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

[Bruce]

Your claim was that is was the domes of the pistons that were what were *the
big* problem.
I was just pointing out, that was just one concern.
Bruce





----- Original Message -----
From: "Bernd Felsche" <bernie at innovative.iinet.net.au>
To: <diy_efi at diy-efi.org>
Sent: Wednesday, July 10, 2002 10:11 PM
Subject: Re: [Diy_efi] Ion Sensing project-Strain Gauges & Basic goal?


> 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.
>
> --
> /"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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