What is a natural gas vehicle?
Dedicated natural gas vehicles (NGVs) are designed to run only on natural gas; bi-fuel NGVs have two separate fueling systems that enable the vehicle to use either natural gas or a conventional fuel (gasoline or diesel). In general, dedicated NGVs demonstrate better performance and have lower emissions than bi-fuel vehicles because their engines are optimized to run on natural gas. In addition, the vehicle does not have to carry two types of fuel, thereby increasing cargo capacity and reducing weight.
Natural gas vehicles are fueled with compressed natural gas (CNG) or liquefied natural gas (LNG). These fuels are considered alternative fuels under the Energy Policy Act of 1992 and qualify for alternative fuel vehicle tax credits. As a new twist, tests are being conducted using natural gas vehicles fueled with HCNG, a blend of CNG and hydrogen.
Compared with vehicles fueled with conventional diesel and gasoline, NGVs can produce significantly lower amounts of harmful emissions. In addition, some natural gas vehicle owners report service lives two to three years longer than gasoline or diesel vehicles and extended time between required maintenance.
The driving range of NGVs generally is less than that of comparable gasoline- and diesel-fueled vehicles because of the lower energy content of natural gas. Extra storage tanks can increase range, but the additional weight may displace payload capacity. NGV horsepower, acceleration, and cruise speed are comparable with those of an equivalent conventionally fueled vehicle.
Other benefits of NGVs include increasing U.S. energy security and paving the way for fuel cell vehicles.
How Natural Gas Vehicles Work
Light-duty natural gas vehicles work much like gasoline-powered vehicles with spark-ignited engines. This schematic shows basic CNG fuel system components.
CNG enters the vehicle through the natural gas fill valve (A) and flows into high-pressure cylinders (B). When the engine requires natural gas, the gas leaves the cylinders and passes through the master manual shut-off valve (C). The gas travels through the high-pressure fuel line (D) and enters the engine compartment. Gas enters the regulator (E), which reduces the gas pressure used for storage (up to 3,600 psi) to the required vehicle fuel injection system pressure. The natural gas solenoid valve (F) allows natural gas to pass from the regulator into the gas mixer or fuel injectors. The solenoid valve shuts off the natural gas when the engine is not running. Natural gas mixed with air flows down through the carburetor or fuel-injection system (G) and enters the engine combustion chambers where it is burned to produce power, just like gasoline.
Schematic of a natural gas vehicle fueling system. Adapted from Compressed Natural Gas: A Suite of Tutorials (PDF 44 KB). Download Adobe Reader. Courtesy of Thomason & Associates, Inc.
Some heavy-duty vehicles use spark-ignited natural gas systems, but other systems exist as well. High-pressure direct injection engines burn natural gas in a compression-ignition (diesel) cycle. See Development of the High-Pressure Direct-Injection ISX G Natural Gas Engine (PDF 617 KB). Heavy-duty engines can also burn diesel and natural gas in a dual-fuel system. See City of Los Angeles Bureau of Sanitation LNG Heavy-Duty Trucks (PDF 352 KB). Download Adobe Reader.