Thread: Dual fuel-propane and gasoline

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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