Two Stroke SI and CI Engines

1. I.C. Engines
2 Stroke Petrol & Diesel Engines
1
Dr. S. VIJAYA BHASKAR
Professor in Mechanical Engineering
Sreenidhi Institute of Science &
Technology, Hyderabad

2. Working Principle of IC Engines
All IC Engines must have the following
FOUR EVENTS/Operations:
1. Suction
2. Compression
3. Expansion / Power and
4. Exhaust

3. Two Stroke Cycle Petrol Engine
 A two-stroke cycle-petrol engine was devised by
Duglad Clerk in 1880.
 In this Cycle, the suction, compression,
expansion and exhaust takes place during two
strokes of the piston.
 It means that there is one working stroke after
every revolution of the crank shaft.
 A two stroke engine has ports instead of valves
X

4. Working Principle of 2 Stroke Engine
 TWO stroke engine rotates only 360 or
ONE revolution of crank shaft rotation
 Suction and Exhaust strokes are two unproductive
strokes in any engine
 But these operations are must in order to operate the
engine
 By combining productive and unproductive strokes in
ONE revolution of the crank shaft or 2 strokes of the
engine in order to obtain one power stroke

5. Operation of 2-Stroke Petrol Engine
5
INLET

EXHAUST

6. Suction Stage
 In this stage, the piston, while going down towards BDC,
uncovers both the transfer port and the exhaust port.
 The fresh fuel-air mixture flows into the engine cylinder from
the crank case, as shown in the figure below.

7. Compression Stage:
 In this stage, the piston,
while moving up, first
covers the transfer port
and then exhausts port.
 After that the fuel is
compressed as the piston
moves upwards as shown
in the figure.
 In this stage, the inlet
port opens and fresh fuel-
air mixture enters into the
crank case.

8. Piston uncovers transfer port
Beginning of the first stroke During first stroke

9. Expansion Stage:
 Shortly before this piston reaches the
TDC (during compression stroke), the
charge is ignited with the help of a spark
plug.
 It 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 great force as shown
in the figure.
 The hot burnt gases expand due to high
speed of the piston. During this
expansion, some of the heat energy
produced is transformed into mechanical
work.

10. Exhaust Stage
 In this stage, the exhaust port is opened as the piston moves
downwards.
 The products of combustion, from the engine cylinder are
exhausted through the exhaust port into the atmosphere, as
shown in the Figure.
 This completes the cycle and the engine cylinder is ready to
suck the charge again.

11. Transfer port covered Compression commenced

13. 13
TWO STROKE ENGINE - CYCLE OF OPERATIONS
• Air charge simultaneously inducted into the crank case
through spring loaded inlet valve, as the pressure in the
crank case drops due to the upward motion of the
piston during the compression stroke.
• After the compression and ignition, the expansion
follows in the usual way. During the expansion stroke,
the charge in the crank case is compressed.
• Near the end of the expansion stroke, the piston
uncovers the exhaust ports and the cylinder pressure
drops to atmospheric pressure, as the combustion
products leave the cylinder.

14. 14
• Further movement of the piston uncovers the transfer
ports, thus permitting the slightly compressed charge in the
crank case to enter the engine cylinder.
• The top of the piston has usually a projection to deflect the
fresh charge towards the top of the cylinder before flowing
through the exhaust ports.
• This has the dual purpose of
A. Scavenging the upper part of the cylinder of the
combustion products and
B. Preventing the fresh charge from flowing directly
towards the exhaust ports.
TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.)

15. 15
• Same objective can be achieved without
piston deflector through proper shaping of
the transfer port.
• During the upward motion of the piston
from BDC, the transfer ports close first and
then the exhaust ports close when the
compression of the charge begins and the
cycle is repeated.
TWO STROKE ENGINE - CYCLE OF OPERATIONS (contd.)

16. 2-Stroke Petrol Engine
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17. 2-Stroke Petrol Engine
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18. 18

19. 19

20. 2-Stroke Diesel (CI) Engine
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21. Essentially Required 2 Strokes
21

22. As per Text Book….
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23. 23

24. 24

25. First Stroke
25

26. First Stroke in 2-Stroke CI Engine
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 Figure 2.1 shows one of the simplest two-
stroke engines, viz., the crankcase
scavenged engine
 As the piston starts moving from its BDC to
TDC position, it closes the transfer port and
the exhaust port.
 The air which is already there in the cylinder
is compressed.
 At the same time with the upward movement
of the piston, vacuum is created in the crank
case.
 As soon as the inlet port is uncovered the

27. First Stroke in 2-Stroke CI Engine
27
 Figure 2.1 shows one of the simplest two-
stroke engines, viz., the crankcase
scavenged engine
 As the piston starts moving from its BDC to
TDC position, it closes the transfer port and
the exhaust port.
 The air which is already there in the cylinder
is compressed.
 At the same time with the upward movement
of the piston, vacuum is created in the crank
case.
 As soon as the inlet port is uncovered the

28. 28

29. Second Stroke in 2-Stroke CI
Engine
29
 Slightly before the completion of the
compression stroke a very fine spray of diesel
is injected into the compressed air (which is at
a very high temperature).
 The fuel ignites spontaneously due to high
pressure and temperature in combustion
chamber
 Pressure is exerted on the crown of the piston
due to the combustion of the charge and the
piston is pushed in the downward direction
towards BDC to produce some useful power.

30. SECOND STROKE
30

31. Second Stroke in 2-Stroke CI Engine
(Cond..)
31
 The downward movement of the piston will first
close the inlet port and then it will compress
the air already sucked in the crank case.
 Just at the end of power stroke, the piston
uncovers the exhaust port and the transfer
port simultaneously.
 The expanded gases start escaping through the
exhaust port and at the same time the fresh air
which is already compressed in the crank case,
rushes into the cylinder through the transfer
port and thus the cycle is repeated again

32. 32

33. S
N
o
Four Stroke Engine Two Stroke Engine
1. The thermodynamic cycle
is completed in four strokes
of the piston or in two
revolutions of crank shaft
or 720° of crank angle.
The thermodynamic cycle is
completed in two strokes of
the piston or in one
revolution of the crank shaft
or 3600 of crank angle
2. One power stroke is
obtained in every two
revolution of crank shaft /
4-strokes of the Piston.
One power stroke is obtained
in each revolution of crank
shaft.
3. Because of above, turning
moment is not so uniform
and hence a heavier
flywheel is needed.
Because of above, turning
moment is more uniform and
hence a lighter flywheel can
be used.
4. Power produced for same
size of engine is less. In
Power produced for same size
of engine is twice, or for same

34. S.
N
o
Four Stroke Engine Two Stroke Engine
5. Lesser cooling and lubrication
requirements. Lower rate of wear
and tear.
Greater cooling and lubrication
requirements. Higher rate of wear
and tear.
6. It has valves and valve actuating
mechanisms for opening and
closing of the intake and exhaust
valves.
It has no valves but only ports
7. Because of comparatively higher
weight and complicated valve
mechanism, the initial cost of the
engine is more.
Because of light weight and
simplicity due to the absence of
valve actuating mechanism, initial
cost of the engine is less.
8. Volumetric efficiency is more due
to more time for induction.
Volumetric efficiency is low due to
lesser time for induction.
9. Thermal efficiency is higher, part
load efficiency is better.
Thermal efficiency is lower, part
load efficiency is poor.
1
0.
Application : Cars, Buses, Trucks,
Tractors, Aero planes and Power
Application : Mopeds, Scooters,
Motorcycles, Hand sprayers etc.,