Factors affecting knock
What is Knocking?
Knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous ignition of a portion of the end-gas-the fuel, air, residual gas, mixture ahead of the propagating flame occurs. When this abnormal combustion process takes place, there is an extremely rapid release of much of the chemical energy in the end-gas, causing very high local pressures and the propagation of pressure waves of substantial amplitude across the combustion chamber.
Effect of Knock:
1. Knock has the following effects on engine operation:
2. Noise and Roughness.
3. Mechanical damage: increase in engine wear, cylinder head and valves may be pitted.
4. Carbon deposits.
5. Increase in heat transfer.
6. Decrease in power output and efficiency.
7. Pre-ignition: combustion Occurs before the spark.
Effect of engine variables on Knock:
To prevent Knock in the S.I. engine the end gas should have:
A- Low temperature.
B- Low density.
C- Long ignition delay.
D- Non- reactive combustion.
When the engine conditions are changed, the effect of the change may be reflected by more than one of the above variables.
A- Temperature factors:
The temperature of the unburned mixture is increased by the following factors:
1. Raising the compression ratio.
3. Raising the inlet temperature.
4. Raising the coolant temp.
5. Increasing load.
6. Advancing the spark.
7. Raising the temperature of the cylinder and combustion chamber walls.
B- Density factors:
Increasing density by any of the following methods, will increase the possibility of Knock:
1. Increasing load.
2. Increasing compression ratio.
4. Advancing the spark.
C- Time factors:
Increasing the time of exposure of the unburned mixture to auto-ignitions by any of the following factors will increase tendency to knock:
1. Increasing the distance of the flame travel.
2. Decreasing the turbulence of mixture.
3. Decreasing the speed of the engine.
The probability of Knock in S.I. engines is decreased by:
1. Increasing the octane rating of the fuel.
2. Either rich or lean mixtures.
3. Stratifying the mixture.
4. Increasing the humidity of the entering air.