LNG stations have refrigeration equipment to maintain the LNG at a constant temperature. When a vehicle is refueled, the LNG in the vehicle's tank will have gained some temperature and will be at a warmer temperature than the LNG supply temperature. When fuel is added to the vehicle's tank, the fresh LNG sinks to the bottom of the tank, which results in stratified fuel. The warmer LNG remains on top and provides the tank pressure while the newly added colder LNG sinks to the bottom. However, once the vehicle returns to the road, the LNG within the tank mixes and the resulting colder vapor temperature causes a collapse in tank pressure. The lower tank pressure affects the operation of the vehicle because the fuel system requires a minimum supply pressure. Too low a supply pressure can cause the fuel mixture to lean out.
Because the heat leaking into the LNG tank causes the pressure to rise, industrial cryogenic LNG tanks are commonly equipped with an economizer valve (aka, tank pressure control regulator). The economizer valve is basically a control valve that draws methane vapor from the tank and blends it into the liquid line to the LNG vaporizer. This affects the operation of the engine in three ways.
- The immediate composition of fuel supplied to the engine changes from the characteristic composition of local LNG to nearly 100% methane, which has immediate effect on the air-fuel ratio.
- The vapor from the economizer can cause the primary regulator's output pressure to drop because the upstream LNG vaporizer is receiving blend of vapor and liquid rather than a constant flow of liquid fuel.
- The composition of the liquid LNG remaining in the tank becomes less predominately methane and the other constituent gases become slowly more predominant, with a subtler, longer term effect on the air-fuel ratio. The change in LNG fuel composition due to the operation of the economizer is known as weathering.
Although "industrial gas" based tanks are commonly described as having a single line fill valve, they often cannot be filled without venting the vapor from the tank. To prevent natural gas from becoming a fire hazard in the vicinity of LNG refueling stations and to prevent greenhouse gas emissions as well, vent gases must be recovered. The vent gas recovery requires a second line so all LNG refueling operations are 2-line systems whether they are described that way or not. Venting is necessary because systems that operate at higher pressures must vented so as to allow the LNG transfer pump to be able to fill the tank.
Unless there is a mass flow meter on the vent line, the vehicle operator is giving the vented fuel back to the LNG station for free. For on-site refueling operations, the vented fuel goes back to the fleet operator. However, when refueling at public LNG stations, the loss of vented fuel can be a measurable cost to the vehicle operator.