Liquefied natural gas (LNG) use as a heavy-duty vehicle fuel is increasing. Current generation high-horsepower natural gas engines used in trucks and buses typically require fuel supply pressures in the range 75 to 120 psig. LNG delivered to the fueling station usually has a saturation pressure of roughly 10 psig. A variety of approaches may be used to provide the required fuel pressure increase. Each approach involves a different on-vehicle fuel system design, and LNG station capabilities must accommodate vehicle fuel system requirements. This paper describes various LNG vehicle fuel system design strategies including key tradeoffs, implications on station requirements, and developmental status.
The most commonly used fuel system design receives and stores the LNG in the vehicle tank at a saturation pressure at least equal to the engine fuel supply pressure requirement. This design is relatively simple, but it requires fuel conditioning (saturation pressure and temperature increase) at the station. This increases station cost and decreases fuel density and hence vehicle range. Saturated LNG fuel systems typically include a pressure-reducing function which can increase the tank hold time for most applications.
LNG vehicle fuel systems based on non-saturated LNG theory are also being developed and tested. These seek to maintain the liquid in the fuel tank in a subcooled state (at lower temperatures and higher densities than saturated systems). This approach employs a pressure-building circuit. A key issue for pressure-building circuits, which are also used in certain saturated fuel system concepts, is the time needed to build initial pressure following refueling. Pumped systems have been used for direct-injection natural gas engines with very high fuel pressure requirements that cannot be met with practical pressure-transfer systems. Cryogenic pumps may also be used for lower pressure requirements, but there are concerns regarding their relative cost and durability for this application.