LNG Vehicle Fuel Tanks



One of the major barriers to using LNG as an alternative fuel is that very specialized, expensive cryogenic containers are required to store and deliver the fuel to the vehicle engine. Unlike the passive fuels, such as gasoline, diesel, compressed natural gas or propane, LNG is continually changing state as heat enters the liquid fuel through the tank plumbing, suspension system and insulating materials.

Conventional, "industrial gas" type cryogenic containers have been used almost exclusively as vehicle fuel tanks by transit fleets in the U.S. since 1996 as LNG vehicle fuel tanks. Unfortunately, almost all of these projects have been abandoned due to operational problems caused by excessive heat leak that is inherent in these tanks. These problems include weathering of the LNG, variations in tank pressure, fuel density and engine failure.

The LNG vehicular fuel tanks designed and patented by Cryogenic Fuels Inc are engineered and certified specifically for end-use in the transportation fuels market. The tanks are of all stainless steel construction and consist of an inner pressure vessel suspended within an outer tank by means of a non-metallic central support beam.

This rugged beam design is combined with multi-layer insulation and a high order vacuum is provided to reduce the amount of "heat leak" into the cold LNG in the inner tank. The result is a thermal protection system that is, at a minimum, four (4) times more efficient than, conventional cryogenic containers.

Fuel Delivery

The most outstanding performance feature of the CFI vehicle fuel storage tank is the capability to withdraw "liquid only" fuel from the tank for delivery to the engine. This delivery mode assures pressure stability from tank full to tank empty conditions and consistent "fuel density" to the engine.

Conventional "industrial gas" cryogenic containers must withdraw both vapor and liquid at the same time in order to maintain the operating pressure of the tank. This, results in excessive pressure variation in the tank and changes in fuel density delivered to the engine.

Pressure Management

The CFI tank operating pressure is maintained totally independent of the saturation temperature of the LNG and it is not necessary to precondition (heat) the LNG in order to maintain the vehicle fuel tank operating pressure. A conventional pressure build regulator, on the tank, is preset to maintain the specified engine operating pressure. The regulator is combined with a short length of "heat exchange" tubing, to maintain the tank operating pressure within 5 PSI of the preset pressure from tank full to tank empty.

Fill Control

The Cryogenic Fuels tank design provides a reliable means of controlling the ullage space within the tank in order to comply with Federal and State codes. The vent return line serves as a device that provides an indication of when the tank is filled to the maximum allowable liquid level, of 90 % full and will allow for the expansion of the LNG after the fill of the tank.

The configuration of the vent return line above the liquid, vapor, interface provides a flow path for the liquid out of the tank at the point of maximum fill, while at the same time establishing a pressure pad at the top of the tank that prevents the tank from being overfilled.