In a spark-ignition piston engine, the fuel-air mixture inside the cylinder burns evenly, starting at the spark plugs. The temperature and pressure increase inside the cylinder, and the expanding gas pushes the piston down at just the right moment in the power stroke. If the temperature and pressure rise too high, however, the controlled burn becomes an explosion, a phenomenon known as detonation. This pounds the piston down with much greater force and can cause catastrophic engine damage in minutes.
One cause of detonation is using a lower grade fuel than the engine requires. (Today, 100LL is the only widely available fuel that high-compression aircraft engines can safely use, but AOPA is supporting industry efforts to transition to a high-octane unleaded alternative; see aopa.org/100UL for more information.) Operating at high power settings with an excessively lean mixture is another. Rich mixtures generally don’t detonate as readily as lean ones, and fuel flow problems such as a partially clogged fuel injector nozzle may cause the fuel/air mixture to be overly lean. Other potential causes include an intake leak or a mistimed magneto. Operating at extremely high manifold pressures with low propeller rpm, hot spots from damaged spark plugs, and extended operations with reduced cylinder cooling also may lead to trouble.
To protect against detonation, keep your engine cool. Use the right octane of fuel and use a richer mixture at high power settings. Avoid prolonged climbs at high power and keep cowl flaps open during phases of flight when the engine needs additional cooling. Engine monitors can provide warning of rising cylinder head temperatures, and if you suspect the engine may be detonating, act quickly to bring the cylinder head temperatures (CHTs) back down: Reduce power, enrich the mixture, and open cowl flaps if you have them. Engine detonation is an emergency, and it’s important for both you and your engine to stay cool.
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