Diesel/electric systems have been used for years to power large haul trucks and wheel loaders in the mining industry. But the push for greater fuel efficiency and cleaner exhaust emissions has motivated researchers, including automobile and on-road truck manufacturers, to develop and test hybrid systems for general use The success of these tests eventually could change the on-road dump trucks and mine-support vehicles used in the rock products industry.

Hybrid systems combine internal-combustion engines, electric motors and rechargeable batteries. On the small end, Toyota introduced a gasoline-electric hybrid car, now being sold in Japan, that it says emits only half the carbon dioxide of a regular gas-powered car and 90% less of other tailpipe emissions. The gas engine recharges on-board batteries plus provide propulsion at fast speeds.

On a larger scale, Navistar and Lockheed Martin Control Systems developed and are field testing diesel-electric hybrid trucks. Built on a medium-duty truck chassis, the hybrid electric vehicle (HEV) has a conventional diesel engine driving a generator, which produces electricity to charge the batteries and to power the electric motor (Figure 1).

The engine, which is not connected to the wheels, is operated at a steady state condition, which optimizes fuel economy and minimizes emissions. The batteries provide power during peak power needs. Engine speed is electronically controlled to maintain proper voltage and current.

Use of an electric motor for propulsion eliminates the need for a transmission. The electric motor has a constant torque curve up to the motor's base speed, where it provides constant power up to maximum speed, Navistar said. The motor is reversible, so no reverse gear is required. Startability and gradeability is equivalent to conventional diesel trucks; acceleration is greater (Table 1).

In addition, regenerative braking uses the motor as a brake-from full power to zero speed-when the driver releases the accelerator pedal. Energy from the vehical's motion is captured to recharge the batteries. This also extends the life of the truck's service brakes.

Navistar expects the diesel-electric trucks to achieve 20% to 30% better fuel economy in urban stop-and-go traffic, about 10% to 15% better economy in suburban driving, and slightly worse fuel economy than a standard truck in highway applications. Regenerative braking is key to improving fuel economy.

The HEV also can provide stationary electric power-220-volt service at 50 or 60 Hz.