This document discusses knocking combustion in internal combustion engines. It defines knocking combustion as abnormal combustion in the combustion chamber that leads to a sudden pressure rise and hammering sound, resulting in reduced performance and potential engine damage. It then discusses the causes and effects of knocking in spark ignition (SI) engines and compression ignition (diesel) engines. Some of the key factors that influence knocking include fuel type, compression ratio, ignition timing, and air-fuel ratio. The document also covers related topics like octane rating, cetane rating, and strategies to prevent knocking.

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Enlightment of Knowledge
Knocking in SI & CI engines
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Definition
Abnormal combustion occurring in the
combustion chamber give sudden pressure rise
accompanied by a metallic, hammer like sound
leading to loss of performance and possible
engine damages. This abnormal combustion is
commonly known as “Knocking Combustion”
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Comparison
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Classification of IC Engine
• SI engine ( Spark Ignition)or Gasoline or Petrol
Engine
• CI engine (Compression Ignition) or Diesel
Engine
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Engine Cylinder

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What is Engine Knock?
1. Fuel burns unevenly in your engine’s
cylinders.
2. Incorrect balance of air and fuel, fuel will
burn in small, regulated pockets instead of all
at once.
3. Shocks go off at the wrong time.
4. An annoying noise and potential damage to
your engine’s cylinder walls and pistons.
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SI ENGINE
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What Could Cause Engine Knocking in SI
engine?
Faulty Spark Plugs
Low-Octane Fuel
Carbon Deposits
High compression ratio
Advanced Ignition timing.
Knocking takes place at end of combustion.
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What Could Cause Engine Knocking in SI
engine?
Decrease in Turbulence
Low Engine Speed
Increase in flame travel distance
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CI ENGINE
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What Could Cause Engine Detonation or Pinking in
CI engine?
Increase in Ignition lag
Increase in Engine speed
Reducing the flame travel distance
Increasing in delay period of the fuel.
Reduce in cetane Number.
Detonation takes place at the beginning of the
combustion.
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Factors Tending to reduce knocking in IC engine
S.No Factors S.I Engine C.I Engine
1. Self Ignition temp of fuel High Low
2. Time Lag or Delay period
of fuel
Long Short
3. Compression ratio Low High
4. Inlet Temperature Low High
5. Inlet Pressure Low High
6. Combustion chamber of
wall temperature
Low High
7. Speed High Low
8. Cylinder size Small High
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OCTANE NUMBER
• Its the % of iso-octane in a mixture of iso-
octane and n-heptane.
• It is a standard measure of the performance of
an engine fuel.
• It is the measure of resistance of gasoline
against knocking or pre-ignition of fuel .
• Octane number is related to petrol.
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• Higher the octane number, better the quality
of petrol, and hence the vehicle gives higher
efficiency.
• The octane no. of iso-octane is 100, while that
of n heptante is zero(0).
• Increase in octane number reduces the
knocking.
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CETANE NUMBER
• Its the % of n-hexadecane in a mixture of n-
hexadecane and 1-methyl naphthalene.
• It is an indicator of combustion speed of
Diesel Engine.
• It is the measure of ignition quality of fuel.
• Alkyl nitrates and Di-tert-butyal peroxide are
used as additives to raise the Cetane number.
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• High cetane number fuels have low octane
numbers, and vice versa. This is one reason why
we can't use petrol in diesel engine and vice
versa.
• High cetane number reduces the pinking or
detonation in CI engine.
• In India Current BIS specification of Anti-Knocking
Index of petrol is 84 (unleaded regular) and 88
(unleaded premium), And cetane index of diesel
is 45-50.
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COMPRESSION RATIO:
The ratio of the maximum to minimum volume
in the cylinder of an internal combustion engine.
• For SI Engine : 10 to 14
• For CI Engine :18 to 23
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Air-Fuel ratio (AFR) for various fuel:
S.NO FUEL AFR
1. Methanol 6.47:1
2. Ethanol 9:1
3. Butanol 11.2:1
4. Diesel 14.5:1
5. Gasoline 14.7:1
6. Propane 15.67:1
7. Methane 17.19:1
8. Hydrogen 34.3:1
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Air fuel ratio
It is the ratio between mass of the air to the mass
of fuel.
AFR = ma / mf
Inverse ratio = 1/AFR is called as Fuel – air ratio.
The theoretical air – fuel ratio for a complete
combustion is called as stoichiometric air fuel ratio.
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For petrol engine fuel Stoichiometric ratio is
14.7:1
i.e In order to burn completely 1 kg of fuel, we
need 14.7 kg of air.
AFR> Stoichiometric ratio = Lean Mixture
AFR< Stoichiometric ratio = Rich Mixture
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AFR for S.I Engine - 12: 1 (Rich Mixture)
20: 1 ( Lean Mixture)
AFR for C.I Engine – 18:1 to 70:1 (Lean Mixture)
C.I Engine – Air and fuel are mixed randomly in
CC, so it is a Non –Homogenous mixture
S.I Engine – Air and Fuel is mixed well as a single
mixture so it is a Homogenous mixture
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AFR GRAPH
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S.I Engine AFR
For Engine starting ,
AFR = 9: 1 ( Rich Mixture)
For Warming up ( Idling )
AFR = 12: 1(Rich Mixture)
For Accelerating,
AFR =11:1 – 13:1( Rich Mixture)
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C.I Engine AFR
For Heavy load,
AFR = 12: 1 ( Rich Mixture)
For Constant speed,( Cruising )
AFR = 14.7: 1(Stoichiometric Mixture)
For Decelerating
AFR = 17:1( Lean Mixture)
Air-fuel ratio of 12:1 is considered as maximum
output ratio, where as the air-fuel ratio of 16:1 is
considered as maximum fuel economy ratio.
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• Physical Delay : Time b/w beginning of the
injection and attainment of chemical reaction.
• Chemical Delay: Time b/w beginning of the
combustion reaction and then accelerating.
• C.I Engine – Physical + Chemical delay ( Paraffins)
• S.I Engine – Chemical delay ( Aromatics)
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• Air–fuel equivalence ratio (λ)
The ratio of actual AFR to the Stoichiometric AFR is
given by symbol Lambda
If < 1 - Rich Mixture.
= 1 - Stoichiometric
> 1 - Lean Mixture.
Fuel–air equivalence ratio (ϕ)
It’s a Vice versa of
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GATE QNS
A diesel engine is usually more efficient than a
spark ignition engine because (Gate ME-03)
a. Diesel being a heavier hydrocarbon, releases
more heat per Kg than gasoline.
b. The air std.efficiency of Diesel cycle is higher
than the Otto cycle, at a fixed compression ratio.
c. The compression ratio of a diesel engine is
higher than of an SI engine.
d. Self Ignition temperature of diesel is higher than
that of gasoline.
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At the time of starting, idling and low speed
operation, the carburetor supplies a mixture
which can be termed as (Gate ME 04)
a. Lean
b. Slightly leaner than stoichiometric
c. Stoichiometric
d. Rich
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Knocking tendency in a SI engine reduces with
increasing (GATE ME 93)
a. Compression ratio
b. Wall temperature
c. Supercharging
d. Engine speed
Presentation prepared by C.PRASATH
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