Four Stroke SI and CI engines

1. I.C. Engines
4 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
NOT TALKING ABOUT 2 OR 4 STROKE OR
PETROL OR DIESEL ENGINE
JUST WORKING OF ANY IC ENGINE
 TWO stroke engine rotates only 360 or
ONE revolution of crank shaft rotation
 FOUR stroke cycle completed through 720 of crank
rotation or TWO revolution of crank shaft rotation

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

4. Spark plug
Inlet valve/Port
Exhaust valve/
Port
Cylinder
Piston
Fuel Injector

5. Inlet valve/Port
opens
SUCTION STROKE
Exhaust valve/Port
Closed

6. Piston down
INTAKE STROKE
Inlet valve/Port
opens
Exhaust valve/Port
Closed

7. Inlet valve /
Port open
Piston down
INDUCTION STROKE
Air/Fuel Mixture InAir ONLY In Exhaust valve/Port
Closed

8. Inlet valve/Port
closes
COMPRESSION STROKE
Piston up
Exhaust valve/Port
Closed

9. Inlet valve/Port
closed
COMPRESSION STROKE
Piston up
Exhaust valve/Port
Closed

10. POWER STROKE
FIRING
Inlet valve/Port
closed
Exhaust valve/Port
Closed

11. POWER STROKE
Piston down
powerfully
Inlet valve/Port
closed

12. Inlet valve/Port
closed
POWER STROKE
Piston down
powerfully

13. Inlet valve/Port
closed
POWER STROKE
Exhaust valve/Port
Closed

14. Inlet valve/Port
closed
EXHAUST STROKE
Exhaust valve/Port
opens

15. EXHAUST STROKE
Exhaust valve/Port
opens
Piston up
Exhaust gases
out
Inlet valve/Port
closed

16. EXHAUST STROKE
Exhaust valve/Port
opens
Piston up
Exhaust gases
out
Inlet valve/Port
closed

17. Inlet valve/Port
opens
INDUCTION STROKE
Exhaust valve/Port
Closed

18. 4-Stroke Spark Ignition (SI)
Engine
18

19. Inlet valve
opens
SUCTION STROKE
Four-stroke SI Engine

20. Inlet valve
open
Piston down
SUCTION STROKE
Four-stroke SI Engine

21. Inlet valve
open
Piston down
INDUCTION STROKE
Four-stroke SI Engine
Air/Fuel Mixture In

22. Inlet valve
closes
COMPRESSION STROKE
Four-stroke SI Engine
Piston up

23. Inlet valve
closed
COMPRESSION STROKE
Four-stroke SI Engine
Piston up

24. Inlet valve
closed
POWER STROKE
Four-stroke SI Engine
Combustion

25. Inlet valve
closed
POWER STROKE
Four-stroke SI Engine
Piston down
powerfully

26. Inlet valve
closed
POWER STROKE
Four-stroke SI Engine
Piston down
powerfully

27. Inlet valve
closed
POWER STROKE
Four-stroke engine

28. Inlet valve
closed
EXHAUST STROKE
Four-stroke engine
Exhaust valve
open

29. Inlet valve
closed
EXHAUST STROKE
Four-stroke SI Engine
Exhaust valve
open
Piston up
Exhaust gases
out

30. Inlet valve
closed
EXHAUST STROKE
Four-stroke SI Engine
Exhaust valve
open
Piston up
Exhaust gases
out

31. Inlet valve
opens
INDUCTION STROKE
Four-stroke SI Engine
Exhaust valve
closed

34. Operation of 4-Stroke Petrol
Engine
3
4
INLET

EXHAUST

35. 4-Stroke Petrol Engine
35

36. 4-stroke petrol engine

37. 4-S Petrol
Engine
37
Works on Otto
Cycle

38. Power Cycle: Otto cycle
The air standard Otto Cycle is an ideal cycle that
approximates a spark- ignition internal combustion engine.
It assumes that the heat addition occurs instantaneously
while the piston is at TDC.

39. Process
(1-2) Isentropic Compression
Compression from ν1 to v2
↓
BDC(β=180º )
↓
TDC (θ=0º)
(2-3) Constant Volume Heat Addition: QH
•While at TDC
•Ignition of fuel (chemical reaction takes place)
(3-4) Isentropic Expansion
•Power is delivered while s = const.
(4-1) Constant volume Heat Rejection process
Otto Cycle

40. Otto Cycle

41.  Process 1–2 is an isentropic compression of the air as the
piston moves from bottom dead center to top dead center.
 Process 2–3 is a constant-volume heat transfer to the air
from an external source while the piston is at top dead
center. This process is intended to represent the ignition of
the fuel–air mixture and the subsequent rapid burning.
 Process 3–4 is an isentropic expansion (power stroke).
 Process 4–1 completes the cycle by a constant-volume
process in which heat is rejected from the air while the
piston is at bottom dead center.

42. 4-S Petrol Engine-Otto
Cycle
42

43. THEORETICAL VALVE TIMING DIAGRAM
FOR FOUR STROKE ENGINE
43

44. 44

45. 45

46. Typical Theoretical P-V and Valve Timing Diagrams of a Four-
Stroke Spark Ignition Engine
46
Observations:
 P-V diagram shows sharp edges
i.e., valves open/close instantaneously at dead centres

47. Actual Case:
Inlet Valve (IV) and Exhaust Valve (EV )open/close
before and after dead centres
 Mechanical Factor
 Dynamic Factor of Gas Flow
 Valves are opened and closed by cam mechanism
 Valves will bounce on its seat if closed abruptly
 Opening/closing of valves spread over a certain crank angle
Every Corner in the P-V
diagram is ROUNDED
47

48. Suction or Intake Stroke
 Suction stroke starts when piston is at
top dead center and about to move
downwards.
 The inlet valve is open at this time and
the exhaust valve is closed.
 Due to the suction created by the motion
of the piston towards the bottom dead
center, the charge consisting of fuel air
mixture is drawn into the cylinder.
 When the piston reaches the bottom
dead center the suction stroke ends and
the inlet valve closes.
4-Stroke Spark Ignition (SI)
Engine
Detailed Notes

49. Compression Stroke
 The charge taken into the cylinder during the
suction stroke is compressed by the return
stroke of the piston.
 During this stroke both inlet and exhaust
valves are in closed position. The mixture
which fills the entire cylinder volume is now
compressed into the clearance volume.
 At the end of the compression stroke the
mixture is ignited with the help of a spark plug
located on the cylinder head. During the
burning process the chemical energy of the
fuel is converted into heat energy.
 The pressure at the end of the combustion
process is considerably increased due to heat
from the fuel.

50. Expansion or Power Stroke
 The high pressure of the burnt
gases forces the piston towards
the BDC, both the valves are in
closed position. Of the four strokes
only during this stroke power is
produced.
 Both pressure and temperature
decrease during expansion.

51. Exhaust Stroke
 At the end of the expansion stroke
the exhaust valve opens and the
inlet valve remains closed.
 The pressure falls to atmospheric
level a part of the burnt gases
escape.
 The piston starts moving from the
bottom dead center to top dead
center and sweeps the burnt gases
out from the cylinder almost at
atmospheric pressure.
 The exhaust valve closes when the
piston reaches TDC.

52. Four Stroke Diesel / CI
Engine
 Diesel Engine was invented by Rudolph Diesel.
It is ignited by compression of charge, so it’s also
called as compression ignition (CI) engine.
 It is similar to four stroke petrol engine but operates at a
much higher compression ratio. The compression ratio of an
SI engine is between 6 and 10:1 while for a CI engine it is
from 16 to 20:1.
 A high pressure fuel injector is used to inject the fuel into
the combustion chamber.

53. Suction / Intake Stroke
 Suction stroke starts when
piston is at top dead center and
about to move downwards.
 The inlet valve is open at this
time and the exhaust valve is
closed.
 Due to the suction created by
the motion of the piston
towards the BDC, Air alone is
inducted during the suction
stroke.

54. Compression Stroke
 Air inducted during the suction
stroke is compressed into the
clearance volume due to return
stroke of piston.
 Both valves remain closed
during this stroke.
 The air in the combustion
chamber is at high
temperature and high pressure
with a decrease in volume.
Both Valves Closed

55. Expansion Stroke or Power
Stroke
 At the end of compression stroke, the
fuel is injected into the cylinder in the
form of fine spray through the nozzle and
is ignited by the temperature of hot
compressed air in the chamber.
 So that combustion process is started at
the end of compression stroke.
 The combustion of gases expands inside
the cylinder so that piston start to move
towards BDC.
 Both the valves remain closed during this
stroke
Both Valves Closed

56. Piston Crown
56

57. Exhaust Stroke
 The piston traveling from
BDC to TDC pushes out
the product of
combustion.
 The exhaust valve is
open and the intake
valve is closed during
this stroke.

58. 4-Stroke Diesel Engine
58

59. 59

60. 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

61. 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.
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