Knock Detection

[Gary Derian]

snip
>>
>>High octane fuel is hard to ignite and therefore resists this
autoignition.
>>Too much compression and/or too much spark advanve will cause detonation.
A
>>poorly shaped combustion chamber which takes too long to burn or low
>>tubulence or no quench area or other not easily adjustable factors also
>>affect the propensity to detonate.
>>
>
>Good description and very similar to others that I have read.  However, in
a
>brief instant the description does gloss over _how_ the extra spark advance
>will cause the detonation.
>
>Assuming the engine is turning 4000 RPM when maximum advance is reached
>then, as you stated the flame propogates nicely through the entire chamber.
>Advance the timing by, say, two degrees, and detonation results.  This
would
>imply, by your description, that if the ignition starts 82 microseconds
>earlier, (2 crankshaft degrees at 4000 RPM), the temperature rise would
>somehow increase and we'd pass the autoignition temperature of the fuel.
>Does this happen before TDC then or after TDC?
>
snip

According to the research done by Harry Ricardo, peak combustion pressure
occurs at 12 degrees after top dead center.  This was measured empirically
on engines with spark timing set for best torque.  I seems that this angle
holds true for all engines.  Therefore, detonation must occur sometime
between the spark and 12 deg ATDC.

My description of a flame front burning through a chamber was over
simplified.  There is actually a lot of turbulence which breaks up the flame
front into little pieces and physically moves them around the chamber.  This
turbulence decreases the combustion time.  At higher rpm, there is more
turbulence.  The net effect is than combustion time occupies nearly the same
crank rotation regardless of rpm.  The reason timing is advanced with rpm is
that there is a delay time between the initial spark and the growth of a
good flame front.  The delay time is nearly the same time regardless of rpm
so the spark is advanced with rpm so that the begining of the flame front
occurs at nearly the same crank position regardless of rpm.

Gary Derian <gderian at cybergate.net>