Pre-ignition

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Pre-ignition (or preignition) in a spark-ignition engine is premature ignition of the air/fuel mixture in the cylinder, before the spark plug fires. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbonaceous deposits in the combustion chamber heated to incandescence by previous combustion events. It is technically a different phenomenon from engine knocking, which refers to uneven combustion caused by displaced or multiple ignition fronts, though knocking often follows pre-ignition. The phenomenon is also referred to as 'after-run' or 'run-on', or sometimes dieseling, when it causes the engine to continue running even after the ignition is shut off. This effect is more readily achieved on carbureted gasoline engines, because the fuel supply to the carburetor is typically regulated by a passive mechanical float valve and fuel delivery can feasibly continue until fuel line pressure has been relieved, provided the fuel can be somehow drawn past the throttle plate. The occurrence is rare in modern engines with throttle-body or electronic fuel injection, because the injectors will not be permitted to continue delivering fuel after electric power to the engine is shut off, and any occurrence may indicate the presence of a leaking (failed) injector. In the case of highly supercharged or high-compression multi-cylinder engines, pre-ignition can quickly melt or burn pistons since the power generated by other still-functioning pistons will force the overheated ones along no matter how early the mix pre-ignites. Many engines have suffered such failure where improper fuel delivery is present. Often one injector will clog while the others carry on normally, allowing mild detonation in one cylinder that leads to serious detonation, then pre-ignition. The challenges associated with pre-ignition have increased in recent years with the...