Internal combustion engines can operate on four main cycles: Otto, Diesel, dual combustion, and Atkinson. The Otto cycle uses spark ignition of a premixed fuel-air mixture at constant volume. Diesel engines ignite fuel injected into compressed hot air at constant pressure. Dual combustion engines combine aspects of Otto and Diesel cycles. Two-stroke engines complete the combustion cycle in two strokes while four-stroke engines take four strokes - intake, compression, power, and exhaust. The document then provides detailed explanations and diagrams of the operating cycles and processes within four-stroke petrol and diesel engines and two-stroke engines.

1. IC ENGINE
Guide:
DEBOLINA MUKHERJEE,
MECHANICAL ENGG. DEPARTMENT
Texted from: IC ENGINE, R K RAJPUT

2. Working Cycles of IC ENGINE
Internal Combustion Engine can work on the following
cycles:
❑ Constant Volume or Otto Cycle
❑ Constant Pressure or Diesel Cycle
❑ Dual Combustion Cycle
❑These may be either Two stroke Cycle or Four Stroke
Cycle engine.

3. Constant Volume or Otto Cycle
❑ The cycle is called constant volume cycle as heat is supplied
at constantvolume.
❑ Petrol, gas and light weight engine work on Otto Cycle.
❑ In case of Petrol Engine, proper mixing of air and petrol takes
place in Carburetor.
❑ Light engines make use of Otto cycle.

4. Constant Pressure or Diesel Cycle
❑ In constant pressure or Diesel cycle, only air is drawn into the
engine cylinder during the suction stroke.
❑ The air gets compressed during the compression stroke and its
pressure and temperature increases.
❑ After compression of the air, a metered quantity of diesel fuel is
sprayed into the compressedair with the help of fuel injector.
❑ Due to high pressure and temperature of the air, the fuel
ignites.
❑ Heavy engines makes use of Diesel cycle.

5. Duel Combustion Cycle
❑ This engine is also called semi-diesel cycle.
❑ In duel combustion cycle, heat is partially added at partially at constant
pressure and partially at constant volume.
❑ In this cycle only air is drawn into the engine cylinder during suction stroke.
❑ The air is then compressed into the combustion chamber at a pressure of
about 26 bar.
❑ The heat of compressed air together with the heat of combustion chamber
ignites the fuel.
❑ Fuel is injected into the engine cylinder just before the end of compression
stroke where it ignites.

6. Two stroke v/s Four Stroke Engine
❑ In a two-stroke engine, the working cycle is completed in two
strokes of the piston or one revolution of the crankshaft.
❑ This is achieved by carrying out the suction and compression
processes in one stroke (or more precisely in inward stroke),
expansion and exhaust processes in the second stroke (or more
precisely in outward stroke).
❑ In a four-stroke engine, the working cycle is completed in
four-strokes of the piston or two-revolutions of the
crankshaft.
❑ This is achieved by carrying out suction, compression,
expansion and exhaust processes in each stroke.

7. Four Stroke Cycle Petrol Engine
❑ In four stroke cycle
petrol engine, the
mixture of air and fuel
is drawn in the engine
cylinder.
❑ Since ignition in petrol
engine is due to spark,
hence it is called Spark
Ignition Engine.
❑ The various stages of
Four stroke cycle Petrol
engine is shown in the
diagram.

8. Steps of Four Stroke Cycle Petrol Engine
❑ SUCTION STROKE: During this stroke the piston moves from TDC to BDC ,
the inlet valve opens and air fuel mixture is supplied to engine cylinder.
Suction stroke is represented by line 5-1 in PV diagram. Exhaust valve
remains closed.
❑ COMPRESSION STROKE: In this stroke, the piston moves 1-2 towards TDC
and compresses the enclosed fuel air mixture drawn into the engine
cylinder during suction stroke. The pressure of the air fuel mixture rises to
a value above 8 bar. Just before the end of the compression stroke the
spark plug initiates a spark which ignites the mixture and combustion
takes place at constant volume (2-3). Both exhaust and inlet valve remain
closed.
❑ EXPANSION STROKE or WORKING STROKE: When the air fuel mixture is
ignited by the spark plug, hot gases are produced which drives the piston
from TDC to BDC and thus work is obtained in this stroke. It is during
expansion stroke that we get work from the engine, hence it is also
known as working stroke. The flywheel mounted on the engine shaft
store energy during this stroke and supply it during the idle strokes. The
expansion of gases are shown by 3-4 PV diagram. Both the valves remain
closed during start of this stroke, but when the piston reaches BDC the
exhaust valve opens.
❑ EXHAUST STROKE: During exhaust stroke, the hot gases after the
expansion and combustion are exhausted to the atmosphere. The piston
moves from BDC to TDC and the burnt gases are removed from the
engine cylinder. This operation is shown by the line 1-5 in PV diagram.

9. Steps of Four Stroke Cycle Petrol Engine

10. Four Stroke Cycle Diesel Engine

11. ❑ SUCTION STROKE: With the movement of the piston from TDC to BDC, during
this stroke, the inlet valve opens and air at atmospheric pressure are drawn
inside the engine cylinder. The exhaust valve remains closed. This operation is
represented by 5-1 in PV diagram.
❑ COMPRESSION STROKE: The air drawn at atmospheric pressure during the
suction stroke is compressed to high pressure and temperature ( to the value of
35 bar and 600⁰C) as the piston moves from BDC to TDC. This operation is shown
by 1-2 in PV diagram. Both the exhaust and the inlet valve remains closed.
❑ EXPANSION STROKE or WORKING STROKE: As the piston starts moving from
TDC, a metered quantity of diesel fuel is injected into the hot compressed air in
form of fine spray by the fuel injector. Then fuel air mixture starts burning at
constant pressure as shown by the line 2-3 in the PV diagram. At point 3 the fuel
supply is cut-off. The fuel is injected at the end of compression stroke but in
actual practice the ignition of fuel starts before the end of compression stroke.
The hot gases of combustion expands adiabatically to point 4 as shown in PV
diagram. The expansion is shown by 3-4.
❑ EXHAUST STROKE: The piston moves from BDC to TDC and the exhaust gases
escapes to the atmosphere through the exhaust valve. When the piston reaches
the TDC, the exhaust valve closes and the cycle is completed. This stroke is
represented by the line 1-5 in the PV diagram.
Steps of Four Stroke Cycle Diesel Engine

12. Two Stroke Cycle Engine
❑ In 1878, Dugald-clerk a british
engineer introduced a cycle
which could be completed in two
strokes of piston instead of four
strokes as in case of four stroke
cycle engines.
❑ In this engine the suction and
exhaust stroke are eliminated.
Here instead of valves, ports are
used. The exhaust gases are
driven out from the engine
cylinder.
❑The diagram shows a two
stroke petrol engine used in
scooters , motor cycles , etc.

13. ❑ The cylinder L is connected to
a closed crank chamber CC.
❑ During the upward stroke of
the piston M, the gases in the
cylinder L, are compressed
and at the same time fresh air
and fuel (petrol) mixture
enters the crank chamber
through the valve V.
❑ When the piston moves
downwards, V closes, and the
mixture in the crank chamber
is compressed.
Two Stroke Cycle Engine

14. ❑ STEP1: Piston is moving upwards and compressing
an explosive charge which has been previously
supplied to cylinder L. Ignition takes place at the
end of the stroke. The Piston then travels
downwards due to expansion of gases.
❑ STEP2: Near the end of the expansion stroke the
Piston uncovers the exhaust port (EP), and the
burnt exhaust gases escapes through the exhaust
port.
❑ STEP3: The transfer port TP is then uncovered
immediately and the compressed charge from the
crank chamber flows into the cylinder and is
deflected upwards by the hump provided at the
end of the piston.
❑ The incoming air petrol mixture, helps the
removal of gases from the engine cylinder.
❑ The piston again starts moving from BDC to TDC
and the charge get compressed when the exhaust
port EP and the transfer port TP is uncovered by
the piston, thus the cycle is repeated.
Two Stroke Cycle Engine