At part throttle and low loads (cruise), very lean mixtures are possible with things such as high energy ignitions, charge straitification or special combustion chamber shapes. Knock sensors are not normally needed under these conditions. When increased power is needed, the mixture is richened closer to the stoichiometric ratio, and the potential for detonation increases as cylinder filling increases.
When the peak pressure inside the combustion chamber occurs too early (as the piston is still rising or at TDC in the bore), knocking occurs. Knocking is the collision of two flame fronts in the comubstion chamber (also known as pre-ignition). Pre-ignition can be caused by:
- carbon deposits or a sharp edge (a nick in the head or piston) causing combustion to start away from the plug.
- a lean mixture pre-igniting due to combustion chamber pressure (the lean mixture requires less pressure to spontaneously ignite) prior to the spark plug firing.
- Too early spark advance.
- Fuel with too-low resistance to combustion (low octane rating).
- This hammer blow is the actual knocking, which in conjunction with higher temps can destroy the piston head (or an apex seal in a rotary engine). It also places a heavy load on the rod bearings and the hydrodynamic oil cushion, which is why lugging the engine (low revs, high load), can wipe out bearings and journals.
If the peak pressure wave occurs early, but the cylinder head is lightly filled, the piston can still compress the fireball without problem (sometimes light pinging).
The results of heavy detonation, especially in turbocharged/supercharged applications can be found on the desktops, and in garbage cans of racers everywhere