Up to 25% increase in LPG consumption over petrol ??

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carlhunt · #1 Post by **carlhunt** » Tue Mar 07, 2006 7:30 am

I have seen on one of the LPG equipment supplier sites the following FAQ Q What about my MPG on LPG? A Again, as a rule of thumb - expect to lose between 0 and 25% of your equivalent petrol MPG figures per gallon. I am curious to know what the factors that might lead to an increase of as much as 25% in LPG consumption over petrol ? {Especially as after conversion I am at the 25% end of the spectrum} Appologies to Clsdualfuel for lifting their text.

richy_boy · #2 Post by **richy_boy** » Tue Mar 07, 2006 8:05 am

Hi Carl, I don't know the answer, but I too loose 25% mpg when running on LPG. I suspect it's due to rich mapping though. My car normally returns 29mpg on petrol, yet only about 21 on LPG. Other people with the same car, yet with a different LPG system seem to get VERY little difference in mpg! Richard

boswell · #3 Post by **boswell** » Tue Mar 07, 2006 9:12 am

I get between 15-20% less mpg on LPG in my 2.5 5 cyl VW T4 here an explaination why nicked off some ones website: "Empty LPG fuel tanks are constructed of heavier material and must be 20% more bulky than a petrol tank, simply because an LPG tank may never be filled to capacity. Some 20% of the tank volume must be left empty for expansion of the contents. Another factor is that LPG tanks are often fitted in addition to a vehicle's petrol tank as LPG is not as freely available as petrol (to date). Because of this most vehicles retain dual fuel capability and the vehicle's overall weight is increased and internal load space may be reduced. Propane has a lower calorific value than petrol when vapourised. In real terms that means the driver must put his or her 'foot down' more to achieve the same speed or rate of speed increase (acceleration). As the calorific value of LPG is some 20% lower than that of petrol the resulting consumption per litre or gallon is worse than petrol by a similar amount. This slight disadvantage is far outweighed by the price advantage (less than half of the cost). You'll cover 10 - 20% less miles per gallon or Km's per litre or gallon of liquid fuel when using LPG instead of petrol, but your overall fuel costs will still be more than halved."

henleyman · #4 Post by **henleyman** » Tue Mar 07, 2006 10:21 am

Hi I have the same problem too, perhaps even worse. My 3.2 Jag XJ6 did 19-25 mpg on petrol but only 12-15 on Gas - it's hardly worthwhile! I assumed it was too rich but the internal diagnostics (millenium) says it's too weak! It runs well enough though. Is it worth having it set up on a exhaust gas analyser? Anybody know someone competent in the East Midlands area that can do this? Peter

p38arover · #5 Post by **p38arover** » Tue Mar 07, 2006 10:35 am

After recent tuning by an LPG specialist, my 4.6 litre Range Rover is now using 27 litres/100km on LPG - but it only uses 13-15 litres/100km on petrol. It's cheaper to run it on petrol because LPG is less than 1/2 the price of petrol. Ron

sm200sx · #6 Post by **sm200sx** » Tue Mar 07, 2006 11:19 am

I get about 23mpg on fuel, and 20mpg on LPG at the moment. I work on 10% loss.

carlhunt · #7 Post by **carlhunt** » Tue Mar 07, 2006 1:11 pm

Andy(Boswell) - I can understand the calorific value arguement but the weight issue only comes into play if your comparing petrol mpg pre conversion with lpg mpg post conversion. I did not measure my petrol mpg pre-conversion so I have no particular reference but the "car" is so heavy that the additional tank has a minor impact. Kev/Richard - I am more suprised by the difference in consuption between the two Nissan's. Given that they are on different lpg systems you do start to wonder if that is a driver. Would I have been better off with a OMVL rather than a BRC is an interesting question. The installer is still investigating to see if consumption can be improved.

richy_boy · #8 Post by **richy_boy** » Tue Mar 07, 2006 1:27 pm

Yeah, my installer has offered to retune my car to try and claw back some LPG efficiency - so there is hope! More worryingly he stated he tuned my car for 300bhp when it was running nearer 240bhp! Something I have never quite understood, so I assume he must of just *guestimated* the fuelling and hence why it's using so much fuel. He also said it'll take about 1 hour to remap my car on the road, which from reading BRC installation documents this isn't strictly true. Appartnely it can self map through the software and hence where the 'fast' model name comes from. This, of course is yet to be seen and is probably marketing 'exageration'?! Any installers have experience with this system? Rich

richy_boy · #9 Post by **richy_boy** » Tue Mar 07, 2006 1:33 pm

Oh! I should also add that Kev's system currently switches back to petrol when revving over 4k rpm where mine stays on LPG to the red line. This obviously will play havork with any comparisons between our cars as I can 'drive enthuastically' on LPG where Kev will be running on petrol... So, without looking at the numbers, Kev 'should' be seeing a better LPG mpg then me I guess *confused* but will be using more petrol per week then I... maybe! Rich

boswell · #10 Post by **boswell** » Tue Mar 07, 2006 6:32 pm

The fact is you won't get the same mpg on LPG as petrol with the conversion equipment available today, that may change with future developments. The weight issue in the above excert from someones webpage can be a problem if you have a large tank and you keep your petrol tank partially filled as well. I often find peoples mpg figures in accurate, how are measuring? is the maths by calculator, in your head? No disrespect intended but people have a habit of "rounding" up or down and when talking less than 20mpg the "rounding" can make all the difference. I use this the calculate after each tank full: <a href="http://www.eforecourt.com/calc_mpg.htm" target=_top>http://www.eforecourt.com/calc_mpg.htm</a> the main reason being my speedo/odometer is in km so using the site cuts down on my maths adding in errors.

go_lpg · #15 Post by **go_lpg** » Tue Mar 07, 2006 11:23 pm

Peter W - Reading your post reminds me of another X300 we saw recently, one we were asked to 'troubleshoot'. The Millenium controller had be configured backwards, reading lean when the mixture was rich and vice versa. The implications of this are plain. We are a little further away but have huge experience with both the model and the equipment. We serviced a 4.0 model last week that we converted 18 months ago - The owner has now done 56,000 miles on LPG and averages 20 mpg on LPG. You'd be welcome if you want to bring it along for us to review your set up (by appointment). Regards Steve (Used to be Steve_96 until I moved) <a href="http://www.go-lpg.co.uk" target=_top>www.go-lpg.co.uk</a>

carlhunt · #16 Post by **carlhunt** » Tue Mar 07, 2006 11:39 pm

Boswell, I use an Excel spreadsheet upon which I record all my lpg purchases. There is some minor automation to calculate the overall mpg and the ratio of lpg/petrol consumption. The calculations are to two decimal places as that is the accuracy of the pump readings. I work in mpg if only becuase those are figures that I am comfortable with and also because the car's trip computer uses mpg. The trip computer I found useful as it appears to be accurate to within a few percent for petrol so using the average consumption reading and my .77 ratio gives a good indication of the amount of lpg used. The four lights can be a little vague. My installer has had a look at the calculations and is happy that the figures are realistic. Just have to wait for them to finish their investigations .....

sm200sx · #17 Post by **sm200sx** » Wed Mar 08, 2006 9:51 am

@Andy I use excel to calculate MPG, every drop of fuel or LPG I put in the tank is logged on the spreadsheet, I monitor each tank as well as trending. As most of my miles consist of the same route and time (105mile round trip to work) then I can spot if anything suddenly changes. Remember this car also does trackdays - you can spot them with the 6mpg dips <img src="http://www.my90.co.uk/lpgforum/clipart/proud.gif" border=0> Some links, and I'm going try out the upload pic thingy ... Graphs - Big pic <a href="http://www.suicidesquad.demon.co.uk/pic ... 0graph.jpg" target="_blank">http://www.suicidesquad.demon.co.uk/pic ... aph.jpg</a> Small pic <a href="http://www.suicidesquad.demon.co.uk/pic ... -small.jpg" target="_blank">http://www.suicidesquad.demon.co.uk/pic ... all.jpg</a> <img src="http://www.my90.co.uk/lpgforum/messages/898/2607.jpg" alt="My MPG graph (small version)">

richy_boy · #20 Post by **richy_boy** » Wed Mar 08, 2006 7:55 pm

Keith, you are a God (or close to!) "Hi Rich, i find it interesting that your installer could "map" for 300bhp .The BRC system is very good i rate it in the top 3 systems available. If i may i will try and explain the set up procedure of the sequent fast." So, basically the car couldn't have been mapped for 300bhp as you can't map to theoretical values - just as I suspected either a) he just mapped it as per usual b) he made the mixture extra rich to try to guess the fuelling required?! Just to clarify things, I initially asked the installer to source a system that can run 300bhp though LPG (ready for future modifications, even though my car was running 240bhp ish at the time!) Currently during full throttle my system occasionaly switches to petrol which is worrying me. Would this be a sign that the LPG system is maxing out? or would a re-map fix the issue? As the system is simply piggy-backing my cars ECU I can't see any other reason it would switch back over?! My current car ECU map is a modified 'stage 1' chip which has a very accurate fuel and ignition timing for the modifications I have (for super unleaded fuel). So, thanks again Keith you've been incredably helpful and have confirmed my thoughts. I shall persist with my installer for now to try and follow this procedure (with a wuick drive to fine tune), but if I have no luck I shall call you. Then again, I can't get a cable/dongle anyway to even check the mixture <IMG SRC="http://www.my90.co.uk/lpgforum/clipart/sad.gif" ALT=":-(" BORDER=0> Rich ps. Where abouts in the UK are you based Keith?