The document discusses combustion in spark-ignition (SI) engines. It defines combustion as a chemical reaction in which fuel combines with oxygen, liberating heat energy. In an SI engine, fuel and air are mixed and inducted into the cylinder where combustion is initiated by a spark at the spark plug near the end of the compression stroke. There are three stages of combustion: ignition lag, flame propagation, and after burning. Abnormal combustion phenomena like pre-ignition and knocking can occur if conditions are not suitable. Factors like turbulence, fuel-air ratio, temperature and pressure, compression ratio, and engine variables affect the flame speed and combustion process.

1. Combustion in SI Engines
Sreenidhi Institute of Science and Technology
Yamnampet, Ghatkesar, Hyderabad, Telangana 501301
UNIT – III

3. 3
As per Oxford Dictionary: The process of
burning something (from Latin: comburere)
Chemistry: Rapid chemical combination of a
substance with oxygen, involving the
production of heat and light.
Thermo Dynamics: Combustion is a chemical
reaction in which certain elements of the fuel
like hydrogen and carbon combine with
oxygen liberating heat energy
and causing an
increase in temperature of the gases.
Meaning of Combustion

4. 4
It is a chemical reaction- Hydrogen and
Carbon in the fuel combine with oxygen,
liberating heat energy and causing an
increase in temperature of the gases.
Combustion Process
Biodiesel releases
higher NOx than
Diesel fuel

5. 5
Combustion Process

6. Combustion Mixture

7.  In any conventional spark-ignition (SI)
engine, the fuel and air are homogeneously
mixed together in the intake system, inducted
through the intake valve into the cylinder
where it mixes with residual gases and is
then compressed.
 Under normal operating conditions,
combustion is initiated towards the end of the
compression stroke at the spark plug by an
electric discharge.
 A turbulent flame develops following the
Combustion in SI Engines

8.  If the spark occurs too early, combustion
takes place before the compression stroke is
completed. So, the pressure developed
opposes the piston movement and the engine
power is reduced.
 If the spark occurs too late, the piston would
have already completed a portion of the
expansion stroke before the pressure rise
occurs and a corresponding amount of
engine power is lost.
 The correct instant for the introduction of the
spark is decided by the ignition lag.
Combustion in SI Engines

9. Conditions Necessary for
Combustion
9
1.Presence of combustible mixture
2.Means for initiating combustion.
3.Stabilization and propagation of flame in
Combustion Chamber

10. Combustion is dependent upon the
rate of propagation of flame front or
flame speed.
Flame Front: Boundary or front
surface of the flame that separates
the burnt charges from the unburnt
one.
Flame Speed: The speed at which
the flame front travels.
10
Boundary

11. Combustion is dependent upon the rate of propagation of flame
front or flame speed.
Flame Front: Boundary or front surface of the flame
that separates the burnt charges from the unburnt
one.
Flame Speed: The speed at which the flame
front travels.
Flame speed affects the combustion
phenomena, pressure developed
and power produced.
Burning rate of mixture
depends on the flame speed
and shape/contour of
combustion chamber.
11

12. 12
Flame Speed
Burning
Rate
Steady and Progressive Flame

13. 13
FYI: Engine Speed and A/F Ratio - Flame Speed
As engine speed increases, the flame speed also
increases
For lean mixture, the flame speed have slower speeds
while for the rich mixture, the fast flame speed

14. Stages of Combustion in SI engine
(Theoritical p-θ Diagram)
In an ideal engine the entire
pressure rise during
combustion takes place at
constant volume i.e., at TDC.
However, in an actual engine
it does not happen as shown
in Fig.
Compression (a b),
Combustion (b  c)
Expansion (c  d)

15. Stages of Combustion in SI engine
A is the point of
passage of spark
(say 20° bTDC),
B is the point at
which the beginning
of pressure rise can
be detected (say 8°
bTDC)
C the attainment of
peak pressure.
Thus AB represents
the first stage, BC
the second stage
and CD the third
stage.
PRESSURE VARIATION DUE TO COMBUSTION IN A PRACTICAL ENGINE

16. Stages of Combustion in SI
engine
Sir Ricardo describes the combustion
Process as 3 stages:
1. Ignition lag stage
2. Flame propagation stage
3. After burning stage

17. Ignition Lag
 There is a certain time interval
between instant of spark and
instant where there is a noticeable rise in pressure
due to combustion. This time lag is called IGNITION
LAG.
 Ignition lag is the time interval in the process of
chemical reaction during which molecules get heated
up to self ignition temperature, get ignited and
produce a self propagating nucleus of flame.

18. Ignition Lag….
The period of ignition lag is
shown by path AB.
Ignition lag is very small and lies between
0.00015 to 0.0002 (2ms) seconds.
This is a chemical process depending upon the
nature/properties of fuel, temperature and
pressure in combustion chamber, proportions
of exhaust gas and rate of oxidation or burning.

19. Flame propagation stage
 The second stage (BC) is a physical
one and it is concerned with spread of the flame
(progressive) throughout the combustion chamber.
 The starting point of the second stage is where the first
measurable pressure rise is seen on the indicator
diagram i.e., the point where the line of combustion
departs from the compression line (point B).
During the second stage the flame propagates
practically at a constant velocity.

20. Flame propagation stage
 Heat transfer to the cylinder wall is low, because only a
small part of the burning mixture comes in contact with
the cylinder wall during this period.
 The rate of heat-release depends largely on the
turbulence intensity and also on the reaction rate which
is dependent on the mixture composition.
 The rate of pressure rise is proportional to the rate of
heat release because during this stage, the combustion
chamber volume remains practically constant (since
piston is near the top dead center).
Rate of pressure rise α Rate of heat release

21. After burning
 The starting point of the third stage
is usually taken as the instant at
which the maximum pressure is reached on the
indicator diagram (point C).
 The flame velocity decreases during this stage.
 The rate of combustion becomes low due to lower
flame velocity and reduced flame front surface.
 Since the expansion stroke starts before this stage of
combustion, with the piston moving away from the top
dead center, there can be no pressure rise during this
stage.

25. Difference in Comb. Process of SI and CI
Engines

26. Combustion in SI
Engine 26
When the flame travels
evenly or uniformly
across the combustion
chamber.
Normal Combustion Abnormal Combustion
When the combustion gets
deviated from the normal
behaviour and resulting in
loss of performance or
damaging the engine

27. NORMAL AND ABNORMAL COMBUSTION

28. Normal and Abnormal Combustion

29. Reasons for Abnormal
Combustion

30. Abnormal Combustion in SI Engine
30
Pre-ignition (self-
ignition) occurs when
the fuel mixture in the
cylinder burns before
the spark-ignition event
at the spark plug.
Pre-Ignition Knocking
Knocking is due to
auto-ignition of end
portion of unburned
charge in
combustion
chamber.

32. PRE-IGNITION
Pre-ignition is the ignition of the
homogeneous mixture of charge as it comes
in contact with hot surfaces, in the absence
of spark.
Pre-ignition is initiated by some overheated
projecting part such as the sparking plug
electrodes, exhaust valve head, metal
corners in the combustion chamber, carbon
deposits or protruding cylinder head gasket
rim etc.
Auto ignition may overheat the spark plug
and exhaust valve and it remains so hot that

33. FYI: PRE-IGNITION
 Engine efficiency will decrease due to pre-
ignition
 pre-ignition causes holes melted in pistons,
spark plugs melted away, and engine failure
happens pretty much immediately.

36. PHENOMENON OF KNOCK IN SI ENGINES
 The Figure shows the cross-section of the combustion
chamber with flame advancing from the spark plug location
 In the normal combustion the flame travels across the
combustion chamber from A towards D. The advancing flame
front compresses the end charge BB'D farthest from the
spark plug thus raising its temperature.
 The temperature is also increased due to heat transfer
from the hot advancing flame-front.

37.  If the temperature of the end charge had not reached
its self-ignition temperature, the charge would not
auto-ignite and the flame will advance further and
consume the charge BB‘D.
PHENOMENON OF KNOCK IN SI ENGINES

38. KNOCKING
38
Now, if the final temperature is greater
than and equal to the auto-ignition
temperature, the charge BB´D auto-
ignites (knocking).
A second flame front develops and
moves in opposite direction, where the
collision occurs between the flames. This

39. ABNORMAL COMBUSTION
PHENOMENON OF KNOCK IN SI ENGINES
 However, if the end charge BB'D reaches its auto-ignition temperature
and remains for some length of time equal to the time of preflame
reactions the charge will auto ignite, leading to knocking combustion.
 In Figure, it is assumed that when flame has reached the position BB',
the charge ahead of it has reached critical auto ignition temperature.
During the preflame reaction period if the flame front could move
from BB' to only CC’ then the charge ahead of CC' would auto-ignite.

40. ENGINE DAMAGE FROM SEVERE KNOCK
Damage to the engine is caused by a combination
of high temperature and high pressure.
Piston Piston crown
Cylinder head gasket Aluminum cylinder head
40

42. Pre-Ignition and Knocking

43. COMPARISON OF ABNORMAL COMBUSTION
Pre-ignition
 Pre-ignition is the ignition of the
homogeneous mixture of charge as it
comes in contact with hot surfaces, in
the absence of spark.
 Pre-ignition is initiated by some
overheated projecting part such as the
sparking plug electrodes, exhaust valve
head, metal corners in the combustion
chamber, carbon deposits or protruding
cylinder head gasket rim etc.
 Auto ignition may overheat the spark
plug and exhaust valve and it remains
so hot that its temperature is sufficient to
ignite the charge in next cycle during the
compression stroke before spark occurs
and this causes the pre-ignition of the
charge.
Knocking
 Knocking is due to auto ignition of end
portion of unburned charge in
combustion chamber.
 The pressure and temperature of
unburned charge increase due to
compression by burned portion of
charge. This unburned compressed
charge may auto ignite under certain
temperature condition and release the
energy at a very rapid rate compared
to normal combustion.
 This rapid release of energy during
auto ignition causes a vibrations and
pinging noise.

44. IMPORTANCE OF FLAME SPEED AND
EFFECT OF ENGINE VARIABLES
44
Factors affecting/influencing the flame speed:
1. Turbulence
2. Fuel-Air Ratio
3. Temperature and Pressure of Intake
4. Compression Ratio (CR)
5. Engine Output
6. Engine Speed
7. Engine Size

45. Turbulence
45
Dictionary Meaning: Violent or unsteady movement of fluid

46. Turbulence….
 Turbulence is necessary to prepare the
homogeneous air fuel mixture
 It breaks the flame front into pieces so that
each and every part of combustion
chamber gets flame to ignite the
homogeneous air fuel mixture.
 If there is uneven flame distribution then
there is incomplete combustion which gives
less torque and also causes pollution

47. Turbulence ….
 The flame speed is quite low in non-turbulent
mixtures and increases with increasing
turbulence.
 This is mainly due to the additional physical
intermingling of the burning and unburned
particles at the flame front which expedites
reaction by increasing the rate of contact.
 Turbulence is mainly dependent on
• Design of combustion chamber
(Geometry of cylinder head)

48. Turbulence ….
 The turbulence in the incoming mixture is
generated during the admission of fuel air mixture
through comparatively narrow sections of the
intake pipe, valve openings etc., in the suction
stroke.

49. Turbulence ….
 Turbulence is created by
• Swirl Motion and
• Tumble Motion
 When flame speed increases
due to
turbulence that reduces the
combustion duration and
hence minimizes the
tendency of abnormal
combustion

50. Fuel-Air Ratio
50
The highest flame speeds (minimum time for
complete combustion) are obtained with slightly
rich mixture at Point A as shown in Figure.
When the mixture is linear or richer, the flame
speed decreases, because
• a lean mixture releases less
thermal energy that causes lower
flame temperature
• while a rich mixture leads to
incomplete combustion, and hence
releases less thermal energy.

51. Effect of F/A Ratio on Flame Temperature

52. Inlet Temperature and Pressure
52
A higher initial pressure and
temperature may help to form a
better homogeneous air-fuel vapour
mixture which helps in increasing
the flame speed.
This is possible because of an
overall increase in the density of
the charge.
Flame speed increases with an

53. Compression Ratio (CR)
53
CR is the ratio of total cylinder volume to clearance
volume.
CR = Total volume
Clearance volume
Value of “CR” for,
Petrol engine lies between 6 to 10 :1
Diesel engine lies between 12 to 22 : 1

54. Compression Ratio (CR)
54
A higher compression ratio increases
the pressure and temperature of the
working mixture which reduce the initial
preparation phase of combustion and
hence less ignition advance is
needed.
High pressures and temperatures of the
compressed mixture also speed up the
second phase of combustion.

55. Compression Ratio (CR)…
55
Increased compression ratio,
reduces the clearance volume and
therefore increases the density of the
cylinder gases during burning.
This increases the peak pressure and
temperature and the total combustion
duration is reduced.
Thus engines having higher
compression ratios have higher

56. Engine Output
 The cycle pressure increases when the engine
output is increased. With the increased throttle
opening the cylinder gets filled to a higher density.
This results in increased flame speed.
 When the output is decreased by throttling, the
initial and final compression pressures decrease
and the dilution of the working mixture increases.
The smooth development of self-propagating
nucleus of flame becomes unsteady and difficult .

57. Engine Output
 The main disadvantages of SI engines are the poor
combustion at low loads and the necessity of mixture
enrichment (between 1.2 to 1.3) which causes
wastage of fuel and discharge of unburnt
hydrocarbon and the products of incomplete
combustion like carbon monoxide etc. in the
atmosphere.

58. Engine Speed
 The flame speed increases almost linearly with engine
speed since the increase in engine speed increases the
turbulence inside the cylinder.
 The time required for the flame to traverse the
combustion space would be halved, if the engine speed is
doubled.
 Double the engine speed and hence half the original time
would give the same number of crank degrees for flame
propagation.
 The crank angle required for the flame propagation
during the entire phase of combustion, will remain
nearly constant at all speeds.

59. Engine Size
 The size of the engine does not have much effect on
the rate of flame propagation.
 In large engines the time required for complete
combustion is more because the flame has to travel a
longer distance.
 This requires increased crank angle duration during
the combustion. This is one of the reasons why large
sized engines are designed to operate at low speeds.

60. 60
1. Compression Ratio
2. Mixture Strength
3. Throttle Opening
4. Engine Temperature
5.Combustion chamber Design and
6. Engine Speed
Factors affecting Ignition Lag

61. 61
Knock in S.I. Engines:
After the combustion is initiated with the help of
the spark plug, the flame spreads across to the
other end of the combustion chamber
• The heat released during the combustion
increases the temperature and consequently
the pressure of the burned part of the mixture
above those of the unburned part.
• This process continues as the flame front
advances through the mixture – the
temperature and pressure of the unburnt
mixture are increased further.
PHENOMENON OF KNOCK IN SI ENGINES

62. 62
If the temperature of the unburnt mixture exceeds
the self-ignition temperature of the fuel above this
temperature during the period of pre-flame
reactions (Ignition Lag)
-Spontaneous ignition – or Auto-ignition
occurs at various pin-point locations.
This Phenomenon is called Knocking.- The process
of Auto-ignition leads to the engine knock.
In normal combustion- the pressure changes are
smooth and there is No Knock.
In Abnormal Combustion- due to auto-ignition, a
second flame will initiate and move in the opposite
direction- resulting in vibration
Knock in SI Engines Contd.

63. 63
Onset of Knocking- the two flame fronts
collide.
A severe pressure pulse is generated.
The gas in the chamber is subjected to compression
and rarefaction along the pressure pulse until
pressure equilibrium is restored.
The order of vibration is ~ 5000 cps.
The onset of knocking is dependent on the
properties of the Fuel.
If the un-burnt fuel does not reach auto-ignition
temperature, there will not be knocking.
Knocking in SI Engines (contd.)

64. 64
Similarly if the ignition lag period is
longer than the time required for the
flame front to burn through the
unburnt charge- there will be no
Knocking.
If the critical temperature is reached
and maintained, and the ignition lag
is shorter than the time taken for the
flame front to burn through the un-
burnt charge -
The End Charge will Detonate.
Knocking in SI Engines (contd.)

65. Knock-limited Parameters
It should be the aim of the engine designer to reduce
the tendency of knocking in the engine.
1. Knock Limited Compression Ratio
2. Knock Limited Inlet Pressure
3. Knock Limited Indicated Mean Effective
Pressure

66. • 1. Knock Limited Compression Ratio
▫ Compression ratio is gradually increased till incipient
knocking is observed
▫ Any change in operating conditions which improve the
knock limited CR is said to reduce the tendency
towards knocking
• 2. Knock limited Inlet Pressure
▫ The inlet pressure is gradually increased and when the
incipient knock is observed- this is Knock Limited Inlet
Pressure
• 3. Knock Limited Indicated Mean
Effective Pressure
▫ Klimep where the incipient knock is observed
66

67. • Performance Number
▫ Performance number is defined as the ratio of
Klimep with the given fuel to Klimep with Iso-
octane with the inlet pressure kept constant.
▫ This performance number is related to octane
number and can also be applied to fuels whose
knocking characteristics are superior to that of
Iso-octane. Thus it extends the octane scale
beyond 100
• Relative Performance Number
▫ rpn = Actual performance number / Perf. No.
corresponding to imep of 100
67
Knock-limited Parameters

68. Effect of Engine variables on Knock
• A. Density Factors B. Time Factors
• C. Composition Factors
Density Factors
▫ 1.Compression Ratio
▫ An increase in C.R. increases Press and Temp.-
reduces ignition delay and increases tendency for
knocking and vice versa
▫ The increased density of the charge increases pre-
flame reactions in the end charge and increases the
knocking tendency in engine
68

69. Knock in SI Engines (contd.)
▫ ii. Mass of Inducted Charge- A reduction by throttling
etc. reduces the temperature and density of the charge
and reduces tendency of knocking
▫ iii. Inlet Temperature of the Mixture
▫ Increase in inlet temp increases tendency of knocking
▫ iv. Temperature of the combustion chamber walls
▫ The hot spots in the combustion chamber walls
should be avoided.
▫ Since the spark plug and exhaust valves are two
hottest spots in the engine, the end charge should not
be compressed against them.
69

70. Knock in SI Engines (contd.)
▫ v. Retarding the spark timing
▫ By retarding the spark timing from the optimized
timing – that is, by having the spark closer to TDC
▫ The peak pressures are reached farther down on the
power stroke with a relatively lower magnitude- this
might reduce knocking
▫ However, this is different from MBT timing and affects
the Brake torque and engine power output
▫ vi. Power output of the Engine
▫ A decrease in output of the engine decreases the
temperature of the cylinder and the combustion
chamber walls, pressure of the charge and end gas
temperature
▫ This reduces the tendency to Knock
70

71. Knock in SI Engines (contd.)
• B. Time Factors
• to reduce knocking tendency:
▫ i. Increasing Flame speed
▫ ii. Increasing Duration of Ignition Lag
▫ iii. Reducing the time of exposure of un-burnt
mixture to Auto ignition
▫ iv. Turbulence – depends on combustion chamber
design and engine speed.
▫ Increasing turbulence increases the flame speed
and reduces the time available for the end charge
to reach auto-ign. Conditions and decreases the
tendency to knock
71

72. Knock in SI Engines (contd.)
▫ Engine Speed- An increase in speed increases the
turbulence of the mixture – increased flame speed-
reduces time available for pre-flame reactions-
decreases knocking tendency.
▫ Flame Travel Distance- A shorter distance is
beneficial- Engine size and spark plug positioning play
critical role
▫ Combustion chamber shape- A compact combustion
chamber reduces flame travel length – Thus spherical
shape preferred plus measures to improve turbulence
▫ Location of Spark Plug- Centrally located spark plug
reduces the flame travel distance hence better
▫ Use of two or more spark plugs (!) in case of large
engines
72

73. Knock in SI Engines (contd.)
C. Composition Factors
i. Fuel-Air Ratio
▫ Equivalence ratio~1.1-1.2 gives minimum reaction
time for auto ignition – Better
▫ ii. Octane value of the fuel
▫ iii. A higher self-ignition temperature of the fuel and
▫ iv. A low pre-flame reactivity reduce the tendency to
knock.
▫ v. Paraffin series have higher tendency and aromatic
series minimum tendency to knock
▫ vi. Compounds with more compact molecular
structure are less prone to knock
73

74. Knock in SI Engines (contd.)
74
Effect of Equivalence Ratio on
Knock Limited Compression Ratio

75. 75
Summary of Parameters affecting Knock in SI Engines
Sl.
No.
Increase in
variable
Major effect
on unburned
reduced
charge
Action to
be taken
to control
Knocking
Can
Operator
usually
control ?
1 Compression
Ratio
Increases
Temperature
and Pressure
Reduce No
2 Mass of
Charge
Inducted
Increases
Pressure
Reduce Yes
3 Inlet
Temperature
Increases
Temperature
Reduce In some
cases
4 Chamber
Wall
Temperature
Increases
temperature
Reduce Not
ordinarily

76. 76
Summary of Parameters affecting Knock in SI Engines (Contd.)
Sl.
No
.
Increase in
variable
Major effect on
unburned
reduced charge
Action to be
taken to
control
Knocking
Can
Operator
usually
control?
5 Spark
Advance
Increases temp.
and pressure
Retard In some
cases
6 A/F Ratio Increases temp
and Pressure
Make very
rich
In some
cases
7 Turbulence Decreases time
factor
Increase Some what
(through
engine
speed)

77. 77
Summary of Parameters affecting Knock in SI Engines
(Contd.)
Sl.
No.
Increase in
variable
Major effect on
unburned
reduced charge
Action to be
taken to
control
Knocking
Can
Operator
usually
control?
8 Engine
speed
Decreases time
factor
Increase Yes
9. Distance of
Flame
travel
Increases time
factor
Reduce No

78. 78
i. Reduce the Distance of Flame Travel
ii. - Centrally locate the spark plug
iii. - Avoid pockets of Stagnant Charge.
iv. Cooling of the Spark plug and Exhaust valve area
These are sources of hot spots in the majority of the
combustion chambers.
v. Reducing the temperature of the charge
- adopting a high surface –to- volume ratio in the part
where the last portion of the charge burns
- Heat transfer to the combustion chamber walls can be
improved by using high surface-to-volume ratio – there
by reducing the temperature
Reducing the Possibility of Knocking

79. 79
- Use of Measures as below:
i. A High Degree of Turbulence:- This improves the flame velocity
- Introduction of SQUISH (or Rapid radial movement of the gas
trapped in between the piston and the cylinder head)
- with a bowl in the piston
- and dome shaped cylinder head
ii. A High Volumetric Efficiency – (more charge during suction stroke)
– Results in Increased Power output
iii. Design of Combustion Chamber for improved anti-knock
characteristics
Developing Engines for High Power output and Increased
Thermal Efficiency