Fuel Economy

[Bob Edwards]

haha i think my fuel readout is broke its read 3.8 since i had it nearly 3yrs ago it did go up tp 7.3 once i dont push the buttons any more

 

[MrJoshua]

If I can get over 20mpg on a run, I’ll be
OK. My other car averages 15mpg per tank

 

[stevieb]

Just tested my e51 3.5 with 60k and getting 11-12 around town. Possibly lower cuz, 3-4 mile trips.

Sent off for obd so might check a few things, and get her a fresh fill of oil for the summer

 

[Simon58]

Had mine up to 10.7 the other day then 7.8 then the usual 4.8 to 5.4 . Not sure what's going on ? . It just came up like this while driving normally as I do , hasn't done it again , would be nice to get 10 or 7 but usually down in the 4 or 5s . New MAF sensor fitted the other week and I thought woopy but then came back to earth with a bang but still runs like a Swiss clock.

 

[Bob Edwards]

well all i can say is mine has been stuck at 3.8 since i bought it nearly three years ago i did reset and got 7.9 but now its back to 3.8 so i ignore it if i worried about fuel consumption i would not have driven delicas for 10years they were about the same and my Audi and my camper enjoy oh and im on a limited income

 

[Simon58]

well all i can say is mine has been stuck at 3.8 since i bought it nearly three years ago i did reset and got 7.9 but now its back to 3.8 so i ignore it if i worried about fuel consumption i would not have driven delicas for 10years they were about the same and my Audi and my camper enjoy oh and im on a limited income

I don't worry about fuel consumption, just wondering why the fuel screen jumped about like it did . I'm on gas so no worries about cost .

 

[JohnK]

I think you'll find the onboard computer gives you totally different readings on petrol than it will on lpg. Neither are particularly accurate.

 

[Ian H]

Indeed, I just use mine as a benchmark, not too concerned about the actual figure, but it is useful to know if you are doing better/worse than usual.

 

[George322]

Mine always seems to end up on 4.6 after about 5k miles. That's about 17mpg when I take into account that LPG is about 12% less efficient than petrol is. and thats what I'm getting when I work it out tankfull to tankfull of LPG.
That's the equivalent of about 36mpg and that's been fairly constant for the last four and a half years.

 

[dognero]

If you convoy behind @Madfish you can get this.
Motorway at 56

 

[Madfish]

If you convoy behind @Madfish you can get this.
Motorway at 56
View attachment 52570

Was a boring drive but we touched 30mpg that day

 

[dognero]

It had its fun moments.
And we looked so cool in a row

 

[Gwilo365]

I note a few LPG runners in here are saying their fuel consumption computer is showing mid 4s.....
To me that's counter-intuitive, as surely you're still covering distance, but not touching the petrol that you have in board a great deal, so surely this figure should be higher, as it's doing more on less?
Could someone please explain to my why it is going down instead?
Remember, man-brain. Make it simple.

 

[JohnK]

I note a few LPG runners in here are saying their fuel consumption computer is showing mid 4s.....
To me that's counter-intuitive, as surely you're still covering distance, but not touching the petrol that you have in board a great deal, so surely this figure should be higher, as it's doing more on less?
Could someone please explain to my why it is going down instead?
Remember, man-brain. Make it simple.

It's a long time since I checked my on-board computer. Just don't bother these days as its pretty meaningless anyway. The most accurate way to work out mpg is to completely fill tank and measure how far you go on that amount of fuel then do that over a number if fills to average out any inconsistencies. The computer works by measuring flow of fuel into the engine against distance covered. It works out how much fuel is going in based on the density of the liquid passing through and has nothing to do with how much fuel is in the tank. When running on lpg it will show a completely different reading as the density of lpg is different to petrol. I'm sure @Lpgc would give you a far more accurate reason than my simple brain can though!

 

[Gwilo365]

It's a long time since I checked my on-board computer. Just don't bother these days as its pretty meaningless anyway. The most accurate way to work out mpg is to completely fill tank and measure how far you go on that amount of fuel then do that over a number if fills to average out any inconsistencies. The computer works by measuring flow of fuel into the engine against distance covered. It works out how much fuel is going in based on the density of the liquid passing through and has nothing to do with how much fuel is in the tank. When running on lpg it will show a completely different reading as the density of lpg is different to petrol. I'm sure @Lpgc would give you a far more accurate reason than my simple brain can though!

Oh, I thought it was based on what was in the tank with a fuel level sensor or something, because after filling up, the minute you start up, the range is massively increased.
I check the on board for working out how far too my next fuel stop.

 

[Ian H]

Oh, I thought it was based on what was in the tank with a fuel level sensor or something, because after filling up, the minute you start up, the range is massively increased.
I check the on board for working out how far too my next fuel stop.

The range is based on fuel tank level and the average fuel economy. The fuel economy is based upon the amount of fuel the ECU thinks it is using. Some, if not all LPG conversions take the petrol injector pulse, add a few milliseconds to compensate for the extra fuel required and send it to the LPG injectors. This means that the economy figures won't correspond so well to the amount of LPG used, because the ECU thinks it is using a different amount of fuel to what it actually is, but they are still ok to use as a reference point if you don't worry about the actual number.

 

[Lpgc]

Available range is calculated litres of fuel in the petrol tank (according to the fuel gauge tank level sender, which itself isn't very accurate) multiplied by calculated average km per litre which also isn't usually very accurate.

Km per litre isn't actually measured, there is no fuel flow meter as such. The ECU calculates momentary fuel flow (in maybe litres per hour) by knowing how long it is pulsing petrol injectors for per 4 stroke cycle, multiplying this by rpm and dividing this by road speed. This calculation isn't very accurate because how much fuel flows per given length of injector pulse depends on manufacturing tolerances of injectors and on guestimated difference between fuel pressure and manifold pressure (where fuel pressure is affected by manufacturing tolerances of the fuel pump setup and where manifold pressure isn't directly measured but is a function of the MAF reading divided by the engine's volumetric efficiency at given rpm, and volumetric efficiency can change too).

The way the momentary reading is calculated means that there's quite a lot of scope for error - an engine with negative fuel trims will usually give a higher fuel economy reading than an engine running with positive fuel trims, because all other things being the same (including real mpg) the engine with negative fuel trims will be pulsing petrol injectors for less time per engine rotation. The engine with negative fuel trims will still be using the same amount of fuel as an engine with positive fuel trims, just that it might be getting that fuel in slightly shorter pulses of slightly higher pressure fuel, or just that it's MAF might be reading slightly more voltage for the same airflow and engine load.

The primary input for an LPG system is length of petrol injection pulses. The LPG system measures length of petrol injection pulses and calculates how long to pulse gas injectors for. If the LPG system is calibrated to give exactly the same fuel trims as the petrol system gives under all conditions then if you hold the accelerator steady and switch between fuels the 'instant mpg' reading won't change (it may fluctuate slightly during the change but will then settle down to show the same reading). But it isn't always the best idea to calibrate the LPG system to give exactly the same fuel trim readings as the petrol system... E.g.If fuel trims on petrol are quite negative or positive a good installer might calibrate the LPG system to give fuel trims closer to zero, then if trims were negative on petrol the mpg reading might increase when switched to LPG, or vice versa, but none of these situations actually change real mpg... it's just that the fuel computer only uses the petrol injector pulse duration when calculating mpg.

The 'momentary' comparison where you hold accelerator pedal in the same position and switch fuels may show whether fuel trims are slightly more negative or positive compared to petrol but the results only apply to that single load condition (engine temp, rpm, airflow), it could be that the same test under different conditions would give different results because the LPG system is calibrated at many different conditions. It is only a momentary condition - If you were driving on a flat road with no wind and doing 60mph on petrol then switched to LPG while keeping your foot in the same position on the accelerator, then if the fuel trims were the same on LPG as on petrol the fuel computer might still read it was using 13 litres of fuel per hour but the car might gradually slow down to 59mph, so to keep at 60mph the driver might very slightly increase throttle probably without even realising it to stay at 60mph and then with the new throttle position the fuel computer might show 13.5 litres of fuel per hour.

I won't go into much detail about why LPG injectors might pulse for more or less duration than petrol injectors but there's a lot more to it than the LPG system just adding a couple of milliseconds onto petrol injector pulse duration and even if an LPG system does pulse gas injectors for longer than petrol injectors it isn't because the engine needs more fuel for correct mixture on LPG... actually the engine needs less fuel for correct mixture on LPG.

 

[Gwilo365]

Mind. Blown.
He's converting my Elgrand AND providing an online technical masterclass at the same time.
Is there anything @Lpgc can't do?

 

[Steve Thompson]

Just my halfpennies worth.
When I was 'interested' in MPG on LPG I was measuring from an empty LPG tank to an empty LPG tank.
I decided this was more accurate than a brim to brim.
Brim to brim works for petrol but the LPG fill is too much of a variable.

 

[Lpgc]

Just my halfpennies worth.
When I was 'interested' in MPG on LPG I was measuring from an empty LPG tank to an empty LPG tank.
I decided this was more accurate than a brim to brim.
Brim to brim works for petrol but the LPG fill is too much of a variable.

Exactly!

Brim to brim is always the best method...

But whereas you might get quite an accurate average with just a couple of tanks of petrol you might have to use several tanks of LPG to arrive at a figure more representative of true average mpg.