Discovering Benefits of Electricity In 644K Hybrid Loader

Oct. 28, 2013

When Jose´ Perez exited the cab of the new Deere 644K Hybrid wheel loader, he had two thumbs up. That was pretty much the overall impression of both professional operators who ran the new machine during an early September Construction Equipment field test at the Operating Engineers’ Local 150 training center in Wilmington, Ill. Deere had loaned us two new machines, the hybrid and a conventional 644K, which we then handed over to Perez, a Local 150 member and a loader operator for Terrell Materials in Chicago, and Chris Tomblin, an instructor/operator on the Local 150 staff.

During the day at Wilmington, we solicited impressions about the hybrid from the operators and frequently heard “operator friendly,” “powerful,” “smooth,” “easy to operate,” and “I hardly used the brakes,” the latter said with a mild sense of disbelief that one could run a wheel loader for an extended time in both short-cycle truck loading and load-and-carry applications and seldom touch the service brakes.

John Chesterman, product marketing manager, four-wheel-drive loaders, for John Deere’s Worldwide Construction and Forestry Division, began the day at Wilmington with a walk-around of the 644s, explaining the fundamental differences in the two power trains and what those differences—which add 15 to 17 percent to the hybrid’s price—mean in terms of machine operation and cost of ownership.

Saving fuel by design

Deere estimates that the hybrid will use 25 percent less fuel than the conventional 644K in typical applications, although some hybrid-using customers, Chesterman told us, have reported savings of 40 to 50 percent.

Fuel savings begin, he said, with a considerably smaller engine in the hybrid, a 6.8-liter John Deere Power Tech PSX 6068, versus the 9-liter Power Tech PVX 6090 in the conventional machine. Net horsepower is close, though, 229 for the 6068 and 232 for the 6090, and both meet Tier 4-Interim standards.

Also, said Chesterman, the hybrid’s engine runs at a constant speed (1,800 rpm at the top end, with additional settings of 1,500, 1,200 and 900 rpm), eliminating the typical engine-speed acceleration/deceleration that burns more fuel in the conventional 644K. Fuel savings at the engine are then complemented by the efficiency of the hybrid’s drive train, said Chesterman.

The conventional 644K’s engine drives a 5F/3R power-shift transmission with a lock-up torque converter, and the drive train is completed with inboard-planetary axles, which have a wet-disc locking differential in the front. (A rear differential lock is optional, as is an automatic-locking feature that uses radar to measure ground speed, compares it to driveshaft speed, and then applies the locks accordingly).

By contrast, the hybrid’s engine drives a brushless AC electric generator, which sends power to a brushless AC electric motor via a “power electronics” module, which, said Chesterman, serves as “the brain of the system.” The electric motor drives a simplified, three-speed, power-shift transmission, which has no reverse clutch and no torque converter. (The hybrid changes direction by changing the electric motor’s rotation.) Axles are identical to those in the conventional machine.

“An area where the hybrid drive excels in efficiency and saves fuel over the conventional drive,” said Chesterman, “is when the machine is pushing hard—as when crowding the bucket in the pile.”

In this situation, he said, the conventional loader’s torque converter has high slippage as it multiplies torque to generate the needed rimpull. This places the engine under heavy loads that burn added fuel, create heat, and strain the cooling system. But the hybrid, said Chesterman, can generate very high crowding forces with low engine loads, since the electric motor generates near-maximum torque at low speeds.

After Perez had done a few loading cycles with the hybrid, he commented that “it seems to have more power to fill the bucket than the conventional—there’s no hesitation.”

“We hear that a lot,” said Chesterman. “Technically, the hybrid doesn’t have more power, but it uses power much more efficiently. Some operators also tell us that the hybrid is more responsive, and that’s understandable, because we’re adjusting performance with electrical circuits, not by engaging and disengaging clutches or creating and relieving pressures as in hydro-mechanical systems, such as hydrostatics or continuously variable transmissions. Hydro-mechanical systems work well, of course, but not instantaneously like electronics.”

Captured energy, dynamic braking

When the operator of the conventional 644K releases the accelerator and applies the service brakes to slow the machine, the kinetic energy of the decelerating machine is converted to heat in the brakes.

By contrast, when the hybrid operator releases the accelerator (which only controls ground speed, not engine speed), the kinetic energy of the decelerating machine is transmitted back through the mechanical drive train and momentarily drives the electric motor (attached to the transmission) as a generator, thus converting kinetic energy into useful electrical energy. This process generally is known as “regenerative braking.”

The electrical energy produced is sent to the power electronics module, then to the engine-mounted generator, which now momentarily functions as an electric motor to “lighten the load on the engine,” says Deere. It does this by assisting the engine (via an engine-mounted gear box) in driving the hydraulic pumps that power boom, bucket, steering, and cooling-fan functions, and by assisting in driving parasitic loads on the engine, such as the water pump, air-conditioner compressor, oil pump, and alternator.

The regenerative-braking process creates resistance in the drive train, which results in a “dynamic braking” effect on the machine, similar to the inherent braking capability of a hydrostatic-drive machine when it decelerates. Any unused electrical power produced when the 644K Hybrid decelerates is scrubbed off via a water-cooled brake resistor located below the right cab platform.

“The system continually looks at engine speed,” said Chesterman, “and if it senses a potential over-speed condition, such as might occur in a downhill descent, it sends power to the brake resistor, similar to those in locomotives and large mining trucks. This creates a load on the system, and dynamic braking slows the machine.”

Actually, Deere has refined the dynamic-braking aspect of the 644K Hybrid by developing a four-stage “coast-control” system, which allows the operator to dial in the amount of braking effect to suit the application. Operator Chris Tomblin experienced the effectiveness of dynamic braking first hand during a load-and-carry exercise that took the machine up and over a steep hill at high speed.

“When coming down the hill,” said Tomblin, “I didn’t touch the brake one time. If you take your foot off the accelerator, the hybrid system slows you down to a manageable speed without the brakes. Same idea when you’re approaching a turn—slows you down to a nice manageable speed.”

But Tomblin did question Chesterman about whether continued used of the dynamic-braking system would eventually cause problems: “Is there a downside—any risk of wear or damage—from using coast control to slow the machine, especially on steep downhill grades?”

Chesterman said no. He told us that the hybrid’s generator and drive motor are relatively simple in design and built to last the life of the machine. A primary benefit of the electric drive system over that of a hydrostatic, he said, is that the electric components maintain constant efficiency over their lifetime, while hydrostatic components eventually wear, lose efficiency, and create excessive heat. 

Tomblin and Perez quickly picked up on the coast-control’s design: “The coast control setting depends on what you’re doing,” said Perez. “I found that setting three was good for loading trucks in a tight pattern—it keeps you from backing up too far. If you’re in a situation where you need more glide, then maybe you’d turn it off.”

Tomblin liked the basic simplicity and efficiency of the system: “Coast control is fantastic when you’re loading trucks,” he said, “because you can basically run the machine with only the right pedal—you’re not going back and forth between the brake and accelerator.

“With a conventional machine, I usually pop the transmission in neutral when I back up, so I can throttle up and raise the boom without using the disconnect. But with the hybrid engine running at a constant speed, you always have full hydraulics, so all you have to do is use the right pedal to feather your speed and braking when backing away from the pile or approaching the truck.”

It’s in this situation—V-pattern truck loading—that the hybrid system is working at its best, said Chesterman, because it’s capturing energy at the end of the back-up from the pile and simultaneously using the energy for hydraulic functions—boom raise, steering, and fan drive. Then, when the machine decelerates as it approaches the truck, the system again captures energy that’s used for final boom raise and bucket opening. And in both instances, captured energy is helping drive the engine’s parasitic loads.

Rimpull control

The pushing power and hydraulic speed of a conventional machine, said Chesterman, are correlated to engine speed, which means that a skilled operator in hard digging must balance engine speed and resistance at the bucket in order to push effectively into the pile without spinning the wheels. But in the hybrid, he said, what the operator does with the accelerator has nothing to do with hydraulic speed; the accelerator is simply adjusting rimpull, which increases as the pedal is depressed.

Tomblin and Perez quickly noted (and adjusted to) the distinct difference in operation of the two machines in the pile. They also quickly picked up on another of the hybrid’s design elements, rimpull control, which proved, in their estimation, second only to coast control as the hybrid’s most useful feature.

Rimpull control, basically, allows the operator to select how much torque the electric drive motor produces in a given situation. With the system switched off, the motor generates maximum torque, probably the best setting for heavy backfilling or excavating. But for tasks not requiring such brute force, settings one through four progressively step down the torque sent to the wheels. 

Tomblin experimented with the settings when loading the truck and actually preferred one of the least aggressive.

“As far as digging in the pile, I thought both machines had equally good power, so they’re pretty much comparable there,” said Tomblin. “But I did like the rimpull control, because usually, if you try to push into the pile in first gear, you’re going to spin the tires, so you come off the throttle, shift up a gear, or take a smaller bite with the bucket.

“But with the rimpull button set to the third light, I could load in first gear with virtually no wheel spin. The machine had plenty of power to load, but it wasn’t eating up the tires. The control takes the edge off—makes the machine less aggressive, which is a good thing in this situation.”

(The conventional 644K was equipped with Deere’s optional wheel-spin-control system, which reduces engine speed in heavy pushing situations in order to reduce wheel torque. The system is active only in first gear.)

Lever steering and other features

Both machines arrived with optional lever steering, and our operators had differing opinions about this feature.

“I really liked the lever steering,” said Perez. “It’s a very nice control in tight spots—it seems to give a very tight radius. I also used it on the load-and-carry cycles, and it was perfectly comfortable. I thought it might be too sensitive at higher speeds, but it seems to adjust to conditions. It takes a lot of strain off your arms.”

Tomblin was less enthusiastic: “Personally, I like the steering wheel—that’s what I’m used to. I find that I tend to over-steer with the lever. But there are operators here who absolutely love lever steer. If the machine didn’t have a steering wheel? I’d adapt.”

Tomblin did, however, very much like the conventional machine’s  lock-up torque converter.

“I’d leave the lock-up on all the time,” he said. “I tried climbing the hill in load-and-carry with the lock-up turned off, and again with it on. Big difference; absolutely no hesitation coming up the hill when it was on.”

Another feature the operators agreed was indispensable was ride control. The Deere system can be set to activate at any speed between 1.5 and 15 mph in 0.5-mph increments. The wide range of settings, said Chesterman, accommodates applications in which the operator might be loading at higher speeds.

Hybrid impressions

After a day in the field with the two 644Ks, we sat down for a few minutes to get the operators’ overall impressions about how the two models compared.

“The conventional is a great machine,” said Tombin, “plenty of power, smooth—I certainly wouldn’t turn it away. But I do like the hybrid. If I spent the day in one machine or the other, I’d load more trucks and be less tired with the hybrid, and a big part of that is not having to use the brake very often.”

Perez was of like opinion: “I liked both machines, but the hybrid seemed smoother and seemed to have more power—it seemed to push into the pile with less effort. It was great when loading trucks, because you don’t have to use the brakes. Just out of habit I was initially hitting the brake when I shifted directions, but then realized that the hybrid lets you make a smooth shift with no braking. I also thought the hybrid was quieter than the conventional.”

Chesterman was interested in the operators’ opinions about adjusting to the hybrid’s different mode of operation—how long did it take them to get accustomed to the machine?

“It was an adjustment at first getting used to the engine running at a constant rpm,” said Tomblin. “In a conventional loader, you’re always listening to the engine to judge how the machine is reacting. But it didn’t take long to feel comfortable with the hybrid, and I really liked having constant hydraulic speed.”

Perez said that he felt comfortable with the hybrid in probably 10 minutes: “The controls are virtually identical, and once it registers that the accelerator is only controlling how fast you’re traveling or how hard you’re pushing into the pile, it’s a very smooth machine to operate.”

We asked the operators how they thought contractors would react to buying a machine with a hybrid drive system—something out of the ordinary. Both thought that contractors might be hesitant at first, because of the different technology, but likely would be swayed by potentially lower operating costs of the hybrid.

“I’m sure that for many contractors,” said Tomblin, “fuel is eating them up. With the hybrid, fuel consumption has to be far less, because it’s running at a constant speed—not always accelerating and decelerating—that’s where you lose fuel efficiency. And I’d guess that maintenance costs would be lower, as well, because you’d definitely have less brake wear.”

Tomblin did raise the issue about the contractor’s ability to work on the hybrid after the warranty expires, and wondered if this might be a “sticking point” in the purchase decision. Chesterman conceded that the technology would probably make the hybrid somewhat less serviceable by the contractor, but then made the point that many general contractors really don’t dig into the repair of major components in their conventional machines.

Our final question to Chesterman was about hybrid design. Comparing the 644K’s design to other electrically based hybrids, we were curious why the 644K does not store captured—but unused—electrical energy in batteries or capacitors for use when the machine is not in a regenerative-braking mode.

It was a design decision, said Chesterman, based on weight, packaging, and incremental costs. But he added that Deere has an open mind about future hybrid designs.