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Originally Posted by Pure_Insanity8
Detonation is basically the fuel/air mixture igniting before it is supposed to. Usually "hot spots" occur somewhere in the combustion chamber, on the piston, etc.. During compression the fuel/air mix is being heated enough to where the hot spot can ignite it (before the spark plug fires). The flame front originating from the hot spot will usually end up "running into" the flame front created once the spark plug fires. So, of course, the mix is going to burn faster than it should, not to mention the violent collision of the two (or possibly more) flame fronts.
Even though under normal circumstances the burn for a single compression to power stroke is VERY fast (can't remember of hand but I think in the few thousandths of a second), this "unintentional" type of burn is going to start leaning closer to an explosion instead of a burn. When this "explosive" force is pushing down on the piston while it is coming up on the compression stroke... let's just say not cool. 
I'm not the best at explanations... but I hope that gives you some idea of how the piston could be blown apart among other problems (like to your crankshaft or connecting rods).  |
Not easy to explain is it Aaron. You have explained
Preignition As you say it is started by a hot spot that starts the burn before the spark has happened. This is not too bad a thing,as the gas still burns evenly, just a bit earlier.
I all ways liked the expression that said the noise was from the 'collision of the two flame fronts'
I think this is what Champion plugs put out.
Detonation is when the spark has ignited the charge, but before it has burnt all the way across the chamber the gas contained in the space furthest away from the plug, reaches flash point and explodes. So instead of a steadly riseing cylinder pressure that can be turned into motion, by the piston/con rod, the sudden rise just produces extream pressure on the piston/rod assemble.
This usually starts in a small way, with a small amount of gas exploding. This causes the pinging/pinking sound, as it continues the area gets hotter, to cause a larger amount to explode next stroke, till the amount that is exploding becomes what we call 'detonation'. The sound is the sudden rise in pressure hitting the chamber walls, and the piston/rod assemble sufferes a huge hammering that will blow pieces away from the edge of the piston, breaking the top rings and eventually breaking the piston. .
Depending of the number of cycles that this process takes, the knock detectors will hear it and retard the spark to prevent it from developing to the detonation stage. Ofcourse if you have handed it a huge dose of Nitrous, it can happen in the one cycle, and the knock sensors won't help.
Harvey.