10 Things You Always Wanted to Know About Engines

This article is copied from the September 1999 issue of Hot Rod Magazine

By Ray T. Bohacz

    During normal combustion, the air/fuel mixture waits for the spark plug to initiate burn. The flame travels across the bore, consuming the fuel and expanding in only one direction. Anything other than this process is abnormal and is usually referred to as spark knock or ping and surface ignition.

    Knock is the spontaneous ignition of the end gas ahead of the flame front. When this occurs there is an extremely rapid release of energy from the end gas that creates very high localized pressures and the collision of the multiple flame fronts. These localized pressure peaks and the colliding flame fronts send shock waves throughout the combustion chamber that cause the piston and connecting rod to oscillate and create a metallic, pinging sound. Severe detonation, on the other hand, will destroy a piston by either burning a hole in it or shattering the ring-land area.

    Surface ignition is the ignition of the air/fuel mix from a hot spot or other object in the combustion chamber acting like a spark plug. When it occurs before spark plug ignition, it is called pre-ignition; if it follows the arc of the plug, it’s called post-ignition. A turbulent flame front that would normally occur from the spark plug follows in the wake of surface ignition.

    Octane ratings establish the ability of the fuel to thermally resist combustion and await ignition from the spark plug. During knock, the fuel is exploding instead of burning, much like it would in a diesel engine. Factors that usually affect abnormal combustion beyond the fuel quality and spark advance are all related to temperature. Any factor that raises combustion temperature will increase the propensity towards abnormal combustion.