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CAT went Hybrid: Excavators need less Fuel

by Hendrik Stellmach

PEORIA(ILLINOIS)/USA – October 14, 2014 - Caterpillar introduced the first hydraulic excavator in the company's history at Bauma 2013. According to the manufacturer the 336 E hybrid consumes less fuel than its conventionally driven equivalent.
The surprise came off perfectly when Caterpillar at their headquarters in Peoria, Illinois (USA) showcased the machine exclusively to a group of international journalists. Caterpillar even kept it secret internally, almost until the last moment, to reduce the risk of data leakage.

hybrid excavator 336 E H from Caterpillar

Hydraulics brought the breakthrough: The 336 E H is as powerful as its conventional counterpart but has a significantly higher fuel efficiency. | photo: bi/Ste

Product manager Ken Gray, responsible for development, production and distribution strategies of the excavators division oversaw the project from the very outset. His pride, presenting their latest innovation, was clear as was the relief that everyone involved in the project had held their tongue and the need for secrecy had finally come to an end. Caterpillar, after Komatsu, is the second manufacturer to bring an excavator with hybrid drive technology to series-production and they did this in their own, individual way:

When developing the 40-ton excavator 336 E H the Americans, unlike the Japanese manufacturers, focused on the perfect functional interaction of diesel engine and hydraulics to reach the reduction of fuel consumption and operating costs as intended. Caterpillar are so confident of their approach that they want to equip a complete series with hybrid drive technology.

Ken Gray introducing the new hybrid excavator 336 E H from Caterpillar

“We don't develop a new technology because it's 'cool'.“ – Ken Gray, product manager for big hydraulic excavators at Caterpillar. | photo: bi/Ste

Hydraulic Systems were the key to Success

The 336 E Hybrid is the first model of a new production series of excavators which Caterpillar would like to bring to the market over the coming years. In contrast to Asian competitors Caterpillar relies on a hydraulic hybrid concept.  The Americans had also done research on electric hybrids since the projects inception in 2009 but the concept was abandoned in 2011.

The reasons for this are that no other drive technology reaches the power density of hydraulics and that Caterpillar almost entirely have recourse to well-proven components when it comes to the hydraulic hybrid concept in the 336 E H. 98 percent of the excavators were standard components, Grey said. From the start it was important to Caterpillar's developers that the customers would not need to make any compromises when purchasing the hybrid excavator. The new model delivers the same power and performance as the conventional 336 E and at the same time is more cost- efficient in operation.

the new 336 E H hybrid excavator form Caterpillar in detail

From the outside the 336 E hybrid is not any different from the standard model except the logo. But also under the hood there are  hardly any differences apart from some key components. | photo: bi/Ste

A significant decrease in operation costs

"Big excavators work in high-performance applications where fuel is a significant part of operating costs,“ Grey said, adding, that "the hybrid variation of our  "workhorse“ 336 E is ideal for the introduction of our new hydraulic hybrid technology.“ The technology finally enables companies to reduce operating costs which Caterpillar had defined from the outset as a development objective. The 336 E has a little higher engine performance than the standard 336 E, more lifting capacity and digging force and thanks to lower rotational speeds, runs much quieter than its conventional brother.  On the basis of current fuel prices Caterpillar calculated that in a high- performance application the return of the investment in a 336 E H will be seen in less than a year.

Brilliantly simple

The 336 E H drive concept is based on three pillars: fuel savings, performance optimization and energy recovery. The latter is performed by  a surprisingly simple concept.: the hybrid excavator uses so called accumulators at the rear which save energy, when the upper structure is slowed down, which again is instantly made available for the next swiveling operation. The unit in question is a cylinder filled with hydraulic oil and gas. During the energy saving process the gas is first compressed and then expands again wear-free and loss less when the energy is released. „It's really simple“, Gray says. „That's one of the reasons why it is so economical“ he adds. Caterpillar developed the component and the whole drive concept themselves and patented both. Admittedly, in its construction and mode of operation the component is not unlike the storage cylinder which won the innovation prize at the BAUMA 2010.

The project's second pillar, the new ESP- Hydraulic Pump (electronic standardized programmable pump), saves energy fuel through engine management, meaning it works with lower rotational speeds and ensures smooth transitions between the hybrid energy sources, diesel motor and accumulator.

The ACS- valve (Adaptive Control System) is the third pillar of the intelligent oil flow management within the hydraulic system. It saves fuel by reducing pressure loss and makes the machine operate without jerks and without power loss. „It is the frictionless management of saving and releasing energy which is the elegant part of the solution“ Gray says.

the new 336 E H hybrid excavator form Caterpillar in action

In applications like truck loading companies can save a lot of money with the hybrid 336 EH. Thanks to the three- steps fuel economy concept it is not important how much swiveling is done to save fuel. | photo: Cat

Energy recovery is only one component

The combination of these three technical components reduces the fuel consumption of the 336 H by up to 25% compared to the standard machine. Fuel efficiency is not only based on energy recovery, as the far east competitor practices it. This is another benefit for the customers since  the excavator also saves fuel during lesser intensive operations thanks to motor management and ACS-valve. „You were surprised at what we can get out of these two components“, Grey goes on. „We can save as much fuel with low rotational speeds and the optimized distribution of energy as we can with the energy recovery through the hybrid.“

We wanted to know, if the driver could tell the difference between the standard model. „If you don't point them at it they don't have a clue“ he responds, although some drivers were that skilled that they could hear and feel the difference. „Some drivers told us that the excavator swivels more easily. In some operations that is very good but in others, where precise operating is needed, possibly not. We sat down with the drivers and tuned the software according to the feel they had wanted.

layout of the new hybrid excavator 336 E H from Caterpillar

The hydraulic hybrid driven swivel system recycles energy from the movements of the upper structure, which normally would be wasted. The hydraulic accumulator contains non-combustible nitrogen. | graphics: Cat

Technology is not an end unto itself

Industry insiders might not be surprised that the world's leading construction machinery manufacturer would not give up the field of alternative drive technology to the competitors peacefully. Even though they had let the Asian competitors come to the fore in this case. For Caterpillar it is not about publicity in its developments but about offering the customers a genuine added value. Therefore the (working!) electric hybrid concept was abandoned after three years of development time. And that is also why they resisted the temptation to present a prototype of it at BAUMA 2010 like other manufacturers did, only to let it disappear afterwards without much ado. „We don't just only work on new technologies like the hydraulic hybrid as an end unto itself or only to prove that we have the abilities to do it. Our technology strategy always focuses on what our research and development efforts can contribute to the success of our customers“, Chief Technology Officer Tana Utley puts it in a nutshell. „We even thought about not calling the machine 'hybrid'“; Kenneth Gray adds. „Because there is an image problem: The image of the hybrid amongst construction companies is that it operates more slowly, it's expensive and it does save fuel but enough to pay off. That's the image of the hybrid on the market.“ In the end there was no better way to describe the machine's properties.

The 336 E H by no means is the only machine with a new drive technology to achieve drastic fuel savings. Already at the INTERMAT 2013  Caterpillar introduced their wheel loader 966 K XE whose power split drive also reaches up to 25 percent reduction in fuel consumption. Distribution has been started in Belgium, France, Germany, Italy, the Netherlands and Great Britain as the first countries.