For a given compression ratio, to reduce the tendency to knock in an S I engine:

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 compression ratio as a reduction in compression ratio will result in the generation of lower peak temperature. In SI engine, knocking occurs before TDC and because of early combustion of charge/more than one flame propagation.

Supercharging in SI and CI engine:

Supercharging increases the pressure and temperature of the charge at the end of compression. This reduces ignition delay in CI engine, thereby the combustion becomes smooth, and the tendency for knocking is avoided. In SI engine the short ignition delay promotes detonation.

Hence supercharging is preferred in diesel engine than in petrol engine.

Factors affecting knocking in SI engine:

i) Compression ratio:- Higher compression ratio will result in the higher temperature of the charge, the tendency to knock will increase.

ii) 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.

iii) 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.

iv) 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.

v) Engine Speed: TO avoid the knocking in SI engine the engine speed should be high.

vi) Octane rating: It should be as high as possible