Can anyone tell me how the meters that lots of cars have now, apparently displaying the instantaneous mpg, work?
Do they actually measure the rate of fuel supply to the engine, at a given instant, compare that figure with the road speed, and then calculate the mpg at that moment?
Or is it simply recording some other engine indicator, eg inlet manifold pressure, and re-calibrating this into a notional mpg?
Thanks,
Cliff Pope
|
They don't work accurately. They can't.
HJ
|
|
|
|
Come on HJ, don't leave it at that please, we are all agog.
|
|
On My SAAB it seems to sample fuel used, measured from the diesel metering, and then the distance travelled, over about 5 to 10 minutes. On the same basis it measures and displays 'Distance to Empty'. That figure and the mpg would obviously vary wildly if it was instantaneous. You would get 5 mpg when accelerating and 999mpg when going downhill with the throttle 'off'. To even out these fluctuations the calculation is down over some relatively short period of time. They aren't that accurate but they give you a clue (IMHO)
|
|
|
|
But they could work accurately, surely? It must be technically possible to have an in-line fuel meter, say at the pump, sending an instantaneous signal to a computer that was also reading the electric speedo, and then simply dividing one figure by the other and displaying the result?
That is what they purport to do, anyway.
Cliff
|
|
|
Exactly my thoughts Cliff, if the computer is determining to the nth degree how much fuel to inject on each compression stroke, or at least altering it every nth part of a second its just a simple matter of this figure against revs to get a total fuel figure consumed. Against miles travelled then to get a consumption.
Look at many modern cars, digital odometer as well as the engine management, OK so there are calibration errors in all the measurements but at least it should be better than guessimetric.
I understand that the real fuel economy experts like Stuart Bladon have a very accurate fuel meter installed in the cars when they are doing consumption runs, though this has to be calibrated in each installation.
|
|
|
on the current BMW 5 series, the odometer and on board computer are calibrated more accurately than the speedo. The speedo has the much discussed 10%+3mph inaccuracy which would give a false low fuel consumption reading and also a false high mileage figure, thus affecting warranty and service intervals (albeit by only a small margin).
To prove the point, when travelling at say an indicated 77mph, reset the "average speed" reading on the computer and it will display an actual speed of 70mph. I think it uses the more accurate odometer reading to calculate fuel consumption.
|
They work of manifold vaccum ie BMW ,years ago people fitted vaccum gauges now they are called economy gauges
Navigation systems that work of gps that give performance details can be accurate to +/-01% the one fitted to my Z3 is gps activated and is as accurate as can be ,soas to make the petrol gauge obselete
|
Andy
The BMW system uses a bit more than manifold vacuum. You can clearly see when the overun fuel cut-off comes in for example. I understood they (like many others) used the fuel flow data from the ECU combined with the speed/distance inputs to give the indication. Also some Vauxhalls shift to a galls/hr figure when stationary, so they are surely using fuel flow data. As for accuracy - well, most are not too bad - my saab seemed fairly close to a 'traditional' calculation.
Regards
john
|
|
|
|
|
i know the older ford ones (sierra for example) had a metering device in the fuel line and a seperate road speed sensor. how accurate they were i dunno.
|
|
|
|
one thing about the bmw economy guage. Years ago it was a vacuum guage, but nowadays its far more sophisticated and takes its readings from a variety of sensors around the engine.
|
|
|
