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.”