Horizon Lines Inc. will upgrade two of its container ships that serve Hawaii with cleaner-burning engines, the company announced today.

The new engines, capable of burning diesel or liquified natural gas, will be more efficient and produce fewer emissions than the existing steam turbine engines that use bunker fuel, said Mike Avara, chief financial officer for the North Carolina-based shipping company.

The two ships are the Horizon Spirit and the Horizon Reliance, which operate between the West Coast and Hawaii. The company plans to eventually retrofit its entire fleet with new engines, Avara said.

Horizon will seek bids from six U.S. and six foreign shipyards to do the work. Horizon expects to begin work on the first vessel in January 2015, with work on both ships to  be completed by the end of 2015 or early 2016, according to a news release from the company. Horizon will has “spare” ships that will fill in on the Hawaii route while the retrofitting work is done, Avara said.