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[albitz]

MPG on the MID is calculated from engine ECU & injectors, it measures the consumption used compared to distance travelled. When LPG cuts in, its the LPG ECU doing the control, petrol no longer comes into it.

The mpg you get on the MID is from when its only running on petrol, and given that it only runs for a few mins on petrol, thats the mpg you see. Mine says 29mpg but there is no way its doing that on gas.

Wrong.

Then tell me, how does the engine/petrol ECU know how much gas is being used  :-?

It can't, the MID tells rubbish when running on gas. Its designed for petrol use!

Not saying its dead accurate, but it operates while running on gas giving instant consumption and the average consumption changes as well, over a longish run. Imo its almost as accurate or inaccurate on gas as it is on petrol.

[mathewst]

When you say more, do you mean more comsuption, that may be right on yours, cant remember now what Opels display on the MID

Yep Zirk more consumption, it shows an increase in consumption about 10 to 12% on lpg then it normally shows on petrol

[Kevin Wood]

What the MID reads when on LPG is pure luck of the draw. In theory it should show the same as on petrol, i.e. it should show how much petrol you would have been using. It has no knowledge that the car is not running on petrol.

In practice, what it displays will depend on how the LPG system is calibrated. If it's calibrated slightly rich on LPG, it'll read higher MPG, and vice versa.

I don't bother to look at it on LPG.

Kevin

[albitz]

But it isnt purely taking its readings from when it has run on petrol as Tunnie stated ?

[zirk]

When you say more, do you mean more comsuption, that may be right on yours, cant remember now what Opels display on the MID

Yep Zirk more consumption, it shows an increase in consumption about 10 to 12% on lpg then it normally shows on petrol

Then thats about right then, it will use about 10 - 12 % more lpg than petrol per Km, when you go long run on lpg next time do a LPG brim to brim, ie fill up before and after preferably at the lpg pump, before setting off, reset the consumption, trip Km, trip litres on the MID and do a comparrision.

As said, the only thing thats not right for me, is the Range display, on a short trip it counts down to zero when on gas, on a long trip if I reset it calculates it correctly for petrol and stays the same ie shows my how much range is left on petrol, handy to know if you low on Gas.

Chris     

[Kevin Wood]

But it isnt purely taking its readings from when it has run on petrol as Tunnie stated ?

Nope. It doesn't know when you're not running on petrol. It works on the basis that you're running on petrol all the time. Range indication will always be a mess because it can't reconcile the petrol it thinks you're burning against the (unchanged) level in the tank - until you get down to 30 miles or less, when it just works on consumption so counts down the miles until it reckons you're on zero.

Kevin

[2woody]

All of the fuelling is still being calculated by the fuel ecu - the LPG system takes it's fuel usage from the injectors directly.

When you're running on LPG, the injectors are still opening and closing, but there's no fuel going through them. The mpg is still, therefore worked out as if it were running on petrol at the time.

The LPG ecu just takes the petrol injector signals, fiddles with them a bit and connects them to the new LPG injectors.

[tunnie]

so in other words, the mid talks rubbish 

Guess it would be more accurate if LPG had same burn factor of petrol, the inaccuracy is that you need to chuck in more LPG to get the same amount of power from the burn.

[zirk]

Surely one of the ECUs must need to know how much LPG is being squirted in other wise how does my MID show Im using more gas than petrol when running on lpg per mile.

Chris.  :-/   

[Dave Elite]

ignore the trip computer now, it will be talking 'dangle berries'  - Its calculations are assuming your running on petrol.

Infact MPG now your on gas, is not the way to measure economy, its pence per mile 

LPG has less 'bang per litre' compared to petrol, so you need more (hence drop in mpg) So fill it up, zero the trip, and work it out the old fashioned way.

When prices where cheaper (i was paying 57p litre on my commute days) i could get cost down to 10.3p a mile on average, where as petrol was costing me 17p per mile.
 

It seems tunnie is the one who is talking 'dangle berries'.

[tunnie]

Surely one of the ECUs must need to know how much LPG is being squirted in other wise how does my MID show Im using more gas than petrol when running on lpg per mile.

Chris.  :-/   

The LPG ECU's are quite basic, they just 'copy' what the petrol ECU does. When you calibrate it (on an hours drive say with mine) it just learns whats being done at various speeds, it then attempts to match that as close as possible.

My MID says 29mpg average, which is about right for petrol use. But on LPG its more like 25mpg.

But as mentioned at the start, MPG is actually totally irrelevant on gas. Its not quite half the price, it takes more LPG to get the same 'bang', so you really want to look at pence per mile, that indicates the true saving & economy 

[Lazydocker]

Let's put this to bed then... As Kevin has already tried to explain briefly...

The petrol ECU supplies all the info for the Trip Computer. When running on LPG the petrol ECU doesn't know that the injectors aren't actually opening The LPG wiring loom interrupts the signal to the petrol injectors and takes it through the LPG ECU, this (if you are running on LPG) then operates the LPG injectors as per the multipliers set up when it was calibrated the system

So, the trip computer is just giving a petrol equivalent. As for Zirk's reading accurately... You're LPG system must be very well calibrated

[TheBoy]

As Kevin Wood says, and Albs hinted at, the MID will display a figure, even on LPG. How accurate this figure is depends on calibration, but I'd still expect it to read about 10% optimistically.

The MID is fed an injection duration signal from the petrol ECU.  The LPG ECU is just an injector emulator, and drives the LPG injectors from the durations from the Petrol ECU (with a map applied (this is the calibration))

[albitz]

Thanks. Often wondered how the MID "knew".

[cem_devecioglu]

Finally got the mig back from the lpg installers shop.
Must admitt they did a great job.
So was wandering now about one thing.
Average consumption on the trip computer, do the rest of you have it (probably Tunnie could share some light since his is 2.2)?
Mine is showing consumption still although it is increased ( it probably should be when it is working on lpg)
I never tried to figure out how the trip computer works but I was surprised this works on lpg.
Oh by the way lpg is BRC sequent

actually your petrol ecu is still thinking the car is using petrol because lpg ecu uses petrol ecu signals to arrange its injection timing..  although not 1 to 1, still your lpg consumption is proportional to MID reading..

ehem..