Propane as an auto fuel has a high octane value and has key properties required for spark-ignited internal combustion engines. To operate a vehicle on propane as either a dedicated fuel or bi-fuel (i.e., switching between gasoline and propane) vehicle, only a few modifications must be made to the engine. Until recently propane vehicles have commonly used a vapor pressure system that was somewhat similar to a carburetion system, wherein the propane would be vaporized and mixed with combustion air in the intake plenum of the engine. This leads to lower efficiency as more air, rather than fuel, is inducted into the cylinder for combustion (Myers 2009). A newer liquid injection system has become available that injects propane directly into the cylinder, resulting in no mixing penalty because air is not diluted with the gaseous fuel in the intake manifold. Use of a direct propane injection system will improve engine efficiency (Gupta 2009). Other systems include the sequential multi-port fuel injection system and a bi-fuel 'hybrid' sequential propane injection system. Carbureted systems remain in use but mostly for non-road applications. In the United States a closed-loop system is used in after-market conversions. This system incorporates an electronic sensor that provides constant feedback to the fuel controller to allow it to measure precisely the proper air/fuel ratio. A complete conversion system includes a fuel controller, pressure regulator valves, fuel injectors, electronics, fuel tank, and software. A slight power loss is expected in conversion to a vapor pressure system, but power can still be optimized with vehicle modifications of such items as the air/fuel mixture and compression ratios. Cold start issues are eliminated for vapor pressure systems since the air/fuel mixture is gaseous. In light-duty propane vehicles, the fuel tank is typically mounted in the trunk
for medium- and heavy-duty vans and trucks, the tank is located under the body of the vehicle. Propane tanks add weight to a vehicle and can slightly increase the consumption of fuel. On a gallon-to-gallon basis, the energy content of propane is 73% that of gasoline, thus requiring more propane fuel to travel an equivalent distance, even in an optimized engine (EERE 2009b).