The piston shows melted areas on the
dome and seizure marks on the skirt.
Detonation results in extremely high combustion chamber temperatures. That rapid heat
rise, if allowed to continue, will heat combustion chamber deposits and spark plug
electrodes to the point that they will ignite the charge before the spark at the plug
does.
This condition is called runaway surface ignition or pre-ignition. Once pre-ignition
occurs, the temperature rise is so rapid that melting point of the piston is reached while
the engine is still running. Metal will melt away beneath the spark plug or in areas of
high heat retention such as at the ring positioning pin.
A complete breakdown of the
lubrication film also occurs resulting in severe seizure. Detonation always precedes
pre-ignition and the causes for this abnormal combustion are the same as those for
detonation.
3B
Ultralight
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