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FACTORS AFFECTING KNOCKING IN S.I ENGINE :

FACTORS AFFECTING KNOCKING IN S.I ENGINE : 

The various engine variables affecting detonation can be classified under four factors, namely, the temperature factors, density factors, time factors and composition factors.

a) Temperature Factors :

    Increasing the temperature of  the  unburnt mixture by any factor in design or operation will increase the possibility of the knock in the S.I Engine. The temperature of the unburnt mixture is increased  by  the  following factors :

 * Raising the compression ratio. 

 * Supercharging 

 * Raising the inlet air temperature.

 * Raising the coolant Temperature.

 * Increasing the load.

 * Raising the temperature of the cylinder and combustion chamber walls.

 * Advancing the spark timing.


 b) Density Factors :

    Any factors which increase the density of  the charge lend to increase in knocking by providing excess energy release. The following method will increase the possibility of Knock on S.I Engine.

 * Increasing the compression ratio.

 * Opening the throttle.

 * Supercharging the engine.

 * Increasing the inlet pressure.

 * Advancing the spark timing.


c) Time Factors :

Increasing the flame speed or reducing the duration of  the ignition delay period or reducing the time of exposure of  the unburnt mixture to auto ignition condition it will tend to reduce knocking. The following factors will increase the possibility of  Knocking :

* Increasing flame travel distance : Engine size, combustion chamber size and spark plug position are the three factors governing the flame travel distance.

* Decreasing the turbulence of mixture.

* Decreasing the speed of the engine.

* Increasing the time available for pre flame reaction.


d) Composition Factors :

The properties of fuel and the fuel- air ratio are the primary means for controlling knock, once the compression ratio and engine the dimensions are selected. The probability of knock is increased by the following factors :

* Low Octane Number.

* Lean Mixture of air fuel ratio

* Decreasing atmospheric humidity.

* Low self-ignition Temperature.

* Short Ignition Delay.



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