The fuel accumulated during the ignition delay period is burned at one time during the flame propagation period. Thus the pressure inside the combustion chamber rises sharply. The pressure inside the combustion chamber rises sharply in proportion to the amount of the fuel injected during the ignition delay. This pressure wave causes the engine to vibrate and make a noticeable noise. This is called as diesel knock. The diesel engine uses a self-ignition combustion system, so to a certain extent, diesel knock is unavoidable. Causes of diesel knock are as follows:
Engine temperature is low.
Intake air temperature is low.
Fuel ignition temperature is high. (Cetane number is low.)
Injection timing is early. (The fuel is injected when the compression temperature is still low.)
Injection condition is not good. (The fuel does not mix well with the air.)
To prevent diesel knock, shortening the ignition delay, thus avoiding a sudden rise in pressure.
The following methods are employed:
Using fuel with a high cetane number.
Raising the compression pressure and the intake air temperature until the beginning of the fuel injection.
Raising the combustion chamber temperature.