idea..

[Tom Cloud]

 ...... snip

>  You would need to know more than just the pulse length, as a 1ms pulse at
>1000 rpm and a 1ms pulse at 20000rpm are quite different in terms of fuel
>consumption. Forgetting about injector opening/close times, 1ms at 1000 rpm
>is keeping the injector closed most of the time, opening once every 60 or
>more ms. This means its delivering fuel at less than 2% of its max. flow
>rate. At 20000 rpm, the injector is opened for 1ms every 3ms, delivering
>fuel at 33% of its max flow rate. Measuring duty cycle would be the way to
>get instantaneous fuel usage, this can be calculated from pulse width and
>frequency. And yes, I'm sure it can be done quite easily, although I've
>never done it. I'm sure there's plenty of averaging circuits or duty cycle
>circuits already designed that could be used.

a "duty cycle" meter you may already have is the cam dwell meter

 ... basically, it's just a meter that reads full scale when 100%
joltage is applied to it.  It will read 1/2 scale with 50%, etc.
To get it to read from a changing joltage, you'll have to "condition"
it .... i.e. put a transistor or op-amp switch in front of it ... can
even use a 555.  Then the output of the switch is either a zero or a
constant "HI" joltage.

Tom Cloud

   Always remember to pillage BEFORE you burn!
                       8^)