About 60% of the energy an excavator expends in an operation goes to swinging the heavy upper frame (or “house”) into position and then returning to the home position, according to Caterpillar’s Ken Gray, Global Project Manager, Large Hydraulic Excavators. A portion of that energy takes the form of heat, some of which is generated in braking the upper frame’s rotation when it reaches the desired position. In regular machines, the heat energy is shed to the atmosphere.
Doesn’t do any good there.
Caterpillar has come up with a way to capture that energy, store it, and re-use it later. The system is designed such that the upper frame hydraulic braking action forces the fluid into two long cylindrical accumulators. The accumulators contain nitrogen gas, which is compressed as they are filled with fluid. The nitrogen-filled ends of the piston-type accumulators can be thought of as loaded springs when pressurized by the incoming fluid. A valve closes to keep the accumulators under pressure immediately after the upper frame has come to a stop. The valve is opened and hydraulic fluid flow reverses course when the excavator operator commands upper frame movement, powering the hydraulic swing motor to rotate the house.
Aleksandar Egelja, Ph.D., Engineering Manager, Advanced Hydraulic Systems, Caterpillar, declined to reveal precisely how efficient the 336E H hybrid system is, telling SAE Off-Highway Engineering magazine only that “we believe our system to be very efficient in the ability to capture almost all of the available swing kinetic energy.” There are small losses owing to factors including “pressure drop, internal oil leakage, thermodynamics, etcetera.”
“Our system is designed to allow accumulator charge up to 32 MPa, which is right in line with our swing system operating pressure,” Egelja added.
Users will see significant savings from the hybrid technology, according to Caterpillar. There is a price premium for the 336E H, which is a hybrid variant of the base 336E, because of the hybrid development effort and associated hardware on the machine. The premium will vary from dealer to dealer, Gray said, but on average he expects it to be 9%.
Customers can recoup the premium in as little as a year in heavy-use applications, he said. A more typical payback period will be about 18 months. The more often the machine is used, the faster the payback, Gray noted. For some companies, the payback period might run out to 24 months.
“If it’s longer than 24 months,” Gray said at an April 4 media briefing on the 336E H near Cat’s headquarters in Peoria, IL, “I would question that maybe you need a smaller machine … Or it might be a situation where you’re not getting enough work.”
Hybridization constitutes one of the 336E H’s three main “technology themes” or “technology building blocks.” They are the three main ways in which the 336E H is different than the base 336E. Hybridization falls into the “re-use” technology theme.
The others are “conserve” and “optimize.” In addressing the “conserve” theme in his April 4 media presentation, Gray characterized the traditional approach to excavator design: “We’re going to run this machine at a certain speed and we’re going to drop an engine into it that will run the machine at that speed.” For the 336E H, the opposite approach was taken. “The “conserve” element of this program,” he explained, “is running the engine at its optimum speed, and making adjustments to the rest of the machine to accommodate that engine running at its optimum fuel consumption point.”
And so engineers kept the C9 engine that’s in the base 336E and slowed it from 1800 rpm to a maximum 1500 rpm on the hybrid. “And we’ve put in a larger-displacement electronically controlled pump so we get the same hydraulic power output and a lot faster response with electronics controls, but we’re running the engine at its sweet spot—at its lowest fuel-consumption rpm,” said Gray.
Product Application Specialist Brian Stellbrink in a later presentation noted that operators enjoy a quieter cab because the engine runs at lower rpm. Otherwise, they experience 336E H as they do the 336E, including the same cycle time.
Using an electrohydraulic pump with intelligence rather than a hydromechanical one, Stellbrink continued, “enables us to control it more efficiently to provide flow when it’s needed, but more importantly only when it’s needed. That’s a key difference.”
He added that engineering efforts to add electronic controls to the pump “have been in development for some time, and we’re applying them now on the hybrid.”
The “optimize” element of the design is the adaptive control system (ACS), which is a new metering valve that works faster and allows independent control of functions “that before worked as one,” said Stellbrink.
“Nothing moves on an excavator without hydraulics,” he explained. “If you think of why the engine’s in that machine, it’s there to turn those pumps. That’s it. So this valve is really the traffic controller, the flow controller. It’s the brains behind the system that tells that oil where to go: bucket, stick, boom, swing, tracks.”
The conserve and optimize elements of the engineering design deliver efficiency benefits in all machine-use scenarios. They contrast with the re-use (hybrid) element, which is of benefit only when the upper frame is rotating. Gray doesn’t think that is much of a disadvantage given how a typical excavator is used: “It’s not on a jobsite if it isn’t swinging.”
Cat has filed for more than 300 patents for the 336E H. Gray declined to divulge Caterpillar’s full hybrid strategy for excavators, but did say there will be two additional models: 336F H and 336D H. He noted that the company had considered electric hybrid technology for the 336E but in the end opted for hydraulic technology. Electric hybrid technology will be considered for future products, Gray added.
See accompanying tables for details on fuel efficiency, fuel consumption, and productivity of the 336E H.
A video of the 336E H produced by SAE Off-highway Engineering magazine can be viewed here.