How is MPG calculated by trip computer

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bob - 08 Jul 2005 04:04 GMT

I've always wondered how the little trip computer (displays instant mpg,
distance to empty, average mpg, etc.) monitors fuel flow. Doesn't seem the
fuel gauge signal would give enough resolution to give an instant mpg
reading but is that what they use or is there a fuel flow measurement
somewhere?

If they use the injector signal now, how did they do it with a carb as I'm
pretty sure I remember seeing these things back in the 70's.

Just curious,
Thanks,
Bobby

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sdlomi2 - 08 Jul 2005 10:33 GMT

>....

somewhere? >snip>

> If they use the injector signal now, how did they do it with a carb as I'm
> pretty sure I remember seeing these things back in the >70's.

Bob, in the late 70's I used a driving computer from Sears, & its
installation revealed some possible answers to your question. The computer
used a flow-meter which was installed into the tube, which had to be cut,
between the fuel pump and the carb. It had vanes which spun as the fuel
flowed. The more fuel that flowed, the greater the NUMBER of spins. The
faster the flow, the greater the RATE of spin. Both these figures were
monitored.
An interesting feature was built into the system. By the operator
entering actual fuel consumption at each fill-up, it compared actual volume
to the volume reported by the meter, & resulted in a self-correcting factor
which gave a more nearly accurate figure for the next cycle, or fill-up. By
so doing, its reported volume approached true volume as more
cycle-differences were recorded.
It was quite a sophisticated system for the times, and its accuracy was
based on the accuracy of that flow meter. s

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bob - 08 Jul 2005 13:59 GMT

> >....
>
[quoted text clipped - 17 lines]
> It was quite a sophisticated system for the times, and its accuracy was
> based on the accuracy of that flow meter. s

Use of a flow meter makes perfect sense. I have just never seen one so
wondered how it worked. I had an 87 Cougar (fuel injected) with such a
computer and it was very accurate. I didn't see any flow meter in the fuel
rail but injectors are probably fairly accurate flow controllers so their
duty cycle could probably be a good approximation unless they leak. I put
160K miles on it and average MPG per tank was always withing .2 or so of
calculated. That can be accounted for by how full you fill the tank each
time.

I have never seen a kit either but would like to add this feature to an old
car I have with a carb. With gas prices where they are, I'd like to
experiment to see what gave the best results and such a computer would be
helpful.

Thanks for the response.
bb

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Jim Vatunz - 08 Jul 2005 10:55 GMT

>I've always wondered how the little trip computer (displays instant mpg,
>distance to empty, average mpg, etc.) monitors fuel flow. Doesn't seem the
[quoted text clipped - 8 lines]
>Thanks,
>Bobby

Now it all makes sense to me. They used to have quite accurate fuel
flow meters put into the fuel line but i think they've been
unavailable for years. If they use the injector cycle timing it makes
sense.

--
http://members.iinet.net.au/~farmerjim/log/log.html
It's boring but it's something.

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Don Stauffer - 08 Jul 2005 14:36 GMT

> I've always wondered how the little trip computer (displays instant mpg,
> distance to empty, average mpg, etc.) monitors fuel flow. Doesn't seem the
[quoted text clipped - 8 lines]
> Thanks,
> Bobby

They do not have much resolution, so the bandwidth is low. That is,
instantaneous mpg is not really instantaneous, but then why do you need
a truly high bandwidth reading of mpg anyway. For older ones with
carbureted engines, one model I saw had a liquid flowmeter. Of course,
with a float chamber this acts as quite a low pass filter also, so again
the reading was averaged over a fair period.

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