Knock sensors

[James Boughton]

Tom Cloud wrote:

>I don't know if he's saying that those frequencies are the
>resonant frequencies of those blocks -- I cirrusly (a Cloud
>joke) doubt it  ... I'd think the mass of the 5.0 and 5.8 would
>make for a much lower resonance than 9.5k or even 6k.  Does anyone
>know?


It is currently believed that knock is more a relationship of the bore diameter
and the temperature of the burned gases in the combustion chamber as
opposed to the actual resonance frequency of the block itself.
>From Heywood's "Internal Combustion Engine Fundamentals":
	"[autoignition theory] holds that when the fuel-air mixture in the
end-gas region is compressed to sufficiently high pressures and temperatures,
the fuel oxidation process-starting with the preflame chemistry and ending with 
rapid energy release-can occur spontaneously in parts or all of the end-gas
region.....A shock wave propagates from the outer edge of this high-pressure
end-gas region across the chamber at supersonic velocity, and an expansion
wave propagates into the high-pressure region toward the near wall....The shock
wave and expansion wave reflect off the walls of the chamber, eventually producing
standing waves.  Usually these standing waves are due to transverse gas vibration
and are of substantial amplitude.  The amplitude of the pressure oscillations builds
up as the standing waves are established, and then decays as the gas motion is
damped out.  The frequency of the pressure oscillations (normally in the 5 to 10kHz
range) decreases with time as the initially finite-amplitude supersonic pressure waves
decay to smaller amplitude sound waves."

Jim Boughton
boughton at bignet.net