|Normal combustion occurs when the fuel/air mixture ignites in the cylinder
and burns progressively with a normal pressure increase, producing maximum
pressure immediately after the piston passes top dead center of the compression
stroke (Fig. 2-14).
A flame front starts at the spark plugs and travels across the combustion chamber at a speed of approximately 70 to 100 feet per second. The velocity of the flame front is influenced by the type of fuel, the ratio of fuel to air mixture, the pressure on the fuel/air mixture, and the temperature of the fuel/air mixture. The pilot has control of these items by ensuring that (1) the airplane has been serviced with the recommended grade of fuel; (2) the pressure of the fuel/air mixture is properly regulated by propeller and throttle controls; (3) the engine temperature is adequately regulated by use of the cowl flaps and by the engine speed when an internal supercharger is used; and (4) the fuel/air ratio is accurately adjusted by the mixture control. When any of these factors is not properly controlled, abnormal combustion will result.
When the fuel/air mixture is ignited by means other than the normal spark ignition, the result is abnormal combustion. This abnormal combustion is divided into two distinct types - detonation and preignition.