The way fuel is stored on a natural gas vehicle depends on the following factors:
- The availability of compressed natural gas (CNG) or liquefied natural gas (LNG)
- The size and design of the vehicle chassis
- The daily mileage needs for the vehicle
- Available refueling options
- Turning radius and payload needs
Fuel storage systems for natural gas vehicles are installed in different ways. In most cases, truck or bus manufacturers install the storage system at the factory. In some cases, the fuel storage system is installed at a different location, such as at a truck-body builder. This latter method is often used for vehicles, such as refuse trucks, that are manufactured to customer specifications.
Natural gas tank packages come in a variety of options for both CNG and LNG vehicles. Tanks must meet rigorous safety standards. They are designed to last the life of the vehicle, assuming normal maintenance and inspection. Tank location depends on the type of vehicle and on fleet needs. Tanks can be mounted on the roof of a transit bus, behind the cab or on the frame of a truck, or on the body of a refuse vehicle. Tanks for CNG typically come in 50, 60, and 75 diesel gallon equivalent packages. Dealer representatives work with fleets to determine the right tank package.
CNG Tanks and Fuel Systems
CNG is stored on vehicles at 3,000 to 3,600 pounds per square inch in tube-shaped cylinders. There are four types of CNG tanks, but only lightweight tanks are used on medium and heavy natural gas vehicles. A lightweight tank is less than half the weight of a steel tank. CNG tanks are made of high-strength materials designed to withstand impact and puncture. In case of fire, a pressure relief device provides controlled venting of the gas rather than letting the pressure build up in the tank.
A typical CNG fuel system includes storage tanks, fuel lines, a regulator, and engine compartment components. Fuel lines are connected to a pressure regulator that reduces pressure to the engine, where a second regulator controls the engine fuel requirements.
LNG Tanks and Fuel Systems
LNG is stored in double-walled vacuum tanks to keep the fuel in a liquid state at a temperature of – 162 degrees Celsius. These tanks are made of stainless steel and are designed to meet stringent safety requirements. For highway tractors with the Westport LNG engine system, fuel storage tanks are available in 70, 100, and 120 water gallon equivalent sizes. Water gallon refers to the total volume of the tank. The actual usable volume of the tank for LNG is lower because a cryogenic pump is installed in the tank which takes up space. Multiple tanks can be arranged to meet regional and long-haul trucking needs.
The table below shows the actual usable volume of fuel for each available tank size in LNG gallons, diesel gallon equivalent, and diesel litre equivalent. An LNG gallon takes up 1.7 times as much space as a diesel gallon.
The operating pressure of an LNG fuel system depends on whether the vehicle is equipped with a Cummins Westport engine or a Westport HD engine system. In both cases, the LNG needs to be warmed and returned to a gas before delivery to the engine at the required pressure.