A CNG conversion kit alters a vehicle with a conventional gasoline or diesel internal combustion engine to run on compressed natural gas — a less expensive and more environmentally clean alternative to traditional gasoline and diesel. Vehicles may be modified to run manually; exclusively on CNG (dedicated CNG vehicles); or they may automatically switch back and forth between CNG and gasoline or diesel (bi-fuel vehicles). Bi-fuel vehicles incorporate two separate fuel tanks — one for gasoline or diesel fuel and another for CNG — and their engines may utilize either of these fuel types at one time.
CNG conversion kits are available for almost any existing car sold in the United States. The kits are available online and at retail stores. While it is claimed that the conversion process is complicated, consumers who are mechanically adept may choose to install the kit themselves. Others opt to have a professional install them.
Many of CNG conversion kits, however, are not certified by the Environmental Protection Agency (EPA) or the California Air Resources Board (CARB). Moreover, in the US and California, the EPA and CARB mandate the purchase of only EPA- or CARB- certified CNG conversion systems. They also require that CNG conversion kits be installed by properly trained installers, known as qualified system retrofitters (QSRs).
A CNG conversion kit incorporates steel, aluminum, plastic or composite CNG storage cylinders; pressure regulators to convert the gas from storage pressure in the tank to metering pressure; gas injectors or gas mixers to meter the fuel; gas cylinders and valves; and the requisite electronic systems, wiring, high-pressure tubing, fittings, switchgear, and accessories. Depending on the company that sells them, the basic CNG conversion kit may or may not include the storage cylinder or other features.
The savings gained by CNG conversion can cover the CNG conversion kit itself, the process of installation, and whatever accessories the customer desires. A significant variable is the amount of fuel capacity desired, as the cost of CNG storage cylinders is high. However, this expense is often recouped in fuel savings, although this may not be applicable for older cars that don't have a long life ahead of them. In addition, income tax credits are granted to customers who install certified retrofitting systems in their cars, which may make a CNG conversion kit an even more attractive option.
Only CNG conversion kits approved by the EPA or CARB are permitted to be installed on vehicles manufactured since 1994 — the year the EPA began to address the issue of fuel conversion systems. As for vehicles manufactured before 1994, owners or installers are not permitted to modify them in ways that might diminish the vehicles' emissions performance. The EPA considers the installation of non-certified CNG conversion kits a federal violation, specifically “tampering with a federally approved emission control system,” which is punishable by a financial penalty of more than $5,000 US Dollars (USD) daily.