AVOID UNNECESSARY DAMAGE TO YOUR ENGINE

Detonation and Pre-Ignition are two unique conditions that can severely damage an aircraft engine. Forces and severe heat that result from the occurrence of either will usually require the complete teardown and repair of the engine.

To understand the similarities, differences and causes of either requires an understanding of exactly fuel burns within the engine to develop power. Anyone who has ever observed how settled fuel vapors ignite has seen how the flame front progresses from the source of ignition smoothly to the outer edges where flammable vapors reach a point of dilution that stops the burn. This is exactly how fuel must burn within a piston engine to develop power without doing damage. Detonation refers to the condition where remote pockets within the fuel air mixture explode violently due to rising pressure following regular ignition. Pre-Ignition refers to the condition where either a mistimed spark or another source of ignition exists within the combustion chamber, allowing the burn to start well in advance of the normally timed spark. Pre-Ignition and Detonation can often overlap each other, usually from detonation damage causing pre-ignition.

DETONATION

Detonation is uniquely distinguished by the fact that it cannot occur before the spark plug fires. When the spark initiates burning within the cylinder the flame front is expected to progress through the cylinder evenly, creating heat and even pressure to push the piston down. As the burn starts cylinder pressure quickly rises. If combustion chamber parts are hotter than usual this can cause remote pockets within the cylinder to spontaneously detonate. This can also occur when fuel octane is below the requirements of the engine.

Causes of detonation are limited to excessive heat and low octane. Excessive heat can come from improper cooling, high compression from excessive combustion chamber deposits, lean mixture, advanced timing and more. When it is limited to one cylinder the very likely culprit is a partially clogged fuel injector. This allows one cylinder to operate much leaner than the others. Intake leaks can also lean the mixture but are usually noticed during low manifold pressure operation where the symptoms of a leak become much more obvious. Detonation caused by low octane fuel is more likely to affect several cylinders since the contributing factor is present in all cylinders. Detonation can difficult if not impossible for a pilot to detect from within the cockpit.

Detonation can occur for some time before severe damage occurs or it can very quickly progress to severe failure depending on its severity. Minor detonation can cause damage that in time will likely progress to an increasingly severe condition. It occurs at the far reaches of the combustion chamber and usually causes the most damage at the edges of the pistons as a result. It causes rapidly rising temperatures at the piston edges, which can allow subsequent detonation to damage ring lands. It can also trigger pre-ignition due to the hot spots. Once this occurs the damaged piston edge is exposed to severe heat and pressure which can cause a hole to be burned through the corner of the piston. Leaking combustion gasses pushing through broken ring lands also causes torching at the pistons edge which quickly progress to failure of the seal between the combustion chamber and the crankcase.