Internal Combustion Engines

97

7 INTERNAL
COMBUSTION ENGINES
Any type of engine, which derives heat energy from the combustion of fu I d rt
· h · I k · e an conve s
this energy mto mec amca wor , 1s termed as a heat engine.
Heat engines may be classified into two main classes :
I. External combustion (E.C) engine, and
2. Internal combustion (I.C) engine.
If the combustion of the fuel takes place outside the working cylinder then the engine
is called the external combustion engine. If the combustion takes place inside the working
cylinder, the engine is called the internal combustion engine.
The most common examples of external combustion engines are the steam_engines and
steam turbines. Internal combustion engines are used in scooters, cars, buses, trucks,
locotnotives, -agricultural and earth moving machinery, power g·e neration and in may
industrial applications.
Advantages of I. C. engines over E. C. engines:
i) High efficiency ii) Simplicity
iii) Compactness iv) Light weight
v) Easy starting, and vi) Comparatively lower cost
Classification of I.C. engines
I.C. engines are classified as given below:
1. According to cycle of combustion:
i) Otto cycle engine ii) Diesel cycle engine
iii) Dual-combustion cycle engine
2. According to the fuel used :
i) Petrol engine ii) Diesel engine .
iii) Gas engine iv) Kerosene engme, etc.
3. According to the cycle of operation : .
i) Two stroke cycle engine ii) Four stroke cycle engme
4. According to the method of ignition : . . .. .
i) Spark ignition (S.l) engine ii) Compression 1gmhon (C.I) engme
5. According to the number of cylinders: . . . ·
i) Single cylinder engine ii) Multi cyhnder engme
r
98 Elemen ts Of Mecha nical Engineering

6 According to the arrangement of cylinders: .

· i) Horizontal engine ii) Vertical en~me iii) V-eng ine
. In-1me
iv) . engine• v) Radial engine, etc.
7 A ccording to the speed of the engine: .
· i) Low speed engine ··n) Me d.ium speed engme iii) High speed engine
8. According to the metho d of cooling the cylind er : .
i) Air cooled engine ii) Water cooled engine
9. According to their uses:
i) Stationary engine ii) Aut~mobil~ engine iii) Aero engine
iv) Locomotive engine v) Mann e engine, etc.
I.C. engine parts and their functions
A cross section of an air cooled I. C. engine with princi pal parts is shown in fig. 7 .1 .
The functions of different parts of the engine are given below.
✓ Cylinder : The }_leart of the engine is the cylinder and its primary functi on is to contain
the :working fluid un~er pressure and_ the se_condary function is to guide the piston. T~
avoid wear of the cylmder block, cylinder hners are provid ed. ThP ""1;_ ..1--. •
g_@)'_ C<!_~t irc~n. ' · •
 Interna l Combustion Engines 101
Piston speed: The velocity of the piston vis the linear distance traveled by the piston per
unit time.
i.e., v = 2/n' mis
where I = Length of stroke in m
n' = Speed of the engine in rps
Cycle of operation
The number of strokes of the piston required to complete the cycle varies with the type
of the engine. There are two types of engines namely four-stroke cycle engine and two-
stroke cycle engine. A,. _fo_ur-strQ,ke engine requires four strokes of the piston or two
revolutions of the crankshaft to complete one cycle. In a two-stroke cycle engine there are
two strokes of the piston or one revolution of the crankshaft to complete one cycle. The four
stroke and two stroke engines are further classified into petrol engines and diesel engines
according to the type of fuel used.
Four-s troke cycle petrol engine
The four-stroke cycle petrol engine operates on Otto (constant volume) cycle. Since
ignition in these engines is due to a spark, they are also called spark ignition engines. The
four different strokes are :
(i) Suction stroke, (ii) Compression stroke,
(iii) Working or power or expansion stroke, and (iv) Exhaust stroke.
The construction and working of a four-stroke petrol engine is shown in fig. 7.4.
Fig. 7 .5. shows a theoretical Otto cycle.

Spark plug \ Exhaust gas

Air-fuel m i x t u r e ~ ~
Inlet valve ~
Exhaust
i.-------.i valve Air-fuel
Piston
mixture
Cylinder

Connecting rod -

(,-- ( ,- -
' '

Crank
\ '-......__./ ' \ '-......__./ ,

Suction stroke Compression stroke Power stroke Exhaust stroke

.Fig. 7.4 Four stroke cycle petrol engine

I'l
 102 Elements of Mechanical Engineering

• move d from the top dead

o • •
Suction stroke (0 to 180 ') : Durm •
g suction stroke, the piston ts ft is revolved either by the
center to the bottom dead center by the cra~shaft._ The crank sh: .nlet valve
momentum of the flywheel or by the electric starting motor. T _e 1 remains open
. trol mixtu re·
and the exhaust valve is closed during this stroke. T~e proportion~te atr-¥~
sucked into the cylinder due to the downward movement of the P1st0n. ·1s operaffon~s
ts
represented by the line AB on the P-V diagram (fig. 7.5).
D

Volume

Fig. 7. 5 Theoretical Otto cycle

Compression stroke (18(/ to 36(/) : After the piston reaches the lower
limit (botto m dead
center) of its travel, it begins to move upward. As this happens, the inlet
vaLve closes. ~e
exhaust valve is also closed, so the cylinder is sealed. As the piston move
s upwa rd, the arr-
petrol mixture is compressed. Due to compression, the pressure and
temp eratu re are
increased and is shown by the line BC on the P-V diagram.
As the piston reaches the top dead center of its travel on the comp ressio
n stroke, an
electric spark is produced at the spark plug. The ignition system delive
rs a high voltage
surge of electricity to the spark plug to create spark. The spark ignites the
and combustion takes place at constant volume as shown by the line CD
air-pe trol mixture !
I
in the PV diagram. \

Power stroke (36ff - 54(/): The expansion of gases due to the heat
exerts a pressure on the piston. Under this impulse, the piston move
of comb ustion
s from top dead
I
center t~ bottom dead ce~ter and the power is transmitt_ed to the crank
shaft throu gh the I
connectmg rod. Both the mlet and exhaust values remam closed durin
g this stroke. Toe 1
expansion of the gas is shown by the curve DE.
~
ll
Exhaust stroke (54(/ - 72d): During this stroke, the inlet valve remai
ns close d and the
exhaus! valve opens. _The greater part of the burnt g~ses escapes becau
se of their own
expansion. The drop m pressure at constant volume 1s represented I

by the line EB The

piston moves from bottom d~ad center to top dead center and pushe s the
rema ining ga~es to
the atmosphere. When the piston reaches the top dead center the exhau
. . . st valv d
cycle 1s completed. The hne BA on the P-V diagra m repres ents this stroke Th
1
e cose s an
are repea ted over an d over agam . . . .
. e operations
m runnm g the engine.
Thus four strokes are completed in two revolutions (0-720°) of the crank h ft .
powe r strok .
e for every two revo 1utions of the crank shaft. s a 1 e one
· · ·, \

j
Interna l Combustion Engines
103
\
Four stroke cycle diesel engine ~
The four stroke cycle diesel engine operates on diesel cycle or constant pressure
cycle. Since ignition in these engines is due to the temperature of the compressed air,
they are also called compression ignition engines. The construction and working of the
four stroke diesel engine is shown in fig. 7 .6, and fig. 7. 7 shows a theoretical diesel
cycle. The four strokes are as follows.
Suction stroke (0 - 180°): During suction stroke, the piston is moved from the top dead
center to the bottom dead center by the crankshaft. The crankshaft is revolved either by
the momentum of the flywheel or by the power generated by the electric starting motor.
The inlet valve remains open and the exhaust valve is closed during this stroke. Jh~
fresh air is sucked into the cylinder due to the downward movement of the piston. The
line AB on the P-V diagram represent~ this operation. (_ Ff ::;- ,.:; )
Compression stroke (180° - 360°): The air drawn at the atmospheric pressure during
suction stroke is compressed to high pressure and temperature as piston moves from the
bottom dead center to top dead center. This operation is represented by the curve BC on the
P-V diagram. Just before the end of this stroke, a metered quantity of fuel is injected into the
hot compressed air in the form of fine sprays QY mea:Qs of fue_l inject9r. The fuel starts
biimm gat constant pressure shown by the line CD. At point D, fuel supply is cut off. Both
the inlet and exhaust valves remain closed during this stroke.

Inlet valve
Exhaust
valve
Piston Air
Cylinder

Connecting rod -

(' ('

' ' I

'

_J
I

\ \
Crank ,
-
Suction stroke
''-....___.... /',

Compression stroke Power stroke

''-.___/ ,

Exhaust stroke

Fig. 7.6 Four stroke cycle diesel engine

Power stroke (36(! - 54{!): The expansion of gases due to the heat of combustion
exerts a pressure on the piston. Under this impulse, the piston moves from top dead
center to bottom dead center The impulse power is transmitted to the crank shaft
through the piston and through the connecting rod. The crank shaft is rotated due to this
 104 Elements of Mechanical Engineering
force. Both the inlet and exhaust valves rem
ain closed during this stroke. The expansion
of the gas is shown by the curve DE.
C

E
A------=-----'B

Volume

Fig. 7. 7 Theoretical diesel cycle

Ex hau st stroke (54 ff - 72f f): During this
stroke, the inlet valve remains clo sed and
exhaust valve opens. The greater par t of the
the burnt gases escape bec aus e of the ir
expansion. The drop in pressure at constant ow n
volume is represented by the ver tica l line
The piston moves from bottom dead center EB.
to top dead center and pushes the rem ain
gases to the atmosphere. When the piston ing
reaches the top dead center the exh aus t val
closes and the cycle is completed. The line ve
BA on the P-V diagram represents this stro
Thus four strokes are completed in two revolu ke.
tions of the crankshaft. A ,,,.. i. _ --P.~, .--. c
0
 1.8 Application of IC Engines
- The most important application of IC engines is in transport on land, sea and air. Other
applications include industrial power plants and as prime movers for electric
generators. Table 1.3 gives, in a nutshell, the applications of both IC and EC engines.
Table 1.3 Application of Engines
IC Engine EC Engine
Type Application Type Application
Gasoline engines Automotive, Marine, Steam Engines Locomotives, Marine
Aircraft
Gas engines Industrial power Stirling Engines Experimental Space
Vehicles
Diesel engines Automotive, Railways, Steam Turbines Power, Large Marine
Power, Marine
Gas turbines Power, Aircraft, Close Cycle Gas Power, Marine
Industrial, Marine Turbine

1.9 Engine Performance Parameters