Diesel-electric hybrid | If it works for locomotives and mining equipment, it might work in a high horsepower farm tractor
FARGO, N.D. — Farmers were skeptical but hopeful when the diesel-electric autonomous tractor debuted this fall.
“Can this thing really pull my air drill?” was a common question.
Diesel-electric has been powering the 60,000 pound, 235 horsepower Caterpillar D7E dozer for the past five years.
Ed Zwilling, head of the advanced drive train team at Cat and the man in charge of designing the hybrid dozer’s power train, said diesel-electric has advantages.
A big one is 60 percent fewer parts, which simplifies manufacturing and maintenance.
“We expect the D7E drive train to have a 50 percent longer life than the D7R it replaces, plus it has better performance than the D7R,” he said.
“Our goal when we started this project was 10 to 30 percent less fuel consumption along with 25 percent more work performed from each unit of fuel that is consumed. Our customers since 2009 tell us they’re getting better numbers than that.”
Cat has built diesel generator sets for decades. The C9 diesel engine spins a 480 volt AC generator, and the power is converted to 650 volts DC, a common voltage level in the industry. The generator and electric motors are designed by Cat.
Electricity flows to a solid state inverter that sends power to the final drive system. The final drive consists of two electric motors feeding into a common mechanical gearing unit that powers the differential steering system. The two motors behave as if they are one.
Unlike train locomotives or the Spirit Autonomous Tractor that appeared in North Dakota last month, the D7E uses mechanical differential gearing to transfer torque from electric motors to the ground. The D7E does not use electric motors to drive the tracks directly.
Instead, power is transferred mechanically via axles to the left and right, double-reduction final drives. The electric motors serve as an interface between the output of the diesel crankshaft and the input shaft of the differential steering system.
“We kept the differential steering system. Because of the high torque loads, I don’t think you could use direct drive from electric motors in a tractor like this,” Zwilling said.
“If you look at heavy duty electric drive mining and industrial machines, they all run the power through gears.”
He said Cat customers also say they are accustomed to the feel of the differential steer system and want the same feel in any new drive system.
Zwilling said safety was one of the chief design requirements in the new tractor, considering that 650 volts are surging just below the operator’s seat.
Current flows through special armoured cables and military grade connectors to help guarantee the operator’s safety.
“We know our dozers often work in fairly deep water, so every connection is water tight,” he said.
“During the development, we had these machines working in the ocean doing beach front reclamation. Every component was running in salt water, working long hours without a problem.”
The D7E is not a pure diesel-electric system, but routing power through the electric motors provides smooth, infinitely variable force at the tracks and eliminates the need for a transmission. There are no gears or shifting tricks that new operators need to learn.
The D7E does not use hydraulic pumps or hydraulic motors to move power. The abundance of electricity means no belts or gear drives are used for the air conditioning, heating, water pumps and battery charger. The electrical system runs all ancillaries.
Engineers know that powering equipment with electricity eliminates the hydraulic oil, filters and heat exchangers associated with mechanical and hydraulic drive. It’s also more convenient. If it’s necessary to drive a motor or activate a lever, simply route a wire.
Electricity is easier to regulate than hydraulic and mechanical power to meet power demands of the job. It also reduces noise and vibration. The cab on the D7E is 50 percent quieter than the previous D7R.
However, the factor that may be more significant is that a smooth electric motor is gentler on all the mechanical components in the drive system because there are no sudden power surges, power gaps or clunks.
Caterpillar says electric drive simplifies operator training and makes life easier on veteran drivers because much of the lurching is gone. It’s easier to operate because the best torque is in the electric motor’s lower r.p.m. range.
In the conventional D7R, the C9 engine stretched from 1,600 to 2,200 r.p.m. to cope with torque and r.p.m. demands of the tractor.
However, when coupled to a generator, the same engine operates in the comfortably narrow working band of 1,500 to 1,800 r.p.m., where it is most efficient.
Because drive speed and torque are now precisely controlled by the electric motor, the diesel doesn’t need to drop down to low lugging r.p.m. or wind up to higher r.p.m.
Serviceability is another factor. Tractors that rack up big hours on the shop clock obviously spend less time on the work site.
The cab tilts up and out of the way to the right side of the machine, giving technicians easy access.
Zwilling said Cat is evaluating other machines with diesel-electric drive train possibilities. The drive train should have a good fit in many other types of tractors if it can stand up to the rigors of dozer work.
When asked if Cat will introduce a diesel/electric Challenger in the near future, Zwilling said the company’s diesel/electric system is now a proven technology.
“I’m not at liberty to talk about our transmission strategy,” he added. “I can only say that we continue to work with Agco and their group in the transmission area.”