Paper Number Development of a Hydro-Mechanical Hydraulic Hybrid Drive Train with Independent Wheel Torque Control for an Urban Passenger Vehicle James D. Van de Ven † Department of Mechanical Engineering Worcester Polytechnic Institute 100 Institute Rd. Worcester, MA 01609 e-mail: vandeven@wpi.edu Michael W. Olson and Perry Y. Li ‡ Center for Compact and Efficient Fluid Power Department of Mechanical Engineering University of Minnesota 111 Church St. SE Minneapolis, MN 55455 e-mail: {mwo,pli}@me.umn.edu † Previously with the Center for Compact and Efficient Fluid Power, University of Minnesota. ‡ Corresponding author: Perry Li. ABSTRACT This paper presents a hydraulic hybrid vehicle drive train to improve the fuel efficiency of a passenger car. The developed hydro-mechanical drive train enables independent control of the torque at each wheel. The motivation for developing this drive train is a hydraulic hybrid vehicle test bed for the Center for Compact and Efficient Fluid Power at the University of Minnesota. The hydro-mechanical hybrid drive train is modeled and compared to a series hybrid drive train in operation on the EPA Urban Dynamometer Driving Schedule. The hydro-mechanical system demonstrates excellent fuel economy potential, yet requires development work in the area of pump/motors with high efficiency at low displacement fractions. INTRODUCTION A major component of global energy consumption is transportation, which consumes 4.8 billion barrels of crude oil per year. Of the transportation industry, passenger cars consume 2 billion barrels of oil per year with a value of $100 billion, as of 2003 1) . This substantial fuel consumption is the motivation for the development of a hydraulic hybrid drive train that significantly improves the efficiency of a passenger car. This drive train is being developed for a test bed vehicle in the Engineering Research Center for Compact and Efficient Fluid Power at the University of Minnesota. A hybrid vehicle contains two sources of power consisting of an internal combustion engine and a second power source that allows for energy storage. The energy storage is used during braking events and other drive train control strategies to minimize fuel consumption. Two auxiliary power sources have been found most practical: electric motor/generators combined with batteries and hydraulic pump/motors combined with hydraulic accumulators. Electric hybrid vehicles have been the first hybrid technology to be mass produced for the commercial passenger car market. A strength of electric hybrids is the high energy density of electric batteries, allowing for large energy storage in relatively compact and lightweight batteries. A substantial shortcoming of electric hybrids is the relatively low power density of both electric motor/generators and batteries at approximately 30-100 W/kg 2) . Switching the second hybrid power source to hydraulics realizes benefits in a multiple areas. 1) The power density of hydraulic pumps/motors and accumulators is very high at approximately 500-1000 W/kg 2) . 2) Hydraulic components are inexpensive when compared with electrical components, especially advanced battery packs. 3) Certain hybrid architectures allow for independent control of the torque at each wheel, which opens numerous possibilities for vehicle dynamics control. 4) Recent and developing technologies such as digital hydraulic valves and high energy density