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[JamesV6CDX]

I know there is a notable drop in MPG when burning fart juice, but my Estate seems to be drinking the stuff.

On a careful run back from Bedfordshire tonight, mostly A roads, I got 22.7 mpg when I refilled. Coming back from the lakes, steady 70mph, I got just under 26mpg.

I know the 2.0 16v has never been the most economical beast but I would expect better than that, especially in manual form - I know on petrol it can easily exceed 35mpg on a motorway run.

Facts currently known about the LPG system  -

- Injector nozzles bored to 2.5mm and system is calibrated

- Lambda operation is fine and gives 4.5v under heavy load and high revs

- Mapping, whilst possibly not 10000% spot on, is certainly not far off. And I'm under the impression this doesn't play a huge part when cruising carefully?

- LPG injector times - the LPG injector time on the software seems to be much higher than the petrol ones (I'm aware it should be a bit higher, but is nearly double)

- Very confident there are no leaks.

- Engine appears to be running fine and smooth, and not really down on power (as far as a 2.0 goes!)

I'm not expecting mirqcle MPG here, but I would have imagined if on a run on petrol it does 35mpg, it wouldn't be unreasonable to expect 28/29 on gas?

Interestingly, if I absolutely rag it around the city, it does 18mpg on gas which is higher than I'd expect - it's on a careful run the figures seem to be down.

Any thoughts much appreciated, I've not really checked anything yet

[theowletman]

Might sound daft but was the outside temperature hot or cold? Apparently you can get more in when it is cold, never ran LPG but I have heard the temperature thing a few times. My 2.0 CD manual would do close to 40mpg with my old codger driving style.

[Taxi_Driver]

I know there is a notable drop in MPG when burning fart juice, but my Estate seems to be drinking the stuff.

On a careful run back from Bedfordshire tonight, mostly A roads, I got 22.7 mpg when I refilled. Coming back from the lakes, steady 70mph, I got just under 26mpg.

I know the 2.0 16v has never been the most economical beast but I would expect better than that, especially in manual form - I know on petrol it can easily exceed 35mpg on a motorway run.

Facts currently known about the LPG system  -

- Injector nozzles bored to 2.5mm and system is calibrated

- Lambda operation is fine and gives 4.5v under heavy load and high revs

- Mapping, whilst possibly not 10000% spot on, is certainly not far off. And I'm under the impression this doesn't play a huge part when cruising carefully?

- LPG injector times - the LPG injector time on the software seems to be much higher than the petrol ones (I'm aware it should be a bit higher, but is nearly double)

- Very confident there are no leaks.

- Engine appears to be running fine and smooth, and not really down on power (as far as a 2.0 goes!)

I'm not expecting mirqcle MPG here, but I would have imagined if on a run on petrol it does 35mpg, it wouldn't be unreasonable to expect 28/29 on gas?

Interestingly, if I absolutely rag it around the city, it does 18mpg on gas which is higher than I'd expect - it's on a careful run the figures seem to be down.

Any thoughts much appreciated, I've not really checked anything yet

Sounds the same as my 2.2.........

15mpg about town...23 on a run......

Tho the comp reckons av low 30's on the motorway at 70mph.......but you have to take into account getting to the motorway and the stop start before and after....

[Kevin Wood]

If the voltage at the Lambda sensor is cycling and you're not getting trouble codes from the ECU then it's found the correct mixture - for cruising, at least. If the mapping were out you'd perhaps be wasting some fuel when it's not closed-loop but that shouldn't make a huge difference overall and if it's driving fine with no hesitation I'd hazard a guess that it's mapped accurately enough.

The gas injector duration will normally be a bit higher than petrol, especially at idle, because they are pretty hefty injectors which take a little longer to open than petrol injectors. Are you running out of duration at all at high revs? I guess not, if you're still indicating rich.

One thing I'd suggest is to run it 100% on gas rather than to keep switching to petrol. That way the ECU will update its long term fuel trim and things may improve.

I'd look further afield. Air filter, plugs, leads, etc in good nick? Running temperature correct, and not overcooling? No air leaks? Exhaust system sound with no blockages? Tyres OK and pressures correct? Did you do enough miles on petrol in this car to gauge what the consumption on petrol is like?

Kevin

[Field Marshal Dr. Opti]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

[JamesV6CDX]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

Higher calorific value of LPG means you need to burn more of it to get the same output.

[Marks DTM Calib]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

Higher calorific value of LPG means you need to burn more of it to get the same output.

Er, that should mean the opposite!

[Entwood]

All engines actually burn MASS (weight) of fuel .. not volume... calorific value is quoted in energy /gram (or lb is you are old enough !!).

Now LPG has a higher calorific value than petrol, so you need less MASS of LPG  to get the same power.

However, LPG is far LIGHTER (less dense) than petrol so you need more VOLUME, to get the same MASS and all fuels are sold by Volume (litres or gallons).

As comparisons are done per volume ... LPG loses out by about 10%-15% in terms of distance/volume, but. as LPG is about 50% cheaper than petrol .. the savings work out at about 40% overall.

My petrol costs were 26.37 pence/mile .... LPG is working out at 15.73 pence/mile ... a saving of 10.64 pence on every mile travelled .. a saving of 40.35 %  

HTH

[JamesV6CDX]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

Higher calorific value of LPG means you need to burn more of it to get the same output.

Er, that should mean the opposite!

Did I get that the wrong way around?  

[Entwood]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

Higher calorific value of LPG means you need to burn more of it to get the same output.

Er, that should mean the opposite!

Did I get that the wrong way around?  

In a word ...... YES  

[JamesV6CDX]

Been thinking of a LPG coversion.
Why does it do less MPG than petrol.Anyone know?. :-?

Higher calorific value of LPG means you need to burn more of it to get the same output.

Er, that should mean the opposite!

Did I get that the wrong way around?  

In a word ...... YES  

Doh

[Danny]

apparently LPG is 115 octane...?

[Entwood]

apparently LPG is 115 octane...?

octane has nowt to do with calorific content, power or anything .. it is PURELY a measure of how easily (or not)  the mixture detonates ... the higher the octane rating the less likly auto-detonation (pinking) is to occur.

Or to put it another way .. to ignite high octane you need a bigger, fatter spark  ... which is why LPG requires the HT system to be in good condition to work well.

Higher octane fuels are used on advanced engines that run at high compression ratios and advanced ignition timing .. these are usually higher performing engines. The use of a high octane fuel periodically in a "normal" engine can, sometimes, have a beneficial cleaning effect as in general they burn a tad hotter due to the faster flame propagation.