[Diy_efi] Peak cylinder pressure timing

[Hugh Keir]

Phil wrote,

The Holy grail of all closed loop spark systems is a strain gauge on the
gearbox input shaft with a transmitter.

How would such a strain gauge help with ignition timing throughout the rev
range, going up and down hills, hot day / cold day when the engine consumes
less/more oxygen, 1st gear / 6th gear, etc, etc.

My feeling is that if it is possible to resolve cylinder pressure with
respect to crank position to output the area under the curve, it would be a
great start.

If it were possible to then go on to produce such a system as Bernd
describes where the electronics optimised the best cam, fuel and ignition
you would have a pretty spectacular ECU.

Step 1 may just be to try to build a system to see what can be done.

My initial concern was that there was a lack of stable piezo material
available, but the piezo manufacturer I spoke to can offer a material that
has a good response and a working temperature up to 250°C with an absolute
maximum for short periods of 360°C.

Hugh

----- Original Message -----
From: "Phil Lamovie" <phil at injec.com>
To: "List for general do-it-yourself EFI talk" <diy_efi at diy-efi.org>
Sent: Thursday, March 06, 2003 2:35 PM
Subject: Re: [Diy_efi] Peak cylinder pressure timing


>
> Steve wrote..
>
> > There's a thesis that's been posted before from a graduate student using
> > ion sensing to determine the peak pressure point, one of his
> > applications was to use that as feedback for ignition timing so that the
> > peak pressure was always at the "best" crank angle.  That wouldn't
> > require any fancy math, and would eliminate the "spark tables" current
> > used.
>
> I think the problem is way more complex than that.
>
> The peak pressure would always be detonation if the conditions
> were suitable. i.e. WOT and low to medium rpm.
>
> The average pressure is a very hard to understand concept
> for an algorithm. It does not necessarily follow that an increase
> in average pressure would accelerate faster. The degrees of crank
> rotation component is essential for making sense of the pressure.
>
> If the chosen spark timing gave a greater pressure earlier
> (in a rotational sense) and also had a higher peak (to be expected
> due to the smaller chamber volume during the event) it could also
> accelerate more slowly than a later but smaller event.
>
> Thus a "weighted bias" (of degrees of crank) is needed to filter
> the raw data. Each measured unit of pressure would be corrected
> to resolve the applied crankshaft torque. Given that any pressure
> before TDC would have a negative effect then this factor can indeed
> be negative.
>
> The Holy grail of all closed loop spark systems is a strain gauge
> on the gearbox input shaft with a transmitter. This "torque peaking"
> device is devoid of all cylinder pressure inputs and has no inbuilt
> tendency to drift towards detonation unless the peak torque occurs
> at that point, thus requiring an "advance limit table"
>
> phil
>
>
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