Caterpillar’s Electric Driven Dozers

• September
• 10

4:38 am Construction Equipment

Caterpillar’s D7E crawler dozer is the first electrically driven dozer using modern alternating-current (AC) technology, and Cat claims it will move up to 25 percent more material per gallon of fuel than the D7R Series II. The drive system takes advantage of today’s small, less-costly semiconductors that can rectify heavy-duty-drive voltage to AC, eliminating brushes. The drive is developed by Caterpillar specifically for crawler loaders.

A 235-horsepower Cat C9.3 diesel drives a generator to produce electricity that runs to a power module where it is inverted to AC current. The current powers two AC drive motors arranged in series, which drives a differential steering system much like that used on other Caterpillar equipments.

The electric drive train has 60 percent fewer moving parts than the drive in previous D7s. And because power flows from the generator to the electric drives through cables, rather than a hard drive shaft, the tractor’s heaviest components can be positioned optimally in the frame for ideal machine balance.

The C9.3 engine is smaller and achieves rated power at 1,700 rpm, compared to the D7R’s 240-horsepower Cat 3176. Smaller displacement is responsible for some of the up-to-20-percent fuel savings, but the biggest efficiency gains come from eliminating the torque converter and keeping the engine running in a much more narrow range of speeds.

The 3176 diesel coupled to a powershift transmission in the D7R normally operates in a speed range from 1,200 to 2,100 rpm, whereas the C9 and electric drive operates more efficiently near peak torque, staying between 1,450 and 1,700 rpm, according to Mike Betz, Caterpillar engineering manager for track-type tractors.

Electric drive eliminates gear shifting, and is said to perform much like a hydrostatic drive, albeit with a much faster response to load demands. Electric-over-hydraulic joysticks control propulsion and blade. Steering performance is improved 50 percent, partly due to use of a dedicated steering pump equal to that used in the D8’s differential steering system. With no mechanical link between the D10-sized final drives and the electric motors’ refined power, the D7E can lock one track for pivot turns, and even counter-rotate to turn within a diameter no greater than the machine’s overall length.

Efficiency is also gained by eliminating parasitic engine load. There are no belts on the D7E’s engine. The cooling fan is powered by a thermostatically controlled hydraulic motor. (The generator, inverter, and electric motors are liquid cooled.) Twenty-four-volt current is drawn from the generator and diverted to run the water pump, charge the batteries, and power the self-contained air conditioning unit mounted on the cab roof.

The D7E is expected to be commercially available in 2009. Caterpillar intends to replace the R model in North America with the electric-drive innovation.Cat believes the greatest opportunities for spreading its newly developed electric-drive technology are on hard-pulling machines that currently use a torque converter. The minds behind the technology hint that there is enormous potential to add batteries (as they improve) and energy regeneration systems to an electric drive train and create ultra-low-emissions hybrid earthmovers in the future.