It has been shown that the best results are obtained when the valves are of ample head proportions, the inlet being rather being rather larger than the exhaust. The sparking plugs are located in fairly central positions as to give the shorter flame travel paths in all directions as far as possible. Further a certain amount of swirl or turbulence of the compressed mixture is provided by the combustion chamber shape in relation to the piston at the top of the compression stroke, so that the sparking plug will be cooled sufficiently by the mixture movement.
Post-ignition Effect: There is another mixture ignition effect that may occur in a modern car engine, namely, running — on of the engine after the actual ignition has been switched off. The effect — known as post (or after) ignition — appear as to be due the presence of, in the combustion chamber of a metal or carbon projection, or edge which becomes heated during the initial combustion stages, to a temperature above the ignition point of the mixture and ends to supplement the electric ignition. During the exhaust, suction and compression strokes this ‘hot spot’ cools down below the ignition temperature of the charge, but as soon as the electric spark ignites the charge, it heats up again. When the engine is at its working temperature and the ignition is switched off, this “hot spot” continues to act as an auxiliary igniter of the charger, until the temperature of the combustion chamber is reduced sufficiently, to prevent this effect.
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