Detonation can and will cause serious engine damage and is the limiting factor in developing maximum power from an engine. All spark ignited aircraft engines are capable of detonation and if an engine is to make some amount of power it must be run near its detonation margin.
And aircraft engines are more susceptible to detonation due to the use of large bore piston and cylinders. Hence the development of high octane fuel which increases the margin before the engine starts to detonate. On this page we will shed some light on the detonation process and the effects on an engine if the pilot does nothing about it.
After the fuel air mixture is ignited by the two sparks plugs in the combustion chamber it will start to burn at an uniform rate until the flame front reaches the last portion of the charge (end gas) on each side in the cylinder.
This article dives into the causes of detonation, continue with the next article to learn about on avoiding detonation.
Under conditions as high cylinder pressures (compression stroke) and high surface temperatures (high power running) the fuel air charge can auto-ignite and burn at a much higher rate (explosive rates). This will cause the peak cylinder pressures to start early (before TDC) and to levels up to 15% higher than normal, for example: reaching 11 to 15 tons in an O-470 engine.
Variations in the cycles of an engine (power requirements and variation in the fuel air charge) can move the engine in and out of the detonation range. Detonation increases cylinder peak pressures and piston surface temperatures in the area where detonation is taking place. If left unchecked this condition results in preignition and engine damage eventually.
Preignition is self igniting of the fuel air charge at the moment it enters the hot combustion chamber before the spark plugs are able to their job. The result is more heat and higher pressures leading to more detonation and a possible runaway condition.
In the initial stages of detonation the EGT will decrease and the CHT will increase. Without the proper engine gauges this will most likely not be noticed by the pilot. It indicates that heat energy is transferred to susceptible parts of the engine.
The excessive pressures caused by detonation will cause cracks at the spark plug hole, injector port, broken piston rings and burnt exhaust valves. Resulting in expensive repairs for the owner of the aircraft.
The burn rate of the end gas during detonation is some 5 to 25 times faster than the normal burn rate, one could even call detonations tiny explosions with a pressure wave of about 5000 Hz. This can be heard as a knocking sound. This can easily be heard in a car but in the aircraft it is almost impossible to detect knocking by just listening.
The high energy pressure wave increases the heat transfer at the top of the piston and at the exhaust valve causing damage as localized melting, softening and erosion of the piston. In extreme cases a hole could be melted through the piston.
Gas flow over hot internal parts of the engine can and eventually will form carbon deposits on the top of the piston, spark plugs and exhaust valves. These deposits heat up more easily than the cylinder walls or pistons thereby introducing the possibility of preignition. The remedy is to lean the engine during idle, taxi and cruise so that the combustion chamber remains clean.