Knock Question

Robert Harris bob at bobthecomputerguy.com
Sat Mar 14 17:32:19 GMT 1998


Whilst re-reading my Heywood trying to figure out how a slower burning open
chamber is going to make more power without knocking to destruction, I ran
across something obscure I had forgotten about.

Seems that when "knock" occurs, a high intensity flash is observed and it is
accompanied by a sharp increase in ionization.  He then says that optical
probes and ionization detectors can be used to measure this.  In fact, the
spark plug itself.

Now my engine builder is a little reluctant to cut a hole through the
cooling jacket and put a quartz window in for an optical detector, and I am
rather reluctant to undertake a total redesign of my electronic ignition
system.  Which leaves sticking a probe into the exhaust and measuring the
amount of ionization.

Heywood while referring to the measurement of ionization, leaves the
methodology to do so as an exercise for the student.  So what I am asking
for help on is any references to how to measure the ionization - SAE papers,
Reference books not leaving it as an exercise for the student etc.  Circuit
Ideas for modifying the ignition would work. One idea that springs to mind
(I am absolutely positive it has been done before by someone!) is to use a
separate spark plug in the exhaust as an ionization detector.

The reason for this interest is that acoustic knock sensors do work.
However, to work well they must be closely tuned to the acoustics of a
particular engine and speed range and without an excellent processor can
give false reading (mainly knock when there ain't - fail-safe). If an
Ionization Detector could be built that resides in the exhaust or a modified
ignition, then it could be coupled to the engine management system
electronically and be virtually platform independent. Self tuning even.  No
muss fuss dyno time thingee.

Any thoughts, info or references would be appreciated. TIA





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