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. 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.
1 AUTO POWER PLANT AUTO POWER PLANT (U6) 98.604. 2 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 3 (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. 4 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. 5 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 6 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). 7 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 8 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 9 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. 10 (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. 11 (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, 12 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. 13 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 14 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. 15 (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. 16 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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