Improving efficiency of mobile hydraulic systems
Published in: Tribology & Lubrication Technology
Date: 10/1/2008
By: Canter, Neil
New technology is improving energy efficiency in mobile hydraulic fluids with the use of displacement-controlled actuators.
Increases in energy costs are prompting the hydraulic industry to pursue improvements in the energy efficiency of mobile machines. A key example is an excavator that is designed for digging dirt in construction, mining and other applications.
In an excavator, the operator is located in a cabin that can swivel in a 360-degree fashion. The cabin is placed on tracks that can propel the vehicle over virtually any kind of terrain. Three hydraulic actuators (cylinders) control the movement of a toothed bucket to lift, dig, push and generally move material. A typical compact excavator is shown in Figure 1.
Chris Williamson, a doctorate student in the department of agricultural and biological engineering at Purdue University, says, “The trouble with mobile hydraulic systems is that they are controlled with valves. Valves act as fluid resistances, adapting the supply pressure to the load pressure by restricting the flow and converting fluid power to heat. These ‘throttling losses* are the greatest contributor to system inefficiency for a multiactuator system in an excavator or similar machine.
“The most common hydraulic circuit design in mobile equipment today is a load-sensing (LS) system. In this configuration, a single hy- draulic pump supplies (luid to several actuators. The pump is constantly adjusted to the highest pressure required by any of the actuators. Once an actuator needs higher pressure, it provides feedback through a hydraulic line to the pump to increase pump power. Unfortunately, the pump pressure must be reduced by a valve to the lower pressure level required by the other actuators. Essentially there is a mismatch between power supply and power consumption, and the difference is wasted.”
To explain, Williamson makes an analogy to electrical technology. “Years ago, variable speed electric motors operated with direct current and were controlled with resistors. Now variable speed motors are usually frequency controlled with alternating current. It’s an inherently more efficient approach. Hydraulic systems are still controlled with resistances, and we’re now at a point where we need more efficient flow-control technologies.”
An alternative mobile hydraulic system that could offer improved efficiency utilizes displacement-controlled (DC) actuators. Williamson explains, “DC actuation involves the direct control of an individual actuator with a single hydraulic pump. Through this technology, the hydraulic pump can consistently provide the actuator with the optimal amount of fluid power needed. Valves are no longer used to control the actuator power.”
The prospect of realizing efficiency improvement in mobile hydraulic fluids with DC actuators has not been determined until now.
DISPLACEMENT-CONTROLLED ACTUATORS
Previous work by the researchers on wheel loaders and skid-steer loaders showed that DC hydraulic systems display fuels savings between 10% and 20% compared to existing systems. In current work, DC and LS hydraulic systems are compared by mathematical modeling in a simulation study using a Bobcat 435 compact excavator (Figure 1).
Williamson says, “We chose a five-ton miniexcavator because it has the same load-sensing hydraulics found in larger excavators, but it is small enough to be evaluated in the laboratory” Smaller excavators are more versatile than the larger machines used for mining and heavy construction, as they have additional actuator functions and can be operated with other attachments besides a bucket for digging.
The simulation study measured dynamic and steady-state processes that each hydraulic system would use in controlling the excavator. An example of the former is how the excavator responds to an applied force. In the latter, how efficiently the hydraulic systems are using energy is evaluated. Steady-state characteristics were based on experimental measurements of the hydraulic components.
The energy consumed by the excavator in a 60-second digging cycle was measured using both hydraulic systems. Five power distributed categories were evaluated including actuator work, valve losses, pump losses, frictional losses (in the hoses and actuator seals) and other losses for charge and drive pumps and a cooling fan. Williamson says, “We found that the largest difference in energy loss was due to the hydraulic valves. In fact, for the LS hydraulic system, valve losses represent 43% of the total energy consumed.”
In contrast, the DC hydraulic system does not rely on valves for actuator control, which means there is minimal energy loss. Overall, if DC technology is used, the excavator would realize an energy savings of 39%.
The biggest challenge in using DC hydraulics is not the added weight of additional pumps but space and cost. Williamson says, “The additional pumps weigh a few hundred pounds, which is negligible when the excavator weighs five tons. But finding a way to fit more pumps in an already compact machine was not a trivial problem. Also, equipment manufacturers like Bobcat are very sensitive to cost increases. Our argument is that customers are willing to pay a little more up front for much lower operating costs. It’s the same phenomenon that we see with hybrid passenger cars.”
This simulation work was carried out with an ISO VG46, mineral oil-based hydraulic fluid. Williamson believes there is a need to evaluate other fluids in order to determine if more energy savings can be realized. He says, “More pumps in a DC hydraulic system means their characteristics and performance are a dominant factor. Any lubricant that can help to improve lubrication and pump performance would increase energy efficiency.”
Future work will involve the actual installation of a DC hydraulic system in a prototype excavator. Once the machine is complete, system performance will be further improved with new control algorithms for optimizing the operation of the diesel engine and hydraulic system.
Additional information can be obtained from a presentation made by Williamson at a recent conference.1 He can also be contacted at williaca@purdue.edu. The research is funded by the Center for Compact and Efficient Fluid Power (CCEFP) and the National Fluid Power Association (NFPA).