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.

  Honda Civics coming down production line.
  CNG canisters stacked behind the cab

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.

  LNG refueling  

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.

Engine Modifications
When the engine in an NGV is started, natural gas flows from the storage cylinders into a fuel line. Near the engine, the natural gas enters a regulator to reduce the pressure. Then the gas feeds through a gaseous fuel-injection system, which introduces the fuel into the cylinders. Sensors and computers adjust the fuel-air mixture so that when a spark plug ignites the gas, it burns efficiently and very cleanly. For LNG, the liquid is heated, converting it back to a gas. From that point on, the process is similar to CNG engines.

NGV Fueling Fundamentals
At CNG stations, the gas is typically taken from the local gas utility’s line at low pressure, compressed and then stored in the vehicle’s storage tanks at high pressure. There are basically two types of fueling equipment – fast-fill and time-fill. In fast-fill, the combination of a large compressor coupled with a high-pressure storage tank system (called a cascade) fills the tank in about the same amount of time it takes to fuel a comparable petroleum vehicle.

  CNG refueling at the Sky Harbor Airport in Phoenix  

A time-fill system does not have a storage system and has a much smaller (and less expensive) compressor. It typically refuels vehicles overnight at a rate of about one gallon per hour. The Fuelmaker Company has introduced the Phill Home Refueling Appliance. The Phill, which is typically hung on a garage wall, connects to a residential gas line and fuels a single vehicle at the rate of slightly less than a half a gallon per hour.

While LNG can be produced on-site from available natural gas, it is typically delivered to the station via tanker truck. In either case, the LNG is stored onsite in special cryogenic storage tanks. To fuel vehicles, LNG is pumped into the vehicles much like other liquid fuels (although using much more sophisticated cryogenic fueling equipment.

There also are LCNG fueling stations. A typical LCNG stations uses LNG to fuel both LNG and CNG vehicles. LNG vehicles are fueled as described above. For CNG vehicles, the LNG is compressed as a liquid and then gasified. The high-pressure gas is then stored on the vehicle as with a fast-fill CNG station. Since it takes less energy to compress a liquid than a gas, once the LNG is available, LCNG stations are less expensive to operate.