'Knock, Knock.' 'Who's There?'

‘It’s me. Detonation. I’m here to wreck your engine.’ Seriously, though, detonation (also called knock) is no joke. Jake Venter explains why it can cause damage, and what is done to prevent it.

- Jake Venter

In last week’s article, I explained that in a spark ignition petrol engine, the flame travels outwards from the spark plug in the form of a distorted ball. The moving flame front increases the temperature and pressure of the gas in front of it. If the temperature rises fast enough, it can cause the remaining unburnt mixture to explode rather than burning rapidly as it is meant to do. This explosion, called detonation, causes shock waves, resulting in heat and vibration damage to pistons, valves and combustion chamber walls.

Detonation produces a knocking sound, but this may be too faint for the driver to hear.

Factors that promote detonation include ignition timing that is too far advanced, a lean fuel/air mixture, and fuel with a low octane rating.

In addition, an overheated combustion chamber may contain glowing hot spots that will also cause spontaneous combustion. If this occurs after the spark it’s called post ignition; if it occurs before the spark it’s called pre-ignition.