Propane, also known as liquefied petroleum gas (LPG), or propane autogas, is considered an alternative fuel under the Energy Policy Act of 1992. According to the Propane Education and Research Council, there are more than 143,000 on-road propane vehicles in the United States. Many are used in fleet applications, such as school buses, shuttles, and police vehicles.
The availability of new light- and medium-duty propane vehicles has surged in recent years, especially for fleet use. Propane vehicles can either be from an original equipment manufacturer (OEM) or conversions. Engines and fueling systems are also available for heavy-duty vehicles, such as street sweepers and school buses, including some prep-ready engines from OEMs, which are included in equipment packages with components that allow conventional vehicles to run on propane.
Types of Propane Vehicles
Propane vehicles have been widely used and refined for decades. There are two types of propane vehicles: dedicated and bi-fuel. Dedicated propane vehicles are designed to run only on propane, while bi-fuel propane vehicles have two separate fueling systems that enable the vehicle to use either propane or gasoline.
A propane vehicle's power, acceleration, and cruising speed are similar to those of conventionally fueled vehicles. The driving range for dedicated and bi-fuel vehicles is also comparable. Extra storage tanks can increase range, but the tank size and additional weight affect payload capacity.
The potential for lower maintenance costs are one reason behind propane's popularity for use in light- and medium-duty vehicles, such as trucks and taxis, and for heavy-duty vehicles, such as school buses. Propane's low carbon and low oil contamination characteristics may result in longer engine life. Propane performs well in cold weather climates because the fuel's mixture (propane and air) is completely gaseous. This factor allows propane-powered vehicles to avoid many cold-start issues associated with using liquid fuels.
Compared to vehicles fueled with conventional diesel and gasoline, propane vehicles can produce lower amounts of harmful tailpipe emissions, depending on vehicle type, age, and drive cycle.
Propane vehicles are similar to their gasoline counterparts with regard to power, acceleration, cruising speed, and driving range. Bi-fuel vehicles include a secondary fuel system and fuel supply, giv¬ing them a much greater range than dedicated propane or gasoline vehicles have. Likewise, larger storage tanks can increase range, but the additional weight displaces payload capacity. Because a gallon of propane has 27% less energy than a gallon of gasoline, the fuel economy of propane vehicles is slightly lower. However, propane has a higher octane rating than gasoline (104-112 compared to 87-92 for gaso¬line), and some original equipment manufacturers (OEMs) offer dedicated engines optimized to take advantage of this higher rating. This can result in improved performance and fuel economy over non-optimized engines.
How Propane Vehicles Work
Propane vehicles operate much like gasoline vehicles with spark-ignited engines. There are two types of propane fuel-injection systems available: vapor and liquid injection. In both types, propane is stored as a liquid in a relatively low-pressure tank (about 150 pounds per square inch). In vapor-injected systems, liquid propane travels along a fuel line into the engine compartment. The supply of propane to the engine is controlled by a regulator or vaporizer, which converts the liquid propane to a vapor. Liquid propane injection engines do not vaporize the propane and allow more precise control of the fuel delivery, resulting in improved engine performance and efficiency.