Originally Posted by
rutan around
AEFR
Not necessarily. If the octane rating of the fuel is high enough it won't detonate. However if the timing is too far advanced ie the fire is lit too early all the fuel will be burnt at or even before top dead centre. This means peak pressure occurs early and, at first, is doing negative work until the piston is past TDC and can begin to move down and do some useful work. The lengthy time (relatively) that peak pressure (and high heat) are in the cylinder's head transfers much more heat to the enclosing metal before the valve opens and dumps the exhaust gas and cooling can begin. It is the heat doing the damage.
How did you address the problem of different flame front speeds for the different fuels?
I adjusted the ignition timing. Have you never tuned an engine?
A higher octane fuel can tolerate more ignition advance and/or a higher compression ratio. The best output of a spark ignition engine is obtained by experimentation with the timing; it's by no means unusual for the limiting factor on engine performance to be detonation. Retarding the timing is the easiest way to alleviate that problem but obviously it can be solved by use of a higher octane rated fuel where available; it is still possible to buy TEL in the form of an additive. It can also be done by richening the mixture, or in extreme cases by reducing the compression ratio, such as by fitting a decompression plate.