EFI Algorithms

[ducharme at orrqms2.psf.lmco.com]

Peter Fenske wrote:


> Second RPM.
> In the past I used the input capture to give me the time value between
> two ignition pulses. A division of the highest RPM about 400 rpm 
> increment at 6400 rpm in timer value would give me a table of
> 15 rpm increments plus if you use idiv a fraction is also created.
> My four point interpolation would use map fraction and rpm fraction
> to smooth the values in the lookup table.
> What I wish to do is use a 25 rpm increment at 6400 rpm to create 
> a rpm table that runs in 25 rpm increments between 25 and 6375 rpm.
> Since I am currently working on idle control I need the finer resolution.
> Also I have decided to split the look up tables into a lower load
> lower rpm table and a higher load higher rpm tables. The lookup indexs
> for the low load rpm table would be finer increments. This would
> make low rpm operation much better.


Unless I'm missing something, if you use input capture on a 68HC11, your
resolution should improve as the speed decreases (period increases)

         RPM =   Constant/ pulse_period

         dRPM ~  Constant/ (pulse_period^2)

At low speeds, minor changes in speed equal larger incremental changes in the
period.

My personal experience with input capture was less than satisfying, but it does
work, although you have to keep track of overflows if your period exceeds 
2^16 E cycles.


Cliff Ducharme

P.S.  I did finish my EFI B&S, and at the ripe old age of 44 have my BSEE.
(If anyone is interested, I'll put my project paper out on the Web.)