Unit 1
1.5 IC Engines- Terminology ,
Combustion in S.I & C.I Engines
Engine Terminology
➢ Dead centres : In the vertical engine, top of the piston
is called TOP DEAD CENTRE (TDC), when the piston is at
bottom position is called Bottom Dead Centre
➢ Stroke : It is the linear distance travelled by the piston
when it moves from one end of the cylinder to the another
➢ Bore : Inner diameter of the cylinder
➢ Stroke volume : volume displaced by the piston in one
stroke. Also called as swept volume
➢ Clearance volume : volume contained between the piston
top and the cylinder head when the piston at top
➢ Stroke to Bore Ratio
Ratio of the stroke length , L , to the bore diameter ‘d’
d < L - under square engine
d = L – square engine
d > L – over square engine
a) d = L – square engine
Balance both torque and power
Eg :-Hero Honda CBZ Xtreme (57.3mm x 57.8 mm)
b) d < L - under square engine (Long stroke Engine)
Engine makes good torque at relatively lower engine revs.
where torque (pulling capability) is essential at lower/relaxed engine revs
(Eg. commuter bikes, touring bikes)
Eg :-Royal Enfield Thunderbird Twinspark (70mm x 90mm)
c) d > L – over square engine (short stroke engine)
engine revs fast and is good for vehicles where fast build up of power is essential (eg.
Race/track bikes, Super sports bikes)
Eg :- TVS Apache RTR 160 (62mm x 52.9mm)
𝜋
𝑉1 = 𝑑 2 L , where L = stroke length and d- bore diameter
4
4 stroke Engine
S.I Engine ( Spark Ignition Engine)
• Combustion takes place with the using the spark plug
• Inlet – air+ fuel mixture
• Heat addition takes place at constant volume
• Otto cycle
F o u r s t ro k e Petrol E n g i n e
1.Intake/Suction Stroke
In suction stroke piston starts at Top Dead
Centre (TDC) of the cylinder and moves to
the Bottom Dead Centre (BDC).
Exhaust Valve will be closed and intake valve
will be open to allowing the fresh charge of
mixed fuel and air into the cylinder.
2.Compression Stroke
• In compression stroke, Once piston
reaches BDC and moves back TDC,
inlet valve will be closed.
• As the piston moves towards TDC , It
compress air inside the cylinder and
compression takes place. Hence it is
called compression stroke.
3.Power Stroke
• In expansion stroke, Both the valves are
closed. When piston reaches top of its
stroke, a spark is generated with a help of
spark plug and combustion takes place
• temperature and pressure generated
inside the cylinder and push down the
piston to BDC.
• it is known as Power stroke
• Power generated in this stroke is stored
in the flywheel for its further utilisation in
the other strokes.
4.Exhaust Stroke
• In this stroke exhaust valve is opened
when piston reaches to BDC and moves to
upward
• . Piston pushes out the burnt gases to the
atmosphere through the exhaust valve,
hence called exhaust stroke
• engine is ready to begin the cycle again
.
pV Diagram (Theoretical& Actual)
Valve Timing Diagram
• Graphical representation of exact moments, in the
sequence of operation , at which the inlet and
exhaust valves open and close and firing takes place
4 stroke Engine
C.I Engine ( Compression Ignition Engine)
• Inlet – air
• Combustion takes place by spraying the fuel
• Heat addition takes place at constant pressure
• Diesel cycle
F o u r s t ro k e Diesel E n g i n e
pV Diagram (Theoretical & Actual)
4 stroke Petrol Engine Vs 4 Stroke Diesel Engine
4 Stroke Petrol Engine (S.I) 4 Stroke Diesel Engine (C.I)
Fuel A mixture of petrol and air Only fresh air is drawn during the
is drawn during the suction stroke
suction stroke
Fuel supply Carburettor Fuel injector
Ignition Spark plug is used to Auto ignition, the fuel is ignited
ignite the charge when sprayed in air at high
temperature
TD cycle Otto cycle Diesel cycle
Compression 6-10 15-25
ratio
Starting Easy, due to low Difficult, due to high compression
compression ratio ratio
Space Less more
occupancy
4 stroke Petrol Engine Vs 4 Stroke Diesel Engine
4 Stroke Petrol Engine 4 Stroke Diesel Engine (C.I)
(S.I)
Initial cost Light and cheap due to Heavy and costly due to max
low max pressure pressure
Running cost High , due to high cost of Less, due to less cost of diesel
petrol
Maintenance Less high
cost
Application Scooter, motor cycles and Buses, trucks, earth moving
other light duty vehicles machines and other heavy duty
machines
Two Stroke Engine -Petrol
• All four processes completed in Two strokes
• One revolution of crank
• There is a power stroke in every revolution
• In two stroke, three ports are available namely inlet
port , transfer port and Exhaust port
• Suction and compression completed during the
upper movement of the piston
• Expansion and Exhaust completed during the upper
movement of the piston
Suction/ Compression
• Piston moves from BDC to TDC
• Air + fuel mixture inside the cylinder gets
compressed
• Transfer port is covered
• While moving Exhaust port gets covered
• Inlet port opened , and allow the fresh air + fuel
mixture
Expansion/ Exhaust
• Shortly before reaches the TDC ignition starts
• Which moves the piston downwards
• During downward movement, transfer port is
uncovered . Allow the exhaust gases to exit
• Then covers the inlet port
• Now the transfer port opened and allow the air+ fuel
mixture to flow inside the cylinder, which also pushes
the burnt gases outside
4 stroke Petrol Engine Vs 4 Stroke Diesel Engine
4 Stroke 2 Stroke
All the four processes completed in All the four processes completed in
two revolution of crank one revolution of crank
Power stroke in two crank Power stroke in every crank revolution
revolution
Power produced is less Power produced per cycle is more
Inlet, Exhaust valves are used Inlet, transfer and exhaust ports are
used
High thermal efficiency Low thermal efficiency
Disadvantages of Two stroke engines
• Low thermal efficiency
• Pollution
• No separate lubrication system provided
Octane Rating/Number(ON)
• An octane rating, or octane number, is a standard measure
of the performance of an engine or aviation fuel.
• higher the octane number, the more compression the fuel
can withstand before detonating (igniting)
• Octane numbers are used between 0 and 100
• Higher octane number will have shorter ignition delays
• Gasoline engines rely on ignition of air and fuel compressed
together as a mixture, which is ignited at the end of the
compression stroke using spark plugs. Therefore, high
compressibility of the fuel matters mainly for gasoline
engines.
• Use of gasoline with lower octane numbers may lead to the
problem of engine knocking.
Cetane Rating /Number(CN)
• Cetane numbers gives quality of Diesel
• Cetane number is an indicator of the combustion speed of diesel
fuel and compression needed for ignition
• Higher the number implies good quality of Diesel
• Generally, diesel engines operate well with a CN from 48 to 50.
Fuels with lower cetane number have longer ignition delays,
requiring more time for the fuel combustion process to be
completed. Hence, higher speed diesel engines operate more
effectively with higher cetane number fuels
