[Diy_efi] Ion Sensing project-Strain Gauges & Basic goal?

[Bernd Felsche]

Bruce tapped away at the keyboard with:
> From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
> > Tom Sharpe tapped away at the keyboard with:
> > > Can we measure crank acceleration between flywheel teeth and determine
> > > peak pressure?????  TomS

> > No... you'll only get an average pressure. The very brief pressure
> > peaks are only an indication of the real problem.
> > The big danger with knock is the "stripping" of the insulating
> > boundary layer of gases from the surfaces of the combustion chamber
> > by the resonating gas flow. This exposes the surfaces to extreme
> > temperatures resulting in rapid erosion.

> So all that copper looking material on the bearings showing would
> be what?, a small problem?.

Not directly related to knock. Bearing design is probably marginal
in that case. Even hydrodynamic bearings "harden" as the clearance
reduces until the lubricant can no longer flow.  Knock would
exascerbate any problems pre-existing in a marginal bearing.

Increase oil flow and/or pressure, review viscoscity and/or
rotational speed.  Perhaps replace the bearing with a rolling
bearing if shaft speeds are insufficient to support the load.

> What you've mention is but one of the big problems, IMO.
> Bruce

Most of the excessive pressure due to knock is over in under a
millisecond. You might be able to use the change in angular velocity
to detect knock at high speeds (above 3000 rpm); if you can
discriminate between true readings and false-positives or worse;
false-negatives.

Fals-positives mean that you lose power because you're compensating
for non-existent knock. False negatives mean that knock may be
damaging the engine.

The pressure waves inside the combustion chamber will tend to be
more active across the bore because that is the lowest energy point.
Pressure is not uniform throughout the combustion chamber,
especially during knock.

There've been some trials with using load-cell washers under
cylinder head fasteners to determine the pressure peaks. (SAE
1999-01-0544) Cylinder pressures were able to be approximated with
the help of DSP and low-pass filtering.

Crank angle velocity reflect the stroke-wise forces on the piston.
If the pressure waves are sweeping across the surface of the piston,
then only a small component of that will be visible. Near TDC, which
is where knock is most likely, the crank is far less effective, so
the smaller change in forces is exerted on a small moment arm.

Add to the equation that _light_ knock is often acceptable to
maximise efficiency.

-- 
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