1. Analysis of Abnormal Combustion in SI Engines
P M V Subbarao
Professor
Mechanical Engineering Department
Measures to Avoid Uncontrollable Flame Speeds….

2. Unexpected Engine Damage
Damage to the engine is caused by a combination of high temperature and high pressure.
Piston
Piston crown
Cylinder head gasket
Aluminum cylinder head

3. Dangerous Accidents

4. Normal Combustion in SI Engines

5. Abnormal Combustion in SI Engines

6. Signatures of Abnormal Combustion in SI Engine
Knock is the term used to describe a pinging noise emitted from a SI engine undergoing abnormal combustion.
The noise is generated by shock waves produced in the cylinder when unburned gas autoignites.
Severe Abnormal
Marginal Abnormal

7. Physics of Normal Combustion
Under normal conditions the combustion is ignited by a spark at the spark plug.
The flame kernel grows and propagates through the combustion chamber until it reaches the cylinder walls where it extinguishes.
The flame front propagates with a speed much less then the speed of sound.
Therefore the cylinder pressure can be considered nearly constant in the cylinder.
The unburned gas in front of the flame is called the end gas.
P,T
time
end-gas
flame P

8. Physics of Knocking Combustion
Knock is a phenomenon that occurs when high temperature and pressure causes the end gas to self ignite.
This causes a very high local pressure and this generates pressure waves across the combustion chamber.
These pressure waves excites the resonance modes of the cylinder.
The frequency of the oscillations under knocking conditions depends on engine geometry, and is often in the range of 5 to 10 kHz.
shock
time
P,T

9. Reasons for Birth of Knock
The end-gas autoignites after a certain induction time which is dictated by the chemical kinetics of the fuel-air mixture.
If the flame burns all the fresh gas before auto-ignition in the end-gas can occur then knock is avoided.
Therefore knock is a potential problem when the burn time is long.

10. Engine Design Parameters Causing the Knock
There are the two fundamental variables that determine the possibility of occurrence of knock in SI Engines.
The end-gas temperature.
The time available for the end gas before flame arrival.
Engine parameters that effect these two fundamental variables are:
Compression ratio
Spark advance,
Speed,
Inlet pressure and temperature
Coolant temperature
Fuel/air ratio
This phenomenon is a fundamental issue of modern SI engine design methods.

11. Fuel Quality : An Important Reason for Knocking
Compression Fuel Quality Brake Thermal
Ratio Requirement Efficiency 5:
6: %
7: %
8: %
9: %
10: %
11: %
12: %

12. Fuel : The Resource is the Culprit : Knock Scale
To provide a standard measure of a fuel’s ability to resist knock, a scale has been devised by which fuels are assigned an Octane Number ON.
The octane number determines whether or not a fuel will knock in a given engine under given operating conditions.
The higher the octane number, the higher the resistance to knock.
By definition, normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) has a value of 100.
Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90.
A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance

13. Knock Characteristics of Various Fuels
Formula Name Critical r RON MON
CH4 Methane
C3H8 Propane
CH4O Methanol
C2H6O Ethanol
C8H18 Isooctane
Blend of HCs Regular gasoline
n-C7H16 n-heptane
For fuels with antiknock quality better than octane, the octane number is:
where mT is milliliters of tetraethyl lead per U.S. gallon

14. Octane Number Requirement of a Vehicle
The actual octane requirement of a vehicle is called the Octane Number Requirement (ONR).
This is determined by using series of standard octane fuels that can be blends of iso-octane and normal heptane ( primary reference ), or commercial gasolines.
The vehicle is tested under a wide range of conditions and loads, using decreasing octane fuels from each series until trace knock is detected.
The conditions that require maximum octane are full-throttle acceleration from low starting speeds using the highest gear available.

15. Knock limit as a function of CR and ON for moderate and high turbulence combustion chambers.