LPG economy

[carl0s]

My 4.4i valvetronic BMW 545i (n62 engine) achieved 19mpg on LPG on mostly motorway at the weekend, driving at around 60 mph. This is equivalent to ~35mpg running costs if it were petrol. I'd expect better than that. Any ideas for what might be causing it? It's supposed to do 35mpg on petrol anyway!
I am going to calculate the around-town costs this week and see how that looks.

Any thoughts, other than that the BMW quoted figures are stupid?

[hitman]

bmw figures are stupid as you well know

be happy with the 19mpg , its better than I get in my range rover

[scottyf]

What were you getting before.
I must admit I was expecting a bit more out of LPG. But as its not optimized and it is for petrol I can live with it. That and the fuels cheap. I always think on mine it fuels it a git on the rich side but still.
I also think I use the throttle a bit more than I normally would on petrol.

I used to get an average of 34.67mpg on petrol
I get an average of 27.49mpg on lpg

That is a drop of 20.7% (Probably a little less in reality as I drive eco friendly on petrol and not so much so on lpg).

Might be my tuning. Might be my system. Not sure.
But as LPG is about 45% the cost of petrol round by me I am generally not too bothered.

I imagine some more tuning could be done to get it spot on.
But If it aint broke... I mean how much more can i save. Okay if I got it up to 30 mpg that would extend my tank range by 25 ish miles.But that's not enough to make one journey to work.

[minimutly]

How is your tune, is the mixture near enough for the petrol ecu to correct?
A petrol system uses lambda to correct to either side of stoics. This is catalytic converter technology requirement - weak rich weak rich. For non cat engines, you could run constant stoics, or just weak of this, thou saving fuel.
LPG engines don't need a cat (or at least a working one) so in theory you can run weak for best economy, hence saving fuel. Not quite legal, or easy to do...
Huw

[carl0s]

Well, the car is modern and uses closed loop mixture control.
It actually has wideband UEGO sensors, so I'm not sure there's much I can do to alter the mixture?

I replaced one of the wideband sensors a bit ago, and it made no difference to the fuel trims on that bank. The other side looked like a nightmare to get to so I left it alone.

[tomas]

My self have 3 cars.merc cl500 peugeot 607 3.0v6.. and bmw 735 with n62 engine.on long run on petrol 28 to galon on lpg 22/24 ..in town a lot...so for 545 i think not to bad..

[tomas]

Need to check did your valvetronic system working i work on many bmw cars and when its not working in valvetronic mode without diagnostics hard to know..there is no engine managment ligth on... and you think its all ok..

[Gilbertd]

Forget what BMW claim that it is supposed to do, what does it actually do on petrol? I doubt it's anything like the claims.....

[carl0s]

As it happens I have valvetronic minimum valve lift increased to 0.8mm (thru INPA). This means it is not in valvetronic at very low load. It uses throttle for that light load operation. Full valvetronic would fully control engine speed with valve lift right from 0.1 to 8mm, but in my case, their lift starts right at 0.8mm and the throttle comes in to reduce engine speed for lower load requirements than that. This might be hurting the cruising economy. I will have to see what the valve lift is at when cruising at 60mph. If it's showing 0.8 then I know the car is throttling which might reduce economy.

Other than that, the valvetronic is operating fine 95% of the time. It occasionally goes fail-safe due to implausible manifold pressure readings.

It's the only way to stop LPG accumulating in the manifold when at stop lights though. Full valvetronic with no manifold vacuum gives me an occasional hesitation after sitting at the traffic lights.

I'm not sure what it actually does on petrol. It is a heavy car though.
Overall it's not so bad. I just expected a bit better.. like that I was getting 25-30mpg petrol-equivalent around town, and 45 - 50 on the motorway when being very gentle. Seems I'm at about half that.
I'm basically achieving the BMW petrol figures (which no one ever achieves), on LPG. I hoped for more than that though!

Pretty sure I have no leaks but I do have a gas leak detector swan-neck thing. I can have another look.