LESSON 20. Engine Detonation

Engine detonation is an engine refers to inappropriate combustion of fuel in the combustion chamber/cylinder of the engine. Either the compressed air fuel mixture is burnt in the cylinders with help of a spark (in SI petrol engines) or the air alone is compressed during the compression stroke and fuel is injected and burnt due to compression ( in CI diesel engines). To get maximum power from the engine it is required that proper of air fuel mixture or fuel is supplied to the engine and ignited at proper time. Sometimes, where preignition of fuel can happen in the engines, it is also observed that whole of the fuel or air fuel mixture is not burnt at once. Due to this, a pressure wave is set up in the combustion chamber which travels to and fro and hits to the cylinder walls. This disturbance in the cylinder forces the walls at the frequency of gases which produces a very peculiar sound which is known as engine detonation or knocking.

Engine detonation can also be illustrated as it can also occur due to sudden and instantaneous ignition of the unburnt charge when the temperature and pressure is so high and sufficient to ignite the fuel or air fuel mixture. The factors affecting engine detonation can be classified as follows :

i) Engine factors

ii) Air, Fuel and Air-Fuel mixture factors

20. 1 Engine factors

There are engine characteristics which can affect engine detonation include :

- Compression ratio :  Engine detonation increases with increase in compression ratio as it increases the gas temperature and pressure thus lowering the reaction time for charge to get ignited. Every engine is designed for a particular maximum compression ratio and any compression ratio beyond this, causes engine detonation.

- Engine size : Engine detonation increases with increase in cylinder size (bore).

- Spark advance : Retarded spark helps in lowering the detonation whereas over-advance in spark leads to more detonation as pressure gets higher than the normal maximum pressure.

- Design of combustion chamber : The design which produces more turbulence in the combustion chamber, it helps in rapid combustion of the charge and hence decrease the chances to knock or detonate.

- Defective cooling system : If engine cooling system is not working properly due to fault in engine thermostat, water pump etc., it can also increase the engine detonation.

- Engine speed : At higher engine speeds which may also lead to fall in volumetric efficiency, the engine detonation is decreased.

- Valve timing : As the valve timing increases the volumetric efficiency which increases the air-fuel mixture intake and increase the cylinder pressure, the tendency to engine detonation is also increased.

20. 2 Air, Fuel and Air-Fuel Mixture factors

It has been observed that charge characteristics mentioned below can also be significant factors which can cause engine detonation.

- Octane number

20. 3 Effects of detonation

The main effects of detonation are:

1. Inefficient combustion.

2. Loss power.

3. Local overheating.

4. Mechanical engine failure.

20.4 Prevention of Detonation

The different methods used for the prevention of detonation are:

1. Anti-knock agents.

2. Cooling of the charge.

3. Reducing the time factor.

Last modified: Wednesday, 29 January 2014, 5:43 AM