Liquefied natural gas is gaining in importance as an alternative to conventional vehicle fuels (gasoline and diesel). Because LNG consists of cryogenic liquid methane stored and transferred under pressure, its use as a vehicle fuel presents technological challenges different from those associated with conventional fuels. LNG temperatures typically range from -260 to -220°F, and pressures typically range from 25 to 250 psi.
One of the challenges that presents itself to anyone managing an LNG refueling station and a fleet of LNG vehicles is the selection and maintenance of the optimum saturation pressure in the station tank and in the vehicle tanks. The saturation pressure is the pressure at which the cryogenic liquid boils to vapor in the tank. Lower saturation pressure corresponds with lower storage temperature. Thus, the term saturation pressure describes the temperature of the cryogenic liquid. The lower the storage temperature, the longer the hold time (that is, the longer the fuel can be stored before it becomes so warm that the pressure relief valve vents vapor to prevent overpressure). Also, lower storage temperature corresponds with greater fuel density. As much as 40% more fuel can be stored in the same volume if the saturation pressure is very low (about 25 psi, very cold fuel), compared with warmer fuel at a saturation pressure of about 120 psi. An additional advantage is that very cold fuel facilitates the filling process at the fueling station. The introduction of very cold fuel from a station tank into a relatively warmer vehicle tank causes the methane vapor in the vehicle tank to collapse to liquid, reducing the pressure there and making the vehicle tank easier to fill.
The strategy of filling the vehicle tanks with very cold fuel, however, has the disadvantage of sometimes subjecting the engine to fuel starvation. The engine’s fuel control system expects the fuel (methane vapor) to be delivered to the engine at a pressure of about 70 to 120 psi, more or less, depending on the manufacture of the system. At pressures lower than this, the engine is likely to suffer from fuel starvation during acceleration or when operating against a heavy load. If the vehicle tank is filled with very cold (25-psi saturation pressure) fuel, it is very likely that the gauge pressure will be at or near 25 psi at times during the first few days after refueling, low enough to cause fuel starvation.
Researchers at the INL recently developed an invention that makes it possible to operate the vehicle’s engine when the fuel is very cold. The invention includes a device that can be added to existing vehicle fuel tanks/piping. The device automatically creates an artificial head of pressure (pressure higher than the saturation pressure) in the vapor space in the fuel tank, sufficient to meet the demands of the engine’s fuel control system. The device operates passively and continuously whenever the engine is running and the tank pressure is low. A prototype device was recently subjected to preliminary tests in an LNG-powered light-duty truck at the INEEL. The results of those preliminary tests were favorable.
We expect that demonstration testing will confirm the following characteristics:
- Simple and inexpensive
- Passive operation
- Commercially viable
- Effective both in light-duty and in heavy-duty applications.
The concept in general, along with several tentative designs, is included in the patent application recently submitted to the U.S. Patent Office by INL researchers. Plans for a demonstration project are underway, and commercialization will follow.
With successful demonstration testing and commercialization of the technology, we expect that the INL’s on-board LNG pressure-building device will become standard equipment for all LNG vehicles.This outcome will enhance public acceptance of LNG as an alternative vehicle fuel by eliminating one of the difficulties associated with LNG use (fuel starvation after a fill-up with very cold fuel).
Widespread use of the pressure-building device will alleviate concerns about differences in fuel management strategies at different filling stations; any LNG vehicle will be able to refuel at any filling station regardless of its particular fuel management strategy or the temperature of its fuel on that particular day. The incidental occurrence of a fill-up with very cold fuel will not present a problem.
Vehicles using the pressure-building device will be able to fill with very cold fuel, a strategy that simplifies the filling process by collapsing the vapor in the fuel tank and increases the effective storage capacity of the fuel tanks by using fuel of optimum density. This outcome is important, since one of the commercial sector’s reservations about LNG as a vehicle fuel is its lower energy per gallon, compared with gasoline and diesel fuel. By filling with very cold, dense fuel, LNG vehicles can more closely approach the effective fuel-carrying capacity of conventionally fueled vehicles.