I c engines a study

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Module I
Inventions
Spark ignition Engines → Nicolas A. Otto (1876)
Compression Ignition Engines → Rudolf Diesel (1892)
Two Stroke Engines → Duglad Clark (1878)
INTRODUCTION
Heat engines absorb energy in the form of heat and convert part of it into mechanical
energy and deliver it as work, the balance being rejected as heat. These devices
derive the heat energy from the combustion of a fuel. Based on the location of the
combustion process, heat engines are classified into internal combustion and external
combustion engines.
Internal combustion engines (IC engines) are those where the combustion of the fuel
takes place inside the engines – eg. automobile engines. In the case of external
combustion engines, combustion of fuel occurs outside the engines and the working
gas so heated is then admitted into the engines for conversion and work extraction –
eg. steam generated in a boiler is then admitted to steam engines for producing work.
Classification of I C Engines:-
i) On the basis of Basic engine design:-
(1) Reciprocating
(2) Rotary (Wankel)
(ii) On the basis of Working cycle:-
(1) Otto cycle (SI Engine)
(2) Diesel cycle (C I Engine)
(iii) On the basis of Strokes:-
(1) Four stroke Engine
(2) Two stroke Engine

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(iv) On the basis of Fuel:-
(1) Petrol
(2) Diesel, CNG& LPG
(v) On the basis of Fuel supply:-
(1) Carbureted types
(2) Injection types
(vi) On the basis of Ignition:-
(1) Battery ignition
(2) Magneto ignition
(vii) On the basis of Cooling Method:-
(1) Water cooled
(2) Air cooled
(viii) On the basis of cylinder arrangement:-
(1) In line Engine
(2) V Engine
(3) Radial Engine etc.

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(ix) On the basis of valve location:-
(1) Overhead valve
(2) Side valve
(x) On the basis of Application:-
(1) Automobile engines
(2) Marine engines
(3) Aircraft engines
(4) Industrial engines
Parts of an IC engine
The main components of a standard IC engine are briefly described below:

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1. Cylinder head. This is the top cover of the cylinder and holds the inlet and
exhaust valves, their operating mechanisms, and the spark plug or fuel injector, as
the case may be. The valves along with their operating mechanism are together
called the valve gear.
2. Cylinder block and cylinder liner. The cylinder head is fitted over the cylinder
block and liner. The space between the block wall and cylinder liner acts as the
cooling water jacket.
3. Piston. The piston is of cylindrical shape to fit the inside bore of the cylinder. Gas
tightness is ensured by means of the piston rings in the slots on the outer cylindrical
surface of the piston.
4. Connecting rod. This is the link connecting the piston to the crankshaft for
transmission of the forces from and to the piston. The pin connecting it to the piston
is called the gudgeon pin and that connecting it to the crankshaft as the crank pin.
5. Crankshaft. This is a shaft with radial cranks, which converts the reciprocating
motion of the piston into rotary motion of the shaft.

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6. Crank case and sump. Crank case is the engine casing having the main bearings
in which the crank shaft rotates. The bottom cover of the engine is the sump which
usually acts as a lubricating oil reservoir.
Nomenclature of I C Engines
1) Cylinder bore (D):- The nominal inner diameter of the working cylinder.
2) Piston area (A):- Cross sectional area of the piston. This is equal to cylinder bore
area
3) Stroke (L):- The nominal distance between TDC & BDC
4) Dead Center: - End points of the strokes
(i) Top dead center (TDC):- Farthest position of piston from crank shaft. It is also
called, Inner Dead Center (IDC)
(ii) Bottom Dead Center (BDC):- Nearest position of piston form crank shaft. It is
also called Outer Dead Center (ODC)
5) Swept Volume (Vs) :- The nominal volume generated by the piston when
travelling from one dead center to next. i.e., TDC to BDC ,
Vs = A×L
6) Clearance Volume (Vc):- The nominal volume or volume for combustion, which
is just above the TDC.

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7) Cylinder Volume (V) :- The sum of swept volume and clearance volume.
V = Vs + Vc
8) Compression ratio (r) :- Ratio of cylinder volume to clearance volume;
V
r
Vc

Four Stroke I C Engines
In a four-stroke engine, the cycle of operations is completed in four strokes of the
piston or two revolutions of the crankshaft. During the four strokes, there are five
events to be Completed, viz., suction, compression, combustion, expansion and
exhaust. Each stroke consists of 180° of crankshaft rotation and hence a four-stroke
cycle is completed through 720° of crank rotation. The cycle of operation for an ideal
four-stroke SI engine consists of the following four strokes:
1. Suction Stroke (0 -180°)
2. Compression Stroke (180°-360°)
3. Expansion Stroke (360°-540°)
4. Exhaust Stroke (540°-720°)
Working principle of a Four Stroke SI Engine
Suction or Intake Stroke: Suction stroke starts when the piston is at the top dead
centre and about to move downwards. The inlet valve is open at this time and the
exhaust valve is closed. Due to the suction created by the motion of the piston
towards the bottom dead centre, the charge consisting of fuel-air mixture is drawn
into the cylinder. When the piston reaches the bottom dead centre the suction stroke
ends and the inlet valve closes. The charge taken into the cylinder during the suction
stroke is compressed by the return stroke of the piston. During this stroke both inlet
and exhaust valves are in closed position. The mixture that fills the entire cylinder
volume is now compressed into the clearance volume. At the end of the compression
stroke the mixture is ignited with the help of a spark plug located on the cylinder
head. In ideal engines it is assumed that burning takes place instantaneously when
the piston is at the top dead centre and hence the burning process can be

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approximated as heat addition at constant volume. During the burning process the
chemical energy of the fuel is converted into heat energy producing a temperature
rise of about 2000 °C
The pressure at the end of the combustion process is considerably increased due to
the heat release from the fuel. At the end of the expansion stroke the exhaust valve
opens and the inlet valve remains closed. The pressure falls to atmospheric level a
part of the burnt gases escape. The piston starts moving from the bottom dead centre
to top dead centre and sweeps the burnt gases out from the cylinder almost at
atmospheric pressure. The exhaust valve closes when the piston reaches T DC. At the
end of the exhaust stroke and some residual gases trapped in the clearance volume
remain in the cylinder. These residual gases mix with the fresh charge coming in
during the following cycle, forming its working fluid. Each cylinder of a four stroke
engine completes the above four operations in two engine revolutions, one revolution
of the crankshaft occurs during the suction and compression strokes and the second
revolution during the power and exhaust strokes. Thus for one complete cycle there
is only one power stroke while the crankshaft turns by two revolutions. For getting
higher output from the engine the heat release should be as high as possible and the
heat rejection should be as small as possible.
Ideal P-V Diagram of Four Stroke S I Engine
Four Stroke C I Engine:-
The four-stroke CI engine is similar to the four-stroke SI engine but it operates at a
much higher compression ratio. The compression ratio of an SI engine is between 6
and 10 while for a CI engine it is from 16 to 20. In the CI engine during suction
stroke, air, instead of a fuel-air mixture, is inducted. Due to the high compression

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ratio employed, the temperature at the end of the compression stroke is sufficiently
high to self ignite the fuel which is injected into the combustion chamber. In CI
engines, a high pressure fuel pump and an injector are provided to inject the fuel into
the combustion chamber. The carburetor and ignition system necessary in the SI
engine are not required in the CI engine.
The ideal sequence of operations for the four-stroke CI engine is as follows:
i. Suction Stroke: Air alone is inducted during the suction stroke. During this stroke
intake valve is open and exhaust valve is closed.
ii. Compression Stroke: Air inducted during the suction stroke is compressed into the
clearance volume. Both valves remain closed during this stroke.
iii. Expansion Stroke: Fuel injection starts nearly at the end of the compression
stroke. The rate of injection is such that combustion maintains the pressure constant
in spite of the piston movement on its expansion stroke increasing the volume. Heat
is assumed to have been added at constant pressure. After the injection of fuel is
completed (i.e. after cutoff) the products of combustion expand. Both the valves
remain closed during the expansion stroke.
iv. Exhaust Stroke: The piston traveling from EDC to TDC pushes out the products
of combustion. The exhaust valve is open and the intake valve is closed during this
stroke.
Ideal P-V Diagram of Four Stroke C I Engine
Comparison of S I and C I Engine
1. Basis of Cycle Otto Cycle
Constant Volume heat
Diesel Cycle
Constant pressure heat

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addition addition
2. Fuel highly volatile non-volatile
3. Introduction of fuel air + fuel introduced
into the cylinder
only air introduced into
the cylinder
4. Ignition Spark plug Self ignition due to high
temperature
5. Compression ratio 6 - 10
Bikes ,cars
16 - 20
Diesel cars & trucks
6.Speed Due to light weight, they
are high speed engine
low speed engines
7. ηth Because of lower CR ηth
is lower th r 1
1
r 
 
ηth is higher or
 
c
r 1
c
r 11
1
r r 1r 
 
  
  
8. Weight lower peak pressure,
engines are lighter
Heavier
Actual indicating diagram of S I Engine

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

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During the expansion stroke the charge in the crankcase is compressed. Near the end
of the expansion stroke, the piston uncovers the exhaust ports and the cylinder
pressure drops to atmospheric pressure as the combustion products leave the
cylinder. Further movement of the piston uncovers the transfer ports, permitting the
slightly compressed charge in the crankcase to enter the engine cylinder.
The top of the piston has usually a projection to deflect the fresh charge towards the
top of the cylinder before flowing to the exhaust ports. This serves the double
purpose of scavenging the upper part of the cylinder of the combustion products and
preventing the fresh charge from flowing directly to the exhaust ports.
Advantages of two-stroke engines
1. A two-stroke engine has a power stroke every revolution of the crankshaft.
Therefore its power to weight ratio is higher than that of a four-stroke engine.
2. The torque is more uniform in a two-stroke engine, hence it requires a lighter
flywheel than that for a four-stroke engine.
3. Two-stroke engines are simpler in construction than four-stroke engines due
to the absence of valves and their operating mechanism.
4. The friction loss is less in two-stroke engines, therefore it gives higher
mechanical efficiency than four-stroke engines.

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5. The capital cost of two-stroke engines is less than that of four-stroke engines.
6. The starting of two-stroke engines is easier than starting of four-stroke
engines.
Disadvantages of two-stroke engines
1. The overall efficiency is less than that of four-stroke engines due to (i)
inadequate scavenging as some combustion products are left in the cylinder (ii)
loss of fresh charge during scavenging, and (iii) less effective compression ratio
for same stroke long.
2. The engine is always overheated due to power stroke in every revolution.
3. The consumption of lubricating oil is higher as it is subjected to higher
temperatures.
4. The exhaust of two-stroke engines is noisier needing more baffling in the
silencers.
Internal combustion Engines
Advantages:-
- Greater thermal efficiency .
- Lower weight to output ratio.
- Lower initial cost.
- Compact and most suitable for portable applications.
- Lesser cooling requirements.
Parts
Cylinders → cast iron, alloy steel
Cylinder head → cast iron, aluminium alloy
Piston → cast iron, aluminium alloy
Piston rings → silicon, cast iron
Judger pin → steel
Valves → specially alloy steels

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Connecting rod → steel
Crank shaft → alloy steel
Crank case → steel, cast iron
Cylinder timer → nickel alloy steel, cast iron
Bearing → white metal
Valve Timing diagram
Actual valve timing of 4 stroke petrol engine:-
Valve timing is the regulation of the points in the cycle at which the valves are set
to open and close. In ideal cycle inlet and outlet valves are open and close at dead
centers, but in actual cycle they open and close before and after dead centers.
Reasons for actual valve timing:-
(1) Mechanical Factor: - valves cannot be closed and opened abruptly because they
are operated by cams. It can left the tappet slowly. (gradual lifting). So that the
opening of the valve must commence ahead of the time. (designed dead center)
(2) Dynamic Factor: - actual valve timing is set taking into considering the dynamic
effects of gas flow.
Intake valve timing:-
As the piston moves out in the suction stroke, the fresh charge is drawn in through
the intake valve, when the piston reaches the BDC and starts to move in the
compression stroke, the inertia of the entering fresh tends to cause it to continue to
move into cylinder. To take this advantage, inlet valve is closed after TDC so that
maximum air is taken in. This is called ram effect.
Exhaust valve timing:-
Opening of exhaust valve earlier reduces the pressure near the end of the
power stroke and thus causes some loss of useful work on this stroke. But it results in
overall gain in output.

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Valve overlap
A period when both the intake and exhaust valves are open at the same time. 15o
for
low speed 30o
for high speed. This overlap should not be excessive otherwise it will
allow the banned gases to be sucked into the intake manifold, or the fresh charge to
escape through exhaust valve.
Valve timing of four stroke spark ignition engines
Advantages of actual valve timing
(1) In creasing the volume efficiency because IVO 10o
before TDC, so that more
amount of fresh charge is entering to the cylinder.
(2) Increasing the amount of air inside cylinder by ram effect.
(3) Reduce the work required to expel the gas as EVO 45o
before BDC
(4) Increase the scavenging effect since EVO after 10o
from TDC.
Comparison of Four stroke and two stroke cycle engines
1. The cycle is completed in Four stroke
of the piston or two revolutions. i.e., one
power stroke is obtained in every two
revolutions.
1. The cycle is completed in two-strokes
of the piston or in one revolution of
crankshaft. i.e., one power stroke is
obtained in one revolution of crank shaft.

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2. Turning moment is not so uniform and
hence heavier flywheel is needed.
3. Power produced for same size of
engine is small.
4. Four stroke engine contains valves and
valve mechanisms.
5. Heavy weight and complication of
valve mechanism.
6. Volumetric efficiency more due to
greater time of induction. (one stroke for
suction stroke)
7. Thermal efficiency high.
8. Cars, buses, trucks, industries etc.
2. More uniform turning movements and
hence lighter flywheel is needed.
3. Power produced for same size of
engine is more (theoriticaly twice,
actually about 1.3 times)
4. No valves but only ports.
5. Light weight and simplicity due to the
absence of valve mechanism.
6. Less volumetric efficiency due to
lesser time for induction.
7. Thermal efficiency lower
8. Compact
scooters, bikes etc (petrol)
Two-stroke diesel engines used in very
large sizes, more than 60 cm base. (ship)
because low weight and compactness.
eg:- Marine Engine, Fork lift etc.
FUELS
Most common hydrocarbon fuels are Alkyl Compounds and are grouped as:
Paraffins - Paraffins are straight chained hydrocarbons, also called alkanes. Some
examples are propane and butane. Isoparaffins have a branched chain structure.
Aromatics - Aromatics are high octane blending hydrocarbons that have a benzene
ring in their molecular structure. Examples are benzene, toluene, xylene.

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Olefins - Olefins are gasoline hydrocarbons resulting from several refining
processes. Examples are ethylene, propylene, butylene. Olefins often contribute to
the formation of gum and deposits in engines and the induction system. Olefins are
also called alkenes.
Fuel Types
Gasoline and diesel fuel are both produced from crude oil. Together, gasoline and
diesel fuel power approximately 99% of the motor vehicle fleet. However,
alternative fuels are being used more and more to reduce vehicle emissions.
Indolene - Indolene is used as the standard gasoline emission test fuel for spark
ignition engines. Indolene is a well refined gasoline with low levels of sulfur,
phosphorus, and vapor pressure.
Diesel Fuel - The diesel fuel is commonly used in relatively large displacement
compression ignition engines. Diesel fuel is used in a broader range of engine sizes
in Europe and other areas of the world. The average molecular weight and boiling
point of diesel fuel is greater than that for gasoline, which makes it suitable for use in
compression ignition engines, characterized by higher in-cylinder temperatures and
pressures.
Compressed Natural Gas - Compressed natural gas (CNG) is comprised primarily
of methane (CH4). CNG vehicles generally produce lower emissions than their
gasoline counterparts. However, there are tradeoffs in engine power and efficiency.
Methanol (CH3OH) - Methanol is a promising alternative fuel because it generally
produces lower tailpipe emissions than gasoline and can be manufactured at prices
comparable to gasoline. A blend of 85% methanol and 15% unleaded gasoline (M85)
is typically used. However, M85 vehicles are virtually phased out of new vehicle
manufacture in Brazil. Vehicles that operate on methanol consume more fuel than if
they were operating on 100% gasoline because its energy content (calorific value) is
less.
Ethanol:

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Ethanol is an important component of automotive fuel used in Brazil. A mixture of
22% ethanol with gasoline (E22) is commonly used. Ethanol is also used in the USA
as an octane enhancer for gasoline (up to 10%). It is also used for flexible fuel
vehicles as a blend of 85% ethanol and 15% unleaded gasoline. Ethanol is produced
from corn, sugar cane or other crops but is currently more expensive than gasoline.
WANKEL ENGINE
Dr. Felix Wankel was the founder of the first successful rotary engine. He was
invented in 1957. The engine has a three lobe rotor which is driven eccentrically in a
casing in such a way that there are thrice separate volumes trapped between the rotor
and the casing. These three volumes perform induction, compression, combustion,
expansion and exhaust process in sequence. Sealing, seal wear ad heat transfer were
some of the development problems of Wankel engines.
The reciprocating piston has been replaced by a triangular-shaped rotor. With on
complete revolution of the rotor the power pulses will occur. There are three
complete four-stroke cycles (revolutions of a rotor). The gear ratios are such that the
output shaft rotates at three times the speed of rotor.
-Passenger cars are manufactured by Mazda, Japan & Rolls Royees Ltd.
- Compression ratio is 18.

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Advantages:-
(1) Power output/weight ratio is higher because of its compactness.
(2) Simple in design - no valve problems.
(3) No. of parts is much less than a conventional four stroke S I engine. Therefore it
is less costly.
(4) Mechanical efficiency is better because of lower frictional losses.
Disadvantages:-
(1) The engine has lower efficiency because higher heat transfer rate.
(2) Exhaust emissions are higher because of poor combustion chamber shape.
(3) There may be starting trouble.
(4) Efficient operation of the engine requires efficient seal between two sides of the
rotor and its casing.
(5) The spark plug life is short without effective cooling.
Stirling Engine:-
The basic components of the stirling engine is cylinder - and piston mechanical
arrangement and a heat source that is external to the cylinder.
Robert Striling developed the original engine in 1816. The engine is quite costly
because of the complexity of rhombic drive and the heat exchanges elements.
It consists of two reversible isothermal process and two constant vol.
processes. The total quantity of heat received from the external sources is supplied
isothermally at temperature T1. The heat is rejected to the sink isothermally at
temperature T2. The regenerator issued for reversible heat transfer to and from the
working fluid during the constant volume process. The thermal efficiency of stirling
cycle, 2
1
T
1
T
 
   
 
The two pistons-power piston and displacer piston have coaxial rods connected to
different point of the rhombic drive. The loosely fitted displacer divides the enclosed
vol. into two main regions- the expansion space and compression space. The closed

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system is changed with a permanent gas, preferably hydrogen which is considered to
be the most suitable working fluid for stirling engine.
When the displacer is at the top, all gases lie in the cool space between two pistons.
The power piston from its lowest position moves up and the gas is compressed at
constant temperature. Then the displacer moves down forcing the gas to move from
cold space through the regenerator into hot space. The net effect of heating the gas is
the rapid development of a higher gas pressure in the expansion space. The heated
gas expands and the power piston moves downwards. After the power stroke, the
displacer returns to the top piston and the hot gases return to the cool space, through
the regenerator and the cycle repeats.
Advantages
- Thermal efficiency - 35% - 45% - better than SI engine.
- Multi fuel capacity
- solar energy also can be used for thermal engine
- lower exhaust emission
- low noise and smooth operation.
- no lubrication needed.
Disadvantages
- Big radiator , about 2.5 time the size of normal one.
- Complex design
Stratified charge Engine
This is a modified SI engine. This engine gives lower exhaust emissions and better

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fuel economy than the convention homogeneous change engines. Hony m Ricardo
modified the SI Engine into stratified one.
- Stratification of the charge mixture means providing different rich and lean fuel
ratios.
- Relatively rich air fuel ratio in the vicinity of the ignition source and a leaner
mixture in the rest of the combustion chamber. The whole mixture is distributed in
“stratas” or “layer” of different air fuel mixture strength.
- Relatively high compression ratio.
- Ability of direct cylinder fuel injection.
Advantages
1. A stratified charge obtained by injecting fuel late in the compression stroke,
decreases knocking.
2. Low octane fuels (cheaper fuels) can be used a high compression ratios.
3. Load control can be achieved with out air throttling.
4. Fuel economy at part load is excellent.
5. Quiet in operation.
Disadvantages
1. Maximum output is not achieved (complete utilization of air is not possible)
2. The operational speed range is less compared to conventional SI engine.
3. Cost is high for modified combustion system.
4. Added complication of injection and spart ignition system.
Free Stratified Combustion Chamber
This chamber uses a shrouded intake valve. This causes an swirl around the cylinder
axis. The fuel is sprayed slightly ahead of the spark plug. The air swirl moves the air
fuel vapour mixture towards the spark plug. The flame front is established between
the spark plug and fuel spray. Mixture stratification by this method results in low
specific fuel consumption at past loads.

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Pre Chamber Stratified Charge Engine
The ford motor company has designed and developed a naturally aspirated torch pre
chamber. This chamber is formed by dividing the combustion chamber into main ad
auxiliary section with an orifice in between them. The auxiliary chamber serves as
the torch or pre chamber. During engine operation the inlet mixture introduced into
the cylinder. The movement of the piston compresses the mixture and forces part of
it through the orifice into auxiliary chamber. This creates a great deal of turbulence
with in auxiliary chamber. The mixture is then ignited by the spark produced in
auxiliary chamber. Because of the turbulence, the air fuel mixture in the auxiliary
chamber burns very quickly. This causes a very rapid rise in pressure in chamber.
The gases rapidly expand through the orifice and generates turbulence in the main
chamber. The ejection hot gases initiates the combustion of mixture in the main
chamber.
- Pre chamber volume is 8-15% of total chamber volume.
Variable Compression Ratio Engine
One method of solving the high peak pressure problem encountered
when the specific output is increased is to reduce the compression ratio at full load
but at the same time keeping the compression ratio sufficiently high for good starting
and part load operation. The new development for solving this, Variable
Compression Ratio (VCR) is developed. Diesel Engines are more suitable for VCR
Engines.
In the VCR engine a high compression ratio is used for good stability and low load
operation and a low compression ratio is used at full load.
The VCR piston, was developed by British Internal Combustion Engines Research
Institute (BICERI) in collaboration with Continental aviation and Engineering
Corporation. The AVCR - 1100 Engine is used in Main Battle Tank.
It consists of two main pieces A and B called shell and the carrier respectively. The
carrier is mounted on a gudgeon pin in the conventional manner while the shell A
slides over the carrier B to vary the clearance volume. These two parts of the piston
are so arranged that two chambers C and D are formed between them which are kept

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full of lubricating oil supplied via a hole in the connecting rod and non returns valve
F from the lubricating system.
The gas load is carried by the oil in the upper chamber C. With the increase in load
the gas pressure is increased to a pre-set valve, the spring loaded relief valve „L‟
opens and discharges oil to the main sump. The piston shell slides down to a position
decided by the relationship between the oil pressures in two chambers and the
cylinder gas pressure. And thus a change in compression ratio is affected.
Advantages
- high power output compactness
- lower thermal and structured loads.
- high specific output
- thermal efficiency. reduces
- good cold starting & idling performance
- multi fuel capacity
Free Piston Engines
The total unit consists of a reciprocating compressor, reciprocating engine and the
turbine. The air density is increased by compressing the air prior to the engine
entrance with the help of a reciprocating compressor. The power required to drive the
compressor can be obtained by the reciprocating engine. If the reciprocating engine
simply drives the compressor and produces high temperature and high pressure gas
for the turbine, it is known as a gas-generator and the total unit is called “Free-piston
Engine”. Advantage is the high thermal efficiency of reciprocating engine and high
power/weight ratio of the turbine. But the fixed cost for the entire set up is high.

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The main elements of the engine are a pair of free floating pistons arranged
in such a way that it forms an opposed piston engines, the compressor is directly
attached to the diesel engine pistons with necessary bounce chamber. (Bounce
chamber air is compressed between them as they move together) and the injector.
When the engine cylinder, produces power in power stroke, the compressor piston
moves to the inner position. The compressor acts as a scavenging pump for the
cylinders, and the exhaust gases are supplied to the power turbine through a suitable
duct. The turbine delivers the useful work. This entire system is quite large & heavy.
Advantages
1. Since opposed - piston engines are perfectly balanced problems of balancing does
not arise.
2. Lubricating mechanism & vibration controls are simpler than conventional
reciprocating engine.
3. A single power turbine can be connected to a number of gas generator units
arrange in parallel, it can use low ignition-quality fuels than the ordinary one because
the compression stroke inside the engine continues till the ignition occurs.
Disadvantages
1. The gas power turbine running cost is high.
2. The system has poor fuel economy.

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3. System is very inefficient in part load especially at light loads. (Fuel consumption
is very high)
4. Entire system is large & heavy.
Fuel - Air Cycle
In the air cycle approximation it was assumed that the working fluid is nothing but
air and this was a perfect gas and had constant specific heats. In actual engine the
working fluid is not air but a mixture of air, fuel and residual gases, that means the
specific heats of the working fluids are not constant but increases as temperature
rises, and finally, the products of combustion are subjected to dissociation at high
temperatures.
F - A Cycle
Fuel-air cycle approximation represents a nearly attainable idea with actual
performance. The fuel air cycle calculation takes into consideration the following:
1. The actual composition of the cylinder gases, i.e., (Fuel + air + water vapour in air
+ residual gas). Fuel-air ratio is changed during the operation of engine.
2. The variation in the specific heat of these gases with temperature. CP & CV
increases with temperature except for monoatomic gases. ( γ also changes with
temperature)
3. The fact that the fuel-air mixture does not completely combine chemically at high
temperature.
4. The number of molecules present after combustion depends upon Fuel-air ratio
and upon the pressure & temperature after the combustion.
Assumptions
1. There is no chemical change in either fuel or air prior to combustion.
2. The change is always in chemical equilibrium.
3. There is no heat exchange between the gases and the cylinder walls in any
process. i.e. isentropic
4. For reciprocating engine the velocities are negligibly small.

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Use of fuel air cycle
The air standard cycle shows the general effect of only CR on engine efficiency
where as Fuel-air cycle may be calculated for various F/A ratios are very important
of engine.
“The actual efficiency of a good engine is about 85% of the fuel air cycle efficiency
i.e. a very good estimate of the power to be expected from the actual engine can be
made from Fuel-air cycle analysis. Also, peak pressure and exhaust temperatures can
be very closely approximated.
Variation of Specific Heat
All gases except mono atomic gases, show an increase in specific heat at high
temperature. This increase in specific heat does not follow any particular law. The
specific heat curve is nearly on straight line which may be approximately expressed
in the form
Cp = a + KT,
Cv = b + KT
where a, b & K are constants.
At higher temperatures specific heat increases rapidly and may be approximately
expressed in the form
Cp = a + k1T + K2T2
,
Cv = b + k1T + K2T2
The physical explanation of increase in specific heat is that as the temperature is
raised, larger and larger fractions of the heat input go to produce - motion of the
“within” the molecules. Same heat energy is goes to moving the atoms so that more
heat is required to raise the temperature of unit mass through are degree.
For air Cp = 1.005 at 0o
C and 1.264 at 2000o
C
Dissociation or chemical equilibrium loss:
Dissociation is the name given to the disintegration of burnt gases at high
temperature. Dissociation, as against decomposition, is a reversible process.

27
Dissociation increases with temperature. During dissociation a considerable amount
of heat is absorbed. This heat will be liberated when the elements recombines the
temperature falls. Thus the general effect of dissociation is a suppression of a part of
the heat during combustion period and the liberation of it as expansion proceeds.
The effect of dissociation is much smaller than that of change of specific heat.
The dissociation mainly is of C02 into CO.
(1) 2CO2 + heat  2CO + O2
The dissociation of CO2 commences at about 1000o
C and at 1500o
C it amounts to
1%.
(2) 2H2O + heat  2H2 + O2
Though during recombination the heat is given back, but it is too late and some of the
heat given back is lost in the exhaust. With no dissociation maximum temperature is
attained with correct mixture strength. With dissociation maximum temperature is
obtained when mixture is about 10% rich. Dissociation reduces the maximum
temperature by about 300o
C at correct mixture strength. If there is no dissociation the
b.p is maximum when the mixture strength is chemically correct. Shaded area shows
the loss of power. When the mixture is lesser there is no dissociation. As the mixture
becomes rich dissociation effect commences to decline due to the increased quantity
of CO.
ACTUAL CYCLES
The Major factors causing the difference between a real cycle and its equivalent F-A
cycles are
(i) The time loss factor :- loss due to time required for mixing of fuel and air and
also for combustion.
(ii) Heat loss factor :- loss of heat from gases to cylinder walls.
(iii) Exhaust blow down factor :- loss of work on the expansion stroke due to early
opening of the exhaust valve.
(1) Time loss factor
In air-standard cycles the heat addition is assumed to be an instantaneous process

28
where as in an actual cycle it is over a definite period of time. Some change in
volume takes place during the combustion process. The time required for the
combustion is more. The crank shaft will usually turn about 30o
- 40o
between the
initiation of spark and the end of combustion. There will be a time loss during this
period and is called time loss factor.
(2) Heat loss factor
During the combustion process and the subsequent expansion stroke the heat flows
from the cylinder gases through the cylinder walls and cylinder head into the water
jacket or cooling fins. Some heat is carried away by the lubricating oil which
splashes on the under side of the piston.
(3) Exhaust blow down
The cylinder pressure at the end of the exhaust stroke is about 7 bas(depends). If the
exhaust valve is opened at the bottom dead center, the piston has to do work against
high cylinder pressure during the early part of the exhaust stroke. If the exhaust valve
is opened too early, a part of expansion stroke is lost. So we set 40o
-70o
opening of
exhaust valve- before BDC.

29
Module 2
CARBURETION
In the SI engine a combustible fuel-air mixture is prepared outside the engine
cylinder. The process of preparing this mixture is called “carburetion”. The
carburetor is a device which atomizes the fuel and mixes it with air and is most
important part of the induction system. The pipe that carries the prepared mixture to
the engine cylinder is called the intake manifold.
During suction stroke vacuum is created in the cylinder which causes the air to flow
through the carburetor and the fuel to be sprayed form the fuel jets. Because of the
volatility of the fuel, most of the fuel vaporizes and forms a combustible fuel-air
mixture. However, some of the larger droplets may reach the cylinder in the liquid
form and must be vaporized and mixed with air during the compression stroke before
ignition by the electric spark.
Four important factors which significantly affect the process of combustion are:
1. The time available for the preparation of the mixture.
2. The temperature of the incoming air of the intake manifold.
3. The quality of the fuel supplied.
4. The design of the induction system and combustion chamber.
Properties of the air-fuel mixtures
Range of air-fuel ratios = 7: 1 to 20 : 1
(1) Mixture requirement for maximum power
- Maximum power is obtained at about 12.5: 1 A/F
- Maximum energy is released when slightly excess fuel is introduced so that the
oxygen present in t he cylinder is utilized.
Disadvantage is partial combustion & less energy release.
(2) Mixture requirement for maximum specific fuel consumption
- Maximum efficiency occurs at as A/C of about 17:1

30
-Maximum efficiency occurs at a point slightly leaves than the chemically correct
A/F ratio because excess air requires complete combustion of fuel when mixing is
not perfect.
A/P ratio (mass) Designation Power output Specific fuel consumption
18 - 22
16 - 18
15 approx.
12 - 14
Very weak
Weak
Chemically correct
Rich
Very Rich
40 % less
10% less
4% less
Max. Power
20 % less
Low
Maximum (economical)
4% more
25 - 30% more
35% - 50% more

31
A SIMPLE CARBURETOR
It consists of a float chamber nozzle with metering orifice, venturi and throttle valve.
The float and a needle valve system maintain a constant height of petrol in the float
chamber.
During suction stroke air is drawn through the venturi .The air passing through the
venturi increases in velocity and the pressure in the venturi threat decreases. From
the float chamber, the fuel is fed to a discharge jet, the tip of which is located in the
throat of the venturi. Now because the pressure in the float chamber is atmospheric
and that at the discharge jet below atmospheric a pressure differential, called
“carburetor depression, exists between them. This causes discharge of fuel into the
air stream and the rate of flow is controlled or metered by the size of smaller section
in the fuel depression is 4 - 5cm below atmospheric.”
Essential Parts of a Carburetor
A carburetor consists essentially of the following parts, viz.
i. Fuel strainer
ii. Float chamber
iii. Main fuel metering and idling nozzles
iv. Choke and throttle
The various parts mentioned above are discussed briefly in the following section.

32
The Fuel Strainer
As the gasoline has to pass through a narrow nozzle exit there is every possibility
that the nozzle may get clogged during prolonged operation of the engine. To prevent
possible blockage of the nozzle by dust particles, the gasoline is filtered
by installing a fuel strainer at the inlet to the float chamber. The strainer consists of a
fine wire mesh or other type of filtering device, cone shaped or cylindrical shaped.
The Float Chamber
The function of a float chamber in a carburetor is to supply the fuel to the nozzle at a
constant pressure head. This is possible by maintaining a constant level of the fuel in
the float bowl. The float in a carburetor is designed to control the level of fuel in the
float chamber. This fuel level must be maintained slightly below the discharge nozzle
outlet holes in order to provide the correct amount of fuel flow and to prevent
leakage of fuel from the nozzle when the engine is not operating. When the float
rises with the fuel coming in, the fuel supply valve closes and stops the flow of fuel
into the chamber.
The Main Metering and Idling System
The main metering system of the carburetor controls the fuel feed for cruising and
full throttle operations (Fig.16.l0). It consists of three principal units:
i. The fuel metering orifice through which fuel is drawn from the float chamber
ii. The main discharge nozzle
iii. The passage leading to the idling system
The three functions of the main metering system are
i. To proportion the fuel-air mixture
ii. To decrease the pressure at the discharge nozzle exit
iii. To limit the air flow at full throttle
The automobiles fitted with SI engine require a rich mixture for idling and low speed
operation. Usually air-fuel ratio of about 12:1 is required for idling. In order to
provide such rich mixture, during idling, most of the modern carburetors incorporate
special idling system is their construction. This system gets operational at starting,
idling and very low speed running of the vehicle engine and is non operational when
throttle is opened beyond 15% to 20%.

33
When the throttle is practically closed or marginally open, the very small quantity of
air creates very little depression at the throat of the venturi, and that is not enough to
suck any fuel from the nozzle. But very low pressure caused on the downstream side
of the throttle due to suction stroke of the piston makes the fuel rise in the idling tube
and the same is discharged through the idling discharge port, directly into the engine
intake manifold. Due to the low pressure through idling air-bleed a small amount of
air also is sucked. The idling air bleed mixes air with gasoline drawn from float
chamber and helps it to vaporize and atomize it and pass on through the idle passage.
The air bleed also prevents the gasoline in the float chamber getting drained off
through the idling passage due to syphon action, when the engine is not in operation.
With the opening of throttle and the engine passing through the idling range of
operation, the suction pressure at the idle discharge port is not sufficient to draw the
gasoline through the idling passage. And the idling system goes out of action. There
after main air flow increases and the cruising range of operation is established. The
desired fuel-air ratio for idling can be regulated by idling adjustment shown in
Hot Idling Compensator
Some modern automobiles have this system in the carburetor unit. Under certain
extremely not operating conditions (with increased engine room temperature and also
a high carburetor body temperature) there is a tendency for the idling mixture to
become too rich. This causes idling instability. The hot idling compensator system
(HIC) incorporates bi-metallic valve that admits air directly into the manifold in
correct quantity when needed. Thus the mixture richness is adjusted and stable idling
is ensured.

34
The Choke and the Throttle
When the vehicle is kept stationary for a long period during cool winter seasons, may
be overnight, starting becomes more difficult. As already explained, at low cranking
speeds and intake temperatures a very rich mixture is required to initiate combustion.
Sometimes air-fuel ratio as rich as 9:1 is required. The main reason is that very large
fraction of the fuel may remain as liquid suspended in air even in the cylinder. For
initiating combustion, fuel-vapour and air in the form of mixture at a ratio that can
sustain combustion is required. It may be noted that at very low temperature vapour
fraction of the fuel is also very small and this forms combustible mixture to initiate
combustion. Hence, a very rich mixture must be supplied. The most popular method
of providing such mixture is by the use of choke valve. This is simple butterfly valve
located between the entrance to the carburetor and the venture throat .When the
choke is partly closed, large pressure drop occurs at the venturi throat that would
normally result from the quantity of air passing through the venturi throat. The very
large depression at the throat inducts large amount of fuel from the main nozzle and
provides a very rich mixture so that the ratio of the evaporated fuel to air in the
cylinder is within the combustible limits. Sometimes, the choke valves are spring
loaded to ensure that large carburetor depression and excessive choking does not
persist after the engine has started, and reached a desired speed. This choke can be
made to operate automatically by means of a thermostat so that the choke is closed
when engine is cold and goes out of operation when engine warms up after starting.
The speed and the output of an engine is controlled by the use of the throttle valve,
which is located on the downstream side of the venturi. The more the throttle is
closed the greater is the obstruction to the flow of the mixture placed in the passage
and the less is the quantity of mixture delivered to .the cylinders. The decreased
quantity of mixture gives a less powerful impulse to the pistons and the output of the
engine is reduced accordingly. As the throttle is opened the output of the engine
increases. Opening the throttle usually increases the speed of the engine. But this is
not always the case as the load on the engine is also a factor. For example, opening
the throttle when the motor vehicle is starting to climb a hill mayor may not increase
the vehicle speed, depending upon the steepness of the hill and the extent of throttle

35
opening. In short, the throttle is simply a means to regulate the output of the engine
by varying the quantity of charge going into the cylinder.
The choke and the throttle
Compensating Devices
An automobile on road has to run on different loads and speeds. The road conditions
play a vital role. Especially on city roads, one may be able to operate the vehicle
between 25 to 60% of the throttle only. During such conditions the carburetor must
be able to supply nearly constant air-fuel ratio mixture that is economical (16:1).
However, the tendency of a simple carburetor is to progressively richen the mixture
as the throttle starts opening. The main metering system alone will not be sufficient
to take care of the needs of the engine. Therefore, certain compensating devices are
usually added in the carburetor along with the main metering system so as to supply
a mixture with the required air-fuel ratio. A number of compensating devices are in
use. The important ones are
i. Air-bleed jet
ii. Compensating jet
iii. Emulsion tube
iv. Back suction control mechanism
v. Auxiliary air valve
vi. Auxiliary air port

36
Types of Carburetors
(1) Updraught type: - in which the air enters at the bottom and leaves at the top.
So that the direction of its flow is upwards. The disadvantages of the
updraught carburettor are that it must left the sprayed fuel droplet by air
friction. Hence it must be designed to relatively small mixing tube and throat
so that even at low engine speeds the air velocity is sufficient to left and carry
the fuel particle along. Otherwise, the fuel droplets tend to separate out.
(2) Down draught Carburetor: - consists of a horizontal mixing tube with a float
chamber on one side of it. By using a cross-draught carburetor in engines,
one-right angled turn in the inlet passage is eliminated and the resistance to
flow is reduced.
(3) Constant choke Carburetor:- the air and fuel flow areas are always constant. But
the pressure difference or depression which causes the flow of fuel and air. eg. Solex
and Zenith Carburetors.
(4) Constant Vacuum Carburetor:-variable chock carburetor - air and fuel flow areas

37
are being varied as per the demand on the engine, while the vaccum is maintained to
be always same. eg. S U and Carter carburetor.
Multiple Venturi Carburettor
Multiple Venturi system uses double or triple venturi. The boost venturi is located
concentrically within the main venturis. The discharge edge of the boost venturi is
located at the throat of the main venturi. The boost venturi is positioned up stream of
the throat of the layer through the boost venturi. Now the pressure at the boost
venturi exit equals the pressure at the main venturi throat. The fuel nozzle is located
at the throat of the boost venturi.
- high depression is created in the region of the fuel nozzle.
- improved atomization are possible
- better control
Multi Jet & Multi based Ventur’s carburetor
Advantage
1. Duel carburetor supplies a charge of the mixture to the cylinder which is uniform
in quality.
2. Distribution is better
INJECTION SYSTEMS
A typical arrangement of various components for the Solid Injection System use in a
C I engine is shown in figure. Fuel from the fuel tank first enters the course filter
from which is drawn into the plunges feed pump where the pressure is raised very
slightly. Then the fuel enters the fine filter where all the dust and dirt particles are
removed. From the fine filter the fuel enters the fuel pump where it is pressurized to
about 200 bar and injected into the engine cylinder by means of the injector. Any
spill over in the injector is returned to the fine filter. A pressure relief valve is also
provided for the safety of the system.
Functional requirements of an injection system
(1) Accurate metering of the fuel injected/cycle. The quantity of the fuel metered
should vary to meet changing speed and load requirements.

38
(2) Timing of the fuel injection in the cycle.
(3) Proper control of rate of injection.
(4) Proper atomization of fuel into very fine droplets.
(5) Uniform distribution of fuel droplets through out the combustion chamber.
(6) To supply equal quantities of mixed fuel to all cylinders in case of multi cylinder
engines.
Types of injection systems
(1) Air injection system :-
The fuel is metered and pumped to the fuel valve by a cam shaft driver fuel pump.
The fuel valve is opened by means of a mechanical linkages operated by cam shaft
which controls the timing of injection. The fuel valve is also connected to high
pressure air live feed by a multi stage compressor which supplied air at a pressure of
about 60-70 bar.
When the fuel valve is opened the blast air sweeps the fuel along with it and a well-
atomized fuel spray is sent to the combustion chamber.
(2) Solid injection:-
Injection of fuel directly into the combustion chamber without primary atomization.
Every solid injection system must have,
(1) a pressure unit (pump)
(2) an atomising unit (Injector)
Classification
(a) Individual pump & injector or jerk pump system.
(b) Common rail system.
(c) Distributor system
(a) Individual pump & injector or jerk pump system.
In the individual pump and injector or jerk pump system a separate metering and
compression pump is used for each cylinder. The pump which meters the fuel also

39
times of injection.
(b) Common rail system
A high pressure fuel pump delivery fuel to an “accumulator”, whose pressure is kept
constant with the help of pressure regulating valve. The high pressure pump usually
has a number of plugs and unlike the individual pump system none of the plugs in
identified with a particular cylinder. Accumulator is connected to different
distributing elements of each cylinder.
(c) Distributor system
In this system the pump which pressurizes the fuel and also meters it. Timing of
injection also set by the pump accessory. The fuel pump after meeting the required
amount of fuel supplies it to a rotating distributor at the correct time for supply to
each cylinder. The number of injection strokes /cylinder of the pump is equal to the
number of cylinders.

40
Bosch fuel injection pump or Jerk pump
When the plunger is at bottom of its stroke the fuel flows through the inlet part into
the barrel and fills the space above the plunger and also the vertical groove and the
space below the helix.
When the plunger starts moving up, a certain amount of fuel goes out of the fuel
chamber through the parts until plunger closes the parts. On further upwards
movement of the plunger the trapped fuel is compressed and is forced out through
the delivery valve to the pipe leading to the injector which immediately injects the
fuel in to the combustion chamber. The injection process continues till the end of the
upward stroke of the plunger when the lower end of helix uncovers the spill part.
When the spill part is up covered the pressure of the fuel in the bowel suddenly drops
as the fuel travels back to the suction chamber via the vertical slot on the plunger.
Both the spring loaded injector as well as spring loaded delivery valve are suddenly
closed, there by terminating the injection process.
The amount of the fuel delivered/stroke is controlled by rotating the plunger by
means of a control rod. As the plunger is rotated by moving the control rod different
portion of the helix came in front of the spill port, thus varying the effective stroke of
plunger, the actual plunger travel remaining constant.

41
Injection Nozzles
A complete fuel injection nozzle consists of two parts.
(1) Nozzle valve and
(2) The nozzle body
The main requirements of an injection nozzle
(i) To inject fuel at a sufficiently high pressure so that the fuel enters the cylinder
with high velocity.
(ii) Penetration of the droplets should not be high so as to impinge on cylinder walls.
This may result in poor starting.
(iii) The fuel supply and cut off should be rapid.

42
Types of nozzle
The type of the nozzle used in greatly depends on the type of combustion chamber in
use. The relative movement of air and may be of two types;
(i) open combustion chamber
(ii) Pre-combustion chamber.
Various types of nozzles
The main types of nozzles use with different types of combustion chamber are:
(i) Single hole nozzle
(ii) Multi-hole nozzle
(iii) Pintle nozzle
(iv) Pintax nozzle
1. Single hole nozzle for open combustion chamber
dia  0.2 mm
 high injection pressre is needed
 high velocity is needed for proper mixing
2. Multiple nozzle  usually 4 to 18 nozzles.
3. Pintle nozzles :- The stem of the nozzle valve is extended to from a pin or
pintle which protrudes through the mouth of the nozzle  spray ---- is 60o
Advantages Avoid weak injection
Prevents the carbon disposes on the nozzle end.
4. Pintaux Nozzle:- which has an auxilary hole drilled in the nozzle body. It
injects a small amount of the fuel through the additional hole (pilot injection)
straightly before the main injection.
Advantages  Better cold starting performance

43
Injection in S I engine :-
Fuel injection systems are commonly used in C I engines. Presently gasoline
injection systems used in S I engine due to the following drawbacks of the
combustion.
(i) Non uniform distribution of mixture in multi cylinder engines
(ii) Loss of volumetric efficiency due to restriction for the mixture flow.
Methods :-
(a) direct injection of fuel into the cylinder.
(b) injection of fuel close to the inlet valve
(c) injection of fuel into the inlet manifold
Why Gasoline injection?
It may be noticed that the intake valve is open in cylinder 2. Now the gasoline
moves to the end of the manifold and accumulate there. This enriches the mixture
going to the end cylinders.
MULTI-POINT FUEL INJECTION SYSTEM (MPFI)
The main purpose of the multi-point fuel injection system is to supply a proper ratio
of gasoline and air to the cylinder. There are two basic arrangements.
(i) Post injection (II) Throttle body injection
Every cylinder is provided with an injectors Similar to the carburetor throttle body;
with throttle valve controlling the amount of air entering the intake manifold, Injector
is placed slightly above the throat of the throttle body
IGNITON
Is considered as the beginning of the combustion process. The ignition process must
add necessary energy for starting and sustaining burning of the fuel till combustion
takes place. With in the range of the mixtures normally use, which varies from air-
fuel ratio 12-13 : 1 a park energy under 10 MJ is sufficient to initiate combustion.

44
Basic requirements
1. The system must have a source of electrical energy
2. The system must produce a peak voltage greater by safe margin than the spark
plug break down voltage at all speeds.
3. The duration of the spark must be long enough with sufficient energy to ensure the
ignition.
4. The system must distribute this high voltage to each of the spark plugs.
Battery ignition System
The ignition coil consists of two coils - one primary and the
other secondary. The primary winding is connected to the battery through an ignition
switch and the contact breaker. The secondary winding is connected to spark plugs
through the distributor. A ballast resister is provided in series with the primary
winding to regulate primary current. For starting purposes this resister is bypassed so
that more current can flow in the primary circuit. A rotating can shaft speed operates
the contact brakes and causes the breaker points to open and close.
When the ignition switch is as and the contact breaker points one closed current
flows from a magnetic field. When the current flow n the primary winding stopped
by opening the contact breaker points the magnetic field collapses, cuts across the
secondary winding and induces a voltage, which is accompanied by a current. This
magnetic field, however, also cuts the primary winding and induces a voltage in this

45
as well as in the secondary.
In order to obtain the highest voltage in the secondary circuit a quick collapse of the
magnetic field is essential. It is also necessary to prevent the axing and consequent
burning of the contact points. These are activated by providing a condenser across
the contact breaker. When the contact points open, the circuit instead of passing
across the points in the form of an arc, flow in to the condenser and is stored by it as
it becomes changed. The change in the condenser immediately discharges back into
the primary circuit in a direction reverse to the flow of a battery current, thus
assisting in a quicker collapse of magnetic field when the contact points open.
Due to the rapidly collapsing magnetic field, high voltage is induced in the primary
circuit and still higher voltage of the order of 11 KV to 22KV in the secondary
circuit. This high voltage in the secondary circuit passes through the distributor roter
to one of the spark plugs leads, into the spark plug and if this voltage is higher than
the breakdown voltage a spark occurs across the spark plug gap causing ignition of
the combustible mixture in the combustion chamber.
Magneto ignition system
Magneto is a special type of electric generator. It is mounted on the engine and
replaces all the components of the coil ignition system except distributor spark plug.
Magnet can be either rotating armature type or rotating magneto type.

46
Comparison
Battery (Coil)
(1) Battery is must. Low battery
starting is impossible
(2) Current for primary obtained by
battery
(3) A good spark is available at spark
plug at low speed
(4) Starting is easy
(5) Occupies more space
(6) Used in petrol cars and buses
Magneto
(1) No battery is needed
(2) Generated by the magneto
(3) Quality of spark is poor
(4) Difficult
(5) Lesser space
(6)Used in racing car, motor cycles,
scooters etc.
Firing Order
The order in which various cylinder of a multi-cylinder engine fire is called the firing
order.
Factors considered
(1) Engine vibrations
(2) Engine cooling
(3) Development of Back pressure
Commonly used firing orders are
3 cylinder 1 - 3 - 2
4 cylinder 1 - 3 - 4 - 2
6 cylinder 1 - 5 - 3 - 6 - 2 - 4
8 cylinder 1 - 6 - 2 - 5 - 8 - 3 - 7 - 4
Ignition Timing
15o
before TDC

47
Super charging
The method of increasing the inlet air density is called supercharging or increase in
the amount of air inducted / unit time is obtained by supercharging, The high density
air or large amount of air helps to burns a greater amount of fuel in a given engine
and thus increase its power output.
Types of super charges
(i) Centrifugal type compressor
(ii Vane type (blower)
(iii) Roots blower
Effects of super charging
1. Higher power output
2. Greater induction of charge man
3. Better atomization of fuel
4. Better mixing of fuel & air
5. Better scavenging of products
6. Quicker acceleration of vehicle
7. More complete and smoother combustion
8. Poor ignition quality fuel can be use
9. Reduction in diesel knock tendency
10. Increased efficiency in S I engine
11. Improved cold starting
12. Reduced exhaust smoke
13. Reduced specific fuel combustion
14. Increased mechanical efficiency
15. Increased thermal stresses
16. Increased heat loses due to increased turbulence

48
17. Increased cooling requirements of piston & valves.
Turbo charging
In turbo charging , the super charger is being driven by a gas turbine which uses the
energy in the exhaust gases. There is no mechanical linkage between the engine and
super charges.
Assignment questions
1. Explain the rating S I engine fuels, S C engine fuels.
2. Explain the important properties of lubricants
3. Important qualities of C I engine fuels & S I engine fuels
4. Why alternative fuels are being considered to I C engines; give any three
alternative fuels in detail
Cooling systems
In large capacity engines water cooling is provided as its heat absorbing
capacity is much higher than that of air. Water is circulated through passages around
the cylinder and combustion chamber. These passages are called water jackets. The
water circulation can be natural or forced.
In natural circulation (thermosyphon) systems the water circulation occurs
due to the difference in density of hot and cold water. In forced circulation systems,
water is circulated through the water jackets with the help of a pump.
Automobile engines use the same water for cooling by recirculation. A
radiator is used to cool the water to its initial temperature after cooling the engine. A
fan blows air through the radiator fins to cool the water. A pump is used to
continuously circulate the water through the engine cooling system.
Types of cooling systems
1. Liquid or indirect cooling system.
(a) Direct or non-return system
(b) Thermosyphon system
(c) Force circulation system

49
(d) Evaporative cooling system
(e) Pressure cooling system
2.Air cooling system (direct).
Lubrication System
Due to a large number of moving parts in an IC engine power loss due to friction is a
major issue. To reduce the friction losses engines are provided with lubrication
system. Its functions are:
i) To reduce friction and wear between moving parts.
ii) To provide sealing between piston rings and cylinder wall to prevent gas leakage.
iii) To cool piston heads, valves, etc.
iv) To wash away carbon and metal particles.
In small capacity engines, the lubricating oil is mixed with the fuel supply. In
medium size stationary engines splash lubrication is adopted. In this, a projecting fin
at the big end of the connecting rod splashes up the lubricating oil stored in the crank
case. In multi-cylinder heavy duty engines pressure feed lubricating systems are
used.

50
In pressure feed lubrication, oil is pumped from its reservoir through
pipelines to various parts of the engine. Bearings on crankshaft get lubricating oil
through small diameter holes drilled in it. Oil to the piston is supplied through the
hole drilled in the connecting rod. Since the lubricating oil is carried through small
diameter holes and pipes it must be pumped at a high pressure. Usually gear pumps
and piston pumps are used for this purpose.
Usually, the engine sump acts as the lubricating oil reservoir. The excess oil supplied
to the engine parts flows back into the sump, which is recirculated. The lubricating
oil needs to be topped up frequently and replaced periodically when its quality
deteriorates due to contamination with combustion products and wear particles.
Pressure feed lubrication systems are of two types: wet sump lubrication system and
dry sump lubrication system.

51
MODULE: 3
Combustion in S I Engines
Combustion is a chemical reaction in which certain elements of the fuel like
hydrogen and carbon combine with oxygen liberating heat energy and causing an
increase in temperature of gases. The conditions necessary for combustion are
(i) The pressure of a combustible mixture
(ii) Initiation for combustion
(iii) Stabilization and propagation of flame in combustion chamber.
In S I engines combustible mixture is generally supplied by the carburetor and the
combustion is initiated by an electric spark given by spark plug.
Ignition Limits
Ignition of charge is only possible within certain limits of fuel-air ratio. These
“ignition limits” correspond approximately to those mixture ratios, at lean and rich
ends of the scale, where the heat released by spark is sufficient to initiate combustion
in the neighboring unburnt mixture. The flame will propagate only if the temperature
of the burnt gases exceeds approximately 1500k in the case of hydrocarbon air
mixture.
The lower and upper ignition limits of the mixture depend upon mixture ratio and
temperature.
Stages of combustion in S I Engines
A typical theoretical Pressure- crank angle diagram, during the process of
compression (a b) combustion (bc) and expansion (c  d) in an ideal for
stroke spark ignition engine is shown
In an ideal engine, combustion takes place at constant volume. i.e. at TDC. But in
actual engine this does not happen.
The pressure variation due to combustion in a practical engine is shown in
figure given below. „A‟ is the point of producing spark (say 20o
before TDC). „B‟ is
the point at which the beginning of pressure rise can be detected (say 8o
before TDC)
and „C‟ the attainment of peak pressure. Thus AB represents the first stage and BC

52
the second stage and CD the third stage.
The first stage (A B) is referred to as the ignition lag or preparation phase
in which growth and the development of a self propagating nucleus of flame take
place. This is a chemical process depending upon temperature and pressure, the
nature of fuel and the proportion of the exhaust residual gas.
The second stage (BC) is a physical one and it is concerned with the
spread of flame throughout the combustion chamber. During the second stage the
flame propagates practically at a constant velocity. The rate of heat release depends
largely on the turbulence intensity and also on the reaction rate which is dependent
on 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.
Fuel air Ratio
Expressed as the ratio of mass of fuel to that of air.

53
Stoichiometric fuel-air Ratio
A mixture that contains just enough air for complete combustion of all the
fuel in the mixture is called a chemically correct mixture or stoichiometric fuel-air
ratio. A mixture having more fuel than that in a chemically correct mixture is termed
as rich mixture and a mixture that contains less fuel is called a lean mixture.
Equivalence Ratio
Actual Fuel-air Ratio
( )
Stoichiometric Fuel air Ratio
 
 = 1 stoichiometirc
 < 1 lean mixture
 > 1 rich mixture
Homogeneous Mixture
In homogeneous gas mixture the fuel and oxygen molecules are more or less
uniformly distributed,  = 1
Flame front Propagation
For efficient combustion the rate of propagation of flame front within the
cylinder is quite critical. The two important factors which determine the rate of
movement of flame front across the combustion chamber are the “reaction rate” and
“the transposition rate”. The reaction rate is the result of a purely chemical
combination process in which the flame eats its way into the unburnt charge. The
transposition rate is due to physical movement of flame front relative to cylinder and
is also the result of pressure differential 6‟n the burning gases and unburnt gases in
combustion chamber.
The flame front progresses relatively slowly due to a
low transposition rate and low turbulence. The transposition of flame front is very
little since change burned at the start is very little. The reaction rate also low. Since
spark plug is to be necessarily located in a quicent layer of gas i.e. close to the
cylinder wall, the lack of turbulence reduces the reaction rate and hence the flame
speed.
Then the flame front leaves the quicscent zone and proceeds into more turbulent

54
areas (Area II) where it consumes a greater mass of mixture. So it progresses rapidly
and at a lowest rate(BC).
The volume of unburnt charge is very much less towards the end of flame
travel and so the transposition rate again becomes negligible thereby reducing the
flame speed. The reaction rate is also reduced again. Since the turbulence is
relatively low Area III (C D).
Factors affecting flame speed
The flame velocity influences the rate of pressure rise in cylinder and it is related to
certain types of abnormal combustion that occur in Spark-ignition Engine.
(i) Turbulence
The flame speed is quite low in non-turbulent mixtures and increasing with
increase in turbulence.
(ii) Fuel-Air Ratio
The highest flame velocities are obtained with somewhat richer mixture (iii)
Temperature and Pressure
Flame increases with increase in temperature and pressure.
(iv) Compression Ratio
Flame speed increases with increase in Compression Ratio
(v) Engine output
The cycle pressure increases when the engine output increased.
(vi) Engine speed
The flame speed increases almost linearly with engine speed since the
increase in engine speed increases the turbulence inside cylinder.
Concept of Combustion Quality
Concept of Combustion quality for Otto cycle engines on the basis of low closely the
actual cycle approaches ideal Otto cycle.

55
Effect of Engine variables on ignition Delay
The ignition lag in terms of crank angle is 10o
to 20o
and in terms of seconds, 0.0015
seconds. The duration of the ignition lag depends on following factors.
1. Fuel
Higher self ignition temperature of fuel, the longer the ignition lag.
2. Mixture Ratio
The ignition lag is smallest for mixture ratio which gives maximum
temperature.
3. Initial Temperature and Pressure
Ignition lag decrease with an increase in the temperature and pressure of gas at the
time of spark. Thus increasing the intake temperature and increasing the compression
Ratio and retarding the spark, all reduces the ignition lag.
4. Electrode Gap
If gap is too small, quenching of flame nucleus may occur and range of fuel-air ratio
for development of a flame nucleus is reduced.
5. Turbulence
Ignition lag is not much affected by turbulence intensity. Turbulence is directly
proportional to Engine speed. When the speed is increased the crank angle measured
is increased.
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. This
is called abnormal combustion or knocking.
Consequences are
(i) Loss of power
(ii) Pre-ignition

56
(iii) Mechanical damage to Engine.
Phenomenon of knock in S I Engine
Heat release due to combustion increases the temperature and consequently
the pressure of burnt part of mixture above those of the unburned mixture. For the
pressure equalization the burned part of mixture will expand, and compress the
unburned mixture adiabatically thereby increasing the pressure and temperature. This
process continues as the flame front.
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 ignition lag. Spontaneous ignition or auto ignition occurs at various
“Pinpoint” locations. This phenomenon is called knocking. The process of auto
ignition leads towards engine knock.
The advancing flame front compresses the end charge, thus raising its temperature.
Also some preflame oxidation may take place in the end charge leading to further
increase in temperature. If the temperature of end charge is not self ignition temp, the
charge will not auto ignite.
However if the end charge reaches its auto ignition temperature the charge will auto

57
ignite, leading to knocking combustion. During the preflame reaction period flame
front could move from BB‟ to only CC‟. Because of auto ignition another flame front
starts travelling in the opposite direction to the main flame front. When the two flame
fronts collide, a severe pressure pulse is generated. This pressure wave produces
combustion chamber vibrations. The human ear can detect the resulting thudding
sound and consequent noise from vibrations.
Effects
1. Noise and Roughness.
2. Mechanical Damage.
3. Carbon Deposits.
4. Increase in heat transfer.
5. Decrease in power output and efficiency.
6. Pre ignition.
THEORIES OF DETONATION
Two general theories of Knock are;
(a) The auto-ignition Theory
(b) The Detonation Theory

58
(a) Auto-Ignition Theory
Auto ignition refers to initiation of combustion without necessity of flame. The auto-
ignition theory of Knock assumes that the flame velocity is normal before the onset
of auto ignition and that gas vibrations are created by a number of end-gas elements
auto-igniting almost simultaneously.
Extensive decomposition of the fuel can take place during the
preflame reactions, producing aldehydes peroxides, hydrogen peroxide and free
radical. The energy released by these reactions and the presence of active chemical
species and free radicals greatly accelerate the chemical reactions and leads to auto
ignition.
(b) Detonation Theory
A true detonation wave formed by pre-flame reactions has been proposed as the
mechanism for explosive auto-ignition. Such a shock wave would compress the
gases to pressures and temperatures where the reaction should be practically
instantaneously.
CHEMISTRY OF KNOCK AND DETONATION
Complex preflame reactions proceed the auto ignition. In the preflame
reactions many intermediate products appear which are an aid to auto-ignition. By
spectrum analysis of burning gases Ricardo and Thornycraft detected the presence of
aldehydes in the cylinder contents immediately prior to combustion. Others formed
that knocking occurred when aldehydes and peroxides were present in the cylinder
gases but no knocking occurred when they were absent. It will be noticed that the
amounts of CO2 and CO increases and amount of O2 falls as combustion proceeds
No positive presence of aldehydes and peroxides in detonation wave.
EFFECT OF ENGINE VARIABLE ON KNOCK OR DETONATION
To prevent knock in S I Engine, the endgas should have,
(a) Low temperature
(b) Low density
(c) Long ignition

59
(d) Non-reactive composition
(a) Temperature Factors
When temperature increases delay period are lower and greater formation of
chemical species are accelerated by an increase in temperature loss. The temperature
of unburned mixture is increased by following factors.
1. Raising Compression Ratio
2. Supercharging
3. Raising the inlet temperature
4. Raising coolant Temperature
(b) Density Factors
Increasing the density of the unburned mixture by any of the following methods will
increase the possibility of knock in engine.
1. Increasing the Compression Ratio
2. Opening the throttle
3. Supercharging the engine
4. Increasing the inlet pressure
(c) Time Factors
Increasing the time of exposure of the unburned mixture to auto-ignition conditions
by any of the following factors will increase the possibility of knock in S I engine.
1. Increasing flame travel distance.
2. Decreasing the turbulence of mixture.
3. Decreasing the speed of engine.
(d) Composition
The properties of the fuel and the fuel-air ratio are the primary means for controlling
knock.
(i) Octane rating of fuel

60
(ii) Fuel air Ratio
(iii) Humidity of Air
CONTROL OF DETONATION
To get maximum efficiency the engine must be designed for highest
Compression Ratio. But that can be used is limited by detonation. The engine is,
therefore, so designed that detonation take place at low engine speed and high
manifold pressure, i.e. full throttle. To prevent detonation the ignition is
automatically retarded, say 20o
to 10o
before TDC.
Knocking can be controlled by
(i) Increasing Engine rpm
(ii) Rotating spark
(iii) Reducing pressure in inlet manifold by throttling
(iv) Using too lean or too rich
(v) Water injection
Following are certain design features which reduces knock
(1) Use of Lower Compression Ratio
(2) Increasing Turbulence
(3) Relocating spark plug or use of two or more spark plug
(4) Suitable Combustion chamber design to reduce flame length and temperature of
end gas.
S I Engine Combustion Chamber Designs
The design of combustion chamber involves the shape of combustion chamber, the
location of spark plug and disposition of inlet and exhaust valve. Bring requirements
of a good combustion chamber.
(i) High power output with minimum octane requirement
(ii) High thermal Efficiency
(iii) Smooth engine operation

61
Various factors to achieve these requirements are
(a) High power output requires
1. High compression ratio
2. Small or no excess air
(b) High thermal efficiency requires
1. High Compression Ratio
2. Small heat loss during combustion
(c) Smooth Engine operation requires
1. Moderate rate of pressure rise during combustion
2. Absence of Detonation
Octane number
It is a comparison between the reference fuels consisting of mixture of
isooctane and n-heptane to unknown petrol and gasoline. Isooctane is low boiling
point branched chain compound has a very slight tendency to knock. Octane number
for an unknown fuel is defined as the percentage of isooctane in the primary
Reference fuel that gives the same knock intensity.
Flash point
The temperature at which the vapours of oil flash when subject to a naked
flame is known as flash point. If container is closes it is called closed flash point and
if open it is called open flash point.
Fire point
It is the temperature at which the oil , if once lit with flame, will burn steadily
at least for 5 seconds. This is usually 11o
C higher than open flash point.
Viscosity index
The velocity of an oil is affected by its temperature. Higher the temperature
lower the viscosity. This variation of viscosity of an oil with changes in temperature
is measured by its viscosity index. The oil is compared with two reference oils

62
having same viscosity at 99o
C one, a paraffinic based oil is arbitrarily assigned as
index of zero and the other, a naphthenic base oil, is assigned as index 100.
High viscosity index relatively smaller changes with temperature
Paraffin  Cn H2n+2
Naphthalene  C2 H2n
Factors affecting the delay period :-
1. Compression ratio
2. Engine speed
3. Output
4. Atomization of fuel and duration of injection
5. Injection timing
6. Quality of the fuel
7. Intake temperature
8. Intake Pressure
Compression ratio
Increase in compression ratio reduces the delay period because it raises both
temperature and density. With increase in compression ratio the temperature of the
air increases .At the same time the minimum auto-ignition temperature decreases due
to increased density of compressed air, resulting in closer contact of the molecules
which, thereby, reduces the time of reaction when fuel is injected.
When the compression ratio is high the delay period is low and therefore the
rise of pressure on ignition is lower. The volumetric efficiency and power also
reduces when the CR is high because it increases the unused percentage.
Speed
The delay period can be given either in terms of absolute time (in
milliseconds) or in terms of crank angle rotation. At constant speed, delay time is
proportional to delay angle. But in variable speed operation delay period may

63
decrease in terms of millisecond but increase in terms of crank angles.
Time delay is increased due to increase in speed because
(i) The loss of heat during compression decreases with the result that both
temperature and pressure of the compressed air tend to rise.
(ii) The increase in turbulence.
Output
With an increase in engine output the air-fuel ratio decreases, operating
temperature increase and hence delay period decreases.
Atomization and duration of injection (High Fuel injection Pressure)
Higher fuel-injection pressure increases the degree of atomization. The
fineness of atomization reduces ignition delay, due to higher surface volume ratio.
Air atomization factor will be reduced due to fuel spray path being shorter. Also with
smaller droplets, the aggregate area of inflammation will increase after ignition,
resulting in higher pressure rise during the second stage of combustion so we select
the “optimum group mean diameter of droplet”.
Injection timing
As the temperature and pressure at the beginning of injection are lower for
higher ignition advance, the delay period increases with increase in injection
advance. “The optimum angle of injection advance depends on many factors, but
generally it varies between 12o
to 20o
TDC.
Quality of fuel
Self ignition temperature is the most important property of the fuel which
affects the delay period. Also fuels with higher octane number giver lower delay
period and smoother engine operation. Other properties of the fuel which affect the
delay period are volatility, latent heat, viscosity and surface tension. First two affect
the time taken to form an envelope of vapour. The other two influence the fitness for
atomization.
Temperature
Pressure of intake air increases, the delay is decreases. Pressure increases the

64
total pressure produced by combustion is high. So output is high. So we can inject
more fuel, because cylinder contain more air.
Fuel temperature :-
Reduces both physical & chemical delay.
Air-fuel Ratio:-
With increase in air-fuel ratio (leaner mixture) the combustion temperature
are lowered and cylinder wall temperature are reduced and hence the delay period
increases.
Engine Size:-
Large engine having low speed.

65
MODULE : 4
COMBUSTION IN CI ENGINES
Air-fuel ratio in CI engines
In the C I engine, for a given speed, and irrespective of load, an approximately
constant supply of air enters the cylinder. The CI engine therefore can be termed
constant air supply engine. With change in load the quantity of fuel is change, which
changes the air-fuel ratio. The overall air-fuel ration may thus vary from about 100:1
at no load and 20:1 at full load.
What ever may by the overall air-fuel ratio in a CI engine due to injection of fuel,
there is a heterogeneous mixture with air-fuel ratio varying widely in different areas
within the chamber. There would be area where the mixture is very lean or very rich.
However there would be certain areas where the local-air-fuel ratio is within
combustible limits and there under favourable conditions of temperature, ignition
occurs.
In full load condition the mixture slightly leaner than stoichiometric. The poor
distribution of fuel and its intermixing with air results in objectionable smoke if
operated near chemically correct ratio and (Air fuel ratio 20-23, i.e. excess air 35 to
50%) hence the CI engine must always operate with excess air.
DIESEL KNOCK
In CI engines the ignition process takes place over a definite interval of time. First

66
few droplets is injected and that droplets are passing through the ignition delay
period, at the same time additional droplets are being injected into the chamber. If
the ignition delay of the fuel being injected is short, the first few droplet will
commence the actual burning phase in a relatively short time after injection and a
relatively small amount of fuel will be accumulated in the chamber. Then the
pressure rise will be moderate.
If the ignition delay is quite long, so much fuel can accumulate, that cause rapid rate
of pressure rise in cylinder, Such situation produces the extreme pressure
differentials and violent gas vibrations known as “knocking”. In the SI engine,
knocking occurs near the end of combustion where as in C I engine, knocking occurs
near the beginning of combustion. In order to decrease the tendency of knock it is
necessary to start the actual burning as early as possible after the injection begins. i.e.
reduce the Ignition delay and thus the amount of fuel present when the actual burning
of the first few droplets start.
Comparison of Knock in SI and CI Engines
1. In spark- ignition engines, the autoignition of the end gas away from the spark
plug, most likely near the end of the combustion causes knocking. But in
compression ignition engines the autoignition of the charge causing knocking is at
the start of combustion.
2. In spark-ignition engine, the charge that auto ignites is homogeneous and therefore
intensity of knocking or the rate of pressure rise at explosive auto ignition is likely to
more than that in compression-ignition engines.
3. In compression-ignition engines, only air is compressed during the compression
stroke and the ignition can take place only after fuel is injected just before the TDC.
Thus there can be no preignition in compression-ignition engines as in spark-ignition
engines.
4. In the SI engine it is relatively easy to distinguish between knocking and non-
knocking operation as the human ear easily finds the distinction. In the CI engine
there is no definite distinction between normal and knocking combustion.

67
Factors tending to reduce knocking SI and CI Engines
Sl No. Factors SI Engine CI Engine
1 Self-ignition temperature of fuel High Low
2. Time lag or delay period for fuel Long Short
3. Compression ratio Low High
4. Inlet temperature Low High
5. Inlet pressure Low High
6. Combustion chamber wall temperature Low High
7. Speed High Low
8. Cylinder size Small Large
Cetane number
The cetane rating of a diesel fuel is a measure of its ability to autoignite quickly
when it is injected into the compressed and heated air in the engine. “The cetane
number of a fuel is the percentage by volume of cetane in a mixture of cetane and 
methyl napthalane (C10H7CH3) that has the same performance in the standard test
engine as that of the fuel.
Cetane (C16 H34)  100
 Methyl Napthalane  0
Higher the cetane rating of fuel lesser is the property for diesel knock.

68
Methods of controlling diesel knock
Reducing delay period by
1. Reducing heat loss  Increase speed.
2. Adding chemical dopes, called ignition accelerators.
eg:- ethyl-nitric and amyl-nitrate. The chemical dopes increase the preflame reactions
and reduce the flash point.
3. Knocking is due to high rate of pressure rise because fuel collection in the cylinder
at that time is maximum. It can be reduced by arranging the injector so that only a
small amount of fuel is injected first.
The CI Engine Combustion Chambers:-
The most important function of the CI engine combustion chamber, is to provide
proper mixing of fuel and air in a short time. In SI engine this process is performed
by carburetor. For this purpose an organized air movement, called air swirl, is
provided to produce high relative velocity between the fuel droplets and air.
Methods of generating air swirl in the CI Engine
1. By directing the flow of the air during its entry to the cylinder, known as induction
swirl. This method is used in open combustion chambers.
2. By forcing the air through a tangential passage into a separate swirl chamber
during the compression stroke, known as compression swirl. This method is used in
swirl chambers.
3. By use of the initial pressure rise due to partial combustion to create swirl
turbulence, known as combustion induced swirl. This method is used in pre-
combustion chambers and air-cell chambers.
I. Induction Swirl
In four-stroke engines induction swirl can be obtained by two methods.
(i) By careful formation of the air intake passages, and
(ii) By making a portion of the circumference of the inlet valve. The angle of mask is
90- 140o
of circumference.

69
In the stoke engine the induction swirl is created by suitable inlet part forms.
The induction swirl is usually augmented by secondary air movement called
“squish”. Squish is the flow of air radially inwards the combustion recess. If a
marked inlet valve is used, it provides an obstruction in the passage which reduces
the volumetric efficiency. With induction swirl we have to use multiple-orifice
injector (number of holes from 4 to 8).
Advantages :-
1. The high excess air allows lower average combustion temperature. i.e. low heat
losses permits high thermal efficiency.
2. In the open combustion chamber the intensity of swirl is low. i.e. easy cold
starting.
3. The swirl is obtained during induction stroke no additional work is done in
producing the swirl.
Disadvantages:-
1. Swirl induced is generally weak in intensity, so multi, orifice nozzles with high
injection pressure are require
2. Small nozzle opening are more frequently closed by carbon deposits.
3. Use of shrouded valve lowers the volumetric efficiency.
4. Weak swirl necessitates excess air, i.e. low air utilization, say about 60% . This
reduces mean effective pressure and produce lower output power.
Direct injection quiescent chamber Direct injection swirl in chamber

70
Compression Swirl
A divided combustion chamber is defined as one in which the combustion
space is divided into two distinct components connected by restricted passages. The
create considerable pressure differences between them during the combustion
process.
“Swirl chamber”:-
Swirl chamber consists of a spherical shaped chamber separated from the engine
cylinder and located in the cylinder head. Into this chamber, about 50% of the air is
transferred during compression stroke. A throat connects the chamber to the cylinder
which enters the chamber in a tangential direction, so that the air coming into this
chamber is given a strong rotary movement inside the swirl chamber and after
combustion, the products rush back in to the cylinder through the same throat at
much higher velocity. This causes considerable heat loss.
Advantages:-
1. Due to strong swirl a single orifice injector with low pressure for injection is
required (1 to 2mm, 125 to 150 bar)
2. Due to strong swirl there is a greater utilization of air
3. Swirl is proportional to speed.
4. The swirl chamber produces smoother engine operation because the small
chamber absorbs initial shock of peak pressure and saves the piston from extreme
pressure variations.
Disadvantages:-
1. The work done during compression is considerable and there is a corresponding
loss during expansion. There fore mechanical efficiency is lower.
2. Greater heat loss to the combustion chamber walls.
3. Cylinder construction is more expensive.
Combustion induced Swirl:-
(a) Pre-combustion chamber:-

71
The precombustion chamber is located in the cylinder head and its volume
accounts for about 20% of the total combustion space. During the compression stroke
the piston forces the air into the precombustion chamber. The fuel is injected into the
prechamber and combustion in injected. The resulting pressure rise forces the
flaming droplets together with air and their combustion products to rush out into the
main cylinder at high velocity through the small holes. Thus it creates both strong
secondary turbulence and distributes the flaming fuel droplets though out the air in
the main combustion chamber. The rate of pressure rise and the maximum pressure is
lower to those of open type chamber.
Air-cell chamber
In this chamber the clearance volume is divided into two parts, one in the main
cylinder and the other called the energy cell. The energy cell is divided into two
parts, major and minor, which are separated from each other and from the main
chamber. Nozzle injects the fuel across the main combustion chamber towards the
open neck of the air cell.
During compression, the pressure in the main chamber is higher than that inside the
energy cell due to restricted passage area between the two. at the TDC, the difference
in pressure will by high and air will be forced at high velocity through the opening
into the energy cell and this moment the fuel-injection also begins. Combustion starts
initially in the main chamber where the temperature is comparatively higher but the
rate of burning is very slow due to absence of any air motion. In the energy cell, the
fuel is well mixed with air and high pressure is developed due to heat-release and the
hot burning gases blow out through the small passage into the main chamber. This
high velocity jet produces swirling motion in the main chamber and thereby
thoroughly mixes the fuel with air resulting in complete combustion.

72
MODULE : 5
AIR POLLUTION
Air pollution can be defined as addition to our atmosphere of any material which will
have a deleterious effect on life upon our planet. The main pollutants contributes by
automobiles are carbon monoxide (CO), unburned hydrocarbons (UBHC), oxides of
nitrogen (NOx) and lead and other particulate emissions. Automobiles are not the
only source of air pollution, other sources such as electric power generating stations
(which mainly emit sulphur oxides, nitrogen oxides, and particulates), industrial and
domestic fuel consumption, refuse burning, industrial processing etc., also contribute
heavily to contamination of our environment.
POLLUTANTS FROM GASOLINE ENGINES
There are four possible sources of atmospheric pollution from a petrol engine
powered vehicle the fuel tank, the carburetor, the crankcase and the exhaust pipe.
The contribution of pollutants, by source, as shown is as follows:
1. Evaporative loss : 15 to 25% of HC
2. Crankcase blowby: 20 to 35% of HC
3. Tail pipe exhaust: 50 to 60% of HC and almost all CO and NOx
The evaporative losses are the direct losses of raw gasoline from the engine fuel
system; the blowby gases are the vapours and gases leaking into the crankcase from
the combustion chamber and the pollutants from the exhaust pipe are due to
incomplete combustion.
Evaporative Losses. Evaporative emissions account for 15 to 25 percent of total
hydrocarbon emission from a gasoline engine. The two main sources of evaporative
emissions are the fuel tank and the carburetor.
(i) Fuel tank losses: Fuel tank losses occur by displacement of vapour during filling
of petrol tank, or by vaporisation of fuel in the tank, forcing the vapour through a
breather vent to the atmosphere. Where the temperature is low the fuel tank breathes
in air. When the temperature goes high it 'breathes out' air loaded with petrol vapour.
Fuel tank losses occur because the tank temperature is increased during the vehicle

73
operation which causes an increase in the vapour pressure and thermal expansion of
tank vapour.
The mechanism of tank loss is as follows:
When a partially filled fuel tank is open to atmosphere the partial pressure of the
vapour phase hydrocarbons and vapour pressure of the liquid are equal and they are
in equilibrium. If the temperature of the liquid is increased, say by engine operation,
the vapour pressure of the liquid will increase and it will vaporize in an attempt to
restore equilibrium. As additional liquid vaporizes, the total pressure of the tank
increases and since the tank is open to atmosphere the vapour will flow out of the
tank. This outflow to the vapour will increase if in addition to liquid temperature rise,
the vapour temperature is also increased.
The evaporation from the tank is affected by a large number of variables of which the
ambient and fuel tank temperature, the mode of vehicle operation, the amount of fuel
hi the tank and the volatility of the fuel are important. Other significant factors are
the capacity, design and location of the fuel tank with respect to the exhaust system
and the flow pattern of the heated air underneath the vehicle.
Less the tank fill, greater is the evaporation loss. This reflects the difference in the
tank vapour space. Also when a car is parked in a hot location the evaporation of the
gasoline in the tank accelerates, so the evaporation loss is greater. The operational
modes substantially affect the evaporation loss. When the tank temperature rises the
loss increases. The vapour which vent from a partially filled tank during vehicle
operation called soak, is a mixture of air and hydrocarbon. After a prolonged high
speed operation the HC per cent in the soak is as high as 60 per cent as compared to
about 30 per cent after an overnight soak.
(ii) Carburetor losses: Carburetor losses result from (a) external venting of the float
bowl relieving the internal pressure as the carburetor heats, and (b) 'hot soak1
losses
which occur after the engine has been stopped, as a result of evaporation of petrol
stored in the bowl, loss being through vent pipe or through the air cleaner. Most of
the loss from the carburetor occurs due to direct boiling of the fuel in the carburetor
bowl during hot soak. Carburetor bowl temperature during hot, soak rises 15°C to
45°C above the ambient. This can cause fuel boiling and the front end gasoline

74
components. In some designs the small passage from bowl leading to the throat after
'heating causes siphon action leading to HC loss.
If the pressure in the fuel line becomes greater than the pressure holding the needle
valve closed, after supply will occur. One of the possible reasons may be fuel
evaporation pressure in the carburetor bowl which presses down the bowl and
increase pressure in the fuel line. If the after-supply is more than the bowl volume
the losses from the carburetor will change drastically. Thus bowl volume and
maximum bowl temperature both significantly affect the evaporative losses from the
carburetor.
Crankcase blowby: The blow by is the phenomenon of leakage past the piston and
piston rings from the cylinder to the crankcase. The blowby HC emissions are about
20 per cent of the total HC emission from the engine. This is increased to about 30
per cent if the rings are worn.
The mechanism of leakage past the piston is as follows Air-fuel mixture trapped in
the top land clearance and behind the top ring is unable to burn due to wall
quenching effect. The cylinder forces this quenched gas past the piston ring and into
the crankcase, along with some burned gases. In the blowby gas about 85 per cent
carbureted mixture in the form of raw HC is present and rest 15 per cent is the
burned gases.
The blowby rate is greatly affected by the top land clearance and the position of the
top ring because some of quenched gas is recycled in the combustion chamber and
the ability of this to burn will depend on nearness to spark plug and the flame speed,
etc., and it will burn only when favourable conditions are there, otherwise, it will go
in the form of HC.
Exhaust Emissions: Tail pipe exhaust emissions are the major source of automotive
emissions. Petrol consists of a mixture of various hydrocarbons and if we could get
perfect combustion then the exhaust would consist only of carbon dioxide and water
vapors plus air that did not enter into the combustion process. However, for several
reasons combustion is incomplete and hence we also get carbon monoxide, a deadly
poisonous gas, and unburnt hydrocarbons (UBHC) in exhaust. Hydrocarbons play an
active part in the formation of smog.

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