From:
http://www.worldcarfans.com/2070322.001/new-bmw-m3-v8-engine-in-detail
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An outstanding highlight in engine management: ion flow technology
A particular highlight in engine management is ion flow technology
detecting any knocking in the engine as well as the risk of misfiring or
miscombustion. Contrary to conventional processes and technologies, this
function is now performed directly where it counts, that is right there
in the combustion chamber itself. To provide this highly efficient
control, each cylinder is monitored and controlled via the spark plug to
determine any knocking tendency. At the same time the system checks the
ignition for smooth and correct operation, and recognises any misfiring.
The spark plug therefore serves as an actuator for the ignition and as a
sensor observing the combustion process, distinguishing in this way
between miscombustion and misfiring. And through this double function
performed by the spark plug, diagnostic requirements in maintaining and
servicing the engine are also facilitated.
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Previously, i think the methods to detect knock have been piezo
detectors on cylinder/rotor head that listen to 6000-8000 Hz knock
signature, EGT/CHT trend monitoring, and pressure sensors routed to
chamber eg. thru spark plug body.
This new method seems to be quite reliable and can be implemented easily
to ECU boxes that have ignition control.
This leads to think..
How apex seals will hold together when engine is on dyno and repeatedly
run against knock limit - to create clear figure where the limits
(schlimits!) are....
If seal has got cracks, it is likely that it's resonant frequency will
change - just like it happens with eg. wine glass..
Testing might be done - without engine breakdown - by fabricating a
probe that attach to spark plug hole and contact seal when it is rotated
on position. Then probe will test it's integrity by exciting it with
sound sweep, and monitoring response when sweeping through it's normal
resonant freq.
Small surface cracks don't necessarily create big difference, but
depending on which mode the seal is vibrating the change in part
stiffness might be detectable.
Cheers,
Petri
Saab was fooling around with this quite a few years ago as I recall.
Also you want to monitor the intake air temperature and keep it below
about 140 F. When the intake air/fuel mixture is compressed 10 to 1 by the engine
and or a turbo charger the temperature rises. It can self ignite before top
dead center by the increase in heat due to the compression. The piezo knock
detectors are after the fact.
One should be looking for a knock or self ignition predictor. The intake
temperature is the best predictor I have found so far. This was known prior
to WW II and P38 pilots were trained to monitor the intake air temp.
Paul Lamar
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