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Definition for detonation goes as spontaneous combustion of the end gas remaining in the cylinder chamber, and it always occurs after normal combustion is initiated by the spark plug.

The definition also says that severe heat and pressure causes the end gas in the chamber to spontaneously combusts.

From the textbook, we learned that the detonation is also caused by using lower grade of fuel and the lower fuel grade is less resistant to combustion compared to higher octane rated fuel(AVGAS80 vs 100LL). Also high manifold pressure is one of the cause to detonation.

If we used lower fuel grade, it will burn easier, so why do we have unburnt fuel/mixture in the cylinder that burns after initial combustion?

Secondly, how does the high manifold pressure induces the detonation?



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1 Answers

  1. Skyfox on Jun 02, 2016

    Normal combustion in an engine cylinder starts at the spark plug and spreads outward across the cylinder at a smooth, although very fast, rate. Detonation is when the entire fuel/air mixture instantaneously combusts, ie. explodes, instead of burning from one end to the other. This is hard on an engine because instead of the pressure inside the cylinder increasing smoothly to push the cylinder down, it instantly reaches maximum pressure and hammers the cylinder down. Compare it to one parent smoothly pushing their kid on a swing like normal, and another parent pushing their kid on a swing by smacking the kid hard in the back. It’s an unhappy time for all.

    When I used to work at an FBO and talked with a customer who worked in the petroleum/fuels industry, he told me that higher octane fuels resist burning and so burn more slowly (still a fraction of a second in a cylinder), making them resistant to dieseling (burning due only to high compression) which makes higher octane fuel required for higher compression engines, and also making high octane fuel resistant to detonation. That’s why we’re always told that if the fuel required for our airplane isn’t available, always go with the next higher fuel, never a lower grade.

    I’m not familiar enough with manifold pressure to confidently comment on how that affects detonation.

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