Integrated Modeling and Hardware-in-the-Loop Study for Systematic Evaluation of Hydraulic Hybrid Propulsion Options.

Abstract

The fuel economy benefits of any given hybrid technology depend greatly on the vehicle type, size, supervisory control and driving schedule. The main goal of this work is to develop a comprehensive methodology for up-front strategic assessments of the best hybrid system for a given vehicle platform, and to explore the impact of vehicle driving schedules on the final decision. Several other objectives enabled achieving the main goal, including modeling, optimization of design and power management of several hydraulic hybrid systems developed for a 4x4 light truck. The parallel, series and power-split hybrid configurations are modeled and analyzed. The unique issues related to matching of components and interactions in the system with a high-power density of pump/motors and the energy storage (accumulator), but relatively low energy density of the storage and limited motor speed range are investigated. The design optimization is carried out to maximize the fuel economy while satisfying vehicle performance constraints. An Engine-in-the-Loop capability is developed for each of the hybrid architectures, integration issues are resolved and the EIL is subsequently used for validation of simulation predictions and studies of the impact of hybrid system configuration and control on diesel emissions. For the power management optimization, the deterministic dynamic programming technique provides the fuel economy benchmark. Stochastic dynamic programming technique is explored next, in order to develop an implementable sub-optimal supervisory control policy based on the vehicle power demand probability distribution sampled from various driving schedules. The simulation results obtained over the wide range of driving schedules from aggressive city cycles to mild highway cycles provided fuel economy trends and comparison of hybrid propulsion options. Fuel economy improvements of ~80% (up to 150% with engine shutdowns) are shown for aggressive city-cycles, while the gains diminish for high-speed highway driving. Verification of the emission reduction potential is enabled by synergistic experiments using a newly developed engine-in-the-loop capability. The results provide insight into the effects of the hybrid power management on transient emissions of soot and nitric oxides from a diesel, and provide guidance for the development of strategies for achieving both clean and efficient hybrid propulsion.