SAE TECHNICAL PAPER SERIES2007-24-0080 Series Hydraulic Hybrid Propulsion for a Light Truck – Optimizing the Thermostatic Power Management Y. J. Kim, Z. Filipi Automotive Research Center, University of Michigan Capri, Naples Italy September 16-20, 2007 2007-24-0080 Series Hydraulic Hybrid Propulsion for a Light Truck – Optimizing the Thermostatic Power Management Young Jae Kim and Zoran Filipi Automotive Research Center, University of Michigan Ann Arbor, MI, USA; E-mail: [email protected] Copyright © 2007 SAE International ABSTRACT The global energy situat ion, the dependence of the transportation sector on fossil fuels, and a need for rapid response to the global warming challenge, provide a strong impetus for development of fuel efficient vehicle propulsion. The task is particularly challenging in the case of trucks due to severe weight/size constraints. Hybridization is the only approach offering significant breakthroughs in near and mid-term. In particular, the series configuration decouples the engine from the wheels and allows full flexibility in controlling the engine operation, while the hydr aulic energy conversion and storage provides exceptional power density and efficiency. The challenge stems from a relatively low energy density of the hydraulic accumulator, and this provides part of the motivation for a simulation-based approach to development of the system power management. The vehicle is a 4x4 truck weighing 5112 kg and intended for both on- and off-road use. The development of the com ponent models and system integration in SUMULINK are discussed before addressing the configuration (single propulsion motor or two) and component sizing. The power management is based on a thermostatic state-of-charge (SOC) approach, but the optimum threshold power and SOC for accumulator charging are determined based on detailed system analysis, rather than the conventional wisdom of operating the engine at the “sw eet spot”. The results indicate significant advantages of reduced threshold power. Relatively low target SOC led to improved ability to capture the braking energy . Engine shut-downs are considered too. The fuel economy predictions for the optimized hybrid system i ndicate improvements in excess of 50% under urban driving conditions, and tangible benefits in highway driving. INTRODUCTION Concerns about global crude oil supplies and growing environmental concerns prov ide a strong impetus for developing fuel-efficient vehicle technologies. The bulk of fossil fuels is being consumed by the transportation sector, and ground vehicles constitute a major part of it [1]. The expected explosive economic growth in Asia, particularly China and India, is going to put even more pressure on the fuel supply side. Improved fuel efficiency and conservation ar e the best “new sources” of energy in the short to mid term. In addition, there is a growing awareness of the effect of green-house gases, such as carbon-dioxide, on global warming and realization that reducing CO 2 emissions needs to be addressed on either voluntar y or mandatory basis. In the case of vehicles using fossil fuels, reduction of CO 2 emission is linked directly to consumption, thus the additional stimulus for a strong research focus on energy efficiency of vehicle propulsion. The overall vehicle fuel economy fundamentally depends on the efficiency of the fuel converter (e.g. IC engine), vehicle mass and losses, e.g. air drag, rolling resistance, driveline losses et c. While in the case of passenger cars there are obvious opportunities for improvements in all of afore mentioned categories [2], in the case of trucks the choices become severely limited. Most of the heavier trucks al ready use highly efficient diesel engines and fuel conversion efficiency can be improved only incrementally. Tr uck weight is dictated by the payload: development of li ght weight structures is likely to increase the payload carr