AIRCRAFT SYSTEMS - COMBUSTION

During normal combustion, the fuel/air mixture burns in a very controlled and predictable manner. Although the process occurs in a fraction of a second, which the mixture actually begins to burn at the point where it is ignited by the spark plugs, then burns away from the plugs until it is all 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.

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. Some common operational causes of detonation include:

• Using a lowers fuel grade than that specified by the aircraft manufacturer.
• Operating with extremely high manifold pressures in conjunction with low rpm
• Operating the engine at high power settings with an excessively lean mixture.
• Detonation also can be caused by extended ground operations, or steep climbs where cylinder cooling is reduced.
  
  
  Detonation may be avoided by following these basic guidelines during the various phases of ground and flight operations:

  • Make sure the proper grade of fuel is being used.
  • While on the ground, keep the cowl flaps (if available) in the full-open position to provide the maximum airflow through the cowling.
  • During takeoff and initial climb, using an enriched fuel mixture, as well as using a shallower climb angle to increase cylinder cooling can reduce the onset of detonation.
  • Avoid extended, high power, steep climbs.
  • Develop a habit of monitoring the engine instruments to verify proper operation according to procedures established by the manufacturer.
    
    
    Pre-ignition 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.
    
    Pre-ignition causes the engine to lose power, and produces high operating temperature. As with detonation, pre-ignition may also cause severe engine damage, because the expanding gases exert excessive pressure on the piston while still on its compression stroke.
    
    Detonation and pre-ignition often occur simultaneously and one may cause the other. Since either condition causes high engine temperature accompanied by a decrease in engine performance, it is often difficult to distinguish between the two. Using the recommended grade of fuel and operating the engine within its proper temperature, pressure, and rpm ranges reduce the chance of detonation or pre-ignition.