pulse width / pulse granularity

[Greg Hermann]

>At 01:41 PM 5/28/00 +0000, Neil Bradley wrote:
>
>>* What is the shortest digital pulse that would ever be needed for an
>>  injector? Would 250 microseconds be long enough? I'm trying to nail down
>>  injector pulse length granularity so we can figure out our requirements
>>  for the "slave" pulse width generator.
>
>So far I saw a couple people address the minimum pulse width question,
>but I don't think I've seen anyone say anything about the granularity,
>IOW the smallest increment by which the pulse can be adjusted. It
>seems that most everyone agrees that 250usec is way shorter a pulse
>width than you're ever likely to need, but if your pulse width is
>only adjustable in 250usec steps, I think you'll find that way too
>coarse. (Ok, maybe not for a dedicated dragster, which need not idle
>politely or pass emissions.)  Consider that if your minimum pulse
>width is 1msec, and you'd like to be able to adjust fuel in 2% steps
>at idle, you need a 20usec resolution. (At least, probably a lot
>finer due to injector flow-vs-PW nonlinearity at low PW.)
>
>I recall 64usec and 16usec being tossed around, as step sizes used
>by some aftermarket FI box and some OE box respectively. Also I
>think someone (Ludis?) mentioned some OE boxes using an even finer
>resolution in software, though I was not clear if the hardware could
>realize that resolution or not.
>
>For a race car probably none of this matters. I just wanted to make
>sure that minimum pulse width was being addressed separately from
>granularity.
>
As I recall, Accel and Haltech use 64 us increments, GMecms use 16 us, and
MoTec, Autronics, and FelPro use 8 us. It IS indeed a significant thing.

Greg


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