Research On The Characteristics And System Application Of A Novel Power-split Hydraulic Transmission

With the continuous growth of vehicle ownership in the world,the problems of energy exhaustion and environment pollution become increasingly prominent.Hybrid powertrain,which can improve fuel economy and reduce engine installed power,has been considered as effective approaches to improving the fuel economy of modern vehicles.Hydraulic hybrid system is more suitable for heavy-duty vehicles due to its high power density,regenerative braking,engine downsizing and optimal engine operations,etc.In the power split hybrid,the power split device,typically a three-shaft planetary gear set,is the key component to split the engine power into two mechanical power branches.A variable displacement hydraulic pump is needed to convert one branch mechanical power to hydraulic power.As the planetary gear set has three shafts,there are multiple system configurations due to different shaft combinations among the engine,final drive and the variable displacement pump.This makes the system structure and controls complicated.Therefore,it is of great significance to study new power split devices.Therefore a novel power split hydraulic transmission has been proposed to replace the planetary gear set and the variable displacement pump.The new hydraulic transmission is based on a double-acting vane pump but with a floating ring.By coupling the floating ring to its output shaft,it becomes a hydraulic transmission,splitting the input shaft power into output shaft power and outlet hydraulic power.By integrating the power split function of planetary gear set and the mechanical-hydraulic power conversion of hydraulic pump in a single unit,it provides a new power split solution for hydraulic hybrid system.In this paper the characteristics of the new power split hydraulic transmission have been investigated through both theoretical analysis and experimental studies.The characteristics of the hydraulic transmission were obtained as follows:1)understand the transmission operating mechanism and analyze the factors detennining the transmission characteristics;2)conduct experimental studies to obtain the transmission performance data;3)combine analysis and experimental results to develop the mathematical models of the hydraulic transmission.Besides,the systematic approach of applying the power split hydraulic transmission to the powertrain of passenger vehicle is studied,which provides a comprehensive understanding of the new transmission applications to off-road vehicles.The contents of each chapter are as follows:The first chapter introduces the research background,reviews the development history and presents situations of hydraulic hybrid powertrain and power split hydraulic transmission,and at the same time introduces the importance of power split hydraulic transmission in hybrid vehicle.The main research contents of this paper are shown in the end.The second chapter focuses on the mechanism and characteristics of the power split hydraulic transmission.Structure and operating principle of the transmission are introduced.Definition of efficiency and power split ratio of the transmission and analyzed.And the mechanical characteristics and volumetric characteristics of the power split hydraulic transmission are developed.The third chapter focuses on the experimental study of the power split hydraulic transmission.A test platform for the transmission is built and each hydraulic components on the platform is selected.The method of experimental study is introduced and the experimental results are shown in the end.The fourth chapter focuses on the characteristic model of power split hydraulic transmission.The research methods of mechanical characteristics and volumetric characteristics are introduced.According to the experimental results and the theoretical analysis,mathematical models of the mechanical characteristics and volumetric characteristics of the power split hydraulic transmission are finally obtained through data fitting.The fifth chapter applies the power split hydraulic transmission to the vehicle powertrain system.The structure of the hydraulic hybrid powertrain system based on the new transmission is introduced.A dynamic simulation model is developed in Simulink and the U.S.EPA’s urban cycle is used as the test driving cycle.A rule-based energy management strategy(EMS)for the hydraulic hybrid powertrain has also been developed.The system parameter design,controller design and the energy management strategy are evaluated through simulation.The sixth chapter summarizes the main research results of this paper.The shortcomings of the power split hydraulic transmission in the process of theoretical modeling,characteristic identification and system application are developed.The future work is shown in the end.