How is the fuel consumption calculation supposed to work?

[Ricky318i]

How is the fuel consumption calculation supposed to work?

When my car (535i '91) is at idle, and I reset the average fuel consumption consumption calculation on the OBC, it reads a max value of >30l/100km for the entire time the car is at standstill. Of course this will affect the average immensely and give you a false average calculation. When sitting in traffic, I can see the average climb and climb.

It it supposed to work like that, or is something wrong? All my other BMW's I've owned/own does not do this.

 

[Lizzard]

That is right, mine does the same. When standing still in traffic and reset the average fuel consumption the indicator will initially just show "--" until you move like a car or more then it will be stuck at 30+ until you are driving again (normal, not sitting in traffic bumper to bumper) and go lower. Yes this indicator is a problem as it reflect that these cars are monsters on idle but that is not the case.

The average fuel consumption is suppose to be taken over a period of 100km minimum to show some real value, see as some of the guys reset this fuel consumption when cruising on the highway it will show for instance 5 or 6 and not really move from that until you hit the traffic in the urban areas again and then slowly creep up to where it will stabilize. This is why you should only take the reading after doing a 100km in mixed traffic to indicate true fuel consumption. Unless you want to see what you get separately in open road and urban areas then this method will work by resetting it every time.

But sitting in bumper traffic those are the figures you will see.

 

[Ricky318i]

I can tell you how I'd like it to work: like any other BMW where on idle (or speed <20km/h) it reads 0.

From the wiring diagrams i can see it comes from the DME where the value is calculated from the injector signals. But i would like to know how it knows to calculate when vehicle speed = 0kmh?

 

[Lizzard]

That is a good question and hope someone technical will be able to answer that one, but your fuel can't be 0 on idle.

 

[Ricky318i]

That is a good question and hope someone technical will be able to answer that one, but your fuel can't be 0 on idle.

Hi, technically it is not 0, it would be a value x litres/hour. This I've seen in VW's.

In my E90, I can reset the average in traffic and it will only read fuel consumption when moving above ~20km/h.

 

[Woodies]

Its an average? If it didn't measure consumption on idle it would be incorrect. Consumption = distance covered divided by fuel used. If you don't cover any distance (or very little) you will get some super crap consumption numbers.

 

[Dewald Basson]

If the engine is on fuel is being used, if you travel at 100km/h or 0km/h you are using fuel. Actual speed only influences the amount used(relates to engine speed), but consumption is l/km not speed/liter - higher engine speeds use more fuel cause there is more revolutions per min so more fuel gets used per min.

If you let the car idle all day and night it could get an consumption figure of 60l/0km.

 

[Lizzard]

Never thought of that, thanks Dewald :=):

 

[jas.vanwyk.7]

If I understand you right , then here is my explanation. Your car measure the amount of fuel used only when it's moving, even if your engine is idling out of gear and freewheeling down a hill. It will indicate the amount of fuel used during the distance traveled. Therefor the calculation is done with fuel used during a distance traveled. If you stand still and engine is idling, no distance was traveled and the calculation is 0. It is different with aircrafts as they calculate liters/ hour. It is calculate between time and fuel used. There is instruments available to calculate l/ hr that you can use the n a car. Go to MGL aviation and you can buy a accurate instrument that you can fit in your car, and it is not that expensive .


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