What is "detonation" in a piston engine?

Question

Recently, I saw this question, which included a rather alarming quote:

Full throttle operation with carburetor heat on can cause detonation.

Woah! That doesn't sound good. In fact, I would have figured aviation engine manufacturers would have worked hard to prevent their engines from exploding, and possibly destroying the rest of the aircraft and/or killing the pilot in the process.

I also found this question, with a rather calm quote:

What if I have a noise cancelling headset on, and the engine undergoes detonation?

It seems like there should be other cues that your engine just blew itself to bits...

I'm figuring that perhaps "detonation" in this context doesn't actually mean that your engine block turns into shrapnel. I also figure that it doesn't refer to the normal process of piston engines, where fuel is injected and mixed with fuel before detonating. So what does it mean?

Answer 1

Detonation, as the name suggests, is an explosion of the fuel-air mixture inside the cylinder. Under normal operation, the spark plugs each ignite a point in the fuel/air charge, which then propagate through the cylinder and provides a consistent, regulated burn in a process called deflagration. This extends the time that the burning fuel pushes on the cylinder, providing a gentle power stroke.

During detonation, the fuel/air charge (or pockets within the charge) explodes rather than burning smoothly. Because of this explosion, the charge exerts a much higher force on the piston and cylinder, leading to increased noise, vibration, and cylinder head temperatures. The violence of detonation also causes a reduction in power. Mild detonation may increase engine wear, though some engines can operate with mild detonation regularly. However, severe detonation can cause engine failure in minutes. Because of the noise that it makes, detonation is called "engine knock" or "pinging" in cars.

^{(Source: FAA PHAK, page 6-18)}

Detonation can be caused by a number of factors, but generally they relate to a situation outside of the engine's design. Using the wrong fuel can cause detonation: the octane rating of a fuel is basically the amount of pressure it can take before it detonates. Similarly, high cylinder temperatures can cause detonation(1). This is why mixture is set very rich when at high power levels: the excess fuel cools the cylinders, preventing detonation. As noted above, full power with carburetor heat on can cause detonation, as the increased temperature mimics the effect of high cylinder temperatures. Full carburetor heat is required to provide at least a 90 degree F increase in intake air temperature.

Further reading:

• Page 6-19 of the Pilot's Handbook of Aeronautical Knowledge.
• An article at Savvy Analysis

1) Detonation is distinct from pre-ignition, where the fuel/air charge is ignited by something other than the spark plug. Notably, detonation and pre-ignition often cause each other, as a detonating engine will have a very high CHT (potentially enough to cause pre-ignition), and if the charge pre-ignites as the cylinder is still rising, the continued compression may cause the charge to detonate.

Answer 2

Detonation is the instaneous combustion that occur due to unburnt gases left in the combustion chamber resulting in high temperature, sufficient for self-ignition

Answer 3

I'd say, sorry, can't retrieve the reference now, that: 'detonation', is not the same as: 'knocking', even if you consider: 'knocking', as the audible expression of a detonation, or preignition or other forms of undesirable combustion inside an Internal Combustion Piston Engine, be the piston alternative or a rotating rotor.

Toyota tested successfully, 7% fuel economy improvement, a cleaner exhaust, in a Wankel Rotary Engine, substituting a glow-plug in the leading site for the ordinary spark plug. Another concept cited here is the presence of two plugs in aviation engines, my idea was that the purpose of a double plug and double ignition system, e.g. two magnetos, is just a redundancy for safety reasons, having a replacement ignition system in the case of failure of one, but the rate and pace of combustion, or flame front speed inside the working chamber of an ICE is also influenced by having one or more plugs, by the presence of hot spots, swirl or quenching inside the combustion chamber, and by the shape of combustion chamber and valve mechanism, if there's any, but the issue is complex.

The books by Harry Ricardo: 'The high-speed internal combustion engine', a classic of ICEs, deal with this and other combustion issues. *link to download the 1931 edition: http://www.mediafire.com/download/gn1o0vyvzaz/The+High+Speed+Internal+Combustion+Engine+Ricardo+1931.pdf

Also available was: 'Aircraft Propulsion', by C. Fayette Taylor, Smithsonian Annals of Flight, 1971. Hope it's useful for you. Regards. Salut †