MoTeC Knock OKM - Manual (Page 32)

32 Appendices
Combustion chamber at TDC
Abnormal Combustion – Detonation
If conditions for combustion are not ideal, detonation can occur. This usually
happens first at points of amplification of the pressure waves.
For example at the edges of the piston
crown where reflecting pressure waves
from the piston or combustion chamber
walls can constructively recombine – this
causes very high local pressures. If the
speed at which this pressure build-up to
detonation occurs is greater than the
speed at which the mixture burns, the
pressure waves from both the initial
ignition at the spark plug and the
pressure waves coming from the
problem spots can set off immediate
explosions in the remaining air/fuel
mixture, rather than smooth combustion.
The remaining fuel in the end gas simply
lacks sufficient octane rating to withstand
this combination of heat and pressure.
Combustion Pressure with detonation