drop in mpg considerably.

[josewick]

Hi all
This is the second winter of me running LPG. I have a BRC system

I did not notice as much last winter but this time round since October the overall mpg had fallened approx 20%.

I read somewhere that it is normal that mpg changes over temp but I do not know how much is classified as normal.

And whether it is to do with the actual gas itself that shell is supplying in winter being different to summer?

Any tutorial would be appreciated

Thanks

[nokia_aid]

im wondering about the lpg i get from shell, i swear i get less mpg from it, im currently experimenting to see if, il know more in a few weeks

[Brian_H]

Gas stays the same the year round here - in europe its a butane/propane mix, here its just propane.

Is the performance on petrol still the same? The major extra loss is down to cold starting but you'd normally see this on the petrol consumption (the mixture gets richer at cold to compensate for fuel condensing on the piston walls when they are cold - but gas doesn't do this as its already vapour rather than tiny droplets of petrol mixed with air).

Also how are you calculating the mpg?

[josewick]

I understand that I will use a bit more petrol as it takes longer to heat up to approx 35oC so the LPG system kicks in

I work out the mpg not only using the trip computer but the actual amount I fill up in litres against how many miles I can get out from it

I use to get approx 6 miles to a litre and now I struggle to get 5 miles per litre even I try driving very gently.

I know give or take it may not be 100% accurate but it will show an indication at least.

[scottyf]

Try setting the kick in temp to be lower.
I'm currently set at 25 degrees with not issues or problem. Obviously its a different car. But using LPG is still far cheaper than petrol even on the warm up.

I've sacrificed a bit of mpg on a trip and range for the cheaper start up. Might even reduce the injector lifespan. But that's a risk I am willing to take.

[LPGC]

Besides the already mentioned running a bit longer on petrol (colder engine takes longer to warm to switchover temp) there shouldn't be any difference in mpg - Unless there is a problem with the engine thermostat causing cold engine enrichment.. Unlikely.

Seems more likely that:

People don't manage to empty the tank quite as fully (particularly big engined vehicles), because tank pressure is lower in the cold weather - But this should mean they don't manage to fill with as much gas either..

Driving habits change.. Defrost the windows in the morning with the engine running (on many cars when you set the defrost position the aircon turns on automatically) / using lower gears than usual (some autos won't engage torque convertor lockup until the gearbox is up to temp).
Thicker engine and gearbox oil when cold / tyres at a bit lower pressure and rubber less flexible giving greater roll resistance.

Some people assume fuel computer figures are correct - Sequential LPG systems compensate for gas temperature, the colder the measured gas vapour temp, the denser it is, so to compensate for the higher density the gas injectors pulse for a shorter time. If a gas system overcompensates for lower temps (and many systems do by default) then this has the effect of leaning the mixture - except the petrol ECU compensates for the leaner mixture by again increasing pulse duration, and then the fuel computer sees the longer pulse and the fuel computer reads lower mpg.

Having said, there are conditions where cars don't run so well on gas in the cold - this is again due to gas vapour temps.. some injectors don't seem as capable at accurately dosing gas when cold / some systems are set up with too much pressure which although might be OK for warm weather means the injectors are nearing or below their minimum pulse duration when cold (the cold dense gas pulse duration compensation).

Simon

[Zebedee]

The mpg on my basic open loop carb'd setup drops from my yearly 18.5 average to 13-14 during the time between beginning of November to end of February.
It's always started on gas, and is rarely run on petrol.
I've also noticed its a little rough running below 2k revs during the winter too (which may or may not be related). It's only used for short (sub 5 mile) journeys with one or two longer ones each month.

[Brian_H]

Only other thing that might cause a drop that doesn't seem to be covered is if you have a leak somewhere - doesn't have to be much of one to make a noticable difference. Although you should see the loss removed if that was the cause if you can do a comparison to the petrol usage (if thats worse than usual then its not down to something on the lpg system but other factors as suggested above)