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pmiulian 08-15-2011 04:07 PM

Hypermilling on LPG
Hello everybody,

I'm new here, but I've been reading with interest for a while the threads on the forum.
I own a Hyundai Accent with a 1.3 SOHC engine, which has a pretty good mpg rating, but I always try to improve it. My personal best was a 51 mpg and the official rating is 45 mpg for highway driving.
The fact that I live in Romania, makes hypermilling a very different thing from what I've read here. Usually extra-urban driving implies a legal speed limit of about 55 mph and lots and lots of villages and curvy roads to drive through.
It has been a month now since I installed on my car a lpg system. This gives me a financial advantage, because lpg is about 60% the price of petrol. Also lpg burns cleaner.
All is good, except the fact that when switching on lpg, the car has worse mpg.
It is not a big difference ( about 10% ), but I don't like it. I think that this is also because the car feels lazy when running on lpg, but I did manage to get the difference in mpg to a 3-5% compared to gasoline.
Is there any special advice for driving a lpg car ?
Are the usual hypermilling techniques different for this kind of fuel ?

320touring 08-15-2011 04:47 PM

I#d be interesetd in this too- an LPG conversion will be on the horizon shortly for the 3218iS, aS I cant ignore the 0.70/litre cost as opposed to the 1.34 a litre of unleaded

D.O.G. 08-15-2011 07:34 PM

I drove a dual fuel (Petrol / LPG) van for years and unfortunately, less power and higher fuel consumption are normal for this type of conversion. Lower fuel costs still make it worthwhile though.

I only used engine on coasting with mine due to it's auto gearbox. However, engine off coasting may have been a problem anyway because of the delay in starting with the setup I had.
Mine would start straight away on petrol, but the LPG only turned on after engine vacuum was detected, so the engine needed to be cranked longer.

If you're lucky, your more modern system may not have this issue?


JRMichler 08-15-2011 08:33 PM

Gasoline has about 128,000 BTU's per gallon, while propane has only 92,000. On the other hand, propane is 110 octane. Propane will get fewer miles per gallon, but could get more miles per BTU if the engine is knock limited.

niky 08-15-2011 08:47 PM

If the kit is a venturi type, you can have it adjusted, old-school style, by tuning the flow, as on carburetor. You can look for adjustment tips online, I suppose. A lot of taxi drivers around here do it themselves. But it's suggested that you let an expert do this... preferably one with a wideband O2 sensor... as those same taxi drivers blow their engines by tuning too lean.

The problem with LPG is that it runs "drier" and hotter than gasoline... so on cars that rely on gasoline to cool down at high load (by running slightly rich), LPG may cause problems if you tune too close to the edge... tune too lean and the engine simply melts spark plugs. Tune way too lean and you're going to experience an epic meltdown.


If it's electronic, they simply need to plug it into a laptop to do the same. Preferably at the shop that installed the unit.

I've some friends who get better economy on LPG after tuning... sometimes as good as or better than on gasoline... but this presupposes that your engine was poorly tuned on gasoline... in one case, one owner experienced a gain in economy because his fuel-injected Toyota van was at 10:1 (air-fuel ratio) at idle! This is far from the ideal 14:1 ratio! On an "economy" car like your Accent, finding an increase or parity in fuel consumption is unlikely... but possible. One Corolla owner sees the exact same economy as on gas.

My LPG unit is retuned to give me the same power as on gasoline (I still make more power on gasoline because we retuned the gasoline system after that with a piggyback controller). Fuel economy is still slightly worse (this is a modified engine on racing cams, can't expect much), but as said... the price difference really makes up for it.

sgtlethargic 08-16-2011 01:39 AM

Was your car converted to dedicated propane or is it dual fuel (propane/gasoline)? Propane has different tuning characteristics than gasoline, so dual fuel is typically a compromise.

Is it fuel injected or a gaseous carburetor (mixer)?

The proverbial they say that propane also gets worse mileage because it's entering the engine as a gas (versus a liquid) so it displaces more air.

pmiulian 08-16-2011 02:59 AM

Wow, thanks for all the replies.

First of all, the lpg system was retrofitted on the car, after I drove it for about 6 years and 90k km. It wouldn`t seem that much for 6 years, but the commute here is very short, with daily trips of about 10 to 30 km. The setup for the lpg system is exactly like this:

The engine is an MPI ( not carbureted ), and the lpg system is Tomasetto complying Euro4 standards. This is funny because the car is Euro3 ratted, but I guess that there is no harm done, except for the money in my pocket. The cost of retrofitting the system was about 900$, but there are systems for carbureted cars that can be bought with only 200$.
The lpg system is very simple - the engine starts on gasoline and when the coolant temperature reaches about 30-40 degrees C, the controller automatically switches to lpg.

The initial tune-up of the system was done at the shop, with a OBD assisted computer, and it did show. The car needs a tune-up, since I`ve covered about 4000 km. Initially, the lpg mpg was very close to the gasoline mpg, but still, not the same. This is the area that I want to investigate.

I mean, maybe insulating the engine block would help - synce lpg absorbs a lot of heat when changing state from fluid to gas.

I did try several aero mods also - a partial undertray front and back. But it seems to me that the one in the back is making things worse for the mpg.
I have a vacuum gauge installed and I use it when driving.
I did a full lower grill block: the aerodynamic on highway speed was great - the car felt very stable and the handling was great, but the mpg dropped a little because the engine wouldn`t get enough air to the air box and the cooling fan would kick in very often.
I sealed the seams on the headlights, front doors and trunk lid.

Also I have some more planned.
I wanted to implement a coolant insulated heat battery - like the one in the hybrid cars. But it seems to me that it is not worth the trouble. Instead I want to use wax to store heat - just place about 1 kg of wax on top of the engine manifold and insulate it. This way, it would absorb heat and slowly release it during the cooling period.

Great forum - I love it :)

niky 08-16-2011 09:20 AM

After 4k, you ought to be checking your filter and your vaporizer... and looking at your plugs for signs of premature wear or poor running.

LPG likes colder spark plugs, so if you haven't changed already, do so. Doesn't seem to do much for economy, but it helps reliability and power.

If your car is distributor type, you can add a little more advance on LPG... we had to do it with a piggyback. But then, my engine doesn't seem to knock even with extreme ignition advance on gasoline. If yours does, better to not take the risk.

I wouldn't muck around with cooling and engine insulation much. LPG does absorb a lot of heat in the state change, but that occurs in the vaporizer, which only helps with the coolant temperature, which is already managed through the thermostat and the radiator fans switching on and off. What you have to be worried about is the temperature inside the combustion chamber, which is hotter because it doesn't get any of the benefits of the cooling from the phase change, gasoline enters the engine as a liquid spray, hence the cooling effect as the heat makes it turn to vapor. LPG enters the engine as vapor already.

This is why LPG tends to burn out spark plugs that aren't "cold" enough.

LPG will only be superior to gasoline overall when the next generation fuel systems with liquid injection come online... (I doubt yours is if it's only $900...) as those will definitely run cooler than gasoline, with attendant benefits in power and possible parity or even superiority in economy... but I have yet to see a system like this on the market... still hoping...

Then again, as we both know... with LPG dirt cheap, it's still a better alternative, as long as your local filling station carries it. ;)

pmiulian 08-16-2011 11:00 AM

Until now, in 6 years, the car didn`t need any repair or engine tune-up. Everything worked fine - all I did was the regular maintenance ( actually I did the oil change on shorter intervals ), and I drove it easy - without racing or demanding too much from the engine. It`s a cheap car, so I don`t have high expectations from it - I just want it to run and not to ask for money :).
I expected the lpg system to be self-tuning, as the car engine is, especially because it is an expensive model, and it`s installed using the car`s software.

niky 08-17-2011 05:13 AM

LPG systems in that price range usually self tune via the stock ECU. But that's programmed to maintain an air-fuel ratio that's beneficial for gasoline, not LPG. Still... if you don't want to overcomplicate, I won't go into O2-sensor intercepts...

You still might see a little power gain via timing adjustment, though.

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