This document discusses combustion in spark ignition (SI) engines. It defines combustion as the rapid chemical combination of a substance with oxygen, involving heat and light production. The key stages of combustion in an SI engine are discussed as ignition lag, flame propagation, and after burning. Factors that affect flame speed like turbulence, fuel-air ratio, temperature and pressure are also covered. Abnormal combustion phenomena like pre-ignition and knocking are described along with their effects. Turbulence is explained as important for preparing the air-fuel mixture and distributing the flame front uniformly in the combustion chamber. Questions asked during the presentation related to using different fuels, increasing pressure ratio, the role of flame speed and turbulence, spark plug positioning, and
1. National Institute of Technology, Patna
Department of Mechanical Engineering
Thermal Engineering(M.Tech)
Presented by:-
Saket Kumar(1936022)
Supervisor:-
Dr. Om Prakash
Presentation Title
Combustion in SI Engines
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2. 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
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3. 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
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5. Conditions Necessary for Combustion
1. Presence of combustible mixture.
2. Means for initiating combustion.
3. Stabilization and propagation of flame in Combustion
Chamber.
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7. 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)
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8. Stages of Combustion in SI engine
PRESSURE VARIATION DUE TO COMBUSTION IN A
PRACTICAL 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.
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9. Stages of Combustion in SI Engine
1.Ignition lag stage
2. Flame propagation stage
3. After burning stage
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10. fig:- %of mass burned vs %of volume burned
in SI engine.
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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.
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12. IMPORTANCE OF FLAME SPEED AND
EFFECT OF ENGINE VARIABLES
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
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FlameSpeed vs A/F Ratio -
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
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15. Fig:- Flame Speed vs Engine Speed
Flame Speed vs Engine Speed in SI Engine
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16. Combustion in SI Engine
Normal Combustion Abnormal Combustion
When theflame travels evenly of
uniformly across the combustion
chamber.
When the combustion gets deviated from
the normal behavior and resulting in loss of
performance or damaging the engine
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18. Abnormal Combustion in SI Engine
Pre-ignition (self- ignition) occurs
when the fuel mixture in the
cylinder burns before the spark-
ignition event a
t the spark plug.
Pre-Ignition Knocking
Knocking is due to
auto-ignition of end portion of
unburned charge in combustion
chamber
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19. PRE IGNITION AND THEIR EFFECT
pre-ignition causes holes melted in
pistons, spark plugs melted away, and
engine failure happens pretty much
immediately.
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22. 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.
Turbulence is mainly dependent on
1. Design of combustion chamber.
2. Geometry of cylinder head
3. Geometry of piston crown
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25. QUESTION ASKED IN PRESENTATION:-
1.Can kerosene oil put inside the combustion chamber of SI Engine?
2.We can increase the pressure ratio like diesel engine in SI Engine?
3.How the flame speed play the importance role inside the combustion chamber?
4.How the tumbling is effect the engine speed in CI and SI Engine?
5.Position of spark plug can affect the velocity of flame front?
6.What is the suitable place of spark plug inside the combustion chamber to avoid
detonation?
7.In SI engine we already used Homogeneous mixture ,why we need of tumble in
SI Engine?
8.Is Any effect of engine Speed with ignition lag?
9.Can be used Two Sparkplug to avoid the detonation if yes to how?
10.What is Different b/w squeeze and Tumble?
11.If speed of engine increase than flame front speed also increase because of
proper mixing of A/F?
12.Combustion Without oxygen is possible or not if not Why?
13.Role of any adiabatic index in SI and CI Engine?
14.How much Sparkplug gap in combustion chamber?
15.The effect of radiation heat transfer on combustion process is helpful for
Reducing the knocking?
16.Effect of EGR in combustion Chamber in SI Engine? 25
26. REFERENCES:-
-Willard W. Pulkrabek-Engg Fundamentals of the IC Engine
-V Ganesan-Internal Combustion Engine
-Heinz Heisler-Advance Engine Technology
-Internet
-My Notes
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