Natural Gas Vehicles, General Information

The vehicle can be equipped with a compressed natural gas (CNG) fuel system, or a liquefied natural gas (LNG) fuel system. CNG is made by compressing natural gas to less than 1% of its volume at standard atmospheric pressure. LNG is created by condensing natural gas into a liquid by cooling it to approximately -259°F (-162°C). CNG often contains an odor-producing chemical, while LNG does not have any odor.

When vaporized at ambient temperatures, natural gas is less dense than air, and it rises and disperses. However, LNG is intially heavier than air due to its cryogenic temperature. When released in large amounts, it may drop or pool on the ground before its temperature rises enough to vaporize. Cold atmospheric conditions may prevent natural gas from dispersing quickly when released in large amounts.

Natural gas is nontoxic, but it can cause asphyxiation if present in very large concentrations.

For natural gas to burn, it must first vaporize, then mix with air in the proper proportions (flammable range is 5 to 15% by volume in air), and then be ignited.

A typical natural gas fuel system consists of:

• Fuel supply cylinders that store compressed gas at high pressure (CNG), or a tank that stores liquefied gas at an extremely low temperature (LNG).

• A vaporizor or heat exchanging device that changes LNG to gaseous form (LNG fuel systems only).

• Pressure relief and manual fuel shutoff valves.

• A filling connection with a check valve that prevents the gas from flowing back out of the fuel filling line.

• High-pressure and low-pressure fuel filters.

• A pressure control regulator that reduces the high fuel tank or cylinder pressure to the lower pressure needed for the engine.

• A fuel contents gauge that indicates the fuel supply in the tank or cylinders.