ice notes (unit 2)
TRANSCRIPT
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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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Air Fuel Mixtures (for different throttle operations)
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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Air Fuel Mixtures (for different throttle operations)
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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Stages of Combustion in SI engine
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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Stages of Combustion in SI engine
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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Stages of Combustion in SI engine
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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Factors influencing the flame speed
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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Factors influencing the flame speed
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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Factors influencing the flame speed
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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Phenomenon of knock in SI engines
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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Phenomenon of knock in SI engines
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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Phenomenon of knock in SI engines
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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Phenomenon of knock in SI engines
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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Combustion Chamber Design
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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Combustion Chamber Design
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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Types of Combustion chambers
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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Types of Combustion chambers
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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Types of Combustion chambers
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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Types of Combustion chambers
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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Parts of a Battery Ignition System
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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Parts of a Battery Ignition System
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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Parts of a Battery Ignition System
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Parts of a Battery Ignition System
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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Parts of a Battery Ignition System
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.
Methods of controlling 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.
Methods of controlling Timing
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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)
Methods of controlling Timing
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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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