Detonation can and will cause serious engine damage and is the limiting factor in developing engine power. 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.
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 a 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.
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.
EGT and CHT
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.
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.
If engine can be operated in such a way that detonation and knocking are avoided, then the contrary is true also. In fact, each engine must be able to operate to its rated power in the intended conditions without detonation as required by FAR 33. This test requires an engine to operate without detonation with a 12% leaner mixture from full rich at 100% power, max CHT and at a hot standard day (ISA).
When leaning aircraft engines at or near peak EGT the engine is most susceptible to detonation. Operating at maximum of 100°F LOP or on the ROP side will normally not cause the engine to detonate, the margins are greater there.
This detonation margin at peak EGT is caused by the amount of turbulence in the combustion chamber (swirl) by the intake and compression stroke. This results in atomization and mixing of the fuel air charge. High domed pistons, high pressure atomizing fuel injectors, turbulence by the intake valve will all produce more turbulence and thus a wider margin of detonation.
All these above factors have led engine manufacturers to recommend leaning below 75% power, where the margins are even wider for detonation to occur. An engine running with a full authority digital engine control (FADEC) should not have these problems as FADEC controls leaning, fuel injection and igniting.
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 introducing the possibility of preignition. The remedy is to lean the engine during idle, taxi and cruise so that the combustion chamber remains clean.
Indicate increased surface temperatures in the combustion chamber thus increasing the change of detonation.
Inlet air temperature
Especially with non intercooled turbo charged engines (Rotax 914), this will increase the end gas temperature and really contributes to detonation. On the other hand, non turbo charged engine can benefit from a little carb heat as this improves the fuel evaporation and so lowering the change of detonation. It is recommended that a carburetor temperature gauge is installed to keep the temperature at 10°F above ISA or 70°F whichever is less.
Advancing the spark timing increases the peak cylinder pressures and the possibility of detonation, as pressure increases the heat of the fuel air charge.
High humidity reduces detonation, as the water vapor cools the fuel air charge. Water injected engines are an example of this.
Fuel with a higher octane rating will reduce detonation. Having said that, if you use a low octane fuel and start using a high octane fuel without changing anything else, you will increase the safety margin before detonation will occur. By itself, using a higher octane fuel will not increase the power of an engine. If it does, the engine was likely to be detonating and reduced in power already.
Increasing the compression ratio increases the power of an engine. However, it also increases the pressure and combustion temperatures and thus the possibility of detonation. Thus the need for a higher octane fuel.