idea..

[Brad Sheridan]

>>
>>   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.
>
>Thats not true.  In simplistic terms, (i.e. close to ideal...)  A
>injector has a nearly constant pressure drop across it, due to fuel
>pressure on one side ( assuming its constant), and manifold on the inside.
>( I'm neglecting flow differences due to vacuum..)  The injector will flow
>a certain amount of fuel per millisecond of open time, minus the varying
>flows during opening and closing of the injector,  but once it is open it
>will flow a constant rate of fuel.  All the ECU has to do is record the
>total "open time" of the injectors into a register, adn do a bit of math
>on it with the input from the distance sensor ( pulse wheel on the drive
>axle)

So are you saying that ALL you need is the pulse width and distance sensor
to determine instantaneous fuel consumption? If so you're wrong. The pulse
width, like I said, is not what determines rate of fuel consumption, but
rather the duty cycle(pulse width over period) of the injector is what is
the determining factor. If you sum the pulse widths you get nothing more
than the sum of the pulse widths. This is useless for determining fuel
usage unless you have some sort of time base for it. So you have to either
know the duty cycle of the injector over 1 cycle, or sum the injector on
times over a set time(the set time is key, using just the sum of the
injector on times over 1 cycle is useless, unless you know RPMs), either
method gives you basically the injector duty cycle. This will give you
instantaneous fuel usage(consumption), which is what the other post seemed
to be trying to get at from what I could tell. Your method would work for
fuel mileage though, as long as you're getting the distance during one full
injector on/off cycle.


Brad