CNG (Compressed Natural Gas) is composed primarily of methane. It is stored under pressure on the vehicle at pressures up to 250 bar (3600 pounds per square inch). CNG cylinders are manufactured to internationally approved standards and made from light, high strength composite materials.
There are three types of Natural Gas Vehicle: Dedicated, Bi-Fuel and Dual Fuel. Dedicated vehicles run on natural gas only.
Bi-Fuel vehicles operate on CNG whilst retaining the ability to use petrol as a reserve fuel. The engine can operate on either fuel but not on both simultaneously. The compression ratio of the engine must remain at a level suitable for petrol. Currently this type of engine is used almost exclusively on vehicles below 3,500kgs.
Dual Fuel engines are derived from diesel engines. A small amount of diesel is retained as a pilot source of ignition. The primary fuel Natural Gas, is mixed with the incoming air as the bulk fuel. (Natural Gas Vehicle Assoc Website – www.ngva.co.uk )
Because of its relatively high Hydrogen content it produces less CO2 (and more H2O) than Gasoline or Diesel which have a higher proportion of Carbon. It also burns at a lower temperature and so generates less NOx.