On 27 Sep 97 at 2:53, Andrew Warren wrote:

> 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).

That sucks.

>
> > .... 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.

Interesting.  Flow is normally tied fairly tightly with turbulence
effects, and dirty injectors would seem to increase turbulence.

>
>     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.

True, but if you use the gadget to work out how far you can go before
refuelling, 6% could be a damned long walk.  Also, if you have a
'don't care' attitude towards the tolerance, then when you tell the
computer that you've just put 30 litres in, the tank error will be
cumulative.

>
> > - 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.

OK

>
> > - 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.

I've got you here - the up market Holdens and Fords produced here all
had built in trip computers - before injection was used (say 12 or so
years ago)(models Ford Fairlane, LTD; Holden Statesman)  Ask some
Aussies for verification...
I trust the beer will be digitally signed and encrypted... <g>

>
> > 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.

Neither did I.  I was assuming the engine computer would provide it
as an aux output to the trip computer.



PS you've got some interesting music in your play list.
MikeS
<mikesmith_ozNOSP*M.relaymail.net>
(remove the you know what before replying)