DIY_EFI Digest V5 #718 - knock sensor

[Mark Agnello]

> > TVS wrote:
> > >
> > > I assume the frequency has some dependence on
> variables
> > > like bore diameter and combustion chamber shape,
> but is the knock detection
> > > circuitry likely to 'listen' for a wide range of
> frequency's or just a
> > > narrow band?
> >

These are just educated guesses, but I suspect they
apply.  Detonation is closely coupled to the pressure
wave in the combustion chamber.  The pressure wave is
an acoustic event.  As such, it should be influenced
by the geometry of the combustion chamber at the
instant of the event.  The dominant mode's wavelength
will probably be proportional to the cylinder bore.  

If the event timespan is too short to propagate
laterally, then the dominant mode will probably be
related to the combustion chamber height at the time
of the event.  Even if there is a lateral mode, I
wonder if the high frequency content in a knock signal
is due to the vertical mode(s)?

WRT the cylinder head and/or block structure
influencing the dominant frequencies, I agree there
will be some interaction.  Unlike the 10 Hz bell
analogy, I don't believe there will be a dominant
vibrational mode of the engine structure.  These
structures are too complex to have a primary mode and
I'll bet auto manufacturers spend a lot of time
designing engine structures that don't have
significant vibrational modes.  I know aircraft engine
manufacturers do this.  If they didn't, the life of
the engine mounted fuel pumps, actuators, sensors, and
accessories would be severely shortened.

What do y'all think?  Remember, I'm just a lonely old
EE that likes to read SF.;)  

Autospeed published an article based on an Aussie grad
student's research into engine knock earlier this
year.  Does anyone have a copy and does it shed light
on this issue?


=====
Mark Agnello
Washington Crossing, PA (USA)
88 Toyota Celica All Trac Turbo (restoration project)
92 Toyota Celica All Trac Turbo
96 Honda VFR
89 Honda Hawk GT (restoration project)

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