ICE Notes (Unit 2)

8/2/2019 ICE Notes (Unit 2)

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Internal Combustion Engines

Faculty - Er. Ashis Saxena


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IndexUnit 1

Introduction to I.C Engines

Fuels

Unit 2

SI Engines

Unit 3

CI Engines

Unit 4

Engine Cooling

Lubrication

SuperchargingTesting and Performance

Unit 5

Compressors


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Unit - 1

Chapter 2

SI Engines


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Carburetion

In an engine, carburetion is the process of preparation of a proper ratio of oxygen with a gaseous form of

a fossil fuel, like natural gas or gasoline, so it can combust.

Internal combustion engines run by igniting fuel that has been sprayed into a fine vapor and mixed

with air. This mixture, called an emulsion, will burn with the right amount of energy to fuel the

engine. Carburetion usually involves all these stages, from vaporizing the gasoline to letting in the air

and finally moving the mixture to where it can be combusted.

Carburetion is responsible for allowing an engine to perform at an optimal level whether it is

starting, running at full throttle, or idling.

What if carburetion process is absent?

If there is too much fuel or too little oxygen, the engine runs "rich" and wastes fuel, produces smoke,

creates too much heat, or ruins parts of the engine.

If there is too little fuel or too much air, the engine runs "lean" and might sputter, stop, or cause

engine damage.


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Air Fuel Mixtures

An engine is generally operated at different loads and speeds. For this proper air-fuel mixture should

be supplied to the engine cylinder. Fuel and air are mixed to form three different types of mixtures.

Chemically correct mixture: Chemically correct or stoichiometric mixture is one in which there is

just enough air for complete combustion of the fuel. Complete combustion means all carbon in the

fuel is converted to CO2 and all hydrogen to H2O.

For example, to burn one kg of octane (C8H8) completely 15.12 kg of air is required. Hence

chemically correctA/Fratio for octane is 15.12:1; usually approximated to 15:1.

Rich mixture: A mixture which contains less air than the stoichiometric requirements is called a

rich mixture (example,A/Fratio of 12:1, 10:1 etc.).

Lean mixture: A mixture which contains more air than the stoichiometric requirements is called a

lean mixture (example,A/Fratio of 17:1, 20:1 etc.).

There is however, a limited range ofA/F ratios (approximately 9:1 (rich) to 19:1 (lean)), in which

combustion in an SI engine will occur. Outside this range, the ratio is either too rich or too lean tosustain flame propagation.


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Air Fuel Mixtures


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Air Fuel Mixtures (for different throttle operations)

For successful operation of the engine, the carburetor has to provide mixtures which follow the

general shape of the curve ABCD (single cylinder) A'B'C'D' (multi-cylinder) in Fig.

The three general ranges of

throttle operation are:

Idling : (mixture must be

enriched)

Cruising (mixture must

be leaned)

High Power (mixture

must be enriched)


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Idling range

Idling range:

An idling engine is one which operates at no load and with nearly closed throttle. Under idling conditions, the engine requires a

rich mixture. This is due to the existing pressure conditions within the combustion chamber and the intake manifold which cause

exhaust gas dilution of the fresh charge.

Since, the clearance volume is constant, the mass of exhaust gas in the cylinder at the end exhaust stroke tends to remain

constant throughout the idling range. The amount of fresh charge brought, in during idling, however, is much less than that during

full throttle operation, due to very small opening of the throttle. This results in a much larger proportion of exhaust gas being

mixed with the fresh charge under idling conditions.

Further, with nearly closed throttle the pressure in the intake manifold is considerably below atmospheric due to restriction to the

air flow. When the intake valve opens, the pressure differential between the combustion chamber and the intake manifold results

in initial backward How of exhaust gases into the intake manifold. As the piston proceeds down on the intake stroke, these

exhaust gases are drawn back into the cylinder, along with the fresh charge. As a result, the final mixture of fuel and air in the

combustion chamber is diluted more by exhaust gas & the contact of fuel and air particles is obstructed resulting in poor

combustion & in loss of power. Hence more fuel is required by richening the air-fuel mixture which improve the probability of

contact between fuel and air particles and thus improves combustion.

As the throttle is gradually opened from A to B, the pressure differential between the inlet manifold and the cylinder becomes

smaller and the exhaust gas dilution of the fresh charge diminishes.


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Idling range

Cruising Range:

In the cruising range from B to C the exhaust gas dilution problem is relatively insignificant. The primary

interest lies in obtaining the maximum fuel economy. Consequently, in this range, it is desirable that the

carburetor provides the engine with the best economy mixture.Power Range:

During peak power operation the engine requires a richer mixture, as indicated by the line CD for the following

reasons:

To provide best power: Since high power is desired, it is logical to transfer the economy settings of the cruising

range to that mixture which will produce the maximum power, or a setting in the vicinity of the best powermixture, usually in the range of 12:1.

To prevent overheating of exhaust valve and the area near it: At high power, the increased mass of gas at

higher temperatures passing through the cylinder results in the necessity of transferring greater quantities of

heat away from critical areas such as those around the exhaust valve. Enriching the mixture reduces the flame

temperature and the cylinder temperature. This reduces the cooling problem and also reduces the tendency todamage exhaust valves at high power.


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Principle of carburetion

Both air & gasoline are drawn through the carburetor and into the engine

cylinders by the suction created by the downward movement of the piston.

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 combustion chamber picks up fuel

discharged from a tube. This tube has a fine orifice called carburetor jet

which is exposed to the air path.

In order that the fuel drawn from the nozzle may be thoroughly atomized,

the suction effect must be strong and the nozzle outlet 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

due to increase in velocity of flow, a suction effect is created.

The restriction is made in the form of a venturi to minimize throttling

losses.


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Principle of carburetionVenturi tube operation

Venturi tube has a narrower path at the centre so that the flow area through

which the air must pass 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

suction is proportionately increased.


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Carburetor - Construction & Working

A simple carburetor mainly consists of a float chamber, fuel discharge

nozzle & a metering orifice, a venturi, a throttle valve and a choke.

The float & needle valve system maintains a constant level of gasoline in

the float chamber. If the amount of fuel in the float chamber falls below thedesigned level, the float goes down, thereby opening the fuel supply valve

& admitting fuel. When the designed level has been reached, the float

closes the fuel supply valve thus stopping additional fuel flow from the

supply system.

During suction stroke air is drawn through the venturi. 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 the venturi the velocity increases

reaching a maximum at the venturi throat. Correspondingly, the pressure

decreases reaching a minimum.


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Choke Valve

Fuel discharge

nozzle

Venturi

Throttle Valve


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Carburetor - Construction & Working

From the float chamber, the fuel is fed to a discharge 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 as

carburetor depression, fuel is discharged into the air stream. 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 which is situated after the 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 cylinder and hence power

output is reduced.

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 engine power output.


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Combustion in SI engine

The combustion is defined as a rapid chemical reaction between the hydrogen and

carbon with oxygen in the air and liberates energy in the form of heat.

The conditions necessary for combustion are the presence of combustible mixture

and some means of initiating the process. The process of combustion in engines generally takes place either in a

homogeneous or a heterogeneous fuel vapor-air mixture depending on the type of

engine.

In an SI engine Fuel and air are homogeneously mixed together in the intake system,

inducted through the intake valve into the cylinder where it mixes with residual

gases and is then compressed. Under normal operating conditions, combustion is initiated towards the end of the

compression stroke at the spark plug by an electric discharge. A turbulent flame

develops following the ignition and propagates through this premixed charge of fuel

and air, and also the residual gas in the clearance volume until it reaches the

combustion chamber walls.


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Stages of Combustion in SI engine

A typical pressure-

crank angle diagram,

during the

compression (a b),combustion (b c)and expansion (c d) in an ideal four

stroke spark-ignition

engine is shown in

Fig. In an ideal

engine the entire

pressure rise during

combustion takes

place at constant

volume i.e., at TDC.

However, in an

actual engine it does

not happen.


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The pressure variation

due to combustion in a

practical engine is

shown below. In this

figure, A is the point ofpassage of spark (say 20

bTDC), B is the point at

which the beginning of

pressure rise can be

detected (say 8 bTDC)

and C the attainment of

peak pressure. Thus AB

represents the first

stage, BC the second

stage and CD the third

stage.


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Sir Ricardo, known as the father of engine research, describes the

combustion process in a SI engine as consisting of three stages:

Thefirst stage (AB), is referred to as the ignition lag or preparation phase

in which growth and development of a self propagating nucleus of flametakes place. This is a chemical process depending upon both temperature

and pressure, the nature of the fuel and the proportion of the exhaust

residual gas. Further, it also depends upon the relationship between the

temperature and the rate of reaction.

The second stage (BC) is a physical one and it is concerned with spread ofthe flame throughout the combustion chamber. The starting point of the

second stage is where the first measurable rise of pressure is seen on the

indicator diagram i.e., the point where the line of combustion departs from

the compression line (point B). This can be seen from the deviation from

the motoring curve.


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During the second stage the flame propagates practically at a constant

velocity. Heat transfer to the cylinder wall is low, because only a small part of

the burning mixture comes in contact with the cylinder wall during this period.

The rate of heat-release depends largely on the turbulence intensity and alsoon the reaction rate which is dependent on the mixture composition. The rate

of pressure rise is proportional to the rate of heat release because during this

stage, the combustion chamber volume remains practically constant (since

piston is near the top dead centre).

The starting point of the third stage is usually taken as the instant at which the

maximum pressure is reached on the indicator diagram (point C). The flame

velocity decreases during this stage. The rate of combustion becomes low due

to lower flame velocity and reduced flame front surface. Since the expansion

stroke starts before this stage of combustion, with the piston moving away

from the top dead centre, there can be no pressure rise during this stage.


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Factors influencing the flame speed

The study of factors which affect the velocity of flame propagation is important

since the flame velocity influences the rate of pressure rise in the cylinder and it

is related to certain types of abnormal combustion that occur in spark-ignition

engines. Turbulence: The flame speed is quite low in non-turbulent mixtures and

increases with increasing turbulence. This is mainly due to the additional

physical intermingling of the burning and unburned particles at the flame front

which expedites reaction by increasing the rate of contact. The turbulence in

the incoming mixture is generated during the admission of fuel air mixture

through comparatively narrow sections of the intake pipe, valve openings etc.,

in the suction stroke.

The increase of flame speed due to turbulence reduces the combustion

duration and hence minimizes the tendency of abnormal combustion.

However, excessive turbulence may extinguish the flame resulting in rough

and noisy operation of the engine.


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Fuel-Air Ratio: The fuel-air ratio has a very significant influence on the flame

speed. The highest flame velocities (minimum time for complete combustion)

are obtained with somewhat richer mixture which shows the effect of mixture

strength on the rate of burning as indicated by the time taken for completeburning in a given engine. When the mixture is made leaner or richer the

flame speed decreases. Less thermal energy is released in the case of Iean

mixtures resulting in lower flame temperature. Very rich mixtures lead to

incomplete combustion which results again in the release of less thermal

energy.

Temperature and Pressure: Flame speed increases with an increase in intake

temperature and pressure. A higher initial pressure and temperature may help

to form a better homogeneous air-vapor mixture which helps in increasing the

flame speed. This is possible because of an overall inert in the density of the

charge.


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Compression Ratio: A higher compression ratio increases the pressure and

temperature of the working mixture which reduce the initial phase of combustion and

hence less ignition advance is needed. High pressures and temperatures of the

compressed mixture also speed up the second phase of combustion. Increased

compression ratio reduces the clearance volume and therefore increases the density of

the cylinder gases during burning. This increases the peak pressure and temperature

and the total combustion duration is reduced. Thus engines having higher compression

ratios have higher flame speeds.

Engine Output: The cycle pressure increases when the engine output is increased. With

the increased throttle opening the cylinder gets filled to a higher density. This results in

increased flame speed. When the output is decreased by throttling, the initial and final

compression pressures decrease and the dilution of the working mixture increases. The

smooth development of self propagating nucleus of flame becomes unsteady and

difficult. The main disadvantages of SI engines are the poor combustion at low loads

and the necessity of mixture enrichment which causes wastage of fuel and discharge of

unburnt hydrocarbon and the products of incomplete combustion like carbonmonoxide etc. in the atmosphere.


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Engine Speed: The flame speed increases almost linearly with engine speed

since the increase in engine speed increases the turbulence inside the cylinder.

The time required for the flame to traverse the combustion space would be

halved, if the engine speed is doubled. Double the engine speed and hencehalf the original time would give the same number of crank degrees for flame

propagation. The crank angle required for the flame propagation during the

entire phase of combustion, will remain nearly constant at all speeds.

Engine Size: The size of the engine does not have much effect on the rate of

flame propagation. In large engines the time required for complete

combustion is more because the flame has to travel a longer distance. This

requires increased crank angle duration during the combustion. This is one of

the reasons why large sized engines are designed to operate at low speeds.


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Abnormal Combustion

In normal combustion, the flame initiated by the spark travels

across the combustion chamber in a fairly uniform manner.

Under certain operating conditions the combustion deviates from

its normal course leading to loss of performance and possible

damage to the engine. This type of combustion can be termed as

an abnormal combustion or knocking combustion.

The consequences of this abnormal combustion process are the

loss of power, recurring preignition and mechanical damage tothe engine.


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Phenomenon of knock in SI engines

In a spark-ignition engine combustion which is initiated between the spark plug

electrodes spreads across the combustible mixture. A definite flame front which

separates the fresh mixture from the products of combustion travels from the spark

plug to the other end of the combustion chamber.

Heat release due to combustion increases the temperature and consequently thepressure, of the burned part of the mixture above those of the unburned mixture. In

order to effect pressure equalization the burned part of the mixture will expand, and

compress the unburned mixture adiabatically thereby increasing its pressure and

temperature. This process continues as the flame front advances through the mixture

and the temperature and pressure of the unburned mixture are increased further.

If the temperature of the unburnt mixture exceeds the self-ignition temperature of the

fuel and remains at or above this temperature during the period of preflame reactions

(ignition lag), spontaneous ignition or auto ignition occurs at various pin-point

locations. This phenomenon is called knocking. The process of auto ignition leads

towards engine knock.


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This phenomenon of knock may be explained by referring to Fig. (a) which shows the cross-section

of the combustion chamber with flame advancing from the spark plug location A without knock

whereas Fig. (c) shows the combustion process with knock.

Normal Combustion

In the normal combustion the flame travels across the combustion chamber from A towards D.

The advancing flame front compresses the end charge BB'D farthest from the spark plug thusraising its temperature. The temperature is also increased due to heat transfer from the hot

advancing flame-front. Also some preflame oxidation may take place in the end charge leading to

further increase in temperature. In spite of these factors if the temperature of the end charge had

not reached its self-ignition temperature, the charge would not auto ignite and the flame will

advance further and consume the charge BBD. This is the normal combustion process which is

illustrated by means of the pressure-time diagram, Fig. (b).


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Abnormal Combustion

However, if the end charge BB'D reaches its auto ignition temperature and remains for

some length of time equal to the time of preflame reactions the charge will auto ignite,

leading to knocking combustion. In Fig. (c), it is assumed that when flame has reached

the position BB', the charge ahead of it has reached critical auto ignition temperature.During the preflame reaction period if the flame front could move from BB' to only CC

then the charge ahead of CC' would auto ignite.


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Because of the auto-ignition, another flame front starts traveling in the opposite

direction to the main flame front. When the two flame fronts collide, a severe pressure

pulse is generated. The gas in the chamber is subjected to compression and rarefaction

along the pressure pulse until pressure equilibrium is restored. This disturbance can

force the walls of the combustion chambers to vibrate at the same frequency as thegas. Gas vibration frequency in automobile engines is of the order of 5000 cps. The

pressure-time trace of such a situation is shown in (d).

It is to be noted that the onset of knocking is very much dependent on the properties

of fuel. It is clear from the above description that if the unburned charge does not

reach its auto-ignition temperature there will be no knocking. Further, if the initial

phase i.e., ignition lag period, is longer than the time required for the flame front to

burn through the unburned charge, there will be no knocking. But, if the critical

temperature is reached and maintained, and the ignition lag is shorter than the time it

takes for the flame front to burn through the unburned charge then the end charge will

detonate. Hence, in order to avoid or inhibit detonation, a high auto-ignition

temperature and a long ignition lag are the desirable qualities for SI engine fuels.


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In summary, when autoignition occurs, two different types of vibration may be

produced. In one case a large amount of mixture may autoignite giving rise to

a very rapid increase in pressure throughout the combustion chamber and

there will be a direct blow on the engine structure. The human ear can detectthe resulting thudding sound and consequent noise from free vibrations of the

engine parts. In the other case, large pressure differences may exist in the

combustion chamber and the resulting gas vibrations can force the walls of the

chamber to vibrate at the same frequency as the gas. An audible sound may

be evident.

The impact of knock on the engine components and structure can cause

engine failure and in addition the noise from engine vibration is always

objectionable.

The pressure differences in the combustion chamber cause the gas to vibrate

and scrub the chamber walls causing increased loss of heat to the coolant.


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Effect of engine variables on knock (Density Factors)

Density Factors

Any factor which reduces the density of the charge tends to reduce knocking by providing

lower energy release.

Compression Ratio: Compression ratio of an engine is an important factor which

determines both the pressure and temperature at the beginning of the combustionprocess. Increase in compression ratio increases the pressure and temperature of the

gases at the end of the compression stroke. This decreases the ignition lag of the end

gas and thereby increasing the tendency for knocking. The overall increase in the

density of the charge due to higher compression ratio increases the preflame reactions

in the end charge thereby increasing the knocking tendency of the engine. The increase

in the knocking tendency of the engine with increasing compression ratio is the main

reason for limiting the compression ratio to a lower value.

Mass of Inducted Charge: A reduction in the mass of the inducted charge into the

cylinder of an engine by throttling or by reducing the amount of supercharging reduces

both temperature and density of the charge at the time of ignition. This decreases the

tendency of knocking.


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Effect of engine variables on knock (Density Factors)

Inlet Temperature of the Mixture: Increase in the inlet temperature of the mixture makes

the compression temperature higher thereby, increasing the tendency of knocking. Further,

volumetric efficiency will be lowered. Hence a lower inlet temperature is always preferable

to reduce knocking. It is important that the temperature should not be so low as to cause

starting and vaporization problems in the engine. Temperature of the Combustion Chamber Walls: Temperature of the combustion chamber

walls play a predominant role in knocking. In order to prevent knocking the hot spots in the

combustion chamber should be avoided. Since, the spark plug and exhaust valve are two

hottest parts in the combustion chamber, the end gas should not be compressed against

them.

Retarding the Spark Timing: By retarding the spark timing from the optimized timing, i.e.,having the spark closer to TDC, the peak pressures are reached farther down on the power

stroke and are thus of lower magnitude. This might reduce the knocking. However, this will

affect the brake torque and power output of the engine.

Power Output of the Engine: A decrease in the output of the engine decreases the

temperature of the cylinder and the combustion chamber walls and also the pressure of the

charge thereby lowering mixture and end gas temperatures. This reduces the tendency toknock.


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Effect of engine variables on knock (Time Factors)

Time Factors

Increasing the flame speed or increasing the duration of the ignition lag or reducing

the time of exposure of the unburned mixture to auto-ignition condition will tend to

reduce knocking.

Turbulence: Turbulence depends on the design of the combustion chamber and on

engine speed. Increasing turbulence increases the flame speed and reduces the

time available for the end charge to attain auto-ignition conditions thereby

decreasing the tendency to knock.

Engine Speed: An increase in engine speed increases the turbulence of the mixture

considerably resulting in increased flame speed, and reduces the time available forpreflame reactions. Hence knocking tendency is reduced at higher speeds.

Flame Travel Distance: The knocking tendency is reduced by shortening the time

required for the flame front to traverse the combustion chamber. Engine size

(combustion chamber size), and spark plug position are the three important factors

governing the flame travel distance.


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Effect of engine variables on knock (Time Factors)

Engine Size: The flame requires a longer time to travel across the combustion

chamber of a larger engine. Therefore, a larger engine has a greater tendency for

knocking than a smaller engine since there is more time for the end gas to auto-

ignite. Hence, an SI engine is generally limited to size of about 150 mm bore.

Combustion Chamber Shape: Generally, the more compact the combustion

chamber is, the shorter is the flame travel and the combustion time and hence

better antiknock characteristics. Therefore, the combustion chambers arc made as

spherical as possible to minimize the length of the flame travel for a given volume.

If the turbulence in the combustion chamber is high, the combustion rate is high

and consequently combustion time and knocking tendency are reduced. Hence,the combustion chamber is shaped in such a way as to promote turbulence.

Location of Spark Plug: In order to have a minimum flame travel the spark plug is

centrally located in the combustion chamber, resulting in minimum knocking

tendency. The flame travel can also be reduced by using two or more spark plugs in

case of large engines.


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Effect of engine variables on knock (Composition Factors)

Composition Factors

Once the basic design of the engine is finalized, the fuel-air ratio and ill# properties of the fuel,

particularly the octane rating, play a crucial role in controlling the knock.

Fuel-Air Ratio: The flame speeds are affected by fuel-air ratio. Also the flame temperature and

reaction time are different for different fuel-air ratios. Maximum flame temperature is obtainedwhen 1.1 to I.2 whereas = 1 gives minimum reaction time for auto-ignition.

Figure below shows the variation of knock limited compression ratio with respect to equivalence

ratio for iso-octane. The maximum tendency to knock takes place for the fuel-air ratio which gives

minimum reaction time.


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Effect of engine variables on knock (Composition Factors)

Octane Value of the Fuel: A higher self-ignition temperature of the fuel and a

low preflame reactivity would reduce the tendency of knocking.

In general paraffin series of hydrocarbon have the maximum and aromatic

series the minimum tendency to knock & the naphthene series comes inbetween the two.

Usually, compounds with more compact molecular structure are less prone to

knock. In aliphatic hydrocarbons, unsaturated compounds show lesser

knocking tendency than saturated hydrocarbons, the exception being

ethylene, acetylene and propylene.


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Combustion Chamber Design

The design of the combustion chamber for an SI engine has an important influence

on the engine performance and its knocking tendencies.

The design involves the shape of the combustion chamber, the location of spark

plug and the location of inlet and exhaust valves.

Combustion chambers must be designed carefully, keeping in mind the following

general objectives.

Smooth Engine Operation

The aim of engine design is to have a smooth operation and a good economy. These

can be achieved by the following: Moderate rate of pressure rise: The rate of pressure rise can be regulated such

that the greatest force is applied to the piston as closely after TDC on the power

stroke as possible, with a gradual decrease in the force on the piston during the

power stroke. The forces must be applied to the piston smoothly, thus limiting the

rate of pressure rise as well as the position of the peak pressure with respect to

TDC.


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Reducing the Possibility of Knocking: Reduction in the possibility of knocking,

in an engine can be achieved by,

i. Reducing the distance of the flame travel by centrally locating the spark plug

and also by avoiding pockets of stagnant charge.ii. Satisfactory cooling of the spark plug and of exhaust valve area which are the

source of hot spots in the majority of the combustion chambers.

iii. Reducing the temperature of the last portion of the charge, through

application of a high surface to volume ratio in that part where the last portion

of the charge burns. Heat transfer to the combustion chamber walls can be

increased by using high surface to volume ratio thereby reducing the

temperature.


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High Power Output and Thermal Efficiency

The main objective of the design and development of an engine is to obtain high power as

well as high thermal efficiency. This can be achieved by considering the following factors:

i. A high degree of turbulence is needed to achieve a high flame front velocity.

Turbulence is induced by inlet flow configuration or squish. Squish can be induced inspark-ignition engines by having a bowl in piston or with a dome shaped cylinder head.

Squish is the rapid radial movement of the gas trapped in between the piston and the

cylinder head into the bowl or the dome.

ii. High volumetric efficiency, i.e., more charge during the suction stroke, results in an

increased power output. This can be achieved by providing ample clearance around the

valve heads, large diameter valves and straight passages with minimum pressure drop.

iii. Any design of the combustion chamber that improves its antiknock characteristics

permits the use of a higher compression ratio resulting in increased output and

efficiency.

iv. A compact combustion chamber reduces heat loss during combustion and increases

the thermal efficiency.


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Types of Combustion chambers

Different types of combustion

chambers have been developed over a

period of time.

T-Head Type: The T-headcombustion chambers Fig. (a) were

used the early stage of engine

development. Since the distance

across the combustion chamber is very

long, knocking tendency is high in this

type engines. This configurationprovides two valves on either side of

the cylinder. requiring two camshafts.

From the manufacturing point of view,

providing two camshafts is a

disadvantage.


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L-Head Type: A modification of the T-head

type of combustion chamber is the L-head

type which provides the two valves on the

same side of the cylinder and the valves are

operated by a single camshaft. Figures (b) &(c) show two types of this side valve engine.

In these types, it is easy to lubricate the

valve mechanism. With the detachable head

it may be noted that the cylinder head can

be removed without disturbing valve gear

etc.

In Fig. (b) the air flow has to take two right

angle turns to enter the cylinder. This causes

a loss of velocity head and a loss in

turbulence resulting in a slow combustion

process.


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The main objectives of the Ricardos turbulent head

design, Fig. (c) are to obtain fast flame speed and

reduced knock. The main body combustion chamber is

concentrated over the valves leaving a slightly

restricted passage communicating with the cylinderthereby creating additional turbulence during the

compression stroke. This design reduces the knocking

tendency by shortening the effective flame travel

length by bringing that portion of the head which lay

over the farther side of the piston into as close a

contact as possible with the piston crown, forming a

quench space. The thin layer of gas (entrappedbetween the relatively cool piston and also cooler

head) loses its heat rapidly because of large enclosing

surface thereby avoiding knocking. By placing the spark

plug in the center of the effective combustion space

slightly towards the hot exhaust valve, flame travel

length is reduced.


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I-Head Type or Overhead Valve: The I-head type is

also called overhead valve combustion chamber in

which both the valves are located on the cylinder head.

The overhead valve engine Fig. (d) is superior to a side

valve or an L-head engine at high compression ratios.Some of the important, characteristics of this type of

valve arrangement are:

i. Less surface to volume ratio and therefore less heat

loss

ii. Less flame travel length and hence greater freedom

from knock

iii. Higher volumetric efficiency from larger valves or

valve lilts

iv. Confinement of thermal failures to cylinder head by

keeping the hot exhaust valve in the head instead of

the cylinder block.


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F-Head Type: The F-head type of valve

arrangement is a compromise between L-

head and I-head types. Combustion chambers

in which one valve is in the cylinder head andthe other in the cylinder block arc known as F-

head combustion chambers Fig. (e). Modern

F-head engines have exhaust valve in the

head and inlet valve in the cylinder block. The

main disadvantage of this type is that the

inlet valve and the exhaust valve are

separately actuated by two cams mounted on

two camshafts driven by the crankshaft

through gears.


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Ignition

In SI engines, as compression ratio is lower, and the self-ignition temperature of

gasoline is higher, for igniting the mixture for the initiation of combustion an

ignition system is a must.

The electrical discharge produced between the two electrodes of a spark plug by

the ignition system starts the combustion process in a spark-ignition engine. This

takes place close to the end of the compression stroke. The high temperature

plasma kernel created by the spark, develops into a self sustaining and propagating

flame front. In this thin reaction sheet certain exothermic chemical reactions occur.

The function of the ignition system is to initiate this flame propagation process. It

must be noted that the spark is to be produced in a repeatable manner viz., cycle-by-cycle, over the range of load and speed of the engine at the appropriate

moment in the engine cycle.

By implication, ignition is merely a prerequisite for combustion. Therefore, the

study of ignition is a must to understand the phenomenon of combustion so that a

criterion may be established to decide whether ignition has occurred.


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Ignition System

In principle, a conventional ignition system should provide sufficiently large

voltage across the spark plug electrodes to effect the spark discharge. Further,

it should supply the required energy for the spark to ignite the combustible

mixture adjacent to the plug electrodes under all operating conditions. As air is a poor conductor of electricity an air gap in an electric circuit acts as a

high resistance. But when a high voltage is applied across the electrodes of a

spark plug it produces a spark across the gap. When such a spark is produced

to ignite a homogeneous air-fuel mixture in the combustion chamber of an

engine it is called the spark-ignition system.

The ignition systems are classified depending upon how the primary energy for

operating the circuit is made available as:

Battery ignition systems

Magneto ignition systems


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Requirements of an Ignition System

It should provide a good spark between the electrodes

of the plugs at the correct timing.

It should function efficiently over the entire range ofengine speed.

It should be light, effective and reliable in service.

It should be compact and easy to maintain.

It should be cheap and convenient to handle. The interference from the high voltage source should

not affect the functioning of the radio and television

receivers inside an automobile.


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Battery Ignition System

Most of the modern spark-ignition engines use battery ignition system. In this

system, the energy required for producing spark is obtained from a 6 or 12 volt

battery.

The construction of a battery ignition system is extremely varied. It depends

on the type of ignition energy storage as well as on the ignition performance

which is required by the particular engine. The reason for this is that an

ignition system is not an autonomous machine, that is, it does not operate

completely by itself, but instead it is but one part of the internal combustion

engine, the heart of the engine. It is therefore extremely important that the

ignition system be matched sufficiently well to its engine. Passenger cars, light trucks, some motorcycles and large stationary engines are

fitted with battery ignition systems. The details of the battery ignition system

of a six-cylinder engine are shown in Fig below.


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Battery Ignition System

The essential components of the system are-:

i. battery

ii. ignition switch

iii. ballast resistor

iv. ignition coil

v. contact breaker

vi. capacitor

vii. distributor

viii. spark plug


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1. Battery, 2. Ignition and starting switch, 3. Ignition Coil, 4. Ignition

distributor, 5. Ignition capacitor, 6. Contact breaker, 7. Spark plugs, and Rv

ballast resistor for boosting the starting voltage.


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Parts of a Battery Ignition System

Battery

To provide electrical energy for ignition, a storage battery is used. It is charged

by a dynamo driven by the engine.

Owing to the electro-chemical reactions, it is able to convert the chemical

energy into electrical energy. It must be mechanically strong to withstand the

strains to which it is constantly subjected to.

Two types of batteries are used for spark-ignition engines, the lead acid

battery and the alkaline battery. The former is used in light-duty commercial

vehicles and the later on heavy duty commercial vehicles.

Ignition Switch

Battery is connected to the primary winding of the ignition coil through an

ignition switch and ballast resistor.

With the help of the ignition switch the Ignition system can be turned on or

off.


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Ballast Resistor

A ballast resistor is provided in series with the primary winding to regulate the

primary current. The object of this is to prevent injury to the spark coil by

overheating if the engine should be operated for a long time at low speed, or

should be stalled with the breaker in the closed position.

This coil is made of iron wire, and iron has the property that its electrical

resistance increases very rapidly if a certain temperature is exceeded. The coil

is therefore made of wire of such size that if the primary current flows nearly

continuously, the ballast coil reaches a temperature above that where this

rapid increase in resistance occurs. This additional resistance in the primarycircuit holds the primary current down to a safe value. For starting from cold

this resistor is by-passed to allow more current to flow in the primary circuit.


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Ignition Coil

Ignition coil is the source of ignition energy in the conventional ignition system. The purpose of the

ignition coil is to step up the 6 or 12 volts of the battery to a high voltage sufficient to induce an

electric spark across the electrodes of the spark plug.

The ignition coil consists of a magnetic core of soft iron wire or sheet and two insulated

conducting coils, called primary and the secondary winding.

The secondary coil consists of about 21,000 turns of 38-40 gauge enameled copper wire

sufficiently insulated to withstand the high voltage.

The primary winding, located outside the secondary coil is generally formed of 200-300 turns of 20

gauge wire to produce a resistance of about 1.15. More heat is generated in the primary than in

the secondary and with the primary coil wound over the secondary coil, it is easier to dissipate the

heat.Contact Breaker

This is a mechanical device for making and breaking the primary circuit of the ignition coil. When

the points are closed the current flows and when they are open, the circuit is broken and the flow

of current stops.

An eight cylinder engine running at 3000 rpm requires 12000 sparks per minute, i.e. 200 sparks

per second.


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Distributor

The function of the distributor is to distribute the ignition surges to the individual spark

plugs in the correct sequence and at the correct instants of time. Depending on

whether a particular engine has 4, 6 or 8 cylinders there are 4, 6 or 8 ignition pulses

(surges) generated for every rotation of the distributor shaft. The use of a distributorrepresents a considerable simplification in a battery ignition system because in most

cases we want use only a single ignition circuit. The contact breaker and the spark

advance mechanism are combined with the distributor in a single unit because the

absolute necessity that the distributor operate in synchronism with the crankshaft.

Each of the segments of the distributor is connected to a sparking plug and as the rotor

presses it, the contact breaker opens, the high tension current is passed through therotor and brass segment through the high tension wiring to the appropriate spark plug.

Obviously, the order in which the sparking plugs are connected to the distributor head

will depend on the firing order of the engine.


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Spark Plug

The spark plug provides the two electrodes with a proper gap across which the high

potential discharges to generate a spark and ignite the combustible mixture within the

combustion chamber.

A spark plug essentially consists of a steel shell, an insulator and two electrodes. Thecentral electrode to which the high tension supply from the ignition coil is connected, is

well insulated with porcelain or other ceramic materials. The other electrode is welded

to the steel shell of the plug and thereby is automatically grounded when the plug is

installed on the cylinder head of the engine. The electrodes are usually made of high

nickel alloy to withstand the severe erosion and corrosion to which they are subjected

in use.


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IGNITION SYSTEM Spark Plug

Cap Connector

Outer Casing

Hexagon

Ceramic Body

Copper Sealing Gasket

OuterElectrode

GapSecuring Thread


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IGNITION SYSTEM Spark Plug

Centre Electrode

These surfaces must bekept clean

Seal

Change Spark Plugsat specified times

Make sure the correctSpark Plug is fitted

Cap Connector

Outer Casing

Hexagon

Ceramic Body

Copper Sealing Gasket

OuterElectrode

Securing Thread


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Battery Ignition System - Working

The source of the ignition energy in the battery ignition system is the ignition coil. This coil stores

the energy in its magnetic field and delivers it at the instant of ignition (firing point) in the form of

a surge of high voltage current (ignition pulse) through the high tension ignition cables to the

correct spark plug.

When the ignition switch is closed, the primary winding of the coil is connected to the positive

terminal post of the storage battery. If the primary circuit is closed through the breaker contacts, a

current flows, the so called primary current.

This current, flowing through the primary coil produces a magnetic field in the core. A cam driven

by the engine shaft, is arranged to open the breaker points whenever an ignition discharge is

required. When the breaker points open, the current which had been flowing through the points

now flows into the condenser, which is connected across the points. As the condenser becomes

charged, the primary current falls and the magnetic field collapses. The collapse of the fieldinduces a voltage in the primary winding, which charges the condenser to a volt much higher than

battery voltage. The condenser then discharges into the battery, reversing the direction of both

the primary current and the magnetic field. The rapid collapse and reversal of the magnetic field in

the core induce a very high voltage in the secondary winding of the ignition coil. The high

secondary voltage is led to the proper spark plug by means of a rotating switch called the

distributor, which is located in the secondary or high tension circuit the ignition system.


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Magneto Ignition System

Magneto is a special type of ignition system with its own electric generator to provide the

necessary energy for the system. It is mounted on the engine and replaces all the

components of the coil ignition system except the spark plug. A magneto when rotated by

the engine is capable of producing a very high voltage and does not need a battery as a

source of external energy. The working principle of the magneto ignition system is exactly the same as that of the coil

ignition system. With the help of a cam, the primary circuit flux is changed and a high voltage

is produced in the secondary circuit.

Since the cranking speed at start is low the current generated by the magneto is quite small.

As the engine speed increases the flow of current also increases. Thus, with magneto there is

always a starting difficulty and sometimes a separate battery is needed for starting. Themagneto is best at high speeds and therefore is widely used for sports and racing cars,

aircraft engines etc.

In comparison, the battery ignition system is more expensive but highly reliable. Because of

the poor starting characteristics of the magneto system invariably the battery ignition system

is preferred to the magneto system in automobile engines. However, in two wheelers

magneto ignition system is favored due to light weight and less maintenance.

Magneto Operation


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IGNITION SYSTEMIGNITION SYSTEM

g p

Coil

Soft IronCore

SecondaryWindings

Primary

Windings

EngineDriven Rotorand Magnets


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S N

IGNITION SYSTEM Faraday Law

S NS NS NS NS NS NS N

Principal -

Each time the magnetic field washes through wires

VoltMeter

An electrical current is produced

Called an EMF Electro Motive Force

CoilWindings

Magneto Operation


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IGNITION SYSTEMIGNITION SYSTEM

The magneticfield passesthrough softiron core

EMF produced inthe coil windings


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IGNITION SYSTEM

No magneticfield in soft ironcore

No EMF producedin the coil windings


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IGNITION SYSTEM

The magneticfield passesthrough softiron core again

EMF produced inthe coil windings

again

S k G ti

Magneto Unit Rotor Arm


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IGNITION SYSTEM Magneto System

IgnitionSwitch

Distribution

ContactBreaker

Coil

Magneto

Condenser

PowerGeneration

Spark Generation


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Firing Order

The order in which various cylinders of a multi-

cylinder engine fire is called the firing order.

There are three factors which must beconsidered before deciding the optimum firing

order of an engine.

i. Engine Vibrationsii. Engine Cooling

iii. Development of Back Pressure


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Firing Order Engine Vibrations

Engine Vibrations

Consider Cylinder -1 fires first. Pressure, p, generated in cylinder -1 will give rise to

force equal to pA[b/(a+b)] & pA[a/(a+b)] on the two bearings A & B respectively.

Load on bearing A would be much more than bearing B.

If the next cylinder fired is cylinder -2, this imbalance in load on the two bearingswould aggregate the problem of balancing of the crankshaft vibrations.

Firing cylinder -3 after Cylinder -1 the load may be more or less evenly distributed.


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Firing Order Engine Cooling

Engine Cooling

On firing Cylinder -1 its temperature increases. If next cylinder

fired is no. 2 , the portion of the engine between the cylinder no.

1 & 2 gets overheated. If then the 3rd cylinder is fired the overheating is shifted between

cylinder no. 2 & 4.

The task of cooling system becomes very difficult because it is

then, required to cool more at one place than at other places andthis imposes great strain on the cooling system.

If the third cylinder is fired after the first the overheating problem

can be controlled to a greater extent.


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Firing Order Back Pressure Development

Development of Back Pressure

After firing the first cylinder, exhaust gases flow out to the exhaust pipe.

If the next cylinder fired is cylinder no. 2, we find that before the gases

exhausted by the first cylinder go out of the exhaust pipe the gasesexhausted from the second cylinder try to overtake them. This require

exhaust pipe to be bigger.

Otherwise the back pressure in it would increase and the possibility of

back flow would arise.

It instead of firing cylinder no 2 cylinder no 3 is fired then by the timethe gases exhausted by the cylinder 3 come into the exhaust pipe, the

gases from cylinder 1 would have sufficient time to travel the distance

between cylinder 1 and cylinder 3 and thus the development of a high

back pressure is avoided.


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Firing Order 4 & 6 cylinder engines

For a four cylinder engine the possible firing

orders are:

1-3-4-2 or 1-2-4-3

For a six cylinder engine the possible firing

orders are:1-5-3-6-2-4 or 1-5-4-6-2-3 or 1-2-4-6-5-3 or

1-2-3-6-5-4

Ignition Timing


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g gHow early or late the spark plug fires in relation to the position of theengine piston.

Ignition timing must change with the changes in engine speed, load,and temperature.

Ignition Timing


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g g

Timing Advance occurs when the plug fires sooner on compressionstroke (High engine speed)

Timing Retard occurs when plug fires later on compression stroke(Lower engine speed)

BASE TIMING Timing without vacuum or computer control.

Methods of controlling Timing


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g gDistributor Centrifugal Advance

Controlled by engine speed.

Consists of two weights and two springs.

At high speeds the weights fly out(held by the springs), rotating thecam, hence advancing the timing.



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g g

Vacuum Advance

Controlled by engine intake manifold vacuum and engine load.

The vacuum diaphragm rotates the pickup coil against the directionof distributor shaft rotation.



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g gElectronic Advance Sensors input influences the ignition timing.

Crank shaft Position Sensor (RPM)

Cam Position Sensor (tells whichcylinder is on compression stroke)

Manifold Absolute Pressure (MAP)(engine vacuum and load)

http://images.google.com/imgres?imgurl=http://www.cranecams.com/images/downloadable/FireBall_Ignition/MAP%20Sensor%20(PN%2090000110).jpg&imgrefurl=http://www.cranecams.com/?show=imageDownload&dir=FireBall_Ignition%2F&h=684&w=900&sz=216&hl=en&start=10&tbnid=Zf7R0KXDc7phVM:&tbnh=111&tbnw=146&prev=/images?q=MAP+sensor&ndsp=20&svnum=10&hl=en&rlz=1T4GFRC_enCA204CA205&sa=N


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g gElectronic Advance Sensors input influences the ignition timing.

Intake Air Temperature Sensor

Knock Sensor (Retards timing when pinging

or knocking is sensed)

Throttle Position Sensor(TPS)

Engine coolant Temperature
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Documents

ICE Notes (Unit 2)