Making the HEMTT a hybrid: Oshkosh Truck develops diesel-electric version of workhorse Heavy Expanded Mobility Tactical Truck

Now in its third generation, Oshkosh Truck Corp. has developed a diesel-electric hybrid-powered version of its Heavy Expanded Mobility Tactical Truck, the HEMTT A3. The truck is one of two hybrid HEMTT A3 vehicles developed by Oshkosh under a development contract with the Department of Defense (DoD). The first was a proof-of-concept model released in 2003 while the second is a production-ready vehicle featuring a 60,500 lb. GVWR, 13-ton payload and components spec'd to military requirements such as extreme temperature requirements.

"We took what we had in '03 and put a lot of engineering into it and a lot of development time into turning this into something that went from a proof-of-concept to an actual vehicle to fit within the military's applications," said Dan Binder, technical director for Oshkosh, in Oshkosh, Wis. "That's where we're at today.

"We have a truck that is diesel-electric hybrid powered and it does virtually everything its conventional counterpart vehicle does today. It meets the grade climbing requirements, speed requirements, it's able to carry the payload and meet the temperature requirements and high expectations for cooling capability." Binder added that the hybrid version is 3000 lb. lighter than the second generation HEMTT currently in production.

o shave pounds on the HEMTT A3, Oshkosh has replaced a number of conventional truck components with lighter and smaller parts. "We've taken out the 8V92TA Detroit Diesel engine and Allison transmission, both very capable pieces of equipment, but big and heavy," Binder explained. "We replaced that engine with a small ISL engine and generator."

The A3 truck features a 9 L six-cylinder Cummins ISL diesel engine rated 400 hp at 2200 rpm which is incorporated into Oshkosh's ProPulse diesel-electric drive system. The ProPulse system incorporates a 305 kW Marathon generator driven off the engine, which produces 460 V of alternating ac current that power four axle mounted Moog wheel motors. The system uses an ac power circuit vs. dc to accommodate the varying conditions and terrain the truck may be placed in, said Binder. "In our truck you don't know whether you're driving down the road or across the field or up a mountain," he said. "The military drive cycle includes huge variations as compared to an on-road application where you can set it to run on that start/stop mode whether it's hilly or level."