pulse width / pulse granularity

[Chris Conlon]

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.

   Hope this helps,
   Chris C.

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