Diesel engine knock

What is engine knock?

Engine knock is the noise produce by the engine due to abnormal combustion. So what are the reasons for knock in a diesel engine?

Diesel Knock

ž If the delay period is long, a large amount of fuel will be injected and accumulated in the chamber.

ž The auto-ignition of this large amount of fuel may cause a high rate of pressure rise.

ž This high rate pressure rise and high maximum pressure leads to a knocking. 

The comparison of SI and CI engine knock

                                                                                  

Figure 1 Knocking in CI Engine

Figure 2 Knocking in SI  Engine

As we have seen from the figure the SI engine knock starts at the end of the combustion because of auto-ignition temperature whereas in diesel engines the knocking starts at the start of combustion because of longer ignition delay. 

Ignition delay is a time taken by the fuel mixture from point of injection to the first occurrence of the auto-ignition on the P-θ diagram. 

We also have seen from figure in SI engine the pressure is very high at the time of detonation whereas in CI engine pressure rise is lower. 

Pre-ignition is the main cause of detonation in the SI engine whereas diesel is injected at the end of compression stroke so there is no chance for pre-ignition of diesel. 

The combustion on the CI engine is based on the auto-ignition of the fuel mixture. If fuel mixture is not uniform over the area of cylinder there is a chance of knock

There are some factors which affect the engine knock in SI and CI engine are follows:

Self-ignition temperature

The self-ignition temperature in the Si engine is high to avoid the pre-ignition tendency whereas in diesel engines the self-ignition temperature is low to achieve auto-ignition as soon as possible to reduce the knock.

Time lag of delay period

The time lag of delay period, in SI engine time lag is high because, after the spark, flame spread slowly and gain strength for further ignition whereas in diesel engine delay period is low because of the self-ignition temperature of diesel is low to reduce the knock.

Compression ratio

In SI engine compression ratio should be low to avoid the pre-ignition of fuel, whereas in diesel engine the combustion starts with the help of self-ignition, so the compression ratio is high to achieve the self-ignition of the mixture as soon as possible to avoid sudden pressure rise as well as engine knock.

Inlet temperature low high

In SI engine the inlet temperature of the wall should be low to avoid pre-ignition which leads the detonation whereas in diesel engine the inlet wall temperature is high to increase the inlet air temperature which the help to attain the self-ignition of fuel as soon as possible and reduce the ignition delay to avoid engine knock.

Inlet pressure

Inlet pressure in SI engine is low to avoid the engine knock, for same compression ratio, the pressure and temperature at the end of the compression stroke is high for high inlet pressure with leads pre-ignition of fuel and engine knock whereas in diesel engine inlet pressure should be high to achieve the self-ignition temperature which reduces ignition delay to avoid engine knock.  

Combustion chamber wall temperature low high

If the wall temperature of a combustion chamber is high in SI engine it causes pre-ignition of fuel which lead engine knock, so it is recommended to maintain the low temperature of the cylinder wall in Si engine whereas high wall temperature in CI engine helps to reduce ignition delay and reduce the engine knock.