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So detonation is explained brilliantly in the answer of this question. In the accepted answer, NathanG goes on to say the following:

the octane rating of a fuel is basically the amount of pressure it can take before it detonates

From what I understand, the higher the number on the octane value, the more pressure the fuel can take, which is why it is used in engines with higher compression ratios.

Assuming octane works exactly the same way in aircraft piston engines as in cars, this has left me confused about a question I was faced with on a small test:

Which of the following situations may lead to detonation in an aircraft engine?

A: Running the engine at too rich a mixture setting

B: Running the engine at too lean a mixture setting

C: Using fuel of too high an octane rating

D: Using fuel of too low an octane rating

I thought the correct answers to this were B and D, yet the correct answers were apparently A and C. Can someone explain why this is the case? I'm just not seeing it.

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  • $\begingroup$ We are not seeing that either. This question is not about piston engines, it's more about how to find trustworthy information. $\endgroup$ – user3528438 Aug 16 '18 at 12:53
  • $\begingroup$ I think that 'detonation' as used in the question has a different meaning to the one you started off with. Taking your initial meaning, if detonation is not happening then the engine is not running. That (initial) meaning renders the question nonsensical, so the 'detonation' in the question must be talking about something else. Think about what 'back-firing' is. $\endgroup$ – simon at rcl Aug 16 '18 at 14:45
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    $\begingroup$ "if detonation is not happening then the engine is not running" - I think you're referring to combustion there. $\endgroup$ – hiigaran Aug 16 '18 at 14:58
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    $\begingroup$ You are correct, the answer is B and D. Another common error is warning signs that say "danger high octane fuel". They should read, "danger LOW octane fuel!"*. $\endgroup$ – jwzumwalt Aug 16 '18 at 16:48
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    $\begingroup$ So the examiner probably just made a mistake when marking, then? $\endgroup$ – hiigaran Aug 16 '18 at 16:51
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Yes, the writers of test questions are human too. It would appear B and D are the correct answers.

According to the Lycoming website, for pilots, the greatest danger for detonation in aircraft engines is too lean a mixture. Running rich at full throttle keeps cylinders cooler. Answer B is correct. Excessively lean mixtures can also be caused by clogged fuel injectors or air intake leaks.

Lower octane also increases risk of detonation because it's "autoignition" temperature is lower than higher octane fuel. Autoignition can happen without spark due to higher temperature and pressure. This throws the "timing" of piston power stroke and ignition off, causing a "ping" or "knock". Engine damage may result. Answer D is correct.

So running rich at high power settings, while producing slightly less peak power, greatly extends the life of your all important engine.

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  • $\begingroup$ Forgot to go back and accept this answer. If anyone is interested, the instructor who graded that test was fired after clipping a wing on a fence (and two aircraft on two previous occasions). $\endgroup$ – hiigaran Jun 14 at 11:21
  • $\begingroup$ Might be the eyes. Mine are not as good as they used to be. $\endgroup$ – Robert DiGiovanni Jun 14 at 23:48

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