Pre-ignition and detonation are not the same, Kevin Cameron notes, which is why you must not use one term to describe the other phenomenon.
In normal combustion, the piston is on its way up the cylinder toward Top Dead Center. It’s compressing a mixture of fuel and air above it. At a point, before TDC, a spark passes across the gap of the spark plug electrodes and that sets the mixture to burning, creating a tiny flame kernel. If the fuel-air mixture is adequately turbulent, the flame kernel is broken up, shredded, and distributed rapidly so that it has the effect of the flame spreading from the spark plug toward the cylinder walls at rates of 50 feet per second and up.
Bear in mind that an actual explosion would be thousands of feet per second, so normal combustion is simply burning, like a forest fire, by a process chemists call deflagration. As in the case of a forest fire, the heat of the flame front ignites the unburned fuel. This desirable outcome of a spark igniting normal combustion proceeds to the cylinder wall in an orderly fashion-as the Supreme Court would say, with all deliberate speed-consuming the entire charge.
There are other possibilities. Pre-ignition is just what you would think from the name: ignition of the fuel-air mixture before the spark by some hot object in the cylinder or combustion chamber. It may be overheated spark plug electrodes or glowing carbon deposits in the combustion chamber, but the result is always the same: After one or two cycles, the part of the piston farthest from the cool cylinder wall, namely the center of the dome, becomes hot very quickly. When aluminum becomes hot very quickly, it loses strength and eventually sags. And then it punches through. That is pre-ignition.
There is another phenomenon, entirely different from pre-ignition, known as detonation. Pre-ignition and detonation are not the same, so you must not use one term to describe the other. In detonation, ignition is normal. It takes place at the spark plug and the flame front moves normally outward. But if the air-fuel mixture gets hot enough as that flame front expands against and compresses the unburned mixture near the cylinder wall, chemical changes occur that transform it into a sensitive explosive.
If the process goes far enough, at some point around the edge of the piston, a small volume of unburned mixture explodes. It burns at the local speed of sound, creating a shock wave, which strikes the inside of the cylinder head, causing the sound we call “knock.” Modern cars and motorcycles have knock detection systems, so we don’t hear them knocking. To have encountered detonation, you might have to be an older person who has ridden in a three-speed taxi running on cheap gas. When the engine is lugging at low rpm, it provides the extra time necessary for detonation conditions to develop.
What you will see on a piston that is lightly detonated is the appearance of having been sandblasted at some point around its edge. That sandblast effect is the aluminum itself being blown off the surface by the waves from detonation. When this particular piston seized, the whole piston was so weak that the connecting rod tore the wrist pin and the wrist-pin bosses right out of the piston and left it as a pitiable wreckage. Pre-ignition and detonation: Both conditions are well worth avoiding.
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