I've fitted a new fuel injection system to the old car - the original was analogue and not too brilliant.
It's working well but has an interesting side effect. There is an OBC which calculates fuel consumption etc which is now wildly out. It worked by counting the time one injector was open. The new system uses PWM with feedback current limiting to drive the injectors which obviously confuses it.
Any ideas?
Is it reading well under on the fuel consumption? Response time of the OBC to detect the injector openness could be slow, and hence a bit too lazy if the the thing is PWM'ing...
Just a guess?
Yes. With the ECU set to two squirts per cylinder cycle (same as the original) it reads something like 1/2 the true consumption. Set to 4 squirts per cycle approx 2 times. So what I'm after is some way of converting the pulses into a signal it can recognise. After all the amount of petrol being injected will be the same.
Hmm, interesting. From your description, it sounds like you need something to convert PWM to a proportional voltage, then a second stage to essentially carry out voltage to frequency conversion (giving pulses of fixed width but varying frequency according to variation in the injector pulse width - a VCO)? Possibly with some sort of adjustment between so you can fine-tune things...
In the early 90s I'm sure there were circuits in the Maplin catalogue to do both the PWM->analogue and the VCO part, but of course the catalogues got less comprehensive over the years (and the one I'm remembering is unfortunately stuck in storage).
A simple RC filter would probably be good enough, just Google PWM to analogue - it'll only cost you pennies to try and it'll probably be good enough.
SteveW
The latter description of just counting pulses doesn't match the orginal description of measuring the duration of a pulse...
If it's just a pulse counter and assumes a fixed duration you just need to stretch the first pulse to the correct duration (and possibly ignore following ones for a period afterwards). Try a google on monostable. The good old 555 chip can do the first part but will retrigger straight away.
I *think* the original analogue system just measured the time the injector was open for. So just a variable width DC pulse. The fuel rail pressure is varied by the manifold depression so the fuel flow per unit of time open would be relatively constant.
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