ME210 THERMAL ENGINEERING - I
Objective of the Course:
To familiarize students with the working of Compressors and reciprocating piston engines and
to enable them to develop a deep appreciation of the concepts of thermodynamics and fluid
mechanics underlying in the design and development of these machines.
UNIT - I
Introduction : Introduction, Comparison of Air Standard and Actual Cycles, Actual and FuelAir Cycles Of IC Engines, Classification - Working principles, Valve and Port Timing
Diagrams.
Engine systems – Fuel Carburetor, Fuel Injection System, Ignition, Cooling and Lubrication.
Introduction to Fuels –Conventional and Alternate fuels-Characteristics.
UNIT - II
Combustion in S.I Engines : Normal Combustion, Importance of flame speed, pre-ignition
and knocking, anti knock additives, combustion chamber, types.
Combustion in C.I. Engine: Stages of combustion – Delay period and its importance –
Diesel Knock– Need for air movement, suction, compression and combustion induced
turbulence.
UNIT – III
Performance of I.C Engines : Measurement of cylinder pressure, fuel Consumption, air
intake, exhaust gas composition, Brake power – Determination of frictional losses and
indicated power – Performance test – Heat balance sheet.
UNIT – IV
Reciprocating Compressors : Classification and working principle, work of compression
with and without clearance. Volumetric efficiency, Isothermal efficiency and isentropic
efficiency of reciprocating air compressors. Multistage air compressor and inter cooling –
working of multistage air compressor.
UNIT - V
Centrifugal compressors : Principle of operation – velocity and pressure variation. Energy
transfer-impeller blade shape-losses, slip factor, power input factor, pressure coefficient
velocity diagrams, power.
Axial Flow Compressors - Mechanical details and principle of operation – velocity triangles
and energy transfer per stage degree of reaction, work done factor - isentropic efficiency.
Polytrophic efficiency.
TEXT BOOKS
1.
John B. Hey Wood” “Fundamentals of I.C. Engines”, 2nd ed., Mc.Graw-Hill, 1988.
2
GANESAN V., “Internal Combustion Engines”, 2nd ed., TMH. 2007.
REFERENCE BOOKS
1.
Sarkar B.K, “ Thermal Engineering”, Tata McGraw-Hill, 1998.
2.
R.YADAV, “Thermal Engineering” , Central Book Depot, 1995.
3.
Mathur R.P., and Sharma, “Internal Combustion Engines”, 8th ed., Dhanpath Rai &
Sons, New Delhi, 1996.
UNIT-I
ENGINES
(i)
(ii)
(iii)
Heat Engine
E.C Engines (External Combustion Engines)
I.C Engines (Internal Combustion Engines)
 Engine:
An engine is a device which transforms one form of energy into another form.
(or)
The machine which does this job of energy conversion is called an engine
 Heat Engine:
Heat engine is a device which transforms the chemical energy of a fuel into
thermal energy and utilizes this thermal energy to perform useful work.
 The most widely used ones are the reciprocating internal combustion engine.
The gas turbine and the steam turbine.
 The reciprocating internal combustion engine enjoys some advantages over
the steam turbine due to the absence of heat exchanges in the passage of the
working fluid.
 Absence of heat exchanges results in a considerable mechanical simplicity and
improved efficiency of the internal combustion engine.
Heat Engine
IC Engine
Rotary
Reciprocating
Wankel Opencycle Gasoline diesel
Engine gas turbine engine
engine
EC Engine
Reciprocating
steam
engine
Rotary
stirling stem closedcycle
engine engine gas turbine
 Reciprocating internal combustion engine components work at an average
temp. which is much below the maximum temperature of the working fluid in
the cycle.
 In internal combustion engine, thermal efficiency can be obtained with
moderate maximum working pressure of the fluid.
 Disadvantage: Problem of vibration caused by the reciprocating components
and also, it is not possible to use a variety of fuels in these engines. Only
liquid (or) gaseous fuels of given specification can be efficiently used.
 Reciprocating internal combustion engines have been found suitable for use in
automobiles, motor-cycles and scooters, power boats, ships, slow speed air
craft, locomotives and power units of relatively small output.
 External combustion engines are those in which combustion takes place
outside the engine.
Ex: Steam engine (or) a steam turbine.
 Internal combustion engines combustion takes place within the engine.
Ex: Gasoline (or) diesel engine.
In Case of I.C Engine analysis, there are three types of cycles as follows
a) Air standard cycles
b) Fuel-Air cycles
c) Actual cycles
About Air standard cycles :
The accurate analysis of I.C Engine process is very complicated. In order to understand
them it is advantageous to analyze the performance of an idealized closed cycle that
closely approximates the real cycle.
Following assumptions are lying under the air standard cycles

The working medium is assumed to be a perfect gas and follows the relation
PV=mRT

There is no change in the mass of the working medium.

All the processes that constitute the cycle are irreversible

Heat is assumed to be supplied from a constant high temperature source and not
from chemical reactions during the cycle

Some heat is assumed to be rejected to a constant low temperature sink during
the cycle

It is assumed that there are no heat losses from the system to the surroundings

The working medium has constant specific heats throughout the cycle.
About Fuel-Air cycles:
By air-standard cycle analysis, it is understood how the efficiency is improved by
increasing the compression ratio. However the analysis could not bring about the
effect of air-fuel ratio on thermal efficiency.
The fuel-air cycle analysis takes into account the following:

The actual composition of cylinder gases: The cylinder gas contains fuel, air, water
vapor and residual gas.

The variation in specific heat with temperature: Specific heats increase with the
increase of temperature.

The effect of dissociation: the fuel and air do not completely combine chemically at
high temperatures and this leads to presence of CO,H2,H and O2 at equilibrium
conditions

The variations in the number of molecules: The number of molecules present after
combustion depend upon fuel-air ratio and upon pressure and temperature of the
combustion.

There is no chemical change in either fuel or air prior to the combustion

Subsequent to combustion, the charge is always in chemical equilibrium

There is no heat exchange between the gases and the cylinder walls in any
process.

The burning takes place instantaneously(constant volume).
Actual cycles:
The actual cycles for internal combustion engines differ from air standard cycles in
many respects. The actual cycle efficiency is much lower than the air-standard
efficiency due to various losses occurring in the actual engine operations. The losses
are mainly due to
 The working substance being a mixture of air and fuel vapour or finely atomized liquid
fuel in air combined with the products of combustion left from the previous cycle.
 The change in the chemical composition of the substance
 The variations of specific heats with the temperature
 The progressive combustion rather than instantaneous combustion.
 The heat transfer to and from the working medium.
 Time loss factor
 Heat loss factor
 Exhaust blow down
 Gas exchange process or pumping loss
Engine Main Components:
(i) Cylinder:
It is a cylindrical vessel (or) space in which the piston makes a
reciprocating motion.
(ii) Piston:
It is a cylindrical component fitted into the cylinder forming the
moving boundary of the combustion system.
(iii) Combustion Chamber: The space enclosed in the upper part of the
cylinder, by the cylinder head and the piston top during the
combustion process.
(iv) Inlet Manifold:
The pipe which connects the intake system to the inlet
value of the engine and through which air (or) air-fuel
mixture is drawn into the cylinder is called the inlet
manifold.
(v) Exhaust Manifold: The pipe which connects the exhaust system to the
exhaust value of the engine and through which the
products of combustion escape into the atmosphere is
called the exhaust manifold.
(vi) Inlet and exhaust Valves: Regulating the charge coming into the cylinder
and for discharging the products of combustion from the
cylinder.
(vii) Spark Plug:
It is a component to initiate the combustion process in
Spark Ignition (SI) engines and is usually located on the
cylinder head.
(viii) Connecting rod: It interconnects the piston and the crankshaft and
transmits the gas forces from the piston to the
crankshaft.
(ix) Crankshaft:
In converts the reciprocating motion of the piston into
useful rotary motion of the output shaft the crankshaft is
enclosed in a crankcase.
(x) Piston Rings:
Piston rings, fitted into the slots around the piston,
provide a tight seal between the piston and the cylinder
wall thus preventing leakage of combustion gases.
(xi) Gudgeon Pin:
It forms the link between the small end of the
connecting rod and the piston.
(xii) Camshaft:
The camshaft and its associated parts control the
opening and closing of the two values. The camshaft is
driven by the crankshaft through timing gears.
(xiii) Cams:
These are made as integral parts of the camshaft and are
designed in such a way to open the values at the correct
timing and to keep them open for the necessary
duration.
(xiv) Fly Wheel:
The net torque imported to the crankshaft during one
complete cycle of operation of the engine fluctuates
causing a change in the angular velocity of the shaft. In
order to achieve a uniform torque an inertia mass in the
form of a wheel is attached to the output shaft and this
wheel is called the fly wheel.
Nomenclature:
(i) Cylinder Bore (d): The nominal inner diameter of the working cylinder is
called the cylinder bore and is designed by the letter ‘d’.
(ii) Piston area (A):
The area of a circle of diameter equal to the cylinder
bore is called the piston area and is designated by the
letter ‘A’
(iii) Stroke (L):
The nominal distance through which a working piston
moves between two successive reversals of its direction
of motion is called the stroke and is designated by the
letter ‘L’.
(iv) 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 is called the dead centre. There
are two dead centres in the engine as (a) Top dead
centre (T.D.C) (b) Bottom dead centre (B.D.C)
(a) Top Dead Centre (TDC):It is the dead centre when the piston has long
distance from the crankshaft. It is designated as TDC. It is also called
as Inner Dead Centre (IDC).
(b) Bottom Dead Centre (BDC): It is the dead centre when the piston has
shortest distance to the crankshaft. It is designated as BDC. It is also
called as the Outer Dead Centre (ODC)
(v) Displacement (or) Swept Volume (VS) or Stroke Volume: The nominal
volume swept by the working piston when it is travelling from one dead centre
to the other dead center is called the displacement volume.

VS  A  L  d 2 L
4
(vi) Clearance Volume (VC): The nominal volume of the combustion chamber
above the piston whent it is at the top dead centre is called the clearance
volume.
(vii) Compression Ratio (r): It is the ratio of the total cylinder volume when
the piston is at the bottom dead centre, VT to the
clearance volume VC.
VT
VC
VC  VS

VC
VS
 1
VC
r
Classification of internal Combustion engines
Classification based on
i) Cycle of operation
a) Otto cycle
b) Diesel cycle
ii) Types of fuel
a) Petrol, Gasoline, kerosene
b) Diesel
c) LPG, Natural gas
d) Solid fuel
e) Dual fuels
iii) Method of charging
a) Naturally aspirated engines
b) Super charged engines
iv) Type of ignition
a) Spark ignition
i)
Battery ignition
ii)
Magento ignition
b) Comp ignition
v) Type of cooling
a) Air cooling
b) Water cooling
vi) Cylinder
a) In-line engine
b) V-engine
c) Opposed cylinder engine
d) Radial engine
e) X-type engine
f) M-type engine
g) U-type engine
vii) Application of engines
a) Transportation
b) Marine
c) Power-plant
d) Space applications
Working of a 4-Stroke spark Ignition engine:
TDC Top Dead Centre
BDC Bottom Dead Centre
Vc
Clearance Volume
SP
Spark Plug
For Horizontal engine IDC Inner Dead Centre
ODC Outer Dead Centre
 Stroke: Movement of piston from one dead centre to other dead centre.
For a 4-stroke engine we get only one power stroke.
For a 4-stroke crank speed – 2 revolutions degrees (360+360=7200)
For two movements of a piston getting one power stroke is good.
The diagram is Value Timing Diagram (VTD)
 Working of a 4-stroke compression ignition engine same as Spark Ignition
(SI) engine but spark plug is replaced as fuel injector. It discharges diesel and
it combusts itself due to high pressure.
 Diesel has low self ignition temperature than petrol.
SI – Petrol – Quantity Governing (can control)
CI – Diesel – Quality Governing.
The compression ratio for
SI
6-10
CI
10-20
CI is costlier & weightier
We can regulate power in
CI
by regulating the quantity of diesel.
SI
by regulating the quantity of fuel.
 Four – Stroke spark Ignition engine: 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.
(i)
Suction
(ii)
Compression
(iii) Combustion
(iv)
Expansion
(v)
Exhaust
 Each stroke consists of 1800 of crankshaft rotation and hence a four-stroke
cycle is completed through 7200 of crank rotation.
 The cycle of operation for an ideal four-stroke SI engine are
(i)
Suction (or) Intake Stroke
(ii)
Compression Stroke
(iii) Expansion (or) Power Stroke
(iv)
Exhaust Stroke
(i) Suction (or) Intake Stroke: Suction, stroke 0
1 starts when the piston is
at the top dead centre and about to move downwards. The inlet value is open
at this time and the exhaust is closed.
(ii) Compression Stroke: The charge taken into the cylinder during the suction
stroke is compressed by the return stroke of the piston 1
2 during this
stroke both inlet and exhaust values are in the closed position. A spark plug
located on the cylinder head burning takes place the fuel is converted into heat
energy producing a temperature rise of about 20000C (2
3).
(iii) Expansion (or) Power Stroke: The high pressure of the burnt gases forces
the piston towards the BDC, stroke 3
4 with both the inlet and exhaust
values remaining closed.
(iv) Exhaust Stroke: At the end of the expansion stroke the exhaust value
opens and the inlet value remains closed. The piston moves from the bottom
dead centre to top dead centre (stroke 5
0).
(ANIMATION)
 Four-Stroke compression Ignition 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 varies from 6 to 10 while for a CI engine it is from 16 to 20.
 Due to the high compression ratio employed, the temp. at the end of the
compression stroke is sufficiently high.
 The ideal sequence of operation 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 value is being closed.
(ii)
Compression Stroke: Air inducted during the suction stroke is
compressed into the clearance volume. Both values remain closed
during this stroke.
(iii) Expansion Stroke: Fuel injection starts nearly at the end of the
compression stroke. Heat is assumed to have been added at constant
pressure. Both values remain closed during the expansion stroke.
(iv)
Exhaust Stroke: The piston travelling from BDC to TDC pushes out
the products of combustion. The exhaust value is open and the intake
value is closed during this stroke.
Working principles of four stroke and two stroke engines
(i)
In Four-stroke engines, there is one power stroke for every two
revolutions of the crankshaft.
(ii)
(iii)
(iv)
(v)
There are two non-productive strokes of exhaust and suction which are
necessary for flushing the products of combustion from the cylinder
and filling it with the fresh charge.
If this purpose could be served by an alternative arrangement, without
the movement of the piston, it is possible to obtain a power stroke for
every revolution of the crankshaft
(increases) the o/p of the engine.
However, in both SI and CI engines operating on four-stroke cycle,
power can be obtained only in every two revolution of the crankshaft.
Since, both SI and CI engines have much in common, it is worth while
to compare then based on important parameters like basic cycle of
operation, fuel induction, compression ratio etc.
Two-Stroke engine:
(i)
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.
(ii)
In such an arrangement, theoretically the power o/p, of the engine can
be doubled for the same speed compared to a four-stroke engine.
(iii) In two-stroke engines the cycle is completed in one revolution of the
crankshaft.
(iv)
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.
(v)
In the four-stroke engine these operations are performed by the engine
piston during the suction and exhaust stroke respectively.
(vi)
In a two-stroke engine the filling process is accomplished by the
charge compressed in crankcase (or) by a blower.
(vii) The induction of the compressed in charge moves out the product of
combustion through exhaust parts. Therefore, no piston strokes are
required for these two operations.
(viii) Two strokes are sufficient to complete the cycle, one far compression
the fresh charge and the other far expansion (or) power stroke.
ACTUAL INDICATOR DIAGRAM FOR A TWO STROKE CYCLE
PETROLENGINE
The actual indicator diagram for a two -stroke cycle petrol engine is shown
in suction is shown by the line 1 -2-3, i.e. from the instant transfer port
pens (TPO) and transfer port closes (TPC).
We know that during the suction stage, the exhaust port is also open. In the first
half of suction stage, the volume of fuel -air mixture and burnt gases
increases. This happens as the piston moves from I to 2 (i.e. BDC).
In the second half of the suction stage, the volume of charge and burnt gases
decreases. This happens as the piston moves upwards from 2 to 3. A little beyond
3, the exhaust port closes (EPC) at 4. Now the charge inside the engine cylinder is
compressed which is shown by the line 4-5.
At the end of the compression, there is an increase in the pressure
i n s i d e t h e e n g i n e cylinder. Shortly before the end of compression
(i.e. TDC) the charge is ignited (IGN) with the help of spark plug. The sparking
suddenly increases pressure and temperature of the products of combustion. But
the volume, practically, remains constant as shown by the line 5-6. The
expansion is shown by the line 6-7. Now the exhaust port opens (EPO) at 7, and the
burnt gases are exhausted into the atmosphere through the exhaust port. It
reduces the pressure. As the piston is moving towards BDC, therefore
volume of burnt gases increases from 7 to 1. At 1, the transfer port opens
(TPO) and the suction starts.
Fig Two-Stroke Engine (ANIMATION)
Description
Basic cycle
SI Engine
Otto cycle (or) constant volume
heat addition cycle.
CI Engine
Diesel cycle (or) constant
pressure heat addition cycle.
Fuel
Gasoline, a highly volatile fuel,
self-ignition temp. is high.
Diesel oil, a non-volatile fuel
self-ignition temp. is
comparatively low.
Introduction
A gaseous mixture of fuel and
Fuel is injected directly into the
of fuel
air is introduced during the
suction stroke. A carburetor is
necessary to provide the
mixture.
combustion chamber at high
pressure at the end of the
compression stroke. A fuel pump
and injector and 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 as ignition system
with spark plug in the
combustion chamber. Primary
voltage is provided by a battery
or a magneto.
Compression 6 to 10 upper limit is fixed by
ratio
antiknock quality of the fuel.
Self-ignition occurs due to high
temp. of air because of the high
compression. Ignition system
and spark plug are not necessary.
Speed
Due to light weight and also the
homogeneous combustion, they
are high speed engines.
Due to heavy weight & also due
to heterogeneous combustion,
they are low speed engines.
Thermal
efficiency
Because of lower CR, the max.
value of thermal efficiency that
can be obtained is lower.
Because of higher CR, the max.
value of thermal efficiency that
can be obtained as higher.
Weight
Lighter due to lower peak
pressures.
Heavier due to higher peak
pressures.
16 to 20 upper limit is limited,
by weight of the engine.
Comparison of Four Stroke and Two-Stroke Cycle Engines.
Four-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.
Two-Stroke Engine
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 fly
wheel is needed.
Because of the above, turning moment is
more uniform and hence a lighter fly
wheel can be used.
Again, because of one power stroke for
two revolutions, power produced for
some 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 (or) far the same power
the engine is lighter and more compact.
Because of one power stroke in two
revolutions lesser cooling and lubrication
Because of one power stroke in one
revolution greater cooling and lubrication
requirements. Higher rate of wear and
requirements. Lower rate of wear and
tear.
The four-stroke engine contains values
and value actuating mechanisms to open
and close the values.
Because of the heavy weight and
complicated value mechanism, the initial
cost of the engine is more.
Volumetric efficiency is more due to
more time for induction.
tear.
Two-stroke engines have no values but
only parts.
Because of light weight and simplicity
due to the absence of value mechanism,
initial cost of the engine is less.
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 fourstroke cycle engine.
Used where efficiency is important, viz,
in cars, buses, trucks, tractors, industrial
engines, aero planes, power generation
etc.,.
Used where low cost, compactness and
light weight are important, viz, in
mopeds, scooters, motor cycles, hand
sprayers etc.,.
In Four-stroke we have valve mechanism.
Costlier
In two-stroke we have port mechanism.
Cheaper.
Valve Timing Diagram:
A valve timing diagram is a graphical representation of the exact moments, in the
sequence of operations, at which the two valves (i.e. inlet and exhaust
valves) open and close as well as firing of the fuel. It is, generally, expressed in
terms of angular positions of the crankshaft. Here we shall discuss theoretical valve
timing diagrams for four stroke and two stroke cycle engines.
Fig: Theoretical valve timing diagram
1. Theoretical valve timing diagram for four stroke cycle engine
The theoretical valve timing diagram for a four -stroke cycle engine is
shown Inthis diagram, the inlet valve opens at A and the suction takes
place from A to B. The crankshaft revolves through 180 0 and the piston moves
from T.D.C. to B.D.C. At B, the inlet valve closes and the compression takes place
from B to C. The crankshaft revolves through 1800 and the piston moves from
B.D.C. to T.D.C. At C, the fuel is fired and the expansion takes place from C to D.
The crankshaft revolves through 1800 and the piston again moves from T.D.C. to
B.D.C. At D, the exhaust valve opens and the exhaust takes place from D to E.
The crankshaft again revolves through 1800 and the piston moves back to T.D. C.
2. Theoretical Port timing diagram for two-stroke cycle engine
.
The theoretical Port timing diagram for a two-stroke cycle engine is shown. In this
diagram, the fuel is fired at A and the expansion of gases takes place from
A to B. The crankshaft revolves through approximately 1200 and the piston moves
from T.D.C. towards B.D.C. At B, the valves open and suction as well as exhaust
take place from B to C. The crankshaft revolves through approximately 120 0
a n d t h e p i s t o n m o v e s f i r s t t o B.D.C
and then little upwards. At C. both the valves close and co mpression takes
place from C to A. The crankshaft revolves through approximately 1200 and the
piston moves to T.D.C
ACTUAL VALVE TIMING DIAGRAM FOR A FOUR
S T R O K E C Y C L E P E T R O L ENGINE:
In the valve timing diagram, as shown we see that the inlet valve opens before
the piston reaches TDC o r i n o t h e r w o r d s , w h i l e t h e p i s t o n i s s t i l l
m o v i n g u p b e f o r e t h e beginning of the suction stroke. Now the pi ston
reaches the TDC and the suction stroke
starts.
The piston reaches the BDC and then starts moving up. The inlet valve
closes, when the crank has moved a little beyond the BDC
This is done as the incoming charge continues to flow into the cylinder
although the piston is moving upwards from BDC
Now the charge is compressed (with both valves closed) and then and temperature)
push the piston downwards with full force and the expansion or working
stroke takes place. Now the exhaust valve opens before the piston again
reaches BDC and the burnt gases start leaving the engine cylinder. Now the
piston reaches BDC and then starts moving up, thus performing the exhaust stroke.
The inlet valve opens before the piston reaches TDC to start suction stroke. This is
done as the fresh incoming charge helps in pushing out the burnt gases. Now the
piston again reaches TDC, and the suction stroke starts. The exit valve
closes after the crank has moved a little beyond the TDC.
This is done as the burnt gases continue to leave the engine cylinder although the
piston is moving downwards. It may be noted that for a small fraction of a
crank revolution, both the inlet and outlet valves are open. This is known as
valve overlap.
PORT TIMING DIAGRAM FOR A TWO-STROKE CYCLE PETROL ENGINE
In the port timing diagram, as shown we see that the expansion of the
charge( a f t e r i g n i t i o n ) s t a r t s a s t h e p i s t o n m o v e s f r o m TDC towards
BDC. F i r s t o f a l l , t h e exhaust port opens before the piston reaches BDC
and the burnt gases start leaving the cylinder. After a small fraction of the
crank revolution, the transfer port also opens and the fresh fuel -air
mixture enters into the engine cylinder. This is done as the fresh incoming charge
helps in p u s h i n g o u t t h e b u r n t g a s e s . N o w t h e p i s t o n r e a c h e s BDC
a n d t h e n s t a r t s m o v i n g upwards. As the crank moves a little beyond BDC,
first the transfer port closes and then the exhaust port also closes. This is done
to suck fresh charge through the transfer port and to exhaust the burnt gases
through the exhaust port simultaneously. Now the charge i s c o m p r e s s e d w i t h
both ports closed, and then ignited with the help of a spark
p l u g before the end of compression stroke. This is done as the charge
requires some time toignite. By the time the piston reaches TDC, the burnt
gases (under high pressure and temperature) push the piston downwards with full
force and expansion of the burnt gases takes place. It may be noted that the
exhaust and transfer ports open and close at equal angles on either side of the
BDC position.
CARBURETTOR :
The carburetor is a device for *atomizing and**vaporizing the fuel and mixing it
with the air in the varying proportions to suit the changing operating conditions of
the engine. The process of breaking up and mixing the fuel with the air is called
carburetion.
There are many types of the carburetors in use, but the simplest form
of the carburetor is shown in fig. It consists of a fuel jet located in the centre of the
choke tube. A float chamber is provided for maintaining the level of the fuel jet
located in the centre of the choke tube. A float chamber is provided for
maintaining the level of the fuel in the jet is controlled by a float and lever which
operates its needle valve. The fuel is pumped into the float chamber and when the
correct level of the fuel is reached, the float closes the needle valve, and shuts off
the petrol supply.
The suction produced by the engine draws air through the choke tube.
The reduced diameter of the choke tube increases the velocity of air and reduces
the pressure. The high velocity and low pressure in the tube facilities the breaking
up of the fuel and its admixture with the air. A throttle valve controls the flow of
the mixture delivered to the engine cylinder.
Fig: Carburettor
FUEL PUMP
The main object of a fuel pump in a diesel engine is to
deliver a fuel to the i n j e c t o r w h i c h s p r a y s t h e f i n e l y d i v i d e d
p a r t i c l e s o f t h e f u e l s u i t a b l e f o r r a p i d combustion. T h e
simplified sketch of a fuel pump is shown. It consist of a plunger
which moves up and down in the barrel by the cam and spring
a r r a n g e m e n t p r o v i d e d f o r pushing and lowering the plunger respectively. The
fuel oil is highly filtered by means of felt-pack filter before entering the barrel of the
pump. The upper end part of the plunger is cut away in a helix shaped piece
forming a groove between the plunger and barrel, which is the most
important one. Therefore, the amount of fuel delivered and injected into
the engine cylinder depends upon the rotary position of the plunger in the
barrel. Figure (b) and (c) shows how the top part of the plunger is designed
so that the correct amount of fuel is delivered to the injector. When the plunger is
at the bottom of its stroke as shown ill Figure (b), the fuel enters the barrel
through the inlet port. As the plunger rises, it forces this fuel up into the
injector, until the upper part cut away comes opposite the sill port. Then the fuel
escapes down the groove and out through the sill port so that injection
ceases, as shown in Figure (c). The plunger can be made to rotate in the barrel
and therefore more fuel is injected. When the plunger is rotated so that the
groove is opposite to the sill port, no fuel at all is injected and thus the engine stops
Classification of injection systems
The injection systems can be classified as follows
(1) Air injection systems
(2) Solid injection systems
Air injection systems
In this system, fuel is forced into the cylinder by means of compressed
air. This system is little used nowadays, because it requires a bulky multi-stage
air compressor. This causes an increase in engine weight and reduces the
brake power output further. One advantage that is claimed for the air injection
system is good mixture of fuel with the air with resultant higher mean
effective pressure. Another is the ability to utilize fuels of high viscosity
which are less expensive than those used by the engines with solid injection
systems. These advantages are off-set by the requirement of a multistage
compressor thereby making the air- injection system obsolete.
Solid injection system
In this system the liquid fuel is injected directly into the combustion
chamber without the aid of compressed air. Hence, it is also called airless
mechanical injection or solid injection system. Solid injection system can be
classified into four types.
(1) Individual pump and nozzle system
(2) Unit injector system
(3) Common rail system
Individual pump and nozzle system
The details of the individual pump and nozzle system are shown in fig.
(a) an (b).In this system ,each cylinder is provided with one pump and one
injector. In this arrangement a separate metering and compression pump is
provided for each cylinder. The pump may be placed close to the cylinder as
shown in fig. (a) or they may be arranged in a cluster as shown in fig.(b). The
high pressure pump plunger is actuated by a cam, and produces the fuel
pressure necessary to open the injector valve at the correct time. The amount
of fuel injected depends on the effective stroke of the plunger.
Figure in pending
Unit injector system
The unit injector system is one in which the pump and the injector
nozzle are combined in one housing. Each cylinder is provided with one of these
unit injectors. Fuel is brought up to the injector by a low pressure pump, where at
the proper time, a rocker arm actuates the plunger and thus injects the fuel into the
cylinder. The amount of fuel injected is regulated by the effective stroke of the
plunger. The pump and the injector can be integrated in one unit as shown in Fig.
(c) Figure in pending
Common Rail System
In the common rail system, fig.(d), a HP pump supplies fuel, under
high pressure, to a fuel header. High pressure in the header forces the fuel to
each of the nozzles located in the cylinders. At the proper time, a mechanically
operated (by means of a push rod and rocker arm) valve allows the fuel to
enter the proper cylinder through the nozzle. The pressure in the fuel to enter
the proper cylinder through the nozzle. The pressure in the fuel header must be
that, for which the injector system was designed, i.e., it must enable to
penetrate and disperse the fuel in the combustion chamber. The amount of fuel
entering the cylinder is regulated by varying the length of the push rod stroke.
A high pressure is used for supplying fuel to a header, from where the
fuel is metered by injectors (assigned one per cylinder). Figure in pending
INJECTOR OR ATOMISER
The injector or atomiser is also an important part of the diesel
e n g i n e w h i c h breaks up the fuel and sprays into the cylinder into a very
fine divided particles. Figure s h o w s t h e t y p e o f a n i n j e c t o r i n
w h i c h f u e l i s d e l i v e r e d f r o m t h e p u m p a l o n g t h e horizontal pipe
connected at A. The vertical spindle of the injector is spring loaded at the top which
holds the spindle down with a pressure of 140 bar so that the fuel pressure
must reach this value before the nozzle will lift to allow fuel to be injected into the
engine cylinder. The fuel which leaks past the vertical spindle is taken off by means
of an outlet pipe fitted at B above the fuel inlet pipe
Fig: Fuel Injector
BATTERY IGNITION SYSTEM:
BASIC IGNITION
SYSTEM(BATTERY)
Fig: Battery ignition system for a six-cylinder engine
Introduction
The electrical discharge produced between the two electrodes of a spark plug by the
ignition system starts the combustion process in a spark ignition engine. The function
of the ignition system is to initiate this flame propagation process.
Energy requirements for ignition
An ignition process obeys the law of conservation of energy. Hence, it can be treated
as a balance of energy between
(i) That provided by an external source
(ii) That released by chemical reaction and
(iii)That dissipated to the surroundings by meaning of thermal conduction
convection and radiation.
The spark energy and duration :
With a homogeneous mixture in the cylinder, spark energy of the order of 1 kv and a
duration of a few micro-seconds would suffice to initiate the combustion process.
Energy requirements for ignition :
An ignition process obeys the law of conservation of energy. Hence, it can be treated
as a balance of energy between
(i) That provided by an external source
(ii) That released by chemical reaction and
(iii)That dissipated to the surroundings by meaning of thermal conduction,
convection and radiation.
The spark energy and duration :
With a homogeneous mixture in the cylinder, spark energy of the order of 1MJ and a
duration of a few micro-seconds would suffice to initiate the combustion process.
Ignition System : A conventional ignition system should provide sufficiently large
voltage across the spark plug electrodes to effect the spark discharge. Further, it
should supply the required energy for the spark to ignite the combustible mixture.
Conventional ignition system should take these factors [optimum spark timing varies
with engine speed inlet manifold pressure & misture composition] into account to
provide the spark of proper energy.
Spark – Ignition System :
Air is a poor conductor of electricity an air gap in an electric circuit act as a high
resistance. But when a high voltage is applied across the electrodes of a spark plug it
produces a spark across the gap. When such a spark is produced to ignite a
homogeneous air-fuel mixture in the combustion chamber of an engine it is called the
spark-ignition system.
The ignition systems are classified depending upon how the primary energy
for operation the circuit is made available as:
(i) Battery ignition systems
(ii) Magneto ignition systems
(i) It should provide a good spark between the electrodes of the plugs at the
correct timing.
(ii) It should function efficiently over the entire range of engine speed.
(iii)It should be light, effective and reliable in service.
(iv) It should be compact and easy to maintain.
(v) It should be cheap and convenient to handle.
(vi) The interference from the high voltage source should not affect the
functioning of the radio & television receives inside an automobile.
Battery Ignition System :
Most of the modern spark –ignition engines use battery ignition system.
Battery – 6 or 12 volt battery
Ex: Passenger cars, light trucks, some motorcycles & large stationary engines are
fitted with battery ignition system.
The essential components of the system are :
(i) Battery
(ii) Ignition switch
Primary side of the ignition coil
(iii)Ballast resistor
(iv) Ignition coil
(v) Contact breaker
(vi) Capacitor
(vii) Distributor
Secondary side of the ignition coil
(viii) Spark plug
Battery
Storage Battery : To provide electrical energy for ignition
Cell connections for 12 volt battery
Two types of batteries are used for spark-ignition engines
(i) The lead acid battery
(ii) The alkaline battery
The former is used in light duty commercial vehicles and the later on heavy duty
commercial vehicles
Ignition switch : Battery is connected to the primary winding of the ignition coil
through and ignition switch and ballast resistor with the help of the ignition switch the
ignition system can be turned on or off.
Ballast resistor :
A ballast resistor is provided in series with the primary winding to regulate the
primary current.
Ignition Coil :
Ignition coil is the source of the ignition energy in the conventional ignition system
Secondary coil consists – 21,000 turns, 38-40 gauge enameled wire Primary winding,
located outside the secondary coil -> 200-300 turns
20 Guage wire
Resistance – 1.15 
Figure is required
At low speed the amount of spark produced are low for H.T magneto efficient system
& it produces many starting troubles the weight is also very high.
Difference between battery ignition & magneto ignition system :
Battery
Magneto ignition
Battery is required & hence there will be
a problem of discharge
Battery is not required & hence there is
no problem of discharge
More maintenance required
Less maintenance
Charge flows form battery to the primary
circuit
Magneto generates the electric charge
Good spark is available at plugs even at
low speed
Spark is poor at the starting & lower
speed
At correspondingly higher ranges of
speed the performance will be reduced
At correspondingly higher speed ranges
performance will 
Occupies more space
Occupies less space
Used in cars & UCV (light commercial
vehicles)
In racing cars & two wheelers
Contact Breaker :
This is a mechanical device for making and breaking the primary circuit of the
ignition coil usually tungsten and each point has a circular flat fact of about 3mm dia.
Capacitor :
The construction of the ignition capacitor is the same as that of every electrical
capacitor.
Distributor :
The function of the distributer is to distribution the ignition surges to the individual
spark plugs in the correct sequence and at the correct instants in time. There are two
types of distributors, the brush type and the gap type.
Spark Plug:
The spark plug provides the 2 electrodes with a proper gap across which the high
potential discharges to generate a spark and ignite the combustible mixture within the
combustion chamber. Spark plugs are usually classified as hot plugs or cold plugs
depending upon the relative operating temp range. The type of spark plug used in an
engine depends on the particular requirements.
Operation of a battery ignition system:
The source of the ignition energy in the battery ignition system is the ignition coil.
This coil stores the energy in its magnetic field and delivers it at the instant of ignition
(Firing pt) in the form of a surge of high voltage current (ignition pulse) though the
high tension ignition cables to the correct spark plug. Storage of energy in the
magnetic field is based on an inductive process as a result of which we also designate
the ignition coil as an inductive storage device. The schematic diagram of a
conventional battery ignition system for a four-cylinder engine.
Limitations :
(i) The primary voltage decreases as the engine speed  (increases) due to the
limitations in the current switching capability of the breaker system.
(ii) Time available for build-up of the current in the primary coil and the stored
energy  (decreases) due to the dwell period becoming shorter.
(iii)Because of the high source impedance (about 500K  )
Lubrication:
Lubrication is the art of admitting a lubrication (oil, gases) between two
surfaces that are in contact & in relative motion.
Types of lubrication system
1. Mist lubrication system’
2. Wet – sump lubrication system
3. Dry – sump lubrication system
Basic components of wet sump lubrication system:
Fig Splash Lubrication
Dry-sump lubrication system:
Fig: Dry Sump Lubrication
Properties of lubrication
i) Viscosity
ii) Flash & Fire points
iii) Cloud & Pour points
iv) Oiliness & Film strength
v) Corrosiveness
vi) Detergency
vii) Stability
viii)
Foaming
Types of liquid cooling systems:
1. Thermo psyphon system
2. Forced circulation cooling system
3. Evaporating cooling system
4. Pressure cooling system
Advantages of liquid – Cooling system
1. Compact design & smaller frontal area.
2. Fuel consumption is rather lower when compared to air cooled ones.
3. Uniform cooling of the parts particularly engine barrel & head because of
jacketing & thus ensuring lower temperature at valve seats.
4. Insulation of cooling system is not necessary .
Limitations:
1. This is a dependent system where water circulation has to be ensured by
additional means.
2. Power absorbed by the pump for circulation is considerable & effects the
power output of the engine.
3. In case of failure of cooling systems there will be severe damage in the
system.
4.
Cost is also considerably high.
5. System requires considerable maintenance also.
Advantages of air – cooled system:
1. Design of the engine become simpler has no water jacket air required & they
are cheaper to renew in case of accidents.
2. Less maintenances problems because of absence of cooling pipes, radiator etc.
3. No damage of leakage of the cooling.
4. No freezing trouble & no cold starting trouble.
5. Higher power to weight ratio.
6. Installation is easy.
Limitations:
1. Limited to only small & medium power range.
2. Suitable in cases where ambient temperature are lower.
3. Cooling is not uniform
4. More aerodynamic noise.
5. SFC is slightly higher.
6. Lower maximum allowable compression rations.
Comparison of Air Cooling and Water Cooling Systems:
Air Cooling Systems
Water Cooling Systems
The design of the system is simple and The design of this system
less costly.
complicated and more costly.
is
The mass of the cooling system (per The mass of the cooling system (per
b.p. of the engine) is very less.
b.p. of the engine) is much more.
The fuel consumption (per b.p. of the The fuel consumption (per b.p. of the
engine) is more.
engine) is less.
Its installation and maintenance is very Its installation and maintenance is
easy and less costly.
difficult and more costly.
There is no danger of leakage or There is no danger of leakage or
freezing of the coolant.
freezing of the coolant.
It works smoothly and continuously. If the system fails, it may cause serious
Moreover it does not depend on nay damage to the engine within a short
coolant.
time.
COOLING SYSTEMS FOR I.C. ENGINES
We have already discussed, in the last article, the adverse effects of
overheating of an I.C. engine and characteristics of the cooling system
adopted. The following two systems are used for cooling the I.C. engines these
days:
1. Air cooling system:
The air cooling system, as shown , is used in the engines of motor cycles, scooters,
aeroplanes and other stationary installations. In countries wit h cold
climate, this system is also used in car engines. In this system, the heat is
dissipated d i r e c t l y t o t h e a t m o s p h e r i c a i r b y c o n d u c t i o n t h r o u g h
t h e c yl i n d e r w a l l s . I n o r d e r t o increase the rate of cooling, the outer
surface area of the cylinder and c ylinder head is increased by providing
radiating fins and flanges. In bigger units, fans are provided to circulate the
air around the cylinder walls and cylinder head.
Fig: Air Cooling
2. Water cooling system (Thermosyphon system of cooling).
The water cooling system as shown , is used in the engines of cars, buses, trucks etc.
In this system, the water is circulated through water jackets around each of the
combustion chambers, cylinders, valve seats and valve stems. The water is
kept continuously in motion by a centrifugal water pump which is driven by a
V-belt from the pulley on the engine crank shaft. After
Fig: Thermosyphon system of cooling
passing through the engine jackets in the cylinder block and heads, the
water is passed through the radiator. In the radiator, the water is cooled by air
drawn through the radiator by a fan. Usually, fan and water pump are mounted and
driven on a common shaft. After p a s s i n g t h r o u g h t h e r a d i a t o r , t h e w a t e r
i s d r a i n e d a n d d e l i v e r e d t o t h e w a t e r p u m p through a cylinder inlet
passage. The water is again circulated through the engine jackets.