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FedEx Freight said it has begun testing two tractors powered by liquefied natural gas.

The two trucks, built by Kenworth Truck Co., use Cummins Westport’s ISX12 G engine, a FedEx spokeswoman said.

The pre-production engine, slated for limited release in 2013, “is currently the only all natural gas engine that begins to meet the size and power needs of Class 8 trucks,” FedEx Freight said in a statement.

Based��at the company’s Dallas service center, the trucks are expected to drive close to 1,000 miles per day to test the engine’s ability to meet the demands of day-to-day operations.

FedEx Freight is the less-than-truckload��unit of FedEx Corp. which is ranked No. 2 on the Transport Topics 100 listing of U.S. and Canadian for-hire carriers.