Knock Detection

[Frederic Breitwieser]

>>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.

Most knock sensors on modern vehicles is nothing more than a piezoelectret
microphone attached to the engine block.  The ECM, connected the the
microphone (er, knock sensor) can just read the amplitude of the wave, or
if more sophisticated, read the frequency spectrum as well as the
amplitude.  Most ECMs that I know of (which is limited knowledge quite
honestly for me) measure just the amplitude.  Its common for folks with
headers or other items loosely attached to the block to experience "knock"
when there is in fact no knock.

In fact, I think it would be an excellent project to build a circuit that
has 16 a/d lines connected to 16 24d/b octave bandpass filters, which in
turn connect to the same knock sensor.  One can then use a microprocessor
with the 16 a/d lines built in (or added, doesn't matter) to read the
frequency spectrum of the knock sounds, and make necessary adjustments to
timing, fuel, spark voltage, or what have you.

For those of you in the music industry/field, the above idea is the basic
principle a vocoder is founded on.

>in a controlled manner (detonation??).  The first, it would seem, only
>occurs BTDC. the second, I would think, could occur at any point in the
>cycle.  Both are destructive though I've been told the detonation is
>the worst.

It generally occures BTDC because the fuel/vapor is being compressed, and
under the additional pressure, the heat from the heads or trouble area is
transferred to the mixture.  This is my understanding at least.  This is
why radiusing the edges insides of the heads as it reduces detonation.

Detonation is very distructive.  Makes holes in pistons, which in turn
become ash trays.


Frederic Breitwieser
Homebrew Automotive Mailing List
Bridgeport, CT 06606
http://www.geocities.com/MotorCity/Downs/4605/index.html
1989 AG Hummer 4-Door
1993 Supercharged Lincoln Continental
2000 Mid-Engine Sports Car <smile>

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