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Simplified schematic of conventional excavator swing system. The system utilizes two variable-displacement pumps and a pilot operated valve to control a fixed-displacement motor.

Swing energy recuperation for hydraulic excavators

Following the trend of passenger vehicles, an increasing volume of research is being devoted to improving fuel efficiency and emissions of off-highway vehicles. Among off-highway vehicles, excavators in particular are receiving significant attention. Since excavators play an important role in the construction and mining industry, improving excavator fuel economy reduces operation cost and pollution emissions.

To reduce excavator fuel consumption, various energy recovery methods have been studied extensively. Energy recuperation methods during excavator boom down or swing deceleration show great potential in improving fuel efficiency. Beyond energy recovery, full parallel hybrid systems have also been examined and have shown to greatly improve efficiency. Since most of the proposed methods require significant design changes to the existing system, however, recovery systems and hybrid excavators require high manufacturing cost, which can be offset by many hours of operation.

For normal operations of excavator digging and dumping, an operator frequently makes swing motions, which accompany significant energy losses in both acceleration and deceleration. Electric concepts that replace the conventional hydraulic motor with an electric motor have shown to significantly reduce fuel consumption. The electric motor converts the rotational kinetic energy to a usable form by charging a capacitor during deceleration. Since electric systems require significant changes and expensive hardware, hydraulic recovery systems have been frequently studied.

For example, Caterpillar has made hydraulic hybrids commercially available. Hydraulic recovery systems charge an accumulator during deceleration and then use the stored energy to run the swing motor or other equipment. To further reduce swing energy consumption, an active counterweight system has also been studied. Although hybrid systems studied previously could improve fuel economy, most proposed configurations required significant modifications to the original design, which in turn required changes in the system behavior and operator experience. Change in the operator experience has been one of the biggest hurdles for the market penetration of the new technologies.

To improve excavator energy efficiency with minimal design changes, a simple swing energy recuperation system was proposed and simulated by researchers from Volvo Construction Equipment and the University of Alabama. With a conventional swing system, a significant amount of the fluid passes through a relief valve to the reservoir during rapid swing acceleration and deceleration, which results in an energy loss.

However, the recuperation system recovers the energy by storing the fluid in an accumulator. When swing relief pressure is reached, fluid passes through a recovery relief valve to an accumulator. Therefore, energy losses can be minimized by minimizing the pressure drop across the relief valve, which requires maintaining the accumulator pressure near but under the relief pressure. To utilize the stored energy, a controller then opens a valve to allow the stored fluid to drive a fixed displacement motor connected to the main pump. In this way, the engine workload, and consequently, the consumed energy decreases.

The energy improvement was determined by simulating six cycles of digging and dumping. By varying the accumulator capacity, recovery motor displacement, and control pressure, the recuperation system reduced the swing energy consumption by 48% and the total excavator energy consumption by 17% during digging and dumping operations. As expected, the recovery system efficiency improved when the pressure drop across the relief valve was reduced without allowing the accumulator pressure to reach the relief pressure. Proper recovery motor selection reduced recovery motor valve actuations and reduced swing energy losses through the accumulator relief valve.

This article is based on SAE International Technical paper 2014-01-2402 by Bradley Thompson and Hwan-Sik Yoon, University of Alabama, and Jaehong Kim and Jae Lee, Volvo Construction Equipment.

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