Si & ci engine, applied thermodynamics

1. Comparison of S. I. Vs C. I. Engine
No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
1. Preparation
of A: F
mixture
Outside the cylinder in
Carburetor
Inside the cylinder (Fuel
Injection)
2. During
suction
stroke
Air + Fuel inducted in
cylinder
Only air inducted in cylinder
3. Nature of
mixture
Homogenous Heterogeneous

2. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
4. Method of
Ignition
Spark plug ignition (S. I). Self ignites due to
compression pressure (C. I.)
5. Ignition
occurs at
Single point (Air gap of
Spark plug) and single
flame travels in
combustion chamber
Multiple points (Wherever
mixture reaches SIT it ignites)
6. Spark plug Present
(Due to high SIT – 246
C)
Absent (uses cold plug or
glow plug heater for cold
starting) (Low SIT – 210 C)

3. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
Cost of fuel High Low
Sp. Gravity Low High
Fuel sold Volume basis (in litres) Mass basis (kg)
Performance For the same compression ratio
efficiency of Otto cycle is more
than dual and diesel Cycle
For all other condition efficiency of diesel
cycle is greater than dual and Otto cycle.
Dual cycle efficiency is always in between
Otto and Diesel cycle.

4. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
7. Carburetor Present Absent (Uses fuel injector)
8. Thermal
Efficiency
Low High
9. Air standard
cycle
Otto Diesel
10. Fuel Economy
(km/litre)
Low (12 km/litr) 40 % Higher than petrol engines
(16 to 17 km/litre)

5. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
11. Compression
ratio
6-10 16-20
12. Limit on
Compression
ratio
Yes due to Knocking
(HUCR)
No limit
13. Noise &
vibrations of
engine
Low High
14 Size of the
engine
Small Bulky due to high CR

6. No Parameter Spark Ignition (S. I.) Compressed ignition (C. I)
15. Application Light duty automobiles (Two-
wheeler and low duty car)
Heavy duty (Trucks, buses power
plants, diesel genets etc.)
16. Bore size /
Cylinder
diameter
Limited due to knocking (Since
flame will need more time to
reach end mixture for large size
resulting in knocking) max. 100
mm
No limit (50 – 900 mm)
17. Pre-ignition of
mixture
(before spark)
Possible Not possible (No sparking)

7. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
18. Number of
Valves
2 (Suction and discharge
valve)
3 (Suction, discharge and
Fuel injector)
19. With
accelerator
increase
Throttle valve opening
increases
Fuel injection amount
increases
20. Quantity of
air
Varies with load /Throttle
valve opening
/Acceleration
Deceleration
Constant amount of air
inducted during suction stroke

8. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
21. Type of
governing
Quantity governed Quality governed
22. As the load
changes
Quantity of A: F mixture
changes
Air supply is constant,
quantity of fuel sprayed
changes and hence quality of
mixture or A:F ratio changes
23. Low load
condition
Rich mixture required Lean mixture required

9. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
24. Operating
range of A:F
ratio
12-17
Cold starting: 7:1
Idling condition: 12.5
Normal power: 16
Maximum power: 13.5
20-100, Idling or low load very
lean 100:1
Maximum power 20: 1
(Below 20 A:F ratio problem of
black smoke and odour from the
exhaust even though max.
power possible)
25. Air supply
preference
Turbulence of air
(disorderly random
motion with no direction)
to increase flame
Air Swirl motion (Orderly
motion in circular direction

10. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
26. Stages in
Combustion
Three (Ignition lag,
Flame propagation and
after burning)
Four (Ignition delay, Rapid or
Uncontrolled combustion,
Controlled Combustion, After
burning)
27. Number of
flames
Single definite flame Multiple flames
28. Peak
pressures
25-50 bar 150-200 bar

11. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
29. Injection
pressures
6 – 10 bar 150-200 bar
30. Delay period No physical delay as mixture
prepared outside cylinder.
Only chemical delay (Ignition lag)
Physical + Chemical delay
(Ignition delay)
Physical delay – Mixture preparation phase
Chemical delay – Pre-flame processes
31. Physical
delay period
(Mixture
preparation)
Absent (Mixture
prepared outside
cylinder)
Mixture prepared inside
cylinder (After fuel sprayed)

12. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
32. Ignition Delay
period (Time
after SIT is
reached for
flame to
appear)
High to avoid end
mixture to self ignite
Low – to avoid large amount
of diesel getting accumulated
during delay period

13. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
Combustion
Ideal
Combustion
Curve
- Ignition and Combustion
occurs at TDC
- Pressure rise instantaneous
- Temp. rise instantaneous.
- No un burnt HC.
- No late burning.

14. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
Combustion
Combustion
Curve

15. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
33. Suction Air + Fuel Only Air
34. Stages of
combustion
3 4
35. Point A Spark is supplied Fuel injection starts
36. Point B Flame appears Mixture reaches SIT
37 Point C Maximum Pressure Flame appears
(B-C: Pre-flame processes)

16. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
37. Stage I A-B: Ignition lag A-C: A-B Physical delay (Mixture
preparation)+B-C Chemical delay
(Pre flame processes)
38. Stage II B-C:Flame
propagation
C-D: Rapid or Uncontrolled
combustion of diesel accumulated
during delay period
39. Stage III C-D: After burning D-E: Controlled combustion after
accumulated diesel is burnt
40. Stage IV Absent After burning

17. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
Abnormal combustion / Knocking and factors affecting knocking
Abnormal
Combustion

18. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
Abnormal combustion / Knocking and factors affecting knocking
41. Definition of
Knocking or
abnormal
combustion
Happens at the end of
combustion due to self
ignition of end mixture
(Hence requires long
delay period to avoid
knocking)
Happens at the start of
combustion due to long delay
period large amount of diesel
getting accumulated inside
the cylinder which burns and
creates large pressure during
uncontrolled combustion
stage.
42. Knocking
occurs at
End of combustion Start of combustion

19. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
43. Delay period
required to
avoid
knocking
Long Short
44. SIT to avoid
knocking
High
(To avoid self ignition of
end mixture)
Low
(Avoid large amount of diesel
does not get accumulated)

20. No. Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
45. Octane number to
avoid knocking (ON
is directly
proportional to SIT)
Should be high. High SIT
desirable
Should be low. Low SIT desirable
46 Cetane number to
avoid knocking (CN
is inversely
proportional to SIT)
Should be low. High SIT
desirable
Should be high. Low SIT
desirable

21. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
46. Flame speed High Low
47. Engine size Low (Flame reaches
faster to end of
chamber)
High
48. Density Low High
49. Supercharging
preferred
No Yes

22. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
50. Compression
ratio
Limited by HUCR
(6-10)
High (16-20)
51. Initial pressure
and
temperature
Low
(To avoid burning of
end mixture)
High
(To help self ignition and
decrease delay period)
52. Cylinder Wall
temperature
Low
(To avoid burning of
end mixture)
High to assist self ignition

23. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
53. Engine speed High (To increase
flame velocity)
Low (To reduce delay period)
54. Turbo charging Not desirable Desirable
55. Cooling water
temperature
Low High
56. Exhaust No smoke or odour
from exhaust
Black Smoky and odour
(heavy Soot in exhaust)

24. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
57. Detection of
engine
knocking
Possible by human ear
(Petrol engines are less
noisy)
Difficult to detect by human
ear (Diesel engine are more
noisy)
58. Intake fuel
temperature
Low High
59. Engine load Low (high load increases
operating temp of
cylinder cooling water
and hence knocking)
High (Increase in operating
temperatures helps in Self
ignition and reduces
detonation)

25. No
.
Parameter Spark Ignition (S. I.)
engines
Compressed ignition (C. I)
60 Exhaust gas
temperature
Higher (Due to lower heat
extraction)
Lower (Due to high heat
extraction)