Knock Knock

[robert joseph dingli]

Bruce writes,
> 
> Here's a question I have always wondered about......
> 
> How do the microphinic knock sensors work?  Do they 
> put out a signal proportional to the "knock"?
> Are they narrow-band devices tuned to the frequency
> that the knock generates?
> 
> I have always wanted to know!

Knock sensors are basically little accelerometers.  They come in various
forms including broad and narrow band responses.  Usually an OEM application
will be tuned by placing various sensors of different resonant frequencies
at various places around the engine.  Incorrect sensor placement or choice
will still work but the signal to noise ratio is lessened.

The electronics required would include a notch frequency filter and a time
based filter which passed signals 10 - 80 deg ATDC.  After that it's an
even bigger job of detecting knock above the normal backround noise at
those freqencies (which varies with load, speed etc.).

The knock frequency of an engine is dependent on the cylinder dimensions
and the gas temperature.  It's usually around 5 - 8 kHz.

Robert

-- 
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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 344 7966                           (+613) 344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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