Thank you for your explanation. I'm aware of the different types.
It was not me that asked any question.
I just clarified your error.
Not very sure anyone has managed to correct anybody here sadly, and none of the explanations given are very accurate.
Detonation refers to burning at speeds very much in excess of the normal flame-front speed inside the chamber. It causes very fast rises in pressure and temperature,
which can melt piston crowns and the result of a supersonic pressure wave resonance, which is of a frequency detectable by the human ear.
Pre-ignition simply refers to any hotspot from the previous engine combustion cycle being hot enough to act as the ignition point independantly of the
spark plug, although the spark plug is absolutely capable of being the source for pre-ignition if it overheats, i.e the hot electrode can cook off
the fuel before the spark jumps. Pre-ignition refers to the innopportune timing of the start of combustion, the flame front extending FROM this
pre-ignition MAY not spread forward quickly, like detonation waves, if it does depends on when it the cycle it occurs, detonation
does not occur outside of a very narrow range of crankshaft angles around TDC. This fact may be confirmed by simple
interrogation of a cylinder pressure signal, which will show oscillations from detonation only
in a narrow range at the peak cylinder pressuer zone. Pre-ignition can occur VERY much in advance of this area of crankshaft
angle, and also post-ignition can occur JUST after spark ignition takes place too. Pre and post ignition are just talking about the
timing of spontanious ignition, neither refers to a supersonic flamefront resonance.
However the two terms are so interlinked it is a source of huge confusion, as very often pre-ignition will cause detonation, as the
mixture being pre-ignited can raise the temperature of the mixture (because it can start the burn when the piston is maybe only half way up the
stroke, and so you end up compressing already buring gas, compressing any gas makes it hotter, so its not difficult to imagine what happens
when you compress a burning mixture - of course this always happens a bit in normal engine running, but its like excessive spark advance
if you like, in that too early raises the temmpreature and pressure TOO early, TOO much - although to make life complicated under normal range
of spark advance a BIT of advance lowers temperature as the gas is in the cylinder for longer before being expelled.).
This overheating can in a few engine cycles, heat up the whole cylinder so much that detonation waves occur, which is burning MUCH MUCH
faster than normal, which is what is audible.
However since with pre-ignition, knock very often follows, the two terms end up being used almost interchangably sadly, it is also possible
for knocking to occur on the other side of the chamber from the spark plug, even after the normal spark has already just occured, and for this to heat up
parts of the chamber enough such that in a few cycles its hot enough not just to cause spontanious ignition after the intended sparking
point, but to then cook off the mixture well BEFORE The intended spark point. Thus detonation (which is a supersonic pressure wave
resonance occuring after spontanious ignition) to cause spontainious ignition on subsequent cycles in such a way that it can
be called pre-ignition. This is still, just spontainious ignition before, instead of after the intended sparking point, and it is
only the supersonic audible wave from the VERY high burn speed of detonation which is actually audible.
Nobody makes "pre-ignition" sensors (although you can infer it from temperature sensors)...but these days everyone has a knock sensor.
Just to confuse things further, both knock and pre-ignition are spontaineous ignition, its just that knock happens to by spontanious
ignition which occurs at the point in the compression cycle where the conditions cause it to manifest in a high speed pressure resonance.
It is possible to have pre-ignition without knock (for a short while). Knock terms from the fact that the detonation wave
reaches the speed of sound inside the chamber, which at these temperatures is about 1 kilometer per second, which in
a typical chamber gives a resonance of a few kilohertz, which is well inside the range of human hearing, hence you can
"hear" detonation, audibly and it sounds like a hammer knocking.... hence... "knocking".
You cannot hear pre-ignition, as its simply the state of affairs (often prior to it later CAUSING detonation) when the mixture
starts to burn to early.
It is also possible to have knock occuring AFTER the normal ignition of the charge by the spark plug, but the detonation wave
occuring is still a "spontainous ignition", as it often begins some distance away from the spark plug, often near the hot exhaust valves,
so it cannot be said that it is possible to tie down the words knock OR pre-ignition to, or not to, the actions of the spark plug.
All that can really be said with certainly- simply - is that detonation is a supersonic wave, and that pre-ignition is burning
beginning when you do not wish it, which is just a way of saying spontainious ignition with a qualifier about if it
was before or after the intended ignition time. Post-ignition doesnt really exist as a term very much, as obviously it onlyhas a tinywindow
of time to occur in before the mixture is all burning anyway, but pre-ignition, post-ignition and knock are all just spontanious
ignition, its just that knock (which usually happens near TDC) is just the end result of it which results in the audible
supersonic pressure-wave, to get the rest you need to ask a chemist or physicist, but spontanious ignition does not
seem to result in supersonic speed burning and associated audible resonance ("knock"), at any point other than very
close to piston at the top of its travel.
As to the original question as to at which engine conditions it occurs, this is entrely dependant on the particular engine in
question, but broadly speaking - knock is often very bad at high load, low speed conditions - because boost is high,
but cylinder charge mixing will be of low fidelity due to the low speed of the piston and hence low speed if incoming
gas charge, which makes mixture formation generally poor, and hence promotes a lot of common combustion irregulatities,
typically locally un-atomised areas of fuel and so on. It is LOAD dependant not speed dependant, but if load
(i.e. boost) is fixed - it can be even worse at low engine speed (very very broadly speaking).