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 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.
Pre-Ignition is defined as combustion that begins before it is intended to, before the regular timed spark. Pre-Ignition can occur alone or as a result of detonation. Hot spots from detonation, improper heat range spark plugs and glowing carbon deposits from lean mixtures are common causes of pre-ignition. Carbon deposits do not usually accumulate when the lean mixture is chronic but normal deposits can be quickly heated to glowing temperature when a fuel injector suddenly becomes partially clogged. Carbon tracks within a magneto that allows a cylinder to get the spark from another cylinder can also be a cause.
Most cases of pre-ignition will start at or near the beginning of the compression stroke since a combustible mixture becomes more difficult to ignite as pressure rises. This causes severe stress on the engine and can quickly burn a hole in the piston, most often in the middle. Pre-ignition will cause a sudden loss of power as the affected cylinder is working against normal rotation of the engine. Severe heat results from compressing a burning mixture. No power is extracted from the burn, resulting in all heat energy being absorbed by the cylinder parts.
Damage from either detonation or pre-ignition is severe. Once the seal between the piston and the crankcase is breached pressurization of the crankcase can push crankcase oil overboard causing additional damage from oil starvation. Engine contamination and the severe stress imposed will require the engine to be completely disassembled. All parts must be assessed for contamination issues and all stressed parts must be appropriately NDT tested for integrity.
Airmark Overhaul, Inc. is a full service aircraft engine overhaul facility and we can quickly get your engine back in service to keep you flying.