Natural gas has less energy by volume than diesel or gasoline. This means that natural gas must be compressed or liquefied to store enough energy onboard for adequate driving range. The type of station equipment needed to refuel a natural gas vehicle differs depending on whether the fuel is in a compressed or liquefied form.
Compressed natural gas (CNG) is stored on a vehicle in tanks at high pressure. Compression reduces the volume by 300 times or more compared with natural gas at normal pressure. A CNG-powered vehicle has similar fuel efficiency to a diesel or gasoline vehicle, but needs more space to hold the same amount of energy in its fuel storage tanks. CNG fuel storage tanks are made of durable materials with built-in safety controls.
At present in Canada, there are differences in the pressure at which natural gas can be dispensed. Public stations must dispense fuel at 3,000 pounds per square inch. Private fleet stations can dispense fuel at 3,600 pounds per square inch. This higher pressure allows for greater vehicle driving range and it is also the standard used for all CNG stations in the United States. Harmonizing dispensing pressure with the United States is a priority for natural gas vehicles in Canada.
Liquefied natural gas (LNG) is natural gas that has been cooled to its liquid state at 162 degrees Celsius. Liquefying the gas reduces the space it occupies by approximately 620 times compared with natural gas at normal pressure. LNG is stored in double-wall insulated tanks on vehicles and in large, insulated storage vessels at stations. LNG vehicles have similar fuel efficiency as diesel vehicles, but more space is needed on the vehicle to hold the same amount of energy compared with diesel fuel.
LNG has greater energy by volume than CNG, so LNG vehicles with the same tank volume have a greater driving range than CNG vehicles. Highway tractors that have high daily mileage requirements often use LNG. As LNG warms up, it returns to its gas state.
There are two forms of LNG as a vehicle fuel – saturated LNG and unsaturated LNG. The difference between these forms is temperature. Saturated LNG is warmer which results in a higher tank pressure. Vehicles equipped with Cummins Westport engines require saturated LNG. Vehicles equipped with Westport Innovations engine systems can operate on either saturated or unsaturated LNG, although vehicle range increases with unsaturated LNG.