The tendency of a diesel engine to knock increases, if:

Concept:

• In the normal working of diesel engines, initially, the air is compressed and then the fuel is injected at high pressure.
• Due to the high compression ratio, the fuel ignites and explodes to give the power stroke.
• Now the requirement of this combustion model is that the highly atomized fuel should start burning as soon as it is sprayed in the cylinder. But, if the fuel sprayed initially is not able to mix properly due to a variety of reasons, it keeps on concentrating in the combustion chamber.
• This increases the concentration of fuel in the chamber and at some point, the large quantity of fuel ignites sending pulses throughout the chamber. This leads to knocking in a diesel engine.

Following reasons are for increasing the knocking in a Diesel engine:

• By decreasing the supercharge flow
• By decreasing the inlet temperature
• By increasing the engine speed
• By instantaneous burning of the first part of the fuel
• By decreasing the cooling water temperature
• By using low Cetane number Diesel
• By decreasing the compression ratio

The following are the differences in the knocking phenomenon of the S.I. and C.I. engines:

• In the S.I engine, the detonation occurs near the end of combustion whereas in the C.I engine detonation occurs near the beginning of combustion
• The detonation in the S.I engine is of a homogeneous charge causing a very high rate of pressure rise and very high maximum pressure
• In the C.I engine, the fuel and air are imperfectly mixed and hence the rate of pressure rise is normally lower than that in the detonating part of the charge in the S.I engine
• In the C.I engine the fuel is injected into the cylinder only at the end of the compression stroke, there is no question of pre-ignition as in S.I. engine
• In the S.I. engine, it is relatively easy to distinguish between knocking and non-knocking operation as the human ear easily finds the distinction