The structure of typical vehicle’s fuel storage container for the diesel and gasoline fuel is simple because the fuel stays as liquid at normal temperature and pressure. However, the structure of LNG vehicle fuel tanks is very complex and difficult to handle. That is because the insulation should be assured for -160℃ temperature of cryogenic liquid fuel and the physical strength that must be satisfied by the double bulkheads and vehicle vibration impact at the same time. In addition, fuel boost pump is needed but the cost of the pump is very high. Due to these complex structures and expensive fuel pressure boost pump, most of the vehicle’s LNG vessel transfers the fuel to the vehicle’s engine by its own boost with LNG vaporization without the pressure booster pump. However this boost method may make the fuel to be wasted. As the fuel could be wasted and could not be delivered to the engine properly, the vehicle may lose power or even stop. In order to remove and prevent these problems, we need to understand configurations and interactions of all the related parts needed by LNG tanks, engine and vehicle. .

Test conditions

• Pressure of LNG fuel line (0.9MPa, 1Mpa, 1.1MPa, 1.5MPa)
• Inner sectional area of gaseous fuel tube (39-44mm2;O.D3/8”tube, 82-90mm2;O.D1/2”tube)
• Inner sectional area of coolant hose (71mm2;I.D 9.5mm hose, 201mm2;I.D 16mm hose)

Summary of engine test

• The pressure of LNG fuel line should be over 1.1MPa to keep the engine maximum power.
• The inner-sectional area of gaseous line tube should not be below 80mm2 to keep the engine maximum power.
• The inner-sectional area of coolant hose should not be below 200mm2 to keep the engine maximum power.

Vehicle test

Summary of vehicle test

• Total test distance; 13,000km
• Fuel efficiency; 3.5km/kg
• Occurring issue; A LNG leakage from pressure controller of LNG tank.
• Note; Internal LNG tank pressure should not be below for requiring minimum pressure to engine start.
• When LNG runs out on the road, an emergency refueling equipment is needed.

LNG Tank

• Surface treatment; Painting is better than polishing to reduce heat penetration into LNG tank. But it is not easy to maintain condition for a long time.
• Fixing bracket;The bracket has to enough strength to protect the slip and rotation of LNG tank by vehicle’s vibration.

Pressure controller

• To supply LNG in stable to engine as per circumstances, appropriate selection of LNG tank’s pressure controller is required basically.

Fuel delivery

The tank has the capability to withdraw "liquid only" fuel from the bottom of the tank for delivery to the engine. Gaseous methane is never withdrawn from the tankduring normal operation of the vehicle. This operating capability eliminates any possibility of weathering of the LNGin the fuel tank and standard LNG is recommended.

The vehicle fuel tank uses a pressure–build regulator to maintain the tank operating pressure totally independent of the saturation temperature and pressure of the LNG. The tank pressure will slowly decrease as liquid is withdrawn from the tank to operate the vehicle. As the pressure in the tank drops past the set point the regulator will open and liquid will be drawn into the pressure build-up coil and build pressure, on demand.

Liquid fuel line

• LNG runs with liquid state from the tank’s fuel outlet to the vaporizer entrance.
• Bigger tube is better, but O.D3/8”(9.5mm)tube is more effective than O.D1/2” (12.7mm)tube.

Vaporizer

• The vaporizer(Heat exchanger) changes LNG from liquid to gaseous state. And this state change is increases the pressure of fuel line.
• The size of vaporizer can be determined by the heat exchange area, flow rate andtemperature of coolant.
• Total heat exchange area of 700cm2 is proper to vaporize about 50kg/h LNG flow.

Vaporizer continue

• The coolant hose of I.D 16mm is more proper than those of I.D 9.5mm, because the flow rate of coolant is affected by the size
• Parallel-flow of the coolant for fuel is better than counter-flow to get more stable fuel pressure

Gaseous fuel line

• LNG runs with gaseous state from the vaporizer outletto the engine.
• Bigger tube is better, but O.D1/2”(12.7mm) tube is more effective than other size tube.

Pressure regulator

• Additional pressure regulator is required formore accurate supply of gaseous fuel.
• The size of regulator’s orifice should be bigger than those of equivalent CNG applications.
• If temperature of gases fuel at this point is higher to unfreeze the regulator, heating to warm it up is not needed.

Conclusion

Key in LNG tank installing to vehicle is

• Appropriate selection and configuration of related fuel pressure control parts.
• Secure enough sectional area of fuel line from LNG tank to fuel injector.
• Secure enough heat exchange area and coolant flow rate of vaporizer.