Instant fuel economy reading

[Cgrigg]

Hey team

When driving normally I watch the instant economy meter move as expected, getting higher with acceleration and then dropping when easing off throttle. But I noticed today that when at a stop point such as traffic lights, the instant economy goes up to 30 ltrs per 100.

Is that normal? Surely idling at the lights could not be registering that economy.

Any help appreciated as always.

 

[Old.Tony]

It could be a glitch in the way it does the math. I've thought of writing the algorithm myself for this but can't be bothered writing the program that goes around it!

Basically it works like this: the ECU reports how much fuel it's injecting and how far you've travelled for that. A bit of mathematical magic and extrapolation ensues and it comes out with a figure that's close-ish.

It's the ish that ***** me to tears. Trouble is, when you're stopped, fuel still has to go to the motor to turn it over (and keep the aircon compressor and alternator running), but there's no distance being clocked up, so when you try dividing the litres consumed by zero distance you get that big stupid looking number 8 that's fallen on its side and turned all calculators into zombies. The program has no idea what to do with it, so an instantaneous figure goes out the window.

A smart programmer might then try to add a little "historical" data to it, so it accumulates data over the last few minutes and tries to find a balance betwen that and current (maybe like a weighted average).

The only thing I've learned is that you can't rely on the instant figure, just the long-term average, and even that is suspect because it's prone to issues like idling in traffic giving it bad data.

 

[BarneyB]

When is any engine at it's worst "economy" ?
(not to be confused with efficiency)

Pulling a trailer uphill at highway speeds?
Accelerating hard from a standing start?
Stop/start traffic jams?

None of the above.

As Tony said/typed...
It's whenever the vehicle is stationary with the engine running, because the distance travelled, per unit of fuel consumed, is zero.

 

[MANNING]

..the instant economy goes up to 30 ltrs per 100.
Is that normal?

Yours is doing excatly what mine does - so definetly acting normal
with the explanation from @Old.Tony & @BarneyB its makes sense why

 

[redkite]

I've driven a Volvo V40 that had a smarter approach: When idling, it didn't show liters/100km (or km/l), but liters/hour.

Which is really the only sensible thing to do.

 

[Old.Tony]

My D40 has the figure for litres per hour in the ECU, but I don't know if the trip information display can cater for this. If it's like most cars - and we have a 2022 vehicle now as well (not a Navara) - this isn't going to be available.

I use Torque to pull that info from the ECU. It's far more reliable, but there's no long-term average for that figure. I suppose I could retain it and graph it, but once I start approaching 1,000km on a tank of fuel I'm looking for a servo regardless, just to be on the safe side.