New Hybrid Technology Brings Promise of Fuel Savings and Smaller Engines for Mobile Machines

Achievement date: 

Researchers affiliated with the Center for Compact and Efficient Fluid Power (CCEFP), an NSF-funded Engineering Research Center (ERC) headquartered at the University of Minnesota, have developed a prototype displacement-controlled excavator. Based on independent measurements of a truck-loading cycle performed by Caterpillar, the prototype showed 40% fuel savings compared to a standard machine with control valves. 


Due to improved efficiency of a displacement-controlled hydraulic system, the average engine power required for a mobile machine can be dramatically reduced. Thus it is possible to use a smaller and less powerful engine to do the same work, assuming an energy storage device is added. (The high energy density of batteries is not needed because duty cycles for this type of machine are relatively short [10 to 20 seconds] and result in small energy storage requirements.) The CCEFP’s industry members continue to show keen interest in this technology through their support for research on the excavator prototype, and in its implications for other construction and agricultural equipment and the resulting potential for economic and environmental benefits.


Displacement control is an alternative hydraulic actuation technology that uses variable displacement pumps to control actuator motions rather than valves. Displacement control reduces power losses and allows energy recovery from gravitational and braking loads. A new hybrid technology that takes advantages of displacement-controlled actuation combined with energy storage in hydraulic accumulators is currently under development by CCEFP researchers. Hydraulic accumulators are optimal energy storage devices for excavators and similar mobile applications because, unlike batteries, energy can be captured or discharged quickly and efficiently with these accumulators.

A patent has been filed for the hybrid design, which uses displacement-controlled actuation for the hydraulic cylinders and a technology known as secondary control (having a hydraulic accumulator to store energy) for rotary actuators. Simulations of the new hybrid excavator system predict that the maximum engine power could be reduced by 50% for a truck-loading cycle, and fuel savings of greater than 50% could be achieved.