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Lycoming Aircraft Engine

Avoiding Engine Detonation

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.



Engine settings

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).

Lycoming Engine Leaning

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 in that region.

This detonation margin at peak EGT is caused by the amount of turbulence in the combustion chamber (aka swirl) caused 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 only, where the margins are even wider for detonation to occur. An engine operating with a full authority digital engine control (FADEC) should not have these problems as FADEC controls leaning, fuel injection and igniting.

High CHTs

EGT

Indicate increased surface temperatures in the combustion chamber thus increasing the change of detonation. By maintaining proper airflow during climb you will avoid this.

Inlet air temperature

Especially with non intercooled turbocharged engines (Rotax 914), this will increase the end gas temperature and really contributes to detonation. On the other hand, a non turbocharged 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.

Spark timing

Advancing the spark timing increases the peak cylinder pressures and the possibility of detonation, as pressure increases the heat of the fuel air charge.

Humidity

High humidity reduces detonation, as the water vapor cools the fuel air charge. Water injected engines are an example of this, with this system more power can be developed.

Fuel

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.

Compression ratio

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 and a more robust crankcase and other engine parts.


written by EAI.