GREEN BAY, WI – Kenworth is rolling out a new diesel-electric hybrid system. But what sets this vehicle apart from others is a gray box, about the size of an oversized piece of luggage, attached to the chassis frame on the driver’s side. That box contains a 340-volt lithium-ion battery pack, according to

The delivery truck’s system switches power between several modes: electric only; combined diesel and electric; and diesel power, depending on driving conditions. For example, electric power is used on acceleration to get the truck moving, and when more horsepower is needed, the diesel engine engages. Electricity generated through regenerative braking is stored and used for acceleration, assisting the diesel engine.

The hybrid system, manufactured by Eaton, is expected to boost fuel efficiency by about 30 percent when used in the stop-and-go slow traffic of an urban delivery environment where the system was designed to offer the biggest return to customers.

Kenworth is also rolling out a utility model of the truck, the T370, which uses the same modes when driving, but also allows users to shut off the truck’s diesel engine when using machinery on the back in favor of the truck’s electric motor.

The hybrid version of the trucks run about $40,000 more per unit over the cost of the base vehicle with savings expected to pay for the truck over about eight to 10 years. There are also federal tax credits, some ranging from $6,000 to $12,000, available for purchases of hybrid units.

Peterbilt — which is owned by the same parent company as Kenworth — is developing a hybrid class 8 long-haul truck into production late next year or early 2010. Peterbilt has also begun production on a line of medium-duty trucks and will start production next year on a refuse collection vehicle using the same power system as Kenworth.

Coca-Cola is one of the early adopters of the T270 delivery truck, buying 100 of them with about 50 in service so far.

The Kenworth hybrid lines are expected to go into full production in November for delivery late this year or in early 2009.