DIY_EFI Digest V1 #303

William Boulton boultonw at OntheNet.com.au
Tue Oct 8 03:34:27 GMT 1996



On 7 Oct 1996, Heneghan,Paul wrote:

> From: "Chaxel Frederic (QUALITE)" <chaxel at cran.u-nancy.fr>
> >I get some TOP signals from a captor. This information
> >is used to calculate the engine RPM and to calculate the
> >Ignition time (by the way of a table).
> >But of course I get the RPM at T time and used it for
> >ignition at T+DT time (1 engine round after).
> >
> >My solution is actually to use RPM(T-1) and RPM(T) to calculate 
> >RPM(T+1) :
> >  RPM(T+1)=RPM(T)-RPM(T-1)+RMP(T)=2*RMP(T)-RPM(T-1)
> >
> >It's a simple linear approximation (and simple to program
> >in assembly language).
> >
> >This give good results (much more better than using RPM(T+1)=
> >RPM(T))
> >
> >Maybe somebody know a better solution (Kalman Filter
> >for instance, or ...) ?
> 
> You could try RPM(t+1) = 3 x RPM(t) - 3 x RPM(t-1) + RPM(t-2)
> This will work for curves (quadratics only) and straight lines.
> If you need higher order (cubics etc.), let me know.
> The extra complexity might not be worth it though.
> 
> >Sorry for my dirty English !
> 
> Looks pretty good to me (you should see my French).
> 
> Paul
> paul.heneghan at bbc.co.uk
> 
> 

Hi folks,

Like to add 2cents to the debate.  The best way (IMHO) to achieve any
degree of accuracy with timing prediction is to run as many reference
points per revolution as possible. I've see the result of 1/rev on a twin
and that was a disaster. Engines just do not maintain consistent angular
velocity during a revolution. I developed a simulator some time ago to
test theory on this and found that good results are obtained with at least
6 reference points per rev. I tried a number of basic equations and
settled on a variation of the one Chaxel used. Just multiplied the result
by some large fraction just less than 1. Still easy in integer assembler.
The problem in predicting timing stems mostly from dA and not dV.

Paul, I'd be interested in how you arrived at your equation. RSVP

Bill Boulton.






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