Mack Trucks was one of several manufacturers announcing breakthroughs for commercial hybrid operations. Source: Mack Trucks. ENLARGE image

Diesel- or gasoline-electric hybrids, the most common hybrids today, integrate motor generators into vehicle architecture, usually within the transmission housing. As a motor, the unit draws energy from the batteries and supplements or replaces the internal combustion engine.

When driven by the engine, the generators charge the vehicle's battery. When braking, the generator is driven by kinetic energy, which converts it to electricity to be stored in the batteries. By contrast, brakes on conventional vehicles convert kinetic energy to heat, energy that's lost as the brakes cool.

Cost/Benefit Analysis

It's the ability to reclaim and reuse energy that makes hybrid vehicles fuel-efficient. They get their best fuel mileage in city traffic, where frequent starting and stopping lets them reclaim the most energy. Some trucks have gotten up to 50% more miles/gallon in urban environments than nonhybrid vehicles operating under identical conditions.

The latest generation of engine controllers processes information fast enough to allow hybrid vehicles to transition smoothly between operating modes. High-speed controls seamlessly blend mechanical and electric power according to demand and conditions. Drivers never feel any change.

In addition, the nickel-metal hydride and lithium-ion batteries that hybrids use are lighter weight, less expensive, and have more storage capacity than lead-acid batteries. In virtually all applications, the combination of these batteries and an electric motor allows trucks to run with smaller, more fuel-efficient engines. When more power is needed, the electric motor supplements the diesel engine for acceleration and hill-climbing. The truck can operate with a smaller, less powerful—and therefore more economical—engine.

Hybrid vehicles also require less maintenance.

Currently, energy conversion for stopping is done by brake shoes creating friction that converts kinetic energy to heat in the brakes. With hybrids, the load on brakes is lessened as more energy is converted to electricity that is stored, and less into heat that is wasted. Because brakes do less work, they require less-frequent adjustment and replacement.

The military is at the forefront of hybrid truck development. The Army is looking at hybrids for power take-off operations, and Mack Trucks rolled out a hybrid construction truck developed with the Air Force. It uses an integrated starter alternator motor (I-SAM) that saves up to 35% on fuel depending on application.

To take advantage of the growing demand for hybrids, truck manufacturers are partnering with drive train suppliers to perfect systems for light-, medium-, and heavy-duty service. Wal-Mart, for example, is working with ArvinMeritor on a heavy-duty hybrid propulsion system with a Cummins engine in an International ProStar. That truck is part of the experimentation involved in the fleet's announced efforts to double fuel mileage within the next decade.

— Abelson is a former director of the Technology and Maintenance Council of the American Trucking Associations, a board member of Truck Writers of North America, and active in the Society of Automotive Engineers.

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