What ping/knock/detonation REALLY is
This is a repost from another thread, but I thought it might be useful is the list had a good idea of what ping/knock/detonation really is. -ch
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The most basic meaning of the word detonation is an expanding cobustion zone with a supersonic flame front. This is opposed to deflagration which is a relatively slow, controlled, and gentle combustion. The latter is what a spark-ignition internal combustion engine is supposed to do.
When a fuel-air mixture detonates, the combustion process happens much, much faster than normal which produces extremly high combustion chamber pressures. Also, the supersonic flame front creates shock waves which can shatter things like piston ring seals and other fragile components. This is also what produces the audible ping or knock assoicated with detonation. (It is a tiny sonic boom, just like a jet in the sky or the tips of a helicopter rotor.)
Detonation can be caused by several factors. These include spontaneous combustion or "pre-ignition" where the fuel-air mixture burns without a spark, and sometimes in several places at once. This is caused by abnormally high intake charge temperatures, very lean mixtures, or high compression ratios.
Another way detonation happens is when the spark fires too soon in the piston cycle (i.e., the spark is too "advanced" in timing). This causes the maximum cylinder pressure to occur as the piston is moving upward in the stroke. This again causes pressures that are too high, and the unburnt mixture will spontaneously ignite. As the flame fronts meet, detonation occurs. (For reference, under normal circumstances it takes about 30-50 degrees of crank angle for combustion to happen. During detonation, this can happen is only 5 degrees. BANG!)
You can think of detonation as bad like this: if a normal combustion process expends it's energy in 1/50th of a second, detonation does the same in 1/500th of a second. Think of it as a 150 man stepping on your toe with tennis shoes. Now try a 150 woman with high-heels. (That's a force/area problem, not exactly the same, but it's not a bad analog.)
Detonation can also be induced by "hot spots" caused by poor head/piston crown design or carbon buildup, although this usually causes issues like dieseling where your car keeps running a few seconds after you shut the key off.
It should be noted here that diesel engines are *designed* to work this way--they are a "compression-ignition" system that actually uses the high compression ratio of the combustion chamber (as high as 24-1, unlike your STi at something like 9.5-1) to ignite the mixture spontaneously. Diesel motors do not use spark plugs to ignite the mixture. Diesel combustion chambers are designed to promote ingition from a single location to avoid harmful detonation, but as anyone who has been beside a diesel motor before, it actually sounds a lot like your car when it's knocking.
I did a few searches for visualizations of combustion chamber detonation and found this site. It's not a bad write up:
http://www.stanford.edu/~bmoses/knock.html
This is why "even a little knock" under WOT can lead to cumulative and significant damage. It's also why it's imperative that Subaru fix the problem. Having your Geo Metro knock at 750 RPM is very different than WOT at 4500 RPM. If this problem is not fixed soon, I predict that we will begin to see a large number of complete engine failures.
Just my $0.02.
-ch
This is a repost from another thread, but I thought it might be useful is the list had a good idea of what ping/knock/detonation really is. -ch
----
The most basic meaning of the word detonation is an expanding cobustion zone with a supersonic flame front. This is opposed to deflagration which is a relatively slow, controlled, and gentle combustion. The latter is what a spark-ignition internal combustion engine is supposed to do.
When a fuel-air mixture detonates, the combustion process happens much, much faster than normal which produces extremly high combustion chamber pressures. Also, the supersonic flame front creates shock waves which can shatter things like piston ring seals and other fragile components. This is also what produces the audible ping or knock assoicated with detonation. (It is a tiny sonic boom, just like a jet in the sky or the tips of a helicopter rotor.)
Detonation can be caused by several factors. These include spontaneous combustion or "pre-ignition" where the fuel-air mixture burns without a spark, and sometimes in several places at once. This is caused by abnormally high intake charge temperatures, very lean mixtures, or high compression ratios.
Another way detonation happens is when the spark fires too soon in the piston cycle (i.e., the spark is too "advanced" in timing). This causes the maximum cylinder pressure to occur as the piston is moving upward in the stroke. This again causes pressures that are too high, and the unburnt mixture will spontaneously ignite. As the flame fronts meet, detonation occurs. (For reference, under normal circumstances it takes about 30-50 degrees of crank angle for combustion to happen. During detonation, this can happen is only 5 degrees. BANG!)
You can think of detonation as bad like this: if a normal combustion process expends it's energy in 1/50th of a second, detonation does the same in 1/500th of a second. Think of it as a 150 man stepping on your toe with tennis shoes. Now try a 150 woman with high-heels. (That's a force/area problem, not exactly the same, but it's not a bad analog.)
Detonation can also be induced by "hot spots" caused by poor head/piston crown design or carbon buildup, although this usually causes issues like dieseling where your car keeps running a few seconds after you shut the key off.
It should be noted here that diesel engines are *designed* to work this way--they are a "compression-ignition" system that actually uses the high compression ratio of the combustion chamber (as high as 24-1, unlike your STi at something like 9.5-1) to ignite the mixture spontaneously. Diesel motors do not use spark plugs to ignite the mixture. Diesel combustion chambers are designed to promote ingition from a single location to avoid harmful detonation, but as anyone who has been beside a diesel motor before, it actually sounds a lot like your car when it's knocking.
I did a few searches for visualizations of combustion chamber detonation and found this site. It's not a bad write up:
http://www.stanford.edu/~bmoses/knock.html
This is why "even a little knock" under WOT can lead to cumulative and significant damage. It's also why it's imperative that Subaru fix the problem. Having your Geo Metro knock at 750 RPM is very different than WOT at 4500 RPM. If this problem is not fixed soon, I predict that we will begin to see a large number of complete engine failures.
Just my $0.02.
-ch
