Knock Detection

[Gary Derian]

To my knowledge, when a spark ignites a compressed fuel air mixture, a flame
front moves across the chamber.  Behind the front is burned gases, In front
of the front is unburned gases.  The pressure is the same throughout the
chamber.  As the front progresses, the pressure rises.  If the flame front
gets to the end, everything is OK and the chamber is filled with very high
pressure gas which pushes the piston.  But if the temperature of the
unburned gases rises above the autoignition temperature of the fuel the
remaining unburned gases will spontaneously explode before the flame front
gets there.  This is detonation.  The explosion causes the combustion
chamber to "ring" like a piece of metal struck with a hammer.

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.

Pre-ignition is when something glowing in the combustion chamber ignites a
flame front before the spark.  This has an effect similar to too much
advance and can lead to detonation.

Gary Derian >gderian at cybergate.net>


-----Original Message-----
From: John Dammeyer <johnd at islandnet.com>
To: diy_efi at coulomb.eng.ohio-state.edu <diy_efi at coulomb.eng.ohio-state.edu>
Date: Wednesday, November 05, 1997 11:46 PM
Subject: Knock Detection


>Hi,
>
>I have a question regarding the detection of knocking from too great an
>ignition advance and too low an octane fuel.
>
>If I understand the process correctly if the ignition is advanced too far
>and the octane level is too low the fuel will burn unevenly creating uneven
>pressure wave fronts resulting in the pinging noise.  Backing off on the
>timing corrects this for a given RPM/Throttle Position.  The fundamental
>frequencies are 5KHz to 11Khz and the Knock Detection devices produce a
>voltage out based on the amount of Knock Noise detected.
>
>So here is the question.  For an over advanced engine does the knock only
>occur between the time of ignition and Top Dead Center or does it carry
over
>into the Power Stroke?  Note I am curretnly not interested in Engine knock
>due to lean fuel or othe rconditions.  Only knock that shows up when the
>ignition is advanced too far.
>
>>From the ECU perpesctive, I wish to automatically back off, say 5 degrees,
>when a cylinder starts pinging and then 8 revolutions later check it again
>before starting a gentle move back, a degree at a time, in the direction of
>maximum advance.
>
>Thanks,
>
>John
>
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