Ignition module

[Peter Orban]

...
> Knock detection.
> I'm interested in hearing more on this. The recent discussions on spark
...
> My understanding of 'knock sensors' is that they rely on the resonant
> frequency characteristics of the block. How make specific is this ?
...

I have no hands on experience whith these, but I can tell how VW developed
their knock sensing DGIFANT EFI. They used an instrumented engine block
for the development. In the combustion chamber they installed a piezo
pressure transducer to verify the knock condition. They also installed a
piezo accelerometer on the side of the block, that is the sensor for the
production version. For finding the placement of this sensor they
used holographic photo techniques to find the vibrational node points on the
block. That was the place where they put the sensor. When the engine was
knocking, they could identify that easily from the time domain pressure
signals, just by visual analysis. They did FFT analysis of the accelerometer
signal and correlated that signal with the fact if the engine was knocking or
not. When the engine was knocking, they got peaks at 7.5KHz in the frequency
domain. On the production version that filtering is done by an analog
electronic filter. The event is also windowed, the output of the filter
monitored only at ignition time.

I would imagine, that the 7.5 KHz would be the resonant freaquency of the
block with everything on it, and knocking would give the exitation of
the system. Would be interesting to hook up a DSO to the output of a
knock sensor on a VW, and hit the block with something, and analyze the
output to see if this is the case.

Peter
--
Peter Orban
National Research Council of Canada
Internet: peter.orban at nrc.ca