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Thus, the switchingscheme saved about 10% more than did the HST scheme.5.2. Applicability of the system to construction equipmentThe proposed system was developed for general construction ap-plications in the form of a test bench and assessed via three generalspeed profiles used for vehicle testing. Our results indicate that thesystemis applicable to several fields, especially for use in constructionmachinery, for the following reasons. As shown in the previous sub-section, the system achieved the highest energy saving potential forthe 10 — modified mode profile with high acceleration and decelera-tion. Excavators and wheel-loaders are heavy and must therefore ac-celerate or brake quickly. Our system was suitable for use in thedriving systems of wheel-loaders or excavators. In winch systems,controlling the lifting and lowering of loads can also be achieved byadjusting torque at the winch shafts. By directly controlling the dis-placement of pump/motor PM2, torque at the shaft of PM2 was suc-cessfully controlled via our system for use in winch systems.Furthermore, the pump/motor PM2 was able to work in all four quad-rants and therefore apply regenerative braking while lowering theload.6. ConclusionsA novel hydraulic energy-saving method was developed frommodeling through simulation and finally to experimental validation.The experimental results confirmed the validity of the employedmathematical model, the effectiveness of the control schemes andthe energy-recovery potential of the system.The study indicated that the round-trip efficiency of the systemvaried from 22% to 59% depending on the operating pressure, dis-placement, and speed of the secondary unit. As a result, the energysaving potential of the system varied based on the desired missionprofile. The results showed that the test bench system saved 10% to20% more energy than did a traditional HST system for the 10-modeand modified 10-mode profiles, respectively. Furthermore, non-revertible fluid in the system was confirmed via experiments.In the test bench, u2 was low in themajority of the time because ofthe limitations on the selection of laboratory equipment. Thus, theenergy-saving potential of the system could be increased via optimi-zation of system parameters for a particular application.When the hybrid system is applied in real applications, such as anexcavator, the combustion engine must be recalculated. A global opti-mal control scheme must also be designed to reduce fuel consump-tion and exhaust fumes. Thus, the increases in cost due to the use ofhydraulic accumulators and variable displacement hydraulic pumps/motors can be compensated for by reducing fuel consumption duringthe cycle life of the excavator. Environmental benefits will also berealized.In the near future, the system presented in this paper will be im-proved as follows.
Fast response logic valves will replace the main di-rectional control valve to reduce the response time of the main valve and guarantee the necessary back pressure for braking. In this study,the time required to activate the main valve coincides with the brakingsignal, which reduces the time needed to activate regenerative brakingbut may generate vibration. Dynamic valves should be included whendesigning a valve control algorithm to reduce vibration or tracking er-rors. Furthermore, the system will be modified for applications to hy-draulic excavators in which energy can be recovered not only fromthe inertia of the excavator but also the down stroke of the boom.AcknowledgmentsThis work was supported by a grant (No. R01-2006-000-11390-0)from the basic research program of the Korea Science and Engineer-ing Foundation (KOSEF).References[1] L. Guzzella, A. Sciarretta, Vehicle Propulsion Systems: Introduction to Modelingand Optimization, Springer, 2007.[2] X. Zhang, C. Mi, Vehicle Power Management: Modeling, Control and Optimiza-tion, Springer, 2011.[3] H. Shimoyama, S. Ikeo, E. Koyabu, K. Ichiryu, S. Lee, Study on hybrid vehicle usingconstant pressure hydraulic system with flywheel for energy storage, SAE 2004-01-3064, 2004.[4] R. Johri, Z. Filipi, Low-cost pathway to ultra efficiency car: series hydraulic hybridsystem with optimized supervisory control, SAE, 2009-24-0065.[5] P.Matheson, J. Stecki,Modeling and simulation of a fuzzy logic controller for a hy-draulic hybrid powertrain for use in heavy commercial vehicles, SAE, 2003-01-3275, 2003.[6] Y.J. Kim, Z. Filipi, Simulation study of a series hydraulic hybrid propulsion systemfor a light truck, SAE, 2007-01-4151, 2007.[7] A.R. Enes,W.J. Book, A hardware in loop simulation testbed for emulating hydrau-lic loads representing the complete dig cycle of a construction machine, Proceed-ings of IMECE 2008, ASME International Mechanical Engineering Congress andExposition, 2008, Boston, Massachusetts, USA, 2008.[8] M. Ochiai, S. Ryu, Hybrid in Construction machinery, Proceedings of the 7th JFPS,International Symposium on Fluid Power, Toyama, 2008.[9] H. Yang, W. Sun, B. Xu, New investigation in energy regeneration of hydraulic el-evators, IEEE/ASME Transactions on Mechatronics 12 (2007) 519–526.[10] K.E. Rydberg, Energy efficient hydraulic hybrid drives, The 11th Scandinavian In-ternational Conference on Fluid Power,