8188 39051 ic engines notes

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Dr.T.V.Rao, Professor, KL UniversityCourse Coordinator forIC Engines & Gas Turbines

INTERNAL COMBUSTION ENGINES

1. What is a Heat Engine? How they ae !"assi#ie$? Gi%e e&a'("es #o the sa'e.

A. Heat engine is a thermal prime mover that converts the chemical energy of a fuel in to heat

energy by combustion of fuel and utilizes this energy to perform mechanical work. Heat

engines are broadly classified into two categories

1) External ombustion Engines!E.. Engines) " #n these engines$ combustion of fuel

takes place outside the engine cylinder. Ex" %team engines$ %team turbines$ closed

cycle gas engine$ etc. &he E engines are mainly used for large electric power

generators.

') #nternal ombustion Engines !#.. Engines)" #n # Engines$ the combustion of fuel

takes place inside the engine cylinder. Ex" (etrol Engines$ iesel Engines$ *as

Engines$ etc. # Engines are mainly used for transport vehicles.

). How ae IC Engines !"assi#ie$?

i) According to the type of fuel used

1. (etrol Engines

'. iesel Engines

+. *as engines,. -i/uel Engines

ii) According to the number of strokes per cycle

1. /our stroke engine

'. &wo stroke engine

iii) According to the method of ignition

1. %park #gnition engine

'. ompression #gnition engine

iv) According to the cycle of working

1. 0tto ycle engine !onstant olume cycle engine)

'. iesel ycle engine !onstant (ressure cycle engine)

+. ual ombustion cycle engine

v) According to the number of cylinders

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1. %ingle cylinder engine

'. 2ulti cylinder engine

vi) According to the arrangement of cylinders

1. ertical engine'. Horizontal engine

+. #nline engine

,. 3adial engine

4. engine

5. 0pposed piston engine

vii) According to the method of cooling

1. Air cooled engine

'. 6ater cooled engine

viii) According to the method of fuel in7ection

1. arburetor engine

'. Air in7ection engine

+. Airless or solid in7ection engine

ix) According to their uses

1. %tationery engine

'. Automobile engine

+. 2arine engine

,. 8ocomotive engine

4. Aircraft engine

*. E&("ain the #o""owing te's with e#een!e to IC engines

i+ Boe, #t is the inside diameter of engine cylinder

ii+ Sto-e , #t is the linear distance traveled by the piston i.e. it is the distance between

&op ead entre and -ottom ead entre

iii+ Swe(t o"/'e 0s+ , #t is the olume swept by the piston as it moves between &

and-

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i%+ C"eaan!e o"/'e0!+, #t is volume of the cylinder above the top of the piston when

piston is at &.

%+ Co'(ession Ratio , #t is ratio of the total cylinder volume to the clearance volume

Compression Ratio r= (Vc +Vs)/Vc

%i+ iston S(ee$, &he average linear speed of the piston is called piston speed and is

given by % 9 ' l : where l stroke and : ;speed of the engine.

2. E&("ain with a neat s-et!h the (in!i(a" (ats o# an IC engine.

1) ylinder" &he heart of the engine is the cylinder in which the fuel is burnt and the

power is generated. #ts function is to contain the working fluid under pressure and to

guide the piston while reciprocating inside the cylinder. &he cylinder is usually made of

*rey ast #ron or steel alloys in order to withstand the high pressure and temperature

during combustion.

') ylinder Head" &he top end of the cylinder is closed by a removable cylinder head. &he

cylinder head consists of two valves i.e. inlet and outlet valves. #t is usually made of

ast iron or alloys of cast iron

+) (iston" #t is close fitting hollow cylindrical plunger moving to and fro in the cylinder.

&he main function of the piston is to compress the charge during compression stroke

and to receive the impulse produced by the combustion of the fuel and to transmit the

energy to the crankshaft.

,) (iston rings" &hese are the metallic rings inserted into the circumferential grooves

provided at the top end of the piston. &hese rings maintain a gas tight 7oint between the

piston and cylinder while the piston is reciprocating in the cylinder. &hey are two sets

of piston rings. 0ne set is called compression rings which press hard against the

cylinder walls forming a tight seal between piston and the cylinder. &his prevents the

high pressure gases from escaping into the crankcase. Another set of rings called oil

rings which are used to extract the lubricating oil from the cylinder walls and to send it

back to the oil sump through holes provided in the piston.

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4) onnecting rod" #t forms a link between piston and the crankshaft. #t converts the linear

motion of the piston into rotary motion of the crankshaft.

5) rank and crankshaft" &he crank is a lever that is connected to the end of the

connecting rod by a pin 7oint with its other end connected rigidly to a shaft calledcrankshaft. #t rotates about the axis of the crankshaft and causes the connecting rod to

oscillate.

) alves" &hese are the control devices that allow the air?fuel to enter into the cylinder

and also discharge the burnt gases to the atmosphere. &he inlet and outlet valves serve

these purposes. &he valves are actuated by means of springs$ cam and lever

mechanisms and are made to open and closed so that the charge enters?discharges at the

right time

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3. E&("ain the wo-ing (in!i("e o# #o/ sto-e (eto" engine 0Otto Cy!"e engine o

Constant o"/'e !y!"e engine o 2 sto-e SI Engine+

#t is also known as 0tto cycle or constant volume cycle engine. #t re@uires four strokes of

the piston to complete one cycle of operations in the engine cylinder. &he four strokes are as

follows"

1) S/!tion o !haging sto-e" #n this stroke$ the inlet valve opens and the fresh charge

!petrol and air mixture from carburetor) is sucked into the engine cylinder as the piston

moves from & to -. #t continues till the piston reaches -. uring this travelof the piston the crankshaft revolves by half rotation. -ecause of the pressure difference

between the atmosphere and the inside of the engine cylinder$ petrolair mixture will be

drawn into the cylinder from the carburetor. At the end of this stroke$ the cylinder will

be completely filled with petrolair mixture and inlet valve is closed.

') Co'(ession Sto-e" #n this stroke$ both inlet and outlet valves are closed and charge

is compressed as the piston moves from - to &. As a result of compression$

pressure and temperature of the charge increases considerably. &his completes one

revolution of the crank shaft. %hortly before the piston reaches the & during this

stroke$ the charge is ignited with the help of a spark plug.

+) owe Sto-e 0Wo-ing sto-e o E&(ansion sto-e+" uring this stroke$ both the

valves are closed and because of the combustion of charge$ the burnt gases expand. ue

to this expansion$ the hot gases exert pressure on the piston and as a result$ the piston is

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pushed from & to -. &he power impulse is transmitted down through the piston

to the crankshaft. &his causes the crankshaft to rotate at high speeds. &hus$ work is

obtained in this stroke. At the end of this stroke$ the exhaust valve opens which will

release the burnt gases to the atmosphere.,) E&ha/st Sto-e" #n this stroke$ the exhaust valve is open as piston moves from - to

&. &his movement of piston pushes out the burnt gases from the cylinder to the

atmosphere. &his completed one cycle of operation and the engine is ready for the next

cycle.

4. E&("ain the wo-ing #o/ sto-es $iese" engine 05iese" Cy!"e engine o Constant ess/e

!y!"e engine o 2 sto-e CI engine+

&he , stroke diesel engine is similar to that of a , stroke petrol engine except that a fuel

in7ector is fitted in place of a spark plug and air only enters into the engine cylinder during

suction stroke. Air alone is compressed during the compression stroke and at end of this stroke$

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the fuel in7ector in7ects fuel into the hot compressed air. &he fuel is then ignited as it comes in

contact with the hot compressed air. Hence these engines are called compression ignition

engines. &he working principle of , stroke diesel engine is based on iesel cycle$ hence they

are called diesel cycle or constant pressure cycle engines. &he four different strokes of the ,stroke diesel cycle is as under"

i) %uction %troke" uring this stroke$ the piston moves from & to -. &he inlet valve

opens and exhaust valve will be closed. &he downward movement of the piston creates

a suction in the cylinder and as a result atmospheric air from the air filter is drawn into

the engine cylinder through the inlet valve. 6hen the piston reaches -$ the suction

stroke ends with closure of the inlet valve.

ii) ompression %troke" uring this stroke$ the piston moves from - to & and both

the inlet and exhaust valves are closed. As the piston moves upwards$ the air inside the

engine cylinder is compressed to high pressure and temperature. &he compression

process is adiabatic in nature and the compression ratio will be from 14"1 to ''"1. At

the end of this stroke$ the fuel is in7ected into the engine cylinder in the form of fine

sprays by a fuel in7ector. ombustion of fuel takes place at constant pressure.

iii) (ower %troke" uring this stroke$ both inlet and exhaust valves remain closed. ue to

the combustion of fuel$ the burnt gases expand and exert a large force on the piston.

ue to this$ the piston is pushed from & to -. &he power impulse is transmitted

down through the piston to the crankshaft through the connecting rod. &his causes the

crankshaft to rotate at high speeds. &hus power is obtained during this stroke. &he

expansion gases is adiabatic in nature and when the piston reaches - the exhaust

valve opens. A part of the burnt gases escapes through the exhaust valve due to their

expansion. &he drop in pressure is at constant volume.

iv) Exhaust %troke" uring this stroke$ the piston moves from - to & and the inlet

valve will be closed and exhaust valve will be opened. As the piston moves upwards$ it

forces the burnt gases out of the engine cylinder. 6hen the piston reaches &$ the

exhaust valve close and the next cycle of suction stroke begins.

&hus in this engine$ four operations are completed in two revolutions of the crank shaft or

, strokes of the piston and the power is obtained during only in power stroke i.e. one power

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stroke for two revolutions of the crankshaft.

6. E&("ain the wo-ing o# two sto-e (eto" engine 0 ) sto-e SI engine7 ) sto-e Constant

%o"/'e !y!"e engine o ) sto-e Otto !y!"e engine+

#n a ' stroke petrol engine$ there are no valves instead the cylinder is provided with ports. &he

ports acts as valves and are opened and closed by the moving piston. #n ' stroke engine$ all

operations are completed in one revolution of the crankshaft or ' strokes of the piston. #n these

engines the two strokes of the piston are as under"

i) 5ownwa$ Sto-e 0Inwa$ Sto-e+ " At the beginning of this stroke$ the piston is at

&. At this position$ the charge !Air and (etrol 2ixture) present in the previous

cycle$ is compressed to high pressure and temperature. &he compressed charge in the

cylinder is ignited by means of a spark plug. As combustion takes place$ the burnt gases

expand and exert a large force on the piston. ue to this$ the piston is pushed

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downwards from & to -. &he power impulse is transmitted to the crankshaft

from the piston through the connecting rod. &his causes the crankshaft to rotate at high

speeds.&he movement of the piston from & to - uncovers the exhaust port . A part of

the burnt gases escape through the exhaust port. /urther downward movement of thepiston uncovers the transfer port and fresh charge enters from the crank case into the

engine cylinder. uring the downward movement of the piston$ the charge in the

crankcase is compressed by the underside of the piston. &he compressed charge from

the crankcase rushes into the engine cylinder through the transfer port driving away the

remaining exhaust gases through the exhaust port. &his process of sweeping out the

exhaust gases with the help of fresh charge is called scavenging. &he top of the piston

ogeesha H 8ecture :otes on E2E1,?',

:agar7una ollege of Engineering B &echnology$ -angalore

has a pro7ection called deflector that deflects the fresh charge coming through transfer

port to move upwards to the top end of the cylinder.

ii) U(wa$ Sto-e o O/twa$ Sto-e" uring this stroke$ the piston moves from -

to &. uring this movement$ the piston covers the transfer port$ thereby stopping the

flow of fresh charge into the engine cylinder. /urther upward movement of the piston

covers the exhaust port and actual compression of the charge begins. #n the mean time$

the inlet port is uncovered by the piston and upward moving piston creates suction in

the crankcase thus admitting fresh charge into the crankcase. &he compression of the

charge in the cylinder continues till the piston reaches &. &hus$ one cycle is

completed. #n the next cycle$ the compressed charge is ignited by the spark plug and the

cycle continues.

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8. Co'(ae two sto-e an$ #o/ sto-e engines.

-asis for comparison /our %troke &wo stroke

1. :umber of strokes per

cycle

, '

'. :umber of revolutions

of the crank shaft per

cycle

' 1

+. &or@ue fluctuations High. Hence heavy

flywheel is needed

2ore uniform. Hence lighter

flywheel is needed.

,. &hermal efficiency High low

4. 2echanical efficiency 8ow Highs

5. Entry and Exhaust of

harge

&hrough alves &hrough ports

. (ower stroke 0ne power for two

revolutions of the crank

shaft

0ne power stroke for every

revolution of the crank shaft.

1C. 6eight Heavy and bulky 8ighter

11. ooling and lubrication 3e@uires lesser cooling and

lubrication since

combustion takes place in

alternate revolutions of the

crankshaft

3e@uires greater cooling

and lubrication since

combustion takes place in

every revolution of the

crank shaft

1'. uses Dsed in cars$ 7eeps$ -uses$

&rucks$ (ower generators$

etc.

Dsed in light vehicles such

as motor cycles$ + wheelers.

8awn movers$ marine

engines$ etc

9. Mention the a$%antages an$ $isa$%antages o# two sto-e engines o%e #o/ sto-e

engines

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Advantages of ' %troke Engines

A ' stroke engine has the twice the number of power strokes than a four stroke engine

at the same speed. Hence theoretically power developed in ' stroke engine is double

than the four stroke engine. &he weight of the ' stroke engine is less than four stroke engine because of the lighter

flywheel due to more uniform tor@ue on the crankshaft

&he work re@uired to overcome friction of the suction and exhaust is saved

%ince there are no mechanical valves and valve gears$ the construction is simpler

#t occupies less space

8ess maintenance since less number of moving parts

&he ' stroke engine is much easier to start

#nitial cost of the engine is less

isadvantages of ' %troke engines

&hermal efficiency is less

%ince the firing takes place in every revolution$ the time available for cooling is less$

which results in overheating of the piston and other engine parts.

onsumption of lubricating oil is more

#ncomplete scavenging results in mixing of fresh charge with the exhaust gases which

will reduce the power output and thermal efficiency.

%ince the transfer port opened for only short duration$ it is likely that less @uantity of

charge may be admitted into the engine cylinder reducing the power output

/or a given stroke and clearance volume$ the effective compression ratio is less

3unning cost will be more as the fuel consumption is more as compared to , stroke

engine

1C. alve timing diagram for /our stroke engineF

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11. (ort &iming diagram for two stroke engineF

8188 39051 ic engines notes

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