Thread: Dual fuel-propane and gasoline

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This is an interesting thread...Probably because of the different experiences you guys have had in the past with propane/LPG..
R Pope,,was it dual fuel??and it didnt like running in the cold??Different..mine loves the cold temperatures better than the warm,in fact,its a real hassle when the day is hot..so much so,that I am going to rig up a cold air box and ram air to the carby..Yes,on petrol,my little 302w goes better than LPG,but,I would rather run LPG because of the savings on a trip..so its tuned for LPG..
NTFDAY,,Dont know about you guys,but we have the mother of all inspections,tests down here,,in fact,you can fit your own system,but it has to be tested/checked out by a registered LPG fitter before it can be used..and a yearly warrant of fitness check,[and the tanks are pressure tested every ten years,,just did mine yesterday,,systems all go again.]to make sure everything is hunky dory,,plus,as we have a Warrant of fitness check every 6 months on cars/vans etc over 5 years old,if you dont have a current LPG WOF,you dont get a wof for the vehicle..$150 fine if caught without one..
Dave S..We also ran CNG[compressed natural gas] down here,,has an even higher octane rating,from memory..was dirt cheap,and only did about HALF the distance that LPG or petrol did..in fact,I think it worked better in the tests they did on our aging diesel bus fleet,,although that didnt last long either....Ill stick to what I have got,it works for me..

lamin8r, my Dodge was supposed to be dual fuel but the PO never used the gasoline EFI so it was all clogged up and disconnected. At -40, I had to drive something else, the propane wouldn't flow.
Another thing...with any pressurized gas, you can't use underground parking, and they get all hyper when you drive on a closed ferryboat. They make you shut off the tank valve as soon as you stop. And I've been told, but never looked in to it, that if your garage goes up in flames with a propane vehicle in it, there's no insurance. Which all seems strange when you consider how many little forklifts there are in big warehouses that are running on propane so the monoxide doesn't kill everybody!
BTW, you are talking LPG gas, which is a different animal altogether from propane. Thousands of pounds pressure in the tank instead of less that a hundred with propane. Around here, you need a megabuck compresser to use that stuff. A neighbour had a test setup installed for free, and he still said it didn't pay! He ran all his tractors on LPG, as well as his road vehicles. In busy farming seasons, the compresser couldn't keep up with the demand of all the machines needing fuel every night.
No, thanks, I'll stick with gasoline!

Rereading posts, I think I have mixed up LNG with LPG a few times.

Originally Posted by R Pope

Rereading posts, I think I have mixed up LNG with LPG a few times.

I know where you are coming from,,each to his own,and yes,regarding mix up,,I THINK the LNG is a problem child,,we dont have that down here[I dont think so],but there has been talk of an LNG installation at our little port of Taranaki,and no one wants a bar of it... because of that reason,,the pressure,and what would happen if it all went tits up..New Plymouth would disappear..[only get mauled a bit with LPG]I think that one of the reasons why the LPG has been a real good deal down here,is the distance we are from oil producing countries,even though we have the oil down here,its the cost of exploration,so the gas side has been put to good use..

To answer the OP's question about dual fuel systems and TBI, the propane system still requires the use of a mixer to supply propane to the engine. The difference between a propane mixer and a propane carburetor is that the carburetor has throttle valves (usually using the throttle body from a gasoline carburetor) in addition to the mixer. Therefore, a straight (mono-fuel) propane system replaces the gasoline carburetor with a propane carburetor. A dual fuel system adds a mixer (typically upstream) of the gasoline carburetor and solenoid valves on the fuel lines determine which system supplies fuel to the engine. As described by lamin8r, you switch back & forth between fuels with a 3-position switch that energizes the appropriate solenoid valves.

Mixers are used with carburetor and TBI engines. Multiport EFI engines are best converted with injection conversions. That is, rather than having a mixer fumigate propane into the intake manifold, an injection conversion adds a parallel injection system to the engine and has propane injector nozzles installed at the intake ports. Although theoretically possible to have propane port-injection with a gasoline TBI system, there are no commercial systems that do this. Technocarb still makes dual fuel TBI conversions.

On an energy basis, a gallon of propane has about 74% of the energy of gasoline. However, open-loop propane mixer systems typically get more like 75-80% of the vehicle's gasoline economy. Feedback systems generally get better fuel economy than open-loop systems. Because of the lower energy content of propane, propane needs to priced much less than 74% of the price of gasoline to have a reasonable payback on the conversion. The lower price should still make up for the slightly lower fuel economy from the additional weight of the propane tank.

The retail price of propane is usually very high compared to gasoline because there is high margin on small volume sales (like 20# BBQ cylinders). To get a much better price on motor fuel, you would have to get an account with a propane supplier. For example, Ferrell Gas in Buffalo, NY is selling propane for $2.35/gallon (winter price), which is their first quantity break (1000-5000 gallons/year). If you would have locked-in the price last summer when the market was low, your price for this year would have been $1.95/gallon. The taxes on motor fuel in NY state amount to $0.2635/gallon so your total price for propane could have been $2.2135/gallon.

Do NOT use a 100# cylinder in an on-road vehicle. It is illegal and will not pass NPFA 58. Only use ASME motor fuel tanks for on-road applications.

Propane burns marginally hotter than gasoline. It has a different flame speed than gasoline so it is very important to use a propane-specific advance curve. This is a bit more challenging to do with dual fuel systems now that Dual Curve is gone but there are alternatives.

Although you can increase your CR to take advantage of propane's 104 octane rating, it is not necessary. For street engines, a 10:1 CR will work well and you keep high-test gasoline in the tank for dual fuel systems. With a dual fuel system, you would primarily be using gasoline to keep it from becoming stale so the added cost of high test would really be negligible.

If you travel long distances, it is a good idea to plan your trips around fuel stops. Propane is plentiful in the USA, but cheap fuel is what you're after. Check the following sites for alternative fuel locations:

• Alternative Fuel Fill Stations and Prices
• Alternative Fuels and Advanced Vehicles Data Center: Alternative Fueling Station Locator
• Pilot Flying J - Fuel Pricing