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Detonation in aircraft engines

You'll hear that one of the reasons AVGAS must be used is that it's higher octane rating is essential to stop detonation in aircraft engines. I've never really bought into this argument (that's not to say it's wrong!)

Aircraft engines aren't incredibly high performance engines in terms of specific power output. In terms of bhp/litre, they are compete on a par with garden lawnmowers (approx 30bhp/litre), with the highest compression ratio's that I have seen being approx 8:1.

Many other reciprocating piston engines these days operate perfectly well on 95RON pump fuel running compressions of 13.5:1.

So why is it that aircraft engines can only control detonation by using 100+ octane fuel, when they have such (relatively speaking) low compression ratios?

The highest compression GA engine I know of is 10.5:1, with the most common being 8.5:1, and really low 7:1. I have done instrumented detonation testing on an 8.5:1 Continental O-520. I did cause and measure detonation for brief periods, which was required to validate my measuring equipment.

To create measurable (though happily not damaging) detonation in this engine, I operated it as follows on several separate occasions: Oil and cylinder head temps at maximum (by baffling), RPM 2000 to 2100, MP full throttle, fuel used: 80/87 Avgas. My test conditions to demonstrate compliance were 2 inches of manifold pressure over square relative to the RPM selected, same high temps, and 91 AKI mogas. There was no detonation.

100LL is the default "certified" aviation gasoline, because some aircraft require it for octane, though other aircraft for other characteristics. The Avgas manufacturers just will no longer make two grades of Avgas for a tiny market. Back when I research such things, Avgas was 0.25% of gasoline production, with Mogas being all the rest. Small wonder Avgas is priced and distributed as a specialty product.

100LL is very certainly a compromise gasoline, but compromised high, to keep the big pistons flying. It has other differences which are less in the forefront, with volatility (and no seasonal change being a big one) compared to Mogas.

The 13:1 engines which operate well on 95AKI are probably liquid cooled? Liquid cooled engines operate with more stable temperatures in the combustion chambers, which is a big factor in preventing detonation. Air cooled aircraft engines are terrible for uneven temperatures in local areas, which can reduce detonation margins.

Like car engines, aircraft engines are best operated to prevent detonation by keeping the RPM faster, and power lower. This is why "over square" operation is discouraged or prohibited for most engines with constant speed propellers. Very simply, detonation takes time to happen. The fuel air charge will only self ignite if the conditions are right, and there is time. Time is directly related to RPM - faster RPM, less time between combustion strokes. Less time between combustion strokes, less time for detonation to occur. If you can normally ignite the fuel air charge before it can detonate, you still have normal combustion, ans all is well.

Detonation has two harmful elements: Very high combustion pressures at the wrong time - explosion going down meets piston coming up = lots of force. The other is that to origin of the detonation (original ignition point) could be at a place in the cylinder other than the spark plug, where it is intended. In such a case, the flame front can 'wash" across combustion chamber surfaces in a path other than that intended. When that happens, those surfaces can be subjected to flame much hotter than intended. A flame which moves out evenly, cools at the front as it goes, so it does not subject the surfaces it contacts to the most intense temps. A washing flame can have a very different flame front, and local areas can get really hot. Extra hot, and early extra pressure can blow holes in pistons.

Interestingly, while I was detonation testing the 520, I found that having achieved detonation speeding up the engine (fine the prop) and pulling the power back quickly did not immediately end the detonation, it continued out of my control until the engine cooled a little.

Do not operate aircraft which are not approved so, on Mogas. Detonation is only one reason. Vapour lock due to the wrong volatility is a much greater risk. It too, can be mitigated, but if it goes bad, it'll be at the very worst time, and the consequences will be total.

Home runway, in central Ontario, Canada

Interestingly, while I was detonation testing the 520, I found that having achieved detonation speeding up the engine (fine the prop) and pulling the power back quickly did not immediately end the detonation, it continued out of my control until the engine cooled a little.

Yes, very interesting about the continued effect of overheating on detonation...

why is it that aircraft engines can only control detonation by using 100+ octane fuel, when they have such (relatively speaking) low compression ratios?

Detonation is encouraged by all of the following factors:

  • High compression ratio
  • Large bore cylinders
  • Simplicity - 2V per cylinder
  • Potential full power operation with one dead mag/plug
  • Supercharging
  • Low engine speed
  • Air cooling

All those factors (in addition to CR) require higher octane fuel to prevent detonation but also produce engines which are some combination of lighter, simpler, and/or more efficient. That's why the fuel was developed and why engines were designed around it.

That said, many of the simpler lower powered aviation engines (virtually everything 150 HP and below) were developed for 80/87 octane fuel and accordingly don't need 100 octane or even close.

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