Off-Road Electric Drives Gain Traction

April 10, 2008
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. (, 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.

Hybrid Wheel Loader

Volvo Construction Equipment, Asheville, N.C. (, unveiled a preproduction prototype of its L220F Hybrid wheel loader. Company officials say it offers more power, better performance, and a 10% reduction in fuel consumption, compared with current versions.

The heart of the parallel-hybrid system is an electric motor called an Integrated Starter Generator (ISG). Fitted between the engine and transmission, the ISG is coupled with a lithium-ion battery with a several hundred watt capacity.

The ISG works in a number of ways, explains Arvid Rinaldo, manager of global markets. “In many applications, up to 40% of a wheel loader’s time is spent idling.” When stationary, the hybrid system automatically shuts down the engine but still runs electronic and climate-control systems for up to an hour. And as soon as the operator hits the throttle, the ISG restarts the engine.

The ISG also overcomes the problem of low torque at low engine speeds inherent in diesel engines. “Traditionally, you have to rev up the engine to get more torque,” says Rinaldo. “But the electric motor is just the opposite; from 0 rpm we get high torque. At start-up we boost the diesel engine with the electric motor, which gives us much faster machine response at lower revs. The operator gets more power, so it’s easier to drive as well.” The ISG’s electric motor generates up to 516 lb-ft of torque from standstill. Put in engine power terms, it adds up to 67 hp of mechanical energy.

The combination of these two attributes means the diesel engine can remain off for long periods when it would otherwise be idling — and the operator doesn’t need to over-rev the engine to get sufficient working torque. The battery recharges when full engine output is not needed, with the ISG acting as an alternator.

Other energy-saving innovations on the L220F Hybrid include a switch from belt-driven to electrically powered a/c compressor and fan. The same goes for the ISG replacing the starter motor and generator. “Eliminating belt-driven accessories saves a lot of power,” adds Rinaldo. The next-generation version, set for delivery in 2009, will also feature regenerative braking.

Volvo Construction Equipment’s L220F features a parallel-hybrid drive that reduces fuel consumption and delivers more low-speed torque than conventional wheel loaders.

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