This may be due to any or more of the following:
(i) Bent connecting rod, which produces a lateral rocking movement on the piston as well as the pin
causing wear of circlip grooves. The sharp end thrust of the pin against the circlips, then removes
them completely, damaging the bosses also.
                                                                                                   33
(ii) Crankpins that are tapered or out of parallel with the crankshaft journals will produce the
similar effect as described above in (i).
(iii) Too much end play in the crankshaft will also produce lateral rocking motion of the piston pin.
(iv). The circlips may be installed loose. This usually happens because of over-compressing the
same while installing, so as to cross yield point causing a loose ring.

PISTON RINGS

The piston rings in I.C. engine have to perform the following functions:

1. To form a seal for the high pressure combustion gases from leaking into the crank case.
2. To provide easy passage for heat flow from the piston crown to the cylinder walls.
3. To balance the side tilting of the piston and to save its life to a certain extend.
4. To maintain sufficient lubricating oil on cylinder walls throughout the entire length of the piston
travel, minimizing the ring and cylinder wear, and at the same time, control the thickness of the oil
film so that satisfactory oil control is maintained. The oil is not to be allowed to go up into the
combustion chamber where eventually it would bum to leave carbon deposits.

Ring Construction

The construction of a piston ring and the
nomenclature of its various parts are shown in
Fig. The ring is generally cast individually and
machined carefully so that when in position, it is
able to exert uniform pressure against the cylinder
walls. A gap has to be cut at the ends so that while
inserting the ring onto the piston, it can be
expanded, slipped over the piston head and
released into the ring groove. Further, the gap is
almost closed when the piston is inside the
cylinder, due to which the ring is able to exert pressure on cylinder walls, which is a must for
sealing purpose. Moreover, any circumferential expansion of the ring at higher operating
temperatures may also be accommodated by the end gap. Some differential expansion of the ring
with respect to the cylinder is always likely to occur in spite of the equal coefficients of cylinder
and ring materials due to the fact that the ring is
always operating at higher temperatures than the
cylinder walls; that is why direction of heat flow
is from the rings to the walls. The sealing action
of the top ring is due to the fact that the high
pressure in the combustion chamber presses the
top ring tightly on the base of' the piston ring
groove, thus sealing the ring. However some
leakage does take place through the end gap of
the top compression ring. This leakage is useful
in that it provides the pressure for scaling action
of the second piston ring where sealing action
takes place in the same way as in case of the top compression ring described above.
The amount of end gap should, however, be determined cautiously. Excessive end gap would result
in blow-by and scuffing of' the rings. On the other hand, lesser clearance would cause the ring ends
                                                                                                      34
to butt at higher temperatures, resulting in excessive and non-uniform pressure on the cylinder
walls, causing excessive wear. In practice piston ring end gap when installed, is kept about 0.30 to
0.35 mm.
The ring end gaps may be either straight butt type or tapered or seal cut type. Out of these butt type
is most common mainly on account of its cheapness. The tapered and the seal-cut types are more
effective in preventing leakage, but are more costly. Therefore, such joints are used only in case of
some low- speed engines, where high pressure combustion gases have more time to leak through
the gap.

Piston Ring Materials
The material generally used for piston rings is fine-grained alloy cast iron containing silicon and
manganese. It has good heat and wear resisting qualities. The hardness on Rockwell B scale is
about 100. Chromium-plated rings are usually used as the top ring, which is subjected to the highest
working temperature and the corrosive action of the combustion products. However, for heavy duty
pistons, other compression rings may also be chromium-plated. Chromium plated rings have
resulted in considerable saving in the cylinder bore life. Although chromium itself is very hard due
to which it is normally expected to wear the cylinder walls rapidly, yet it does the trick because of
very fine finish of its coating on the ring. Chromium plating further helps the rings to resist scuffing
because it is difficult to be welded to cast iron cylinder. However, these rings should not be used
when the cylinder bore itself is lined with chromium or any such hard material.
The rings are provided generally, a porous phosphate coating to reduce the scoring of the surfaces
during running-in. These coatings are formed by immersing the ring in a bath of phosphoric acid
and manganese, which deposits a layer of iron manganese phosphate on the ring surface. The
porous surface has cavities for the worn particles and also acts as oil reservoir, which remains even
after the coating has worn away. Thus the rubbing fiction between the rings and cylinder walls is
reduced.
Rings with Molybdenum- filled face have also been introduced recently. The molybdenum surface
has larger oil carrying capacity. It, therefore, provides better cylinder wall lubrication with resultant
longer engine life. The higher melting point of molybdenum (26200C) enables the ring to stand
higher temperatures than other ring metals and thus resist scuffing. A more recent development is
thermo-chemically treated chromium (TC chrome) especially suited for top ring application where
lubrication is marginal. Alloy steels have also been used as ring material. Stainless steel, oil rings
resist pitting and corrosion to remain clean and do not clog with carbon as quickly as other rings.
Further, these resist excessive tension loss at engine operating temperatures.

Number of Rings
The number of rings to be used on a piston varies depending upon the requirements. Earlier two to
four compression rings and one to two oil control rings were used, but with modem design trends of
decreased car and engine heights, the number of rings is restricted to usually three, out of which
one is the oil control ring. A minimum of two compression rings are required because of the high
pressure difference bet the combustion chamber and the crankcase at the beginning of the power
stroke. This difference may be as high as 70 atmospheres. A single, piston ring cannot take such
high pressure, which necessitates the use of at least two compression rings, which divide the
pressure between themselves. Increasing the number of rings (which is restricted by the maximum
piston height) also reduces the design pressure between the rings and the cylinder walls which
results in decreased wear and consequently increased life.
Types of Rings
The piston rings are, generally speaking, of two types
1. Compression rings 2. Oil control rings
                                                                                                 35
Compression Ring:

The top compression ring (i.e.
the ring nearest to the
combustion chamber) has to do
the hard work of gas sealing
and transfer of heat from the
piston crown to the cylinder
walls. The compression rings
perform a double role. They
seal and transfer heat, and also
assist the oil rings in controlling
oil.
How the compression rings perform their primary function of sealing the compression charge and
combustion pressure is made clear by means of Fig above. When the piston is moving up during
compression stroke, the pressure in the combustion chamber acts on top of the compression ring,
which forces the ring to flatten against the bottom of the ring groove and the pressure behind the
ring will also force the ring against the cylinder wall Fig (a). This provides a very effective seal
without leakage. However, for this the surface finish of the ring, groove and cylinder wall must be
of adequate quality. A similar situation exists during the power stroke in which the piston actuates
downward Fig (b). In this case, the upper ring face is flattened against the top of the ring groove.

Compression Rings Design considerations and trends:

1. Ring width. The trend is towards reducing ring width. During the past 40 years, the ring widths
have been reduced to about half (to about 1.5 mm). Coupled with advantages of reduced ring width,
these are also having its disadvantages.

(a) Advantages :

(i) Better resistance to ring scuffing.

(ii) Reduced ring width means lower piston
height and consequently lesser engine height.
(iii) Better resistance to ring flutter.
(iv) The problems of ring inertia are reduced.
Ring inertia is the tendency of the ring to
continue its motion as the piston changes
direction. The problem is particularly severe
at high speeds.

(b) Disadvantages:

(i) Machining very narrow grooves in the
piston accurately is quite difficult.

(ii) Rings with too much reduced width and
without a satisfactory thickness / width ratio
become unstable in the ring grooves.
                                                                                                      36



2. Shape.
The basic ring shapes in present day use are given in Fig. The advantage of the taper face Fig (b)
over the plain ring fig (a) is that is reduces the contact with the cylinder wall to a narrow line,
which affords high unit loading on the face of the ring to accelerate ring sealing. It also provides a
downward scraping action which results in relatively good oil Machining of the inside upper comer
makes it a torsional twist ring (c). The Internal forces are
changed due to machining, so that when it is installed in the
cylinder, it turns about its axis so as to provide a line contact
between the ring and its groove and also contact with the
cylinder wall at its lower edge, which contributes towards a
better control of passage of both the combustion gases and the
engine oil. The scraper type torsional twist ring (d) functions like
the beveled torsional-twist ring discussed above. However, due
to its narrow face, it has the advantage of the higher unit face
loading in the untwisted position. The taper-face, torsional twist
ring (e) is a combination of the taper face and torsional-twist
designs. Keystone rings (f) have inclined side faces and operate
in grooves; of similar geometry. Relative movement between the ring and the groove in the
transverse direction discourages the build up of carbon deposits and therefore, prevents ring stick.
Rings of this form are most commonly used for the top ring in turbo charged engines.
Another recent improvement in compression ring design, which has become quite popular, is the
radius-face. The advantage of the radius shape is that there is high unit loading due to the narrow-
line contact and secondly the contact with the cylinder wall will remain intact even if the piston
ring groove straightness is slightly off. Further, this contact is not lost even when the ring changes
direction at the extreme of its stroke when a rocking action can lakes place. Thus, it results in good
oil and blow-by control on high speed output engines. Moreover, the wear at the top of the cylinder
wall where the top ring changes direction, is also reduced. The radial face ring may be beveled to
make it positive twist or reverse twist type if so desired.

3. Rings for worn cylinders.
An ordinary piston ring meant for correctly bored cylinder will not work efficiently if fitted in a
bore which is worn oval. In such cases, spring expander piston ring may be employed. They may be
two-piece, three-piece, or four-piece type. Spring expanders are made of spring steel with crimps
spaced uniformly along the circumference. The outer cast iron ring exerts only a part of the total
pressure on the walls, the rest being contributed by the spring expander which is put inside the
outer cast iron one. In three-piece type, apart from the cast iron ring and the spring expander, a
spiral steel side rail consisting of two turns of thin flat steel is located below the cast iron ring. In
four-piece type, there are two such side rails placed one below and the other above the cast iron
ring. This type of ring adapts itself to the irregularities of the cylinder bore due to comparatively
better flexibility of the ring and also radial pressure due to the internal spring expander.

Oil control rings
The function of the oil control rings is evident from their name. They scrap the excess lubricating
oil from the cylinder walls and deliver it through the ring slots and the piston oil drain holes to the
oil pan. To perform this function effectively, they must prevent excessive amounts of oil from
passing:
                                                                                                  37

(i) Between the ring face and the cylinder
wall.
(ii) Through the ring end gap, and
(iii) Around behind the ring.

         To understand the action of the oil
control ring, consider the Fig shown.
When the piston is moving up, the lower
face of the piston ring groove is pressed
against the lower ring face which makes
the outer ring face to slide over the oil on the cylinder wall. While doing so it scrapes some oil in
front of it, which goes to the clearance between the ring and the groove face and then through a
number of holes in the groove, to the sump. Similarly when the piston moves down, the sharp edge
of the oil control ring scrapes the oil on the cylinder wall, the excess oil returning to the sump
through the holes in the groove. The oil holes in the oil ring groove must be adequate to ensure free
flow of all the scraped oil, otherwise some pressure may build-up in the groove which would cause
the oil to move up to the combustion chamber side and ultimately get burnt there.

Oil control rings Design considerations and trends:
Commonly used oil control rings are
shown in Fig. The beveled ring (a) is
installed with the bevel side towards the
top. This is the most simple of oil
control ring. In the stepped scraper ring
(b), due to the decreased width of the
outer face the pressure between the ring
and the cylinder wall is increased,
resulting in better scraping. The slotted
scraper ring, (c) has a recess on the outer
face, which results in the formation of
narrow ring lands providing a higher
radial pressure against the cylinder walls
and also two- stage scraping action. The
oil holes or slots are also provided
between this recess and the inner ring
face, which facilitates adequate flow of
excess oil to the inside of the piston and
back to the sump. In the delayed action
scraper ring (d) there is a central raised
land, which wears out during the run-in
period until both the outer lands control
the scraping. In the 'double action
scraper ring'- (e), there are two scraper
rings in the same groove, with an expander at the back to increase the radial pressure against the
cylinder wall. The 'composite rail scraper ring' (f) is highly useful for worn out cylinders having
ovality or taper in the bores. There are a number of steel rails which are chrome- plated and have
rounded ends, a spacer in the from of a crimped spring to keep the rails apart and an expander. The
rounded ends provide a smooth scraping action reducing wear of cylinder wall, the spacer pushes
                                                                                                    38
the rails against the sides of the ring groove and the expander provides additional radial force
between the ring and the cylinder wall.
In later designs, expanders were combined with spacers. Such a construction is light having less
inertia, is open so that oil can easily flow to return to the crankcase, provides very good oil control
and has a long service life. It consists of a stainless steel expander spacer that is used to exert
uniform outward pressure on two steel rails. The narrow width of steel rails and the tension of the
stainless steel expander results in high unit pressure loading. The steel rails provide very good side
sealing because the expander exerts side pressure to the rails as well as outward pressure to the
rails.

Causes of Ring Failures

I. Rapid wear: When the abrasive content in the intake air is very high and particles are also very
hard, wear is very rapid. Excessive ring or ring groove-wear is caused by the scraping action of
abrasive in the engine. This condition is evident from the excessive oil consumption and poor
engine performance. It is found that rapid abrasive wear is the type of ring failure most commonly
experienced.

2. Scuffing: Scuffing or scoring is caused by breakdown of protective lubricating oil film on the
cylinder wall. This allows metal to metal contact of the ring faces on the cylinder wall producing
heat due to friction causing surface welding. The material from the ring face gets transferred onto
the cylinder wall causing ring scuffing, which gets worse with further additional running. The
factors promoting scuffing may be listed as:

(a) Overheated engine - This may be due to
       (i) Cooling system, which may be restricted, clogged, or otherwise inefficient.
       (ii) Leakage of coolant or defective thermostat valve.
       (iii) Incorrect ignition timing, weak air-fuel mixture causing detonation.

(b) Lack of or deficient lubrication-This may be caused by
       (i) Less bearing clearances. (ii) Low oil pressure.
       (iii) Water leaking in cylinders causing breakdown of lubrication film.
       (iv) Defective oil pump        (v) Dilution of oil in crankcase.
       (vi) Low oil level in the sump.

(c) Insufficient clearance in cylinder bore. The following may be the sources of the trouble:
        (i) Wrong size piston or piston rings. (ii) Distorted cylinder bores.
        (iii) Wrong size cylinder bore.        (iv) Cylinder liner distorted or loose in the cylinder
block.

(d) Improper cylinder finish which may be due to honing not having been done after boring.

3. Ring breakage
Sometimes the ring breaks during service. This may be on account of overstressing due to shock
loading, fatigue, etc. The following are the main causes of such over stressing:
(i) Use of ring of incorrect size.     (ii) Ring sticking due to the carbon deposits.
(iii) Detonation or pre-ignition.      (iv) Excessive ring side clearance.
(v) Uneven wear of ring grooves.       (vi) Insufficient end gap of rings.
(vii) Excessive ring flutter cased by over speeding.
                                                                                                    39

CONNECTING ROD

The function of the connecting
rod      is    to    convert     the
reciprocating motion of the
piston into the rotary motion of
the crankshaft. A typical
connecting rod is shown in Fig.
A combination of axial and
bending stresses act on the rod in
operation. The axial stresses are produced due to cylinder gas pressure and the inertia force arising
on account of reciprocating motion, whereas bending stresses are caused due to the centrifugal
effects. To provide the maximum rigidity with minimum weight, the cross-section
of the connecting rod is made an I-section. The small end of the rod has either a 'solid' eye or a
'split' eye; this end holding the piston pin. The big end works on the cranks pin and is always split
on the multi cylinder engines. In some connecting rods, a hole is drilled between two ends for
carrying lubricating oil from the big end to the small end for lubrication of piston and the piston.
         The connecting rods are generally, made by drop forging of steel or duralumin. However,
with the progress of technology, the connecting rods are also cast from malleable or spheroidal
graphite (SG) cast iron. In the later method, a closer weight tolerance can be maintained compared
to the forging method. In general, forged connecting rods are compact and lighter which is an
advantage from inertia view point, whereas cast connecting rods are comparatively cheaper, but on
account of lesser strength their use is limited to small and medium size petrol engines.

PISTON PIN




Piston pin or wrist pin or gudgeon pin as it is so often called connects the piston and the connecting
rod. For lightness it is made in tubular form. It passes through the bosses in the piston and the small
end of the connecting rod. It is made of low carbon case hardened steel having 0.15% carbon, 0.3%
silicon, 0.5% manganese and the remainder iron. The pin is carburised at 9000 C, hardened by
                                                                                                    40
quenching from 7800 C and finally tempered at 1500 C. The piston pins are usually lapped to a very
fine surface finish of about 0. 1 microns, without which it is likely to fail very early due to fatigue
caused by surface irregularities. Piston pin operating clearances are generally kept about 7.5
microns, the larger clearance would result in more noise and decreased life. Piston pin-connecting
rod connections are of three types:

1. Piston pin is fastened to the piston by set screws through e ton bosses. It has a bearing on the
connecting rod small end (Fig. 2.97).
2. The pin is fastened to the connecting rod by means of a bolt, while it forms bearings in the piston
bosses. one such piston pin is shown in Fig. 2.98. These days, however, bolt has been replaced by
interference fit. This avoids a discontinuity into the connecting rod small end or the piston pin
itself, which is always a source of weakness.
3. The pin floats both in the piston bosses and the small end of the connecting rod. This is the
arrangement most commonly used. To prevent end movement, circlips are used as shown in Fig. 2.
99. Alternatively, piston pin may be prevented from sliding, by using spherical end-pads made of
aluminium, brass or bronze. In heavy duty engines, semi-floating pins as described at serial nos. 1
and 2 above, may freeze in their bearings. This does not happen in case of pins floating both in the
piston bosses and the connecting rod small end, because of the double swivel action of such pins.

CRANKSHAFTS




Crankshaft is the engine component from which the power is taken. It receives the power from the
connecting rods in the designated sequence for onward transmission to the clutch, gear box,
propeller shaft, differential and subsequently to the wheels.

Construction
Crankshaft is a shaft consisting of the following major parts

1. Main journals       2. Crank pins 3. Crank webs 4. Counter weights              5. Oil holes
A simplified sketch of the crankshaft for a 4 cylinder in-line engine is shown in Fig. Main journals
are supported in main bearings in the crankcase. These form the axis for the rotation of the
crankshaft. Their number is always one more or one less than the number of cylinders. The
crankpins are the journals for the connecting rod big end bearings and are supported by the crank
webs. The distance between the axis of the main journal and the crankpin centre lines is called the
'crank-throw', which determines the crankshaft turning effort. Oil holes are drilled from main
journals to the crankpins; through crank webs to provide lubrication of big end bearings.
                                                                                                     41

When the engine is running, the centrifugal
forces acting at each crankpin due to rotation
of both the crankshaft as well as the big end
of the connecting rod tend to bend the
crankshaft. To counter this tendency,
counterweights are either formed as integral
part of the crank web or attached separately,
on the side opposite to the crankpin. As the
separate weights can be made to overlap the
crank webs, more mass can be concentrated at a smaller radius, thus preventing the inertia from
being excessive. However, in that case very high accuracy is also needed while attaching them,
otherwise it would result in unbalanced crankshaft in spite of the counterweights.
On one of the main bearing journals, thrust bearing is located so as to support the loads in the
direction of the shaft axis. Such loads may arise due to clutch release forces, etc. On the front of the
crankshaft usually the following attachments are there.
                (i) Timing gear or sprocket which drives the cam shaft.
                (ii) Vibration damper.
                (iii) Pulley for driving the water pump, fan and the generator.
On the rear end is mounted the flywheel, which serves as energy reservoir. Though there is one
working stroke in two revolutions of the crankshaft in four stroke engines, the flywheel absorbs
excess energy during the power stroke and gives out the absorbed energy during the other three
strokes. Thus it is the, flywheel which keeps the crankshaft rotating at the uniform speed
throughout. The flywheel also has teeth on its outer periphery which mesh with the pinion, of the
starting drive to start the engine.
The crankshafts are generally of two types, viz., one piece or integral and built up. In the built up
construction the crank pins and journals are bolted to crank arms, which also serve as flywheels.
One-piece construction is almost universally used for automotive crankshafts.

Design features
For car engines, which are petrol engines in majority of the cases, the crank pin length is at least
30% of its diameter, which itself is usually not less than 60% of the cylinder bore. The thickness of
the crank web is usually about 20% of the cylinder bore size, the main journal diameter is bigger
than the crank pin diameter and is usually 75% of the cylinder bore whereas its length is about half
its diameter. The angles between crank throws are selected from the consideration of smooth power
output. In 6-cylinder in-line engines the angle is 1200, whereas for V-8 engines it is 900.
Materials and manufacture
Earlier the crankshafts were forged, but with the improvements in foundry techniques, the casting
of crankshafts has become quite common. S.A.E. steels 1045 and 3140 are the commonly used
steels for forged crankshafts. S.A.E. 1045 contains manganese (0.60-0.90%), whereas S.A.E. 3140
contains nickel (1.10-1.40%) and chromium (0.55--0.75%) besides manganese (0.70-0.90%).
Chrome-vanadium and chrome molybdenum steels have also been used. In case of cast crankshafts
both cast steel as well as SG iron has been used. The ultimate tensile strength of the crankshaft
material is about 600 MPa. Typical cast steel for crankshafts has the following chemical
composition:
Carbon 1.35-1.6 %, Chromium 0.05-0.5%, Silicon 0.85-1.1%, Manganese 0.6-0.8%, Copper 1.5-
2.0% Phosphorus        0.10% (max.), Sulphur 0.9% (max.)
The Spheroidal Graphite type or commonly called S.G. iron is a high strength cast iron in which the
carbon present is in the form of spherical modules of graphite (compared to flakes in the gray cast
                                                                                                   42
iron). Due to spherical form, the S.G. iron has high strength, improved ductility and larger
toughness than the gray cast iron and is thus able to take up the type of stresses that are imposed on
the crankshaft while running.
In case of forged crankshafts, the drop forging process with closed dies is used. The forging may be
done either in-place or alternatively, the crankshaft is forged in one plane and then wound to place
the crank throws at desired angles. The blank so produced is then heat treated to remove residual
stresses and then machined to final dimensions. The machining includes mainly the rough turning,
final grinding and final lapping of main journals and crankpins. The cast crankshaft is
manufactured by the shell-moulding process, in which a thin shell-like mould is made from the
sand and synthetic resin. This mould is then used for further casting. This method has the advantage
of high accuracy in terms of closer tolerances which reduces the machining cost.
The crankshafts produced by the forging process are very dense and tough with grains parallel to
the principal directions. The cast crankshafts have a uniform and random structure throughout. The
cast crankshaft has also to be heat treated and machined as in case of forged ones. They are slightly
less dense compared to forged crankshafts, but are also cheaper due to reduced machining. Due to
these reasons, both the types are in use.

VIBRATION DAMPERS

Vibration damper is also called a harmonic balancer. It is mounted on
the front end, of the crankshaft. It consists of a damper flywheel
crankshaft pulley and a driving flange connected together with a
rubber ring in between. During the power stroke in an engine cylinder,
the force of explosion is applied to its crankpin, which tends to twist
the crankshaft. This force disappears during the other strokes; while
during the next power stoke it again reappears due to which the
crankshaft is untwisted. The same cycle is repeated at every crankpin
on the engine crankshaft. These twisting and untwisting of crankshaft
set up torsional vibrations, which may become amplified if their
frequency becomes equal to the natural frequency of the vibrating
parts due to resonance. The vibration damper reduces the torsional
vibrations by means of the dragging effect produced by the inertia of
damper flywheel. On account of this dragging effect the rubber ring is
also flexed which tends to avoid the vibrations and keep the crankshaft speed uniform. Engines
with belt-driven crankshafts do not need vibration
damper. The belt acts as a damper.

MAIN BEARINGS

Crankshaft is supported in main bearings which
are located in the lower portion of the engine
block. The number of bearings depends upon the
number of cylinders in the engine and their
arrangement. For example in the Hindustan
Ambassador car engine which is 4-cylinder in-
line type, there are three main bearings, whereas
in the Ashok Leyland vehicles engines, which are 6-cylinder in-line type, the number of main
bearings is seven. V-type-8 cylinder engines generally have five main bearings. The main bearings
used are mainly of two types. One is the precision insert type which is available both in finished as
                                                                                                      43
well as semi finished condition. This type is installed directly in the bearing saddle bores with the
crankshaft removed or even when the crankshaft is in position in the engine. The finished type is
installed when the crankshaft journals are of the standard size. When the journals have worn out
and are ground to some undersize, so that available standard size bearings do not give the required
bearings clearance within the permissible limits, the semi-finished bearings are used which are
rebored to the desired size. The second type of main bearing available is the cast type in which the
bearing metal is sprayed directly onto the saddle bores and the inside of the bearings caps after
which they are line bored to the desired dimensions. A number of shims on each side of the
bearing, within it and the cap are also provided in the cast type for adjustment of clearances. Taking
out a number of shims reduces the bearings clearance.

To prevent the crankshaft from moving to and fro in the endwise direction, one of the bearings is
made as a thrust bearing i.e., it is provided with flanges on both sides, which fit accurately with the
sides of the crankpin on which it is installed. Modem bearing of the insert type usually has a back
of steel on which there is lining of suitable bearing material. For example, Sealed Power bearings
are of the following types:

(i) Standard duty. Babbitt lining with steel or bronze back is mainly used. Babbitt metals or white
metals are basically either antimony-tin or antimony-lead alloys. They have very good
conformability, embeddability and corrosion resistance, but have low fatigue strength. A typical
composition for a tin-based Babbitt metal is antimony 7.5%, lead 0.2%, arsenic 0.1 %, copper 3.0%
and tin remainder.
(ii) Medium duty. Copper alloy lining with steel back is mainly used. Copper- based alloys are
harder and have higher fatigue strength than babbitt metals, but less conformability, embeddability
and corrosion resistance.
(iii) Heavy duty. Tin flash coating, Babbitt over-plate, brass barrier plate, copper alloy lining and
steel back are used.
(iv) Extra heavy duty. Tin flash plate, Babbitt over-plate, aluminium alloy lining and steel back are
used. A typical aluminium alloy used for turbo-charged heavy duty diesel engines contains 11%
silicon, 1% copper and the remainder aluminium. These alloys do not suffer from corrosion as is
the case with copper-based alloys.

BEARING OIL CLEARANCES
As bearings wear, more and more oil is thrown onto the cylinder walls. The piston rings cannot
handle so much oil. Part of it works up into the combustion chambers, where it bums and forms
carbon. Carbon deposits in the combustion chambers reduce engine power and cause other engine
troubles. Excessive oil clearances can also cause some bearings to fail from oil starvation. An oil
pump can deliver only a certain amount of oil. If the oil clearances are excessive, most of the oil
will pass through the nearest bearings. There won't be enough for the more distant bearings. Then
these will probably fail from lack of oil. An engine with excessive bearing oil clearances usually
has low oil pressure. The oil pump cannot build up normal pressure because of the large oil
clearances in the bearings.
If bearing oil clearances are too small, there will be metal to metal contact between the bearing and
the shaft journal. Very rapid wear and quick failure will result. Also there will not be enough oil
throw off to lubricate cylinder walls, pistons, and, rings. The greater the oil clearance, the faster oil
flows through the bearing. Proper clearance varies with different engines, but 0.0015 inch [0.037
mm] is a typical clearance. As the clearance becomes greater (owing to bearing wear, for example),
the amount of oil flowing through and being thrown off increases. With a 0.003inch [0.076mm]
                                                                                                 44
clearance (only twice of 0.0015 inch) [0.037 mm], the oil throw off increases as much as five times.
A 0.006 inch [0.152-mm] clearance allows 25 times as much oil to thrown off.

CAMSHAFT

The camshaft provides a means of
actuating the opening and controlling
the period before closing, both for the
inlet as well as the exhaust valves. It
also provides a drive for the ignition
distributor and the mechanical fuel
Pump. The camshaft consists of a
number of cams at suitable angular
positions for operating the valves at approximate timings relative to the piston movement and in a
sequence according to the selected firing order.
The camshaft is forged from alloy steel or cast from hardenable cast iron and is case hardened. A
typical cast iron alloy for a camshaft would consist of 3.3%
carbon, 2% silicon, 0.65% manganese, 0.65% chromium,
0.25% molybdenum and the remainder iron. After chilling
this, the surface hardness attained is about 500 to 600 DPN.
In modern engines, cam lobes are ground with a slight
taper across the face. The tappets used with such camshafts
have spherical base and are slightly offset from the cam
face. This provides tappet rotation and a wear pattern
preventing edge loading which is a major cause of failure.
Cam shaft is supported in a number of bearings. Timing
gears are made of cast iron, steel, aluminium or laminated
fibre. Chains are made with links of alloy steel with ground
and case hardened pins. Sprockets are made of nylon,
aluminium, steel or iron.

DRIVING THE CAMSHAFT




The camshaft is driven by gears, by sprockets and chain, or by sprockets and toothed belt. The
crankshaft must turn two times to turn the camshaft once. There are four piston strokes to a
complete one cycle of actions in the engine. This requires two crankshaft revolutions. Each valve
                                                                                                      45
must open only once during a complete cycle. Since each valve opens every camshaft revolution,
the camshaft must rotate only once while the crankshaft rotates twice. This 1:2 gears ratio is
achieved by making the camshaft gear or sprocket twice as large as the crankshaft gear or sprocket.
The gears are called timing gears. The chain is called the timing chain. The toothed belt is called
the timing belt. The reason for this is that the gears, chain and sprockets, or belt and sprockets
"time" the opening and closing of the valves. There are timing marks on the sprockets or gears of
the camshaft and the crankshaft to ensure correct valve timing. The camshaft is mounted either in
the cylinder block (camshaft in block) or on the cylinder head (OHC). In either position, the
camshaft is supported by bushings or sleeve bearings. Many camshafts have a spiral gear to drive
the distributor and oil pump, and an eccentric to drive the fuel pump.

ENGINE VALVES

To admit, the air-fuel mixture in the engine cylinder and to force the exhaust gases out at correct
timings, some control system is necessary, which is provided by the valves. The engine valves may
be broadly divided into 3 main categories
1. Poppet valve        2. Sleeve valve       3. Rotary valve
Out of these three, Poppet valve is the one which is being universally used for automobile engines
due to many advantages.

POPPET VALVES
The Poppet valve derives its name from its motion of popping up and down. This is also called
"mushroom valve" because of its shape which is similar to a mushroom. It consists of a head and a
stem as shown in Fig. It possesses certain advantages over the other valve types because of which it
is extensively used in the automotive engines

1. Simplicity of construction
2. These are self-centering.
3. These are free to rotate about the stem to new position.
4. Maintenance of sealing efficiency is relatively easier in their case.

Generally inlet valves are larger than the exhaust valves, because speed
of incoming air-fuel mixture is lesser than the velocity of exhaust gases
which leave under pressure. Further because of pressure, the density of
exhaust gases is also comparatively high. Moreover, smaller exhaust
valve is also preferred because of shorter path of heat flow in this case
and consequently reduced thermal loading. Generally inlet valves and
exhaust valves are 45% and 38% of the cylinder bore respectively.
Further, to improve heat transfer to the cylinder head, the stem diameter
of the exhaust valve is generally 10 to 15% greater, than that of the inlet
valve. Moreover, the valve lift in both inlet and exhaust valves should be at least equal to 25% of
the valve head diameter which would provide the annular valve-opening area equal to the port
throat area. If the valve lift is less, the volumetric efficiency of the engine will be decreased. On the
other hand if it is excessive, the inertia of the valve
actuating mechanism would be unduly large resulting in
excessive noise and wear.
The valve face angle (with the plane of the valve head) is
generally kept 450 or 300. A smaller face angle provides
greater valve opening for a given lift, but poor sealing
                                                                                                   46
because of the reduced seating pressure for a given valve spring load. Due to this reason in some
engines, the inlet valve face angle may be kept 300 or 450 whereas the exhaust valve face angle is
only 450, as this increases its heat dissipation. In some cases, a farther differential angle of about
1/2 deg to 1 deg is provided between the valve and its seating, which results in better sealing
conditions. Now the common practice is to keep the inlet and exhaust valve face angle to 450.

Each cylinder has two valves, an intake valve and an exhaust valve. Some high-performance
engines have four valves per cylinder like two intake valves and two exhaust valves or two inlet
valves and one exhaust valve.
Modern engines using low-lead or no lead gasoline makes the working of the valves harder. The
reason for removing the lead from gasoline is that excessive amounts of lead can pollute the
atmosphere. The fact that there is little or no lead in gasoline creates a potential wear problem for
valves and valve seats. Lead in gasoline forms
a fine coating on the valve faces and seats.
This coating acts as a lubricant. Without the
lead, the valve faces and seats lack lubricant
and can wear rather rapidly. For this reason,
many engines have valves with special
coatings on their faces. These coatings reduce
wear. Normally these types of valves will not be reconditioned, but will be replaced. For severe
operating conditions, the valve faces may be made of stellite, a very hard metal. In addition, some
valves for modern, high-performance engines have a chrome-plated stem and a hard-alloy tip that
has been welded onto the stem. Stellite is a cobalt-base alloy having 1.8% carbon, 9% tungsten,
29% chromium and the remainder cobalt. This reduces wear on these two critical areas. Also, some
valves are made with a hollow stem. This reduces the valve weight so it has less inertia. Engine
power and responsiveness are increased.

MATERIALS
The materials used for inlet and exhaust valves are generally different because of the different
operating conditions to which these are subjected. Inlet valves operate at temperatures of up to
about 5000C. However, the exhaust valves have to operate in more severe conditions. As such the
material for exhaust valve must have the following mechanical properties:

1. Sufficient strength and hardness to resist tensile forces and the wear.
2. Adequate fatigue strength.
3. High creep strength.
4. Resistance to corrosion.
5. Resistance to oxidation at the high operating temperatures.
6. Small coefficient of thermal expansion to avoid excessive thermal stresses in the head on account
of high temperature gradient them.
7. High thermal conductivity for good heat dissipation.

Silicone-chrome steel containing about 0.4% carbon, 0.5% nickel, 0.5% manganese, 3.5% silicon
and 8% chromium is the material generally used for inlet valves. For early exhaust valves,
molybdenum was added to it. The more recent materials for exhaust valves are the austenitic steels
and precipitation hardening steel. A typical austenitic steel is the ‘21-12’, which contains 0.25 %
carbon, 1.5% manganese, 1% silicon, 12% nickel and 21% chromium. Another improved austenitic
steel '21-4N' contains 0.5% carbon, 9% manganese, 0.25% silicon, 4% nickel, 21% chromium and
0.4% nitrogen.
                                                                                                    47
To avoid corrosion, the valve is given an aluminium coating. Aluminium oxide formed separates
the valve steel from the cast iron seat to keep the face metal from sticking. The stems of both the
inlet as well as exhaust values may be chromium plated to make them wear resistant.
The use of different material compositions for the inlet and the exhaust valves will also allow
keeping the same valve clearance for both the inlet and the exhaust valves.

VALVE COOLING

The intake valve runs relatively cool, since it passes only the air-fuel mixture. But the exhaust valve
must pass the very hot exhaust gases. The exhaust valve may actually become red-hot in operation,
at temperatures well above 10000F [537.80C]. The valve stem is coolest, and the part nearest the
valve face is next coolest. The valve stem passes heat to the valve guide, which helps to keep the
valve stem cool. Likewise, the valve face passes heat to the valve seat. This helps to keep the valve
face cool. The valve seat and valve guide are cooled by the engine cooling system. The cylinder
head is carefully designed to permit good coolant circulation through the water jackets around the
seat and guide. Some I-head engines have nozzles that force coolant circulation around the valve
seats. In some engines, deflectors in the head improve coolant circulation around the valve seats.
To assist in valve cooling, some heavy-duty engines have an exhaust valve with a fat hollow stem
partly filled with the metal sodium. Sodium melts at 2080F [97.80C]. When the engine is operating,
the sodium is a liquid. The liquid is thrown up and down in the stem by the valve movement. This
circulation takes heat from the valve head and carries it down to the stem, which is cooler. The
action keeps the head cooler for longer valve life. However, sodium cooled valves are very rarely
used in automotive engines.

VALVE SEAT INSERTS
Valve seat inserts are simply rings of plain, grooved, stepped or slotted shape as shown in Fig.
They are pressed into the cylinder block or cylinder head to reduce wear and to prevent leakage of
the gases. Seat insert for exhaust valve differs from that of inlet valve due to its higher operating
temperature and different condition. The valve seat inserts should be well finished and ground at an
equal angle to the valve face, then pressed or shrinked into the recess of the blocks. The fitting of
the inserts should be at perpendicular to the abutment face so that a good mesh between the seat
and valve face can be obtained. Therefore another advantage of the insert is that it reduces the
frequent grinding of the valve face. The materials used for the inserts ate phosphor bronze, special
heat resisting cast iron and stellited steel.
 The valve seats must be faced very accurately, so that there is
complete contact between the valve and the valve seat when the
valve closes. Valve seat face is thus ground to the same angle to
which the valve face is ground. This may have any value from 300 to
450. For cylinder blocks or heads made of gray cast iron, the valve
seats are directly machined on the cylinder blocks or heads as the
case may be because working conditions are not severe. These are
called integral seats. However, where aluminium blocks or heads are
used, separate valve seat inserts are employed even for inlet valves.
For the exhaust valves, always the separate valves seats are used, the
operating conditions being very severe. Insert seats are also used as salvage procedure when badly
damaged integral seats are reconditioned. Valve seat inserts are simply rings made of alloy steel
consisting of chromium, silicon, tungsten or cobalt. When worn, these inserts can be easily replaced
by press fitting.
                                                                                                   48
VALVE GUIDES

The stem of the Poppet valve needs a guide for the alignment of its up and down motion so that the
face of the valve is maintained in a central position with respect to the valve seat while opening and
closing. The simplest valve guide is a reamed hole in the cylinder block or head in which the valve
stem moves. This type of integral valve guide is cheaper to provide and the heat transfer is also
better. However, the modem tendency is to provide separate valve guides, which are inserted into
the holes in the cylinder block or head as the case may be. These are particularly required where the
valve stem and cylinder head materials are not compatible. These inserts are of cylindrical shape
and they are made from pearlitic cast iron having minimum hardness of the order of 220 Brinell
hardness number. Sometimes alloy irons containing nickel and chromium are also used. Silicon,
aluminium or bronze guides offer maximum resistance to fatigue, corrosion and heat.

ROCKER ARMS

There are several different
types of rocker arms as
shown in the above figure.
Some rocker arms have a
means of adjustment. The
purpose is to provide a
minimum valve tappet
clearance or gap in the
valve train. The most
common type is the shaft
mounted adjustable rocker
arm (fig-B). On valve
trains with hydraulic valve
lifters, this is not important
because the hydraulic valve
lifter automatically takes
care of any clearance.
Some rocker arms have an
adjusting screw. It can be
turned in or out to adjust
the valve-tappet clearance.
The stud or pedestal mounted rocker-arm valve train is adjusted by turning the stud nut or attaching
bolt. Valve-tappet clearance should be kept to a minimum to reduce noise and wear from parts
hitting together when the valves are opened. The clearance should be large enough to assure
complete closing of the valves. If the adjustment were made to give no clearance when the engine
is cold, trouble would result. When the engine warmed up, the valve-train parts would expand
enough to prevent the valves from closing completely. Hot combustion gases would flow between
the valves faces and valve seat. This would result in burned valves and seats and an expensive
engine repair job.

Overhead-cam engines may also use rocker arms. In some engines, the rocker arms are mounted on
shafts. In others, the rocker arm floats. One end of the rocker arm rests on the hydraulic valve lash
adjuster. This device takes up any clearance that occurs. The other end of the rocker arm rests on
the valve stem. The cam on the camshaft rides on a pad on the rocker arm. When the cam lobe
                                                                                                   49
moves around above the rocker arm, it pushes down on the rocker arm. The rocker arm pivots on
the valve lash adjuster so the valve-stem end is pushed down. This opens the valve. When the cam
lobe moves out from above the rocker arm, the valve spring closes the valve. Meantime, the valve-
lash adjuster takes up any lash, or clearance.

VALVE SPRINGS
Helical springs are used to keep the valve in constant contact with the tappet and the tappet with the
cam. Since the spring is subject to compressive loads, it is ground flat at each end to ensure even
distribution of pressure. The coil ends are also placed diametrically opposed to avoid the bending
tendency of the spring under compression. The arrangement for the retention of the springs is
simple. A ring split into two halves with internal projection to fit into the valve spring retaining
groove and the outer surface tapered is employed. Over the split ring, another ring is inverted which
supports the spring. The valve springs are subject to heavy service. These are, therefore, made from
high grade spring steel wire, the materials being generally hard-drawn carbon steel or chrome-
vanadium steel. Valve springs are often shot peened to make them fatigue resistant.

VALVE ACTUATING MECHANISMS

In the entire valve actuating mechanisms, a cam driven at half the crankshaft speed is, used to
operate each valve, inlet or exhaust. However, there are different methods of operating the valves
by the cam. These may be broadly divided into two types viz, mechanisms with side camshaft (This
type of valve train uses pushrods and the engine is called a pushrod engine) and the mechanisms
with overhead camshaft (OHC engines).
1. Mechanisms with side camshaft

In these, the crankshaft is on the side of the engine and the
valves are operated either directly by the cams or through
the push rods and rocker anus. These may be further
classified as:

(a) Double row side valve mechanism (T-head). This is
the oldest type of valve actuating mechanism. In this the
inlet and the exhaust valves are operated by separate cam
shafts, which make the mechanism complicated. Moreover, the
shape of the combustion chamber provides poor combustion
and low engine performance, due to which this type of
mechanism is obsolete.
(b) Single row side valve mechanism (L-head). In this, the
inlet and the exhaust valves are all arranged in a single row and
operated from the same camshaft. This method was once quite
popular on account of the following advantages:
(i) Low engine height. (ii) Low production cost. (iii) Quiet
operation. (iv) Ease of lubrication.
This mechanism is, however, no more in use, because it is very
inefficient on account of the complicated shape of the combustion chamber which is more prone to
detonation. There were also restrictions of space on the size of inlet valves that would be used.
Moreover, difficulties were experienced in cooling the exhaust valves.
                                                                                                 50
(c) Overhead inlet and side exhaust valve mechanism (F-head). This is a combination of the two
systems described above. Overhead valve mechanism is used for the inlet valve operation and the
side valve mechanism for the exhaust valve. It is used in F-head engines. This mechanism is
simpler than the overhead crankshaft operated types and allows the use of larger inlet valves, but
larger valves being heavier, there is also a limitation on the maximum speed of the engine that
could be a lowed. F-head engines were found to be less efficient and were also more expensive.
Hence these have also become obsolete.

(d) Single row overhead valve mechanism (I-Head): This type is very extensively used now. The
cam operates the valve lifter which in turn actuates the push
rod. The push rod further operates the rocker arm, which
actuates the valve. This type of mechanism is having the
following advantages.

(a) Higher volumetric efficiency than the side valve design.
(b) Higher compression ratios can be used.
(c) Leaner air-fuel mixtures can be burnt.
(d) The rocker arm leverage makes it possible to use smaller
cam lobes compared to the side valve mechanism.



2. Mechanisms with overhead camshaft




The OHC engine uses various means of carrying the cam lobe action to the valve. The simplest
arrangement is shown in. It uses a bucket tappet. The operation of the bucket-tappet Valve train is
also shown. The cam lobe is directly above the bucket tappet. As the lobe comes around, it pushes
the tappet and the valve down. Then valve opens. As the lobe passes the tappet, the spring pulls the
valve closed. A second arrangement, using a rocker arm, is shown. As the cam lobe comes up under
the valve lifter it causes the rocker arm to rock. This pushes down on the valve stem so that the
valve moves down and opens when the lobe passes out from under cycle of actions in the engine.
                                                                                                     51
This requires two crankshaft revolutions. Each valve must open only once during a complete cycle.
Since each valve opens every camshaft revolution, the camshaft must rotate only once while the
crankshaft rotates twice. This 1:2 gear ratio is achieved by making the camshaft gear or sprocket
twice as large as the crankshaft gear or sprocket. The gears are called timing gears. The chain is
called the timing chain. The toothed belt is called the timing belt. The reason for this is that the
gears, chain and sprockets, or belt and sprockets "time" the opening and closing of the valves. The
camshaft is mounted either in the cylinder block (camshaft in block) or on the cylinder head
(OHC). In either position, the camshaft is supported by bushings, or sleeve bearings. Many
camshafts have a spiral gear to drive the distributor and oil pump, and an eccentric to drive the fuel
pump.

The valves operating mechanisms with overhead single or double crankshafts are highly efficient.
However, with these considerably more lubricating oil is needed to flood the cam profiles as
compared to the overhead valves operated by side crankshafts. Moreover, they have the
disadvantage of higher initial costs. Fig. shows single row valves operated by a single overhead
crankshaft and an inverted bucket type follower. With this type of follower, the camshaft is
arranged directly over the valve stems.

VALVE STEM OIL SEALS

There is always considerable -oil on top of the cylinder head. Oil is needed to lubricate the valve
stems, rocker arms, and pushrods (where used). Oil- must be prevented from getting past the valve
stems and into the combustion chamber. The oil would burn, leaving carbon deposits. Valves and
piston rings would not work properly. Compression ratio could go so high that detonation would
occur. Spark plugs would foul and misfire. To prevent all this from happening, valve stems are
protected from oil by either an oil shield or an oil seal. The shield or seal allows just enough oil to
get on the stem to provide proper stem lubrication.

VALVE ROTATION

When the exhaust valve rotates as it opens, there is less chance of valve-stem deposits causing the
valve to stick. In addition, valve rotation results in more even valve-head temperature. Some parts
of the valve seat may be hotter than others. So hot spots may develop. If the same part of the valve
face continues to seat on the hot spot, a hot spot develops on the valve face. The hot spot on the
valve face wears or burns away faster. But if the exhaust valve rotates, no one part is always
subjected to the higher temperature. Therefore longer exhaust valve life results.
In the typical engine design, the rocker arm is slightly offset from the centerline of the valve. Every
time the valve is opened, there is an off-center push on the valve stem. This tends to rotate it. There
are also special valve rotators that are part of some valve trains. These are of two types, free and
positive.

VALVE LIFTERS

There are two types of valve lifters (also called valve tappets), the solid or mechanical lifter and the
hydraulic lifter. The solid lifter is just a cylinder placed between the cam (on the camshaft) and the
pushrod. The valve lifter is rotated in much the same way that the valve is rotated by the rocker
arm. The face of the lifter is offset slightly from the center of the cam. This rotation of the valve
lifter prevents sludge from accumulating in the lifter bore in the cylinder block. At the same time,
the lifter rotates the pushrod. This keeps clean the pushrod bearing surfaces with the lifter and
                                                                                                   52
rocker arm. A rough or worn cam lobe can often be located by seeing which ' pushrod is not
turning, or not turning at the same speed as the other pushrods. This rotation distributes the wear
from the cam over the face of the lifter. The hydraulic valve lifter has an internal construction that
reduces noise and valve-train clearance.

HYDRAULIC VALVE LIFTERS

The hydraulic valve lifter is used in many
engines. It is very quiet because it assures
zero tappet clearance (or valve lash). Also, it
usually requires no adjustment in normal
service. Variations due to temperature
changes or to wear are taken care of
hydraulically.
Figure shows the details of a hydraulic valve
lifter. Oil is fed into the valve lifter from the
oil pump, through an oil gallery that runs the
length of the engine. When the valve is
closed, oil from the pump is forced into the
valve lifter through oil holes in the lifter
body and plunger. The oil forces the ball-
check valve in the plunger to open. Oil then
passes the ball-check valve and enters the
space under the plunger. The plunger is forced upward until it touches the valve pushrod. This takes
up any clearance in the system.
When the cam lobe moves around under the lifter body, the lifter is raised. Since there is no
clearance, there is no tappet noise. The raising of the lifter and the opening of the valve suddenly
increases the pressure in the body chamber under the plunger. This causes the ball-check valve to
close. Oil is therefore trapped in the chamber. Because liquids such as oil are not compressible, the
lifter acts like a simple one piece lifter. It moves up as an assembly and causes the valve to open.
Then, when the valve closes, the lifter moves down, and the pressure on the plunger is relieved. If
any oil has been lost from the chamber under the plunger, oil from the engine oil pump causes the
ball-check valve to open. Engine oil can then refill the chamber.
VALVE TAPPET (valve lifter or cam follower)
                                                                                                  53
The valve tappet i.e. cam follower follows the shape of the cam lobe on the camshaft. Thus it
provides a method of converting the angular movement of the cam into a reciprocating motion in
the valve train which is directly proportional to the amount the cam lobe deviates from the base
circle.

Valve spring Retainer locks

The different types of valve spring retainer locks are shown in the figure.


VALVE TIMING

Figure 12-22 shows a typical valve-timing diagram. In
this diagram, the exhaust valve starts to open at 47
degrees before BDC on the power stroke. It stays open
until 21 degrees after TDC on the intake stroke. This
gives more time for the exhaust gases to leave the
cylinder. By the time the piston reaches 47 degrees
before BDC on the power stroke, the combustion
pressures have dropped considerably. Little power is
lost by giving the exhaust gases this extra time to leave
the cylinder.
In a similar manner, the intake valve starts to open at 12
degrees before TDC and remains open for 56 degrees
past BDC after the intake stroke. This gives additional
time for air-fuel mixture to flow into the cylinder. The
delivery of adequate amounts of air-fuel mixture to the
engine cylinders is a critical item in engine operation.
Actually, the cylinders are never quite "filled up" when
the intake valve closes.
The exhaust valve closes 21 degrees after the intake valve opens in Fig. This provides an overlap of
33 degrees during which both valves are open at the same time. Most automotive engines have
valve overlap during the end of the exhaust stroke and the beginning of the intake stroke. Then the
exhaust gases are moving rapidly from the cylinder into the exhaust port. Holding the exhaust valve
open well past TDC after the exhaust stroke gives the gases more time to leave. At the same time,
starting to open the intake valve before TDC on the exhaust stroke gives the incoming air-fuel
mixture a "head-start" toward entering the cylinder.
Timing of the valves is controlled by the shape of the lobe on the cam and the relationship between
the gears or sprocket and chain on the camshaft and crankshaft. Changing the relationship between
the driving and driven gears or sprockets changes the timing at which the valves open and close.
For example, suppose the timing chain is worn and this allows the chain to "jump time." Then the
chain slips a tooth and this causes the camshaft to fall behind that one tooth. The valves would then
open and close later. Now, for example, the valve action has been moved back 15 degrees. The
exhaust valve will open at 32 degrees before BDC on the power stroke. It will close at 36 degrees
after TDC on the exhaust stroke. The intake-valve actions would likewise be moved back. These
valve-action delays would reduce engine performance and cause engine overheating. The gears or
sprockets are marked so that they can be properly aligned on assembly to get the correct valve
timing.
                                                                                                   54
MANIFOLDS

There are separate sets of pipes attached to the cylinder head which carry the air-fuel mixture and
the exhaust gases. These are called manifolds.

INTAKE MANIFOLD

The intake manifold is a tube type hollow
casting of cast iron or aluminium. As
shown in Fig. It consists a inlet port to
receive the air and fuel mixture from the
carburetor. The carburettor is attached to it
by means of studs. The outlet ports of the
intake manifold are attached to the intake
ports of the engine block or engine head
by means of studs and clamps. In case of
V-8 engines, the intake manifold consists
of two intake ports. A two-barrel
carburettor is used on these ports. Each barrel feeds to four separate cylinders. A good design of the
intake manifold consists of a smooth and short path from the carburettor to the cylinders thereby
minimizing the chances of condensing and collecting the fuel on the manifold walls.

EXHAUST MANIFOLD

The exhaust manifold is similar to the intake
manifold but it carries the burned gases away
from the engine cylinders. It is also attached
to the side of the engine block or engine
head, whatever the arrangement of exhaust
valves may be. The inlet ports of the
manifold are clamped to the outlet ports of
the exhaust valves by means of studs and
clamps. The gaskets are provided between
the seats of the engine block and manifold
ports to prevent the leakage of the gases. As
shown in Fig it consist a outlet port from
where the burned gases are passed to the
exhaust pipe, muffler and tail pipe and finally to the atmosphere. In case of V-8 engines, there aree
two exhaust manifolds, one for each bank of cylinders. Sometimes these two manifolds are
connected by a crossover pipe and exhaust through a common muffler and tail pipe. The material
used for exhaust manifold is cast iron or aluminium alloy.

The exhaust manifold is so designed that it may avoid the possibility of overlapping of exhaust
strokes thereby minimizing the back pressure. Therefore some manifolds are divided into two or
more branches so that not more than two cylinders may exhaust into the same branch at the same
time. Back pressure is also minimized by providing large-radius bends thereby eliminating
restrictions and obtaining a streamlining flow of the burned gases through the exhaust manifold.
                                                                                                   55
                       FUEL SUPPLY SYSTEM IN PETROL ENGINES

(Syllabus: Types of fuel feed systems, fuel Pumps & filters, construction, air filter types &
construction, carburetion, simple carburetor, Different circuits in carburetor, types of
carburetors.)

FUEL FEED SYSTEMS

The basic fuel supply system in an automobile with petrol engine consists of a fuel tank, fuel lines,
fuel pump, fuel filter, air cleaner, carburetor and inlet manifold. Following are the types of systems
which have been used for the supply of fuel from the fuel tank to the engine cylinder:

(1) Gravity system      (2) Pressure system   (3) Vacuum system     (4) Pump system
(5) Fuel injection system.

Out of these the first four systems make use of the carburetor while in the fuel injection system the
carburetor has been dispensed with altogether. The gravity system is confined to two wheelers
while the pressure and the vacuum systems are almost obsolete now and the pump system is being
used widely on automobiles. Fuel injection has been employed with certain advantages in some
modern vehicles.

1. Gravity System: In this fuel tank is mounted at the highest position from where the fuel drops
into the carburetor float chamber by gravity. The system is very simple and cheap but the rigidity of
placing the fuel tank necessarily over the carburetor is a disadvantage.
2. Pressure System: In the pressure system a hermetically sealed fuel tank is used. The pressure is
created in the tank by means of engine exhaust or a separate air pump, For starting, the pump is
primed by hand. It is under the pressure thus produced that the fuel flows to the float chamber of
the carburetor. There are chances of pressure leak but the advantage lies in the fact that the fuel
tank can be placed at any suitable location.
3. Vacuum System: This system is based upon the simple fact that the engine suction can be used
for sucking fuel from the main tank to the auxiliary fuel tank from where it flows by gravity to the
carburetor float chamber.
4. Pump System: In this system, a steel pipe carries petrol to the fuel pump which pumps it into the
float chamber of the carburetor through a flexible pipe. If the fuel pump is mechanical, it has to be
driven from the engine camshaft and hence placed on the engine itself. However, electrically
operated fuel pump can be placed anywhere, the rear location (away from the hot engine) reducing
the tendency of forming vapour lock. This system is used most commonly in the present day cars.
5. Fuel Injection System: The petrol injection system has been used successfully on many modem
vehicles. In this carburetor is dispensed with altogether. The fuel is atomized by means of an
injector nozzle and then delivered into an air stream. Separate fuel injectors are used for each
cylinder while the mixture under different load and speed conditions is controlled either
mechanically or now more often electronically. This is most accurate fuel supply system. Since
exact quantity of the fuel can be metered as per the engine requirement, complete combustion can
be obtained and there by substantial reduction in the exhaust emissions can also be attained.

FUEL TANK

It is made of steel or aluminium alloy sheet. The steel tank is usually coated on the inside with a
lead-tin alloy to protect against corrosion. In case of aluminium, there is saving in weight and a
                                                                                                    56
high resistance to corrosion; however, the repairs are comparatively more difficult. Recently
synthetic rubber compounds and flame resistant fibre reinforced plastics have also been employed
to make fuel tanks by moulding.
         Fuel tank is placed in the vehicle at any suitable location. For front engine vehicles, the
usual locations are the underside of the luggage compartment at the rear, or directly above the rear
axle. The latter location has been favoured recently because of the protection here by the rear
wheels and the surrounding body structures in case of any rear-end collision. For cars with engines
at the rear, fuel tank may be located behind the compartment at the front. Fuel tank is divided into
interconnected compartments by means of baffle plates. This arrangement reduces surging of fuel
on account of sudden braking or cornering. If the surging is un-checked, it may interrupt the fuel
supply to the carburetor. Holes are made in the sides of the tank for fixing the fuel gauge sensor, the
supply and the return pipes and the primary filter. The supply line from the fuel tank is taken at
such a level that there is always some dead fuel storage, for the sediments to settle down. A drain
plug is provided at the bottom of the tank for periodic removal of the sediments. The cap over the
fuel filter tube is vented to atmosphere so that the pressure inside the tank is always atmospheric. In
case this vent is choked, a vacuum may be created in the tank with the consumption of fuel by the
engine as a result of which the normal fuel delivery, which is calculated on the assumption of
atmospheric pressure in the tank, may be affected. This vacuum may even cause the tank, to
collapse. Recently in U.S.A., they have started venting the fuel tank to a ‘vapour recovery system'
instead of to the atmosphere for reducing air pollution caused by the evaporation of fuel from the
conventional fuel tank especially in hot season.

FUEL PUMPS

Two main types of pumps most generally used are:
1. A.C. Mechanical Pump. 2. S.U. Electrical Pump.

A.C. Mechanical Pump:

This is a diaphragm type of
pump as shown in Figure. The
diaphragm used is made out of
high-grade cotton impregnated
with synthetic rubber. The
valves are made of bakelite,
which being lighter, keeps the
inertia stresses minimum.

The drive for the pump is
taken from camshaft by means
of an eccentric or cam. The
eccentric operates the rocker
arm which in turn pushes the
diaphragm up and down. Downward movement of the diaphragm causes vacuum in the chamber
which causes the inlet valve to open and the fuel then goes through the strainer to the chamber. The
next upward movement of the diaphragm causes the inlet valve to close while the outlet valve
opens and the fuel goes out to the carburetor float chamber.
                                                                                                     57
When the float chamber of the carburetor is completely filled up, there is no need of pumping more
fuel till some of it is consumed. But if the engine continues to run at light load, the camshaft will be
running all the time and if no other means are provided, the pump will build up excessive pressures
which may damage the pump itself. To avoid this, however, the connection between the rocker arm
and the pull rod is made flexible and when the float chamber is full, the diaphragm is not operated
though the camshaft is running all the time. Reliability is the main advantage of mechanical pumps.
However, these have some disadvantages also:

1. They have to be situated close to the engine due to which they are exposed to engine heat, which
may result in vapour locking in the fuel supply system.
2. They operate only after the engine has started.

S.U. Electrical Pump:

In this type also a diaphragm is
used. Alternate vacuum and
pressure are produced due to the
movement of the diaphragm which
is caused electrically in this case.
Closing the ignition switch
energizes the solenoid winding,
magnetic flux is generated which
pulls the armature to which the
diaphragm is attached. Thus the
diaphragm moves to cause suction
in the pump chamber and the fuel
is drawn into the chamber. But as soon as the armature moves, it interrupts the electric supply by
disconnecting the breaker points, the solenoid is de-energized and the armature falls back, causing
the diaphragm to move to create pressure in the pump chamber which opens the outlet valve and
the fuel goes out to the carburetor float chamber. This movement of the armature, however,
completes the circuit again and the solenoid again gets energized. The whole cycle is again
repeated in this way and the fuel continues to be pumped.
        Electrical pumps need not be situated necessarily close to the engine; mostly these are
located near the fuel tank. Thus these are not subjected to engine heat. Further an electrical pump
need not wait for the engine to start. It starts operating immediately as the ignition is switched on.

FUEL PUMP TESTING

There are three main tests which are performed to judge the performance of a fuel pump. These are
pressure test, volume test and vacuum test.

Each pump must produce certain pressure on the outlet side, which is specified by the
manufacturer. To test a given pump for pressure connect a suitable pressure gauge in between the
pump and the carburetor and run the engine at specified speed, say 500 r.p.m. If the gauge indicates
pressure which is lower or higher than that specified, even after checking that there is nothing
wrong with pipes and connections etc., the pump has to be replaced or repaired and checked again
to see that gives the specified pressure reading only.
The volume test is performed by disconnecting the pump from the carburetor and measuring the
discharge separately by means of a graduated container and stop watch, while the engine is running
                                                                                                       58
at idling speed. It is to be noted here that the engine in this case will be run only by the petrol in the
carburetor float chamber. If the observed rate of flow is less than the specified one, the pump needs
repairs or replacement.
The fuel pump should obviously create some vacuum on its inlet side, which is specified by the
manufacturer. To test it, connect a suitable vacuum gauge in between the fuel tank and the pump
and run the engine at idle speed. The reading of the vacuum gauge must conform to the
specifications. Now stop the engine and see for how much time the vacuum is held in the lines. The
vacuum must be retained for at least 10 seconds after closing the engine. Otherwise the pump has to
be taken out for repairs or replacement.

VAPOUR RETURN LINE

The petrol under the heat of the engine tends to form into vapour. An additional factor aiding the
formation of vapour in the fuel pump is the creation of vacuum during suction of fuel, which lowers
its boiling point. To guard against this, a vapour return line, connecting a special outlet of the fuel
pump to the fuel tank, is provided in some engines. Any vapour formed in the pump, goes back to
the fuel tank where it condenses. Thus the vapour does not go to the carburetor side. This
arrangement also helps to decrease the formation of vapours, because in addition to vapours, the
excess fuel pumped by the fuel pump is also returned to the fuel tank. This continual circulation of
the fuel keeps the pump cool and thus avoids the vapour formation.

AIR CLEANERS           I

As hundreds of cubic meters of air per hour are used by the engine of an automobile, it is very
important that this air should be very clean. Impurities like dust in the air cause a very rapid wear of
the engine, particularly of the cylinders, pistons and rings. It is, therefore customary to install air
cleaner on the intake system of automotive engines. Apart from filtering the air, air cleaner also
performs other functions:
(i) It acts as a silencer for the carburetion system. i.e., it reduces the engine induction noise to an
acceptable level.
(it) In case the engine back fires, the air cleaner also acts as a flame arrester.
Air cleaners offer, however, a resistance to air flow which is increased as the air cleaners get
clogged with dirt. These should, therefore, be cleaned regularly or replaced periodically, say, every
20,000 Km.




The air cleaners generally used are of two types, like:
1. Heavy duty type     2. Light-duty type
                                                                                                    59

Heavy duty type air cleaner

This is of oil bath type. It contains a filter element C saturated with oil. At the bottom there is
separate oil pan D. The air from the atmosphere enters through circumferential gap A. At the corner
B when the air takes a turn, it leaves large, particle impurities there. Next, impinging on the surface
of the oil relieves the air further of impurities. Final cleaning is done by means of filter C and the
clean air passes through passage E as shown.

Light duty types air cleaner: Figure shows a light duty type of air cleaner. It consists of a cleaning
clement only. The clement consists of a cylindrical cellulose fiber material, over which is put a fine
mesh screen to provide strength. Sides of the element are sealed against dust. The air passes
through the element and any dust contained is left outside.

Thermostatic control of air cleaner:

In modem engines, lean fuel-air mixture is employed to reduce air pollution. However, such an
engine would not perform efficiently when cold, e.g., during cold start or idling. To remedy this
incoming air is heated and then sent to the air cleaner having a thermostatic control, sensitive to a
spring which controls an air bleed valve depending upon the temperature of the air entering the
carburetor. When the engine is cold, the bleed valve starts opening as a result of which cold air
from the outside mixes with the hot air, thus increasing its temperature. At about 400C, the bleed
valve opening is maximum when the hot air supply is cut off and only outside air enters the
carburetor.

FUEL FILTERS

The most commonly used filter for cleaning petrol is the fine mesh gauge. It has worked very well,
where large dust particles are involved, but has not proved much effective in preventing the fine
particles and the water from going inside the cylinder. A very simple and effective device used is
the ordinary chamois leather, which if first moistened with petrol will allow only the petrol to pass
through it, and the water will be intercepted. Fine grit, of course, cannot pass through it.

COMBUSTION IN SI ENGINES

Combustion may be defined as a relatively rapid chemical combination of hydrogen and carbon in
the fuel with the oxygen in the air resulting in liberation of energy in the form of heat. Combustion
is a very complicated phenomenon and has been a subject of intensive research for many years. In
spite of this, combustion phenomenon is not fully understood even today.
The conditions necessary for combustion are:

(i) The presence of a combustible mixture; (ii) Some means of initiation combustion, and
(iii) Stabilization and propagation of flame in the combustion chamber. In SI engines the
combustible mixture is generally supplied by the carburetor and the combustion is initiated by an
electric spark given by a spark plug.

A chemical equation for combustion of any hydrocarbon can be easily written. For C18 H18 (iso-
octane) the equation is
                       C8H18 + 12.5 02 = 8 C02 + 9 H20
                                                                                                     60
It is well known that the combustion process is not a simple and direct combination of atoms as
indicated by the chemical equation. As a rule, oxidation reactions have a multi-stage nature and are
chain reactions in which an important role is played by the active intermediate products formed
during the reaction. The actual sequence of stages in the oxidation reactions and combustion of IC
engine fuels has not been fully understood so far.

IGNITION LIMITS

Experiments have shown that ignition
of the charge is only possible within
certain limits of fuel-air ratio. These
'ignition       limits'        correspond
approximately to those mixture ratios,
at lean and rich ends of the scale,
where the heat released by spark is no
longer sufficient to initiate combustion
in the neighbouring unburnt mixture. It
is generally agreed that the flame will propagate only if the temperature of the burnt gases exceeds
approximately 1500 K in the case of hydrocarbon-air mixture. Thus at room temperature, for
example, for the required temperature of 1500 K to be reached, the relative fuel-air ratio (actual
fuel-air ratio divided by Stoichiometric or chemically correct fuel-air ratio) must lie between 0.5
and 2.1. For hydrocarbon fuel the Stoichiometric fuel-air ratio is about 1: 15 and hence the fuel-air
must be between about 1: 30 and 1: 7
The lower and upper ignition limits of the mixture depend upon mixture ratio and temperature. The
ignition limits are wider at increased temperatures because of higher rates of reaction and higher
thermal diffusivity coefficients of the mixture.
CARBURETION

In the SI engine a
combustible fuel-
air mixture is
prepared outside
the          engine
cylinder.       The
process           of
preparing       this
mixture is called
carburetion. This
complicated
process is achieved in the induction system, which is shown in figure. The carburetor, a device
which atomizes the fuel and mixes it with air, is the most important part of the induction system.
The pipe that carries the prepared mixture to the engine cylinders is called the intake manifold.
During the suction stroke vacuum is created in the cylinder which causes the air to flow through the
carburetor and the fuel to be sprayed from the fuel jets. Because of the volatility of the fuel, most of
the fuel vaporizes and forms a combustible fuel-air mixture. However, some of the larger droplets
may reach the cylinder in the liquid form and must be vaporized and mixed with air during the
compression stroke before ignition by the electric spark. Four important factors which significantly
affect the process of carburetion are:
                                                                                                61
1. The time available for the preparation of the mixture.
2. The temperature of the incoming air of the intake
manifold.
3. The quality of the fuel supplied.
4. The design of the induction system and combustion
chamber.

Atomization, mixing, and vaporisation, are the processes
which require a finite time to occur. In high speed engines
the time available for mixture formation is very small. For
example, for an engine running at 3000 r.p.m. the induction
process lasts less than 0.02 seconds. To complete these
processes during such a small period requires great
ingenuity in designing the carburetion system. Because of
the short time available complete and efficient mixing,
vaporisation, and distribution is difficult to achieve.
Temperature is a factor which controls the vaporisation
process of the fuel. A high temperature results in high rate
of vaporisation. The temperature of the mixture can be
increased by, say, heating the induction manifold but this
would reduce power due to reduction in mass flow. The
volatility of the fuel affects the vaporisation and
distribution of fuel. The design of the induction manifold
will affect the uniform distribution of the mixture among the cylinders and the constancy of its
composition under variable operating conditions.

The design of carburetion system of the SI engine is complicated because the optimum air-fuel ratio
required by it varies widely over its operational range, particularly in the case of an automobile
engine. When the engine is idling, a richer mixture is required due to dilution of mixture by
products of combustion. Again at full load
condition a richer mixture is required for
maximum power. Therefore, to study engine
air-fuel mixture requirements in detail it is
necessary to study the properties or
characteristics of the various air-petrol
mixtures in the SI engine.

PROPERTIES OF THE
AIR-PETROL MIXTURES



There is a limited range of air-fuel ratios, in
a homogeneous mixture, which can be
ignited in a SI engine. These limits are about
7: 1 A/F by mass (0.14: 1 F/A) on the rich
side, and about 20: 1 A/F (0.05: 1 F/A) on
the lean side in single cylinder engines.
Figure shows the effect of A/F ratio on
power output and specific fuel consumption
for a SI engine for full throttle and constant speed condition.
                                                                                                    62

Figure below shows a typical fuel consumption loop for various A/F ratios (see Table also). The
most important hint to note from these figures is that the A/F ratio for maximum power is not the
same as the A/F ratio for maximum economy. Further, the A/F ratio for maximum power and
maximum economy varies with load as shown in fig.

(a) Mixture requirements for maximum power. Figure shows that the maximum power is obtained
at about 12.5 - 1 A/F (0.08: 1 F/A). The maximum energy is released when slightly excess fuel is
introduced so that all the oxygen present
in the cylinder is utilized. More fuel than
this does no help. In fact it is
disadvantageous because the combustion
of a large excess of fuel with the same
amount of oxygen results in smaller
energy release due to partial combustion,
and more carbon monoxide is formed.
Mechanical efficiency is maximum at
maximum power position.

(b) Mixture requirement for minimum
specific fuel consumption.

Figure above shows that at full throttle,
maximum efficiency occurs at an A/F
ratio of about 17: 1 (0.06: 1 F/A ratio. The
maximum efficiency occurs at a point
slightly leaner than the chemically correct
A/F ratio because excess air requires
complete combustion of fuel when mixing
is not perfect; and the power maximum
temperatures associated with the inlet
mixture favourably affect the chemical
equilibrium and specific inlet of the gases.
However, if the mixture is made too lean,
the flame speed is reduced so much that
the large time losses overcome the above-
mentioned beneficial effects, and the
efficiency falls off.

Various investigators have experimentally
determined the properties of air-petrol mixtures which are summarized in a Table. For practical
purposes the portion of the fuel consumption loop between B and D is important.

Fuel Consumption Loop or Hook Curve: The properties of the air-petrol mixture can be found
by conducting a test with air/fuel ratio as the only variable; speed, throttle opening and igniting
timing being constant. The air-fuel ratio is varied in the range a petrol engine can operate-from
every rich, A/F ratio of 10: 1 to very lean, A/F ratio 22: 1. At the two extremes, of curse, the engine
may not run satisfactorily. The results are plotted-specific fuel consumption to a base of m.e.p. and
a fuel consumption loop or hook curve is obtained. For a single cylinder engine the curve is well
                                                                                                    63
defined as shown in Fig. 11.3. Point A represents the weakest mixture (maximum A/F ratio. 18-22)
at which specific fuel consumption is high and m.e.p. is low. The engine may run unsteadily and
may be so slow that the gases continue burning in the clearance volume until the next suction stroke
begins, causing popping back through the carburetor. As the air-fuel ratio is decreased, specific fuel
consumption decreases and m.e.p. increases. B, with an A/F of about 16 is the point of maximum
economy but the power is about 10-15% less than the maximum. Reducing the A/F further
stoichiometric condition (A/F = 15) is reached, which is the best compromise between maximum
economy and maximum power, fuel consumption is about 4% more than minimum and power
about 4% less than the maximum. Further reducing the air-fuel ratio, say to 12: 1 to 13: 1, gives the
point of maximum power D, but the sfc. is 25 to 30% more than minimum. Further reducing the
A/F ratio say 9-11, point E, adversely affects both sfc and m.e.p. At E running may be unsteady and
there may be combustion of mixture in the exhaust system.




             MIXTURE REQUIREMENTS FOR STEADY STATE OPERATION

A carburetor is a mechanical device designed to fulfill the following conditions:

(1) Meter the liquid fuel in such quantities as to produce the air-fuel ratio required by the engine at
all speeds and loads (steady as well as transient conditions).
(2) Atomize the fuel and mix it homogeneously with the air.
                                                                                                  64
In stationary engines the desired air-fuel
ratio is that which gives the maximum
economy. Automotive engines, operating
under variable speed and load conditions,
present the most difficult requirements to
carburetors. The air-fuel ratios must change
depending on whether maximum economy
or maximum power is required. Also,
required air-fuel ratio must be provided for
transient conditions like starting, warm-up,
and acceleration. Further, in all conditions
the consideration of minimum exhaust
emission should be kept in mind. For
automotive engines there are three main
areas of steady state operation requiring
different air-fuel ratios. (Steady state
operation means continuous operation at a
given speed and power output with normal
engine temperature). These are given in
Table 11.2 and figure shown.

1. Idling and low load. (From no load to about 20% of rated power):
 The no load running mode of the engine is called idling. During idling, the air supply is restricted
by the nearly closed throttle and the suction pressure is-very low. This condition of low pressure
gives rise to backflow of exhaust gases and air leakage from the various parts of the engine intake
system.

At idling and during part load operation backflow during the valve overlap period occurs since the
exhaust pressure is higher than the intake pressure. This increases the amount of residual gases.
During the suction Process the residual gases expand, thereby, reducing the fresh mixture inhaled.
Increase dilution causes the combustion to be erratic or even impossible. Irregular and slow
combustion, so obtained, results in poor thermal efficiency and higher exhaust emissions. The
problem of dilution by residual gases becomes more pronounced at low loads and idling because
the exhaust temperature reduces with decreasing load, i.e. the density and hence mass of the
residual gases increase.

Further dilution of the charge occurs due to air leakage past valves, etc., at low inlet manifold
pressures obtained at idling and low loads. The above two phenomena require that the air-fuel
ratios used for idling and low loads, say up to about 20 per cent of full load, should be rich for
smooth engine operation (F/A ratio 0.08 or A/F ratio 12.5: 1). Up to this point the amount of fuel
burned is quite small and, hence, fuel economy is not important.

How rich mixture improves combustion needs some explanation. The amount of fresh charge
brought in during idling is much less than that during full throttle operation. The presence of
exhaust gas tends to obstruct the contact of fuel and air particles - a requirement necessary for
satisfactory combustion. The richening of mixture increases the probability of contact between fuel
and air particles and thus improves combustion.
                                                                                                     65
2. Normal power range or cruising range. (From about 20% to 75% of rated power):

As the load is increased above 20% of rated load, the dilution by residual gases as well as leakage
decreases, and therefore in the normal power range the prime consideration is usually the fuel
economy. As already stated maximum fuel economy occurs at A/F ratio of about 17: 1 (F/A - 0.06).
Practical values are about 5% higher (AIF ratio 16.7: 1). Due to manufacturing tolerances provided
in the carburetor the air-fuel ratio supplied by it varies during operation. A value of about ± 6% is
typical of a standard carburetor. A closer control of these tolerances, say to a value of about ± 3%,
would allow leaner mixtures to be used and thereby, improving both the fuel economy as well as
the exhaust levels. (The mixture ratios for best economy are very near to the mixture ratios for
minimum emissions).

3. Maximum power range. (From about 75% to 100% rated power):

 As already stated the mixture requirement for maximum power is a rich mixture, of A/F about 14:
1 or (F/A = 0.07). Besides providing maximum power, a rich mixture also prevents overheating of
exhaust valve at high load and inhibits detonation. At high load there is greater heat transfer to
engine parts. Enriching the mixture reduces the flame temperature and the cylinder temperature,
thereby reducing the cooling problem and lessening the chances of damaging the exhaust valves.
Also, reduced temperature tends to reduce detonation. Aircraft engines have elaborate arrangement
for enrichment of mixture, as detonation can wreck the engine in a matter of seconds. (In
automobile engines elaborate arrangement for enrichment is not required as these engines generally
operate well below full power. Further in these engines detonation is audible as 'knock' and the
driver can take corrective measure by decreasing the load or increasing the engine speed by shifting
to a lower gear ratio).

DISTRIBUTION

In multi-cylinder engines the carburetor supplies the air-fuel mixture to each of several cylinders.
Ideally each cylinder should receive mixture of the same air-fuel ratio. However, in practice it is
very difficult to achieve this ideal condition.

In the carburetor, complete atomization
and vaporization of the fuel is not
achieved. Therefore, the mixture passing
through the intake manifold generally
contains a certain amount of petrol in the
droplet form. These droplets have greater
inertia than the gaseous mixture and,
hence, whenever the direction of flow is
changed abruptly, the droplets tend to
continue in their original direction of
movement, as shown in Fig. As a result,
there is a variation in the A/F ratio
between cylinders, the outer cylinders getting richer mixture than the inner cylinders, the outer
cylinders getting richer mixture than the inner cylinders. There is another factor which contributes
to this uneven distribution-the existence of thin film of liquid fuel adhering to the inner walls of the
intake manifold. The remedies adopted to partially correct this imbalance in A/F ratio in different
cylinders are the following:
                                                                                                     66
(i) The mixture in the intake manifold is heated to vaporize the liquid droplets. However, this
reduces the mass of the charge with consequent reduction in power output.

(ii) A rich overall air-fuel mixture is supplied so that the leanest cylinder receives the required A/F
ratio. Of course, this would result in other cylinders getting a richer mixture than necessary.

TRANSIENT MIXTURE REQUIREMENTS

Besides providing a suitable mixture for steady-running a carburetor has to provide mixture for
transient conditions under which speed, load temperature, or pressure change rapidly. The principle
transient conditions of operation are starting, warming up, acceleration, and deceleration.
The transient mixture requirements are different from steady running mixture requirements because
in the transient case the evaporation of the fuel maybe incomplete, the quantity of liquid fuel in the
inlet manifold may be increasing or decreasing, and the distribution of fuel to various cylinders
may be different. To understand why evaporation may be incomplete it is important to remember
that the petrol is a mixture of hydrocarbons having different boiling temperatures. Those which
have high vapour pressures and low boiling points are called 'light ends' and those which are less
volatile are called 'heavy ends'. The temperature of the vapour when 10 per cent of fuel is vaporized
is called the '10 per cent point', and the temperature of the vapour when complete fuel is vaporized
is called the 'end point'.
1.Starting and warm up requirements.

While starting from cold the speeds as well as engine temperatures are low, hence much of the
'heavy ends' supplied by the carburetor do not vaporize and remain in liquid form. Also, the
vaporized fuel may re-condense on coming in contact with cold cylinder walls and piston head.
Therefore even when the fuel-air ratio at the carburetor is well within the normal combustion Units
or petrol-air mixtures, the ratio of evaporated fuel to air in the cylinder may be too lean to ignite.
Therefore during starting a very rich mixture must be supplied, as much as 5 to 10 times the normal
amount of petrol (A/F ratio 3 : 1 to 1.5: 1 or F/A ratio 0.3 to 0.7), so that enough 'light ends' are
available for proper ignition. As the engine warms up the amount of evaporated fuel increases and
hence the mixture ratio should be progressively made leaner to avoid too rich evaporated fuel-air
ratio.
The volatility of the fuel significantly affects the starting and warm up characteristics of the engine.
Lower the 10 per cent point of the fuel (i.e. higher volatility), more is the quantity of evaporated
fuel and hence less rich mixtures would be needed for starting. The 50 per cent point dictates, to
some extent, the warm up characteristics.
Note. Too high or too low volatility, both create difficulties in operation. Too high volatility may
form vapour bubbles in the carburetor and fuel lines particularly when the engine temperatures are
high, which interfere with the supply' and metering of the fuel and may disturb the fuel-air ratio so
seriously as to cause the engine to stop. Too low volatility may cause the petrol to condense on the
cylinder walls, diluting and removing the lubricating oil film. Eventually the petrol may reach the
crankcase past the piston rings and dilute the engine oil. Condensation of petrol on cylinder walls
also causes carbon deposits. Thus petrol should be carefully made to suit the type of the engine and
the climate of the place.

2. Acceleration requirements.

The term 'acceleration' with regard to engines, is generally used to refer to an increase in engine
speed resulting from opening the throttle. However, the main purpose of opening the throttle is to
                                                                                                    67
provide an increase in torque and whether or not an increase in speed follows depends on the nature
of the load.

The fuel evaporated in the intake manifold moves much faster than the liquid film formed on the
induction system walls. Under steady running conditions this does not cause any problem but when
the throttle is suddenly opened, e.g. during acceleration, the liquid fuel lags behind and,
temporarily, the cylinder receives a lean mixture whilst actually a rich mixture is needed to produce
more instantaneous power for acceleration. Therefore, to compensate for the temporary leaning of
the mixture and to provide rich mixture needed during the accelerating period, additional fuel must
be supplied by a suitable mechanism. This mechanism is known as acceleration pump.

A SIMPLE OR ELEMENTARY CARBURETOR

To understand a modern carburetor which is a complicated piece of equipment, it is helpful to first
study a simple or elementary carburetor which provides an air-fuel mixture for cruising or normal
range at a single speed and then to add it other mechanisms to provide for other duties like starting,
idling, variable load and speed operation and acceleration.

Fig.     shows     a    simple
carburetor. It consists of a
float chamber, nozzle with
metering orifice, venturi and
throttle valve. The float and a
needle valve system maintain
a constant height of petrol in
the float chamber. If the
amount of fuel in the float
chamber falls below the
designed level, the float
lowers, thereby opening the
needle of fuel supply valve.
When the designed level has
been reached, the float closes
the needle valve, thus
stopping additional fuel flow
from the supply system. Float chamber is vented to the atmosphere.

During suction stroke air is drawn through the venturi. Venturi is a tube of decreasing cross-section
which reaches a minimum at the throat (Venturi tube is also known as choke tube and is so shaped
that it gives minimum resistance to air flow). The air passing through the venturi increases in
velocity and the pressure in the venturi throat decreases. From the float chamber, the fuel is fed to a
discharge jet, the tip of which is located in the throat of the venturi. Now because the pressure in
the float chamber is atmospheric and that at the discharge jet is below atmospheric a pressure
differential, called carburetor depression, exists between them. This causes discharge of fuel into
the air stream and the rate of flow is controlled or metered by the size of the smallest section in the
fuel passage. This is provided by the discharge jet and the size of this jet is chosen empirically to
give the required engine performance. The pressure at the throat at the fully open throttle condition
lies between 4 to 5 cm of Hg below atmospheric, and seldom exceeds 8 cm Hg below atmospheric.
                                                                                                      68
To avoid wastage of fuel, the level of the liquid in the jet is adjusted by the float chamber needle
valve to maintain the level a short distance below the tip of the discharge jet.

The petrol engine is quantity governed which means that when less power is required at a particular
speed the amount of charge delivered to the cylinders is reduced. This is achieved by means of a
throttle valve of the butterfly type which is situated after venturi tube. As the throttle is closed less
air flows through the venturi tube and less is the quantity of air-fuel mixture delivered to the
cylinders and hence less is the power developed. As the throttle is opened, more air flows through
the choke tube, and the power of the engine increases.

A simple carburetor of the type described above suffers from a fundamental fault in that it provides
increasing richness as the engine speed and air flow increases with full throttle because the density
of the air tends to decrease as the rate of air flow increases. Also, it provides too lean mixtures at
low speeds at part open throttle. This phenomenon can be explained as follows: Since the throttle
regulates the amount of air flowing up the venturi tube, it also checks the quantity of fuel issuing
from the nozzle by regulating the vacuum at the throat. At low engine speeds with part open throttle
for (at low air flow rates) the vacuum at the throat is small and hence we get too lean a mixture. At
high engine speeds the vacuum at the throat is high and hence we get too rich mixture.

COMPLETE CARBURETOR

In order to satisfy the demands of an engine under all conditions of operation the following
additional systems are added to the simple carburetor:

(i) Main metering system
(ii) Idling system
(iii) Power enrichment by economizer system
(iv) Acceleration pump system
(y) Choke.

Main Metering System:

 The main metering system of a carburetor is designed to supply a nearly constant basis fuel-air
ratio over a wide range of speeds and loads. This mixture corresponds approximately to best
economy at full throttle (A/F ratio - 15.6 or F/A ratio 0.064). Since a simple or elementary
carburetor tends to enrich the mixture at higher speeds automatic compensating device are
incorporated in the main metering system to correct this tendency. These devices are:

(i) Use of a compensating jet that allows an
increasing flow of air through a fuel
passage as the mixture flow increases.
(ii) Use of emulsion tube for air bleeding.
In this device the emphasis is on air
bleeding alone.
(iii) Use of a tapered metering pin that is
arranged to be moved in and out of the
main or auxiliary fuel orifice either
manually or by means of some automatic
                                                                                                   69
mechanism changing the quantity of fuel drawn into the air charge.
(iv) Back-suction: suction control or pressure reduction
in the float chamber.
(v) Changing the position or jet in the venturi. The
suction action is highest at the venturi throat, therefore
by raising the venturi the nozzle relatively moves to
points with smaller suction and the flow of fuel is
decreased.
(vi) Use of an auxiliary air valve or port that
automatically admits additional air as mixture flow
increases.

The main devices are explained below in detail.

i.(a) Compensating jet device.

This device is shown in Fig. In this device, in addition
to the main jet, a compensating jet is provided which is
in communication with a compensating well. The
compensating well is open to atmosphere and gets its
fuel supply from the float chamber through a restricting orifice. As the air flow increases, the level
of fuel in the well decreases, thus reducing the fuel supply through
the compensating jet. The compensating jet thus tends towards
leanness as the main jet tends towards richness, the sum of the two
remaining constant as shown in the graph. At even higher rates of
air flow, when the compensating jet has been emptied, air is bled
through the compensating jet to continue the leanness effect, and
incidentally, to assist in fuel atomization.

i.(b) Emulsion tube or air bleeding device.

In the modern carburetors the mixture correction is done by air
bleeding alone. In this arrangement the main metering jet is fitted
about 25 mm below the petrol level and it is called a submerged
jet. The jet is situated at the bottom of a well, the sides of which
have holes which are in communication with the atmosphere. Air
is drawn through the holes in the well, the petrol is emulsified, and
the pressure difference across the petrol column is not as great as
that in the simple or elementary carburetor. Initially, the petrol in
the well is at a level equal to that in the float chamber. On opening the throttle this petrol, being
subject to the low throat pressure, is drawn into, the air. This continues with decreasing mixture
richness as the holes in the central tube are progressively uncovered. Normal flow then takes place
from the main jet.

i.(c) Back-suction control or pressure reduction method.

A common method of changing the air-fuel ratio in large carburetors is the back suction control
shown in Fig. In this arrangement a relatively large vent line connects the carburetor entrance (say
point 1) with the top of the float chamber. Another line, containing a very small orifice line,
                                                                                                   70
connects the top of the float chamber with the
venturi throat (say point 2). A control valve is
placed in the large vent line. When the valve is wide
open, the vent line is unrestricted the pressure in the
float chamber is equal to p1 and the pressure
difference acting on the fuel orifice is (p1 - p2).
 If the valve is closed, the float chamber
communicates only with the venturi throat and the
pressure on the fuel surface will be P2. Then delta
pf will be zero, and no fuel will flow. By adjusting
the control valve, any pressure between p1 and P2
may be obtained in the float chamber, thus changing
the quantity of fuel discharged by the nozzle.


i.(d) Auxiliary valve carburetor.

An auxiliary valve carburetor is illustrated in Fig. With an increase of engine load, the vacuum at
the venturi throat also increases. This causes the valve spring to lift the valve admitting additional
air and the mixture is prevented from becoming over-rich.

i.(e) Auxiliary port carburetor.

An auxiliary port carburetor is illustrated in Fig. By opening the butterfly valve, additional air is
admitted and at the same time the depression at the venturi throat is reduced, decreasing the
quantity of fuel drawn in. This method is used in aircraft
carburetors for altitude compensation.




Auxiliary valve carburetor                                                  Auxiliary            port
carburetor
                                                                                             71
(ii) Idling System.
It has already been shown that at idling and low load the
engine requires a rich mixture (about A/F 12: 1).
However, the main metering system not only fails to
enrich the mixture at low air flows but also supplies no
fuel at all at idling. For this reason, a separate idling jet
must be added to the basic carburetor. An example of
idling jet is shown in fig. It consists of a small fuel line
from the float chamber to a point a little on the engine
side of the throttle. This line contains a fixed fuel orifice.
When the throttle is practically closed, the full manifold
suction operates on the outlet to this jet. In addition, the
very high velocity past the throttle plate increases the
suction locally. Fuel can therefore be lifted by the
additional height up to the discharge point, but this
occurs only at very low rates of air flow. As the throttle is
opened, the main jet gradually takes over while the idle
jet becomes ineffective. The desired air-fuel ratio for the
idling jet is regulated manually by idle adjust, which is a
needle valve controlling the air bleed.

(iii) Power Enrichment or Economizer System.
As the maximum power range of operation (75% to
100% load) is approached, some device must allow richer
mixture (A/F about 13.1, F/A 0.08) to be supplied despite
the compensating leanness. Such a device is the meter
rod economizer shown in Fig. The name economizer is
rather misleading. It stems from the fact that such a
device provides a rich uneconomical mixture at high load
demand without interfering with economical operation in
the normal power range. The meter rod economizer
shown in figure simply provides a large orifice opening
to the main jet as the throttle is opened beyond a certain
point.
The rod may be tapered or stepped. Other examples provide for the opening of auxiliary jets
through some linkage to the throttle movement or through a spring action when manifold vacuum is
lost as the throttle is opened.

(iv) Acceleration Pump System.

It has already been shown that when it is
desired to accelerate the engine rapidly, a
simple carburetor will not provide the
required rich mixture. Rapid opening of the
throttle will be immediately followed by an
increased air flow, but the inertia of the
liquid fuel will cause at least a momentarily
lean mixture just when richness is desired
for power. To overcome this deficiency an
                                                                                                   72
acceleration pump is provided, one example of which is shown in Fig. The pump consists of a
spring-loaded plunger. A linkage mechanism is provided so that when the throttle is rapidly opened
the plunger moves into the cylinder and forces an additional jet of fuel into the venturi. The plunger
is raised again against the spring force when the throttle is partly closed. Arrangement is provided
so that when the throttle is opened slowly, the fuel in the pump cylinder is not forced into the
venturi but leaks past plunger or some holes into the float chamber. Instead of the mechanical
linkage shown some carburetors have a pump plunger held up by manifold vacuum. Whenever that
vacuum is reduced by the rapid opening of the throttle, a spring forces the plunger, down in a
pumping action identical to that of the pump illustrated.
(v) Choke.

During cold starting period, at low cranking speed and before
the engine has warmed up, a mixture much richer than usual
mixtures (almost 5 to 10 times more fuel) must be supplied
simply because a large fraction of the fuel will remain liquid
even in the cylinder, and only the vapour fraction can provide
a combustible mixture with the air. The most common means
of obtaining this rich mixture is by the use of a choke, which
is a butterfly type of valve placed between the entrance to the
carburetor and the venturi throat as shown in Fig. 11.19. By
partially closing the choke, a large pressure drop can be
produced at the venturi throat that would normally result
from the amount of air flowing through the venturi. This
strong suction at the throat will draw large quantities of fuel
from the main nozzle and supply a sufficiently rich mixture
so that the ratio of evaporated fuel to air in the cylinders is
within combustible limits. Choke valves are sometimes made
with a spring loaded by-pass so that high pressure drops and
excessive choking will not result after the engine has started
and has attained a higher speed. Some manufacturers make the choke operate automatically by
means of a thermostat such that when the engine is cold the choke is closed by a bimetallic element.
After Starting and as the engine warms up the bimetallic element gradually opens the choke to its
fully open position.
CARBURETOR TYPES
There are basically two types of carburetors, open choke and constant vacuum type. In the former,
the main orifice, known as the
choke tube or venture, is of fixed
dimensions, and metering is
effected by I would like to bring your
kind attention that varying the
pressure drop across it. In the case
of the constant vacuum type the
area of the air passage is varied
automatically while the pressure
drop is kept approximately
constant.
Almost all carburetors, except S.U.
carburetor are of open choke type.
The important examples of open
                                                                                                  73
choke type are: Zenith, Solex, Carter and Stromberg carburetors. S.U. carburetor is of constant
vacuum type. Carburetors may be up-draught, horizontal or downdraught. The downdraught and
horizontal are most widely used. The downdraught has the advantage that the mixture is assisted.
by gravity in its passage into the engine induction tract, and at the same time the carburetor is
usually reasonably accessible. The horizontal -version has some advantage where under bonnet
space is limited. The up-draught variety is now almost obsolete and is only used where neither of
the other types can be accommodated.

          DESCRIPTION OF SOME IMPORTANT MAKES OF CARBURETORS

   1. Solex Carburetor.




1. Float                             2. Main jet                    3. Choke tube / Venturi
4. Emulsion tube                     5. Air correction jet          6. Nozzles
7. Butterfly valve                   8. Starter disc                9. Starter petrol jet
11. Starting passage                 10. Air jet                    12. Starter lever
13. Pilot jet                        14. Pilot air bleed orifice    15. Idle screw
16. Idle port                        17. Slow speed opening         18. Pump injector
19. Pump lever                       20. Pump jet                    21. Pump inlet valve

The Solex carburetor is famous for ease of starting, good performance, and reliability. It is made in
various models, and is used in Fiat, Standard cars and Willys jeep. Figure shows a schematic
arrangement of a Solex carburetor. The unique feature of this carburetor is the Bi-starter for cold
starting. The various circuits of the Solex carburetor are described below:
                                                                                                      74
(i) Normal running: The Solex carburetor has a conventional float (1) in a float chamber. For
normal running, the fuel is provided by the main jet (2) and the air by the choke tube or venturi (3).
The fuel from the main jet passes into the well of the air bleed emulsion system. (4) is the emulsion
tube which has lateral holes. The correct balance of air and fuel is automatically ensured, by air
entering through and being calibrated by the, air correction jet (5). The metered emulsion of fuel
and air is discharged through the spraying orifice or nozzles (6) drilled horizontally in the vertical
stand-pipe in the middle of choke tube or venturi. (7) is the conventional butterfly valve.

(ii) Cold starting and warming: The unique feature of Solex carburetors is the provision of a bi-
starter or a progressive starter. The starter valve is in the form of a flat disc (8) with holes of
different sizes. These holes connect the starter petrol jet (9) and starter air jet sides to the passage
which opens into a hole just below the throttle valve at (11). Depending upon the position of the
starter lever (12) either bigger or smaller holes come opposite the passage. The starter lever, which
rotates the starter valve, is operated from the dashboard control by means of a flexible cable.
Initially, for starting richer mixture is required and after the engine starts the richness required
decreases. So in the start position (the starter control pulled out fully) bigger holes are the
connecting holes. The throttle valve being-in the closed position the whole of the engine suction is
applied to starting passage (11), sucking petrol from jet (9) and air from jet (10). The jets and
passages are so shaped that the mixture supplied to the carburetor is rich enough for starting.
After the engine has started, the starter lever is brought to the intermediate position, bringing the
smaller hole in the starter valve (8) into circuit, thus reducing the amount of petrol. Also in this
position, the throttle valve is partly open, so that the petrol is also coming out from the main jet.
The reduced mixture supply from the starter system in this situation is however sufficient to keep
the engine running, till it reaches the normal running temperature, when the starter is brought to off
position (control rod completely pushed home).

(iii) Idling and slow running: From the lower part of the well of the emulsion system a hole leads
off to the pilot jet (13). At idling the throttle is almost closed and hence engine suction is applied at
the petrol jet. Fuel is drawn there-from, and mixed with a small amount of air admitted through the
small pilot air bleed orifice (14) and forms an emulsion which is conveyed down the vertical
channel and discharged into the throttle body past the idling volume control screw (15). The slow
running adjustment screw allows the speed of the engine to be varied. The volume control screw
(15) which permits variation of the slow running jet's delivery of petrol, allows the richness of the
mixture to be corrected with accuracy.

To ensure the smooth transfer from idle and low speed circuit to the main jet circuit without the
occurrence of flat spot, bypass orifice (17) is provided on the venturi side of the throttle valve. As
the throttle is opened wide, the suction at idle port (16) is decreased. But suction is also applied
now at the slow speed opening (17), which more than offsets the loss of suction at the idle port and
thus flat spot is avoided.

(iv) Acceleration: To avoid flat spot during acceleration a diaphragm type acceleration pump is
provided. (This is also known as economy system). It delivers spurts of extra fuel needed for
acceleration through pump injector (18). Pump lever (19) is connected to the accelerator so that
when the pedal is pressed, the lever moves towards left, pressing the membrane towards left-thus
forcing the petrol through pump jet (20) and injector (18). When the pedal is left free, the lever
moves the diaphragm back towards right creating vacuum towards left which opens the pump inlet
valve (21) and thus admits, the petrol from chamber into the pump.
                                                                                                       75
Carter Carburetor.




Carter carburetor is -an American make carburetor and is used in jeep. A diagrammatic view of a
downdraft type carburetor is shown in Fig. 11.22 and is described below.

The petrol enters the float chamber (1) which is of the conventional type. The air enters the
carburetor from the top, a choke valve (2) in the passage remains open during normal running. The
Carter carburetor has a triple venturi diffusing type of choke, i.e., it has three venturies, the smallest
lies above the level in the float chamber, other two below the petrol level, one below other. At very
low speeds, suction in the primary venturi (3) is adequate to draw the petrol. The nozzle (4) enters
the primary venturi at an angle, delivering the fuel upwards against the air stream securing an even
flow of finely divided atomized fuel. The mixture from the primary venturi passes centrally through
the secondary venturi (5) where it is surrounded by a blanket of air stream and finally this leads to
the third (main) venturi (6), where again the fresh air supply insulates the stream from the second
venturi. The mixture reaches the engine in atomized form. Multiple venturies result in better
formation of the mixture at very low speeds causing steady and smooth operation at very low and
also at very high engine speeds.

In this carburetor a mechanical metering method is employed. In the fuel circuit there is a metering
rod (7) actuated by a mechanism connected with the main throttle. The metering rod has two or
more steps of diameter. The area of opening between the metering rod jet and metering rod governs
the amount of petrol drawn into the engine. At top speed the metering rod is lifted, the smallest
section of the rod is in the jet and the maximum quantity of petrol flows out to mix with the
maximum amount of air corresponding to full throttle opening.

Starting circuit. For starting a choke valve (2) is provided in the air circuit. The choke valve is of
butterfly type, one half of which is spring controlled. The valve is hinged at the centre. When the
                                                                                                     76
engine is fully choked, the whole of the engine suction is applied at the main nozzle, which then
delivers fuel. As the air flow is quite small, the mixture supplied is very rich. Once the engine fires,
the spring-controlled half of the choke valve is sucked open to provide correct amount of air during
warming up period.

Idle and low speed circuit. The idling circuit is shown in the figure. For idling rich mixture is
required in small quantity. In idling condition throttle valve (8) is almost closed. The whole of the
engine suction is applied at the idle port (9). Consequently the petrol is drawn through the idle feed
jet (10) and air through first bypass (11) and a rich idle mixture is supplied. In low speed operation
the throttle valve is opened further. The main nozzle also starts supplying the fuel. At this stage the
fuel is delivered both by the main venturi and low speed port (12) through idle passage.

Acceleration pump circuit. The acceleration pump is meant to overcome flat spot in acceleration.
The pump consists of a plunger (13) working inside a cylinder consisting of inlet check valve (14)
and outlet check valve (15). The pump plunger is connected to accelerator pedal by throttle control
rod (16). When the throttle is suddenly opened by pressing accelerator pedal, pump is actuated and
a small quantity of petrol is spurted into the choke type by a jet (17). Leaving the accelerator pedal
causes the pump piston to move up, thereby, sucking fuel from the float chamber for next
operation. The purpose of acceleration pump is only to provide an ex1ra spurt of fuel during
acceleration to avoid flat spot and not to supply fuel continuously for heavy load.


   2. S.U. Carburetor.

Carburetors in general are
'constant choke' type. Zenith,
Solex and Carter carburetors are
examples of this type. S.U.
carburetor differs completely
from them being 'constant
vacuum or depression' type with
automatically variable choke.
Fig.      11.23     shows       the
diagrammatic view of the basic
components of a horizontal S.U.
carburetor. The carburetor has a
conventional system of float
chamber (not shown) which
feeds fuel into a vertical channel
in which is situated the jet sleeve
(1). The sleeve bears a number
of holes in its side, so that the
fuel will enter the sleeve and
thus stand at the same level as in
the float chamber.

In to the jet sleeve (1) fits the
tapered metering needle (2)
which is secured by a grub screw
                                                                                                      77
into the choke piston or piston assembly (3). This part of the piston slides up and down in the air
passage, one end of which is connected to atmosphere through air cleaner and the other to the
engine through a conventional throttle valve (4). The upper portion of the choke piston if formed
into a suction disc (5), which is a sliding fit in the suction chamber (6), the whole assembly being
located by a hardened hollow steel guide rod (13), which has its bearing in the long boss in the
centre of the suction chamber as shown. The upper side of suction disc is connected to the air
passage through a suction hole (7) and the lower side to the atmosphere through an atmospheric
hole (8).

The working of the carburetor is as follows:

By means of the suction hole, engine depression is transmitted to the upper face of the suction
piston, while atmospheric air pressure is admitted to the lower chamber through the atmospheric
hole. (The engine depression depends on engine demand adjusted by the degree of throttle
opening). 'Thus the position of choke piston at any instant depends upon the balance of its own
weight and a light spring (9) if provided, (down) against the vacuum force (up). As the weight of
the piston is constant, the vacuum also remains constant. Thus constant vacuum, variable choke
(i.e. variable cross-sectional area of air passage) is obtained. Due to constant vacuum an
approximately constant air velocity is maintained. With the vertical movement of the choke piston,
the metering needle also moves up and down concentrically in the petrol jet orifice (10), thus
varying the effective area of the jet. As the piston rises under increased suction, the tapered needle
also moves upwards and increases the effective jet area, allowing a greater amount of petrol to flow
.into the main air stream and vice versa. Thus approximately constant air-fuel ratio is maintained.

The unique feature of the S.U. carburetor is that it has only one jet. There is no separate idling jet or
acceleration pump. Since a constant high air velocity across the jet is maintained even under idling
condition, the necessity for a separate idling jet is obviated.

For cold starting a rich mixture is required. This is provided by an arrangement to lower the jet tube
away from the needle by means of the jet lever, thereby enlarging the jet orifice. The lever is
operated from the dash board in the car.

There is also an arrangement providing a slightly rich mixture on acceleration. For this purpose an
oil dashpot (12) is provided in the upper part of the hollow piston guide rod (13). In this a small
piston (14) is suspended by a rod from the oil cap nut (15). This arrangement also prevents the
flutter of choke piston.

In tuning the carburetor, variation in mixture strength is obtained by selecting needles having
different tapers, and a wide range of different needles available. The idling speed is adjusted with
the help of adjustment nut (not shown) which moves the jet sleeve up and down.

The S.U. carburetor is used in many British Cars and was used in Hindustan Ambassador car.
                                                                                                    78
                          FUEL SUPPLY SYSTEM IN DIESEL ENGINES

(Syllabus: cleaning system, transfer pump, injection pump, nozzles, their functions & necessity,
simple & multiple unit pump, CAV Bosch pump. Modern distributor type pumps, maximum &
minimum speed governors, injection nozzle & types of injectors.)

INTRODUCTION

The development of the compression-ignition (CI) engine, also known as Diesel engine, was
mainly due to the work of Dr. Rudol Diesel, who got a patent of his engine in 1892. Today the CI
engine is a very important prime mover, being used in buses, trucks, locomotives, tractors, pumping
sets and other stationary industrial applications, small and medium electric power generation and
marine propulsion. The importance of CI engines is due to

(i) Its higher thermal efficiency than SI engines, and

(ii) CI engine fuels (diesel oils) being less expensive than SI engine fuels (petrol or gasoline).
Furthermore, since Cl engine fuels have a higher specific gravity than petrol, and since fuel is sold
on the volume basis (litres) and not on mass basis (kg), more kg of fuel per litre are obtained in
purchasing CI engine fuel.
These factors make the running cost of Cl engines much less than SI engines and hence make them
attractive for all industrial, transport and other applications. However, the passenger cars it has not
found much favour because of the four main drawbacks of a CI engine in relation to SI engine -
heavier weight, noise and vibration, smoke and odour. Because of the utilization of higher
compression ratios (12: 1 to 22: 1 compared to 6: 1 to 11: 1 of SI engines) the forces coming on the
various parts of the engine are greater and therefore heavier parts are necessary. Also because of
heterogeneous mixture, lean mixture (large air-fuel ratio) is used. Both the factors result in a
heavier engine. The smoke and odour are the result of the nature of diesel combustion phenomenon,
i.e., incomplete combustion of a heterogeneous mixture, and droplet combustion.

Compression-ignition engines, because of their varied applications, are manufactured in a large
range of sizes, speeds, and power outputs. The piston diameters of the CI engines vary from about
50 mm to 900 mm, speeds range from 100 to 4400 rpm and power output range from 2 to 40,000
Bhp.

COMBUSTION IN THE CI ENGINE

The process of combustion in the compression- ignition engine is fundamentally different from that
in a spark-ignition engine. In the SI engine a homogeneous carburetted mixture of petrol vapour
and air, in nearly stoichiometric or chemically correct ratio, is compressed in the compression
stroke through a small compression ratio (6: 1 to 11 : 1) and the mixture is ignited at one place
before the end of the compression stroke (say 30 deg before tdc) by means of an electric spark.
After ignition a single definite flame front progresses through the air-fuel mixture, and entire
mixture being in the combustible range. For a given speed the quantity of charge (both air and fuel)
depend on load.

In the Cl engine, air alone is compressed through a large compression ratio (12 : 1 to 22 : 1) during
the compression stroke raising highly its temperature and pressure. In this highly compressed and
highly heated air in the combustion chamber (well above ignition point of fuel) one or more jets of
                                                                                                    79
fuel are injected in the liquid state, compressed to a high pressure of 110 to 200 bars by means of a
fuel pump. Each minute droplet as it enters the hot air (temperature 450-550 degree C and pressure
30-40 bar) is quickly surrounded by an envelope of its own vapour and this, in turn, and after an
appreciable interval, is inflamed at the surface of the envelope.

To evaporate the liquid, latent heat is abstracted from the surrounding air which reduces the
temperature of a thin layer of air surrounding the droplet and some time must elapse before this
temperature can be raised again by abstracting heat from the main bulk of air in its vicinity. As
soon as this vapour and the air in contact with it reach a certain temperature and the local air-fuel
ratio is within combustible range, ignition takes place (though the core is still liquid and relatively
cold). Once ignition has taken place and a flame established, the heat required for further
evaporation will be supplied from that released by combustion. The vapour would be burning as
fast as it can find fresh oxygen.

Thus we see that at first there is a delay period before ignition takes place. The duration of delay
period depends, among other factors, on temperature and pressure of the air and the self-ignition
temperature (SIT) of the fuel. The higher the air temperature or lower the self-ignition temperature,
the shorter the delay period. Higher pressure also results in shorter ignition delay because of
increase in the rate of heat transfer and more intimate contact between the hot air and the cold fuel.
Once the delay period is over and the ignition is established, the rate of burning depends on the
ability of the droplet to find fresh oxygen, i.e. on the rate of which it is moving through the air or
the air is moving past it.

In the CI engine the fuel is not injected at once, but is spread over a definite period of time
corresponding to 20-40 degrees of crank travel. (This period is, in most cases, greater than the
ignition delay period). The initial fuel droplets meet air whose temperature is only little above their
self-ignition temperature and they ignite after the ignition delay. The subsequent fuel droplets find
air already heated to a much higher temperature by the burning of initial droplets and, therefore,
light up much more quickly, almost as they issue from the injector nozzle, but their subsequent
progress is handicapped because of less quantity of oxygen available.

As it is impossible to inject the fuel droplets so that they distribute uniformly throughout the
combustion space, the fuel-air mixture formed in the combustion chamber is essentially
heterogeneous. Under these conditions if the air within the cylinder were motionless, only a small
proportion of the fuel would find sufficient oxygen and even burning of this fuel would be slow or
even chocked as it would be surrounded by its own products of combustion. It is, therefore,
essential to impart an orderly and controlled movement to air and fuel so that a continuous supply
of fresh air is brought to each burning droplet and the products of combustion are swept away. The
effect of this air motion is called air swirl.

It may be recalled here that in the SI engine combustion also motion of the air is essential to speed
up the combustion. However, there is a basic difference in the air motions required in SI and CI
engines. In SI engines we call it turbulence and in CI engines we call it swirl Turbulence which is
required in SI engines implies disordered air motion with no general direction of flow, to break up
the surface of the flame front and to distribute the shreds of flame throughout an externally
prepared homogeneous combustible mixture. swirl which is required in Cl engines implies an
orderly movement of the whole body of air with a particular direction of flow, to bring a continuous
supply of fresh air to each burning droplet and sweep away the products of combustion which
otherwise would suffocate it.
                                                                                                     80

FUEL INJECTION

In diesel engines air is drawn into the cylinder during the suction stroke and compressed to a very
high pressure, thus raising the air temperature to a value required to ignite the fuel injected into the
cylinder. The usual compression ratios for diesel engines vary from 12 : 1 to 22 : 1, the
corresponding pressures and temperatures at the end of compression stroke being 28 bar to 70 bar
and 5200C to 7200C, respectively.

Fuel is injected into the cylinder near the end of the compression stroke, thus requiring a high
injection pressure. During the process of injection the fuel is broken into a fine spray of very small
droplets. These droplets take heat from the hot compressed air. The surfaces of these -droplets form
vapours, which in turn mixes with air to form a fuel-mixture. When due to continued heat transfer
from hot air to fuel the temperature of this mixture rises to a value greater than its self-ignition
temperature, it ignites. The period between the start of the injection and start of ignition, called the
ignition delay, is about 0.001 second for high speed engines and 0.002 second for low speed
engines. After the ignition the temperature and the pressure raises rapidly.
The injection period covers about 25 degrees of crank rotation and, thus, the process of vaporisation
and mixing continues at the increasing rate after the delay period is over. However, the amount of
oxygen available for mixing and burning reduces as the combustion advances, therefore, the heat
release is affected.

REQUIREMENTS OF A DIESEL INJECTION SYSTEM
The above analysis indicates that the following requirements should be fulfilled by the diesel
injection equipment:

   1. The fuel should be introduced into the combustion chamber within a precisely defined
       period of the cycle.
   2. The amount of the fuel injected per cycle should be metered very accurately. The clearances
       between the working parts of a fuel pump as well as the size of the orifice are very small.
       The working clearance is as small as 0.001 mm and the nozzle orifice size of even a big
       engine is as small as 0.625 mm in diameter. If it is enlarged by even 0.075 mm, the output
       would vary by about 35 per cent. This increased output may result in imbalance,
       overheating or smoky exhaust.
   3. The rate of injection should be such that it results in the desired heat release pattern.
   4. The quantities of the fuel metered should vary to meet changing speed and load
       requirements.
   5. The injected fuel must be broken into very fine droplets, i.e., good atomization should be
       obtained.
   6. The spray-pattern must be such that it results in rapid mixing of fuel and air.
   7. The beginning and the end of injection should be sharp, i.e., there should not be any
       dribbling or after-injection.
   8. The injection timing, if desired, should change to suit the engine speed and load
       requirements.
   9. In the case of multi cylinder engines, the distribution of the metered fuel among various
       cylinders should be uniform.
   10. In addition to the above requirements, the weight and the size of the fuel injection system
       must be minimum. It should not be costly to manufacture and expensive to attend to, adjust
       or repair.
                                                                                                  81

To accomplish the objectives of precise metering, distributing, timing and atomising the following
functional elements are required in a fuel injection system.

  1. Pumping elements to move the fuel from fuel tank to cylinder (plus piping, etc.).
  2. Metering elements to measure and supply the fuel according to the requirement of speed
     and load.
  3. Metering controls to adjust the rate of the metering elements for changes in load and speed
     of the engine.
  4. Distributing elements to divide the metered fuel equally among the cylinders.
  5. Timing controls to adjust the start and the stop injection.
  6. Mixing elements to atomize and distribute the fuel within the combustion chamber.
Components of fuel injection system


The fuel injection system in
general includes the following:

1.    Main fuel tank to store
sufficient fuel.

2. Filters to remove dust, dirt and
others from the fuel.

3. Fuel lift pump to suck fuel
from the low level fuel tank and
to supply it at a slight pressure to
the fuel injection pump.

4. Fuel injection pump to supply
proper quantity of fuel to the
injection nozzle at the required
instant and for proper duration
and to pressurize fuel.

5. Injection nozzles to inject fuel uniformly and evenly into the compressed air in a properly
atomized state.

Low pressure, high pressure and leak off lines to carry the fuel from the lift pump to the fuel
injection pump, from the fuel injection pump to the nozzles and to carry back the fuel that leaks off
from the various components to the fuel tank.
The main components of the diesel engine fuel injection system can be seen in figure

Fuel tank

The fuel tank must be of non-corroding material, and must remain free of leaks at double the
operating pressure and in any case at 0.3 bar. Suitable openings or safety valves must be provided
or similar measures taken, in order to permit excess pressure to escape of its own accord. Fuel must
not leak past the filler cap or through pressure-compensation devices. This applies when the vehicle
                                                                                                    82
is subjected to minor mechanical shocks, as well as when cornering, and when standing or driving
on an incline.

The fuel tank and the engine must be so far apart from each other that in case of an accident there is
no danger of fire. In addition, special regulations concerning the height of the fuel tank and its
protective shielding apply to vehicles with open cabs, as well as to tractors and buses

Fuel filtration

Fuel injection equipment must be supplied with sufficiently clean fuel to prolong its life. The
components of the fuel injection unit are made to a high degree of precision. The clearances
provided between the plunger and the barrel in the fuel injection pump and between the needle and
nozzle holders are of the order of 2 to 5 microns (thickness of the human hair is about 100
microns). Therefore, the impurities (gritty matter) if any carried along with the fuel will cause rapid
wear in the injection system and in the cylinder. Hence, fuel is purified by passing it through two
filters.
The most damaging type of contaminant to fuel injection equipment is a hard abrasive particle in
the size range of 5-20 microns. As little as 5g of such material can completely wear out an
automotive type six-cylinder pump, irrespective of the amount of fuel passed. Cleanliness of the
fuel delivered to the pump is essential in achieving long equipment life.
The critical wear areas are those where sharp edged apertures are closed whilst fuel is passing
through them. On inline pumps these are the delivery valve unloading collars and those parts of the
plunger working surfaces in contact with the inlet and spill ports. These finely mated areas are
particularly susceptible to damage by abrasive wear and as the wear increases so does the leakage
rate. Initially this causes inefficient and perhaps uneven running but eventually the leakage at low
speed becomes so great that starting is impaired. Similar remarks apply to spill-metered distributor
pumps, which are more sensitive where one control edge is used for all cylinders and the rate of
wear is proportionately greater. On the inlet-metered type of distributor pump the areas of the rotor
in contact with the ports are the most sensitive. These pumps have the advantage that this wear does
not affect the valve metering process and the leakage paths at the distributor holes are relatively
long when the pressures are high.
Diesel fuel filters initially used felt or cloth as the filtering medium. Paper is now generally used
although some applications still employ felt. Many of the replacement elements sold by spurious
manufacturers do not give effective protection to the fuel injection equipment. Those do fall into
two general classes. The first relies on filtration in depth and is represented by the felt block or
thick paper element, which is usually of star formation, similar to most engine lubricating oil filter
elements. The pores of most of these materials have a relatively wide size range but, because of the
thickness used, there is a good probability that each drop of fuel will meet a small hole during its
passage through the element. The second class of filtering medium is relatively thin paper which
relies on close control of the pore sizes for its efficiency.
With both types of material the manufacturer can strike a balance between filtration efficiency and
element choking life. This balance is important. The optimum is that which gives the lowest
combined cost of equipment maintenance and filter replacement. For a given combination of
element life and initial efficiency thick papers require a smaller area than thin papers but the
efficiency of many of these reduces progressively as the elements become choked. This is thought
to be because if the pore sizes are not carefully controlled the small pores become choked first, and
more of the fuel is routed through excessively large pores as it passes through the material. The
efficiency of thin paper, in terms of accumulative percentage transmission, improves as more
                                                                                                83
contamination is presented to it. This is probably because the contaminants form a bed on the
surface of the paper.
It follows from the above that, with most thick paper and felt elements it is important to replace
them at the manufacturers' recommended service interval. If a good-quality thin-paper element is of
sound construction it would be safe, or even beneficial, to leave it in service until it is choked.
Unfortunately, despite the high degree of quality control employed by most reputable filter
manufacturers, it cannot be guaranteed that no element will have a faulty seam or joint, through
which most of the fuel will pass as the paper becomes choked. It is therefore a good policy to
change these elements also at the recommended time.
FUEL FILTERS




CAV- TYPE FUEL FILTER                                  STAR- FORM PAPER FUEL FILTER ELEMENT


Figure shows the construction of the CAV Type FS fuel filter. Two lengths of thin paper are
spirally wound and the top and bottom edges alternately glued to form a continuous Vee slot. By
this means the maximum paper area is obtained in the space available, 0.35 m2 in the standard
element. This is twice the area of most star-form elements of the same size. The paper is creped to
allow the fuel to run between the surfaces if they are forced together by the pressure drop as the
element becomes choked. The spiral form of construction was originally patented by Lucas CAV
but is now used by other filter manufacturers.

SERIES FILTRATION

On applications such as earth-moving equipment and
those in dusty conditions it is usual to fit two filters in
series. This has the advantage that (1) the overall
efficiency is improved, (2) there is greater protection
against the risk of a faulty element, and (3) if the two
elements are not replaced simultaneously there is
protection against faulty workmanship, which could
otherwise result in the clean side of the system becoming
contaminated by the deposit from the dirty element. The
choking propensity of the fuel is reduced as the wax is
removed by the filter. Although the fitting of a second
filter will reduce the choking life of the first element,
unless the total pressure available is increased, the life of
the second element will be greater than that of the first.
The life ratio will depend on the grade of paper used but
                                                                                                     84
3:1 can be expected from a good quality paper. Where
the two filter elements are separate this means that the
final stage of protection can be left undisturbed for a
longer period. If they are in one unit the second stage
can be of smaller area than the first.

This is the case with the Stanadyne Master filter (see
the figure). The two pleated paper stages are contained
in a glass housing, which allows the element condition
and presence of water to be checked visually. A metal
housing is provided for heavy duties. The element is
fastened to a vertical base plate, with push-on fuel
connections, using a metal strap so that it can be
replaced without the use of tools.

WATER PROTECTION

Water in diesel: Water as well as dirt can be present in the fuel, as a result of poor storage or
handling, or of condensation in the tank. The effect of water on the equipment can be just as
disastrous as that of dirt, with rapid wear, corrosion and possibly seizure. Diesel fuel oil will absorb
water freely. Water in the diesel fuel system can damage the fuel- injection system and cause it to
fail. Many vehicles have a water detector located in the fuel tank. This device is mounted on the
tank fuel pickup tube. When from 1 to 2.5 gallons [3.8 to 9.5 L] of water has collected in the
bottom of the fuel tank, the detector circuit is completed (through the water). This turns on an
instrument-panel light which warns the driver WATER IN FUEL.
        When this light comes on, the water should be removed with a pump or by syphoning. The
pump or syphon should be hooked up to the fuel-return hose. In many cars, this connection can be
made above the rear axle or under the hood near the fuel pump. Syphoning should continue until
fuel starts to come out. Then you have removed all the water. Remove the fuel-tank cap during this
operation. Then reinstall it when the job is done. It is important that measures are taken to ensure
that water does not reach the fuel pump.




The CA V Sedimenter with waterstop.                   CAV Filter Agglomerator
                                                                                                    85
The CAV Filtrap system: This system has been designed for keeping water out of the fuel pump.
The full system comprises two units, a preliminary Sedimenter and a Filter Agglomerator, which
replaces the normal filter.
Sedimenter and Waterstop: The Sedimenter is designed to remove the larger water droplets in the
fuel and for this reason should be placed before the fuel lift pump, if fitted, so that these droplets
are trapped before being dispersed by the action of the pump. A Sedimenter full of water offers no
protection and this can happen if the unit is placed where it can be overlooked. The Waterstop
version therefore contains a float valve to cut off the flow when the water reaches a dangerous
level. Figure shows the construction of the Waterstop. Fuel entering to the top of the unit passes
over the Sedimenter cone and through the narrow annulus between this and the inside of the bowl.
This smoothes out the flow and encourages the water to fall to the base of the unit during the
passage from the annulus to the outlet at the centre of the cone. The float valve sinks in fuel but
floats in water. It therefore rises to cut off the flow through the outlet before the water is high
enough to carry over. A probe can be fitted in the base of the Waterstop to operate a remote light or
buzzer when the cut-off point approaches. Units are available with the probe alone.
Filter Agglomerator: The size of water droplet extracted by the Waterstop depends on the flow rate,
which determines the deposition time available. Inevitably some small droplets will be carried over
with the fuel. Most of these will be removed by the Filter Agglomerator. The high surface tension
of the very small water droplets carried over from the Sedimenter prevents them from being forced
through the filter paper by the pressure drop. They therefore collect on the surface of the paper
where they agglomerate into larger droplets. Eventually the pressure force acting on the surface of
the larger droplet is sufficient to overcome the surface tension and the water is forced through the
paper. It is then large enough to sediment out into the bowl of the Agglomerator before the fuel
exits to the pump. On systems of moderate flow the combination of Sedimenter and filter
Agglomerator will remove at least 98% of water initially in the fuel.

Fuel feed pump

The fuel feed or transfer or lift pump lifts adequate quantity of fuel from the fuel tank and supplies
the same to the fuel injection pump. The fuel is supplied at about 1 bar pressure via the fuel filters.

Camshaft operated gear pump, plunger pump, diaphragm pump, vane pump and hand priming
pump are used depending on the type of fuel system.




The single acting fuel feed pump is shown in fig. In this a revolving cam or eccentric presses the
plunger of the feed pump downwards by means of the roller tappet and pressure spindle. A portion
of the fuel present in the suction chamber is delivered through the pressure valve to the pressure
                                                                                                     86
chamber. Now the plunger spring gets compressed. As soon as the eccentric cam has passes its
maximum stroke, plunger, pressure spindle and roller tappet move upward due to the pressure
exerted by the plunger spring. A portion (quantity delivered per stroke) of the fuel present in the
pressure chamber is thereby delivered to the injection pump through the filter During tills period,
fuel is also sucked from the fuel tank into
tile suction chamber through tile preliminary
filter and the suction valve.

Double-acting fuel-supply pumps

Double-acting fuel-supply pumps have a
higher delivery rate than their single acting
counterparts and are used with injection
pumps having large numbers of barrels
(cylinders) and correspondingly higher
delivery quantities. They are suitable for use
with P- and ZW-pumps.
In contrast to the single-acting pump, the
double-acting pump delivers fuel to the
injection pump during both pump plunger
strokes. In other words twice fuel delivery
per camshaft revolution.
Hand pumps

The hand (primer) pump has the following functions:

                    i. To fill the injection system's intake side before the system is taken into
                       operation for the first time

                   ii. For refilling and bleeding the system after repair or maintenance work

                  iii. For refilling and bleeding the system after the vehicle's fuel tank has run dry.

The hand pump is usually integrated in the fuel supply pump, although there are versions which are
installed in the line between the fuel tank and the supply pump.

Preliminary filters

The function of the preliminary filter is to protect the supply pump from
course contaminants. Under rough operating conditions, for instance
when re-fuelling takes place from barrels, it is recommend that a strainer
is also installed, either in the fuel tank or in the line to the fuel-supply
pump.

The preliminary filter can either be integrated in the fuel-supply pump,
or installed at the supply-pump inlet or between the fuel tank and the
supply pump.

Fuel lines / high-pressure delivery lines
                                                                                                      87

Steel tubing must be used for the high pressure delivery lines in the fuel-injection system's high-
pressure stage. On the other hand, flame-inhibiting, steel braid-armored flexible fuel lines can be
used for the low-pressure stage. These must be routed to ensure that they cannot be damaged
mechanically, and fuel which has dripped or evaporated must not be able to accumulate nor must it
be able to ignite. In the high-pressure stage, the high-pressure delivery lines represent the
connection between the fuel-injection pump and the nozzles. Apart from being routed without sharp
bends, they must be as short as possible, and their bend radii must be at least 50 mm.

On vehicle engines, the high-pressure lines are normally fastened with clamping pieces at regular
intervals. This means that external vibrations are not transferred to the pressure lines. or only to a
limited degree. Seamless steel tubing is used for the high-pressure lines.

These can have different dimensions depending upon pump size, and with respect to their length,
internal diameter, and wall thickness they must be matched to the injection process. This results in
specified line dimensions which must be precisely complied with.

The pipe sealing cone is attached to the end of the high-pressure line (see fig.). Special delivery
lines are required for high-pressure fuel injection (with nozzle pressures of up to 1400 bar). The
compression pulsating fatigue strength of these lines depends upon the material used and upon the
maximum peak-to valley height of the internal wall roughness.

It is also possible to install specially treated high-pressure delivery lines. In order to increase their
internal strength, these lines are bent to the required shape and subjected to very high pressure (up
to 3800 bar) which is then suddenly released. This leads to material compression on the inner walls
with a resulting increase in strength.


AIR CLEANERS

To clean air entering the engine air cleaners are
used, which also act as silencers for the air-intake. A
few of these are described here.

1. Light duty air cleaner

It consists of wire mesh element and oil reservoir
gravitate to the bottom The atmospheric air enters
the air cleaner through the windows, strikes the oil
surface, where heavier impurities are retained by
the oil and ultimately gravitate to the bottom,
whereas air with lighter impurities passes through
the wire mesh element, where the impurities are
retained and the clean air passes out to the engine.

2. Medium duty air cleaner:
 This type of air cleaner consists of a paper filter
element with a row of plastic fins around it. As the
air from the atmosphere enters the cleaner, the
                                                                                                   88
plastic fins give it a high rotational speed between the casing and the filter element. This causes
impurities to separate out from air due to centrifugal action, which is thrown out to the casing walls
from where it flows down. Air without these dust particles then passes through the paper element,
which removes any further impurities and clean air then goes to the engine.

3. Heavy duty air cleaner:
It consists of a centrifugal pre-cleaner and two filtering
elements as shown, along with oil reservoir at the
bottom. The pre-cleaner gives a whirling motion to the
incoming air, which causes the impurity particles in the
air to be thrown out through the slots provided. The pre-
cleaned air then impinges on the oil surface where some
of the impurities are left while an emulsion of air and oil
passes through the wire-wool mesh where most of the
dirty oil is absorbed. This dirty oil then condenses and
falls back into the oil reservoir, where the dirt settles
down. The relatively clean air then passes through the
second wire mesh, which retains any impurities still left
and the clean air is then passed on to the engine.


TYPES OF INJECTION SYSTEMS

Diesel injection systems can be divided into two basic types. They are:
        (1). Air injection
        (2). Solid injection.

Air Injection: Figure shows the schematic diagram
of an air injection system. The fuel is metered and
pumped to the fuel valve by a camshaft driven fuel
Pump. The fuel valve is opened by means of a
mechanical linkage operated by the camshaft which
controls the timing of injection. The fuel valve is
also connected to a high pressure air line fed by a
multi-stage compressor which supplies air at a
pressure of about 60 to 70 bar.
When the fuel valve is opened the blast air sweeps
the fuel along with it and a well-atomised fuel spray
is sent to the combustion chamber.
The advantages and disadvantages of air injection
system are discussed below:

Advantages of air injection system
1. The main advantage of the air injection system is the good atomisation obtained. A high mean
effective pressure can be attained as rapid combustion results due to good mixing of fuel and air.
2. Heavy and viscous fuels, which are cheaper, can also be injected.
3. The fuel pump is required to develop only a small pressure.

Disadvantages
                                                                                                   89
Not withstanding the above advantages air injection system has many disadvantages which have
rendered it obsolete now. These are:

1. It requires a high pressure multi-stage compressor. The large numbers of parts, inter-cooler, etc.,
make the system complicated and expensive.
2. A separate mechanical linkage is required to time the operation of the fuel valve.
3. Due to the compressor and the linkage the total weight of the engine increases. This also results
in reduced bhp due to power loss in operating the compressor (consumes @ 10% of the power
output) and the linkage.
4. The fuel in the combustion chamber burns very near to the injection nozzle which many times
lead to overheating and burning of the valve and its seat.
5. The fuel valve sealing requires considerable skill.
6. In case of sticking of the fuel valve, the system becomes quite dangerous due to the presence of
high pressure air.

Solid Injection.

Injection of fuel directly into the combustion chamber without primary atomisation is termed as
solid injection. This is also called airless mechanical injection.

Every solid injection system must have:

(i) A pressurizing unit (the pump), and
(ii) An atomising unit (the injector)

The different types of solid injection systems vary only in the manner of operation and control of
these two basic elements. The main types of modem fuel injection system are:
1. Common rail system.
2. Individual pump and injector or jerk pump system.
3. Distributor system.

Common rail system:




Figure shows the schematic diagram of a common rail system. A High pressure fuels pump delivers
fuel to an accumulator, whose pressure is kept constant with the help of a pressure regulating valve.
The high pressure pump usually has a number of plungers and unlike the individual pump system
none of the plungers is identified with a particular cylinder. Therefore, the pumping action of the
fuel pump is not required to limit to a short period equal to the duration of the injection and the
                                                                                                  90
noise and stresses on driving mechanism can be reduced by spreading the pumping action over a
long period.

As can be seen from figure a common rail or a pipe starts from the accumulator and leads to the
different distributing elements for each cylinder. For each cylinder there is a separate metering and
timing element which is connected to an automatic injector injecting fuel into the cylinder.

In the common rail system the supply pressure of the fuel is independent of the speed, and, hence,
is not affected by the fuel pump. However, the metering element opening period changes with
speed. As the speed is reduced, it remains open for a longer period and, therefore, the quantity of
the fuel supplied per stroke tends to increase; Thus this system tends to the self-governing. The
main disadvantage of the common rail system is that in case of injection needle sticking in an open
position an excess amount of the fuel may be injected into cylinders.

This system also requires that the nozzles for different cylinders must be accurately matched for
ensuring good fuel distribution between various cylinders. However these problems were rectified
with the help of electronic sensors and actuators and it is widely used in the modern cars.

The Individual pump and Injector or jerk pump system:


In the individual pump and injector or the
jerk pump system a separate metering and
compression pump is used for each
cylinder. The pump which meters the fuel
also times the injection.
Figure shows a schematic diagram of a
jerk pump system. A jerk pump is a
reciprocating fuel pump which meters the
fuel and also furnishes the injection
pressure. There is a separate pump and a
separate injector for each cylinder. Usually
the fuel is supplied with the help of a
multi-unit fuel pump having one plunger for each cylinder. In about 20 degrees of crankshaft
rotation a pressure of about 65 bar to 300 bar is developed. This requires robust and heavy valve
gears and is always accompanied by a jerking noise, hence the name jerk pump.
Each unit of the multi-unit fuel pump is connected to the associated injector by a pressure-line and
for accurate timing the length of these pressure lines must be identical. This requires that the fuel
pump must be placed at the centre of the engine block. The quantity and the time of injection are
controlled by the pump itself. The jerk pump system is universally used for medium and high speed
diesel engines.

Unit injector: The pressure line
connecting the individual pump and
the associated injector is often a source
of trouble and the unit injector design
avoids this line by combining both the
pump and the injector into one unit.
Each cylinder has one such unit
                                                                                                       91
injector. This arrangement requires push rod and rocker arm to operate the unit injector. Such a
system is shown in Figure


Distributor system:




Figure shows a schematic diagram of the individual pump fuel supply system with distributor type
injection pump. In this system the pump which pressurizes the fuel also meters and times it. The
fuel pump after metering the, required amount of fuel supplies it to a rotating distributor at the
correct time for supply to each cylinder. The number of injection strokes per cycle for the pump is
equal to the number of cylinders.

Since there is only one metering element, a uniform distribution is automatically ensured. Not only
that the cost of the fuel injection system also reduces to a value less than two-thirds of that for
individual pumps system. This type is also widely used in modern vehicles.

Comparison of Fuel Injection Systems:




DIESEL-ENGINE GOVERNORS

Why does the diesel engine need a governor?

With a diesel engine, there exists no single control-rack position which would permit the diesel
engine to maintain its speed accurately without a governor. At idle, for example, without a
governor the engine speed would either drop until the engine stalls, or the engine speed would
continue to increase until it races, culminating in self-destruction. The latter possibility is due to the
                                                                                                    92
diesel engine operating with an excess of air, meaning that there is no effective throttling of the
cylinder charge as engine speed increases.
For instance, if a cold engine were started and left to run at idle while the initial fuel-delivery
quantity continued to be injected, the engine's inherent friction would soon start to drop. The same
applies to the drive resistance from engine-driven assemblies such as the alternator, air compressor,
fuel injection pump, etc.

This means that if the
control-rack position were
to remain unchanged, and
the control rack were not
retracted to reduce the fuel-
delivery quantity (as a
governor would do), the
engine's     speed    would
increase more and more
(due to the above drop in
friction) until it possibly
reaches the point of self
destruction.

In other words, it is
imperative that the diesel
engine be equipped with a
governor. Nowadays, either
mechanical       (flyweight)
governors or Electronic
Diesel Control (EDC) are used for the in-line fuel-injection pumps.

Pneumatic governors, controlled by intake-manifold pressure, wore formerly fitted to smaller
injection pumps. These have been discontinued as a result of the increased demands made on
control (governing) precision and on governor functions.

Governing Fuel injection

During its intake stroke, the diesel engine draws in only air. During the compression stroke, this air
is heated to such a high temperature that the diesel fuel injected into the cylinder towards the end of
the compression stroke ignites of its own accord. Fuel is metered to the engine by the fuel-injection
pump. It is injected at high pressure through the injection nozzle into the combustion chamber.
Fuel injection must take place:

   1.   In a precisely metered quantity according to the engine load,
   2.   At the correct moment in time,
   3.   For a precisely defined period of time, and
   4.   In a manner suited to the particular combustion process concerned.

The fuel-injection pump and the governor connected to the control rack are responsible for these
conditions being complied with. The amount of fuel injected per pump-plunger stroke is
approximately proportional to the engines torque.
                                                                                                    93

If a mechanical (flyweight) governor is used in the vehicle, the control rack is connected with the
accelerator pedal via the governor. With an electronic governor (EDC), the accelerator pedal is
equipped with a sensor connected to the ECU. When the accelerator pedal is de-pressed, its
movement is converted into the corresponding rack travel, with the momentary engine speed also
being taken into account.

Governor operation

No matter what the load is on the
engine, the fuel-injection pump
must always provide the engine
with the correct amount of fuel.
All in-line fuel-injection pumps
have      a      plunger-and-barrel
assembly (pumping element) for
each engine cylinder. This
comprises the pump barrel and the
plunger. The plunger is forced in
the fuel delivery direction by an
engine-driven camshaft and back
again by its return spring. Since the plunger lift cannot be varied, the delivery quantity can only be
adjusted by changing the plunger's effective stroke. To this end, the plungers are provided with an
inclined helix so that the desired effective stroke is selected by rotating the plunger. Rotation is by
means of the control rack which engages with the plunger, the control rack itself being shifted
longitudinally by the governor. The plunger rotation positions the plunger helix to control the end
of delivery (otherwise known as spill or port opening) and with it the delivery quantity. Delivery
commences when the top edge of the plunger closes the inlet port in the barrel wall.

In the case of maximum delivery, spill does not take place until maximum effective stroke, in other
words with maximum possible fuel delivery quantity. During partial delivery, spill takes place
earlier depending upon the plunger's rotational setting. In the end position as required for zero
delivery, that is, when the engine is to be switched off, the plunger's longitudinal slot is positioned
directly opposite the inlet port. This means that the pressure chamber above the plunger is
connected with the fuel gallery throughout the complete plunger stroke, so that no fuel is delivered
(see fig.). There are a number of different helix configurations. In the case of plungers with only a
bottom helix, fuel delivery always commences at the same plunger-lift point, whereas end of
delivery takes place sooner or later depending upon the plunger's rotational setting. When the
plunger has a top helix, start of delivery can be varied. There are also plungers available with both
top and bottom helix.

Governor functions

The basic job of every governor is to limit the engine's maximum speed. In other words, the
governor must ensure that engine speed never exceeds the maximum specified by the manufacturer.
Depending upon its type, the governor can have further functions such as the maintenance of
certain fixed speeds, such as idle, or maintaining the speed range between low idle and high idle
(maximum) speed.
                                                                                                    94
The governor can also have other responsibilities, the options provided by the electronic governor
(EDC) being considerably more extensive than those of the mechanical (flyweight) governor. The
various demands made on the governors led to the development of the following governor types:

Maximum-speed governor: These governors are designed to limit the engine's maximum speed
only.

Minimum-maximum-speed governor: In addition to maximum speed, these governors also control
low idle speed.
Variable-speed governor. As well as the maximum and low idle speeds, these governors also
control the speed range in between.

Combination governors A combination of maximum-minimum-speed governors and variable-speed
governors.

Governors for stationary power units Designed for use with engine-generator sets as per DIN 6280.
        Apart from its basic function, the governor also has a number of other control functions.
These include automatically starting and stopping the extra fuel required for starting (start
quantity), and changing the full-load delivery quantity as a function of engine speed (torque
control), charge-air pressure, or atmospheric pressure. Supplementary equipment is required for
some of these functions.


GOVERNOR TYPES

Unlike petrol engines where only carburetor controls both air and fuel delivery at various speed and
load conditions, in diesel engines the fuel delivered depends independently on the injection pump
characteristic, and the air intake on the engine. Generally, the air intake decreases with speed of the
engine, whereas the pump having a rising characteristic, results in over injection at higher speeds.
On the other hand, at idling when the engine speed is less, the fuel delivered is also less when
actually more fuel is required; as a result the engine will stop. Similarly with increased load on, the
engine, the fuel delivered by the pump also increases, causing excessive carbon deposits and high
exhaust temperature. On the other hand, a reduction of engine load will cause the speed to overheat
to overshoot to dangerous values. A governor is, therefore, a necessity in case of diesel engines to
control the fuel injected to ensure optimum conditions at all speeds and loads within the range
specified.

From the point of view of method of operation, the governors may be classified as:

           (i)     Mechanical governor
           (ii)    (ii) Pneumatic governor
           (iii)   (iii) Hydraulic governor

However, from the point of view of principle of governing, they may be of the following types:

(i) Maximum, speed governor
(ii) All speed governor

The principles of working of all the above types will be discussed in the following articles.
                                                                                                  95
Mechanical governor:




The principle of working of a mechanical governor may be explained with the help of Figure
shown. Two spring-loaded weights are mounted on the governor shaft which gets drive from the
engine. At one end, the bell crank levers carry balls whereas their other ends touch the lower
surface of the flange of a sleeve on the governor shaft. As the engine speed increases, the
centrifugal force due to the weights acts against the spring tension. Once the former exceeds the
later, the weights fly apart, causing the other ends of the bell crank levers to raise the sleeve and
hence operating the control lever in the downward direction which further actuates the control rack
on the fuel injection pump in a direction which reduces the amount of fuel delivered and hence
decreases the engine speed. In the same way, the amount of fuel delivery is increased when the
engine speed tends to decrease.


Thus, the spring tension can be so adjusted as to actuate the control rack for decrease in fuel
delivery at some maximum speed. Below this maximum speed, the control rack on the fuel
injection pump may be directly operated by the driver through accelerator pedal and linkage, with a
provision to over ride the driver control at maximum speed which is limited by the amount of
spring tension. Such a governor is called the maximum speed governor.
In another type of governor, however, there is no direct connection between the accelerator pedal
and the pump control rack. The two are linked only through the governor. The driver can, then,
change spring tension with the help of the accelerator pedal and hence fix the speed of the engine,
                                                                                                   96
i.e. of the vehicle. This type of governor is called all speed governors. Such a governor is used in
India in Tata and Ashok Leyland vehicles.
Pneumatic governor




A pneumatic governor is shown in the figure. It consists of two main parts, the venturi unit and the
diaphragm unit. The venturi unit is connected to the engine inlet manifold and the diaphragm unit is
fitted on the fuel injection pump. The two units are connected by a vacuum pipe.


Accelerator pedal controls the position of the butterfly valve in the venturi unit and hence the
amount of vacuum from the inlet manifold, which is applied to the diaphragm via the vacuum pipe.
As the diaphragm is connected to the fuel pump control rack, the rack is operated left or right
depending upon the amount of vacuum applied. Thus the position of the accelerator pedal
determines the position of the pump control rack and hence the amount of fuel injected. Idling
spring adjustment is also provided by means of a separate spring and lever; the spring tension can
be adjusted with an adjusting nut provided as shown. Thus this is an all speed type of governor.


Hydraulic governor
A hydraulic governor, for e.g. the CAN make works on the principle of inverted hydraulic
amplifier, i.e. a small pressure change in some part of the system producing in some other part, an
opposite change of much greater amplitude. This is also an all speed type governor.
                                                                                                  97
DIESEL FUEL INJECTION PUMPS

The function of a fuel injection pump is to deliver accurately, metered quantity of fuel under high
pressure, at the correct instant and in the correct sequence, to the injector fitted on each engine
cylinder. The injection pressures generally employed in case of automotive engine fuel injection
range from 7 to 30 MPa In some systems injection pressures can be as high as 200 MPa. The
injection pump is driven from the engine's timing gears and its output is controlled by the driver
through accelerator pedal. As the volume of fuel to be metered for each injection is very small and
frequency of injection is quite high, the pump has to be manufactured to very high precision. For an
idea, in a 4-stroke 4 cylinder diesel engine, at maximum speed of 6000 r.p.m., about 150 mm3 of
fuel has to be metered and injected 20 times in a second. In a 2-stroke engine, number of injections
per second are twice of this value.

The fuel injection pumps are generally of jerk pump type. However, in many cases, distributor type
pumps are also used. Both of these will be
discussed here.

Jerk pump type fuel injection pump:


Figure shows the construction of a single
cylinder jerk pump type fuel injection pump of
American Bosch type. The main parts of the
pump are the delivery valve, the plunger, the
control sleeve and the control rack. The delivery
valve is of special construction (Pic.A) and is
spring loaded. The plunger contains a helix at its
upper end (Pic.B), which serves to control the
quantity of fuel to be injected, as shall be
discussed later. The plunger is operated by
means of cam and tappet.

To understand the working of the injection
pump, consider the figure shown above. Figure
(A) shows the position of the plunger at the
bottom of stroke. In this position, both the
intake and the spill ports are uncovered. As the
plunger, moves up, it covers the two ports after
which the upward plunger movement exerts
pressure on the fuel at the top and then to the
delivery valve which opens against the spring
force, thereby delivering the fuel under pressure
[Fig (B)].
The extreme position of the plunger is shown in Fig (C). As soon as the helix uncovers die spill
port, the fuel escapes through the vertical slot in the plunger and the spill port and pressure is
released, shutting the valve down on its seat. The delivery valve has an annular groove and four
longitudinal grooves (Fig.A). The lowering of the delivery valve at the end of delivery increases the
space on delivery side by an amount equal to that of the relief plunger, due to which a sudden
                                                                                                        98
decrease of pressure occurs in the delivery line causing the injector to close immediately. Thus any
tendency to 'dribbling' or secondary injection is eliminated.




                Pic-A                                                   Pic-B




We have seen so far that fuel delivery starts when the plunger just covers the intake and the spill
ports and ends when the helix just uncovers the spill port so as to release pressure via the vertical
slot. Out of these events, the first one, i.e., start of delivery cannot be varied, but the later, i.e. end
of delivery cannot be timed to occur earlier or later, thus varying the quantity of fuel injected
according to requirements. This is done by rotating the plunger in the desired direction. Looking
from the top, a clockwise plunger rotation will delay the end of delivery, increasing the effective
plunger stroke and consequently the fuel supply injected. An anti-clockwise rotation, on the other
hand, decreases the fuel injected per stroke. Fig (B) and (C) show the plunger position for
maximum delivery. In this position, the vertical slot is in such a position so that the lowermost
point on the helix is opposite spill port. Once the plunger is rotated in anti-clockwise direction as in
Figs (D) and (E), the lowermost helix point is no more opposite spill port. Consequently, the
effective stroke is decreased. In the no delivery position, Fig (F), the vertical slot is opposite spill
port. Thus no pressure can be built up at any vertical plunger position, reducing the effective stroke
                                                                                                     99
to zero. From the above it is clear that though the plunger stroke is always constant yet the part of it
which actually pumps can be varied.

The arrangement used to control the angular position of the
plunger and consequently the amount of fuel injected per
stroke is shown in Figure. A lug on the plunger fits into a slot
at the bottom of a toothed sleeve whose teeth engage with the
teeth on the control rack, which is further connected with die
accelerator pedal through linkage. To and fro motion of the
control rack causes the plunger to undergo rotational
movement as desired.
         For starting the engine from cold, more fuel, is
required to be injected. This is done by moving the control
rack to a position where excess fuel is delivered by the pump.
However, the rack has to be brought to normal position
immediately after the engine starts. All this is achieved by
means of a governor. In some designs the rack and pinion
type of control mechanism as discussed above is replaced by.
the 'fork-and-lever" arrangement. The advantages claimed for the later are, lesser sources of
backlash and decreased possibility of misalignment and consequent sluggish control-rod movement
occurring.
For multi cylinder engines, usually many such pump assemblies as explained above, one for each
cylinder, are assembled into one unit to give a compact construction. High pressure delivery lines
are then connected to different fuel injectors on various cylinders. Such pumps are then referred to
as 'in-line' type. The main parts of an in-line pump are pump housing, governor housing, camshaft-
tappet assemblies and the pumping elements.
The pump housing is the main housing that contains cambox (which supports cam shaft tappet
assemblies, control mechanism and governor) and the pumping head (contains pumping elements).
It is made either as a single aluminum alloy casting or aluminum alloy cam box is bolted on to a
pumping head of steel: The governor housing is an extension of the cam box.

Distributor type fuel injection pump

The principle of working of a distributor type
fuel injection pump has been illustrated by
means of Figures. Unlike the jerk pump
explained earlier, there is a single pumping
element in this type of pump and the fuel is
distributed to each cylinder by means of a
rotor. The rotor has a central longitudinal
passage and a set of radial holes (suction
ports) each equal to the number of engine
cylinders, four in the figure shown. Similarly
the outer sleeve also has a set of equal
number of holes (delivery ports) at a
different level BB, which are offset from the
suction ports in top view also. Besides there
is a metering port in the sleeve for the fuel
intake at level AA and a distribution port in
                                                                                                    100
the rotor at level BB. This distribution port is connected to the central passage in the rotor. Each of
the delivery ports is connected to the high pressure delivery lines leading to injectors on the engine
cylinders. As the rotor revolves, the suction ports align with the intake metering port one by one,
while the distribution port aligns with the delivery ports in turn, though these alignments of suction
ports and the distribution port with the relevant ports take place at different instants.

   1. The lower end of the
      central passage in the
      rotor opens into a
      chamber in which two
      opposed        pumping
      plungers are housed. As
      the rotor rotates, a
      stationary ring with
      internal cams operates
      the plungers through
      rollers and shoes which
      am placed in slots into the rotor base. The number of lobes on the cam ring is equal to the
      number of engine cylinders and these are evenly spaced around the ring.

As the pump plungers move away from each other, the fuel is drawn into the central rotor passage
from the inlet port through suction ports [Figs (a) and (a)]. The fuel thus charged is delivered to
each cylinder in turn at high pressure, when the distribution port in the rotor coincides with the
delivery port for any cylinder [Figs (b) and (b)].

A metering valve controlled by the governor or hand throttle regulates the amount of fuel entering
the rotor. Pressing the accelerator pedal increases the area of the metering port allowing more fuel
to enter rotor, thereby increasing the engine speed. The maximum amount of fuel intake to the
pump is controlled by adjusting the outward travel of the pump plungers.
Each cam on the stationary ring has usually two peaks. When the roller reaches fast peak, which is
higher, fuel injection stops. The valley next causes a rapid reduction in pressure in the injector pipe,
preventing dribbling. The second peak on the cam prevents the pipe line pressure from collapsing
completely for a time long enough for the distribution port to be cut off from the delivery port. This
ensures a residual pressure in the delivery port and the injector to be maintained during cut off.
The fuel injection timing in this type of pump is controlled by rotating the internal-cam ring. As the
engine speed increases, the fuel feed pump delivery pressure increases. This increase in the pump
pressure is utilized to move the cam in an advance direction, thus providing more timing advance
with the increase of engine speed.

Small size and less weight are the main advantages of this type of pump

FUEL INJECTOR
This is also known as nozzle, atomizer or fuel valve. Its function is to inject the fuel in the cylinder
in properly atomized form and in proper quantity. An American Bosch type fuel injector is shown
in Fig. It consists of mainly two parts, i.e. the nozzle and the nozzle holder, the former being
connected to the later by means of a screwed cap. This facilitates the change of nozzle valve
whenever required.
A spring-loaded spindle in the nozzle holder keeps the nozzle valve pressed against its seat in the
nozzle body, till the fuel supplied by fuel injection pump through inlet passage exerts sufficient
                                                        101

pressure so as to lift the nozzle valve against the
spring force, when a spray of atomized fuel is fed
into the combustion chamber. The fuel spray
continues till the delivery from injection pump is
exhausted when the spring pressure again suddenly
closes the nozzle valve back on its seat. The actual
opening fuel-line pressure can be varied by
adjusting the initial degree of spring compression. A
small quantity of fuel is purposely allowed to leak
between the nozzle valve and its guide for
lubrication purpose. The fuel accumulated around
the spindle in this way is drained back to the fuel
tank through the leak off connection. Adjusting
screw provided at the top serves to adjust the
tension in the spring, and hence the pressure at
which the nozzle valve opens.
                                                                                                   102




The nozzles used may be classified broadly into the hole type, the Pintle type, and the Pintaux type.
Hole type nozzles are generally used in engines with open type combustion chambers, whereas
pintle type nozzles are common in engines with pre-combustion chambers and some special swirl
chambers. The pintle type nozzles carry an extension, which products a hollow cone type spray.
Such nozzles have the advantage of being self cleaning. In the throttling pintle type of nozzle, the
Pintle is much longer and is also shaped like a
truncated cone at its lower end. Such a shape causes
only a small amount of the fuel to be injected as the
injection starts, die rate of injection increasing
gradually as die pintle protrudes further from the
nozzle end. The pintaux nozzle has been developed
specially for the comet typo combustion chamber.
This helps easy starting under cold conditions. The
opening pressure of hole type nozzles varies from 17
to 34 MPa, whereas that of pintle type nozzles varies
from 7 to 15 MPa. A good nozzle should atomize the
fuel uniformly so as to maintain proper injection
angle and direction. Pintaux injection nozzles with
normal design of the pintle or Single hole nozzle the fuel is sprayed tangentially into the spherical
chamber of the engine. In this way the fuel is not sprayed into the hottest zone or towards the centre
of the chamber. Therefore for cold starting the engine needs heater plugs.
The pintaux type injection nozzles are designed as shown. At starting, the nozzle valve is lifted
slightly thus the pintle hole is not cleared and the fuel is only discharged through the auxiliary hole
into the central hot zone, thereby obtaining better cold starting performance. When engine reaches
to its normal speed then the needle valve is lifted from the pintle hole thereby allowing the whole of
the fuel to pass through the pintle hole and entering the chamber tangentially. In this way this type
of nozzle provides both the advantage of cold starting as well as of normal running of the engine.
                                                                                                     103
COLD STARTING DEVICES

Direct injection engines are easier to start because of the lower compression ratios usually
employed in them, while indirect injection engines are difficult to start from cold. Thus special
devices are required to start from cold the indirect injection engines though such devices may also
be used to facilitate easy start in, case of direct injection engines also. Various devices used for this
purpose are:

1. Decompression devices.      2. Heater plugs.        3. Inlet manifold heaters.
4. Chemical sprays.

1.De-compression devices

A lever on the cylinder head causes the compression in the cylinders to be partially released, which
makes it easier to crank the engine, till it gains sufficient momentum to start the engine by itself
when the lever is brought back again to restore full compression.

2. Heater plugs




A heavier plug also called glow plug, having heating element at its end is screwed into the swirl
chamber of each cylinder. Pushing a heater button on the dash board heats the elements which heat
the air in the chamber. The heater push button is then released and the starter is pressed when the
engine is started because of compression of the hot air.

3. Inlet manifold heaters

Instead of heater plugs, the air in the inlet manifold can be heated by means of a heating coil, which
helps the engine to start. However, the coil takes heavy current from the battery and sometimes
diesel oil is burnt in the inlet manifold using special thermostatic device. A crude method of heating
inlet manifold is by a burning rag dipped in the diesel oil and placing the same close to the
manifold.

4. Chemical sprays

Certain chemicals like ether, are available which when sprayed into the inlet manifold vaporize
readily and may be ignited at lower temperature than diesel oil. Special pumps are available to
inject such chemicals.
                                                                                                 104
                                       COOLING SYSTEM

(Syllabus: Cooling system: Necessity of cooling, types of cooling, Air cooling & water cooling,
forced circulation, thermostat, water pump, Radiator, pressurised cooling, antifreeze solution,
liquid cooling & Oil cooling)

        Only a part of the total fuel energy supplied to the Internal Combustion engine is converted
into useful work. The work is delivered at the crankshaft and rest of the fuel energy is rejected as
follows:

(i) Heat from the engine boundaries due to radiation, convection, and to a small extent, conduction,
(ii) Exhaust heat, and
(W) Heat rejected to the coolant.

The exact proportion of the energy supplied to the
engine (as fuel), which is converted into useful work is
critically influenced by many physical characteristics of
the engine such as engine design, type of the fuel used,
cooling system etc. In general about 26 % of the energy
supplied is converted into useful work (BHP); about 33
% is lost as exhaust heat and about 10% as heat to
friction, compression and direct rejection from engine.
The rest of the energy, about 31 %, has to be removed
by the cooling system if it is not to increase the engine
temperature while running

Necessity of Engine cooling:

The cooling of the engine is necessary to for the following reasons.

   1. The lubricating oil used in the engine determines the maximum engine temperature that can
      be used. Depending on the type of lubricating oil used this temperature ranges from 160 to
      200 degree C. Above these temperatures these temperatures the lubricating oil deteriorates
      very rapidly with the temperature increase and it even evaporates and burn, injuring piston,
      and cylinder surfaces. Piston seizure due to overheating resulting from the failure of the
      lubrication is quite common.
   2. The strength of the materials used for the various engine parts usually decreases with the
      increase in temperature and thus establishes an upper limit for temperatures at various
      points of the engine. For example: For the water cooled engines the temperature of the
      cylinder head should not exceed about 270 degree. While for the air cooled engines, which
      use light alloys, this limit is as low as 200 degree C. The high local temperatures in addition
      to decrease the strength of the materials may result in excessive thermal stresses due to
      uneven expansion of various engine parts and may result in cracking.
   3. High engine temperature may result in very hot exhaust valve, which in turn may rise to
      pre-ignition and detonation.
   4. If the cylinder head temperature is high, the volumetric efficiency and hence the- power
      output of the engine is reduced. Thus, it is clear that some form of cooling must be provided
      to keep the temperature of the engine low in order to avoid the loss of volumetric efficiency,
      which leads to loss of power, engine seizure and danger of engine failure.
                                                                                                    105

Disadvantage of overcooling:

For the smooth and efficient operation, the engines must be kept in an adequate operating
temperature. It should never be over cooled. The low engine temperatures increase the viscosity of
the lubricant and hence more friction is encountered in the moving components. This will decrease
the mechanical efficiency and will make the starting difficult. It is also seen that at low temperature
the magnitude of corrosion attains a significant magnitude and hence the life of the engine is
greatly reduced. As the fuel always contains some Sulphur, the formation of Sulphurous and
Sulphuric acid during combustion will promotes the deterioration of the cylinder and the other
engine components at low temperatures. The dew points of these acids vary with pressure and
hence the critical temperature, at which corrosion attains significant proportions, varies along the
cylinder barrel. Hence to avoid the condensation of the acids, the temperature of the coolant should
be more than 70 degree C.

At low temperature the thermal efficiency is decreased due to more loss of heat to the cylinder
walls. Since the vaporization of the fuel at low temperature will be poor, it will result in the fall of
combustion efficiency. So the cooling system should be capable of cooling the engine and also
keep the cylinder liner temperature above a minimum level to avoid corrosion and ensure good
warm up performance of the engine.

Methods of Engine Cooling

The cooling systems are broadly classified as Direct Air cooling and indirect or Liquid cooling. But
according to need and the particular design adopted, there are different cooling methods used in
Automobile engines as

a) Direct air cooling
b) Indirect or water cooling
        i) Thermosiphon system,
        ii) Pump circulation system
c) Liquid cooling
d) Pressure sealed cooling
e) Evoperative cooling or steam cooling

Direct-air Cooling.

This system is being used in light engines such
as scooter, motor cycle, tractor and small Aero-
plane engines. The outer surface of the cylinders
and cylinder heads consists of fins as shown in
Fig, which increase area of contact with the air thus radiated more heat to the atmosphere. The rate
of heat flow through the cylinder walls is expressed by law which may be stated as

        Q= KA t M / L
Where, Q=rate of heat transfer, K=average thermal conductivity, t = temperature difference
between the hot and cold surfaces, A = surface heat transfer area, M=mass flow rate of air, L-length
of the heat flow path.
                                                                                                   106
Therefore for better air cooling, the surface area of the metal which is in direct contact with the air
is increased by increasing the number of fins. The conductivity depends upon the metal used for
cylinder and cylinder head, temperature difference depends upon the atmosphere and combustion
temperature. The mass of air flowing over the fins can be increased by means of providing the fan
or air blower with the engine fly-wheel. The flow of heat is also inversely proportional to the length
of the heat flow path which depends upon the maximum pressure for which engine cylinder is
designed.

Advantages of Air cooling

       (a) As the air cooled engine does not consist of radiator, cooling jackets, coolant etc.
            therefore it is lighter than others.
       (b) Power lost in the cooling water circulating pump is saved in this system.
       (c) The chances of freezing of water are omitted when used in cold climates or places.
       (d) The air cooled engines are advantageous, where there is scarcity of cooling water.
       (e) The cylinders and cylinder head prepared under this system are cheaper than any other
           types.

Disadvantages

        (a) Air cooled engines are less efficient in cooling because the co-coefficient of heat
             Transfer for air is less than that for water.
        (b) As the cooling air can not provide the even cooling around the cylinder, therefore
            distortion in cylinders and cylinder- head is possible due to the localized cooling.
        (c) As the air cooled engines do not have water jackets, they are noisier. Because cooling
            water serves as sound insulator.
        (d) It needs a fast running as well as bulky cooling fan which produces more noise and
            absorbs more power.
(ii) Indirect or water cooling.
In this system the cooling water flows through the water jackets provided around the cylinder,
combustion chamber and valve ports. The heat from the cylinder is absorbed by cooling water
which is circulated through the radiator. From radiator this heat is dissipated to the atmosphere by
means of fins provided on the radiator. To maintain the proper working temperature the system is
provided with a thermostat. The water cooling systems are of two types.
(a) Thermosyphon system. (b) Pump circulation system.

Thermosyphon system.
Referring Fig, this system does not consist of
water pump but the circulation of water through
the jackets and radiator is maintained only by
means of natural convection which is called
Thermosyphon.
This system is very simple and cheap but has
some disadvantages by which it could not be
much popular. Its cooling rate is slow. It always
needs a particular or minimum level of water. It
needs a big size radiator which requires more
space. Therefore these are the main draw back
of this system.
                                                                                                 107
Pump circulation system.

This system is the modification of Thermosyphon system. The system consists of a water pump
provided between the lower tank of the radiator and the water inlet of the engine body. The water
pump driven by the engine crankshaft by means of a belt. The water is circulated with the pump
force and thereby obtaining a rapid cooling.

                                                                At the starting time the engine does
                                                                not need cooling until the
                                                                temperature exceeds the working
                                                                range of the engine. Therefore in
                                                                modern engines the cooling system
                                                                consists of a by-pass, through
                                                                which water circulates in the water
                                                                jackets with out entering the
                                                                radiator at the time of starting and
                                                                below operating temperature.
                                                                The system also consists of a
                                                                thermostat, which is opened
                                                                automatically by means of high
                                                                temperature whenever          engine
                                                                desires cooling. Thus a rigid
                                                                control      over     the    cooling
                                                                temperature can be obtained under
                                                                this system.




                                                                Advantages of pump circulation
system

 (1) The size of the water passage and jackets can be reduced and thereby obtain a compact
      structure of the engine.
(2) Water can be circulated around the hottest spots of the engine such as spark plug and valve
     seat bosses.
(3) It needs less water and small size of the radiator.
(4) As the water is circulated by means of pump force, the radiator can be placed at any where as
    convenient to the designer.
(5) A rapid cooling is obtained by this system.
(6) To increase the cooling efficiency, the system can convert as a pressurized system by minor
    alterations.

(iii) Liquid cooling.

The liquids such as glycerin and ethylene glycol are used as coolant instead of water because they
have higher boiling points, this system is called liquid cooling. The advantages of this system are
that a liquid of higher boiling point increases the heat carrying capacity of the system, reduces the
weight of coolant as well as size of the radiator itself.
                                                                                                108

(iv) Pressure sealed cooling.


The boiling point of the cooling water in the open
atmosphere is always 100 degree C, which can be
raised to higher pressures by sealing the cooling
system. Therefore the closed system reduces the
weights of the coolant and the radiator and
increases the thermal efficiency of the engine.

                                                         The pressure of the system can be raised by
                                                         means of a special radiator cap which
                                                         maintains pressure from 0.5 to 1 kg/cm2.
                                                         The cap consists of Pressure valve or blow
                                                         off valve, vacuum valve over flow pipe and
                                                         gasket for sealing purposes. When the
                                                         pressure in the -system exceeds the
                                                         predetermined value then the spring loaded
                                                         pressure valve be opened automatically.
Similarly when the engine is stopped then the pressure falls below that of the outside atmosphere
and creates a vacuum in the system. The hoses and other thin parts of the cooling system can be
collapsed due to creation of vacuum inside the system. Therefore to overcome this difficulty
another spring loaded vacuum valve opens automatically and thereby admitting atmospheric air
into the system.
The gasket -provided between the cap and the filler neck insures an air-tight seal and the overflow
pipe releases the excessive pressure and steam to the atmosphere.

(v) Evaporative cooling.

The places where there is deficiency of industrial or
soft water, this system of cooling is preferred in
Automobile engines. This system is similar to that for
water cooling but the latent heat of the steam is
considered in it.
Referring Fig, the system consists of a radiator which
acts as a condenser. The water from the radiator is
entered into the jackets of the engine block by means
of a water pump. The water in the jackets is
converted into the steam which flows out at the top of
the engine blocks and enters at the bottom of the radiator. This steam is condensed in the radiator
by means of air flow. The water formed is collected in the bottom of the radiator and re-circulated
by the same pump. The main advantages of this system are that it needs very little quantity of
coolant at start and the engine is also warmed up quickly.

COMPONENTS OF WATER COOLING SYSTEM
The main components of water cooling system are as given under.

(i) Radiator (ii) Water pump (iii) Cooling Fan (iv) Thermostat
                                                                                                   109

Radiator.
The radiator is a heat exchanger that removes heat from coolant passing through it. The purpose of
a radiator is to ensure the close contact of the hot cooling water coming out of the engine with
atmosphere air and hence to cool the water passing through it very efficiently. A radiator consists of
an upper (header) tank which is connected to the water outlet of the engine, and the lower
(collector) tank which is connected to the water inlet of the engine. Both of these tanks are
connected by means of radiator cores. An overflow pipe in the header tank and a drain pipe in the
lower tank are also provided. The radiator upper tank and the lower tanks are connected with the
engine block by means of the flexible water hose pipes. The hot water from the engine enters the
radiator at the top and is cooled by the cross flow of air, while flowing down the radiator. The
cooled water is collected in the collector tank and from there it is pumped again to the engine water
jackets.
                                                                  The radiator cores are of two types,
                                                                  the tubular type and thee cellular
                                                                  type. Referring Fig, in case of
                                                                  tubular type, the cooling water
                                                                  passes through the vertical or zigzag
                                                                  tubes and air passes around them. In
                                                                  case of cellular type air passes
                                                                  through the tubes and water passes
                                                                  in the spaces in between them. Out
                                                                  of these the tubular type core is the
                                                                  most commonly used ones, which
                                                                  are further classified depending
                                                                  upon the shape of the fins
                                                                  (Serpentine, spiral and plate fins)
around the tubes to increase the area for heat transfer from water to the cooling air.




In a typical radiator, there are five fins per
inch [25.4 mm] Radiators used in cars that
have factory-installed air conditioning
have seven fins per inch [25.4 mm]. This
provides the additional cooling surface
required to handle the additional heat load
imposed by air conditioning. Most late-
                                                                                                   110
model cars have a cross-flow radiator. In this type, the coolant flows horizontally from the inlet
tank on one side to the outlet tank on the other side. Basically, the cross-flow radiator is a down-
flow radiator turned on its side. This allows the car body to be designed with a lower hood line. The
outlet tank contains the transmission oil cooler, where used.
On any radiator the inlet tank (above or to one side) serves two purposes. It provides a reserve
supply of coolant. It also provides a place where the coolant can be separated from any air that
might be circulating in the cooling system.
Water Pump:

The purpose of it is to increase the circulation of the coolant in the cooling system of the engine. It
is mounted at the front of the engine in between the engine block and the radiator. Referring Fig it
consists of a casing which contains inlet and outlet ports of water. The impeller, which may be of
rotor or disc type is mounted at the one end
of the pump shaft and encloses in the
casing. A pulley is mounted on the outer
end of the shaft by means of hub. The
impeller shaft itself is supported in the
pump body by a pre-lubricated ball-bearing
assembly as shown in the Fig.

        When pulley gets drive from the
crank shaft by means of a V-belt then the
pump is also rotated within the casing. The
water available at the impeller and its
blades is forced out through the outlet to the
engine water Jackets by means of
centrifugal force. Fresh water from the
lower tank of the radiator always flows through the hose pipe to the pump. The leakage of water
from the casing side to towards bearing side is prevented by means of providing a water seal
between two.

(iii) Cooling Fan:

 When the vehicle is moving under
heavy loaded at slow speed then the
natural air-draft is insufficient to provide
the desired engine cooling. Therefore the
system consists of a cooling fan, which
provides a powerful draft of air through
the radiator. It is mounted behind the
radiator on the water-pump shaft and is
driven by the same V-belt that drives the
pump and the generator. Sometimes the
fan is mounted inside a thin plastic or
metal housing around its periphery. This
housing is attached behind and against
the radiator and is called fan shroud. It
                                                                                                  111
allows fan to pull more air past the radiator. It may have four to seven blades, sometimes spaced
unevenly to reduce noise. It is generally made of sheet metal, but these days moulded plastic
materials e.g., nylon or polypropylene are also being used for making fans.
        In modern vehicles to keep the cooling constant at different speeds and to save the
unnecessary consumption of power in fan drive the cooling systems are equipped with a
thermostatically controlled power booster fan that is driven through a liquid clutch which slips after
reaching a set speed.
For efficient and economical running, it is required that the fan must give adequate air flow at all
the conditions of vehicle load and speed. More flow than the minimum necessary for effecting
cooling at any particular time is simply uneconomical. Thus the commonly used method of running
the fan at one constant speed ratio with the engine is not desirable. If, for example, the fan is
designed to give adequate air flow at low vehicle speeds, say, when going uphill when the air flow
due to vehicle speed is very small, obviously the air draught at high vehicle speeds will be much
more than the desired, when the air flow due to vehicle speed itself is quite high. Thus a fan that is
always running with the engine will be unnecessarily consuming engine power which has been
estimated as much as 5% of the engine Brake Power and producing more noise. This is clearly a
waste and must be avoided. The following methods are currently being employed for this

1. The fan blades are of variable pitch type, which is controlled by the engine speed itself. As the
engine speed increases, the pitch decreases, thus reducing the air flow. Alternatively, the blade
pitch is controlled directly by the cooling water temperature, which has been found to be more
efficient design.

2. The fan is not directly driven by the belt from the crankshaft, but is driven through a fluid
coupling. One rotor of the coupling is driven by the engine, while to the other one is attached the
fan. The spacing between the rotors is controlled by the temperature of the cooling system, which
changes the slip of the coupling thus changing the fan speed according to the requirements.

3. The fan is driven by means of a separate electric motor which is supplied with power directly
from the electric circuit of the engine. This system has been used on Maruti cars. A thermostat
switch is placed at an appropriate place in the cooling system and depending upon the cooling
system temperature it operates to switch on or off the fan motor. It has been found that under
ordinary conditions, only about 5 percent of the time the fan motor remains in 'on' position, while
95 per cent of the time it is 'off.' The saving of engine power thus achieved may, therefore, be well
imagined.

(iv) Thermostat: At start the engine does not need cooling, because overcooling results in
deterioration of engine efficiency. Therefore a thermostat valve is provided in the cooling system,
which keep a rigid control over the cooling and maintains the cooling water temperature at a
predetermined value. Different types of
thermostats are as explained under.

(a) Bellows-type thermostat: Referring Fig, it
consists of metallic bellows filled with acetone
liquid a valve and its seat. The unit is suspended
in the hose and mounted at the outlet of the
engine.
                                                                                                  112
At start the temperature of the cooling water is low and the pressure of acetone inside the bellows is
also reduced thereby retaining the valve on its seat and closing the circulation of water through the
radiator. Under this position the water flows back to the pump through by-pass only.

        When the engine after start is warming up and
temperature reaches to predetermined valve then the liquid
inside the bellows is converted into vapour thereby exerting
a pressure and opening the valve as shown in Fig. Under
this position the flow of water through the radiator. Any
variation in the temperature is automatically controlled by
means of opening and closing of the valve.

(b).Wax Thermostat:

It is also fitted at the some place as previous one. Referring
Fig, it consists of a copper loaded Wax element having
high coefficient of expansion. As the water is heated, the
wax of the element expands. This movement of the
element, along the plunger opens the valve against the
return spring thereby allowing the water through the
radiator and closing through the bypass by means of side shutter attached with the valve. The wax
contracts on cooling thus retained the Valve on its seat and there thereby closing the flow of water

Radiator pressure cap:




                                           The
cooling systems on automobile engines today are sealed and pressurized by a radiator pressure cap.
There are two advantages to sealing and pressurizing the cooling system. First, the increased
pressure raises the boiling point of the coolant. This increases the efficiency of the cooling system.
Second, sealing the cooling system reduces coolant losses from evaporation and permits the use of
an expansion tank.
                                                                                                   113
At normal atmospheric pressure, water boils at 2120F [1000C]. If the air pressure is increased, the
temperature at which water boils is also increased. As the pressure goes up, the boiling point goes
up. Therefore, the coolant can be safely run at a temperature higher than 2120F [1000C] without
boiling. Higher the coolant temperature, greater the temperature difference between it and the
outside air. This difference in temperatures is what causes the cooling system to work. The hotter
the coolant, the faster the heat moves from the radiator to the cooled passing air. This means that
the pressurized, sealed cooling system can take heat away from the engine faster. Therefore, the
cooling system works more efficiently when the coolant is under higher pressure.

However, the cooling system can not be pressurized too much. If the pressure in the system gets too
high, it can damage the radiator and blow off the hoses. To prevent this, the radiator cap has a
pressure relief valve. When the pressure gets too high, then it raises the valve so that the excess
pressure can escape into the expansion tank.
 The radiator pressure cap also has a vacuum vent valve. This valve protects the system from
developing a vacuum that could collapse the radiator. When the engine is shut off and cools, the
coolant volume is reduced. Cold coolant takes up less space than hot coolant. As the temperature of
the coolant drops, a vacuum develops in the cooling system. To prevent excessive vacuum from
developing, the vacuum valve opens to allow outside air or coolant from the expansion tank to flow
into the cooling system. This relieves the vacuum that could otherwise cause outside air pressure to
collapse the radiator.
 Never remove the cap when the engine is hot. Boiling coolant and steam can erupt from the filler
neck. Several types of safety caps are also used on radiators. The safety cap has a button or lever on
top of the cap. By pressing the button, or lifting the lever, the pressure is relieved. This eliminates
the chance that steam or boiling coolant could scald you when the cap is removed. In cooling
systems with an expansion tank the radiator pressure cap is more or less permanently installed. The
cap should not be removed just to check the coolant level or to add coolant. Instead, Coolant level
is checked visually at the expansion tank. Many tanks are marked to indicate normal hot and cold
coolant levels. If coolant Is needed, it is poured into the expansion tank.

ANTI-FREEZE SOLUTIONS

        In cold climates there is always a danger that the water in the cooling system may get
frozen. As the volume of water when converted into ice increases by about 10%, this may result in
the damage of the entire system. Water freezing in the cylinder block or cylinder head could expand
enough to crack the block or head. Water freezing in the radiator could split the radiator seams. In
either case, there is serious damage. A cracked block or head cannot be repaired satisfactorily. A
split radiator is hard to repair. To avoid this some additives are used, which when mixed with water
in suitable proportions, lower the freezing point of water. Such additives are called antifreezes and
the solution thus formed becomes antifreeze solution. Even for hot climates the use of antifreeze
has the incidental advantage that boiling point of the cooling water is also raised
The requirements of on anti-freeze may be enumerated as

1. It should be thoroughly miscible with water and should present the freezing of the coolant drawn
to the lowest ambient temperatures.
2. It should not have any corrosive action on system components, especially the radiator hose pipes.
3. Its boiling point should be high so that there is minimum loss due to evaporation and the coolant
can operate at higher temperatures.
4. It should not deposit any foreign matters in the jackets, hose pipes or radiator core.
                                                                                                    114
5. It should have high specific heat capacity so as to be comparable to the specific heat capacity of
the coolant
6. It should not be inflammable and its flash point should be the maximum possible operating
temperature
7. Its viscosity should not be excessive so that the circulation is not affected.
8. It should be readily miscible with corrosion inhibitor and anti sealing compounds.

       The anti-freezes most commonly used are wood alcohol (methyl alcohol), denatured alcohol
(ethyl alcohol), glycerin, ethylene glycol etc. Each of these has its own advantages and
disadvantages. Alcohol is quite effective, but it is very much volatile and due to this reason
evaporation losses are high. Ethylene glycol corrodes copper, Aluminium and tin-lead solder alloys.
The glycerin is less volatile, but it is comparatively costly and also it attacks rubber hose pipes. The
Calcium chloride is another type of good antifreeze. Moreover, with chromates like sodium
chromate added to it, the corrosion of most metals is drastically reduced. The amount, by which the
freezing Point of cooling water is lowered, depends upon the proportion of the anti-freeze in the
cooling water. For example, a 50% concentration of ethylene glycol (by volume) lowers the
freezing Point of water to about -370C.
       Some antifreeze compounds plug small leaks in the cooling system. These antifreeze
compounds contain tiny plastic beads or inorganic fibers which circulate with the coolant. When a
leak develops, the beads or fibers jam in the leak and plug it. This is the same action provided by
adding "stop leak" or "sealer" to the cooling system in an emergency. Some antifreeze
manufacturers add a foam inhibitor to the ethylene glycol. Air does not conduct heat as well as
coolant. Any air in the cooling system may cause excess foaming of the coolant as it is whipped up
by the water-pump impeller. A foam Inhibitor tends to reduce this problem.
       Antifreeze solutions usually spoil the finish of the paint work. Therefore adequate
precaution should be taken while topping up the Cooling system with antifreeze solution.

TEMPERATURE GAUGES

These are generally provided on the engine as optional accessory. This seems to be only due to
economy reasons, because the installation of the temperature gauge on the dashboard helps the
driver to avert serious consequences, as he has before him the temperature indication all the time.
Supposing the thermostat gets stuck up and does not open, the water circulation will stop and the
engine temperatures will go very high. However, if the temperature gauge is provided, the driver
can stop the engine at once and remedy the situation, before the engine is overheated and some
damage has already occurred.

The temperature gauges are of two types

1. Bourdon tube type.

2. Electrically operated type.

Bourdon tube type:
This has a Bourdon tube inside,
which is connected by a capillary
tube to the element, containing some
volatile    liquid      at       suitable
                                                                                                   115
temperature and which is inserted in the cooling water circuit at an appropriate point, generally on
the engine side of the thermostat. As the temperature of cooling water increases, the liquid in the
element evaporates and exerts its pressure in the capillary, which is further transmitted to the
Bourdon tube. Due to this pressure, the Bourdon tube tries to straighten out and thus moves a
pointer attached to it, to show higher temperature on the scale.


Electrically operated type:




This contains an element made of such a material that its electrical resistance decreases with
increase of temperature. The element is connected to the coils inside the dash unit as shown in Fig
(b) while Fig(a) depicts entire electrical circuit. The gauge element is inserted into the cooling
water at some appropriate place. As the cooling water temperature increases, the resistance of
element decreases, which causes more current to flow in the coil (2), thus increasing the e.m.f. built
up there. The pull of the coil (2) on the armature carrying indicator pointer, therefore, increases and
the pointer moves to show the higher temperature.
                                                                                                    116
                                    LUBRICATION SYSTEM


(Syllabus: Lubrication system: Function of lubrication system, classifications of lubricants, type of
lubricants, properties of lubricants, oil filter, oil pumps, crankcase ventilation, oil additives, and
specifications of lubricants.)

In the motor-vehicles it is possible in some cases that the engine as well as the metallic chassis
components to move under direct contact. This direct contact produces a 'dry' friction and there by
producing a large amount of heat which results serious troubles, such as scoring of cylinders,
burning of bearings, sticking of piston rings, and excess fuel consumption. Therefore to overcome
these difficulties, it is essential to interpose a layer of film of a lubricant between the two surfaces
so that two surfaces may not come in direct contact with each other. Under this condition friction
remains of lubricant itself which is called 'viscous friction'. In case of vehicle chassis Lubrication
where to with-stand the lubricating oil is not possible, under this condition the dry friction is
reduced by applying the grease between the two rubbing surfaces. As this lubricant does not fully
separate the two surfaces, therefore even remains some friction between two which is called
"boundary" friction. Therefore in automotive the role of lubrication and lubricants is an important
factor to minimise wear or low frictional loss between adjacent moving surfaces.

PURPOSES OF LUBRICATION

(i) To minimise wear and friction: If the lubricating system in automotive does not function
properly then sufficient lubricant is not supplied to moving parts, thus dry or boundary friction
occurs between moving surfaces. Therefore a part of power is lost to overcome this friction. If more
friction is being developed between two surfaces then the parts will be worn out and damaged due
to rapid development of heat. In automotive, the friction of lubricant itself also matters, therefore
proper selection of the lubricant according the desire is necessary.

(ii) To remove heat from engine parts: The lubricating oil is filled into the engine oil pan, from
where it is circulated to most of the moving parts of the engine. Therefore in addition to lubrication
purpose, the lubricating oil absorbs heat from the moving parts and transferring it to the atmosphere
by means of surrounding surface of the oil pan and other parts. In this way lubricating oil also
serves the purpose of a cooling agent.

(iii) To absorb shocks occurred between moving parts: When power is transmitted through two
moving or rotating parts then the lubricant available in the clearance between the two parts acts as
cushion and absorb shocks. For example, in an engine as the combustion takes place, a sudden load
is imposed on the top of the piston. This load or pressure is transmitted to the crankshaft through
the piston pin, connecting rod and connecting rod bearings. Therefore lubricating oil or film of the
oil filled around the piston pin and between bearing and journals provides cushioning effect and
absorb shocks. Thus oil also reduces the engine noise and wearing of the parts.

(iv) To provide sealing action: As the lubricating oil film is filled in the gaps and microscopic
irregularities between the piston rings and cylinder walls so that no gas can escape through them.
Thus it will provide a gas tight seal against the leakage from cylinder to the crank case side.
                                                                                                    117
(v) To act as a cleaning agent: As the lubricating oil is circulated through the engine, it washes off
and carries away dirt, carbon particles and foreign matter if any. The large particles of impurities
are removed from the oil pan and the dissolved smaller are purified by filtration.
LUBRICATION PRINCIPLES

Consider a block resting on a flat surface covered with a layer of lubricating oil. If the weight of the
block is very high or the oil is thin, the oil will squeeze out. In other words thick oil can support a
higher load than that supported by thin oil.


(a) Hydrodynamic lubrication: When this block is moved over the surface, a wedge-shaped oil film
is built up between the moving block and the surface. This wedge-shaped film is thicker at the
leading edge than at the rear. In other words the moving block acts as a pump to force oil into
clearance that narrows down progressively as the block moves. This generates appreciable oil film
pressure which carries the load. This type of lubrication where a wedge-shaped oil film is formed
between two moving surfaces is called hydrodynamic lubrication. The important feature of this
type of lubrication is that the load carrying capacity of the bearing increases with increase in
relative speed of the
moving surfaces. This
occurs      because     at
higher speed the time
available to the oil to
squeeze out is less.
The force required to
move the block over the
surface depends upon
the weight of the block,
the speed of movement,
and the thickness or
viscosity of the oil.
This force divided by
the pressure caused by
the weight of the block                                                                           is
called the coefficient of
friction.     A    higher
coefficient of friction
signifies a greater force                                                                         to
move the block.
The       flat    surface
lubrication of the kind referred above exists at places such as thrust bearings, valve tips and cam
lifters. Many other surfaces which use hydrodynamic lubrication are cylinder wall, valve guide,
main bearings, connecting rod bearings, and camshaft bearings.
(b) Elasto-hydrodynamic lubrication: When the load acting on the bearings is very high, the
material itself deforms elastically against the pressure built up of the oil film. This type of
lubrication, called Elasto-hydrodynamic lubrication, occurs between cams and cam followers, gear
teeth, and rolling bearings where the contact pressures are extremely high.
(c) Boundary lubrication: If the film thickness between the two surfaces in relative motion becomes
so thin that formation of hydrodynamic oil film is not possible and the surface high spots or
asperities penetrate this thin film to make metal-to-metal contact then such lubrication is called
                                                                                                   118
boundary lubrication. Such a situation may arise due to too high a load, too thin an oil or
insufficient supply of oil due to low speed of movement. Most of the wear associated with friction
occurs during boundary lubrication due to metal-to-metal contact. A condition of boundary
lubrication always exists when the engine is first started. The shaft is in contact with the bottom of
the bearing with only a thin surface film of oil formed on them. The bearing surfaces are not
perfectly smooth-they have 'hills' and 'valleys' which tear this thin film which is
constantly formed while the crankshaft is turning slowly. As the speed increases it switches on to
hydrodynamic lubrication. Boundary lubrication may also occur when the engine is under very high
loads or when the oil supply to the bearing is insufficient.

(d) Hydrostatic lubrication: In hydrostatic lubrication a thin oil film resists its instantaneous
squeezing-out under reversal of loads with relatively slow motions. The oil film acts as a cushion.
If oil supply is sufficient the oil film thickness is restored before next reversal of load.

In internal combustion engines there are a variety of different types of bearings and motions as
given in Table

                  Motion                                        Surface
1.     Sliding contact                         (a) Journal bearings - crankpins,
       (a) Rotating                            crankshafts, camshafts, valve mechanisms,
                                               etc.

(b) Oscillating                                (b) Journal bearings - piston pins, knuckle
                                               pins, rocker arm bearings etc.

(c) Reciprocating                              (c) Slipper bearings - pistons, piston rings,
                                               valve stems, cross-heads, etc.

2. Meshing contact                             2. Worm, bevel spur and helical gears.

3. Rolling contact                             3. Ball, roller, and needle bearings.



PROPERTIES OF LUBRICANTS

(i) Viscosity. It is an important property of lubricating oil which is usually specified as the time in
seconds that it takes for a given amount of the oil, to flow by gravity through a standard -sized hole
at a particular temperature. As the light oil having low viscosity and heavy oil has high viscosity,
oils are classified in light medium and heavy groups. As the rise of temperature decreases viscosity
and fall of temperature increases, the lighter oils are used in the winter where as heavier in summer.
Therefore a good lubricant should have approximately the same viscosity at all the temperatures.

The oil will become less viscous when it is heated and more viscous when is cooled. Therefore the
automotive engine is harder to start at low temperature because of high oil resistance. In accordance
to change of temperature, the change in viscosity of the oil differs oil to oil. Therefore to identify
oil quality a viscosity index scale is adopted which runs from 0 to 100 numbers. Higher the
viscosity index number, lower the rate of change of viscosity with change of oil temperature.
                                                                                                       119
(ii) Viscosity ratings. Considering temperature, the viscosity of oil is determined by the length of
time required for a definite amount of oil to pass through a hole of particular size. Since the low
temperature increases and high temperature decreases the velocity, the lower numbers represent the
lower viscosity of the oils. The viscosity of the oil is rated in two ways by the Society of
Automotive Engineers (SAE) as the winter grade oils and other than winter. They are of three
grades, (a) SAE5W, SAE10W, SAE20W; (b) SAE20, SAE30, SAE40, SAE50 and (c) say
SAE10W-30. In first the W represents grade winter and the second is without the W suffix. The oils
of third grade have multiple ratings or called multi grade oils. The above stated SAE10W-30 oil is
comparable to oils of SAE10W, and SAE-30 grades.

(iii) Specific gravity. This property represents the density of the oil which is measured by a
hydrometer. The standard of this density has been recommended by the American        Petroleum
Institute, which is indicated by the symbol API-gravity.

(iv) Pour point: It is the lowest temperature at which the lubricant can be poured. This point can be
varied by putting certain additives into the oil. Thus the oil having lower pour points are being used
in cold countries, where as of higher pour points in hot countries.

(v) Flash point and Fire point. The flash point is the lowest temperature at which the oil flash when
light tilted on its surface. If this oil is further heated then a temperature at which it begins to burn is
called fire point. Therefore these points of lubricating oil must be sufficiently high so that it may
not flash or burn during service.

(vi) Resistance against carbon formation. The high temperature of the engine cylinder and piston
breaks down the oil films, thus the oil burns and form the carbon. This carbon may pack at the
piston rings and prevents them to function properly. There by sticking the rings in the grooves,
obtaining poor- compression, excessive oil consumption and scoring of cylinder walls. In this way
formation of the carbon may cause poor performance and damage to the engine. Therefore a
lubricant must be resistant to the heat and form a minimum amount of carbon during service.

(vii) Resistance against oxidation. During service some air mix with the lubricating oil and form
its oxides. These oxides are very sticky, clog oil, oil channels and restrict the piston rings and
valves from proper functioning. In this way the excessive oxidation may cause bearing failures, and
damage to other parts, which result in a serious trouble in the engine. Therefore the lubricating oil
must be well refined with proper additives so that it may not tend to oxide even at high
temperatures.

(viii) Resistance against corrosion and rust formation. At high temperature acids are formed in the
lubricating oil. These acids corrode the whole lubricating system and main parts of the engine.
Similarly water may also be formed in the oil which tends to rust the engine part. Therefore
lubricating oil should have added some chemicals in it, so that formation of rust and corrosion
effect on the engine parts may be avoided.

(ix) Resistance against foaming. Due to churning action in the engine crankcase some foam is
developed in the lubricating oil. These foams overflow through crankcase ventilator and thereby
reducing the oil level in the sump. On the other hand the foaming oil gives poor performance in
lubrication system. Therefore to avoid the foaming action some additives are added in the
lubricating oil.
                                                                                                   120
(x) Resistance against pressure. A high pressure is exerted on the lubricant used at the bearings and
valves of the engine. Under this condition the oil tends to squeeze out from the surfaces of the
moving parts. Therefore some chemicals are added into the oil to resist the pressure and to provide
the slippery films between two rubbing parts of the engine.

(xi) Cleanliness. As through ventilator, carburetor, due to carbon formation and too wear of metal
engine parts, some dirt is deposited in the oil sump and system. This dirt mixes with the oil and acts
like an abrasive paste, which results poor performance of engine and speed up wear of parts. To
overcome this difficulty some detergent additives are put in the oil. The detergent additive in the oil
loosens and detaches the deposits of carbon, gum, dirt etc. Then the larger particles of the loosened
material drop to the sump and the smaller particles are separated by means of oil filter. Very minute
suspended impurities are flushed out when the engine oil is changed. Therefore the oil should be
sufficient clean and stable itself so that the crank case, oil sump and oil lines are kept clean.

(x) Acidity and neutralization number: The oil must have low acidity. The neutralization number is
a measure of acidic or alkaline contents of oil. New oil has low neutralization number, which is the
quantity of alkaline solution or acid solution required to make the oil neutral. Used oil has high
neutralization number.

SERVICE RATING OF OILS

Since SAE grades are based solely on viscosity they do not bear any relationship to oil quality.
Addition of certain additives can materially influence their performance under different operating
conditions and different characteristics of fuel burnt. Therefore, API (American Petroleum
Institute) adopted in 1947 a system which divided crankcase oil into three classes - Regular Type,
Premium Type and Heavy Duty Type - depending upon the properties of oil and the operating
conditions under which it was intended to be used. Generally Regular Type oils were straight
mineral oils, Premium Type contained oxidation inhibitors and Heavy Type contained oxidation
inhibitors plus detergent-dispersant additives.

These early classifications did not recognise that diesel and gasoline engines have different oil
requirements or that the requirements for either type of engine are significantly influenced by the
type of fuel burned. API developed a new classification based on severity of engine service in 1952
which were revised in 1955, 1960, and again in 1970. For gasoline engine oils 5 service ratings are
: SA, SB, SC, SD and SE whereas the 4 service ratings for diesel engine oils are CA, CB, CC, and
CD. The additives and other properties are different for oils having different service ratings. S and
C stand for spark-ignition and compression-ignition engines, respectively. Rating A is for light-
duty service, the severity of service increasing towards rating D which is severe duty

TYPES OF LUBRICANTS

As the conditions of different moving parts such as crank shaft, axles, differential, steering system,
transmission system, brakes, ignition, distributor, leaf springs and shock absorbers differ from each
other, different types of lubricants are used in vehicles. They are classified in three main categories
as (a) fluid, (b) semi-fluid and (c) solid. Fluid oils are used in lubricating system, semi-fluid in
chassis side and the solid lubricants in springs or some bearings of the automobiles. These
lubricants according to their source of availability are classified as under:
                                                                                                     121
(i) Vegetable oils. The most common vegetable oils which can be used as lubricant in automotive
side are olive oil, rape-seed oil, caster oil, hazel-nut oil, palm oil. The caster oil which has viscosity
and film strength has been used as a lubricant in automotive side and given good results. But the
vegetable oils have a drawback that they become gummy after use and also get easily oxidized.
Therefore they are avoided in now a days on automobiles.
(ii)Animal oils: The most common animal oils which can be used as a lubricant are lard oil, tallow
oil, whale oil etc. These are obtained from the animal fats. But due to their gummy and oxidizing
properties after use they are not suitable for automotive uses.

(iii) Mineral oils. The mineral oil used for lubrications are obtained from petroleum, shale or coal
tar. These lubricants are widely used in automobiles. They have all the qualities desired by the
lubricant. They have sufficient advantages over the vegetable and animal oils. The main
constituents of these oils are hydro-carbons. After distillation and purification, they-are further
classified as paraffin, olefins, napthenes and aromatics. During process the gummy property of the
oil is eliminated and few are diluted and used in light duty bearings are called light machine oils.
These lubricants are also used in starters, generators and distributors of the engines, by means of oil
cups. Some lubricants penetrate in the surface of the metals and loosen the rust and resist the
corrosion are used to lubricate the springs.

(iv) Synthetic lubricants: These lubricants are such as poly-organosiloxanes or silicon fluids,
polyglycol ethers, aliphatic diester etc. They are prepared synthetically by mixing the natural oils
and some chemicals under controlled conditions. Actually they are superior than mineral oils, but
have limited uses due to their higher costs.

(v) Greases: Grease is a semi-solid lubricant made by mixing the thickening agents into the
lubricating oils. This lubricant is lubricant is applied in automobiles in places where the retention of
the liquid lubricant is difficult or not possible. It resists pressure and provides lubrication at high
temperatures. It also provides sealing when used in bearings. They are classified according to their
bases such as aluminium grease, soda grease, calcium grease and mixed greases The petroleum
grease is produced by mixing the metallic soap in to the mineral oil which retains well under
sufficient high pressure conditions. The wrong selection and excess quantity of the grease in the
bearings may result a loss in power.

(vi) Solid lubricants: The solid lubricants are used under the condition where the speed is low and
pressure is high. A common and most suitable solid lubricant is graphite. It has a black soft mass in
the form of carbon. It is used either in the natural flake form or suspended in oil, grease or distilled
water in a finely divided colloidal form. It is more suitable under high pressure condition where
film-lubrication is not feasible. Sometimes soapstone is also used as a lubricant, because it is soft
and greasy in touch. It also has the ability to fill up the pores of the metal surfaces and make them
frictionless.

Blending and compounding

As all the desirable characteristics are not possible in the pure oils, therefore to thicken the oil and
to increase its oiliness qualities, some other oils are mixed in it. When petroleum oils are mixed
with the petroleum oils, this mixing process is called as 'Blending'. Similarly when mineral oils are
mixed with the vegetable oils or animal oils then this process is called 'compounding'. Therefore the
blended and compounded oils provide a large range of lubricating oils and are more economical
than mineral oils alone, as the vegetable and animal oils render the mineral oil less volatile.
                                                                                                   122
OIL ADDITIVES

These are the chemical substances which are added to the lubricating oil either to reinforce some of
its natural properties or to provide it with certain new properties which it does not possesses
originally. Oil additives are classified according to the property of the oil which they reinforce or
add. The important additives in current use are:

1. Oxidation inhibitors. These inhibit the formation of varnish by preventing the oxidation of the oil
at the engine operating conditions. These are usually organic compounds such as amines, sulphides
or phenols with metals like tin, zinc or barium.

2. Corrosion inhibitors. These prevent or at least reduce the formation of acids which would cause
bearing corrosion. These consist of oxidation inhibitors with the addition of metal salts of thio-
phosphoric acid and sulphurized waxes.

3. Detergents. There are also called dispersant additives. These inhibit the formation of low
temperature sludge binders and break the sludge particles into finely divided particles, which stay
in the oil in fine suspension and are removed when the engine oil is changed. Thus various engine
parts, like piston rings, main bearings, oil galleries, connecting rod etc. remain clean. The detergent
additives are the polymers and polyalyenyl succinimides.
         However, the additives in two-stroke engine oil act in a different manner. These convert the
particles of carbon into a crumbled powdered form which is easily scavenged through the exhaust
port.

4. Viscosity Index Improvers. These are the additives which do not allow or at least minimise the
decrease of oil viscosity with the increase in temperature. In case of ordinary lubricating oil, these
are mainly hydrocarbon molecules whose internal energy increases with increase in temperature so
that they move faster and further apart and thus become thinner. The additives are usually polymers
such as from acriloid plastics. These have long chain molecules which are coiled up when cold so
that they can then move as freely as the hydrocarbon molecules. However, on heating, the polymer
molecules unwind and hinder the movement of the hydrocarbon molecules, thus compensating for
the decrease of oil viscosity with temperature.

5. Anti foaming additives. The lubricating oils, in general, have a tendency to foam due to engine
vibrations which give rise to churning of oil in the sump. If allowed to foam, the oil cannot properly
lubricate the engine bearings, apart from loss by overflowing. Antifoaming additives are available,
which suppress this foaming tendency of oils. Poly-organosiloxanes are the most common anti-
foaming additives.

6. Extreme Pressure (E.P.) additives. These cater for more difficult conditions of lubrication, e.g.,
the one arising between the highly stressed cams and valve tappets. These prevent metal to metal
contact by forming a chemical film. Polymeric materials such as poly-isobutene form such
additives.

7. Pour point depressants. These serve to lower the pour point of the oil by coating the wax crystals
in the oil so that they would not stick together and thus facilitate oil flow.

8. Other additives: Apart from above a larger variety of additives are available. Some of these are
rust inhibitors, water repellents, emulsifiers, dyes, odour controllers, etc.
                                                                                                    123

EFFECT OF ENGINE CONDITIONS ON LUBRICATING OIL

The severe engine conditions of temperature and pressure to which the lubricating oil is subjected,
cause its deterioration in many ways, which are discussed here.

1. Sludge formation: Sludge is a mushy material composed of oil, water and other combustion
products. It readily clogs the oil lines and galleries. It is caused by the condensation of water in the
engine crankcase, which forms an emulsion with the oil, dirt and the other matter. Obviously this
occurs in the case of engine which seldom runs hot to drive away the water vapour out of the
crankcase. A vehicle, which runs mostly in traffic, running slow and stopping intermittently is thus
more prone to sludge formation. This can be remedied at least partially in such a vehicle by
adjusting its cooling system so that engine remains at a reasonably high temperature.

2. Lacquer formation: Lacquer or varnish is formed when the oil gets oxidised due to high
temperatures. Lacquer is often responsible for sticking valves and clogged piston rings. To avoid
this, use suitable additives and change the oil regularly as per the recommendations of the
manufacturer.

3. Oil dilution: Oil dilution is caused by the leakage of gasoline past the piston into the crankcase,
or by the condensation of water vapour in the crankcase. This happens often when the engine is
cold. i.e. at the time of starting and warming up. In winter, choke is used frequently to start the
engine. The effect of using choke is to introduce a small additional quantity of gasoline in the
cylinder, which contributes towards oil dilution. This it is seen that choke should be applied only
when it becomes a must, as for example, in very, cold weather. Further it is also observed that the
dilution occurs mostly under starting and idling conditions only. It follows, therefore, that engine
should not be raced heavily just after starting because there is no lubrication at that time.
Oil dilution may give sometime a faulty dipstick reading. For example, if the dipstick shows
satisfactory oil level when put into the sump when just the engine has started, the same will be
misleading because the condensed water vapour is also present there in the sump. As the engine
warms up and comes to normal conditions, the water will vaporize and the indicated oil level will
therefore fall. And this will be the actual oil level. Oil dilution reduces the lubricating property of
the oil. Thus, if exceeded beyond limits, it will cause metal to metal contact and hence very rapid
wear and premature failure of engine parts.

4. Carbon deposition: Carbon formation occurs normally on the combustion chamber walls,
pistons, valve stems and piston rings. It results due to incomplete combustion of fuel, particularly
so at the time of starting and idling when richer fuel is supplied.
Carbon is a bad conductor of heat and therefore dissipation of heat from the engine will be less. The
result is higher combustion chamber temperatures, which encourage oxidation of the oil and
thicken out the deposits further. Moreover, the tendency to detonate is increased. The sticking of
rings in the grooves causes blow by of hot gases and the consumption of oil is increased.

CONSUMPTION OF LUBRICATING OIL

The various sources of oil losses in the engine are:
1. Combustion. The oil works up past the piston rings into the combustion chamber, where it is
burnt up. This cannot be avoided altogether since some of the oil has to go past the piston rings to
lubricate the upper portion of the piston and the cylinder walls.
                                                                                                   124

2. Loss through leakage. There may be some loss of oil through leakage at the faulty crankshaft
bearing seals, at the defective joint between the cylinder block (or crankcase) and the oil pan, at the
loose or faulty drain plug, or due to some crack in the crankcase. However, with proper care such
losses can be reduced to negligible.

3. Loss through crankcase ventilation. Some of the oil is carried away from the crank case as
vapour or mist or along with the exhaust gases.

4. Loss on account of wear of engine parts:
(i) When the cylinder bore, the piston skirt or the rings wear out, more oil is pumped past the piston
rings, which causes increased oil consumption. Actually, higher the pressure intensity between the
piston rings and the cylinder walls, the lower the oil consumption. This is achieved by putting steel
expander inside the ring so that even when the ring is worn out, though the pressure due to ring
itself decreases, yet the pressure due to steel expander is not decreased, thus maintaining a higher
pressure intensity between the rings and cylinder walls.
(ii) Worn out big end bearings of the connecting rod means excessive clearances, causing too much
oil to be thrown on cylinder walls.
(iii) Worn out rocker shaft or rocker bushes may also be a source of excessive oil consumption.

5. Loss due to excessive vehicle speed. The oil consumption increases with increase in speed of the
engine. There are several reasons why the oil consumption should increase at high engine speeds:

(i) High speed means high engine temperatures and consequently reduced oil viscosity which
means the oil would be thinner and, therefore, will flow with relative ease. This means more of oil
will pass up the piston rings and bum into the combustion chamber. Further thinner oil will be
susceptible to easy leakage.
(ii) At high speeds there is more centrifugal effect, as a result of which more oil is fed to the oil
lines and more is sprayed on the cylinder walls. This causes more oil consumption
(iii) 'Ring shimmy' is caused at high engine speed which pumps large amounts of lubricating oil
past the rings into the combustion chamber.
(iv) At higher speeds more oil is lost in the form of mist through crankcase ventilation. In an
automobile engine working under normal conditions, average consumption of lubricating oil is
estimated at about 2000 km. per litre. The consumption increases as the pistons and the cylinders
wear out.

                                   LUBRICATING SYSTEMS

Various lubricating systems used for internal combustion engines may be classified as:

1. Mist lubrication system   2. Wet sump lubrication system
3. Dry sump lubrication system.

Mist Lubrication System:

This system is used for 2-stroke cycle engines. Most of these engines are crank charged, i.e. they
employ crankcase compression and, thus, are not suitable for crankcase lubrication.
                                                                                                    125
Such engines are lubricated by adding 2 to 3 per cent lubricating oil in the fuel tank. The oil and the
fuel mixture are inducted through the carburettor. The gasoline is vaporised and the oil, in its form
of mist, goes via crankcase into the cylinder. The oil which impinges on the crankcase walls
lubricates the main and connecting rod bearings, and the rest of the oil which passes on to the
cylinder during charging and scavenging periods lubricates the piston, piston rings and the cylinder.
The 2-stroke engine is very sensitive to particular oil and fuel combination. The composition of
fuels and lubricants used influence the exhaust smoke, internal corrosion, bearing life, ring and
cylinder bore wear, ring-sticking, exhaust and combustion chamber deposits, and one of the most
irritating and difficult problem of spark plug fouling and whiskering. Therefore, specially
formulated ash-less oils are used for 2-stroke engines.
The fuel and oil ratio used is also important for good performance. A fuel and oil ratio of 30 to
50 : 1 is optimum. Higher ratios increase the rate of wear and lower ratios result in spark plug
fouling. The main advantage of this system is simplicity and low cost because no oil pump, filter,
etc., are required. However, this simplicity is at the cost of many troubles some of which are
enumerated below:

1. Some of the lubricating oil invariably burns in combustion chamber. This heavy oil when
burned, and still worse, when partially burned in combustion chamber leads to heavy exhaust
emissions and formation of heavy deposits on piston crown, ring grooves and exhaust port which
interferes with the efficient engine operation.

2. One of the main functions of the lubricating oil is protection of anti-friction bearings, etc.,
against corrosion. Since the oil comes in close contact with acidic vapours produced during the
combustion process, it rapidly loses its anti-corrosion properties resulting in corrosion damage of
bearings.

3. For effective lubrication, oil and the fuel must be thoroughly mixed. This requires either separate
mixing prior to use or use of some additive to give the oil good mixing characteristics.

4. One important limitation of this system is oil starvation of the working parts especially when the
throttle is closed on a descent on a long hill. A dosed throttle means no fuel, and, hence, no oil. The
prolonged absence of oil so produced may result in overheating and piston seizure. This oil
starvation can be controlled if the driver while descending on a hill periodically releases the throttle
to replenish for the complete absence of oil.

5. Due to high exhaust temperature and less efficient scavenging the crankcase oil is diluted. In
addition, some lubricating oil also burns in combustion chamber. This results in about 5 to 15 per
cent high lubrication consumption for two-stroke engines as compared to four-stroke engines of
similar size.

6. Since there is no control over the lubricating oil, once introduced with fuel; most of the two-
stroke engines are over-oiled most of the time.

Some manufacturers use a separate oil injection pump to inject the oil directly into the carburettor
and the amount of the oil is regulated according to engine load and speed. This system completely
avoids the oils starvation problem discussed above. The oil consumption is also reduced. This
results in lesser deposits and less spark plug fouling problems. Since in this system the main
bearings are excluded from the crankcase and instead receive lubricating oil from a separate pump,
the corrosion damage of bearings is also eliminated.
                                                                                              126

Wet Sump Lubrication System: In wet sump lubrication system the bottom part of the crankcase,
called sump, contains the lubricating oil from which the oil is supplied to various parts. Figure
shows three versions of wet sump lubrication, system. These are

(i) Splash system

(ii) Modified splash system

(iii) Full pressure system

Figure shows the splash
system. It is used for small
engines. In this system the oil
level in the sump is so
maintained that when the
connecting rod big end is at its
lowest position the drippers on
the connecting rod end strike
the oil in the troughs which are
supplied with oil from the
sump by an oil pump. Due to
this striking of the drippers, oil
splashes over various engine
parts like crankpin bearings,
piston skirt and rings, piston
pins, etc. Excess oil supplied
drips back to the sump.

The splash system is not
sufficient if the bearing loads
are high. For such cases, the
modified splash system is used.
This is shown in Fig.(c). The
main and camshaft bearings are
lubricated by oil under pressure
pumped by an oil pump. The
other     engine    parts    are
lubricated by splash as shown                                                                   in
Fig.(c).

In the full pressure system,
shown in Fig(a), an oil pump is
used to lubricate all parts of the engine. Drilled passages are used to lubricate connecting rod
bearings. The cylinder walls, piston and piston rings are lubricated by the sprays thrown from the
crankshaft and connecting rod. Full pressure system is used for engines which are exposed in high
engine loads.
                                                                                                     127
Since the bearings are machined to a very close tolerance and are likely to be damaged if any
foreign materials are allowed to enter the lubrication line, a strainer is always used in oil circuit. A
gear type pump or rotor type pump submerged in the oil and driven by the camshaft draws oil from
the sump through a strainer to prevent foreign material from entering the system. A pressure relief
valve is also used to avoid very high pressure built tip in case of filter clogging or if the oil is very
cold or sluggish

Dry-sump system

This system is employed in some racing
car engines for situations where the
vehicle has to be operated at very steep
angles, for example, sports cars, jeeps
etc. If ordinary pressure system of
lubrication is used in such cases, the
situations may arise when there is no oil
at the place where oil pump is installed.
To avoid such instances, dry sump
system is used (Fig. 6.9), wherein two
pumps, instead of one, are used. The
scavenge pump A is installed in the crankcase portion which is the lowest. It pumps oil to a
separate reservoir B, from where the pressure pump C pumps the oil through filter D, to the,
cylinder bearings; a full pressure system of lubrication is employed. The oil pressure is maintained
at 400-500 kPa for the main and big end bearings while about 50-100 kPa pressure is used for
timing gears and camshaft bearings etc.

Pre-Lubrication System


Daimler-Benz of Germany
have devised a system by
means of which lubrication
of the engine is ensured
before it actually starts. The
arrangement is shown in Fig.
A is the main oil pump which
is operated by the engine and
it delivers oil to the oil
gallery through filter B. A
secondary lubricating pump
H is provided which is
operated by an electric
motor. The oil from H is
divided into two branches.
One branch goes through relief valve and filter B to the oil gallery and the other leads the oil
through filter C and non return valve D to the fuel control device G. The main oil gallery is further
connected through pressure switch F and valve E to the fuel control device G.
                                                                                                   128
When the pump H is started, in the pre-lubrication stage the oil flows through filter B to oil gallery.
The switch F is opened at 0.5 atmosphere and the oil goes through E to device G. Simultaneously
oil from pump H is also flowing through filter C and valve D to device G. When the pressure in the
circuit reaches 1.5 atmosphere, the fuel control device is operated and the fuel is delivered to the
engine causing it to start.

In this system double safety is provided. If filter C is blocked the device G can still be operated by
direct circuit through pressure switch F and valve E.

LUBRICATION SYSTEM COMPONENTS

OIL STRAINERS
Oil strainer is attached at the inlet of the oil
pump to guard it against the entry of foreign
particles or dirt, grit etc. The strainer is
made of ordinary wire mesh screen. A good
practice is to install a floating strainer,
which is hinged to the oil pump inlet. The
floating strainer remains at the surface of
oil, whereas the girt, dust etc. remain at the bottom of the crankcase. The result is that very small
amount of impurities goes to the strainer screen and hence the chances of it being blocked up are
minimized. Even then to account for such eventuality, in which whole of the strainer screen is
blocked up, the provision of a by-pass is kept as shown in figure.

OIL PUMPS

Next to oil strainer in the lubrication system sequence, comes the pump. Its function is to supply oil
under pressure to the various engine parts. The oil pump is generally located inside the crank case
below the oil level. However, instances exist where it has been mounted outside the crank case
above the oil level. The pump is usually driven from the end of the distributor shaft, which gets its
drive from the crankshaft through a skew gear

The oil pressure in the engine increases with the increase                                      in
engine speed which would increase the pump speed. The
maximum pressure is limited by means of a pressure
relief valve and this valve is so selected that the pump                                      will
deliver sufficient lubricating oil to all the engine parts.
Minimum oil pressure required is almost 100 kPa.
Usually 15 to 30 liters of oil per minute circulation is
enough for engine lubrication. The size of the pump is so
selected that necessary volume of oil at the desired
pressure is supplied by it at moderate engine speeds
without the occurrence of cavitation.
Further, with wear of bearings and other engine parts,
more oil will start leaking which may result in loss of
engine pressure. The size of the pump should be
sufficient to maintain the desired pressure with reasonable amount of wear. The different types of
pumps used for engine lubrication are
                                                                                                    129
1. Gear pump
2. Crescent type gear pump.
3. Rotor pump
4. Plunger pump
5. Vane pump.

Gear Pump
This is the type once almost universally used in the automotive engines. Its construction is very
simple in that it consists of two spur, or for quieter running, helical gears only which are in mesh
with each other. One gear is mounted on a stub shaft and is driven only whereas the other gear is
the driving gear, itself being driven directly by the cam shaft through the same gear which drives
the distributor shaft. The oil is transported from the inlet to the outlet side in gear spaces between
the gear teeth as shown in the figure and is discharged through the outlet port. Since the teeth start
to mesh at the outlet port, the oil is driven out under pressure




       fig. Gear pump.                                 fig. Gear pump with pressure relief valve

The pump is always submerged in oil in the crankcase and, therefore no priming is necessary. The
pump delivers pressurized oil at about 300 to 400 kPa. A pressure relief valve is also provided in
many oil pumps to relieve the pressure when it becomes excessive due to high engine speeds or the
clogged oil lines. When the pressure exceeds the prescribed value, the spring is compressed and the
ball is lifted off from its seat and the oil passes back to the inlet side, through the bypass provided.
The pressure at which valve opens may be adjusted
by means of the screw provided.

Crescent type Gear Pump

In this an internal ring gear is in mesh with a driving
external gear which is mounted eccentrically with
respect to the ring gear. Due to this eccentricity, there
is a space in between the two gears where a crescent
shaped spacer is placed as shown in Fig. Due to
rotation of the gears, a suction is produced at the inlet
end of the crescent, which draws in oil. This oil is
then trapped in tooth spaces between the crescent and
the driving gear and the crescent and the tooth spaces
                                                                                                    130
of the internal driven gear and is thus carried towards the outlet side of the crescent where it is
discharged into the outlet port. With continuous running the spaces between the gear teeth will be
filled with oil after which the extra oil will cause the oil pressure to increase. The oil pressure will
subsequently depend upon the oil supplied by the pump and the oil escaping from the engine
bearings.

Rotor Pump
This type of oil pump is now widely used in
automobiles. It is similar to the gear pump except that
in this two gears mesh internally. A is the external rotor
having the number of lobes one more than on the
internal rotor B. The axes of rotation of the two rotors
are different which causes the size of the spaces
between them to vary. Rotor B gets its drive from the
engine and causes rotor A to rotate along with it. The
oil enters the pump through inlet port, as the rotor lobes
are moving out of mesh. The oil is then transported
from inlet to the outlet of the pump in the spaces
between rotor lobes. The rotation reduces the clearance
between the lobes and the oil is discharged under
pressure as the lobes of the rotors move into mesh at
the outlet port. This type of pump is about 25% more efficient and compact than the gear type
pump. This is also quieter running since there are comparatively lesser teeth in mesh for each
revolution. Because of these advantages, its use is
on increase in the automobile engines.

Plunger Pump
This is reciprocating type of pump in which the
oil gets communicated from the inlet to the outlet
by alternate suction and pressure created by a
reciprocating plunger. This was once used in
engines using splash lubrication for filling the oil
troughs, but is obsolete now.

Vane pump.

This type consists of a cylindrical housing, within
which is eccentrically mounted a driven rotor. The
rotor contains a number of vanes in the rotor slots,
which are equally spaced around its periphery. These
vanes can slide back and forth into these slots. The
movement of sliding vanes is guided by means of a
control ring as shown.
When the pump is operating, the vanes are pressed
outwards against the housing by their centrifugal force.
The oil enters the inlet port and is swept by the vanes to
the outlet port. This type of pump has an advantage of a
continuous oil flow compared to the pulsating oil flow
in case of gear pump.
                                                                                                    131

OIL FILTERS

From the pump all the oil used for lubrication usually passes through an oil filter before it reaches
the engine bearings. The bearings are machined to a very close tolerance and are likely to be
damaged if any foreign materials are allowed to enter the lubrication line. The filter does not keep
the engine clean. This function is performed by the lubricating oil. The extremely small particles
from cleaning of carbon and gum remain suspended in the oil and are able to pass freely through
the minimum oil film thickness of about 6-7 micron at the bearings and are removed from the
engine only when the oil is drained. The job of the filter is to remove from oil the abrasive particles
that cause wear of the working surfaces. The size of abrasive particles to be removed is more than
about 10 to 15 microns. Filters also prevent sludge deposit to pass to the bearings.
The lubricating oil in use is deteriorated resulting in the formation of sludge, lacquer and carbon.
Further, it is contaminated by various by-products of combustion of fuel, viz, water, acids and un-
burnt fuel. In addition to these the fine particles of metal due to wear, especially during the running
in period and particles of rust formed in the engine are the other impurities presents in the oil.
It has been found from experience that impurity particles of 7 to 15 microns diameter are most
damaging, because the oil film size is bigger than this due to which smaller particles never come
into contact with the mating surfaces separated by the oil. The particles bigger than 15 microns
diameter sizes would be filtered out and will not be able to cause any damages to the engine
components.
Commonly used materials for filtering are wire gauge, felt, paper, plastic impregnated paper etc.
The filtering element must allow the oil to pass through with out much resistance, but prevent the
undesirable particles from entering the oil galleries. The resistance to flow offered by a new filter
element would be about 0.3 to 0.5 bar. However, with use the resistance increases due to clogging
of the filter element. The maximum permissible resistance is about 2 bar.

The filter arrangement may be of

   (i)     By-pass type, or




   (ii)    Full flow type.


In the by-pass type filter arrangement see Fig (b) only a small part of the oil is passed through the
filter and rest of the oil is directly supplied to the bearings. Thus, a part of the oil is continuously
                                                                                                   132
filtered. Since this type of filter does not obstruct oil flow
to the bearings a very fine filtering medium, usually a
special filter impregnated with resin to keep it from
disintegrating in the presence of moisture, can be used.
Such a fine paper will remove all harmful contaminants.
The size of filter used is also smaller.

In the full-flow type filter arrangement see Fig. (a), whole
of the oil is filtered before it is fed to various bearings. The
filter size, therefore, should be large and it is not practical
to remove very small particles from the oil because of very
high pressure needed to pump oil through such a filter. The
filter area ranges from 38 to 57 m2 (600 to 900 sq.in) with
flow rates of up to 14 litres per minute. In normal operation
all the engine oil is filtered about every 30 seconds. This
necessitates the use of a pressure relief valve to prevent
excessive pressure build up after a cold start.
The filters used in automobiles engines are of various kinds. The important one out of them are,
     1. Cartridge type
     2. Edge type
     3. Centrifugal type.
Cartridge type oil filter:
This type is used on most automobile engines and it consists of a filtering element placed in the
metallic casing. The contaminated lubricating oil is made to pass through the filtering element,
which takes up all the impurities. The element is given a pleated form to maximize the surface area
of the filter for a given size of element. Currently filter elements with fine pores have been
employed which has made it practicable to arrest particles of size down to within the region of 5
microns in the filter shown in Fig.

The oil enters the filter at the top and passes through the filter elements as shown by arrows. The
pure oil then goes to the porous metallic tube from where it goes to the outlet for circulation. A
drain plug is also provided as shown. The filtering elements of two types are available, i.e., the ones
which can be cleaned and those which have to                                                         be
placed after certain intervals, say, 10000 kin.

Edge type oil filter

It is also called the stack type. In this the oil is
made to pass through a number of closely
spaced discs. The alternate discs are mounted
over a central spindle, while the discs in
between these are attached to a separate fixed
spindle as shown. The clearance between two
successive discs is a few microns. The oil is
made to flow through the spaces between these
discs and because of the very small spaces
involved the impurities are left on the disc
                                                                                                 133
peripheries from where these are periodically removed by simply operating the central knob. This
may be done either manually or the knob may be connected to the clutch system and operated
periodically by means of clutch action.

Centrifugal type oil filter

In this the impure or dirty oil from the engine enters the
hollow central spindle having holes around its periphery
as shown in Fig. The dirty oil comes out of these holes
and fills the rotor casing after which it passes down the
tubes A at the ends of which jets is attached. The oil
under pressure passes through these jets, the reaction of
which gives the motion to the rotor casing in the
opposite direction so that it starts rotating. The oil
impinges on the outer stationary casing under heavy
pressures, where the impurities are retained and clean oil
falls below from where it is taken out. The filter walls
have to be cleaned at intervals of about 70,000 km.


OIL COOLERS

In all heavy duty engines the engine temperature
and hence the temperature of oil becomes quite
high. As the viscosity of lubricating oil decreases
with the rise in temperature, at higher
temperatures the oil film in the bearings might
break and the conditions of boundary lubrication
may be created instead of fluid lubrication which
is desired. To avoid such thing to happen, the
heavy duty engines are provided with oil coolers.

Oil coolers are nothing but simple heat
exchangers. Either the cold water from the radiator or the air stream is used to cool the oil. The
water type coolers are, however, more common because they can be used as reversible coolers, i.e.
at the start when it is desirable that the oil should not be cooled, rather it should be heated
somewhat to provide complete circulation in the lubrication system as fast as possible. The water
being hotter than the oil, will initially heat the oil and when higher temperatures are reached, the
reverse will happen, i.e. the water will cool the oil. One such oil cooler is shown in Fig. The water
enters the cooler as shown by means of arrows and then cools the oil tubes which are placed in a
direction perpendicular to the flow of water, after which it comes out. The oil is circulated in the
directions shown in the figure.

CRANKCASE VENTILATION

. It is quite possible that products of combustion from the combustion chamber may leak into the
crankcase. These contain mainly nitrogen, water and carbon dioxide, but may consist of traces of
acid due to sulphur present in the fuel which may cause corrosion of engine parts, e.g. piston pins,
cylinder walls, timing gears etc.
                                                                                                  134

Crankcase ventilation was developed as a remedial measure against this. A steady flow of air is
carried through the engine crankcase which carries away the products of combustion along with it
in the form of vapour or mist. The methods of crankcase ventilation that have been developed so far
are
1. Road draft system. 2. Manifold suction system.

Road draft system:




Fig shows a road draft system in which use is made of the air steam due to motion of the vehicle,
and supplemented by the cooling fan. The air enters the engine at the top and is then circulated in
the crankcase where it carries the mist, vapours, etc. after which it comes out through the outlet and
goes into the atmosphere.

Manifold suction system:
In the manifold suction system, which is also
called positive crankcase ventilation system,
filtered air from the engine air intake is drawn
through the crankcase where it gathers water
vapors, un-burnt fuel vapors and exhausts gases
after which it is directed to the inlet manifold and
from there to the engine where any un-burnt fuel
left is burnt. To avoid large air flows to the
manifold during idling, an engine-suction-
operated valve placed in between the crankcase
and the inlet manifold regulates the air flow
depending upon engine speed.
In the Maruti 800 engine blow by gases from the
crankcase are taken to the air cleaner through an
oil separator which removes the oil particles
accompanying the gases. In the air cleaner the
blow-by gases mix with the fresh air intake and
the combined air is passed on to the carburetor.
The main advantage of the manifold suction
system is that being a closed type of system this
does not cause pollution of atmosphere, which
occurs in the road draft system.
                                                                                                   135
Positive Crank Case Ventilation (PCV) Systems

This method of crank case ventilation to remove the piston blow by gases and the fumes uses the
manifold vacuum to establish the air circulation and, with it, the removal of the unwanted vapour
mixtures and burnt and partially burnt products of combustion.

In-Line engine PCV systems




For the in-line engines there are, broadly, two Circulation layouts. Firstly, there is the crankcase to
induction manifold system where air is drawn into the rocker cover via a rubber pipe connected to
the air filter. The air then flows from the rocker cover to the crankcase by way of the push-rod and
camshaft passage ways, it is then drawn from the side of the crankcase to a central point on the
induction manifold via the oil and mist separator. The gas and vapour fumes are then drawn into the
cylinders; on each of their induction strokes so that, in effect, the engine is consuming its own
blow-by gases and fumes. Some of these fumes will contribute to the combustion process and will
then pass through the normal exit of the exhaust port to the exhaust system, where they may be
given further after-burning treatment before being ejected into the atmosphere.

At very high engine speeds, or slightly lower speeds if the engine is worn, the crankcase to
induction-manifold flow path is insufficient to accommodate the amount of blow-by. Hence, the
direction of flow in the rocker cover switches from rocker-cover to crankcase, rocker-cover to air
filter, where it is again drawn (via the induction venturi) into the induction manifold. Thus, the
blow-by gases are now drawn into the combustion chamber by way of the crankcase and rocker
cover.

Secondly, there is the camshaft-cover to induction-manifold circulation system where air is drawn
from the air filter directly to the crankcase via a rubber pipe, the air then circulates around the
crankcase, picks up the blow-by gases and then passes to the camshaft cover space. The air, gases
                                                                                                   136
and fumes then move from the camshaft cover to a central point on the induction manifold, due to
the existing vacuum in the manifold, where they are immediately drawn into the cylinders as part of
the normal induction charge.

Again, at very high engine speeds or, if the engine is worn, at much lower speeds, the cam shaft-
cover to induction-manifold passageway is inadequate to circulate all the air and blow-by products.
Thus, a part of the air and fumes in the crankcase will be forced up the crankcase to the air filter
pipe, it is then drawn into the induction manifold via the carburetor or petrol injection venturi. Note
that at very high engine speeds there will only be a small to moderate amount of vacuum created in
the intake of the venturi.

With the camshaft cover to induction manifold circulation system, oil and mist separation is not
required, as would be the case with the crankcase exit system, where a considerable amount splash
and mist blow-by accumulates.

Open and closed positive crankcase ventilation systems:

Earlier positive crankcase ventilation used the open method of circulating air and did not have a
pipe between the air filter and cover. However, when the engine becomes A piston blow-by pushed
the gas and vapour out from the rocker filler cap directly into the atmosphere. The present positive
crankcase ventilation (PCV) systems have adopted the closed where excess blow-by gas in the
upper range is drawn back into the cylinder by the air filter and venturi route.


OIL LEVEL INDICATORS

To make sure that sufficient oil is there in the engine crankcase some means must be provided for
its measurement. The simplest method most commonly used is the 'dip stick'. It is a long stick with
a handle at one end for holding. The marks are provided on the dip stick indicating full, half full or
quarter full crankcase. The critical mark is also provided below which the level should not fall. It is
the duty of the driver to check the oil level at regular intervals, particularly before undertaking any
long journey. Another type of oil level indicator is based upon the float system. To the float inside
the crankcase, is attached suitable indicator to indicate the oil level outside the crankcase.

OIL PRESSURE GAUGES

If any leakage occurs at any part of the lubrication
system, the pressure in the entire system would fall,
reducing consequently the oil supply to various
bearings as a result of which they are bound to starve
and be damaged. Thus it is very important for the
driver to keep a watch on the oil pressure and any
reduction in the system pressure must come into his
notice.
The gauges in use are of two types, the Bourdon type
and the electrically operated type. The working
principle of these gauges is the same as the
temperature gauges in the cooling system. Only
instead of thermal element in case of electrically operated type, here we have a linear resistance,
                                                                                                   137
which is controlled by the sliding contact attached to a diaphragm, whose deflection is proportional
to the oil pressure.

OIL PRESSURE WARNING LIGHT

A warning light is provided on
the dashboard which lights up
when the oil pressure is lower
than the specified value. This
light is controlled by an oil
pressure switch mounted in the
main oil gallery
It consists of a diaphragm with a
projection at the centre. When
the oil pressure is above the prescribed value, the projection keeps the contact bar pushed up,
thereby separating the contacts so that the warning light is off. However, when the oil pressure
falls, the diaphragm also moves down thereby making the contacts, so that the warning light comes
on.
Oil pressure warning system

Ordinary warning lamps are
sensitive to only one oil
pressure irrespective of engine
speed, though the minimum oil
pressure required depends
upon the engine speed. The
remedy has been provided by
M/s Daimler Benz AG
(Germany) in the from of a 4
stage       diaphragm-operated
switch.
When the oil exerts pressure on the diaphragm, the Projection attached to it presses the uppermost
contact bar, breaking the contacts between the two projections. Further movement causes a
projection from the centre of the first contact bar to deflect a second bar and so on, up to the fourth
stage. Each of these four stages is brought singly into circuit with the warning lamp by a selector
switch operated in conjunction with the speedometer. The warning light in this way lights up only
when the oil pressure falls below the value corresponding to the minimum engine speed.

CHASSIS LUBRICATION

Different lubricants and methods of lubrication are employed for chassis lubrication. The
manufacturer supplies complete information about the location of lubrication points, manner of
lubrication, type of lubricant and the period after which the lubrication must be done, etc. in the
form of lubrication charts. A brief general guideline, however, about the lubrication of only a few
main chassis components is given here.

1. Gear Box Lubrication
                                                                                                   138
Gear box is filled with lubricating oil up to the specified level, which may be checked with a
dipstick. The oil level should be checked after every 1500 km and replenished, if necessary. After
first 800 km and subsequently every 10,000 kin, the gear box should be drained, flushed and
refilled with the fresh lubricant. For moderate temperatures (above 10 degree C), generally S.A.E.
90 oil or hypoid 90 oil is used. For heavy duty applications, S.A.E. 140 oil is used. The exact
specification for a particular vehicle is given by the manufacturer.

2. Steering Box and Rear Axle Lubrication

In the steering box and the rear axle, gears are involved. Therefore, the same oil is generally used as
for gear box lubrication. The lubrication should be done through greasing nipple after every 20,000
kin.

3. Lubrication of Wheel Hubs

Special grease is used for wheel hubs. For example, Mobihub grease is specified in case of
Hindustan Ambassador car and Leyland Specification G grease for Ashok Leyland vehicles.

4. General Chassis Lubrication

S.A.E. 140 oil or L.M. Specification G grease is specified for Ashok Leyland vehicles and
Mobilgrease No. 4 is prescribed for Ambassador Cars. For frequency of lubrication, the manual
should be consulted.

								
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