Caterpillar Hybrid Excavator Technology Sells Itself to Expert Operators

May 3, 2013

After Mike Staley and Mike High spent the best part of a chilly April morning using Caterpillar’s 336E hydraulic excavator and its 336E H hybrid counterpart to trench, load trucks, and maneuver a manhole box, Construction Equipment editors sat down with them to collect their thoughts about how the machines compared.

CE’s senior editor Frank Raczon asked the question that got to the point of our in-the-dirt exercise: “Now that you’ve run the hybrid, what would you tell your contractor friends if they asked ‘what’s this thing all about—is it really going to produce like a standard machine?’”

Staley, a training instructor for Operating Engineers’ Local 649 in Peoria, Ill., had a ready answer: “I’d tell them that there’s no concern about production—if you’ve got serious pipe footage to put in the ground, the hybrid would probably pay for itself very shortly.”

High, training director for Local 649, was of similar opinion: “For contractors not familiar with the machine—like us coming in this morning—they might be skeptical that the hybrid is quicker—that you can move more dirt with it. If I worked for a contractor looking for an new machine, I’d say, ‘Get them both out here and time me on them.’”

Construction Equipment had asked Caterpillar if we could invite a couple of independent operators to run the conventional and hybrid 336Es back-to-back, then to give us their take on comparative performance. The company agreed, and we met up with Staley and High at Caterpillar’s Edwards Demonstration and Learning Center, just outside Peoria, along with our hosts, Brian Stellbrink, product application specialist, hydraulic excavators; Lawrence “L. J.” Tognetti, senior product service engineer, excavation division; and Chad Cremeens, application specialist/field operations coordinator at Edwards.

Two energy-saving approaches

In a 336E H orientation meeting the day before, Caterpillar’s Ken Gray, global product manager for large hydraulic excavators, said that the company decided in 2009 to keep an open mind as it began concurrent development of two hybrid concepts, both aimed at recovering the energy that conventional excavators waste when their swinging upper structures decelerate. One approach would use an electrically based system, the other a hydraulically based system.

In a conventional excavator, when the operator releases the swing lever and the upper structure begins to decelerate, the inertia of the swinging upper begins to drive the hydraulic swing motor, forcing it to functional momentarily as a pump. The resulting oil flow, with no work to do, pushes past a relief valve and exhausts to reservoir, creating resistance that brings the swing motor to a stop, but also dissipating the swinging upper’s kinetic energy as heat.

This wasted energy can be captured and put to work, however, with a regenerative-braking system. If electrically based, the system typically uses an electric swing motor that functions momentarily as a generator during swing deceleration to charge batteries or capacitors. Stored power is then used for swing acceleration or to assist the engine in driving the main hydraulic pumps.

If hydraulically based, oil flow produced by the swing motor (functioning momentarily as a pump during swing deceleration) is stored in high-pressure accumulators (not pushed over relief), where it’s available to help drive the swing motor during swing acceleration.

Both approaches lessen the load on the main hydraulic pump, subsequently reducing engine load and saving considerable fuel.

According to Gray, Caterpillar concluded that the hydraulically based system was the more feasible: it minimized incremental costs for equipping a machine with a system to capture, store, and reuse energy (about 9 percent, compared with the 336E’s sticker); it was deemed less complicated and more reliable; and since it approached 90 percent efficiency in capturing available energy, it provided greater potential for fuel savings (averaging 25 percent, 336E H versus 336E) and, thus, faster payback on the added hybrid investment.

At the heart of the 336E H’s hydraulic regenerative-braking system is the swing-energy-recovery valve (SERV), which functions as the electronic gatekeeper for oil passing between the swing motor and accumulators, while also coordinating flow from the pump.

]We asked Caterpillar engineer Alek Egelja, Ph.D., about the volume of oil flow through the SERV, and he said it would depend on the application, of course, but it could be on the order of 200 to 300 liters per minute. The net result is that the main pump is relieved of producing a significant volume of oil.

Beyond regenerative braking

As Stellbrink explained to operators High and Staley, however, the design of the 336E H goes beyond equipping it with an energy-recovery system. The design philosophy, said Stellbrink, has three major aspects: “conserve fuel, optimize performance, reuse energy,” the “reuse” aspect being the hydraulic regenerative-braking system.

The “conserve fuel” aspect focuses on lowering fuel consumption—by using “free oil” for the swing, said Stellbrink, but also by turning down engine speed, to 1,500 rpm, considerably less than the 336E’s 1,800 rpm. The hybrid’s engine software also was remapped, said Stellbrink, to ensure the same net engine power at the reduced speed.

Both Staley and High were at first slightly incredulous that the 336E H’s engine (a Cat C9.3 rated at 308 net horsepower) was turning so slowly.

“I don’t think I’ve ever done anything on an excavator at 1,500 rpm,” said Staley. “I completely forgot it was running at a low rpm, because there’s no loss of performance or production, compared with the 336E—but you do notice that the hybrid is quieter. I’d think that the slower speed also would give you longer engine life.”

The hybrid can run at a lower speed, said Stellbrink, because the 336E H’s electronically controlled main pump is larger, having greater displacement, which means it can turn slower but still produce adequate flow. The new pump also reacts more quickly to changing flow requirements, he said.

“You don’t think about the lower engine speed while you’re running,” said High, “ because you have all the power, smoothness, and speed that you want. Fifteen hundred rpm? That’s difficult to believe.”

The “optimize performance” aspect of the 336E H’s design, said Stellbrink, centers primarily on a newly designed main-control valve (the Adaptive Control System Valve). The new valve precisely regulates all the 336E H’s hydraulic functions, he said, sending hydraulic flow where it’s needed, only when it’s needed, and continually metering flow to and from each circuit.

The net result of the 336E H’s overall design, said Stellbrink, is that the machine has the potential for significant fuel savings in all operations, with swing-intensive applications yielding the largest fuel savings and the quickest payback.

“Speaking for operators,” said Staley, “most contractors think there’s something wrong if you’re not out there throwing dirt. So as an operator, I’m not concerned about saving fuel—my job is production. I understand that most excavators have different power modes [the 336Es have three: economy, standard, and high], but when you’re moving dirt, why would you want anything less than full out? Of course, if I were on the other side, fuel would be a concern. Seems as if you might get both out of the hybrid.”

“That’s the idea behind the hybrid,” said Stellbrink. “You get your production, and the accountant sees the benefits of fuel savings.”

Perceptions from the 336E H cab

Staley asked to run the 336E before getting on the hybrid, and after trenching for a while, he exited the cab with high praise for the conventional version: “I could move a mountain of dirt with that machine.”

Both Staley and High were initially quite taken with the conventional 336E’s power and hydraulic capability. But after a morning’s work with the two models, both were of the opinion that, although breakout power for the two was equal, the hybrid had the edge in cycle speed and controllability—especially speed.

Stellbrink explained that the design philosophy of the hybrid was to make it as close in performance to its conventional counterpart as possible, but the two operators were not dissuaded from their perception.

“They’re both smooth, they’re both powerful, they both give you all you want, but I definitely like the speed of the hybrid,” said Staley. “Maybe it’s just the swing that’s faster, but I had the impression that it’s faster all around—seemed to me that boom-up and stick-in functions were faster as well.”

High was of the same opinion: “I think the hybrid is more aggressive, and I like that. Seemed to me that with the conventional machine, I had plenty of time to boom-down and stick-out when swinging back to the trench, but with the hybrid, you really have to get on it. The swing on the conventional is like going uphill, then coming down. The hybrid is more like flat ground. It doesn’t take nearly as long to ramp up to speed.”

Staley said he needed a few cycles to get his timing with the hybrid: “The first couple of times, I over-swung the trench, because of the hybrid’s speed, but it was easy enough to pick up a rhythm. It was also faster at truck loading, and I think it tended to carry a little better grade—it was easy to grade with.”

Both operators also shared the perception that the hybrid provided an edge in controllability: “Just an overall impression of finer control,” said High.

In a lifting exercise, the operators used the machines to handle a manhole box. Both machines ran in their heavy-lift mode, which increases system pressure, reduces pump flow, and reduces engine speed on the conventional 336E, but not on the lower-speed hybrid.

“The noticeable drop in the standard machine’s rpm gives you the mindset that maybe the heavy-lift mode is more effective than on the hybrid,” says High. “But I don’t think that was the case; perception accounts for a lot when you’re running a machine.”

Staley, in fact, gave the edge to the hybrid in lifting control: “The hybrid seemed a bit smoother at sliding the box along the ground, which you have to do sometimes to position it under another pipe.”

We asked Staley and High for any last thoughts.

“Both are very nice machines to run, no doubt about that,” said Staley. “The ‘improvements’—as I’d call them—on the hybrid are impressive. Based on what little I knew or heard about the hybrid before running it, I wouldn’t have expected it to perform as well as it did. I thought the hybrid did everything the standard machine did, but with an edge in overall performance—especially speed.”

High echoed an earlier comment from Staley: “As Mike said, operators aren’t concerned with fuel, it’s production, because if I don’t get the footage, the contractor doesn’t make money. If I had the choice of either machine to run all day, I’d take the hybrid, because I think I’d get more production. All else being equal, I’d say that the contractor with the hybrid would have an advantage.”

Based on fuel savings, Caterpillar estimates that the hybrid’s cost premium (compared with the conventional machine) can be recouped in most operations within 18 months, and in as little as a year in some. Staley and High came away convinced that the hybrid also would be more productive in high-volume applications, which would be a further positive consideration for payback.

Fuel aside, we’re guessing that operating expenses for the hybrid would be on a par with the conventional model, except perhaps for accumultaor maintenance, which Caterpillar estimates would be in line with a similar timeframe as that for the hydraulic cylinders.

Staley and High, much impressed with 336E H, asked Stellbrink if the hybrid concept has application for other excavator models.

“The technology has legs,” said Stellbrink, “so there’s opportunity up and down the line.”