Engine detonation, backfiring, knock questions

LongRoadBob

LongRoadBob

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I am a student in ground school in Norway (am american, and learned norwegian at a late age, so I could be missing some nuance in the course book on this subject) and am having trouble understanding detonation specifically in an airplanes motor. I've also read about it int the Pilots Handbook of Aeronautical Knowledge, and Wikipedia, and other places but in all cases it's unclear to me yet how detonation starts, if detonation includes the "engine knock" or if that is separate or not, etc.

Couple of things, I have learned of several conditions that can result in detonation. I believe also that a lean mixture is one main cause (which causes other conditions like high cylinder temp) that contributes. Some aspects really puzzle me so far such as that it is stated that a rich mixture will help keep the engine cooler because "not all the fuel will burn off in combustion, and it will evaporate which will help cool down the cylinder" but not all the fuel burning also is a factor (as I understand it) in contributing to detonation, where it "burns too slowly" and in the next intake cycle, there is premature combustion.

I could write more, but it would just reveal how my thinking goes around and around on this.
Any tips or help on the theory behind this? Also backfiring is separate from detonation but often they happen on alternate cycles?

Any help appreciated.
 
There are a number of types of abnormal combustion events - some are triggered before the spark plug fires and some are triggered after the spark plug fires. It is somewhat complicated by the fact that different authors are not consistent in their use of the terminology.

But, in general, the term detonation is often associated with pre-ignition - the mixture starts burning before the sparkplug fires due to hot spots in the combustion chamber (soot particles, hot spark plug tip...) and can result in significant engine damage.

Knock is usually associated with the sudden combustion of the last of the fuel / air mixture in the cylinder well after the spark plug fires. This is usually less likely to result in engine damage. Causes would be an over advances spark timing for the current conditions, low octane fuel, overly lean mixture (which slows the burn rate and allows the conditions for knock to develop).

Backfiring would be caused by an ignition event while the intake valve is still open resulting in the mixture in the intake manifold burning. But, people also often refer to the combustion of un-burned fuel in the exhaust system (following an event such as a misfire (no combustion)) as a backfire even though it's not.
 
Thanks to both of you for the help! Just after I posted I did more extensive Googling and found a (I think) very helpful piece from the AOPA website on this:
http://www.aopa.org/News-and-Video/All-News/1998/July/1/Airframe-and-Powerplant-(5)

And as you mention, Capt. Thorpe, I did notice many were not consistent in terminology (the Wiki page segues to knock without stating if this is a separate category, etc.) and your description was very helpful too and consistent with what I have been understanding. Also I too always thought the misfire you mention WAS backfiring so that was good to clear up too.

Thanks James_Dean (never thought I'd say that in real life) for the link. I see he mentions a lot about turbos and detonation. Thanks!
 
Just about all of Deakin's articles are worth adding to your library
 
LongRoadBob said:
I am a student in ground school in Norway (am american, and learned norwegian at a late age, so I could be missing some nuance in the course book on this subject) and am having trouble understanding detonation specifically in an airplanes motor. I've also read about it int the Pilots Handbook of Aeronautical Knowledge, and Wikipedia, and other places but in all cases it's unclear to me yet how detonation starts, if detonation includes the "engine knock" or if that is separate or not, etc.

Couple of things, I have learned of several conditions that can result in detonation. I believe also that a lean mixture is one main cause (which causes other conditions like high cylinder temp) that contributes. Some aspects really puzzle me so far such as that it is stated that a rich mixture will help keep the engine cooler because "not all the fuel will burn off in combustion, and it will evaporate which will help cool down the cylinder" but not all the fuel burning also is a factor (as I understand it) in contributing to detonation, where it "burns too slowly" and in the next intake cycle, there is premature combustion.

I could write more, but it would just reveal how my thinking goes around and around on this.
Any tips or help on the theory behind this? Also backfiring is separate from detonation but often they happen on alternate cycles?

Any help appreciated.
Click to expand...
Just think of it as spontaneous combustion of the fuel air mass in the combustion chamber. rather than a smooth progression of the flame front. or in other words it all goes bang at once, causing a spike in pressure.
 
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