United Parcel Service has ordered 48 road tractors that will burn liquified natural gas while pulling trailers up and down Interstate 15 between southern California and northern Utah.

UPS has also contracted with Clean Air Fuels Corp. to build an LNG fueling station in Las Vegas, where most of the new tractors will be based.

The Kenworth tractors with GX engines from Westport HD will replace old diesel-powered vehicles, and join 11 LNG-fueled tractors that have been operating out of Ontario, Calif., since 2000, the UPS announcement said. The giant package carrier now has 1,914 alternative-fuel vehicles in its fleet.

The new, publicly accessible LNG fuel station in Las Vegas will form a midpoint with existing stations in Ontario and Salt Lake City, and Clean Energy plans to expand this Southwest LNG Corridor to San Diego. The firm hopes other carriers will buy and operate LNG tractors on this route. It expects to deliver 1.2 million gallons of LNG per year to UPS.

Wesport HD said that funding for the trucks and fueling stations comes from the U.S. Department of Energy and the South Coast Air Quality Management District. DOE estimates that the 44 LNG tractors alone will help displace about 1.25 million gallons of petroleum annually.

GX engines use Westport fuel systems applied to 15-liter Cummins ISX diesels. Engine and chassis equipment includes proprietary fuel injectors, cryogenic fuel tanks with integrated fuel pumps, and associated electronic components. GXes burn 95 percent LNG with 5 percent diesel fuel, and produce 25 percent less greenhouse gas than straight diesels.

Westport HD says the GX engines deliver strong performance and a tractor's saddle tank is large enough for a 600-mile range. The GX complies with the U.S. Environmental Protection Agency's 2010 exhaust emissions limits and meets California Air Resources Board standards.

The new tractors will be delivered in the first half of this year. With them, UPS will have about 1,100 natural gas trucks. It also has about 800 electric, hybrid-electric and propane-powered trucks.