This document summarizes the key components and operation of internal combustion engines. It discusses: - The definitions of engines and heat engines and classifications of engines as rotary, reciprocating, external combustion, and internal combustion. - The major components of engines like the cylinder, piston, combustion chamber, valves, and their functions. - The operating cycles of 4-stroke spark ignition engines and 4-stroke compression ignition engines through their intake, compression, combustion, and exhaust strokes. - Comparisons between spark ignition and compression ignition engines and between 2-stroke and 4-stroke engines. - Differences between ideal engine diagrams and actual engine performance.

1. INTRODUCTION
1.1 Energy Conversion
· Definitions:
i. Engine – a device which transforms one form of energy into
another form.
ii. Heat Engine – a device which transforms the chemical energy of
fuel into thermal energy and utilizes this thermal
energy to perform useful work (mechanical energy).
· Classification of heat engines - Engines whether internal Combustion or
External Combustion are of 2 types:
i. Rotary engines
ii. Reciprocating engines
· External engine – combustion takes place outside the engine.
· Internal engine – combustion takes place within the engine e.g. steam
engine or turbine, and gasoline or diesel engines.
· The most widely used engines are:
i. The reciprocating internal combustion engines (have been found
suitable for the use in automobiles, motor-cycles and scooters,
power boats, ships, slow speed aircraft, locomotives and power
units of relatively small output.)
ii. The gas turbine
iii. The steam turbine
· Advantages of reciprocating internal combustion engines compare to the
steam engine are:
i. Mechanical simplicity and improved efficiency due to the absence
of heat exchangers in the passage of the working fluid (boilers and
condensers in steam turbine plant).
ii. Higher thermal efficiency due to:

2. a) All its components are worked at an average temperature which
is much below the maximum temperature of the working fluid in
the cycle.
b) Moderate maximum working pressure of the fluid in the cycle
produces less weight to power ratio.
c) The possibility of developing a small power output reciprocating
internal combustion engines.
· The main disadvantages of reciprocating internal combustion engines are:
i. The problem of vibration caused by the reciprocating components.
ii. Only liquid or gaseous fuels of given specification, which are
relatively more expensive, can be efficiently used.
1.2 Basic Engine Components and Nomenclature
Fig. 1.2 Cross-section of a Spark Ignition Engine
The major components of the engine and their functions are:
No Components Definition/Function
1. Cylinder Cylindrical vessel or space in which the piston makes a
reciprocating motion. It is filled with the working fluid and
subjected to different thermodynamic processes during the
engine’s operation.

3. 2. Piston A cylindrical component fitted perfectly into the cylinder
providing a gas-tight space with the piston rings and the
lubricant. It forms the first link in transmitting the gas forces to
the output shaft.
3. Combustion
Chamber
The space enclosed in the upper part of the cylinder, by the
cylinder head and the piston top during the combustion process.
The combustion of fuel and the and the consequence release of
thermal energy results in the building up of pressure in this part
of the cylinder.
4. Inlet Manifold The pipe which connects the intake system to the inlet valve of
the engine and through which air or air-fuel mixture is drawn into
the cylinder.
5. Exhaust
manifold
The pipe which connects the exhaust system to the exhaust
valve of the engine and through which the products of
combustion escape into the atmosphere.
*6. Inlet and
Exhaust
Valve
Provided either on the cylinder head or on the side of the
cylinder for regulating the charge coming into the cylinder (inlet
valve) and for discharging the products of combustion (exhaust
valve) fro the cylinder.
7. Spark Plug A component to initiate the combustion process in spark ignition
(SI) engine and is usually located on the cylinder head.
8. Connecting
Rod
Interconnects the piston and the crankshaft and transmit the gas
forces and is usually from the piston to the crankshaft.
9. Crankshaft Converts the reciprocating motion of the piston into useful rotary
motion of the output shaft. There are a pair of crank arms and
balance weights which provide static and dynamic balancing for
the rotating system. The crankshaft is enclosed in a crankcase
[].
10. Pistons Rings Fitted into the slots around the piston to provide a tight seal
between the piston and the cylinder wall thus preventing leakage
of combustion gases.
11. Gudgeon Pin Forms the link between the small end of the connecting rod and
the piston.
12. Camshaft Control the opening and closing of the two valves with its
associate parts which are push rods, rocker arm, valve springs
and tappets. Also provides the drive to the ignition system and is
driven by the crankshaft through timing gears.

4. *13. Cams Integral parts of the camshaft and are design to open the valves
at the correct timing and to keep them open for necessary
duration.
Fly Wheel A mass inertia attach to the output shaft in the form of wheel to
achieve a uniform torque.
· The general nomenclatures are:
Cylinder bore (d): The nominal inner diameter of the working cylinder and is
usually expressed in mm.
Piston area (A): The area of a circle of diameter equal to the cylinder bore
and is usually expressed in cm2
.
Stroke (L): The nominal distance through which a working piston
moves between two successive reversals of its direction of
motion and is usually expressed in mm.
Dead centre: The position of the working piston and the moving parts
which are mechanically connected to it, at the moment
when the direction of the piston motion is reversed at
either end of the stroke. There are two dead centres in the
engine:
i) Top dead centre
(TDC):
The dead centre when the piston is
farthest from the crankshaft.
ii) Bottom dead centre
(BDC):
The dead centre when the piston is
nearest to the crankshaft.
Displacement or
swept volume (Vs):
The nominal volume swept by the working piston when
travelling from one dead centre to the other. It is
expressed in terms of cubic centimetre (cc):
LdLAVs
2
4
p
=´= (1.1)
Clearance volume
(VC):
The nominal volume of the combustion chamber above the
piston when it is at the top dead centre is the clearance
volume and it is usually expressed in cubic centimetre (cc).
Compression Ratio
(r):
The ratio of the total cylinder volume when the piston is at
the bottom dead centre, VT, to the clearance volume, VC:
C
s
C
sC
C
T
V
V
V
VV
V
V
r +=
+
== 1 (1.2)

5. Fig. 1.3 Top and Bottom Dead Centre
1.3 THE WORKING PRINCIPLE OF ENGINES
1.3.1 Four-Stroke Spark Ignition Engine
· The cycle of operations is completed in 4 strokes of the piston (2
revolutions of the crankshaft).
· 4-stroke cycle is completed through 720 °C of crank rotation as each
stroke consists of 180 °C of crankshaft rotation.
· Five events to be completed, viz., suction, compression, combustion,
expansion and exhaust.
· An ideal 4 stroke SI engine consists:
i. Suction/intake stroke -
ii. Compression stroke
iii. Expansion/power stroke
iv. Exhaust stroke
· Suction/Intake Stroke

6. Ø 0 => 1
Ø Piston at top dead center (TDC) moving downwards to bottom dead
center (BDC).
Ø Inlet valve opens and exhaust valve is closed
Ø The charge consisting of fuel-air mixture is drawn into the cylinder.
Fig. 1.4 Working principle of a Four-Stroke SI Engine
· Compression Stroke
Ø 1 => 2
Ø The return stroke of the piston compresses the mixture into the
clearance volume.
Ø Both inlet and exhaust valves are closed.
Ø The mixture is ignited with the help of an electric spark between the
electrodes of a spark plug at the end of the process.
Ø Burning takes place almost instantaneously when piston at TDC
and the chemical energy of fuel is converted into heat energy (2 =>
3).

7. Ø The pressure at the end of the combustion process is increased
due to the heat release at constant volume.
· Expansion/power Stroke
Ø 3 => 4
Ø High pressure of the burnt gasses forces the piston downward the
BDC thus produces power.
Ø Both inlet and exhaust valves remain closed.
Ø Both pressure and temperature decrease.
Fig. 1.5 Ideal Indicator Diagram of a Four-stroke SI Engine
· Exhaust Stroke
Ø 5 => 0
Ø The piston moves from BDC to TDC and sweeps the burnt gases
out from the cylinder almost at atmospheric pressure.
Ø The exhaust valve opens and the inlet valve remains closed at the
end of the expansion process.
Ø The exhaust valve closes at the end of this stroke and some
residual gasses trapped in the clearance volume remain in the
cylinder.
1.3.2 Four-Stroke Compression Ignition Engine
· Suction/Intake Stroke
Ø Air alone is inducted
Ø Intake valve is open and the exhaust valve is closed.

8. · Compression Stroke
Ø Air inducted is compressed into the clearance volume.
Ø Both valves are closed.
Fig. 1.6 Cycle operation of a Four Stroke CI Engine.
· Expansion/power Stroke
Ø Fuel injection starts at the end of the compression stroke.
Ø The piston movement on its expansion stroke increases the
volume and heat is assumed to have been added at constant
pressure.
Ø After the injection of fuel is completed, the products of combustion
explode.
Ø Both valves remain close.
· Exhaust Stroke
Ø The piston travelling from BDC to TDC pushes out the products of
combustion.
Ø The exhaust valve is open and the intake valve is closed.

9. 1.3.3 Comparison of SI and CI Engines
Description SI Engine CI Engine
Basic cycle Otto cycle or constant volume
heat addition cycle
Diesel cycle or constant pressure
heat addition cycle.
Fuel Gasoline, a highly volatile fuel.
Self-ignition temperature is
high.
Diesel oil, a non-volatile fuel.
Self-ignition temperature is
comparatively low.
Introduction
of fuel
A gaseous mixture of fuel and
air is introduced during the
suction stroke. A carburator is
necessary to provide the
mixture.
Fuel is injected directly into the
combustion chamber at high
pressure at the end of the
compression stroke. A fuel pump
and injector are necessary.
Load control Throttle controls the quantity of
mixture introduced.
The quantity of fuel is regulated
in the pump. Air quantity is not
controlled.
Ignition Requires an ignition system
with spark plug in the
combustion chamber. Primary
voltage is provided by a battery
or a magneto.
Self-ignition occurs due to high
temperature of air because of the
high compression. Ignition
system and spark plug are not
necessary.
Compression
ratio
6 to 10. Upper limit is fixed by
antiknock quality of the fuel.
16 to 20. Upper limit is limited by
weight increase of the engine.
Speed Due to light weight and also
due to homogeneous
combustion, they are high
speed engines
Due to heavy weight and also
due to heterogeneous
combustion, they are low speed
engines
Thermal
efficiency
Because of the lower CR, the
maximum value of thermal
efficiency that can be obtained
is lower.
Because of higher CR, the
maximum value of thermal
efficiency that can be obtained is
higher.
Weight Lighter due to lower peak
pressures.
Heavier due to higher peak
pressures.

10. 1.3.4 Two-Stroke Engine
· Based on the concept where the two unproductive strokes, viz., the
suction and the exhaust could be served by an alternative arrangement,
especially without the movement of the piston, the two-stroke engine was
invented by Dugald Clark (1878).
· The cycle is completed in 1 revolution of the crankshaft.
Fig. 1.7 Cycle operation of a two-stroke engine.
· No piston strokes are required for the two unproductive strokes, viz., the
suction and the exhaust.
i. The filling of fresh charge is accomplished by the charge
compressed in crankcase or by a blower.
ii. The induction of the compressed moves out through exhaust ports.
· Therefore, two strokes are sufficient to complete the cycle, one for
compressing the fresh charge and other for expansion/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 the
upward motion of the compression stroke.
· The ignition and expansion takes place after the compression.
· The charge in the crankcase is compressed during the expansion stroke.

11. · The piston uncovers the exhaust ports and the cylinder pressure drops at
atmospheric pressure as the combustion products leave the cylinder near
the end of the of the expansion stroke.
Fig.1.8 Indicator diagram of a two-stroke SI engine.
· Further movement of the piston uncovers the transfer port, 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.
1.3.5 Comparison of 4-stroke and 2-stroke engines
Table 1.2 Comparison of Four and Two-Stroke Cycle Engines
Four-Stroke Engine Two-Stroke Engine
The thermodynamic cycle is completed in
four strokes of the piston or in two
revolutions of the crankshaft. Thus, one
power stroke is obtained in every two
revolutions of the crankshaft.
The thermodynamic cycle is completed
in two strokes of the piston or in one
revolution of the crankshaft. Thus one
power stroke is obtained in each
revolution of the crankshaft.
Because of the above, turning moment is
not so uniform and hence a heavier
flywheel is needed.
Because of the above, turning moment is
more uniform and hence a lighter
flywheel can be used.
Again, because of one power stroke for
two -revolutions, power produced for
same size of engine is less, or for the
same power the engine is heavier and
bulkier.
Because of one power stroke for every
revolution, power produced for same
size of engine is more (theoretically
twice; actually about 1.3 times), or for
the same power the engine is lighter and

12. more compact.
Because of one power stroke in two
revolutions lesser cooling and lubrication
requirements. Lower rate of wear and
tear.
Because of one power stroke in one
revolution greater cooling and lubrication
requirements. Higher rate of wear and
tear.
The four-stroke engine contains valves
and valve actuating mechanisms to open
and close the valves.
Two-stroke engines have no valves but
only ports (some two-stroke engines are
fitted with conventional exhaust valve or
reed valve).
Because of the heavy weight and
complicated valve mechanism, the initial
cost of the engine is more.
Because of lightweight and simplicity
due to the absence of valve mechanism,
initial cost of the engine is less.
Volumetric efficiency is more due to more
time for induction.
Volumetric efficiency is low due to lesser
time for induction.
Thermal efficiency is higher; part load
efficiency is better than two-stroke cycle
engine.
Thermal efficiency is lower; part load
efficiency is poor compared to a four-
stroke cycle engine.
Used where efficiency is important, viz., in
cars, buses, trucks, tractors, industrial
engines, airplanes, power generation etc.
Used where low cost, compactness and
light weight are important, viz., in
mopeds, scooters, motorcycles, hand
sprayers etc.
1.4 ACTUAL ENGINES
· Actual engines differ from the ideal engines because of various constraints
in their operation.
Ideal Indicator Diagram Actual Indicator
Fig. 1.9 (a) Four-stroke Engine

13. · The indicator diagram also differs considerably from the ideal indicator
diagrams as given in Fig. 1.9 (a) and 1.9 (b) respectively.
Ideal Indicator Diagram Actual Indicator
Fig. 1.9 (b) Two-stroke Engine
1.5.1 Number of strokes
· Classification on the number of piston strokes for completing a cycle of
operation: eg. 2-stroke or 4 stroke
1.5.2 Cycle of operation
i) Constant volume heat addition cycle or Otto cycle engine (Spark
Ignition engine or Gasoline engine)
ii) Constant-pressure heat addition cycle engine or Diesel cycle engine
(also call a Compression Ignition CI engine or diesel engine).
1.5.3 Type of Fuel Used
i) Volatile liquid fuels like gasoline, alcohol, kerosene, benzene etc.
ii) Gaseous fuels like natural gas, petroleum gas, blast furnace gas, town
gas and biogas etc.
iii) Solid fuels like charcoal, powdered coal etc.
iv) Viscous fluid like heavy and light diesel oils.
v) Two fuels (Dual –fuel engines)

14. 1.5.4 Method of Changing
i) Naturally aspirated engines: admission of air or air-fuel mixture at near
atmospheric pressure.
ii) Supercharged engines: admission of air or air-fuel mixture under
pressure i.e. above atmospheric pressure.
1.5.5 Type of Ignition
· Two types of ignition system are normally used to produce the required high
voltage for the initiation of spark plug:
i) Battery ignition system
ii) Magneto ignition system
1.5.6 Type of cooling
i) Air-cooled engine
ii) Water-cooled engine
1.5.7 Cylinder arrangement
i) Cylinder row
ii) Cylinder bank

15. 1.6.2 Two-Stroke Diesel Engine
· Very high power used for ship propulsion (brake power can be up to 37 000
kW).
1.6.3 Four-Stroke Gasoline Engines
· Most important application of small four-stroke engine is in automobiles
(typical output is in the range of 30-60 kW at a speed about 4500 rpm).

16. · Also used for busses and trucks (4000 cc) and big motorcycles with sidecars.
· Small pumping sets and mobile electric generating sets.
· Small aircraft (200 kW range)
1.6.4 Four-Stroke Diesel Engines
· One of the most efficient and versatile prime movers (manufactured in sizes
form 50-1 000 mm of cylinder diameter and engine speeds ranging from 100-
4 500 rpm while delivering outputs from 1-35 000 kW).
· Development is going in the use of diesel engines in personal automobiles, as
they are more efficient compare to gasoline engines. However the vibrations
from the engine and the unpleasant odor in the exhaust are the main
drawbacks.
· Diesel engine are used both for mobile and stationary electric generating
plants of varying capabilities:
i) Tractors for agricultural application (30 kW), jeeps, buses and trucks
(40 – 100 kW).
ii) Engine with higher output like supercharged engine in the output range
of 200 to 400 kW.
iii) Locomotive applications (output 600 – 4 000 kW).
iv) Marine applications, from fishing vessels to ocean going ships (100 –
35 000 kW).
v) Small diesel engines are used in pump sets, construction machinery,
air compressors, drilling rigs and many miscellaneous applications.
1.7 First Law of Engine Cycle
· According to first law energy can neither be created nor destroyed. It can only
transform from one form to another. Therefore there must be an energy
balance of input and output to a system.

17. 1.8 Engine Performance Parameters
i) Indicated thermal efficiency, hith
The ratio energy in the indicated power, ip, to the fuel energy in appropriate units:
[ ]
[ ]kJ/ssecondperfuelinenergy
kJ/sip
ith =h (1.3)
=
fuelofvaluecaloricfuel/sofmass ´
ip
(1.4)
ii) Brake thermal efficiency, hbth
The ratio of energy in the brake power, bp, to the input fuel energy in
the appropriate units:
fuelofvaluecaloricfuel/sofmass ´
=
bp
bthh (1.5)
iii) Mechanical efficiency, hm
The ratio of brake power to the indicated power (power provided by the
piston):
fpbp
bp
ip
bp
m
+
==h (1.6)
bpipfp -= (1.7)
It can also be defined as the ratio of the brake thermal efficiency to the
indicated thermal efficiency.

18. iv) Volumetric efficiency, hv
The ratio of the volume of air actually inducted at ambient conditions to
the swept volume of the engine:
pressureandetemperaturambientat
volumesweptbydrepresentechargeofmass
inductedactuallychargeofmass
=vh (1.8)
volumeswept
conditionsambientat
strokeperaspiratedchargeofvolume
= (1.9)
v) Relative efficiency or Efficiency ratio, hrel
The ratio of thermal efficiency of an actual cycle to that of the ideal cycle and it is
very useful for indicating the degree of development of the engine:
efficiencystandard-Air
efficiencythermalActual
=relh (1.10)
vi) Mean effective pressure, pm

19. The average pressure inside the cylinders of an internal combustion
engine based on the calculated or measured power output. For any
given engine, operating at a given speed and power output, there will
be a specific indicated mean effective pressure, imep, and a
corresponding brake mean effective pressure, bmep. Therefore;
Indicated power:
100060´
=
LAnKp
ip im
(1.11)
Indicated mean effective pressure:
LAnK
ip
pim
´
=
60000
(1.12)
The brake mean effective pressure:
LAnK
bp
pbm
´
=
60000
(1.13)
where
ip = Indicated power (kW)
pim = Indicated mean effective pressure (N/m2
)
L = Length of the stroke (m)
A = Area of the piston (m2
)
N = Speed in revolution per minute (rpm)
n = Number of power strokes
N/2 for 4-stroke and N for 2-stroke engines
K = Number of cylinders
Alternately, the indicated mean effective pressure can be specified from
the knowledge of engine indicator diagram (p-V diagram):
diagramindicatortheofLength
diagramindicatortheofArea
=imp
where the length of the indicator diagram is given by the difference
between the total volume and the clearance volume.
vii) Mean piston speed, ps
_
An engine parameter for correlating engine behavior as a function of
speed.

20. LNsp 2=
_
where L is the stroke and N is the rotational speed of the crankshaft in
rpm.
viii) Specific power output. Ps
The power output per unit piston area and is a measure of the engine designer’s success in using
the available piston area regardless the cylinder size:
Specific power output, Ps..
AbpPs /= (1.14)
_
constant pbm sp ´´= (1.15)
ix) Specific fuel consumption, sfc
An important parameter that reflects the fuel consumption
characteristic of an engine:
[ ]hourfuel/kWofkg
Power
timeunitpernconsumptioFuel
=sfc (1.16)
x) Fuel-air (F/A) or air-fuel ratio (A/F)
In the SI engine the fuel-air ratio practically remain a constant over a
wide range of operation. The CI engines at a given speed the airflow
does not vary with load; it is the fuel that varies directly with load.
Therefore the term fuel-air ratio is generally used instead of air-fuel
ratio.
Stoichiometric fuel-air ratio: a mixture that contains just enough air for
complete combustion of all fuel in the mixture.
Rich mixture: a mixture that having more fuel than that in a chemically
correct mixture.
Lean mixture: a mixture that contains less fuel (or excess air).
The ratio of actual fuel-air ratio to chemically correct fuel-air ratio is
called equivalence ratio:
ratioair-fueltricStoichiome
ratioair-fuelActual
=f
accordingly, f = 1 (chemically correct mixture)

21. f < 1 (lean mixture)
f > 1 (rich mixture)
xi) Caloric value of the fuel
The thermal energy released per unit quantity of the fuel when the fuel
is burned completely and the products of the combustion is cooled
back to the initial temperature of the combustible mixture. Other terms
used are the heating value and the heat of combustion.
1.9 Design and performance Data