Detonation in S.I. Engines:

In an S.I. engine once the spark is supplied at the spark plug and a flame nucleus is formed the flame travel across the combustion chamber in an orderly manner.

The pressure keeps on increasing continuously and uniformly throughout the combustion chamber.

The peak pressures are normally attained when the flame reaches far side of the combustion chamber.

Fig (a) and (b): p- diagram in case of SI combustion engine with knocking

Let us focus our attention on last part of the charge called end gas in the combustion chamber as shown in Fig.

The flame originating at point A travels across the combustion chamber upto point E. The end part of the gas is represented by DED’.

This end gas has experienced rapid compression during the compression stroke and later on by advancing flame front. Therefore the pressure, temperature and density of end part of the gas is high.

If the combustion process is normal the pressure of end gas will almost be equal to peak pressure when the flame front is about to reach to this end gas.

If the ignition lag or ignition delay of the unburned end gas is consumed before the flame front reaches it, the end part of the charge will auto-ignite because it is at much higher temperature than its self-ignition temperature.

This auto-ignition of end gas is completed in almost negligible time and causes a violent pressure rise due to extremely high rate of liberation of chemical energy and almost at constant volume. This rise in pressure of end gas is almost (3-4) times the anticipated peak pressures with normal combustion chamber at very fast speed. The pressure wave is reflected back and forth several times by the cylinder walls and sets the engine parts vibrating, giving rise to a pinging or ringing sound and the detonation is said to occur. We conclude that detonation will take place in case of S.I. engines if, End part of the charge reaches to its self-ignition temperature and its delay period it over.

The flame does not reach the end part of the charge.