Answering the demand for machines that use less fuel and run cleaner, two leading construction-equipment manufacturers wowed crowds with innovative propulsion systems at last month’s Conexpo show in Las Vegas.
Caterpillar, Peoria, Ill. (cat.com), unveiled the industry’s first electric-drive, track-type tractor. Set for delivery next year, its electric-drive system eliminates the traditional mechanical transmission and reportedly gives the 60,000-lb D7E an unmatched balance of power, efficiency, control, and maneuverability.
In essence, a 235-hp Cat C9 diesel engine drives a brushless-ac generator that, in turn, powers motors to propel the tracks, explains Mike Betz, a Caterpillar engineering manager. The two motors, also brushless ac, are sealed and liquid cooled and, thus, suited for the same rugged environments as today’s tractors. All drive and control elements rely on proprietary Cat designs.
While the basic concept is not new dc-drive systems have been used for years in large mining trucks the breakthrough, says Betz, is in ac technology and solid-state semiconductors that control the motors and generator.
Among the benefits, the D7E’s electric drivetrain reportedly delivers excellent low-speed torque and more power to the ground than conventional transmissions. “The continuously variable electric drive is efficient over the entire operating range,” says Betz. With no gears, there are no peaks and valleys in efficiency. The engine runs at lower speeds in a narrower, more optimal band, and that results in better performance and fuel efficiency, he says. “And the system doesn’t generate as much heat, so cooling loads go down and we need less fan power.
“We get 10% more productivity and burn up to 20% less fuel. So in a typical dozing application, we move about 25% more material per gallon of fuel,” Betz emphasizes.
The electric drive system has 60% fewer moving parts than a conventional transmission, adds Douglas Oberhelman, group president. “Having fewer components to wear out or break down provides much longer power-train life,” he says. The electric system also powers auxiliary components such as the water pump and air-conditioning compressor, so no engine belts are needed. “And with no gears to shift, it makes operator training that much easier, particularly in areas with shortages of qualified drivers,” says Oberhelman.