Ignition Controllers

[Tim Drury]

>Does anyone out there have any advice as to the best method to implement
>this ? Ideally what I would like to do is advance the timing to the onset
>of knock and then retard it slightly back into the safe zone. The
>controller will be used on a fuel injected 12A turbo rotary engine.

The first thing I would do (and I already did this), is find out what a knock
sensor really is.  Here's a starting point: a knock sensor is simply an
accelerometer tuned to high frequencies.  Knocking causes the engine
block to vibrate at high frequencies.  Which frequencies?  Where in the
block is the best place to measure?  Good questions.  

As far as where, I've heard the rule of thumb is to locate it fairly centrally.
In the case of large motors with lots of pistons, sometimes 2 knock sensors
are used.

At what frequencies?  It depends on the engine, which is why I didn't include
this feature in my system.  Engines knock and resonate at different frequencies.
You can look at all the technical papers you want (and this is a good idea
to learn how knock detection works) but unless you see one for your application,
the frequencies may be a little off.

Someone mentioned the Harris knock detection chip.  I've seen this (it was years
ago) and believe it could be tuned to detect knock in a narrow band.  It probably
is nothing more than a switched-capacitor bandpass filter, but this would be a
good start.

Without previous knowledge of what frequency to tune to, you'll have to experiment.
We've tried hooking a knock sensor up to a graphic equalizer with 10 bands.  We
didn't have any luck; I believe the bands were too broad.  If cost were no object,
I would install a knock sensor, hook its output to a spectrum analyzer tuned for
narrow bands between 5kHz and 20kHz.  Then advance the timing on the motor
_very_ slowly and look for spikes to occur.  This is _probably_ knock.  Be very 
careful though, too much knock for too long and you'll blow a hole through Mr. Piston.

The cheaper method would be to install the Harris chip or a narrow bandpass filter
to the output of the accelerometer and run the same test as above without the
spectrum analyzer.  Manually tune across the band using the filter and look for
higher voltages coming out of the filter (use an envelope detector).  This method
requires more time and more knocking from the motor.

And to be honest, I really don't have a clue what frequencies knocking occurs at.
5-20kHz is just a guess.  Read some SAE papers to get an idea.

-tim





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