COLUMBUS, Ind. -- Cummins Inc. increased its market share in the heavy-duty engine market during the first half of this year, while sales at competitor Caterpillar Inc. declined, WardsAuto.com reports.

Cummins sold 40,329 over-the-road engines during the first six months of 2008, or just over 40 percent of the market and up from 32 percent a year ago.

Caterpillar, which last year lost its eight-year hold as the top independent truck- and bus-engine supplier, sold 11,906 units for a 12 percent market share during the first half of 08, down from about 20 percent in the first half of 07.

Two years ago, Caterpillar led the big engine market with 28.3 percent, versus 25 percent for Cummins.

In June, Caterpillar announced it would stop selling heavy-duty truck and bus engines in the United States after 2009. (See July 1 Bus & Motorcoach News.)

As the first half 08 figures indicate, many customers already were moving away from Cat prior to the announcement of its withdrawal from the truck and bus market. Some analysts attribute the shift to problems Caterpillar had getting its engine up to standards.

All heavy-duty engine makers introduced new technology in 2007 to meet tightened federal emission standards. Several large trucking fleets complained of various computer and mechanical problems with all of the first new engines, but criticisms about Caterpillars power plants continued throughout 2007.

Caterpillar executives have promised the company will continue technical and parts support for all of its engines throughout their service life.

The market share for other heavy-duty engine brands during the first half of 2008 were: Detroit Diesel and Mercedes-Benz, 24,017 units for a 24.3 percent share; Volvo ABs Mack and Volvo subsidiaries sold 13,772, or 13.9 percent of the market, and Navistar sold 8,654, or 8.8 percent of the market.

The heavy-engine market will see more changes next year. Daimler will stop importing Mercedes-Benz units, producing all its North American engines under the Detroit Diesel brand at its Michigan factory, and Navistar plans to produce 11- and 13-liter models of its MaxxForce engines.