How dose Diesel Knock happens?

If the delay period in C.I Engine is long a large amount of fuel will be injected and accumulated in the chamber. The auto ignition of this large amount of fuel may cause in high rate of pressure rise and high maximum pressure which may cause Knocking in Diesel Engine.A long delay period not only increase the amount of fuel injected by the moment of ignition but also improves the homogeneity of the fuel and air mixture and its chemical preparedness for explosion type self-ignition similar to detonation of S.I engines.

Delay period is directly related to knocking in CI engine. Causes of diesel knock are following factors :-

* A low compression ratio permitting only a marginal self ignition temperature to be reached.

* A low combustion pressure due to worn out piston, rings and bad values.

* Low cetane number fuel.

* Poorly atomized fuel spray preventing early combustion.

* Coarse droplet formation due to malfunctioning of injector parts like spring

* Low intake temperature and pressure of air.

The following methods are employed :-

* To prevent diesel knock, shortening the ignition delay, thus avoiding a sudden rise in pressure.

* 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.

The following are the difference in the Knocking Phenomena of the S.I Engine and C.I Engine :-

1 . 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.

2. The detonation in the S.I Engine is of a homogeneous charge causing very high rate if pressure rise and very high maximum pressure .In the C.I engine, the fuel and air are detonation part of the charge in the S.I Engine.

3. 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.

4. 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.

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