LPG Fuel Economy

[Frank]

I am curious to know what everyone is getting for their fuel economy. When comparing propane with gasoline, propane contains about 72.6% of the energy of gasoline (84,250 vs 116,090 BTU/gallon, AFDC Fuels Table). The US DoE's Transportation Energy Data Book: Edition 29 (page 296) has slightly different values: 83,500 BTU/gal LHV for LPG and 115,400 BTU/gal LHV for conventional gasoline but similarly shows LPG to have 72.3% of the energy of gasoline.

Most propane conversions get much better fuel economy than what would be expected by comparing energy contents. With carbureted systems, I think it is not uncommon for an open-loop propane conversion to get 75-80% of the vehicle's gasoline economy. With feedback carburetor systems, I believe the typical propane fuel economy is about 80-85% of the vehicle's gasoline economy. Injection conversions on OBDII vehicles do even better with expected fuel economies in the 85-95% range.

With the open-loop carburetion systems, my understanding that the better than expected fuel economy is mainly due to improved, leaner fuel mixture distribution and a more ideal ignition advance curve. A gasoline carburetor system can have a significant variation in fuel mixture from cylinder to cylinder so the average fuel mixture must be richer overall to ensure that each cylinder gets a combustible fuel mixture. Closed loop carburetor systems improve on this mainly by ensuring that the fuel mixture remains constant over a wide range of engine temperatures.

Injection conversions on OBDII vehicles can't improve on fuel mixture distribution because of gasoline port injection. Properly configured, the fuel trims on propane should the same as on gasoline. Because this type of conversion works by intercepting the gasoline injector signals, modifying them, and then rerouting them to the propane injectors, theoretically this should result in propane fuel economy that is closer to 72-73% of the gasoline economy. However, they do much better and my only explanation is that the vehicle's PCM allows more ignition advance because of propane's higher octane number.

Although the station where I normally refuel has an erratic meter so I can't really get a good fuel consumption data, my estimate is that I am getting about 85% of my gasoline economy with my 2003 Impala LS. Technocarb's experience is that they often see propane fuel economies closer to 90% or more and this is with actual propane dispenser readings rather than the vehicle's on-board fuel economy display.

• Does anyone have any other explanations as to why LPG economy should be better or worse than expected?
• What fuel economy are you seeing on propane and what would you expect it to be on gasoline?

I made up some Excel spreadsheets that you might find useful. I've included some sample data so that you can see what the chart looks like. The first row of data (row 4) is just to initialize the distance calculation so it's not necessary to add fuel data (cost and amount). I've also included a column for the price of gasoline so that you can have an idea of what kind of mileage you'd be getting on gasoline. The metric calculation includes the actual fuel economy data for my Impala.

• Metric LPG Fuel Economy Calculation
• Metric & Imperial LPG Fuel Economy Calculation (fuel in litres, distance in miles)
• US LPG Fuel Economy Calculation

If I've made any errors, please let me know so I can revise and re-upload them.

[Steptoe]

I am curious to know what everyone is getting for their fuel economy.

69 Camaro
350, turbo 350 trans, 1200 stall, 2.55 diff
performer dual plane, bottom end cam, cast truck rams horns manifolds, compression just under 11:1 with small chamber cast fuelie heads, Std GM HEI recurved for LPG.
Dedicated LPG
Impco L Impco 425 on 450 holley baseplate....the mixer is getting worn..done well over 130,000 miles
Gal = 4.54L

What fuel economy are you seeing on propane....

Around town approx 11.5 to 12.5 mpg
Open road 13 to 14.2 mpg

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and what would you expect it to be on gasoline?

I have no idea what petrol would be....given the low end cam and 11:1 the dynamic CR will be way up for pump gas, be it 91 or 96/98 avalible in NZ.
LPG is approx $NZ 5/gal (4.54L)
Petrol approx $NZ 9.15 gal

[webbew]

'73 Impala wagon
400, '91 700R4 with lockup
Performer heads and matching intake manifold, new cam, Mallory HEI
Dedicated LPG, Technocarb

In petrol-equivalent terms (so, multiply the values below by 0.726 to get miles per actual LPG gallon):
16-17 mpg (US gallons) in mixed driving
Best I ever logged was 18.3 mpg, worst around 14 mpg which included some traffic jams.

Can't say what I'd expect on petrol, since practically everything on the car is altered. I think having the 4-speed with overdrive helps a lot, it already shifts into 4th at 60 kph if cruising.

[DODGEN1]

1975 W-100 short bed 4X4
440 stage 5 heads 2.14 intakes 1.80 exhast 86 cc. 10:1, cast 1978 intake manifold, stock exhast manifolds
dual 2 1/4 with cross over
comp rv range cam.
malory unilite dist. centrifical only. all in 32 deg at 2800 rpm.
i figure my curve is a little slower than most but ive had some detonation problems if its any faster.
727 trans 203 transfer case part time kit
1973 HD44 front axle lock outs D60 rear 4.10 gears detroit locker
285/75/16 tires
5300 lbs.
model E, 425 in a 780 thermoquad base plate. you can smell fuel at an idle. but it that "ya it smells a little."
8-8 1/2 mpg all the time up, down,loaded or unloaded. i'm in the mountains no flat ground, all curves
normal operating RPM 2500 to 3500.

An erratic meter needs to be corrected.

[jono]

Just when I finally get used to expressing in litres per 100 km this question comes along in MPG !

Archaic Subaru boxer 1800cc ute BRAT/Brumby /MV1800 1100 kg on the scale, with similar era Impco Cobra and CA125 fresh air at its best is not highway travel but a mix, oddly is 10.2 litres per 100km (62.5miles) best petrol use was 9.0 per 100km

Turbo EA82 in L Series sedan was slurping at 15 litres per 100km until finer tune got it down to about 11.5, Impco fresh air as well due to feed back controls only designed for non boost applications - best petrol use was 10 litres per 100km

Both tend not to give better economy on highway runs at 110 to 120 kph

whereas my 250ci Ford six at 1460 kg would return 21 litres per 100 miles with fresh air impco

sorry for the non compliant mix of economies, may return with better MPG figures.

I would expect any injected system that piggys the petrol efi system would return better due to the injector cut on decel above about 1500 rpm - surely if petrol injectors are switched off so too the vapour injection?

[Steptoe]

metric is the norm in NZ to....but have never got my head around it..well as far as economy numbers are concerned.

Also British (aussie NZ) and US gals are different in size....
US gal I think is about 4L/gal British 4.54L per gal.

[The_Stuff]

my van takes roughly $30 of propane for 200km highway. Also propane is 107.9 cents/liter in regina and 93.9 cents/liter where i live. that is $4.084 a us gallon. Gasonline is 107.9 and diesel is 110.9 a liter


so $30 = 27.5liters - 7.26 gallons - 125 miles = 17 mpg?

[bumpstart]

3.785 L = 1 US gallon
4.546 L = 1 british imperial gallon

1.61 Km = 1 mile
100 km = 62.11 miles

250 - 300 RWHP 13bt rotary engine fitted with impco 225 feedback system
17 L per 100 km highway
22 L per 100 km city

blended LPG price is 55 to 70 AUS cents per litre

same engine fitted with 2 x 550 primary injectors and 2 x 1000 secondary injectors and run to one bar boost
demand 98 octane petrol at $1.30 to $1.50 $AUS per litre ( perth western australia )
and would go 280-350 KM per 60 litres
= 17.1 L per 100 km highway
= 21.4 L per 100 km city

in US terms
LPG
17/ 3.785 = 4.5 US gallons per 100 km ( 62.11 miles )
= 13.8 MPG
22/ 3.785 = 5.8 US gallons per 100 km
= 10.7 MPG

98 RON petrol
17.1/ 3.785 = 4.5 US gallons per 100 km
= 13.8 MPG
21.4/ 3.785 = 5.7 US gallons per 100 km
= 10.9 MPG

================================================

in british imperial terms
LPG
17 / 4.546 = 3.74 UK gallons per 100 km
= 16.6 MPG
22 / 4.546 = 4.84 UK gallons per 100 km
= 12.8 MPG

98 RON octane petrol
17.1/ 4.546 = 3.76 UK gallons per 100 km
= 16.5 MPG
21.4/ 4.546 = 4.71 UK gallons per 100 km
= 13.6 MPG


from this you may be able to work out
rotary engine with petrol must run around 11.5 :1 mixtures up around 1 bar boost
whilst the same engine is quite happy with leaner LPG mixes at this boost
---- LPG's inherent atomisation/ homogeneous mixture and also higher octane are coming to play
- hence the nearly 1 to 1 economy ratio independent of the 25% losses in BTU

i can run 14/15 : 1 mixtures under 1 bar boost with LPG
but with petrol must maintain 11.5 ( +/- 0.5 ) to 1 mixtures
else 14:1 under boost at 1 bar on petrol would amount to a dead engine come start of day 2
( NEVER run a petrol rotary lean under boost )


looking at the price of the fuels,, i have a 50% reduction in operating cost
and a large improvement in emissions
note LPG at lean mixes = low CO and low hydrocarbons but high NOx,,
rotary engines are inherently low NOx , but high CO and hydrocarbons
--- and so they complement each other ----

with few vices beyond the fact at 300 RWHP
i am on the limit of the model E convertor and not far off the tank safety valves limit


LPG rotary FTW

[The_Stuff]

other then propane being clean burning and my engine is still like new @ 225k kms on it, im debating on converting it to gasoline... shame to do it though being a factory installation.

[Frank]

Some comments to the previous replies in the order posted.

• For those of you who aren't tracking your fuel economy (but would like to), I've added some Excel spreadsheets to my original post to help you.
• [Steptoe]I too grew up with the metric system but I still relate to fuel economy in terms of miles per [imperial] gallon.
• [webbew]If you are trying to estimate gasoline economy on an energy basis compared to propane, you would divide by 0.726 rather than multiply. That is, your 16-17mpg (US) average economy on propane would get more like 22.0-23.4 mpg on gasoline. I would expect a station wagon with a 400 CID engine to get closer to 20-21 mpg (US) on gasoline but the 700R4 probably does help a lot.
• [DODGEN1]I've already spoken with the station owners about trying to increase the amount of propane they pump but they weren't receptive to any suggestions. They must know what they are doing when they sell me 55 litres (sometimes more, sometimes less) to fill my 78 litre tank.
• [Steptoe]The US gallon is 3.7854 litres.
• [The_Stuff]Wow! Propane is super expensive where you live and should be less than 72% of price of gasoline on an energy basis. Is there any chance you can renegotiate your price a bit lower? And yes, your fuel economy works out to 17 miles per US gallon.
• [bumpstart]That is excellent!
• [The_Stuff]Converting your car back to gasoline is a tough call because it will have a cost and the payback on it will depend on how much you drive your van. I know propane isn't as common in SK as it is in AB so finding a better price for propane won't be easy. I would also check the price at bulk propane suppliers (i.e., home heating).

[Steptoe]

Stuff on cost
so $30 = 27.5liters $?? 1.09 /L
NZ at the pumps is up around $NZ 1.30/L
With discount card around $NZ 1.10
exchange rate around .76 to $US = 0.86 ..but petrol...91 octance is close to $2.00/L..higher octanes well over $2.
Thats $US 1.52/L or $US 5.78 per US gal.

other then propane being clean burning and my engine is still like new @ 225k kms on it, im debating on converting it to gasoline... shame to do it though being a factory installation.

Looking at future crude supplies, demands over the next few years...Im very certain if u pull the lpg out , you will regret it.

[webbew]

[*][webbew]If you are trying to estimate gasoline economy on an energy basis compared to propane, you would divide by 0.726 rather than multiply. That is, your 16-17mpg (US) average economy on propane would get more like 22.0-23.4 mpg on gasoline. I would expect a station wagon with a 400 CID engine to get closer to 20-21 mpg (US) on gasoline but the 700R4 probably does help a lot.

What I was trying to say is I get 16-17 miles per energy equivalent to 1 petrol gallon. With 1 LPG-gallon, I would get FEWER miles because it's less energy-dense, hence *multiply* mpg by 0.726. Just for confirmation: In mixed driving I use 19 liters of LPG per 100km, which in energy terms is equivalent to 13.8 liters of petrol = 17 mpg (US).

Just came back from a 5 day holiday which didn't have a lot of city driving, but twice we went up a mountain (from sea level to 900m/3000ft), and the average over the whole trip (> 1000km) was 16.5 LPG liters/100km (14.3 miles per LPG US gallon) ==> 12 petrol-equivalent liters/100km = 19.6 miles per petrol-equivalent US gallon.

[webbew]

[quote="Steptoe"]Stuff on cost
so $30 = 27.5liters $?? 1.09 /L
NZ at the pumps is up around $NZ 1.30/L
With discount card around $NZ 1.10
exchange rate around .76 to $US = 0.86 ..but petrol...91 octance is close to $2.00/L..higher octanes well over $2.
Thats $US 1.52/L or $US 5.78 per US gal.


Agree completely, lately it's been an increasingly good comparison again - our family holiday just now (see my post above) cost NZ$ 18.60 / 100km. For that money, you'd have to try to use only 8.7 petrol liters / 100km, which would be a hard ask for a big car

[Steptoe]

I ca swap between " ' chain mile to metric nps
Mph and kmph
weight nps
gal , pint quart to L and back
Centigrade a Farneheit ok
square measure sort of ok
Cubes ok

mpg L/km km /L never got my head around that...just plain simple mpg imperial gals. lol
And presures still think only in " vac and psi.

[The_Stuff]

still I dont think that 17mpg is very good for my van. I think now that I found out the problem with the idle air valve (thanks C3H8!) I should get it adjusted again. I will have to safety it soon as it runs out in december. (every 5 years the propane system needs a safety in saskatchewan) and i hope it passes!! do not really want to buy new tanks and lines!

Reason i do not find it very good is my little 2l diesel truck does 25mpg minimum, and i had a similar mini van with a 2.2 out of a dodge shadow shoe horned in there and it did 40mpg highway. (foot to floor and you couldnt pass anyone lol)