How to know whether you’re experiencing pre-ignition or detonation

2 Answers

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   santiagoqr1 on Sep 04, 2015

   This is a hard question since they are really hard to tell apart while it’s happening. But I know something that will give you a clue:
   When an engine is detonating, it will make that “explosion” sound, and it can actually run for quite a while. There’s cases where they take the plane for the 100 hour inspection, and they realize it’s been detonating for a long time (piston crowns look “sandblasted”), whereas when the mix is Pre-Igniting, it can only run for a few seconds, then you’ll blow the engine, it’ll literally melt a hole through the piston heads.
   So, if you’re worried about detonation, just try making smooth changes to power and mixture, and follow the procedures, but if you’re worried about pre-ignition, well, let me tell you, you have bigger things to worry about.

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2. 
   

   
   

   Kris Kortokrax on Sep 15, 2015

   Where in the world did you get the information you posted??

   Here is the excerpt from the Pilot’s Handbook of Aeronautical Knowledge concerning detonation and preignition.

   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:

   • 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.
   • Maintaining extended ground operations or steep climbs in which 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 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 shallower 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.

   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.

   Detonation and preignition 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 preignition.

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