Knock Detection

[John Dammeyer]

At 12:15 PM 07/11/1997 -0500, you wrote:
>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.

And yet the TI document, 
(available at:http://www-s.ti.com/sc/psheets/spra039/spra039.pdf)
states that there is a window from 10 degrees After Top Dead Center (ATDC)
to 70 degrees ATDC at which detonation is most likely to occur and so
testing need only occur there.
>
>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.

And yet that still doesn't answer the question as to why such a small
advance in the ignition point of two degrees (82 microseconds at 4000 RPM)
will suddenly result in detonation.  8-(

Or more specifically, a light aircraft tuned for a particular octane fuel
refuels in another part of the countery with a load of slightly lower Octane
fuel, (marked as higher Octane),  and has pre-ignition or detonation on
takeoff with possibly disastrous results.  Yet setting the timing too
conservative so that knocking could never happen may put the aircraft in the
trees due to lack of power.

In either case I'll test for it from TDC to TDC+140 degrees which hopefully
covers the range.

Thanks for your input.

John



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