Mack Trucks division is designing a diesel-electric hybrid engine for military use that the company says eventually may transfer to the commercial sector. The $4 million project – led by the company’s Hagerstown, MD facility - will deliver a prototype to the Air Force by March 2005. The vehicles would be used to refuel aircraft. An obvious commercial application for the hybrid design would be as heavy-duty garbage trucks where speed is not an issue and frequent stops occur. In hybrids, batteries are charged when brakes are applied, so frequent stops are helpful. Mack controls about 70 percent of the commercial heavy-duty refuse truck market. The company expects to see a 20 percent improvement in fuel efficiency, officials said.