Knocking tendency in a SI engine reduces with increasing

Concept:

When the spark is advanced, burning gas is compressed by the rising piston and therefore temperature and pressure are increased, thus, the tendency to knock increases.

In spark ignition (SI) engines, the possibility of knocking can be reduced by decreasing the compression ratio as a reduction in the compression ratio will result in the generation of lower peak temperatures. In SI engine, knocking occurs before TDC and because of early combustion of charge/more than one flame propagation.

Factors affecting knocking in SI engine:

Compression ratio:

• Higher compression ratio will result in the higher temperature of the charge, the tendency to knock will increase.

Inlet air temperature:

• The effect of inlet air temp is the same as compression ratio. High inlet temp of the air-fuel mixture will increase knocking.

Advancing the spark:

• Advancing the spark will result in more compression during sparking. The increase in temperature during the sparking will be higher. Hence tendency to knock will increase and so by retarding the spark, the tendency to knock will decrease.

Coolant water temperature: 

• By increasing coolant water temperature, less heat will be carried from the engine which will increase the engine temperature. So the knocking will increase.

Engine Speed:

• To avoid the knocking in the SI engine the engine speed should be high.

Octane rating:

• It should be as high as possible.