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Caterpillar D7E: Move earth and save the Earth

CNET editor Wayne Cunningham gets to drive the first hybrid dozer, the Caterpillar D7E.

Caterpillar D7E
This big dozer, the Caterpillar D7E, is the first to use a hybrid power train. CNET

On the way to a demonstration of Caterpillar's first hybrid dozer, I was expecting it would be one of the little ones, the kind used to dig pools and landscape suburban back yards. But looming up in the middle of Holt of California, a Caterpillar dealer outside Sacramento, Calif., was a huge beast, a massive yellow earth mover, the metal tracks of which came up to my waist. The Caterpillar D7E was a lot bigger than the little hybrid I was expecting.

A Caterpillar representative jumped in the cab and, metal tracks scraping up the clean concrete floor, pivoted the big dozer around and drove it out to the demonstration area, a field of dirt with one big hill, and strategically placed holes and trenches--not to mention a slalom course marked by orange pylons. As a dramatic start to the demonstration, the driver took the 56,669 pound D7E over the steepest section of the hill, the dozer's blade pointing up toward the sky. At the top, it neatly balanced on the crest before making its descent, demonstrating how easily it maintained control on this loose ground.

The D7E differs from traditional earth-moving equipment in that it uses a locomotive-style series hybrid drivetrain. It still has a big diesel engine in front of the cab, but it merely serves as a generator, powering two electric motors. The 9-liter diesel engine runs at a steady 1,500rpm to 1,800rpm, its most efficient power band. That engine is smaller by a liter than the D7R, Caterpillar's comparable diesel dozer. Net power output from the electric drive is 235 horsepower, and the D7E has almost 100,000 pounds of pulling power. As one of Caterpillar's engineers pointed out, the fact that it only weighs 56,669 pounds, it will exceed its grip before it maxes out on pulling power.

A quieter ride and efficient operation

Some of the advantages of the hybrid system became apparent at the beginning of the demonstration. Where a conventional tractor has gears, and has to rely on the changing power of a revving diesel engine, the electric motors give the D7E a smooth torque curve, creating steady power for moving dirt or just traveling around a work site. And the constant speed of the engine means less noise, for both bystanders and the driver. It's far from quiet, but a lot less noisy than other tractors.

D7E powertrain
The D7E's power train uses a diesel engine to generate electricity, powering drive motors. Caterpillar

During our demonstration, the driver stopped the D7E on the hillside and stepped out of the cab, showing how its seat sensor automatically puts on the electronic parking brake. Getting back in, the driver proceeded to pivot it on the hill side, the loose dirt letting the dozer slide down a little, but demonstrating a turning radius that beats smaller tractors, and is 50 percent better than the D7E's nonelectric counterpart.

Unlike a hybrid car, the D7E doesn't carry batteries to store excess electricity--it would take a huge battery stack to turn the motors. However, it does use a kind of mechanical regeneration when it turns, sending power from the track that's slowing down through a differential to the outside track.

The demonstration proceeded, showing the D7E using its ripper to loosen up hard dirt. It used its blade to fill in a trench, making a number of quick maneuvers to work its way down, then pushed dirt up and over the hill in the demonstration area. Finally, the driver ran it through the pylon slalom, showing the dozer's maneuverability.

Caterpillar used these maneuvers to tout the D7E's advantages. The trench-filling exercise uses up to 30 percent less fuel than with a conventional dozer, while other fuel economy gains were in the 10 to 20 percent range. The most important numbers for the contractors in attendance at the event were the D7E's gains of 25 percent more material moved per gallon of fuel than a comparably sized nonelectric dozer, and 10 percent more material moved per hour.

Novice at the controls

Finally, the Caterpillar representatives let me get in the cab, a dream realized from way back when I used to play with Tonka trucks. Although the size of the dozer seemed overwhelming, I was encouraged, as a woman contractor who took an earlier test drive got out of the cab literally squealing with joy.

D7E steering lever
This lever steers the 56,000 pound D7E. CNET

After a quick run-through with the controls, and after I assured the Caterpillar rep that I wouldn't attempt to rearrange the demonstration area, I set the speed and pushed the forward switch. The D7E started moving slowly, and the rep flagged me. Oh yeah, I forgot to raise the engine speed so the diesel could generate enough electricity to get the thing moving. OK, turn up the engine speed with a dial, set the engine speed with a roller switch, then press the forward switch. The D7E easily got underway. The cab is pretty high tech, with a rearview monitor and an electrically powered air conditioning unit to fend off the Central Valley heat. Steering the beast amounted to pulling or pushing a big lever on the left side of the driver seat.

Given the size of the D7E and the ground I was covering, it wasn't exactly a smooth experience, but it would be a lot rougher and noisier in a pure diesel tractor. In fact, Caterpillar claims 50 percent less noise in the cab. More impressive was the fact that I, a real novice, could drive it after just a couple minutes of instruction.

Some people might figure that the extra complexity of a hybrid system would be a problem for heavy equipment like the D7E, but they would be wrong. The fact that the engine runs at a steady speed means less stress, and Caterpillar engineered it without belts, the parts that wear the quickest. The cables from generator to motors are all armored, and use military grade connectors. The electric drive itself has fewer moving parts than a conventional gearbox.

Then, of course, there are the environmental gains. The D7E's greater fuel economy means lessened greenhouse gas emissions. It meets the EPA's Tier 3 standard regulating nitrogen oxide, hydrocarbons, and particulate matter for nonroad diesel equipment. Caterpillar is also working toward future Tier 4 regulations, taking effect in 2010.

Caterpillar begins production of the D7E this October, and will offer it as a replacement for its existing D7R diesel dozer in the United States and Europe.