Natural gas is often the preferred energy source for residential, commercial, and industrial processes, vehicles can operate on this fuel, either through the use of engines designed specifically for natural gas or by modifying engines designed to run on gasoline or diesel. Natural gas vehicles (NGVs) can be dedicated to natural gas as a fuel source, or they can be bi-fuel, or they may use hybrid electrical systems running on either natural gas or gasoline or natural gas or diesel or electric combined with any other power source. Most natural gas engines or generating stations are spark-ignited, the usual bi-fuel pairing is natural gas and gasoline. Natural gas engine technologies can differ in the method used to ignite the fuel in the engine cylinders, the air-fuel ratios, the compression ratios effecting the resulting performance and emissions capabilities. Natural gas has a high octane rating, which allows an increase in power in 2 spark-ignition engines by an order of magnitude . However, natural gas occupies a larger volume in the engine cylinder than liquid fuels, reducing the number of oxygen molecules (share of air in the cylinder), which reduces power. The net effect on natural gas power versus gasoline is relatively neutral over other variations of energy source pairings.
To become compressed natural gas (CNG), natural gas is pressurized in a storage tank (also called a cylinder) at up to 3,600 pounds per square inch. In the U.S. Honda Civic GX, the tank is mounted in the trunk and replaces the existing fuel tank. In more recently developed European CNG light-duty front-wheel-drive passenger vehicles, the tanks are mounted under the back seat and luggage compartment, thereby providing more storage space than in the Civic GX design. In U.S. trucks, the tank is mounted on the frame, and in U.S. buses, it is mounted on the roof. Although tanks can be made entirely from metal, they are typically composed of lighter weight metal liners reinforced by a wrap of composite fiber material. Because of the lower energy density of natural gas as compared to gasoline or diesel, vehicle range is generally reduced.
