A Study of Hydraulic Hybrid Vehicle Topologies with Flywheel Energy Storage 2017-01-7010
The application of fluid power technology in the United States is widespread, seeing use in industries as diverse as dentistry, military vehicles, and mining. Fluid power is also attracting interest in hybrid vehicle applications, which require an energy storage component. While most hydraulic energy storage is accomplished using hydraulic accumulators, energy storage flywheels also provide an attractive alternative for use in mobile hydraulic systems. The main difference between the system architectures proposed in literature has been whether to include distinct, separate hydraulic pump/motors for the engine and the flywheel. Previous studies have compared the various topographies to traditional drivetrains, using both numerical simulation and experimentation, with favorable results. This study uses numerical simulation based on previously validated models to directly compare performance for the prevalent flywheel hydraulic hybrid vehicle topologies to determine which topology provides higher efficiency over a standard drive cycle. The study also proposes a more efficient control method for such systems, using variable pressure, rather than set-point pressure control. This analysis reveals that energy loss behavior in the hydraulic pump/motors is dominating, that 3 hydraulic pump/motor topologies are more efficient due to their effect on engine operating point and hydraulic unit displacement, and that increased efficiency is achievable using variable pressure control by allowing the hydraulic pump/motors to operate at higher displacements. The principles revealed by this study allow for the potential of significantly increased energy storage density in hydraulic systems while retaining the power density and ruggedness which make hydraulic power attractive in so many applications.