worrying bad MPG

The main forum
Post Reply
topbanana0
Posts: 13
Joined: Sun Jul 13, 2008 2:00 pm

worrying bad MPG

Post by topbanana0 » Tue Aug 26, 2008 8:30 pm

Hi, Just had my 1.2 Clio converted, its used as a driving school car.
Before it was used for this my wife used to drive it, and her mpg was about 45-48 (on petrol) mostly school run etc.
When I started using it for SOM I was getting about 35-36 (on petrol) lots of stop start etc
Now its on gas im getting 25-27mpg (with out taking into consideration the petrol, so it's probably doing less)
Went back to the guy that installed it and the plugged a laptop into it, and we drove around while he adjusted things?
he took 10% of this and that, I couldn’t let him take any more off as the car started to run a bit flat.
I have not checked the mpg since but it looks like its doing similar.
I expected to lose a bit but this is poor, what do you think, I know is a SOM car but it only got a little
engine.

by the way, its a Romano multi point, I think and it had new plugs and air filter also a service

mak-uk
Posts: 288
Joined: Mon Sep 03, 2007 8:48 pm
LPG System: Prins VSI
Car: BMW 540iA Sport
Location: All over the UK

Re: worrying bad MPG

Post by mak-uk » Thu Aug 28, 2008 12:15 pm

Hi

On LPG, despite what some claim, a lot of converted cars experience around a 20% increase in consumption. In your case, assuming 35 MPG on petrol, this would equate to 28 MPG on LPG. Assuming you're paying half the price that would in broad terms equate to 56 MPG.

Ideally, the installer should 'map' the car not only with the aid of his LPG software but an OBD-II code reader which can show live fuel trim data. The short-term fuel trims should be as close to zero as possible (some state -3 to + 3, some say -10 to +10) throughout the rev-range to ensure fueling is correct.
Mak
Site Owner
2001 BMW 540i Auto Sport Saloon
~300 BHP; ~320 ft/lbs, Prins VSI system

runningoldcars
Posts: 47
Joined: Thu May 22, 2008 10:12 pm

Re: worrying bad MPG

Post by runningoldcars » Sun Aug 31, 2008 7:36 pm

If you look through the figures for mpg that are quoted about this forum you can see a big variation between cars. My own car does 25mpg on LPG an about 29 on petrol which I and others think is exceptional, and if they were not my own figures I may even be sceptical.

Others see a huge change.

The only technical answer I could propose is that due to the light throttle and open/close behaviour of the throttle when being used by learners your system may be functioning poorly.
By this I mean your injectors are positioned in your manifold, but you are likely not squirting liquid fuel straight on to the back of the valve and down the inlet port, and hence for the LPG to enter the cylinders in correct doses it requires a decent pressure charging down the manifold runners.
If your learners are only opening the throttle a little you are not getting full atmospheric pressure charging your cylinders, and hence the air velocity through your runners is low. This means that the LPG may not be getting sucked in at the right timing.
This will then be detected by you Lambda sensor as lean mixture and the car will then enrich the mixture, only for the next suck to give a super-rich mixture (new injection plus residual from last one), this then leads to the next pulse being leaned out and the whole cycle runs again. Overall though this may seem like it would average out the net effect is it runs slightly rich, but not enough to cause big, sooty problems (remember Lambda signals are time averaged so a little richness can get overlooked, and off the scale readings can only be corrected so much and so quickly).

The reason this would occur is if the engine is being run in a poor load/speed zone (such as low revs and oscillating throttle) as the reflection waves in the manifold are pushing the gas back, rather than helping it in.

All said though, the chances of the gas dynamic behaviour of your Clio's manifold and LPG injector interaction being that poor are very slim though, unless you are running a hotrod camshaft profile, so i'm probably talking through my hat!

Practical steps to take:
The way to tell may be to drive the car yourself for a good few runs (hundred or so miles?) and then check your consumption.

Get an emissions check at your MOT place and see what your exhaust gas profile is.
As a rough guide 1l of petrol creates 2.3kg of CO2 and 1l of LPG creates 1.5kg. If you can get concentrations of both from the emissions slip you can use that as a ratio and that may indicate any odd behaviour.
____________________________________________________________________________________________
RunningOldCars

Honda Accord Coupe, 2.0l, 4spd automatic, Aldesa RSI+ lpg kit, Steely Dan's 'Katy Lied' on stereo.

topbanana0
Posts: 13
Joined: Sun Jul 13, 2008 2:00 pm

Re: worrying bad MPG

Post by topbanana0 » Wed Sep 03, 2008 2:46 pm

Ok, had the MOT check my emissions, hope you know what it means as I don’t?
Petrol:
Fast idle test
CO: < 0.20% vol 0.01% vol (he said that’s very low)
HO: < 200ppm vol 4ppm vol
LA: 0.970-1.030

Gas:
Fast idle test
CO: <0.20% vol 0.00% (he said that’s no CO emissions)
HO: <200ppm vol 6ppm vol
LA: 0.970-1.030

What you think?

runningoldcars
Posts: 47
Joined: Thu May 22, 2008 10:12 pm

Re: worrying bad MPG

Post by runningoldcars » Sun Sep 07, 2008 5:51 pm

I'm pleased to say it means your car is running very well.

I'm not so pleased to say that I have no idea why it's guzzling lpg.

My only idea is the original one;, it's purely down to driving style and the normal loss in MPG is being amplified.

Did you do the 'big run' test? If it fails that then there must be an installation issue, and without seeing the car i'm at the end of my knowledge.

I'm sorry I can't be more helpful.
____________________________________________________________________________________________
RunningOldCars

Honda Accord Coupe, 2.0l, 4spd automatic, Aldesa RSI+ lpg kit, Steely Dan's 'Katy Lied' on stereo.

topbanana0
Posts: 13
Joined: Sun Jul 13, 2008 2:00 pm

Re: worrying bad MPG

Post by topbanana0 » Sun Sep 07, 2008 7:51 pm

Thanks for your input, i will take it for a good run,when i get some time and i willl let you know.

Post Reply