idea..

[Clare Snyder]

>>>
>>>   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
>
>
>
Sum of pulse width over time gives lbs/hr or gal/hr consumption -
instantaneously (to whatever time period) Duty cycle will give the same
general information.
Sum of pulse width over distance gives MPG or L/100Km instantaneously (to
whatever distance unit) Using a cascade off of the speed sensor, counting
100 pulses, for instance, can calibrate easily for MPG.