How do computers on bikes/cars work out fuel economy?

[monkey]

I guess they work out an equation based on revs, speed and time but do they use a constant value based on the size/power of the engine that's in the computers ROM?

Just curious really. My Daytona said I was doing about 44.7mpg from Woolwich way to West London at low revs (running bike in). That sound possible?
:)

[yorkie_chris]

They know the nominal flow rate of the injectors vs fuel pressure, and it also controls the pulses to the injectors. Simply speed/fuel flow gives it
Wether the computer is accurate or not is a different question.

[monkey]

Is it accurate or not? hE HE HE. You stalking me Chris?
:)

[OldBoy]

One way of measuring fuel consumption would be to measure the total 'on' time of the injectors for a second (fuel pressure is constant),
this will give the fuel used per second. with the number of pulses from the odometer pickup per second for distance (say four pulses per meter).
The instant MPG can be worked out, or totaled up for trip MPG.

Accuracy would depend on how stable the injector pressure is (assuming no lambda sensor) and tyre wear or make for wheel diameter.

[hovis]

see sig

[Toypop]

I have had a number of cars that give you a consumption read-out. I have measured them manually by brimming it at the pump and calculating the figure using the trip counter and then compared it to the computer readings.

On all those cars the computer was over optimistic and claimed that the car was doing around 4 to 6mpg better than reality.

With one car I even kept a spreadsheet for a year and it was very consistent over that time. Computer said 55mpg and the manual calculation said 51. In the winter the computer said 52 and the manual calculation said 47 etc.

[OldBoy]

I have had a number of cars that give you a consumption read-out. I have measured them manually by brimming it at the pump and calculating the figure using the trip counter and then compared it to the computer readings.

On all those cars the computer was over optimistic and claimed that the car was doing around 4 to 6mpg better than reality.

With one car I even kept a spreadsheet for a year and it was very consistent over that time. Computer said 55mpg and the manual calculation said 51. In the winter the computer said 52 and the manual calculation said 47 etc.

Perhaps the readings are conveniently rounded up or down or a 'fiddly factor' added to give a more favorable displayed MPG. Like all speedo's indicating about +10%.

If they were challenged about this they would probably say it was for "indication only".

[Biker_Billy]

Perhaps the readings are conveniently rounded up or down or a 'fiddly factor' added to give a more favorable displayed MPG. Like all speedo's indicating about +10%.

If they were challenged about this they would probably say it was for "indication only".

Yep, I'll agree with that. On a recent rideout with some of the Orgers down in Cornwall, when my bike didnt want to play, I was forced into the car, trying to follow the bikes...my car said I averaged 15mpg, when I filled up, it was more like 12ish...

[monkey]

Yep, I'll agree with that. On a recent rideout with some of the Orgers down in Cornwall, when my bike didnt want to play, I was forced into the car, trying to follow the bikes...my car said I averaged 15mpg, when I filled up, it was more like 12ish...

Flippin eck what were you in a Bentley?!

[Biker_Billy]

No, that wouldnt have cornered....was in the M3...mostly 1st, 2nd and 3rd (good for a ton)

[-Ralph-]

I don't know how these things work, but you'd think if it knows how much fuel you've used (it has a fuel gauge after all) and it knows how far you've travelled (it's got an odometer too), the MPG's a pretty easy calculation.

Not suggesting it uses the fuel gauge and odometer, but the ECU obviously has access to these kind of figures.

Also if the speedo overeads, the mileage will correspond. So the car thinks it's travelled further, so the MPG figure will be artificially high?

Nice thought actually - my pointy overreads by a full 10% so it's actually only done 10350, not 11500 miles :cool:

[muffles]

I thought only speedo's over-read, not odometers? Have heard that several times, though I'm not sure how they do it.

[Alpinestarhero]

see sig

I get about 50 mpg

Sweet!

Matt