Automotive Engines basic

7/27/2019 Automotive Engines basic

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RAJALAKSHMI ENGINEERING

COLLEGE

DEPARTMENT OF AUTOMOBILEENGINEERING

NOTES OF LESSON

FOR

AUTOMOTIVE ENGINES


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UNIT-I

BASIC THEORY

INTRODUCTION

Definition of Engine

An engine is a device, which transforms one form of energy into another form. Normally,most of the engines convert thermal energy into mechanical work and therefore they are

called heat engines.

Heat engines can be broadly classified into two categor ies :i. Internal Combustion Engines (IC Engines)

ii. External Combustion Engines (EC Engines)

External Combustion and Internal Combustion Engines

External combustion engines are those in which combustion takes place outside theengine 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 thecombustion of fuel is employed to generate high pressure steam, which is used as the

working fluid in a reciprocating engine or a turbine. In case of gasoline or diesel engines,the products of combustion generated by the combustion of fuel and air within thecylinder form the working fluid.

Application of IC and EC Engines

IC Engines

1. GASOLINE ENGINEAUTOMOTIVE, MARINE AIRCRAFT2. DIESEL ENGINE - AUTOMOTIVE, MARINE,POWER,LOCOMATIVE3. GAS ENGINESINDUSTRIAL POWER

EC Engines

1. STEAM ENGINESLOCOMOTIVES,MARINE2. STEAM TURBINEPOWER,LARGE MARINE


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Cylinder Block:

The cylinder block is the main supporting structure for the various components. Thecylinder of a multicylinder engine is cast as a single unit, called cylinder block. The

cylinder head is mounted on the cylinder block. The cylinder head and cylinder block are

provided with water jackets in the case of water-cooling with cooling fins in the case ofair-cooling. Cylinder head gasket is incorporated between the cylinder block and cylinder

head. The cylinder head is held tight to the cylinder block by number of bolts or studs.The bottom portion of the cylinder block is called crankcase. A cover called crankcase,

which becomes a sump for lubricating oil is fastened to the bottom of the crankcase. Theinner surface of the cylinder block, which is machined and finished accurately tocylindrical shape, is called bore or face.

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 ofthe engine is filled with the working fluid and subjected to different thermodynamicprocesses. The cylinder is supported in the cylinder block.

Piston

It is a cylindrical component fitted into the cylinder forming the moving boundary of thecombustion 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 forcesto the output shaft.

Combustion Chamber

The space enclosed in the upper part of the cylinder, by the cylinder head and the pistontop during the combustion process, is called the combustion chamber. The combustion offuel and the consequent release of thermal energy results in the building up of pressure in

this part of the cylinder.

Inlet Manifold

The pipe which connects the intake system to the inlet valve of the engine and through

which air or air-fuel mixture is drawn into the cylinder is called the inlet manifold.

Exhaust Manifold

The pipe that connects the exhaust system to the exhaust valve of the engine and through

which the products of combustion escape into the atmosphere is called the exhaustmanifold.


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Inlet and Exhaust Valves

Valves are commonly mushroom shaped poppet type. They are provided either on thecylinder head or on the side of the cylinder for regulating the charge coming into the

cylinder (inlet valve) and for discharging the products of combustion (exhaust valve)

from the cylinder.

Connecting Rod

It interconnects the piston and the crankshaft and transmits the gas forces from the pistonto the crankshaft. The two ends of the connecting rod are called as small end and the bigend. Small end is connected to the piston by gudgeon pin and the big end is connected to

the crankshaft by crankpin.

Crankshaft

It converts the reciprocating motion of the piston into useful rotary motion of the outputshaft. In the crankshaft of a s ingle cylinder engine there is pair of crank arms and balanceweights. The balance weights are provided for static and dynamic balancing of the

rotating system. The crankshaft is enclosed in a crankcase.

Piston Rings

Piston r ings, fitted into the slots around the piston, provide a tight seal between the piston

and the cylinder wall thus preventing leakage of combustion gases

Gudgeon Pin

It forms the link between the small end of the connecting rod and the piston.

Camshaft

The camshaft and its associated parts control the opening and closing of the two valves.The associated parts are push rods, rocker arms, valve springs and tappets. This shaft also

provides the drive to the ignition system. The camshaft is driven by the crankshaftthrough timing gears.

Cams

These are made as integral parts of the camshaft and are designed in such a way to openthe valves at the correct timing and to keep them open for the necessary duration.

Fly Wheel

The net torque imparted to the crankshaft during one complete cycle of operation of theengine fluctuates causing a change in the angular velocity of the shaft. In order to achieve


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a uniform torque an inertia mass in the form of a wheel is attached to the output shaft andthis wheel is called the flywheel.

Four-stroke Spark-ignition Engine

In a four-stroke engine, the cycle of operations is completed in four strokes of the pistonor two revolutions of the crankshaft. During the four strokes, there are five events to be

completed, viz, suction, compression, combustion, expansion and exhaust. Each strokeconsists of 180 of crankshaft rotation and hence a four-stroke cycle is completed through

720 of crank rotation. The cycle of operation for an ideal four-stroke SI engine consistsof the following four strokes:

i. Suction or intake stroke;

ii. Compression stroke;

ii. Expansion or power stroke and

iv. Exhaust stroke.

Working principle of a Four Stroke SI Engine

i. Suction or Intake Stroke: Suction stroke starts when the piston is at the top dead centreand about to move downwards. The inlet valve is open at this time and the exhaust valve

is closed. Due to the suction created by the motion of the piston towards the bottom deadcentre, the charge consisting of fuel-air mixture is drawn into the cylinder. When thepiston 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. During this stroke both inlet and exhaustvalves are in closed position. The mixture that fills the entire cylinder volume is nowcompressed into the clearance volume. At the end of the compression stroke the mixture

is ignited with the help of a spark plug located on the cylinder head. In ideal engines it isassumed that burning takes place 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 energyproducing a temperature rise of about 2000 C. The pressure at the end of the combustion

process is considerably increased due to the heat release from the fuel.

iii. Exhaust Stroke: At the end of the expansion stroke the exhaust valve opens and theinlet valve remains closed. The pressure falls to atmospheric level a part of the burntgases escape. The piston starts moving from the bottom dead centre to top dead centre

and sweeps the burnt gases out from the cylinder almost at atmospheric pressure.

The exhaust valve closes when the piston reaches T.D.C. at the end of the exhauststroke and some residual gases trapped in the clearance volume remain in the cylinder.


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Residual gases mix with the fresh charge coming in during the following cycle, formingits working fluid. Each cylinder of a four stroke engine completes the above four

operations in two engine revolutions, one revolution of the crankshaft occurs during thesuction and compression strokes and the second revolution during the power and exhaust

strokes. Thus for one complete cycle theres only one power stroke while the crankshaft

turns by two revolutions.

The Constant Volume or Otto Cycle

The three cycles of great practical importance in the analysis of piston engineperformance are:

i. The Constant Volume or Otto cycleii. The Diesel cycle

iii. The dual combustion or limited pressure cycle.

In this article we will describe the constant volume or Otto cycle illustrated in Fig. 2. TheOtto cycle is the theoretical cycle for the spark ignition engine.

Figure: 2. The constant volume or Otto cycle

In the air cycle analysis the induction and exhaust processes, represented by line0-1 and 1-0 respectively, are neglected. The work done during both the processes is equal

and opposite and hence cancels each other. After suction stroke 0-1, the piston is atbottom of outer dead center. This is represented by point 1 onp-Vand T-s diagrams. The

air is now compressed by reversible adiabatic process during the inward motion of thepiston until the piston reaches top or inner dead center position (process 1-2).In thisprocess entropy remains constant. At this time heat is added at constant volume so that

the state of air changes from point 2 to 3. In an actual engine it is equivalent to burning of


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fuel instantaneously (by an electric spark) so that heat liberation is at constant volume.The next process is reversible adiabatic expansion3-4, piston moving outwards from top

dead center to bottom dead center. At the end of this expansion process the heat isrejected by gases at constant volume, process 4-1, and the cycle is completed. In actual

engine it corresponds to instantaneous opening of exhaust valve at 4 bringing down the

pressure to atmospheric pressure 1, followed by exhaust stroke 1-0.

Since the processes 1-2 and 3-4 are adiabatic, no heat transfer takes place duringthese processes. The heat transfers are limited to addition of heat during the constant

volume process 2-3 and rejection of heat during constant volume process 4-1.

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 thepiston 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 samespeed 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. Themain 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 enginethese 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 thecharge compressed in crankcase or by a blower. The induction of the compressed chargemoves out the product of combustion through exhaust 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.

Figure 1 shows one of the simplest two-stroke engines, viz., the crankcase scavengedengine. The air or charge is inducted into the crankcase through the spring loaded inlet

valve when the pressure in the crankcase is reduced due to upward motion of the pistonduring compression stroke. After the compression and ignition, expansion takes place in

the usual way.

Figure: 1 Crankcase Scavenged Two Stroke Engine


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stroke diesel engine is used quite widely. Many of the high output diesel engines work onthis cycle. A disadvantage common to all two-stroke engines, gasoline as well as diesel,

is the greater cooling and lubricating oil requirements due to one power stroke in eachrevolution of the crankshaft. Consumption of lubricating oil is high in two-stroke engines

due to higher temperature. A detailed comparison of two-stroke and four-stroke engines

is given in the Table below


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UNIT-II

SI ENGINE FUEL SYSTEM

Carburetion

Introduction

Spark-ignition engines normally use volatile liquid fuels. Preparation of fuel-air mixtureis done outside the engine cylinder and formation of a homogeneous mixture is normally

not completed in the inlet manifold. Fuel droplets, which remain in suspension, continueto evaporate and mix with air even during suction and compression processes. Theprocess of mixture preparation is extremely important for spark-ignition engines. The

purpose of carburetion is to provide a combustible mixture of fuel and a ir in the requiredquantity and quality for efficient operation of the engine under all conditions.

Definition of Carburetion

The process of formation of a combustible fuel-air mixture by mixing the proper amountof fuel with air before admission to engine cylinder is called carburetion and the device

which does this job is called a carburetor.

Factors Affecting Carburetion

Of the various factors, the process of carburetion is influenced byi. The engine speed

ii. The vaporization characterist ics of the fueliii. The temperature of the incoming air and

iv. The design of the carburetor

Principle of Carburetion

Both air and gasoline are drawn through the carburetor and into the engine cylinders by

the suction created by the downward movement of the piston. This suction is due to anincrease in the volume of the cylinder and a consequent decrease in the gas pressure inthis chamber. It is the difference in pressure between the atmosphere and cylinder that

causes the air to flow into the chamber. In the carburetor, air passing into the combustionchamber picks up discharged from a tube. This tube has a fine orifice called carburetor jet

that is exposed to the air path. The rate at which fuel is discharged into the air depends onthe pressure difference or pressure head between the float chamber and the throat of theventuri and on the area of the outlet of the tube. In order that the fuel drawn from the

nozzle may be thoroughly atomized, the suction effect must be strong and the nozzleoutlet comparatively small. In order to produce a strong suction, the pipe in the carburetor

carrying air to the engine is made to have a restriction. At this restriction called throat dueto increase in velocity of flow, a suction effect is created. The restriction is made in the


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form of a venturi to minimize throttling losses. The end of the fuel jet is located at theventuri or throat of the carburetor. The geometry of venturi tube is as shown in Fig.16.6.

It has a narrower path at the center so that the flow area through which the air must passis considerably reduced. As the same amount of air must pass through every point in the

tube, its velocity will be greatest at the narrowest point. The smaller the area, the greater

will be the velocity of the air, and thereby the suction is proportionately increased

As mentioned earlier, the opening of the fuel discharge jet is usually loped where thesuction is maximum. Normally, this is just below the narrowest section of the venturi

tube. The spray of gasoline from the nozzle and the air entering through the venturi tubeare mixed together in this region and a combustible mixture is formed which passesthrough the intake manifold into the cylinders. Most of the fuel gets atomized and

simultaneously a small part will be vaporized. Increased air velocity at the throat of theventuri helps he rate of evaporation of fuel. The difficulty of obtaining a mixture of

sufficiently high fuel vapour-air ratio for efficient starting of the engine and for uniformfuel-air ratio indifferent cylinders (in case of multi cylinder engine) cannot be fully met

by the increased air velocity alone at the venturi throat.

The Simple Carburetor

Carburetors are highly complex. Let us first understand the working principle bf a simpleor elementary carburetor that provides an air fuel mixture for cruising or normal range at

a single speed. Later, other mechanisms to provide for the various special requirementslike starting, idling, variable load and speed operation and acceleration will be included.

Figure 3. shows the details of a simple carburetor.

Figure: 3 The Simple Carburetor


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The simple carburetor mainly consists of a float chamber, fuel discharge nozzle and a

metering orifice, a venturi, a throttle valve and a choke. The float and a needle valvesystem maintain a constant level of gasoline in the float chamber. If the amount of fuel in

the float chamber falls below the designed level, the float goes down, thereby opening the

fuel supply valve and admitting fuel. When the designed level has been reached, the floatcloses the fuel supply valve thus stopping additional fuel flow from the supply system.

Float chamber is vented either to the atmosphere or to the upstream side of theventuri.During suction stroke air is drawn through the venturi.

As already described, venturi is a tube of decreasing cross-section with aminimum area at the throat, Venturi tube is also known as the choke tube and is so

shaped that it offers minimum resistance to the air flow. As the air passes through theventuri the velocity increases reaching a maximum at the venturi throat. Correspondingly,

the pressure decreases reaching a minimum. From the float chamber, the fuel is fed to adischarge jet, the tip of which is located in the throat of the venturi. Because of the

differential pressure between the float chamber and the throat of the venturi, known ascarburetor depression, fuel is discharged into the air stream. The fuel discharge isaffected by the size of the discharge jet and it is chosen to give the required air-fuel ratio.

The pressure at the throat at the fully open throttle condition lies between 4 to 5 cm ofHg, below atmospheric and seldom exceeds8 cm Hg below atmospheric. To avoidoverflow of fuel through the jet, the level of the liquid in the float chamber is maintained

at a level slightly below the tip of the discharge jet. This is called the tip of the nozzle.The difference in the height between the top of the nozzle and the float chamber level is

marked h in Fig.3.

The gasoline engine is quantity governed, which means that when power output is

to be varied at a particular speed, the amount of charge delivered to the cylinder is varied.This is achieved by means of a throttle valve usually of the butterfly type that is situated

after the venturi tube. As the throttle is closed less air flows through the venturi tube andless is the quantity of air-fuel mixture delivered to the cylinder and hence power output isreduced. As the throttle is opened, more air flows through the choke tube resulting in

increased quantity of mixture being delivered to the engine. This increases the enginepower output. A simple carburetor of the type described above suffers from a

fundamental drawback in that it provides the required A/F ratio only at one throttleposition. At the other throttle positions the mixture is either leaner or richer depending onwhether the throttle is opened less or more. As the throttle opening is varied, the air flow

varies and creates a certain pressure differential between the float chamber and theventuri throat. The same pressure differential regulates the flow of fuel through the

nozzle. Therefore, the velocity of flow of air II and fuel vary in a similar manner. At thesame time, the density I of air decrease as the pressure at the venturi throat decrease withincreasing air flow whereas that of the fuel remains unchanged. This results in a simple

carburetor producing a progressively rich mixture with increas ing throttle opening.


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The Choke and The Throttle

When the vehicle is kept stationary for a long period during cool winter seasons, may beovernight, starting becomes more difficult. As already explained, at low cranking speeds

and intake temperatures a very rich mixture is required to initiate combustion. Some

times air-fuel ratio as rich as 9:1 is required. The main reason is that very large fraction ofthe fuel may remain as liquid suspended in air even in the cylinder. For initiating

combustion, fuel-vapour and air in the form of mixture at a ratio that can sustaincombustion is required. It may be noted that at very low temperature vapour fraction of

the fuel is also very small and this forms combustible mixture to initiate combustion.Hence, a very rich mixture must be supplied. The most popular method of providing suchmixture is by the use of choke valve. This is simple butterfly valve located between the

entrance to the carburetor and the venturi throat as shown in Fig.3.

When the choke is partly closed, large pressure drop occurs at the venturi throatthat would normally result from the quantity of air passing through the venturi throat. The

very large depression at the throat inducts large amount of fuel from the main nozzle andprovides a very rich mixture so that the ratio of the evaporated fuel to air in the cylinderis within the combustible limits. Sometimes, the choke valves are spring loaded to ensure

that large carburetor depression and excessive choking does not persist after the enginehas started, and reached a desired speed. This choke can be made to operate automaticallyby means of a thermostat so that the choke is closed when engine is cold and goes out of

operation when engine warms up after starting. The speed and the output of an engine iscontrolled by the use of the throttle valve, which is located on the downstream side of the

venturi.

The more the throttle is closed the greater is the obstruction to the flow of the

mixture placed in the passage and the less is the quantity of mixture delivered to .thecylinders. The decreased quantity ofmixture gives a less powerful impulse to the pistons

and the output of the engine is reduced accordingly. As the throttle is opened, the outputof the engine increases. Opening the throttle usually increases the speed of the engine.But this is not always the case as the load on the engine is also a factor. For example,

opening the throttle when the motor vehicle is starting to climb a hill may or may notincrease the vehicle speed, depending upon the steepness of the hill and the extent of

throttle opening. In short, the throttle is simply a means to regulate the output of theengine by varying the quantity ofcharge going into the cylinder.

Compensating Devices

An automobile on road has to run on different loads and speeds. The road conditions playa vital role. Especially on city roads, one may be able to operate the vehicle between 25to 60% of the throttle only. During such conditions the carburetor must be able to supply

nearly constant air-fuel ratio mixture that is economical (16:1).However, the tendency ofa simple carburetor is to progressively richen the mixture as the throttle starts opening.The main metering system alone will not be sufficient to take care of the needs of the

engine. Therefore, certain compensating devices are usually added


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in the carburetor along with the main metering system so as to supply a mixture with therequired air-fuel ratio. A number of compensating devices are in use. The important ones

are

i. Air-bleed jetii. Compensating jetiii. Emulsion tube

iv. Back suction control mechanismv. Auxiliary air valve

vi. Auxiliary air port

As already mentioned, in modern carburetors automatic compensating devices are

provided to maintain the desired mixture proportions at the higher speeds. The type ofcompensation mechanism used determines the metering system of the carburetor. The

principle of operation of various compensating devices are discussed briefly in thefollowing sections.

Air-bleed jet

Figure: 4 Air bleed principle in a typical carburetor

Figure 4. illustrates a principle of an air-bleed system in atypical modern downdraughtcarburetor. As could be seen it contains an air-bleed into the main nozzle. An orificerestricts the flow of air through this bleed and therefore it is called restricted air-bleed jet

that is very popular. When the engine is not operating the main jet and the air bleed jetwill be filled with fuel. When the engine starts, initially the fuel starts coming through the

main as well as the air bleed jet (A). As the engine picks up, only air starts comingthrough the air bleed and mixes with fuel at B making a air fuel emulsion. Thus the fluidstream that has become an emulsion of air and liquid has negligible viscosity and surface


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tension. Thus the flow rate of fuel is augmented and more fuel is sucked at low suctions.By proper design of hole size at B compatible with the entry hole at A, it is possible to

maintain a fairly uniform mixture ratio for the entire power range of the operation of anengine. If the fuel flow nozzle of the air-bleed system is placed in the centre of the

venturi, both the air-bleed nozzle and the venturi are subjected to same engine suction

resulting approximately same fuel-air mixture for the entire power range of operation.

Compensating Jet

Figure: 5 Compensating Jet device

The principle of compensating jet device is to make the mixture leaner as the throttleopens progressively. In this method, as can be seen from Fig.5 in addition to the main jet,a compensating jet is incorporated. The compensating jet is connected to the

compensation well. The compensating well is also vented to atmosphere like the mainfloat chamber. The compensating well is supplied with fuel from the main float chamber

through a restricting orifice. With the increase in airflow rate, there is decrease of fuellevel in the compensating well, with the result that fuel supply through the compensatingjet decreases. The compensating jet thus progressively makes the mixture leaner as the

main jet progressively makes the mixture richer. The main jet curve and thecompensating jet curve are more or less reciprocals of each other.

Emulsion Tube

The mixture correction is attempted by air bleeding in modern carburetor. In one sucharrangement as shown in Fig.6, the main metering jet is kept at a level of about 25 mm

below the fuel level in the float chamber. Therefore, it is also called submerged jet. Thejet is located at the bottom of a well. The sides of the well have holes. As can be seenfrom the figure these holes are in communication with the atmosphere. In the beginning

the level of petrol in the float chamber and the well is the same. When the throttle isopened the pressure at the venturi throat decreases and petrol is drawn into the air stream.

This results in progressively uncovering the


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Figure: 6 Emulsion Tube

holes in the central tube leading to increasing air-fuel ratios or decreasing richness ofmixture when all holes have been uncovered. Normal flow takes place from the main jet.The air is drawn through these holes in the well, and the fuel is emulsified and the

pressure differential across the column of fuel is not as high as that in simple carburetor.Figure: 6 Emulsion Tube

Acceleration Pump System

Acceleration is a transient phenomenon. In order to accelerate the vehicle and

consequently its engine, the mixture required is very rich and the richness of the mixturehas to be obtained quickly and very rapidly. In automobile engines situations arise whenit is necessary to accelerate the vehicle. This requires an increased output from the enginein a very short time. If the throttle is suddenly opened there is a corresponding increase in

the air flow. However, because of the inertia of the liquid fuel, the fuel flow does notincrease in proportion to the increase in air flow. This results in a temporary lean mixture

ca11singtheengine to misfire and a temporary reduction in power output.

To prevent this condition, all modern carburetors are equipped with an accelerating

system. Figure 7. illustrates simplified sketch of one such device. The pump comprises ofa spring loaded plunger that takes care of the situation with the rapid opening of the

throttle valve. The plunger moves into the cylinder and forces an additional jet of fuel atthe venturi throat. When the throttle is partly open, the spring sets the plunger back.There is also an arrangement which ensures that fuel in the pump cylinder is not forced

through the jet when valve is slowly opened or leaks past the plunger or some holes intothe float chamber.

Mechanical linkage system, in some carburetor, is substituted by an arrangement whereby the pump plunger is held up by manifold vacuum. When this vacuum is decreased by


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rapid opening of the throttle, a spring forces the plunger down pumping the fuel throughthe jet.

Figure: 7 Acceleration pump system

Types of Carburetors

There are three general types of carburetors depending on the direction of flow of air. Thefirst is the up draught type shown in Fig.8(a) in which the air enters at the bottom and

leaves at the top so that the direction of its flow is upwards. The disadvantage of the updraught carburetor is that it must lift the sprayed fuel droplet by air friction. Hence, itmust be designed for relatively small mixing tube and throat so that even at low engine

speeds the air velocity is sufficient to lift and carry the fuel particles along. Otherwise,the fuel droplets tend to separate out providing only a lean mixture to the engine. On the

other hand, the mixing tube is finite and small then it cannot supply mixture to the e ngineat a sufficiently rapid rate at high speeds.

Figure: 8 Types of Carburetors


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In order to overcome this drawback the downdraught carburetor [Fig.8 (b)] is adopted. It

is placed at a level higher than the inlet manifold and in which the air and mixturegenerally follow a downward course. Here the fuel does not have to be lifted by air

friction as in the up draught carburetors but move into the cylinders by gravity even if the

air velocity is low. Hence, the mixing tube and throat can be made large which makeshigh engine speeds and high specific outputs possible.

A cross-draught carburetor consists of a horizontal mixing tube with a float chamber on

one side of it [Fig.8(c)]. By using across-draught carburetor in engines, one right-angledturn in the inlet passage is eliminated and the resistance to flow is reduced.

Constant Choke Carburetor:

In the constant choke carburetor, the air and fuel flow areas are always maintained to beconstant. But the pressure difference or depression, which causes the flow of fuel and air,

is being varied as per the demand on the engine. Solex and Zenith carburetors belong tothis class.

Constant Vacuum Carburetor:

In the constant vacuum carburetor, (sometimes called variable choke carburetor) air and

fuel flow areas are being varied as per the demand on the engine, while the vacuum ismaintained to be always same. The S.U. and Carter carburetors belong to tills class.

Multiple Venturi Carburetor:

Multiple venturi system uses double or triple venturi. The boost venturi is locatedconcentrically within the main venturi.The discharge edge of the boost venturi is located

at the throat of the main venturi. The boost venturi is positioned upstream of the throat ofthe larger main venturi. Only a fraction of the total air flows though the boost venturi.Now the pressure at the boost venturi exit equals the pressure at the main venturi throat.

The fuel nozzle is located at the throat of the boost venturi.


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UNITIII

IGNITION SYSTEM

INTRODUCTION:

In all spark ignition engines which work on the Gasoline either 2-Stroke or 4-Strokecycle principle and utilize a carburetor or fuel injection system, the combustion of the air-

fuel mixture is initiated by an electric spark.

The term Spark Ignition means that a brief electric arc is produced between the

electrodes of a spark plug, the energy for which is derived from an external power source.In most cases this power source is the vehicle battery, which is constantly being

supplemented by the alternate while the vehicle is mobile.

A different method of ignition is employed in diesel engines. This is called compression

ignition and relies on the fact that when air compressed, its temperature rises. In dieselengines, compression ratio of between 16:1 and 25:1 are common, and at the end of a

compression the temperature of the trapped air is sufficiently high to ignite the diesel fuelthat is sprayed into the cylinder at the appropriate time.

The functions of ignition system

The functions of the coil ignition systems in general use on motor vehicle may be dividedinto three areas. These are:

Production of the high voltage necessary to produce a spark at the plug gap.

Distribute the spark to all the cylinders at proper time based on the firing order.

Varying the timing of the spark depending on the various operating conditions ofthe engine like cranking time, varying speed and load, so that the best performance

is obtained from the engine under all operating conditions.

Mechanism of IgnitionIt must be remembered that vehicle battery voltages are usually 12 volt or 24 volt and this

value is too low to produce a heavy spark at the plug gap in a cylinder under

compression. For this reason one of the major functions of the battery ignition system isto raise the battery voltage to the required level and then apply it to spark plugs.

This process is correctly initiated in the primary circuit and completed in the secondary

winding of the ignition coil. Depending on the type of engine and the conditions existingin the cylinders, a voltage of between 5,000 to 20,000 volts is required and this is called

the ionizing voltage orfiring voltage.


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This firing voltage forces the electrons to jump between the electrodes of the spark plugin the gap to produce the required spark. The electr ic spark has sufficient heat energy to

ignite the air-fuel mixture which later continues to burn itself.

The conventional coil ignition system

Inductive ignition systems: that uses an ignition coil to perform the step up transformer

action and to increase the electrical voltage. The ignition coils of the inductive ignitionsystems operate on the principle of electromagnetic induction (EMI) irrespective of

whether it is triggered by contact breakers or by electronic triggering units.

Note:

As a reminder of the principle of EMI, a voltage will be induced into a coil whenever thefollowing factors are present:

(a) a magnetic field(b) a set of conductors(c) a relative movement between the magnetic field and conductors.

The factors affecting the operation of the Ignition system.

The factors that determine the value of the voltages induced into the ignition coil

windings during the ignition cycle are:

(a)The strength of the magnetic field.The stronger the magnetic field produced in the coil primary winding, the greaterthe possibility of producing a high secondary voltage.

(b)The number of conductors on the secondary winding being cut by the magneticfield. This is

important when considering the voltages produced in both coil windings during theignition cycle.

(c) The speed of relat ive movementbetween the magnetic field and the

conductors. The faster the magneticfield can be made to cut the conductors,

the higher will be the value of voltageinduced into the coil windings.

Construction of the Ignition coil

The source of the high voltage pulses ofcurrent produced in the inductive ignition

I nition Coil

_

+

Primary

winding

Secondary

winding

Soft Iron core

Oil filled forCooling

Soft iron

Cover

HT Outlet


22/38

system is in the ignition coil. The coil stores the energy in the magnetic field around theprimary winding and at the required instant of ignition, transforms it into a pulse of high

voltage current in the secondary winding. From here it is delivered to the correct sparkplug via the high tension (HT) cables. This Inductive storage device may vary in des ign

between certain manufacturers, but in general the most common construction is as shown

in figure below.

This coil contains a rod shaped, laminated soft iron core at its centre, and the soft ironcover surrounds both primary and secondary windings. Both of these soft iron

components are used to intensify and maximize the effect of the primary magnetic fieldand thus, the energy stored. The iron core must be laminated to minimize the effects ofeddy currents that are produced during operation and so keep to a minimum the heat

developed. The outer soft iron cover is slotted to allow circulation of the oil filling whichis used for cooling purposes.

Around the laminated core, the secondary winding is wounded. This consists of many

turns of very fine insulated copper wire (generally in the vicinity of 20,000 turns). Oneend of this winding is connected to the HT outlet of the coil via the laminated iron corewhich it used as the pick- up point for this connection. The other end is connected to the

positive (+) low tension primary terminal.

Over the top of the secondary winding the primary winding is wounded with the

insulation. The primary winding consisting of a few hundred turns of relatively heavyinsulated copper wire. The ends of the primary winding are connected to the two low

tension, or primary terminals. A reason for placing the primary winding over thesecondary is that it is in this coil, which caries the full primary circuit current(approximately 2 ampere in standard systems), the secondary winding generates the heat

and by placing it thus, the cooling oil is given ready access to it.

A ceramic insulator at the base of the coil supports the core and winding and at the top isa plastic-type insulator which provides a location point for the high-tension and primaryterminals. This top insulator is sealed into the outer case to prevent the loss of coolant oil

or the energy of moisture.

Operation of an Ignition coil

Electromagnetic induction is the effect of creating the voltage in a conductor by means of

relative movement between the conductor and a magnetic field. In the ignition coil theconductors remain stationary and the magnetic field is moved across them. To developthese necessary conditions, the first requirement in the ignition oil is the production of a

magnetic field. This is the function of the primary winding.

When the ignition switch is closed, the primary winding of the coil is connected to thepositive terminal of the vehicle battery. Now, if the primary circuit is completed through


23/38

the contact breaker points a current will flow in the circuit, creating a magnetic field inthe coil around the soft iron core. This magnetic field grows outwards from the core until

it has reached maximum value and the core is fully magnetized and ceases to growfurther.

To provide the very high voltage necessary to create a spark across the plug gap, thesecondary winding has a very large number of turns.

NOTE: The ratio of the number of secondary turns to the number of primary turns is verylarge approximately 100:1. The effect of this high ration is to produce a very high

voltage in the secondary winding when the magnetic field is collapsed rapidly across it as

the contact breaker points are opened.

To understand the operation of the ignition coil, it is necessary to have the knowledge ofthe effect of winding insulated wire into the form of a coil and then passing a current

through it. In earlier chapters of this course, an explanation was given of how a magneticfield forms around a wire when current flows through it. The development of themagnetic field around a wire wound into a coil was also explained. Also the direction or

polarity of the magnetic field was shown to depend on the direction of current flow in thewire.

THE CAPACITOR

ConstructionThe construction of a capacitor is quite simple. It is made of two strips of metallised

paper, separated by a thin dielectric (insulator), generally of waxed paper or plastic, bothrolled tightly together and fitted into a metal container. An insulated flexible lead isattached to one of the metallised plates and brought out for connection to the insulated

side of the contacts. The other metallised plate is attached to the metal container which

Growing Magnetic field


24/38

has facilities for connecting it to a good earth either inside or outside the distributor, thuseffectively connecting the capacitor across, or in parallel, with the points.

As a general statement it can be said that acapacitor is a device which has the ability to

store an electr ical charge. When a capacitor

is charged, each plate will hold an equalbut opposite charge. That is the plate

connected to the negative side of the circuitwill acquire a negative charge, and the plate

connected to the positive side of the circuit,a positive charge. Once these oppositecharges are stored on the plates, they will

attract each other through the separatingdielectric, and thus tend to prevent the charge escaping or leaking away.

NOTE: The loss of electrical charge from a capacitor is termed the capacitors leakage.

Among the tests applied to a capacitor is a test for leakage, which must be below a certainrate of loss.

Removing the charge from a capacitor is called discharging it. This is accomplished byconnecting a conductor across its plates. The excess electrons are attracted from thenegatively charged plate to the positively electrons are attracted from the negatively

charged plate to the positively charged plate. The electron flow continues until such timeas both charges equalize, i.e. there is no potential difference between the plates.

The factors affecting the capacity of a capacitor:

(a) The area of the plates holding the charges and the number of plates used.(b) The distance the plates are separated, i.e. the thinner the dielectric, the greater

the attract ive force between the charges, and therefore the higher the capacity.(c) The type of dielectric, e.g. plastic, mica, paper, air, etc.

Unit of Capacitor

The amount of charge a capacitor can hold is termed its capacitance (symbol C) which ismeasured in a unit called the farad (symbol F). Since the farad is a large quantity and it is

difficult to have such a big capacitor in real time, the capacitors are generally measuredby micro farad (Symbol F)

Automotive capacitors are in the vicinity of 0.20 to 0.30 microfarads (one microfarad =10-6 farads, or 1 millionth of a farad).

The operation of a capacitor in an ignition circuit is relatively simple, but tends to appear

complex because of the number of events, or changes, that occur simultaneously. Thefollowing explanation presents these changes as a logical, sequential set of events.

_+


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OPERATION OF THE IGNITION CIRCUIT

A simple circuit shown above can illustrate the position of the major components of an

ignition system. With the ignition switch on: when the breaker contacts initially close,a current commences to flow in the primary circuit and the magnetic field builds up

relatively slowly, due to the self induced voltage that is developed at this time

During this closed circuit period of the ignition cycle, the capacitor is in parallel with

the breaker contacts which are closed at this time. As the distributor shaft continues itsrotation the cam lobe lift the breaker gently to open the contacts. It takes a certain

number of degrees of distributor shaft rotation and therefore a measurable period of timefor this to occur.

When the contacts are open and instantly a resistance is presented to the primary circuitbecause of this contact gap. The primary current is interrupted and the magnetic field is

starting to collapse. The current produced by the self induced voltage has to enter theplates of the capacitor. Since the high resistance will be induced across the contacts dueto their separation and it will naturally take the low resistance path.

Capacitor function

When the contacts gap is slowly widening and the self induced voltage is rapidly risingtowards the 200-300 volt level. The capacitor is rapidly charging up. As the capacitor

reaches the fully charged state, the contacts have opened to such a degree that even this

Distributor Cap

Coil

Starter Contact

Motor Breaker

Ignition Ballast

Switch Resistor

Spark plug

Battery


26/38

high voltage cannot jump the gap and so the primary circuit currents comes to an instanthalt.

This sudden stopping of the primary current, produced by the action of the capacitor,

gives an extremely fast collapse of the magnetic field. The mutually induced voltage,

generated in the secondary winding at this instant will be very high. Since the secondarywinding has about 100 times as many turns as the primary winding, the secondary

voltage will be about 100 times higher than the primary voltage (200 to 300 volts).

The secondary voltage at this instant is fed out through the HT circuit to the correct sparkplug where it ionizes the plug gap and forms a spark which ignites the air-fuel mixture.For the period of time of spark duration the capacitor remains fully charged. After the

energy of the secondary circuit has been expended in the HT spark, the capacitordischarges back through the battery, ignition switch and co il primary to the opposite plate

of the capacitor, thus recharging it in the reverse direction. The capacitor then dischargesback again to recharge itself in the original direction but at a lower value. It continues

this oscillating cycle of charge and discharge until all of the stored energy is dissipatedacross the resistance of the primary circuit. The distributor cam continues its rotation, thepoints close again and the whole cycle is repeated.


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UNITIV

COOLING AND LUBRICATION SYSTEM

Need for cooling system

The cooling system has four primary functions. These functions are as follows:

1. Remove excess heat from the engine.2. Maintain a constant engine operating temperature.3. Increase the temperature of a cold engine as quickly as possible.4. Provide a means for heater operation (warming the passenger compartment).

Types of cooling system:

The different Types of cooling system are

1. air cooling system2. liquid cooling system3. forced circulation system4. pressure cooling system

Air-Cooled System :

The simplest type of cooling is the air-cooled, or direct, method in which the heat isdrawn off by moving air in direct contact with the engine Several fundamental principles

of cooling are embodied in this type of engine cooling. The rate of the cooling isdependent upon the following:

1. The area exposed to the cooling medium2. The heat conductivity of the metal used & the volume of the metal or its size in

cross section

3. The amount of air flowing over the heated surfaces4. The difference in temperature between the exposed metal surfaces and the

cooling air

LIQUID-COOLED SYSTEM

Nearly all multicylinder engines used in automotive, construction, and material-handling equipment use a liquid-cooled system. Any liquid used in this type of system is

called a COOLANT.

A simple liquid-cooled system consists of a radiator, coolant pump, piping, fan,thermostat, and a system of water jackets and passages in the cylinder head and blockthrough which the coolant circulates. Some vehicles are equipped with a coolant


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distribution tube inside the cooling passages that directs additional coolant to the pointswhere temperatures are highest. Cooling of the engine parts is accomplished by

keeping the coolant circulating and in contact with the metal surfaces to be cooled. Theoperation of a liquid- cooled system is as follows: The pump draws the coolant from the

bottom of the radiator, forcing the coolant through the water jackets and passages, and

ejects it into the upper radiator tank. The coolant then passes through a set of tubes to thebottom of the radiator from which the cooling cycle begins. The radiator is situated in

front of a fan that is driven either by the water pump or an electric motor. The fan ensuresairflow through the radiator at times when there is no vehicle motion. The downward

flow of coolant through the radiator creates what is known as a thermosiphon action. Thissimply means that as the coolant is heated in the jackets of the engine, it expands. As itexpands, it becomes less dense and therefore lighter. This causes it to flow out of the top

outlet of the engine and into the top tank of the radiator. As the coolant is cooled in theradiator, it again becomes more dense and heavier. This causes the coolant to settle to the

bottom tank of the radiator. The heating in the engine and the cooling in the radiatortherefore create a natural circulation that aids the water pump. The amount of engine

heat that must be removed by the cooling system is much greater than is generallyrealized. To handle this heat load, it may be necessary for the cooling system in someengine to circulate 4,000 to 10,000 gallons of coolant per hour. The water passages, the

size of the pump and radiator, and other details are so designed as to maintain theworking parts of the engine at the most efficient temperature within the limitationimposed by the coolant.

Pressure cooling system

Radiator Pressure CapThe radiator pressure cap is used on nearly all of the modern engines. The radiator cap

locks onto the radiator tank filler neck Rubber or metal seals make the cap-to-neck jointairtight. The functions of the pressure cap are as follows: 1. Seals the top of the radiator

tiller neck to prevent leakage. 2. Pressurizes system to raise boiling point of coolant. 3.Relieves excess pressure to protect against system damage. 4. In a closed system, itallows coolant flow into and from the coolant reservoir. The radiator cap pressure valve

consists of a spring- loaded disc that contacts the filler neck. The spring pushes the valveinto the neck to form a seal. Under pressure, the boiling point of water

increases. Normally water boils at 212F. However, for every pound of pressureincrease, the boiling point goes up 3F. Typical radiator cap pressure is 12 to 16 psi. Thisraises the boiling point of the engine coolant to about 250F to 260F. Many surfaces

inside the water jackets can be above 212F. If the engine overheats and the pressureexceeds the cap rating, the pressure valve opens. Excess pressure forces coolant out of the

overflow tube and into the reservoir or onto the ground. This prevents high pressurefrom rupturing the radiator, gaskets, seals, or hoses. The radiator cap vacuum valve opensto allow reverse flow back into the radiator when the coolant temperature drops after

engine operation. It is a smaller valve located in the center, bottom of the cap. Thecooling and contraction of the coolant and air in the system could decrease coolant

volume and pressure. Outside atmospheric pressure could then crush inward on the hoses


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and radiator. Without a cap vacuum or vent valve, the radiator hose and radiator couldcollapse

ENGINE LUBRICATINGSYSTEMS

All internal combustion engines are equipped with an internal lubricating system.Without lubrication, an engine quickly overheats and its working parts seize due to

excessive friction. All moving parts must be adequately lubricated to assure maximumwear and long engine life.

PURPOSES OF LUBRICATION

The functions of an engine lubrication system are as follows: Reduces friction and wearbetween moving parts. Helps transfer heat and cool engine parts. C leans the inside of the

engine by removing contaminants (metal, dirt, plastic, rubber, and other particles).Absorbs shocks between moving parts to quiet engine operation and increase engine life.

The properties of engine oil and the design of modern engines allow the lubrication

system to accomplish these functions.

TYPES OF LUBRICATING (OIL)SYSTEMS

Now that you are familiar with the lubricating system components, you are ready to studythe different systems that circulate oil through the engine. The systems used to circulateoil are known as splash, combination splash force feed, force feed, and full force-feed.

SplashSystems

The splash system is no longer used in automotive engines. It is widely used in small

four-cycle engines for lawn mowers, outboard marine operation, and so on. In the splashlubricating system, oil is splashed up from the oil pan or oil trays in the lower part of thecrankcase. The oil is thrown upward as droplets or fine mist and provides adequatelubrication to valve mechanisms, piston pins, cylinder walls, and piston rings. In the

engine, dippers on the connecting-rod bearing caps enter the oil pan with eachcrankshaft revolution to produce the oil splash. A passage is drilled in each connecting

rod from the dipper to the bearing to ensure lubrication. This system is too uncertainfor automotive applications. One reason is that the level of oil in the crankcase will vary

greatly the amount of lubrication received by the engine. A high level results in excess

lubrication and oil consumption and a slightly low level results in inadequate lubricationand failure of the engine.

Combination Splash and Force Feed

In a combination splash and force feed, oil is delivered to some parts by means ofsplashing and other parts through oil passages under pressure from the oil pump. The oil

from the pump enters the oil galleries. From the oil galleries, it flows to the main bearingsand camshaft bearings. The main bearings have oil- feed holes or grooves that feed oilinto drilled passages in the crankshaft. The oil flows through these passages to the


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connecting rod bearings. From there, on some engines, it flows through holes drilled inthe connecting rods to the piston-pin bearings. Cylinder walls are lubricated by splashing

oil thrown off from the connecting-rod bearings. Some engines use small troughsunder each connecting rod that are kept full by small nozzles which deliver oil under

pressure from the oil pump. These oil nozzles deliver an increasingly heavy stream as

speed increases. At very high speeds these oil streams are powerful enough to strike thedippers directly. This causes a much heavier splash so that adequate lubrication of the

pistons and the connecting-rod bearings is provided at higher speeds. If a combinationsystem is used on an overhead valve engine, the upper valve train is lubricated by

pressure from the pump.

Force Feed

A somewhat more complete pressurization of lubrication is achieved in the force-feed lubrication system. Oil is forced by the oil pump from the crankcase to the main

bearings and the camshaft bearings. Unlike the combination system the connecting-rod bearings are also fed oil under pressure from the pump. Oil passages are drilled in the

crankshaft to lead oil to the connecting-rodbearings. The passages deliver oil from themain bearing journals to the rod bearing journals. In some engines, these opening areholes that line up once for every crankshaft revolution. In other engines, there are

annular grooves in the main bearings through which oil can feed constantly into thehole in the crankshaft. The pressurized oil that lubricates the connecting- rod bearingsgoes on to lubricate the pistons and walls by squirting out through strategically drilled

holes. This lubrication system is used in virtually all engines that are equipped withsemifloating piston pins.

Full Force Feed

In a full force-feed lubrication system, the main bearings, rod bearings, camshaftbearings, and the complete valve mechanism are lubricated by o il under pressure. In

addition, the full force-feed lubrication system provides lubrication under pressure tothe pistons and the piston pins. This is accomplished by holes drilled the length of theconnecting rod, creating an oil passage from the connecting rod bearing to the piston pin

bearing. This passage not only feeds the piston pin bearings but also provides lubricationfor the pistons and cylinder walls. This system is used in virtually all engines that are

equipped with full-floating piston pins.


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UNITV

COMBUSTION AND COMBUSTION CHAMBERS

Introduction (Combustion in S.I Engines)

Combustion may be defined as a relatively rapid chemical combination of hydrogen andcarbon 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 ofintensive research for many years.

Stageso of Combustion in SI Engine

In a spark-ignition engine a sufficiently homogeneous mixture of vaporized fuel, air andresidual gases is ignited by a single intense and high temperature spark between the spark

plug electrodes (at the moment of discharge the temperature of electrodes exceeds10,000C), leaving behind a thin thread of flame. From this thin thread combustionspreads to the envelop of mixture immediately surrounding it at a rate which depends

primarily upon the temperature of the flame front itself and to a secondary degree, uponboth the temperature and the density of the surrounding envelope. In this manner there

grows up, gradually at first, a small hollow nucleus of flame, much in the manner of asoap bubble. If the contents of the cylinder were at rest, this flame bubble would expandwith steadily increasing speed until extended throughout the whole mass. In the actual

engine cylinder, however, the mixture is not at rest. It is, in fact, in a highly turbulentcondition the turbulence breaks the filament of flame into a ragged front, thus presenting

a far greater surface area from which heat is radiated; hence its advance is speeded up

enormously. The rate at which the flame front travels is dependent primarily on thedegree of turbulence, but its general direction of/movement, that of radiating outward

from the ignition point, is not much affected. According to Ricardo the combustion canbe imagined as if developing in two stages, one the growth and development of a semi

propagating nucleus of flame called ignition lag or preparation phase, and the other, thespread of the flame throughout the combustion chamber [see F ig. 9].

Figure: 9. Stages of combustion in SI engine


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The former is a chemical process depending upon the nature of the fuel, upon

temperature and pressure, the proportion of the exhaust gas, and also upon thetemperature coefficient of the fuel, that is, the relationship between temperature and rate

of acceleration of oxidation or burning. The second stage is a mechanical one pure and

simple. The two stages are not entirely distinct, since the nature and velocity ofcombustion change gradually. The starting point of the second stage is where first

measurable rise of pressure can be seen on the indicator diagram, i.e., the point where theline of combustion departs from the compression line. In Fig. 14.2(b), A shows the point

of passage of spark - (say 28 before TDC), B the point at which the first rise of pressurecan be detected (say, 8before TDC) and C the attainment of peak pressure. Thus ABrepresents the first stage (about 20 crank angle rotation) and BC the second stage.

Although the point C makes the completion of the flame travel, it does not follow that atthis point the whole of the heat of the fuel has been liberated, for even after the passage

of the flame, some further chemical adjustments due to reassociation, etc., and what isgenerally referred to as after burning, will to a greater or less degree continue throughout

the expansion stroke. The first stage AB, by analogy with diesel engines is called ignitionlag, which label is wrong in principle. In spark ignition there is practically no ignition lagand a nucleus of combustion arises instantaneously near the spark plug electrodes. But

during the initial period flame front spreads very slowly and the fraction of burnt mixtureis small so that an increase of pressure cannot be detected on the indicator diagram. Theincrease of pressure maybe just one per cent of maximum combustion pressure

corresponding to burning of about 1.5per cent of the working mixture, and the volumeoccupied by the combustion products may be about 5 per cent of the combust ion chamber

space.

The stage II is the main stage of combustion. The end of second stage is taken as the

moment at which maximum pressure is reached in the indicator diagram (see Fig. 9).However, combustion does not terminate at this point and after burning continues for a

rather long time near the walls and behind the turbulent flame front. The combustion ratein the stage III reduces, due to surface of the flame front becoming smaller and reductionin turbulence. About 10 per cent or more of heat is evolved in the after-burning stage and

hence the temperature of the gases continues to increase to pointD in Fig.9. However, thepressure reduces because the decrease in pressure due to expansion of gases and transfer

of heat to walls is more than the increase in pressure due to combustion.

Effect of Engine Variables on Flame Propagation

A study of the variables which affect the flame propagation velocity is important

because the flame velocity influences the rate of pressure rise in the cylinder, and hasbearing or certain types of abnormal combustion.

There are several factors which affect the flame speed, the most important being fuel-airratio and turbulence.


33/38

1. Fuel-air ratio. The composition of the working mixture influences the rate ofcombustion and the amount of heat evolved. With hydrocarbon fuels the maximum flame

velocities occur when mixture strength is 110% of stoichiometric (i.e., about 10% richerthan stoichiometric). When the mixture is made leaner or is enriched and still more, the

velocity of flame diminishes. Lean mixtures release less thermal energy resulting in

lower flame temperature and flame speed. Very rich mixtures have incompletecombustion (some carbon only burns to CO and not to CO2) that results in production of

less thermal energy and hence flame speed is again low.

2. Compression Ratio. A higher compression ratio increases the pressure and temperatureof the working mixture and decreases the concentration of residual gases. Thesefavorable conditions reduce the ignition lag of combustion and hence less ignition

advance is needed. High pressures and temperatures of the compressed mixture alsospeed up the second phase of combustion. Total ignition angle is reduced. Maximum

pressure and indicated mean effective pressure are increased.. Lastly, use of a highercompression ratio increases the surface to volume ratio of the combustion chamber,

thereby increasing the part of the mixture which after-burns in the third phase. Theincrease in compression ratio results in increase in temperature that increases thetendency of the engine to detonate.

3. Intake temperature and pressure. Increase in intake temperature and pressure increasesthe flame speed.

4. Engine load. With increase in engine load the cycle pressures increase. Hence the

flame speed increases. In SI engines with decrease in load, throttling reduces power of anengine. Due to throttling the initial and final compression pressures decrease and thedilution of the working mixture due to residual gases increases. This makes the smooth

development of self propagating nucleus of flame difficult and unsteady and prolongs theignition lag. The difficulty can be overcome to a certain extent by enriching the mixture

at low loads (0.8 to 0.9of stoichiometric) but still it is difficult to avoid after-burningduring a substantial part of expansion stroke. In fact, poor combustion at low loads andthe necessity of mixture enrichment are among the main disadvantages of spark ignition

engines which cause wastage of fuel and discharges of a large amount of products ofincomplete combustion like carbon monoxide and other poisonous substances.

5. Turbulence. Turbulence plays a very vital role in combustion phenomenon. The flamespeed is very low in non-turbulent mixtures. A turbulent motion of the mixture intensifies

the processes of heat transfer and mixing of the burned and unburned portions in theflame front (diffusion). These two factors cause the velocity of turbulent flame to

increase practically in proportion to the turbulence velocity. The turbulence of themixture is due to admission of fuel-air mixture through comparatively narrow sections ofthe intake pipe, valves, etc. in the suction stroke. The turbulence can be increased at the

end of the compression by suitable design of combustion chamber that involves thegeometry of cylinder head and piston crown. The degree of turbulence increases directly

with the piston speed. If there is no turbulence the time occupied by each explosionwould be so great as to make the high speed internal combustion engines impracticable.


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Insufficient turbulence lowers the efficiency due to incomplete combustion of the fuel.However, excessive turbulence is also undesirable.

6. Engine Speed. The higher the engine speed, the greater the turbulence inside the

cylinder. For this reason the flame speed increases almost linearly with engine speed.

Thus if the engine speed is doubled the time required, in milliseconds, for the flame totraverse the combustion space would be halved. Double the original speed arid hence half

the original time would give the same number of crank degrees for flame propagation.The crank angle required for the flame propagation, which is the main phase of

combustion, will remain almost constant at all speeds. This is an important characteristicof spark ignition engines. However, the increase in engine speed would lead to ignitionadvance due to the first phase of combustion. This can be illustrated with a numerical

example. Consider a petrol engine running at 1500rpm. Let us say for the first stage ofcombustion the ignition lag, the time required in terms of crank angle, is 8 of crank

rotation, and for the second stage, the propagation of flame through the combustionspace, 12oofcrank rotation is required. Thus the total ignition period is20of crank

rotation. Now if the engine speed is doubled from 1500 to 3000 rpm, the time requiredfor the second stage will again be 12 of crank rotation (due to doubling of turbulenceintensity time in milliseconds is halved and in terms of crank angle remains constant), but

for the first stage time in milliseconds is constant and hence in terms of crank angle itwill be doubled, i.e., it would be 16.This would make the total ignition period of 16 + 12= 28 crank rotation at 3000rpm compared to 8 + 12= 20 at .1500 rpm. From this it

follows that with increase in engine speed ignition must be advanced. This is done inpractice by automatic ignition advance mechanism.

7. Engine size. Engines of similar design generally run at the same piston speed. This isachieved by smaller engines having larger rpm and larger engines having smaller rpm.

Due to the same piston speed, the inlet velocity, the degree of turbulence, and flamespeed are nearly same in similar engines regardless of the size. However, in small engines

the flame travel is small and in large engines large. Therefore, if the engine size isdoubled the time required (in milliseconds) for propagation of flame through combustionspace will also be doubled. But with lower rpm of larger engines the time for flame

propagation in terms of crank angle would be nearly same as in smaller engines. In otherwords the number of crank degrees required for flame travel will be about the same

irrespective of engine size, provided the engines are similar.

Rate of Pressure Rise

The rate of pressure rise is a very important aspect of flame development from engine

design and operation point of view. It considerably influences the maximum cylinderpressure, the power produced and the smooth running of the engine. The rate or pressurerise depends on the mass rate of combustion of the mixture in the cylinder. Fig. 10 shows

pressure-crankangle diagrams for three different combustion rates. One is for a high, thesecond for the usual and the third for a low rate of combustion


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Figure: 10. Relationship b/w pressure and crank angle for different rates of combustion

It is clear from the figure that with lower rates of combustion longer time is required forcombustion that necessitates the initiation of burning at an earlier point on thecompression stroke. With higher rates of burning the time required for combustion is

smaller and the rate of pressure rise is higher. Also, the peak pressure produced is close toTDC, which is desirable because it produces greater force acting through a large portion

of the power stroke. But peak pressure and hence peak temperature too close to TDCgives a long time for rapid heat loss from the cylinder. The higher rate of pressure risecauses rough running of the engine because of vibrations and jerks produced in

crankshaft. If the rate of pressure rise is very high it results in abnormal combustioncalled detonation. In practice the engine is so designed that approximately one-half of the

pressure rise takes place as the piston reaches TDC. This results in peak pressure and

temperature 10 to 15 after TDC. In this way very small portion of the expansion strokeis-lost and the gain is smooth engine operation and saving an appreciable period of time

during which loss of heat is rapid. In the old engines with low compression rat ios of 5 to6 a rate of pressure rise of 2 bar per crank degree used to be thought as optimum. Today

with higher compression ratios of the order of8 to 9, a rate of pressure rise of 3 to 4 barper crank degree may be employed if the engine mountings are sufficiently stiff andefficient.

Abnormal Combustion

In normal combustion the flame started by the spark travels across the combustionchamber in a fairly even way. Under certain engine operating conditions abnormal

combustion may take place that is detrimental to, life and performance of the engine.There are a variety of ways in which abnormal combustion can occur. The importantabnormal combustions are detonation or knock, preignition, run-on, etc. Of these

detonation or knock is most important because it puts a limit on the compression ratio atwhich an engine can be operated, which, in turn, controls the efficiency and to someextent, power output.


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QUESTION BANK

PARTA

1. What is the use of air standard cycle?2. Draw the Otto cycle on P-V diagram3. what are the firing order used in 4 and 6 cylinder engines4. what is stiochiometric air fuel ratio5. what are the three basic types of carburetors6. what are the different methods of engine cooling7. what is blow by8. what is flame front9. what are the various types of combustion chambers used in SI engines10.what is scavenging efficiency11.what is short circuiting12.what is carnot cycle and its importance13.What are different air fuel mixtures on which an engine can operate14.Write down the firing order for four and six cylinder engines15.what is indicator diagram16.what is the necessity of firing order in a multicylinder engines17.what are the mixture limits for inflammability18.what is the difference between a constant choke carburetor and a constant vacuum

carburetor19.what is a thermostat20.what is the need for providing crankcase ventilation21.what are the variable s that affect the knock of SI engines22.How is the location of the spark plug can influence knocking tendency23.why scavenging is important in two stroke engines compared to four strokeengines24.Define trapping efficiency25.List the different types of ignition system used in vehicles26.What is the use of condenser in ignition system27.What are the main functions of air injection pump?28.What is meant by crank case ventilation?29.Write the reason for cooling an engine30.what is the advantage of air cooling system31.what is meant by abnormal combustion32.What is delay period and what are the factors that affect the delay period?33.Define enthalpy34.Write down the components of ignition system used in automobiles35.Write down the possible faults in conventional ignition system36.Draw the diagram for an ignition coil and name the components37.Write any four advantage of 4 stroke petrol engine over 2 stroke petrol engine38.write the air standard efficiency of an Otto cycle39. write ant four factor that affects the process of combustion40.what are the various requirement of good carburetor


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41.what is the main function of an spark plug42.write different methods of SI lubricating system43.write the reason for cooling an engine44.what is meant by abnormal combustion45.What is a homogeneous and heterogeneous mixture?46.

what is opposed piston engine47.Define dwell angle and mention its importance

48.Briefly explain about two types of spark plug used in automobiles49.List the advantage of Electronic ignition system with conventional type50.Write down the firing order for four and six cylinder engines51.List the different types of ignition system used in vehicles52.What is the use of condenser in ignition system53.Write down the components of ignition system used in automobiles54.Write down the possible faults in conventional ignition system

PARTB

55.Compare four stroke and two stroke cycle engines and bring out their relativemerits and demerits

56.Describe with suitable sketches the idling system and accelerating pump of amodern carburetor

57.sketch a mechanical fuel pump and describe its working58.Explain with a neat sketch the forced circulation cooling system59.Discuss the different properties of lubricants60.Discuss the design criteria for a S.I engine combustion chamber61.Explain with neat sketches the two different types of two stroke engines62.Explain the various scavenging pumps used in a two stroke engines63.Discuss the two stroke engine construction and operation with neat sketches64.with a neat sketch explain the working principles of a simple carburetor65.Explain briefly the electronic fuel injection system used in some modern cars66.Explain with neat sketches the thermosypon cooling system and pressure cooling

system

67.Discuss the various types of combustion chambers used in SI engines68.Explain with a graph the three possible theoretical scavenging processes69.Explain with neat sketch the cross flow scavenging and uniflow scavenging70.Briefly explain the principle of four stroke petrol engine with a simple sketch71.Draw the ideal and actual indicator diagram for a four stroke petrol engine and

explain why they differ each other.72.Draw the sketch of a downdraught carburetor. How do the idle and low speed

circuits work in this carburetor73.Briefly explain the working principle of line fuel injection pump74.Explain the dry sump lubricating system with neat sketch75.What is an air cooling system and in which type of engine it is normally used.76.Explain the desirable properties of a good lubricant77.What are the various types of combustion chamber used in SI engines? explain

them briefly


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78.Write any one type of scavenging pump used in a 2 stroke petrol engine andexplain briefly.

79.Define the followingi) Delivery ratioii)

Trapping efficiencyiii) Relative cylinder charge

iv) Scavenging efficiency80.A gas working on the Otto cycle has a cylinder of diameter 200 mm and stroke

250 m. The clearance volume is 1570 CC. Find the air standard efficiency.Assume Cp= 1.004 KJ/kg K and Cv = 0.7171 KJ/kg K for a ir.

81.With a suitable sketches explain the various modern automobiles carburetors82.Give brief account of the injection in SI engine83.Clearly explain the various Wet sump lubricating system. Compare wet sump and

dry sump lubrication

84.With suitable sketches explain the liquid cooling system of an Automobile.85.Explain the phenomenon of knock in SI engine and compare with it with CIengines

86.Explain various scavenging pumps used in a two stroke engines87.what are the advantage and disadvantage of 2 stroke engine88.Explain with figures various types of combustion chambers used in SI engines.89.With the aid of a circuit diagram explain the working principles of a battery coil

ignition system.

90.Describe the working principles of a centrifugal spark advance mechanism withneat diagram

91.Describe the necessity of the spark advance mechanism in an engine92.With the aid of a diagram describe vacuum spark advance mechanism93.Draw the diagram for an ignition coil and name the components94.With the aid of a circuit diagram explain the working principles of a battery coil

ignition system.95.Describe the working principles of a centrifugal spark advance mechanism with

neat diagram97. Describe the necessity of the spark advance mechanism in an engine

98. With the aid of a diagram describe vacuum spark advance mechanism

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Automotive Engines basic