12 Thermal Engineering

8/9/2019 12 Thermal Engineering

http://slidepdf.com/reader/full/12-thermal-engineering 1/41



2. SWEPT VOLUME VS

&he volume swept by the piston corresponding to its stroke length is called stroke

volume or swept volume

3. AIR STANDARD EFFICIENCY

!t is the ratio of ideal work done to the heat supplied

Air standard efficiency -Supplied Heat

DoneWork Ideal

4. THERMAL EFFICIENCY

&he thermal efficiency of the engine is defined as the ratio of actual work done to

heat supplied

&hermal efficiency - Supplied Heat

DoneWork Actual

OTTO CYCLE



8park ignition engines operate on theoretical Otto cycle" &he cycle is represented onP9V# and &98# diagrams"

Otto cycle consists of two constant volume and two reversible adiabatic processes as

shown" a# Process 1!2" # :eversible adiabatic compression or isentropic

compression during which air is compressed from state9l to state9" &he

law of process is PV5- C"

b# Process 2!3"# %eat is added to air from a heat reservoir at constant

volume and state changes from state9 to state97"

%eat supplied, ;97 - m Cv &7 9 & # "

c# Process 3!4"# &he air e$pands from state97 to state96 reversible

adiabatically according to law PV5- C"

d# Process 4!1"# During this process heat is re<ected at constant volume and

the system returns to its original state" &herefore, a cycle is completed"%eat re<ected, ;69+ - m Cv &69&!#

=fficiency -Supplied Heat

jected Heat Supplied Heat

ed HeatSuppli

Workdone :e−

=

=fficiency -,7

+6+

T T

T T

−

−

−

( ) +

++

−

−=γ

η r

>ean effective pressure

Pm -( )( )

( )( ){ }++++

++−−

−−

−

pr r r

r P γ

γ



DIESEL CYCLE

&he difference between otto and diesel cycle is in the heat addition process" !ncase of otto cycle the heat is added at constant volume while in case of diesel cycle, the

heat is added at constant pressure"

Process 1!2" # :eversible adiabatic compression or isentropic compression

during which air is compressed from state9l to state9" &he law of process isPV5- C"

Process 2!3"# %eat is added to air from a heat reservoir at constant Pressureand state changes from state9 to state97"

%eat supplied, ;97 - m Cv &7 9 & # "

Process 3!4"# &he air e$pands from state97 to state96 reversible adiabatically

according to law PV5- C"

Process 4!1"# During this process heat is re<ected at constant volume and thesystem returns to its original state" &herefore, a cycle is completed"

%eat re<ected, ;69+ - m Cv &69&!#



DUAL CYCLE



&his cycle is a combination of Otto and Diesel cycles" !t is sometimes called semidiesel cycle because semi diesel engine work on this cycle" !n this cycle heat is absorbed

partly at a constant volume and partly at a constant pressure" &he ideal dual combustion

cycle consists of two reversible adiabatic or isentropic, two constant volume and aconstant pressure processes" &hese processes are represented on PV and &8 diagram"

Process 1!2" # :eversible adiabatic compression or isentropic compressionduring which air is compressed from state9l to state9" &he law of process is PV5- C"

Process 2!3"# %eat is added to air from a heat reservoir at constant volume and

state changes from state9 to state97"%eat supplied, ;97 - m Cv &7 9 & # "

Process 3!4"# %eat is added to air from a heat reservoir at constant pressure and

state changes from state97 to state96"%eat supplied, ;796 - m Cp &6 ? &7 # "

Process 4!$"# &he air e$pands from state96 to state9/ reversible adiabatically

according to law PV5- C"

Process $!1"# During this process heat is re<ected at constant volume and the

system returns to its original state" &herefore, a cycle is completed"

%eat re<ected, ;/9+ - m Cv &/9&+#

=fficiency -Supplied Heat

DoneWork

>ean effective pressure

Pm -

( ) ( )[ ] ( )( )[ ]( ) ( )++

++++

+

−−

−−−+−−

ρ

ρ ρ γ γ γ

r

r r r r r P p p p

GAS TUR%INES



OPEN CYCLE GAS TUR%INE

&he fresh charge is supplied to the compressor in each cycle" &he compressed air

is mi$ed with fuel and combustion occurs in the combustion chamber" &he combustiongases leaving the turbine after e$pansion are e$pelled to the atmosphere" &hus the cycle

becomes an open cycle"

CLOSED CYCLE GAS TUR%INE

!n a closed cycle gas turbine the working substance air undergoes the cycle

repeatedly" &he combustion process is replaced by heat addition from e$ternal source ande$haust process is replaced by heat re<ection process to the surroundings"

&he closed cycle gas turbine work on 0oule@s or Brayton@s cycle as shown



&he process +9 shows the isentropic compression of the air in the compressor"

&he process 97 shows the heating of air in heating chamber at constant pressure" &he

process 796 shows the isentropic e$pansion in the turbine" &he process 69+ shows coolingof air at constant pressure in cooling chamber"

ork done by the turbine per kg of air

& - Cp &7 ? &6#

ork re)uired by the compressor per kg of air C - Cp & ? &+#

et work available

- & 9 C



UNIT II

INTERNAL COM%USTION ENGINES

Classification ? Components and their function ? Valve timing diagram and port timingdiagram ? Comparison of two stroke and four stroke engines ? Carburetor system, Diesel

pump and in<ector system Performance calculation ? Comparison of petrol and diesel

pump and in<ector system ? Performance calculation ? Comparison of petrol and dieselengine ? ubrication system and Cooling system ? Battery and >agneto ignition system9

*ormation of e$haust emission in 8! and C! engines

INTRODUCTION

As the name implies or suggests, the internal combustion engines briefly written

as !C engines# are those engines in which the combustion of fuel takes place inside theengine cylinder" &hese are petrol, diesel, and gas engines" e have seen in steam engines

or steam turbines that the fuel, fed into the cylinder, is in the form of steam which is

already heated or superheated#, and is ready for working in the combustion cycle of the

engine" But, in case of internal combustion engines, the combustion of fuel takes placeinside the engine cylinder by a spark and produces very high temperature as compared to

steam engines" &he high temperature produced may ruin the metal of cylinder, valves,etc" !t is, therefore, necessary to abstract some of heat from the engine cylinder" &he

abstraction of heat or the cooling of cylinder may be effected by the surrounding air as in

case of a motor cycle or aeroplane engine or by circulating water through <acketssurrounding the cylinder barrel and cylinder head" &he water cooling is mostly adopted

for large pistons"

CLASSIFICATION OF IC ENGINES

&he internal combustion engines may be classified in many ways, but the

following are important from the sub<ect point of view

+" According to the type of fuel useda# Petrol engines" b# Diesel engines or oil engines, and c# Eas engines"

" According to the method of igniting the fuel

a# 8park ignition engines briefly written as 8"+" engines#, b# Compressionignition engines briefly written as C"!" engines#, and c# %ot spot ignition engines

7" According to the number of strokes per cycle

a# *our stroke cycle engines, and b# &wo stroke cycle engines"

6" According to the cycle of operationa# Otto" cycle also known as constant volume cycle# engines, b# Diesel cycle

also known as constant pressure cycle# engines, and c# Dual combustion cycle

also known as semi9diesel cycle# engines"/" According to the speed of the engine

a# 8low speed engines, b# >edium speed engines, c# %igh speed engines"

F" According to the cooling systema# Air9cooled engines" b# ater9cooled engines" c# =vaporative cooling

engines"

3" According to the method of fuel in<ection



a# Carburetor engines, b# Air in<ection engines, c# Airless or solid in<ection

engines"

4" According to the number of cylindersa# 8ingle cylinder engines b# >ulti9cylinder engines"

G" According to the arrangement of cylinders

a# Vertical engines, b# %oriHontal engines, c# :adial engines, d# !n9line multi9cylinder engines, e#V9type multi9cylinder engines, <# Opposite9cylinder engines,

g# Opposite9 piston engines"

MAIN COMPONENTS OF IC ENGINES

As a matter of fact, an !C engine consists of hundreds of different parts, which are

important for its proper working" &he description of all these parts is beyond the scope of this book" %owever, the main components, which are important from academic point of

view, are shown and are discussed below'

1. C&'()*er. !t is one of the most important part of the engine, in which the piston movesto and fro in order to develop power" Eenerally, the engine cylinder has to withstand a

high pressure more than /. bar# and temperature more than ...IC#" &hus the materials

for an engine cylinder should be such that it can retain sufficient strength at such a high pressure and temperature" *or ordinary engines, the cylinder is made of ordinary castiron" But for heavy duty engines, it is made of steel alloys or aluminium alloys" !n case of

multiple cylinder engines, the cylinders are cast in one block known as cylinder block"

8ometimes, a liner or sleeve is inserted into the cylinder, which can be replaced whenworn out" As the material re)uired for liner is comparatively small, it can be made of



alloy cast iron having long life and sufficient resistance to rapid wear and tear to the fast

moving reciprocating parts"

2. C&'()*er +e,*# !t is fitted on one end of the cylinder, and acts as a cover to close thecylinder bore" Eenerally, the cylinder head contains inlet and e$it valves for admitting

fresh charge and e$hausting the burnt gases" !n petrol engines, the cylinder head also

contains a spark plug for igniting the fuel9air mi$ture, towards the end of compressionstroke" But in diesel engines, the cylinder head contains noHHle (i.e. fuel valve# for

in<ecting the fuel into the cylinder" &he cylinder head is, usually, cast as one piece and

bolted to one end of the cylinder" Eenerally, the cylinder block and cylinder head aremade from the same material" A copper or asbestos gasket is provided between the engine

cylinder and cylinder head to make an air9tight <oint"

3. P(s-o)# !t is considered as the heart of an l"c" engine, whose main function is to

transmit the force e$e!+ed by the burning of charge to the connecting rod" &he pistons aregenerally made of aluminium alloys which are light in weight" &hey have good heat

conducting property and also greater strength at higher temperatures"

4. P(s-o) r()s# &hese are circular rings and made of special steel alloys which retain

elastic properties even at high temperatures" &he piston rings are housed in thecircumferential grooves provided on the outer surface of the piston" Eenerally, there are

two sets of rings mounted for the piston" &he function of the upper rings is to provide air tight seal to prevent leakage of the burnt gases into the lower portion" 8imilarly, the

function of the lower rings is to provide effective seal to prevent leakage of the oil into

the engine cylinder"

$. Co))ec-() ro*# !t is a link between the piston and crankshaft, whose main function is

to transmit force from the piston to the crankshaft" >oreover, it converts reciprocating

motion of the piston into circular motion of the crankshaft, in the working stroke" &he

upper (i.e. smaller# end of the connecting rod is fitted to the piston and the lower (i.e. bigger# end to the crank" &he special steel alloys or aluminium alloys are used for the

manufacture of connecting rods" A special care is re)uired for the design and manufacture

of connecting rod, as it is sub<ected to alternatively compressive and tensile stresses aswell as bending stresses"

/. Cr,)0s+,-# !t is considered as the backbone of an l"c" engine whose function is to

convert the reciprocating motion of the piston into the rotary motion with the help of connecting rod" &his shaft contains one or more eccentric portions called cranks" &hat

part of the crank, to which bigger end of the connecting rod is fitted, is called crank pin"

!t has been e$perienced that too many main bearings create difficulty of correct

alignment" 8pecial steel alloys are used for the manufacture of crankshaft" A special careis re)uired for the design and manufacture of crankshaft"

. Cr,)0 c,se# !t is a cast iron case, which holds the cylinder and crankshaft of an !"c"

engine" !t also serves as a sump for the lubricating oil" &he lower portion of the crank case is known as bed plate, which is fi$ed with the help of bolts"

. F'&+ee'# !t is a big wheel, mounted on the crankshaft, whose function is to maintain

its speed constant" !t is done by storing e$cess energy during the power stroke, which isreturned during other strokes"



SE5UENCE OF OPERATIONS IN A CYCLE

8trictly speaking, when an engine is working continuously, we may consider acycle starting from any stroke" e know that when the engine returns back to the stroke

where it started we say that one cycle has been completed" &he readers will find different

se)uence of operations in different books" But in this chapter, we shall consider thefollowing se)uence of operation in a cycle, which is widely used"

I. S6c-(o) s-ro0e# !n this stroke, the fuel vapor in correct proportion, is supplied to the

engine cylinder"

2. Co78ress(o) s-ro0e# !n this stroke, the fuel vapor is compressed in the engine

cylinder"

3. E98,)s(o) or or0() s-ro0e# !n this stroke, the fuel vapor is fired <ust before the

compression is complete" !t results in the sudden rise of pressure, due to e$pansion of thecombustion products in the engine cylinder" &his sudden rise of the pressure pushes the

piston with a great force, and rotates the crankshaft" &he crankshaft, in turn, drives the

machine connected to it"

4. E9+,6s- s-ro0e# !n this stroke, the burnt gases or combustion products# are e$haustedfrom the engine cylinder, so as to make space available for the fresh fuel vapor"

TWO STRO:E AND FOUR STRO:E CYCLE ENGINE

!n a two9stroke engine, the working cycle is completed in two strokes of the piston or one revolution of the crankshaft" &his is achieved by carrying out the suction

and compression processes in one stroke or more precisely in inward stroke#, e$pansion

and e$haust processes in the second stroke or more precisely in outward stroke#" !n a

four9stroke engine, the working cycle is completed in four9strokes of the piston or two9revolutions of the crankshaft" &his is achieved by carrying out suction, compression,

e$pansion and e$haust processes in each stroke" !t will be interesting to know that from

the thermodynamic point of view, there is no difference between two9stroke and four9stroke cycle engines" &he difference is purely mechanical"

Advantages and Disadvantage of &wo9stroke over *our9stroke Cycle =ngines

*ollowing are the advantages and disadvantages of two9stroke cycle engines over four9stroke cycle engines'

A*;,)-,es

+" A two stroke cycle engine gives twice the number of power strokes than the four

stroke cycle engine at the same engine speed" &heoretically, a two9stroke cycleengine should develop twice the power as that of a four9stroke cycle engine" But

in actual practice, a two9stroke cycle engine develops +"3 to +"4 times greater

value for slow speed engines the power developed by four9stroke cycle engine of the same dimensions and speed" &his is due to lower compression ratio and

effective stroke being less than the theoretical stroke"

" *or the same power developed, a two9stroke cycle engine is lighter, less bulky andoccupies less floor area" &hus it makes, a two9stroke cycle engine suitable for

marine engines and other light vehicles"

7" As the number of working strokes in a two9stroke cycle engine are twice than the



four9stroke cycle engine, so the turning moment of a two9stroke cycle engine is

more uniform" &hus it makes a two9stroke cycle engine to have a lighter flywheel

and foundations" &his also leads to a higher mechanical efficiency of a two9strokecycle engine"

6" &he initial cost of a two9stroke cycle engine is considerably less than a four9stroke

cycle engine"/" &he mechanism of a two9stroke cycle engine is much simpler than a four9stroke

cycle engine"

F" &he two9stroke cycle engines are much easier to start"

D(s,*;,)-,es

+" &hermal efficiency of a two9stroke cycle engine is less than that a four9stroke

cycle engine, because a two9stroke cycle engine has less compression ratio than

that of a four9stroke cycle engine"

" Overall efficiency of a two stroke cycle engine is also less than that of a four9

stroke cycle engine because in a two9stroke cycle, inlet and e$haust ports remain

open simultaneously for some time" !n spite of careful design, a small )uantity of

charge is lost from the engine cylinder"7" !n case of a two9stroke cycle engine, the number of power strokes is twice as

those of a four9stroke cycle engine" &hus the capacity of the cooling system must be higher" Beyond a certain limit, the cooling capacity offers a considerable

difficulty" >oreover, there is a greater wear and tear in a two9stroke cycle engine"

6" &he consumption of lubricating oil is large in a two9stroke cycle engine becauseof high operating temperature"

/" &he e$haust gases in a two9stroke cycle engine create noise, because of short

time available for their e$haust"

VALVE TIMING DIAGRAM

A valve timing diagram is a graphical representation of the e$act moments, in the

se)uence of operations, at which the two valves (i.e. inlet and e$haust valves# open andclose as well as firing of the fuel" !t is, generally, e$pressed in terms of angular positions

of the crankshaft" %ere we shall discuss theoretical valve timing diagrams for four stroke

and two stroke cycle engines"



1. T+eore-(c,' ;,';e -(7() *(,r,7 or o6r s-ro0e c&c'e e)()e

&he theoretical valve timing diagram for a four9stroke cycle engine is shown !n

this diagram, the inlet valve opens at A and the suction takes place from A to B. &hecrankshaft revolves through +4.. 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" &he crankshaft revolves

through +4..

and the piston moves from B.D. C" to T.D. C" At C, the fuel is fired and thee$pansion takes place from C to D. &he crankshaft revolves through +4.. and the piston

again moves from T.D.C. to B.D.C. At D, the e$haust valve opens and the e$haust takes

place from D to E. &he crankshaft again revolves through +4.. and the piston moves back to T.D.C"

2. T+eore-(c,' ;,';e -(7() *(,r,7 or -o!s-ro0e c&c'e e)()e.

&he theoretical valve timing diagram for a two9stroke cycle engine is shown" !nthis diagram, the fuel is fired at A and the e$pansion of gases takes place from A to B. &he

crankshaft revolves through appro$imately +.. and the piston moves from T.D.C.

towards B.D.C. At B, the valves open and suction as well as e$haust take place from B to

C" &he crankshaft revolves through appro$imately +.

.

and the piston moves first to B.D.C and then little upwards" At C" both the valves close and compression takes place

from C to A. &he crankshaft revolves through appro$imately +.. and the piston moves to&"D"C

FOUR STRO:E CYCLE PETROL ENGINE

!t is also known as Otto cycle" !t re)uires four strokes of the piston to complete

one cycle of operation in the engine cylinder" &he four strokes of a petrol engine suckingfuel9air mi$ture petrol mi$ed with proportionate )uantity of air in the carburetor known

as charge# are described below'

1. S6c-(o) or c+,r() s-ro0e# !n this stroke, the inlet valve opens and charge is sucked



into the cylinder as the piston moves downward from top dead centre (T.D.C.). !t

continues till the piston reaches its bottom dead centre (B.D. C"# as shown in a#"

2. Co78ress(o) s-ro0e# !n this stroke, both the inlet and e$haust valves are closed andthe charge is compressed as the piston moves upwards from B.D. C" to TD. C" As a result

of compression, the pressure and temperature of the charge increases considerably the

actual values depend upon the compression ratio#" &his completes one revolution of thecrank shaft" &he compression stroke is shown in b#"

3. E98,)s(o) or or0() s-ro0e 8hortly before the piston reaches T.D.C" during

compression stroke#, the charge is ignited with the help of a spark plug" !t suddenly

increases the pressure and temperature of the products of combustion but the volume, practically, remains constant" Due to the rise in pressure, the piston is pushed down with a

great force" &he hot burnt gases e$pand due to high speed of the piston" During this

e$pansion, some of the heat energy produced is transformed into mechanical work" !t

may be noted that during this working stroke, as shown in (c), both the valves are closedand piston moves from T.D.C. to B.D.C

4. E9+,6s- s-ro0e# !n this stroke, the e$haust valve is open as piston moves from B.D.C.to T.D.C" &his movement of the piston pushes out the products of combustion, from the

engine cylinder and are e$hausted through the e$haust valve into the atmosphere, as

shown in (d). &his completes the cycle, and the engine cylinder is ready to suck the

charge again"

ACTUAL INDICATOR DIAGRAM FOR A FOUR STRO:E CYCLE PETROL

ENGINE

&he actual indicator diagram for a four stroke cycle petrol engine is shown" &he

suction stroke is shown by the line +9, which lies below the atmospheric pressure line" !tis this pressure difference, which makes the fuel9air mi$ture to flow into the enginecylinder" &he inlet valve offers some resistance to the incoming charge" &hat is why, the

charge can not enter suddenly into the engine cylinder" As a result of this, pressure inside

the cylinder remains somewhat below the atmospheric pressure during the suction stroke"&he compression stroke is shown by the line 97, which shows that the inlet valve closes

lVC# a little beyond i"e" BDC ). At the end of this stroke, there is an increase in the



pressure inside the engine cylinder" 8hortly before the end of compression stroke (i.e.

TDC), the charge is ignited lE# with the help of spark plug as shown in the figure" &he

sparking suddenly increases pressure and temperature of the products of combustion" But

the volume, practically, remains constant as shown by the line 796" &he e$pansion stroke

is shown by the line 69/, in which the e$it valve opens (EV) a little before / (i.e. BDC). ow the burnt gases are e$hausted into the atmosphere through the e$it valve" &he

e$haust stroke is shown by the line /9+, which lies above the atmospheric pressure line" !t

is this pressure difference, which makes the burnt gases to flow out of the enginecylinder" &he e$it valve offers some resistance to the outgoing burnt gases" &hat is why

the burnt gases can not escape suddenly from the engine cylinder" As a result of this,

pressure inside the cylinder remains somewhat above the atmospheric pressure line

during the e$haust stroke

VALVE TIMING DIAGRAM FOR A FOUR STRO:E CYCLE PETROL ENGINE

!n the valve timing diagram, as shown we see that the inlet valve opens before the piston reaches TDC or in other words, while the piston is still moving up before the

beginning of the suction stroke" ow the piston reaches the TDC and the suction stroke

starts" &he piston reaches the BDC and then starts moving up" &he inlet valve closes,



when the crank has moved a little beyond the BDC &his is done as the incoming charge

continues to flow into the cylinder although the piston is moving upwards from BDC

ow the charge is compressed with both valves closed# and then and temperature# pushthe piston downwards with full force and the e$pansion or working stroke takes place"

ow the e$haust valve opens before the piston again reaches BDC and the burnt gases

start leaving the engine cylinder" ow the piston reaches BDC and then starts moving up,thus performing the e$haust stroke" &he inlet valve opens before the piston reaches TDC

to start suction stroke" &his is done as the fresh incoming charge helps in pushing out the

burnt gases" ow the piston again reaches TDC, and the suction stroke starts" &he e$itvalve closes after the crank has moved a little beyond the TDC. &his is done as the burnt

gases continue to leave the engine cylinder although the piston is moving downwards" !t

may be noted that for a small fraction of a crank revolution, both the inlet and outlet

valves are open" &his is known as valve overlap"

FOUR!STRO:E CYCLE DIESEL ENGINE

!t is also known as co!pre""ion i#nition en#ine because the ignition takes pJacedue to the heat produced in the engine cylinder at the end of compression stroke" &he four

strokes of a diesel engine sucking pure air are described below'

1. S6c-(o) or c+,r() s-ro0e# !n this stroke, the inlet valve opens and pure air is sucked

into the cylinder as the piston moves downwards from the top dead centre (TDC). !t

continues till the piston reaches its bottom dead centre (BDC) as shown (a).

2. Co78ress(o) s-ro0e# !n this stroke, both the valves are closed and the air is

compressed as the piston moves upwards from BDC to TDC. As a result of compression,

pressure and temperature of the air increases considerably the actual value depends upon

the compression ratio#" &his completes one revolution of the crank shaft" &hecompression stroke is shown in ($).

3. E98,)s(o) or or0() s-ro0e# 8hortly before the piston reaches the TDC during thecompression stroke#, fuel oil is in<ected in the form of very fine spray into the engine

cylinder, through the noHHle, known as fuel in<ection valve" At this moment temperature

of the compressed air is sufficiently high to ignite the fuel" !t suddenly increases the pressure and temperature of the products of combustion" &he fuel oil is continuously

in<ected for a fraction of the revolution" &he fuel oil is assumed to be burnt at constant

pressure" Due to increased pressure, the piston is pushed down with a great force" &he hot



burnt gases e$pand due to high speed of the piston" During this e$pansion, some of the

heat energy is transformed into mechanical work" !t may be noted that during this

working stroke, both the valves are closed and the piston moves from TDC to BDC.

4. E9+,6s- s-ro0e# !n this stroke, the e$haust valve is open as the piston moves from

BDC to TDC. &his movement of the piston pushes out the products of combustion from

the engine cylinder through the e$haust valve into the atmosphere" &his completes thecycle and the engine cylinder is ready to suck the fresh air again"

ACTUAL INDICATOR DIAGRAM FOR A FOUR STRO:E CYCLE DIESEL

ENGINE

&he actual indicator diagram for a four9stroke cycle diesel engine is shown" &he

suction stroke is shown by the line +9 which lies below the atmospheric pressure line" !t

is this pressure difference, which makes the fresh air to flow into the engine cylinder" &heinlet valve offers some resistance to the incoming air" &hat is why, the air can not enter

suddenly into the engine cylinder" As a result o -+(s 8ress6re ()s(*e -+e c&'()*er

re7,()s so7e+,- <e'o the atmospheric pressure during the suction stroke" &he

compression stroke is shown by the line 97, which shows that the inlet valves closes(%VC) a little beyond (i.e. BDC).At the end of this stroke, there is an increase of

pressure inside the engine cylinder" 8hortly before the end of compression stroke (i.e.TDC), fuel valve opens (&V) and the fuel is in<ected into the engine cylinder" &he fuel is

ignited" Actual indicator diagram for a by high temperature of the compressed air" &he

ignition suddenly increases volume and temperature of the products of combustion" But

the pressure, practically, remains constant as shown by the line 796" &he e$pansion strokeis shown by the line 69/, in which the e$it valve opens a little before / (i.e. BDC). ow

the burnt gases are e$hausted into the atmosphere through the e$haust valve" &he e$haust

stroke is shown by the line /9+, which lies above the atmospheric pressure line" !t is this

pressure difference, which makes the burnt gases to flow out of the engine cylinder" &hee$haust valve offers some resistance to the outgoing burnt gases" &hat is why, the burnt

gases can not escape suddenly from the engine cylinder" As a result of this, pressure

inside the cylinder remains somewhat above the atmospheric pressure during the e$hauststroke"



VALVE TIMING DIAGRAM FOR A FOUR!STRO:E CYCLE DIESEL ENGINE

!n the valve timing diagram as shown we see that the inlet valve opens before the piston reaches TDC' or in other words while the piston is still moving up before the

beginning of the suction stroke" ow the piston reaches the TDC and the suction stroke

starts" &he piston reaches the BDC and then starts moving up" &he inlet valve closes,when the crank has moved a little beyond the BDC"

&his is done as the incoming air continues to flow into the cylinder although the piston is

moving upwards from BDC. ow the air is compressed with both valves closed" *uel

valve opens a little before the piston reaches the TDC. ow the fuel is in<ected in the

form of very fine spray, into the engine cylinder, which gets ignited due to hightemperature of the compressed air" &he fuel valve closes after the piston has come down a

little from the TDC. &his is done as the re)uired )uantity of fuel is in<ected into the

engine cylinder" &he burnt gases under high pressure and temperature# push the piston

downwards, and the e$pansion or working stroke takes place" ow the e$haust valveopens before the piston again reaches BDC and the burnt gases start leaving the engine

cylinder" ow the piston reaches BDC and then starts moving up thus performing thee$haust stroke" &he inlet valve opens before the piston reaches TDC to start suction

stroke" &his is done as the fresh air helps in pushing out the burnt gases" ow the piston

again reaches TDC, and the suction starts" &he e$haust valve closes when the crank hasmoved a little beyond the TDC. &his is done as the burnt gases continue to leave the

engine cylinder although the piston is moving downwards"

COMPARISON OF PETROL AND DIESEL ENGINES

*ollowing points are important for the comparison of petrol engines and diesel engines'

Pe-ro' E)()es D(ese' E)()es

• A petrol engine draws a mi$ture of

petrol and air during suction stroke"

• &he carburetor is employed to mi$

air and petrol in the re)uired

• A diesel engine draws only air

during suction stroke

• &he in<ector or atomiHer is

employed to in<ect the fuel at the



proportion and to supply it to the

engine during suction stroke

• Pressure at the end of compression

is about +. bar

• &he charge (i.e. petrol and air

mi$ture# is ignited with the help of spark plug

• &he combustion of fuel takes place

appro$imately at constant volume"!n other words, it works on Otto

cycle

• A petrol engine has compression

ratio appro$imately from F to +."

• &he startingK is easy due to low

compression ratio"

• As the compression ratio is low, the

petrol engines are lighter andcheaper"

• &he running cost of a petrol engine

is high because of the higher cost of

petrol"

• &he maintenance cost is less"

• &he thermal efficiency is up to

about FL"

• Overheating trouble is more due to

low thermal efficiency"

• &hese are high speed engines"• &he petrol engines arc generally

employed in light duty vehicles

such as scooters, motorcycles, cars" &hese are also used in aero planes

end of compression stroke"

• Pressure at the end of compression

is about 7/ bar"

• &he fuel is in<ected in the form of

fine spray" &he temperature of the

compressed air about F..MC at a pressure of about bar# is

sufficiently high to ignite the fuel"

• &he combustion of fuel takes place

appro$imately at constant pressure"

!n other words" !t works on Diesel

cycle"

• A diesel engine has compression

ratio appro$imately from +/ to /"

• &he starting is little difficult due" to

high compression ratio"

• As the compression ratio is high"

the diesel engine are heavier and

costlier"

• &he running cost of diesel engine is

low because of the lower cost of diesel"

• &he maintenance cost is more"

• &he thermal efficiency is up to

about 6.L

• Overheating trouble is less due to

high thermal efficiency• &hese are relatively low speed

engines"

• &he diesel engines are generally

employed in heavy duty vehicles

like buses" trucks, and earth moving

machines etc"

"

TWO!STRO:E CYCLE PETROL ENGINE

A two9stroke cycle petrol engine was devised by Duglad Clerk in ! :%O" !n this

cycle, the suction, compression, e$pansion and e$haust takes place during two strokes of

the piston" !t means that there is one working stroke after every revolution of the crank



shaft" A two stroke engine has ports instead of valves" All the four stages of a two stroke

petrol engine are described below'

1. S6c-(o) s-,e# !n this stage, the piston, while going down towards BDC, uncovers boththe transfer port and the e$haust port &he fresh fuel9air mi$ture flows into the engine

cylinder from the crank case, as shown (a).

2. Co78ress(o) s-,e# !n this stage, the piston, while moving up, first covers the transfer port and then e$haust port" After that the fuel is compressed as the piston moves upwards

as shown ($). !n this stage, the inlet port opens and fresh fuel9air mi$ture enters into the

crank case"

3. E98,)s(o) s-,e# 8hortly before this piston reaches the TDC during compression

stroke#, the charge is ignited with the help of a spark plug" !t suddenly increases the

pressure and temperature of the products of combustion" But the volume, practically,

remains constant" Due to rise in the pressure, the piston is pushed downwards with a greatforce as shown in (c). &he hot burnt gases e$pand due to high speed of the piston" During

this e$pansion, some of the heat energy produced is transformed into mechanical work"

4. E9+,6s- s-,e# !n this stage, the e$haust port is opened as the piston movesdownwards" &he products of combustion, from the engine cylinder are e$hausted through

the e$haust port into the atmosphere, as shown (d). &his completes the cycle and the

engine cylinder is ready to suck the charge again

ACTUAL INDICATOR DIAGRAM FOR A TWO STRO:E CYCLE PETROL

ENGINE

&he actual indicator diagram for a two9stroke cycle petrol engine is shown insuction is shown by the line +997, i.e. from the instant transfer port opens (TP) and

transfer port closes (TPC). e know that during the suction stage, the e$haust port is also

open" !n the first half of suction stage, the volume of fuel9air mi$ture and burnt gasesincreases" &his happens as the piston moves from ! to (i.e. BDC). !n the second half of

the suction stage, the volume of charge and burnt gases decreases" &his happens as the

piston moves upwards from to 7" A little beyond 7, the e$haust port closes (EPC) at 6" ow the charge inside the engine cylinder is compressed which is shown by the line 69/"



At the end of the compression, there is an increase in the pressure inside the enginecylinder" 8hortly before the end of compression (i.e. TDC) the charge is ignited (I*+)

with the help of spark plug" &he sparking suddenly increases pressure and temperature of

the products of combustion" But the volume, practically, remains constant as shown bythe line /9F" &he e$pansion is shown by the line F93" ow the e$haust port opens (EP)

at 3, and the burnt gases are e$hausted into the atmosphere through the e$haust port" !t

reduces the pressure" As the piston is moving towards BDC, therefore volume of burntgases increases from 3 to +" At +, the transfer port opens (TP) and the suction starts"

VALVE TIMING DIAGRAM FOR A TWO!STRO:E CYCLE PETROL ENGINE

!n the valve timing diagram, as shown we see that the e$pansion of the charge

after ignition# starts as the piston moves from TDC towards BDC. *irst of all, thee$haust 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 fuel9air mi$ture



enters into the engine cylinder" &his is done as the fresh incoming charge helps in pushing

out the burnt gases" ow the piston reaches BDC and then starts moving upwards" As the

crank moves a little beyond BDC, first the transfer port closes and then the e$haust portalso closes" &his is done to suck fresh charge through the transfer port and to e$haust the

burnt gases through the e$haust port simultaneously" ow the charge is compressed with

both ports closed, and then ignited with the help of a spark plug before the end of compression stroke" &his is done as the charge re)uires some time to ignite" By the time

the piston reaches TDC, the burnt gases under high pressure and temperature# push the

piston downwards with full force and e$pansion of the burnt gases takes place" !t may benoted that the e$haust and transfer ports open and close at e)ual angles on either side of

the BDC position"

TWO!STRO:E CYCLE DIESEL ENGINE

A two9stroke cycle diesel engine also has one working stroke after every

revolution of the crank shaft" All the four stages of a two stroke cycle diesel engine are

described below'

1. S6c-(o) s-,e# !n this stage, the piston while going down towards BDC uncovers thetransfer port and the e$haust port" &he fresh air flows into the engine cylinder from the

crank case, as shown in (a).

2. Co78ress(o) s-,e# !n this stage, the piston while moving up, first covers the transfer

port and then e$haust port" After that the air is compressed as the piston moves upwards

as shown in ($). !n this stage, the inlet port opens and the fresh air enters into the crank case"

3. E98,)s(o) s-,e# 8hortly before the piston reaches the TDC during compression

stroke#, the fuel oil is in<ected in the form of very fine spray into the engine cylinder

through the noHHle known as fuel in<ection valve, as shown in (c). At this moment,temperature of the compressed air is sufficiently high to ignite the fuel" !t suddenly

increases the pressure and temperature of the products of combustion" &he fuel oil is

continuously in<ected for a fraction of the crank revolution" &he fuel oil is assumed to be burnt at constant pressure" Due to increased pressure, the piston is pushed with a great

force" &he hot burnt gases e$pand due to high speed of the piston" During the e$pansion,

some of the heat energy produced is transformed into mechanical work "



4. E9+,6s- s-,e# !n this stage, the e$haust port is opened and the piston moves

downwards" &he products of combustion from the engine cylinder are e$hausted through

the e$haust port into the atmosphere as shown in (d). &his completes the cycle, and theengine cylinder is ready to suck the air again"

ACTUAL INDICATOR DIAGRAM FOR TWO STRO:E CYCLE DIESELENGINE

&he actual indicator diagram for a two9stroke cycle diesel engine is shown" &he suction is

shown by the line' +997 i.e. from the instant transfer port opens (TP) and transfer portcloses (TPC). e know that during the suction stage, the e$haust port is also open" !n the

first half of suction stage, the volume' of air and burnt gases increases" &his happens as

'the piston moves from - (i.e. BDC). !n the second half of the suction stage, the volume

of air and burnt gases decreases" &his happens as the piston moves upwards from 97" Alittle beyond 7, the e$haust port closes (EPC) at 6" ow the air inside the engine cylinder

is compressed which is shown by the line 69/" At the end of compression, there is an

increase in the pressure inside the engine cylinder" 8hortly before the end of compression

(i. e. TDC), fuel valve opens (&V) and the fuel is in<ected into the engine cylinder" &hefuel is ignited by high temperature of the compressed air"

&he ignition suddenly increases volume and temperature of the products of combustion"

But the pressure, practically, remains constant as shown by the line /9F" &he e$pansion !8

shown by the line F93, ow thN e$haust port opens (EP) at 3 and the burnt gases aree$hausted into the atmosphere through the e$haust port" !t reduces the pressure" As the

piston is moving towards BDC, therefore volume of burnt gases increases from 3 to +" At

+, the transfer port opens (TP) and the suction starts"

VALVE TIMING DIAGRAM FOR A TWO!STRO:E CYCLE DIESEL ENGINE

!n the valve timing diagram, as shown, we see that the e$pansion of the charge

after ignition# starts as the piston moves from TDC towards BDC. *irst of all, the

e$haust 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 andthe fresh air enters into the engine cylinder" &his is done as the fresh incoming air helps in



pushing out the burnt gases" ow the piston reaches BDC and then starts moving

upwards" As the crank moves a little beyond BDC, first the transfer port closes and then

the e$haust port also closes" &his is done to suck fresh air through the transfer port and toe$haust the burnt gases through the e$haust port simultaneously" ow the charge is

compressed

with both the ports closed" *uel valve opens a little before the piston reaches the TDC.

ow the fuel is in<ected in the form of very fine spray into the engine cylinder, whichgets ignited due to high temperature of the compressed air" &he fuel valve closes after the

piston has come down a little from the TDC. K&his is done as the re)uired )uantity of fuel

is in<ected into the engine cylinder" ow the burnt gases under high pressure andtemperature# push the piston downwards with full force and e$pansion of the gases takes

place" !t may be noted that in a two9stroke cycle diesel engine, like two9stroke petrol

engine, the e$haust and transfer ports open and close at e)ual angles on either side of the

BDC position"

SCAVENGING OF I.C. ENGINES

e have already discussed the se)uence of operations in a cycle of an !"C" engine"

&he last stroke of an !C engine is the e$haust, which means the removal of burnt gases

from the engine cylinder" !t has been e$perienced that the burnt gases in the enginecylinder are not completely e$hausted before the suction stroke" But a part of the gases

still remain inside the cylinder and mi$ with the fresh charge" As a result of this mi$ing,

the fresh charge gets diluted and its strength is reduced" &he scientists and engineers, allover the world, have concentrated on the design of their !C engines so that the burnt

gases are completely e$hausted from the engine cylinder before the suction starts" &he process of removing burnt gases, from the combustion chamber of the engine cylinder, isknown a scavenging" ow we shall discuss the scavenging in four9stroke and two9stroke

cycle engines"



1. Fo6r!s-ro0e c&c'e e)()es# !n a four stroke cycle engine, the scavenging is very

effective as the piston during the e$haust stroke pushes out the burnt gases from the

engine cylinder" !t may be noted that a small )uantity of burnt gases remain in the enginecylinder in the clearance space"

2. To!s-ro0e c&c'e e)()e' !n a two9stroke cycle engine, the scavenging is less effective

as the e$haust port is open for a small fraction of the crank revolution" >oreover, as thetransfer and e$haust port arc open simultaneously during a part of the crank revolution,

therefore fresh charge also escapes out along with the burnt gases" &his difficulty is

overcome by designing the piston crown of a particular shape "

TYPES OF SCAVENGING

&hough there are many types of scavenging, yet the following are important fromthe sub<ect point of view'

1. Cross 'o sc,;e)()# !n this method, the transfer port or inlet port for the engine

cylinder# and e$haust port are situated on the opposite sides of the engine cylinder as is

done in case of two9stroke cycle engines#" &he piston crown is designed into a particular shape, so that the fresh charge moves upwards and pushes out the burnt gases in the form

of cross flow as" 8hown in a#"

2. %,c0'o or 'oo8 sc,;e)()# !n this method, the inlet and outlet ports are situated on

the same side of the engine cylinder" &he fresh charge, while entering into the engine

cylinder, forms a loop and pushes out the burnt gases as shown in ($).

3. U)('o sc,;e)()# !n this method, the fresh charge, while entering from one side )r

sometimes two sides# of the engine cylinder pushes out the gases through the e$it valvesituated on the top of the cylinder" !n uniflow scavenging, both the fresh charge and burnt

gases move in the same upward direction as shown in (c).

DETONATION IN IC ENGINES&he loud pulsating noise heard within the engine cylinder is known as detonation

also called knockin# or pinkin#). !t is caused due to the propagation of a high speed pressure wave created by the auto9ignition of end portion of unburnt fuel" &he blow of

this pressure wave may be of sufficient strength to break the piston" &hus, the detonation

is harmful to the engine and must be avoided" &he following are certain factors which

causes detonation' "



• &he shape of the combustion chamber,

• &he relative position of the sparking plugs in case of petrol engines,

• &he chemical nature of the fuel,

• &he initial temperature and pressure of the fuel,

• &he rate of combustion of that portion of the fuel which is the first to ignite" &his

portion of the fuel in heating up, compresses the remaining unbumt fuel, thus

producing the conditions for auto9ignition to occur"

&he detonation in petrol engines can be suppressed or reduced by the addition of a smallamount of lead ethide or ethyl fluid to the fuel" &his is called dopin#

&he following are the chief effects due to detonation'

• A loud pulsating noise which may be accompanied by a vibration of the engine"

• An increase in the heat lost to the surface of combustion chamber"

• An increase in carbon deposits"

RATING OF SI ENGINE FUELS OCTANE NUM%ER

&he hydrocarbon fuels used in spark ignition 8"!"# engine have a tendency to

cause engine knock when the engine operating conditions become severe" &he knocking

tendency of a fuel in 8"!" engines is generally e$pressed by its octane nu!$er. &he percentage, by volume, of iso9octane in a mi$ture of iso9octane normal heptane ,which

e$actly matches the knocking intensity of a given fuel, in a standard engine, under given

standard operating conditions, is termed as the octane nu!$er ratin# of that fuel" &hus, if

a mi$ture of /. percent iso9octane and /. percent normal heptane matches the fuel under test, then this fuel is assigned an octane number rating of /." !f a fuel matches in

knocking intensity a mi$ture of 3/ percent iso9octane and / percent normal heptane,

then this fuel would be assigned an octane number rating of 3/" &his octane number

rating is an e$pression which indicates the ability of a fuel to resist knock in a 8"+"engine" 8ince iso9octane is a very good anti9knock fuel, therefore it is assigned a rating of

+.. octane number" On the other hand, normal heptane has very poor anti9knock )ualities, therefore it is given a rating of . Hero# octane number" &hese two fuels, i.e. iso9

octane and normal heptane are known as primary reference fuels" !t may be noted that

higher the octane number rating of a fuel, the greater will be its resistance to knock and

the higher will be the compression ratio" 8ince the power output and specific fuelconsumption are functions of compression ratio, therefore we may say that these are also

functions of octane number rating" &his fact indicates the e$treme importance of the

octane number rating in fuels for 8"!" engines"

RATING OF CI ENGINE FUELS CETANE NUM%ER

&he knocking tendency is also found in compression ignition C"!"# engines with

an effect similar to that of 8"+" engines, but it is due to a different phenomenon" &he

knock in C"!" engines is due to sudden ignition and abnormally rapid combustion of accumulated fuel in the combustion chamber 8uch a situation occurs Because of an

!gnition lag in the combustion of the fuel between the time of in<ection and the actual



burning" &he property of ignition lag is generally measured in terms of cetane nu!$er. !t

is defined as the percentage, by volume, of cetane in a mi$ture of cetane and alpha9

methyl9naphthalene that produces the same ignition lag as the fuel being tested in thesame engine and under the same operating conditions" *or e$ample, a fuel of cetane

number /. has the same ignition )uality as a mi$ture of /. percent cetane and /. percent

alpha9methyl9naphthalene" &he cetane which is a straight chain paraffin with goodignition )uality is assigned a cetane number of +.. and alpha9methyl9naphthalene which

is a hydrocarbon with poor ignition )uality, is assigned a . Hero# cetane number"

IGNITION SYSTEMS OF PETROL ENGINES

e have already discussed that the ignition in a petrol engine, takes place by means of a

spark plug at the end of the compression stroke" &he voltage re)uired to produce a spark across the gap between the sparking points of a plug, is about 4... volts" &hus, the

ignition system in a petrol engine has to transform the normal battery voltage F to +

volts# to 4... volts" !n addition to this, the ignition system has to provide spark in each

cylinder at the appropriate time" *ollowing two ignition systems of petrol engines areimportant from the sub<ect point of view'

+" Coil ignition system " >agneto ignition system"

COIL IGNITION SYSTEM

!t is also known as $atter/ i#nition "/"te!, and has an induction coil, which

consists of two coils known as primary and secondary coils wound on a soft iron core, as

shown " &he primary coil consists of a few hundred turns about 7.. turns# of wire" Over

this coil, but insulated from it, are wound several thousand turns about .,... turns# of secondary coil" &he one end of the primary coil is connected to a ignition switch,

ammeter and battery generally of F volts" &he other end of the primary coil is connected

to a condenser and a contact breaker"



A condenser is connected across the contact9breaker for the following two reasons'

• !t prevents sparking across the gap between the points,

• !t causes a more rapid break of the primary current, giving a higher voltage in

the secondary circuit"

&he secondary coil is connected to a distributor in a multi9cylinder engine# with thecentral terminal of the sparking plugs" &he outer terminals of the sparking plugs areearthed together, and connected to the body of the engine" hen the current flows

through the primary coil, it sets up a magnetic field which surrounds both the primary and

secondary coils" As the switch is on, the contact9breaker connects the two ends" &hemagnetic field in coils has tendency to grow from Hero to ma$imum value" Due to this

change in the magnetic field, a voltage is generated in both the coils, but opposite to the

applied voltage of battery#" &hus the primary coil does not give the final value" &hevoltage in the secondary coil is, therefore, not sufficient to overcome the resistance of the

air gap of the sparking plug, hence no spark occurs" hen the current in the primary coil

is switched off by the moving cam, the magnetic field generated around the coil collapses

immediately" &he sudden variation of flu$, which takes place, gives rise to the voltagegenerated in each coil" &he value of the voltage depends upon the number of turns in each

coil" As a matter of fact, the voltage re)uired to produce a spark across the gap, between

the sparking points, is between +. ... to . ... volts" 8ince the secondary coil hasseveral thousand turns, so it develops a sufficient high voltage to overcome the resistance

of the gap of the sparking plug" &his high voltage then passes to a distributor" !t connects

the sparking plugs in rotation depending upon the firing order of the engine" %ence, theignition of fuel takes place in all the engine cylinders" &he coil ignition system is

employed in medium and heavy spark ignition engines such as in cars"

MAGNETO IGNITION SYSTEM

&he magneto ignition system as shown has the same principle of working as thatof coil ignition system e$cept that no battery is re)uired as the magneto acts its own

generator" !t consists of either rotating magnets in fi$ed coils, or rotating coils in fi$edmagnets" &he current produced by the magneto is made to flow to the induction coil

which works in the same way as that of coil ignition system" &he high voltage current is

then made to flow to the distributor, which connects the sparking plugs in rotationdepending upon the firing order of the engine" &his type of ignition system is generally

employed in small spark ignition engines such as scooters, motor cycles and small motor

boat engines"



FUEL IN=ECTION SYSTEM FOR DIESEL ENGINES

&he fol+owing two methods of fuel in<ection system are generally employed with

diesel engines (i.e.co!pre""ion ignition engines# '

+" Air in<ection method, and " Airless or solid in<ection method"&hese methods are discussed, in detail, as follows'

1. A(r ()>ec-(o) 7e-+o*# !n this method of fuel in<ection, a blast of compressed air is

used to in<ect the fuel into the engine cylinder" &his method re)uires the aid of an air compressor which is driven by the engine crankshaft" &he air is compressed at a pressure

higher than that of engine cylinder at the end of its compression stroke" &his method isnot used now9a9days because of complicated and e$pensive system"

2. A(r'ess or so'(* ()>ec-(o) 7e-+o*# &he most modern compression ignition engines

use, now9a9days, the solid in<ection system" !n this method, a separate fuel pump driven

by the main crankshaft is used for forcing the fuel" &he fuel is compressed in this pump toa pressure higher than that of engine cylinder at the end of compression" &his fuel under

pressure is directly sprayed into the combustion chamber of the engine cylinder at the end

of compression stroke, with the help of an in<ector" &he solid in<ection method may be

further divided into the following two commonly used systems"

• Common rail system

• !ndividual pump system

Co77o) r,(' s&s-e7# !n the common rail system as shown a multi cylinder high pressure pump is used to supply the fuel at a high pressure to a common rail or header"



&he high pressure in the common rail forces the fuel to each of the noHHle located in the

cylinders" &he pressure in this common rail is kept constant with the help of a high pressure relief valve" A metered )uantity of fuel is supplied to each cylinder through the

noHHle by operating the respective fuel in<ection valve with the help of cam mechanism

driven by the crankshaft of the engine"

I)*(;(*6,' 8678 s&s-e7# !n the individual pump system, as shown each cylinder of the

engine is provided with an individual in<ection valve, a high pressure pump and a

metering device run by the crankshaft of the engine" &he high pressure pump plunger isactuated by a cam and produces the fuel pressure necessary to open the in<ection valve at

the correct time" &he amount of fuel in<ected depends upon the effective stroke of the

plunger"

COOLING OF I.C. ENGINES

e have already discussed that due to combustion of fuel inside the engine

cylinder of !"C" engines, intense heat is generated" !t has been e$perimentally found that

about 7.L of the heat generated is converted into mechanical work" Out of the remaining

heat about 3.L# about 6.L is carried away by the e$haust gases into the atmosphere"&he remaining part of the heat about 7.L#, if left un9attended, will be absorbed by

engine cylinder, cylinder head piston, and engine valves etc" !t has also been found that

the overheating of these parts causes the following effects'

• &he overheating causes thermal stresses in the engine parts, which maylead to their distortion"

• &he overheating reduces strength aft he piston" &he overheating may

cause even seiHure of the piston"

• &he overheating causes decomposition of the lubricating oil, which may

cause carbon deposit on the engine and piston head"



• &he over heating, causes burning of valves and valve seats"

• &he overheating reduces volumetric efficiency of the engine"

• &he overheating increases tendency of the detonation "

!n other to avoid the adverse effects of overheating, it is very essential to providesome cooling system for an !"C" engine" !n general, the cooling system provided should

have the following two characteristics for its efficient working'• !t should be capable of removing about 7.L of that total heat generated in the

combustion chamber" !t has been e$perienced that removal of more than 01 of heat generated reduces thermal efficiency of the engine" 8imilarly, removal of

less than 7.L of the heat generated will have some adverse effects as mentioned

above"

• !t should be capable of removing heat at a fast rate, when the engine is hot" But

at the time of starting the engine, the cooling should be comparatively slow, so

that the various components of the engine attain their working temperature in a

short time"

COOLING SYSTEMS FOR I.C. ENGINESe have already discussed, in the last article, the adverse effects of overheating of

an !"C" engine and characteristics of the cooling system adopted" &he following two

systems are used for cooling the !"C" engines these days'

1. A(r coo'() s&s-e7# &he air cooling system, as shown , is used in the engines of motor

cycles, scooters, aeroplanes and other stationary installations" !n countries with coldclimate, this system is also used in car engines" !n this system, the heat is dissipated

directly to the atmospheric air by conduction through the cylinder walls" !n order to

increase the rate of cooling, the outer surface area of the cylinder and cylinder head isincreased by providing radiating fins and flanges" !n bigger units, fans are provided to

circulate the air around the cylinder walls and cylinder head"

2. W,-er coo'() s&s-e7 T+er7os&8+o) s&s-e7 o coo'()". &he water cooling system

as shown , is used in the engines of cars, buses, trucks etc" !n this system, the water is

circulated through water <ackets around each of the combustion chambers, cylinders,

valve seats and valve stems" &he water is kept continuously in motion by a centrifugalwater pump which is driven by a V9belt from the pulley on the engine crank shaft" After



passing through the engine <ackets in the cylinder block and heads, the water is passed

through the radiator" !n the radiator, the water is cooled by air drawn through the radiator

by a fan" (sually, fan and water pump are mounted and driven on a common shaft" After passing through the radiator, the water is drained and delivered to the water pump

through a cylinder inlet passage" &he water is again circulated through the engine <ackets"

"

COMPARISON OF AIR COOLING AND WATER COOLING SYSTEMS

&he following points are important for the comparison of air cooling and watercooling systems"

A(r Coo'() S&s-e7 W,-er Coo'() S&s-e7

• &he design of this system is simple

and less costly

• &he mass of cooling system is veryless

• &he fuel consumption is more

• !ts installation and maintenance is

very easy and less costly

• &here is no danger of leakage or

freeHing of the coolant

• !t works smoothly and

continuously" >oreover it does notdepend on any coolant

• &he design of this system is

complicated and more costly

• &he mass of cooling system is muchmore

• &he fuel consumption is less

• !ts installation and maintenance is

difficult and more costly

• &here is a danger of leakage or

freeHing of the coolant

• !f the system fails, it may cause

serious damage to the engine withina short time"

SUPERCHARGING OF IC ENGINES

!t is the process of increasing the mass, or in other words density, of the air9fuel

mi$ture in spark ignition engine# or air in compression ignition i.e. diesel engines#induced into the engine cylinder" &his is, usually, done with the help of compressor or

blower known as supercharger" !t has been e$perimentally found that the supercharging

increases the power developed by the engine" !t is widely used in aircraft engines, as the



mass of air, sucked in the engine cylinder, decreases at very high altitudes" &his happens,

because atmospheric pressure decreases with the increase in altitude" ow9a9days,

supercharging is also used in two9stroke and four9stroke petrol and diesel engines" !t will be interesting to know that a supercharged engine is lighter, re)uires smaller foundations

and consumes less lubricating oil as compared to an ordinary engine" *ollowing are the

ob<ects of supercharging the engines"+" &o reduce mass of the engine per brake power as re)uired in aircraft engines#"

" &o maintain power of aircraft engines at high altitudes where less o$ygen is

available for combustion"7" &o reduce space occupied by the engine as re)uired in marine engines#"

6" &o reduce the consumption of lubricating oil as re)uired in all type of engines#"

/" &o increase the power output of an engine when greater power is re)uired as

re)uired in racing cars and other engines#"

METHODS OF SUPERCHARGING

8trictly speaking, a supercharger is an air pump, which receives air from theK

atmosphere surrounding the engine, compresses it to a higher pressure and then feeds itinto the inlet valve of the engine"

*ollowing two method of supercharging are important from the sub<ect point of view'

1. Rec(8roc,-() -&8e' !t has a piston which moves to and fro inside a cylinder" !t is an

old method and is not encouraged these days, as it occupies a large space and haslubrication problem"

2. Rotary type: !t resembles a centrifugal pump i +its outward appearance, but differs in

action" &here are many types of rotary pumps, but gear type, lobe type and vane type are

commonly used"

LU%RICATION OF I.C. ENGINES

As a matter of fact, the moving parts of an !"C engine are likely to wear off due tocontinuous rubbing action of one part with another" !n order to avoid an early wearing of

the engine parts, a proper lubrication arrangement is provided in !"C" engines"

!n general, following are the main advantages of lubrication of !"C" engines'

+" !t reduces wear and tear of the moving parts"

" !t damps down the vibrations of the engine"

7" !t dissipates the heat generated from the moving parts due to friction"

6" !t cleans the moving parts"

/" !t makes the piston gas9tight"

LU%RICATION SYSTEM FOR IC ENGINES

&he following two lubrication systems of !"C" engines are important from the

sub<ect point of view'



1. S8',s+ '6<r(c,-(o)# &his method is generally employed for lubricating small !"C"

engines" !n this method, an oil sump is fi$ed to the bottom of the crank case and the pump

is immersed, in the lubricating oil, as shown A small hole s drilled in the crank shaft andthe oil is forced through this hole to the bearing" &he oil is also forced along the

connecting rod either through a hole drilled in the rod or along a small copper pipe to the

gudgeon pin and piston"

8urplus oil lubricates the cams, tappets and valve stems" &he whole oil is drained back

into the sump"

2. Force* '6<r(c,-(o)# !n this method, the lubricating oil is carried in a separate tank and

is pumped at a high pressure to the main bearings" !t passes at a lower pressure to the

camshaft and timing gears, as the oil drains with the sump it is pumped back by a pumpknown as scavenge pump through an oil cooler to the oil tank"



GOVERNING OF I.C. ENGINES

As a matter of fact, all the !"C" engines like other engines, are always designed to

run at a particular speed" But in actual practice, load on the engine keeps on fluctuatingfrom time to time" A little consideration will show, that change of load, on an !"C" engine,

is sure to change its speed" !t has been observed that if load on an !"C" engine is decreased

without changing the )uantity of fuel, the engine will run at a higher speed" 8imilarly, if

load on the engine is increased without changing the )uantity of fuel, the engine will runat a lower speed" ow, in order to have a high efficiency of an !"C" engine, at different

load conditions, its speed must be kept constant as far as possible" &he process of providing any arrangement, which will keep the speed constant according to the

changing load conditions# is known as #o2ernin# o3 I.C" en#ine".

METHODS OF GOVERNING I.C.ENGINES

&hrough there are many methods for the governing of !"C" engines, yet the

following are important from the sub<ect point of view'

I. H(- ,)* 7(ss o;er)()# &his method of governing is widely used for !C engines of

smaller capacity or gas engines &his method is most suitable for engines, which arefre)uently sub<ected to reduced loads and ,8 a result of this, the engines tend to run athigher speeds" !n this system of governing, whenever the engine starts running at higher

speed due to decreased load#, some e$plosion are omitted or missed" &his is done with

help of centrifugal governor in which the inlet valve of fuel is closed and the e$plosionsare omitted till the engine speed reaches its normal value" &he only disadvantage of this



method is that there is uneven turning moment due to missing of e$plosions" As a result

of this, it re)uires a heavy flywheel" "

2. 56,'(-,-(;e o;er)()# !n this system of governing, a control valve is fitted in the fueldelivery pipe, which controls the )uantity of fuel to be mi$ed in the charge" &he

movement of control valve is regulated by the centrifugal governor through rack and

pinion arrangement" !t may be noted that in this system, the amount of air used in eachcycle remains the same" But with the change in the )uantity of fuel with )uantity of air

remaining constant#, the )uality of charge (i. e. air9fuel ratio of mi$ture# changes"

henever the engine starts running at higher speed due to decreased load#, the )uantityof fuel is reduced till the engine speed reaches its normal value" 8imilarly, whenever the

engine starts running at lower speed due to increased load#, the )uantity of fuel is

increased" !n automobile engines, the rack and pinion arrangement is connected with the

accelerator"

3. 56,)-(-,-(;e o;er)()4 !n this system of governing, the )uality of charge (i.e. air9

fuel ratio of the mi$ture# is kept constant" But )uantity of mi$ture supplied to the engine

cylinder is varied by means of a throttle valve which is regulated by the centrifugal

governor through rack and pinion arrangement" henever the engine starts running athigher speed cue to decreased load#, the )uantity of charge is reduced till the engine

speed reaches its normal value" 8imilarly, whenever the engine starts running at lower speed due to increased load#, the )uantity of charge is increased" &his method is used for

governing large engines"

4. Co7<(),-(o) s&s-e7 o o;er)()# !n this system of governing, the above mentionedtwo methods of governing (i.e. )ualitative and )uantitative# are combined together, so

that )uality as well as )uantity of the charge is varied according to the changing

conditions" &his system is complicated, and has not proved to be successful"

CAR%URETTOR

&he carburettor is a device for atomising and vaporing the fuel and mi$ing it withthe air in the varying proportions to suit the changing operating conditions of the engine"

&he process of breaking up and mi$ing the fuel with the air is called car$uration" &here

are many types of the carburettors in use, but the simplest form of the carburettor isshown" !t consists of a fuel <et located in the centre of the choke tube" A float chamber is

provided for maintaining the level of the fuel in the <et and is controlled by a float and

lever which operates its needle valve" &he fuel is pumped into the float chamber and

when the correct level of the fuel is reached, the float closes the needle valve, and shutsoff the petrol supply" &he suction produced by the engine draws air through t'le choke

tube" &he reduced diameter of the choke tube increases the velocity of air and reduces the

pressure" &he high velocity and low pressure in the tube facilitates the breaking up of fueland its admi$ture with the air" A throttle valve controls the flow of the mi$ture delivered

to the engine cylinder"



SPAR: PLUG

!t is always screwed into the cylinder head for igniting the charge of petrol

engines' !t is, usually, designed to withstand a pressure up to 7/ bar and operate under acurrent of +.... to 7.... volts" &erminal A spark plug c.+sists of central porcelain

insulator, containing an a$ial electrode length wise and ground electrode welded to it"

&he central electrode have an e$ternal contact at the top, which is connected to the

terminal and communicates with the distributor" A metal tongue is welded to the groundelectrode" which bends over to lie across the end of the central electrode" &here is a small

gap known as spark gap between the end of the central electrode and the metal tongue, as

shown" &he high tension electric spark <umps over the gap to ignite the charge in theengine cylinder" &he electrode material should be such which can withstand

corrosiveness, high temperature having good thermal conductivity" &he electrodes are

generally made from the alloys of platinum, nickle, chromium, barium etc"



FUEL PUMP

&he main ob<ect of a fuel pump in a diesel engine is to deliver a fuel to thein<ector which sprays the finely divided particles of the fuel suitable for rapid

combustion"

&he simplified sketch of a fuel pump is shown" !t consist of a plunger which

moves up and down in the barrel by the cam and spring arrangement provided for pushing and lowering the plunger respectively" &he fuel oil is highly filtered by means of

felt9pack filter before entering the barrel of the pump" &he upper end part of the plunger

is cut away in a heli$ shaped piece forming a groove between the plunger and barrel,which is the most important one" &herefore, the amount of fuel delivered and in<ected

into the engine cylinder depends upon the rotary position of the plunger in the barrel"

*igure ($) and (c) shows how the top part of the plunger is designed so that the correct

amount of fuel is delivered to the in<ector" hen the plunger is at the bottom of its strokeas shown ill *igure ($), the fuel enters the barrel through the inlet port" As the plunger

rises, it forces this fuel up into the in<ector, until the upper part cut away comes opposite

the sill port" &hen the fuel escapes down the groove and out through the sill port so thatin<ection ceases, as shown in *igure (c). &he plunger can be made to rotate in the barrel

and therefore more fuel is in<ected" hen the plunger is rotated so that the groove is

opposite to the sill port, no fuel at all is in<ected and thus the engine stops



IN=ECTOR OR ATOMISER

&he in<ector or atomiser is also an important part of the diesel engine which breaks up the fuel and sprays into the cylinder into a very fine divided particles" *igure

shows the type of an in<ector in which fuel is delivered from the pump along the

horiHontal pipe connected at A. &he vertical spindle of the in<ector is spring loaded at thetop which holds the spindle down with a pressure of +6. bar so that the fuel pressure

must reach this value before the noHHle will lift to allow fuel to be in<ected into the engine

cylinder" &he fuel which leaks past the vertical spindle is taken off by means of an outlet pipe fitted at B above the fuel inlet pipe

12 Thermal Engineering

Documents

Transcript of 12 Thermal Engineering