Propane, also known as liquefied petroleum gas (LPG), or 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 270,000 on-road propane vehicles in the United States. Many are used in fleet applications, such as police cars, shuttles, and school buses.
The development of new light- and medium-duty propane vehicles has surged in recent years for fleet use. Propane vehicles can either be conversions from gasoline vehicles or purchased from OEMs. Engines and fueling systems are also available for heavy-duty vehicles, such a street sweepers and school buses, including some from original equipment manufacturers.
Types of Propane Vehicles
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. There are also two types of fuel-injection systems available: vapor injection and liquid propane injection. In both types, propane is stored as a liquid in a relatively low-pressure tank.
A propane vehicle's power, acceleration, and cruising speed are similar to those of gasoline-powered vehicles. The driving range for dedicated and bi-fuel vehicles is also comparable to that of gasoline vehicles. Extra storage tanks can increase range, but the additional weight displaces payload capacity.
Lower maintenance costs are one reason behind propane's popularity for use in light-duty vehicles, such as pickup trucks and taxis, and for heavy-duty vehicles, such as school buses. Propane's high octane rating (104 to 112 compared with 87 to 92 for gasoline) and low carbon and oil contamination characteristics have resulted in documented engine life of up to two times that of gasoline engines. Because the fuel's mixture (propane and air) is completely gaseous, cold start problems associated with liquid fuel are reduced.
Compared with vehicles fueled with conventional diesel and gasoline, propane vehicles can produce lower amounts of harmful emissions, depending on vehicle type, drive cycle, and engine calibration.
How Propane Vehicles Work
Propane vehicles work much like gasoline-powered vehicles with spark-ignited engines. 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. The vapor is fed to a mixer located near the intake manifold, where it is metered and mixed with filtered air before being drawn into the combustion chamber where it is burned to produce power, just like gasoline.
Liquid propane injection engines do not vaporize the propane. Instead, it is injected into the combustion chamber in liquid form. Liquid injection systems have also proven reliable in terms of power, engine durability, and cold starting.