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Chris, You are about to undertake a project of some
magnitude, regarding a knock sensor.
In concept the idea is quite simple, in execution it gets
quite complex.
1st if you do not tie it in to some automatic anti-knock
method (such as automatically reducing ignition advance, blowing the waste
gate, etc) that reacts very quickly, your human response to the warning and time
to take action - may be too late to avoid damage. Most systems respond by
reducing ignition advance (probably the fastest thing that can be
done).
2nd Separating out the specific frequency (out of all the
ongoing engine noise) that indicates a detonation can be done with a notch
filter providing (important provision) you know that specific knock signal
frequency and magnitude. That generally takes electronic circuits and
sometimes pretty advanced digital filters to separate out the signal from all
the engine noise.
If you are bound to pursue this - then the place to start
is probably with the knock sensor system on the Rx7/8. The "microphone"
you screw into the block would probably be a good place to start, but the rest
of the anit-knock system I would hesitate to take on. Even then,
the sound spectrum from a rotary in an automobile installation and that in
an aircraft may be considerably different - so the filter has to be "tuned" the
the noise environment it will function in.
Now the old brain can be a pretty sophisticate noise
filter and complex processing unit - I am told, but never tried, that hooking
up a knock sensor microphone to a relatively simply audio frequency type
amplifier - that with Practice, one can learn to distinguished the sound of a
knock. The problem with this approach is two fold as I see it - one, your
engine has to go into detonation in order to train the ear/brain (the very thing
you are trying to avoid) and two, you still have to be able to recognize it and
react quickly enough to prevent damage.
There are probably other steps more easily taken to reduce
likely hood of detonation. Like ensuring your static timing is accurately
set, boost levels (if using turbo/super chargers) are kept very modest, Engine
temperatures are kept within bounds, intercooler is used to cool intake air,
etc).
But, I certain wish you success and it would undoubtedly
be an valuable addition to the rotary community (especially those flying with
forced inductions systems) body of knowledge and tools.
Good Luck
Ed
Sent: Friday, March 04, 2011 9:43 AM
Subject: [FlyRotary] Knock sensor and indicator
Since I have had an issue with detonation, I am contemplating
adding a knock sensor. What do y'all know about this addition. I do
not believe Tracy's EM has a provision
for a reading, so, what might be a good route to take. The brief research
I have done shows different frequencies, types etc. What
would be correct for a rotary, both as to a sensor and something
to notify me in the cabin? Thanks. I am about to
replace ALL my apex seals and soft parts and hopefully finally lick this
issue....hopefully....did I mention
hopefully...determinably...persistent? (heck, I wasn't planning
anything else this weekend <g>)
Chris
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