[Diy_efi] Peak cylinder pressure timing

[Phil Lamovie]

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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