Figure 7-21.  Normal combustion and explosive combustion.  Figure 7-20.  Typical starting circuit.  Detonation may be avoided by following these basic  guidelines during the various phases of ground and flight  operations:  Preignition occurs when the fuel-air mixture ignites prior  to the engine’s normal ignition event. Premature burning  is usually caused by a residual hot spot in the combustion  chamber, often created by a small carbon deposit on a spark  plug, a cracked spark plug insulator, or other damage in the  cylinder that causes a part to heat sufficiently to ignite the  fuel-air charge. Preignition causes the engine to lose power  and produces high operating temperature. As with detonation,  preignition may also cause severe engine damage because  the expanding gases exert excessive pressure on the piston  while still on its compression stroke.  by the spark plugs. It then burns away from the plugs until it  is completely consumed. This type of combustion causes a  smooth build-up of temperature and pressure and ensures that  the expanding gases deliver the maximum force to the piston  at exactly the right time in the power stroke.  [Figure 7-21]  Detonation is an uncontrolled, explosive ignition of the  fuel-air mixture within the cylinder’s combustion chamber.  It causes excessive temperatures and pressures which, if not  corrected, can quickly lead to failure of the piston, cylinder,  or valves. In less severe cases, detonation causes engine  overheating, roughness, or loss of power.  Detonation is characterized by high cylinder head temperatures  and is most likely to occur when operating at high power  settings. Common operational causes of detonation are:  • Maintaining extended ground operations or steep  climbs in which cylinder cooling is reduced  • Ensure that the proper grade of fuel is used.  • Keep the cowl flaps (if available) in the full-open  position while on the ground to provide the maximum  airflow through the cowling.  • Use an enriched fuel mixture, as well as a shallow  climb angle, to increase cylinder cooling during  takeoff and initial climb.  • Avoid extended, high power, steep climbs.  • Develop the habit of monitoring the engine instruments  to verify proper operation according to procedures  established by the manufacturer.  • Use of a lower fuel grade than that specified by the  aircraft manufacturer  • Operation of the engine with extremely high manifold  pressures in conjunction with low rpm  • Operation of the engine at high power settings with  an excessively lean mixture  7-19  Explosion Normal combustion  M  A  I  N  B  U  S  Starter  Battery  A L T  B A T  Battery contactor  (solenoid)  Starter  contactor  External  power relay  +  +  OFF  L  R  B  S  Ignition switch  External  power plug 