Hybrid-electric drives aren't just for passenger cars and on-highway vehicles anymore. Here at the CONEXPO/CON-AGG 2008 show in Las Vegas, Caterpillar unveiled a new earth moving machine propelled by an electric drivetrain, the first of its kind.
Called the D7E, this 60,000-lb track-type tractor is powered by a 235-HP Cat C9 diesel engine. Yet unlike traditional tractors, the D7E's diesel engine drives a generator to produce electricity for the two electric motors that drive the tracks via a differential steering system.
The electric drivetrain does away with the need for a traditional mechanical transmission. Instead, the variable speed motors function like a continuously variable transmission, according to Edward Zwilling, engineering manager for advanced drivetrains. “There are no gears to shift,” he says. The D7E, however, does retain a hydraulic system for its implements.
Like its hybrid-electric cousins that travel the highways, the D7E features regenerative capabilities. Energy from the slowing tractor is captured and sent back to the engine flywheel, so that it can run the hydraulic pumps that still move the implements.
Zwilling says the electric drive system produces and captures enough energy to not only propel the tractor and drive the pumps but also to power accessories that would have been driven by an engine belt in the past. These accessories include an electric water pump and an HVAC system. “The D7E is a beltless machine,” he says.
Cat's new electric machine targets heavy dozing and grading applications that require power. And Zwilling points out this 235-machine “delivers more power at the ground” than a conventional D7 tractor using a 240-HP engine. “We take nearly the full power of the engine to the ground,” he says, adding that losses for this type of powertrain are relatively small.
Thanks to the efficiency of electric machines and the fact that hybrid drivetrains can increase the amount of time engines spend running at efficient operating conditions, the D7E also promises big fuel economy benefits. Zwilling reports the D7E offers up to 20 percent better fuel consumption and can move about 25 percent more material per gallon of fuel than similarly sized conventional D7 tractors.
The electric drivetrain is also simpler than its mechanical counterparts, with roughly 60 percent fewer moving parts than previous D7 tractors.
The D7E's electric drivetrain may take center stage — and it did at the show where Cat displayed an exploded view of the system. But the new tractor also features improvements in the cab design to increase visibility, in the steering performance and in the undercarriage design.
For all its benefits, don't look for electric drivetrain across the board. Zwilling says while electric technology makes a lot of sense in this mid-size machine performing dozing and site preparation, conventional drivetrain technologies are still better suited to the smallest and biggest pieces of earth-moving equipment. “The goal is to bring the right transmission to the right application,” he says.