Hybrid Excavators Leave Nothing to Waste

We've all heard about hybrid automotive technology. The concept is to capture energy wasted during braking and reuse it for productive purposes, which increases vehicle efficiency. But hybrids have broken out of the automotive arena and are starting to infiltrate the world of heavy iron. Excavators are at the forefront of this evolution.

Komatsu was the first to introduce a hybrid excavator to the U.S. market, and currently offers a second-generation model, the 23.8-ton HB215LC-1. Recently, Caterpillar announced its own unique hybrid technology with the Cat 336E H. And while it’s not currently offered outside of Japan, Hitachi chose to display the ZH200 hybrid excavator at the Intermat show in Paris this past April.

While the approaches to capturing and reusing wasted energy may differ, the concept is the same. Typically, significant energy is lost as you swing the excavator back and forth on the undercarriage. With a hybrid excavator, the goal is to capture the large amount of energy required to brake the upper structure while it’s swinging, and put that energy back to use.

Getting Into the Swing

The more you swing the upper structure, the greater potential benefit from a hybrid excavator.

“The Cat 336E H will deliver fuel savings in all applications it was designed to perform. However, the best fuel savings will come in high-volume truck loading and trenching where the machine is required to cycle frequently,” says Kent Pellegrini, product application specialist, Caterpillar. “The more it swings or cycles, the more opportunities there are to store and reuse energy, which leads to more savings and quicker payback for customers. ”

The same applies to the Komatsu hybrid. “Since the HB215LC-1 utilizes the energy created during the swing brake motion, applications that require a high amount of swinging maximize the effectiveness of the hybrid technology,” says Brian Yureskes, marketing manager for excavators, Komatsu America Corp. “Provided that a utility machine is operating with similar characteristics, customers can expect the same benefits of hybrid technology as those experienced with larger machines.”

A common misconception about hybrid technology is that performance is sacrificed for fuel savings. “This is not the case with the HB215LC-1, which offers the same high level of performance as its conventional counterpart, the PC200LC-8,” Yureskes asserts. “In addition to the 25% to 40% fuel savings, hybrid technology produces an environmentally friendly machine that reduces CO2 and greenhouse gas emissions significantly. Noise levels are also reduced compared to a conventional machine, making the HB215LC-1 a suitable choice for urban areas or night work.”

According to Pellegrini, “The Cat 336E H uses up to 25% less fuel than the standard 336E without sacrificing power or productivity. Fuel efficiency — the amount of fuel used per ton of material moved — is also a benefit. Compared to our 336D, the 336E H is 50% more fuel efficient. Using less fuel per unit of work performed also results in a lower carbon footprint, and that benefits us all.”

Hitachi also reports substantial fuel savings (20%) from the hybrid system on its ZH200 compared to the standard ZX200-3 excavator, with a corresponding drop in CO2 emissions.

How they work

Basically, hybrids capture and store energy during the braking process, which is then released at the appropriate time to produce useful work. The actual components used depend upon the design of the particular hybrid system.

A few key electrical components form the backbone of Komatsu’s hybrid system. “The electric swing motor, generator motor and ultra-capacitor differentiate the HB215LC-1 from a conventional excavator,” says Yureskes. “The machine body, undercarriage, hydraulics system and work equipment are identical to those found on Komatsu conventional excavators.”

The electric swing motor recovers energy during each swing brake, and stores it in the capacitor to be used when needed. Energy can then be sent to either the swing motor or the generator motor to help the engine accelerate as engine assist. This allows the hybrid to quickly respond to hydraulic power demands. Such responsiveness gives the hybrid the advantage over a conventional machine.

Proprietary machine control logic also contributes to fuel savings. The Komatsu hybrid has a different approach to engine speed management during operation. A low engine idle speed of 700 rpm helps reduce fuel use in between work cycles. The electrical engine assist helps the engine and hydraulic system quickly reach the necessary speed to match hydraulic demand.

In a conventional machine, the engine would lug down to peak torque. But with the hybrid system, the engine ramps up to the needed engine speed based on the hydraulic demand. There are other benefits, such as reduced noise. Due to the engine speed management and electrical engine assist, the engine spends less time at high rpm, making its quieter operation ideal for noise-sensitive areas.

The Hitachi ZH200 also uses a hydraulic/electric swing motor to generate electrical power under braking, which is stored in a capacitor alongside the machine’s cooling pack. The electrical power is used to boost slew speed, and also feeds a secondary hydraulic motor to assist the diesel engine when there is a demand for hydraulic power.

Caterpillar uses a different approach by capturing the swing brake energy with a hydraulic accumulator. A large accumulator, squeezed under the hood between the engine and counterweight, is pressurized by house-swing braking. That energy is then used to accelerate the excavator upper structure back in the opposite direction.

The Caterpillar 336E H has all of the same advantages of the 336E, with the biggest difference being greater fuel efficiency. Its design incorporates three building block technologies.

The Electronic Standardized Programmable (ESP) pump smoothly transitions between the hydraulic hybrid power sources, engine and accumulator for efficient engine power management. It also matches pump and engine load to outperform the 336E, even at lower engine speed.

The Adaptive Control System (ACS) valve intelligently manages restrictions and flows to control machine motion with no loss of power and ensure operators experience no difference in control, hydraulic power or lift capability. The ACS main valve takes the energy wasted in conventional excavators when flow dumps over relief and directs it to circuits that can immediately use the power. It reduces fuel consumption by preventing pressure loss.

The hydraulic hybrid swing system captures the excavator’s upper structure swing brake energy in accumulators, and releases the energy during swing acceleration.

The design of the Cat 336E H also allows it to run at reduced engine speeds, which saves fuel and reduces noise in and outside the cab.

Electric or hydraulic?

There are currently two ways to capture the swing brake energy from the excavator: electrically with a capacitor or hydraulically with an accumulator. The Komatsu and Hitachi excavators use capacitors, while the Caterpillar excavator uses a hydraulic accumulator.

The electric hybrids share little in common with automotive hybrid technology. “The way construction equipment operates played a major role in the development of the electric hybrid technology,” says Yureskes. “Construction applications differ from the automotive industry because there is less time for energy to be captured and converted into energy.”

To illustrate, take a typical 90° swing:

•Swing starts/swing acceleration — approximately 40°

•Swing coasting — approximately 30°

•Swing braking — approximately 20°

“Even if the swing is large (greater than 90°), the degree of swing brake still represents a small portion of the work cycle, resulting in a small window for capturing the energy,” says Yureskes. “By contrast, the automotive industry can use batteries, since the time it takes to accelerate from a stoplight is a few seconds; therefore, the battery has time to convert the chemicals into electricity.”

Capacitors provide the necessary solution. “A capacitor has the ability to capture the energy very quickly so the energy generated in that small window of swing braking is able to be captured and stored,” says Yureskes. “Due to the size of the machine, a large amount of energy is necessary to quickly decelerate the upper structure, and these large forces directly correlate to the amount of energy that is stored. A capacitor allows the machine to convert and use this energy instantaneously.”

Caterpillar weighed the electric and hydraulic options and took a different route. “We considered both the electric and hydraulic paths, and we built prototypes of both and tested them along with competitive machines,” says Pellegrini. “We determined that the most efficient, least complex and easiest to service is the hydraulic hybrid solution.”

By using this approach, there is very little difference between the Cat 336E H and a conventional 336E. “Nearly 98% of the machine is made from the same proven and reliable components used on our standard 336E,” says Pellegrini, “and that gives us economy of scale.” As a result, no special training is required for dealer technicians to service the machine.

“Production machines have been tested by operators globally,” he continues. “Productivity studies and customer applications have validated performance. The ‘brains’ of the system, the ACS valve, has been in the field proving itself with superior results for several years. In fact, the ACS and hybrid system have worked well together over thousands of hours of testing.”

Return on Investment

The additional components needed for the hybrid systems will add to the initial purchase price, but manufacturers expect the return on this investment to be earned through fuel savings.

“The factors that impact the return on investment for the Komatsu HB215LC-1 the most are application, utilization and fuel price,” says Yureskes. “High swing applications, such as truck loading or trenching, maximize the amount of energy that can be stored and made available for high load cycles.

“Ensuring high utilization is important,” he adds, “since fuel savings are realized while the machine is working. The magnitude of fuel savings directly correlates with fuel prices. As fuel prices increase, the level of savings also increase, considering the HB215LC-1 consumes up to 25% to 40% less fuel than a conventional machine.”

Caterpillar’s 336E H will sell at a premium compared to its conventional counterpart. However, because the 336E H burns less fuel and reduces regulated engine emissions, owning and operating costs will be significantly lower, and its carbon footprint will be smaller.

Precisely when a customer will recoup the initial investment depends on fuel prices and the applications. However, assuming today’s fuel prices and a high-production application, Caterpillar estimates customers can realistically expect to see a return on investment in the 336E H hybrid excavator model in as little as one year, or approximately 2,000 hours or less. ET