[Diy_efi] injector fireing

[Brian Dessent]

killroy wrote:

> I was wondering how the injectors are fired during
> intake. Is it a single pulse that varies in length, or
> a series of small pulses. Also dose any body have any
> info on how to construct an injector driver from
> discrete components. I want to build a efi setup run
> by a linux single board computer for a 12 hp b&s
> riding tractor engine. What would be the best method
> to use? would being only one cylinder cause any
> problems.

I'm no expert, I'm sure someone will jump in if I offer inaccuracies.

Yes, it's a single pulse of varying width/time.  The actual timing of
when the pulse occurs relative to the state of the intake valve is not
really all that important.  The problem is that (in a car application at
least) a wide dynamic range is needed from the injector -- the amount of
fuel needed at idle is much less than what's needed WOT near redline. 
If you were to size the injector such that it was only firing when the
intake valve was open, you would hardly be able to idle because the
required PW would be way too small.  So the injectors are sized to be at
max 80-90% duty cycle.  If the injector is open for 90% of the time,
then it hardly matters when that 10% of the time that it's off occurs,
relative to the intake valve.  Additionally, injecting onto a closed
valve is good since it cools the valve and helps vaporize some fuel that
might otherwise have puddled.

Anyway, for an injector driver all you really need is a transistor
switch.  A MOSFET is a good choice these days, compared to a BJT the
gate drive is much simpler.  You may want to look at the megasquirt
schematics to see what they're using and copy that, or look at one of
the hacked schematics of a gmecm.  Looks like megasquirt is using the
IRFIZ34N, rated at 55V, 21A, and the mc34151 gate drive chip.  Both
seems good choices for your project.  The FET has as internal body diode
which will act as snubber for the inductive kick each time the current
is interrupted.  Put a heat sink on the FET and it should be fine.

Brian

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