Natural Gas Vehicle Fundamentals

Natural Gas Vehicle Fundamentals
Natural gas vehicles (NGVs) operate on the same basic principles as gasoline-powered vehicles. The fuel is
mixed with air and fed into the cylinder where it is then ignited by a spark plug to move a piston up and down.
Natural gas can power all the same vehicles currently powered by gasoline and diesel fuel – light-, medium
and heavy-duty; on-road and off-road. However, since natural gas is a gas rather than a liquid at standard
pressure and temperature, some modifications are required to make an NGV work efficiently. These changes
are primarily in the fuel storage tank, fueling receptacle/nozzle and the engine.

Fuel Storage
Most NGVs operate using compressed natural gas (CNG) so the fuel takes up less space. CNG is stored on
board vehicles in high-pressure (3,000-3,600 pounds per square inch) in tube-shaped cylinders that are
attached to the rear, top or undercarriage of the vehicle. The cylinders meet very rigorous safety standards.
They are made of high-strength materials designed to withstand impact, puncture and, in the case of fire,
their pressure relief devices (PRDs) provide a controlled venting of the gas rather than letting the pressure
build up in the tank.Natural gas may also be stored on-board in the form of liquefied natural gas or LNG. To
become LNG,natural gas must be cooled to –260 degrees Fahrenheit. The biggest advantage of LNG over
CNG is space requirements. LNG requires only 30 percent of the space of CNG to store the same amount
of energy. In order to keep the LNG cold, LNG is stored on-board vehicles in thermal storage tanks. In other
words, sophisticated thermos bottles.

Fueling Receptacle
Since gasoline and diesel fuel are liquids at standard temperature and pressure, their storage systems
essentially can be open, that is, no special precautions physically must be taken to keep the fuel from
escaping (other than a non-leaking tank). CNG and LNG storage systems, however, must be closed,
that is, their systems must be designed not to let any fuel escape.So where gasoline and diesel fueling
receptacles are the familiar larger open tube into which a smaller fueling nozzle is inserted, natural gas
nozzles lock onto the receptacles, and form a leak-free seal, similar to the coupling on an air compressor
nozzle. The receptacles are designed so that, when the nozzle is removed, the gas is prevented from escaping.