Russell Kent wrote:

Sorry, I didn't mean to imply that "pressure in the chamber causes detonation."  Prior to
the onset of detonation, which is a macro-scale effect within the combustion chamber, there
a micro-scale events that cause the combustion chamber pressures to dance all over the
place.  What GAMI's system does is to detect this pre-detonation instability, which occurs
as the engine approaches the regime where detonation occurs, and then adjust the engine
controls to back away from that regime.  The engine control adjustment might be to enrichen
the charge, reduce the boost, reduce the throttle, retard the timing, or turn on the
anti-detonation fluid pump :-)

Paul Lamar asked:

How dose GAMI do it? Saab measures the resistance across the spark
plug gap prior to firing the spark plug as I recall. The resistance in
the combustion chamber is going to be a function of the pressure
as air is an insulator. I suppose with a fast enough computer one
could detect high speed variations in  pressure or it could be
pure BS on GAMI's part. That  would not surprise me given their
track record. I mean who is going to replicate the test
set up and destroy an engine to prove them wrong.

When I last spoke with them (2 years ago),  I believe their Prism system was using a special
spark plug that had a small diameter port through the ceramic insulator.  There was a pressure
transducer at the end of the port, so they were directly measuring chamber pressure (albeit a
few microseconds delayed due to wave propagation through the port).  Dunno if they've changed
to the Saab system, although with all of the lead and oil fouling that goes on with aviation
engines (target market for Prism), measuring spark plug gap resistance seems like a recipe for
failure.

Russell Kent

Well perhaps. My guess is the resistance is so high for the air gap
that the deposit resistance's  are a small percentage of the total.

Paul Lamar
 
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