I wrote:

> I don't think that cars with digital "miles per gallon" displays
> have flow meters in their fuel rails.
>
> I mean, think about it... If YOU were designing a fuel-consumption
> display for a fuel-injected car, would you get the information by
> installing two expensive flow-meters, measuring flow to the
> injectors and subtracting the flow back FROM the injectors, or would
> you go for the software-only method of simply accumulating the
> widths of the injector pulses and multiplying that by the known flow
> rate of each injector?
>
> I know which method _I_ would use...

and Mike Smith <mikesmith_ozNOSP*M@relaymail.net> replied:

> I *know*  I'm arguing with an expert here,....

    Not really, Mike; I've only designed 75% of one engine management
    system (the client ran out of money before we finished).

> .... but consider some drawbacks...
> - dirty/ partially blocked injectors

    Good point... My only arguments to this one would be:

    1.  It's rare for an injector to get so badly clogged that flow
        through it is significantly reduced... Usually, the worst
        that happens is that the spray pattern degrades.

    2.  No one expects the fuel-consumption display to be a
        laboratory-accuracy device anyway... Even if one of your
        injectors loses HALF of its flow rate, the display will only
        be off by 6% (on an eight-cylinder engine); that's well
        within the tolerance that most people would expect.

> - offspec signal applied to injector/s (battery/alternator issue)

    All electronic engine-management systems of which I'm aware
    monitor the battery voltage to compensate for this.

> - not all emission-controlled cars are injected - some are
> conventional carburettors.

    True... And if you can find a carbed car with a digital
    fuel-consumption display, I'll buy a beer at the Embedded
    Systems Conference and MAIL it to you.

> For a mass-produced item like a car, is a flow-meter really
> expensive? (lets ignore rip-off spare parts prices, I'm thinking
> cost)

    I don't know... Although I suspect that flow-meters accurate
    enough to detect the difference between the flow to the
    injectors and the return flow back to the tank wouldn't be
    cheap.

    Note the following excerpt from my original message:

        "I don't think that cars with digital 'miles per gallon'
        displays have flow meters in their fuel rails"

    I didn't mean to imply that injector-pulse measurement was the
    ONLY way (or even necessarily the best way); I was just saying
    that I don't believe that EXISTING fuel-consumption displays use
    anything more complicated than that pulse-measurement... It
    doesn't seem reasonable to me that an auto manufacturer would
    burden its cars with the cost of ANY extra hardware just to
    provide a function as minor as a fuel-consumption display.

    The fact that fuel-consumption displays seemed to appear right
    around the same time that microprocessor-controlled fuel
    injection became commonly available, AND the fact that I've
    never seen a fuel-flow meter on an automobile engine, AND the
    fact that I know of no carbureted cars that provide a digital
    fuel-consumption display, would seem to bear out my hypothesis
    that the fuel-consumption "measurement" is handled indirectly by
    a software-only process that just looks at injector
    pulse-widths.

    Just my opinion, of course... I could be wrong.

    -Andy

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===
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=== Andrew Warren - fastfwd@ix.netcom.com
=== Fast Forward Engineering - Vista, California
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