The development of a power management strategy for a hydraulic hybrid passenger vehicle

Abstract

The amount of energy being consumed is increasing each year, with the highest sector being the transportation industry. Within the transportation sector, the highest area of oil consumption is in the small and lightweight vehicle category. With increasing oil prices and decreasing supply, methods of reducing oil consumption have been studied. One is by developing a hybrid vehicle, which combines the internal combustion engine with an additional power source. For lightweight vehicles, electric hybrid vehicles have been thoroughly studied. While hydraulic hybrids have been studied for larger applications such as delivery trucks and buses, little research has been done in the area of small, lightweight vehicles. Hydraulics have a higher power density than electronics, so hydraulic hybrids can get better performance than electric hybrids while reducing fuel consumption.In this research, a series and power-split architecture is studied for a passenger vehicle. Because of the additional hydraulic power source along with energy storage, the optimal way to control these vehicles is not known. Therefore, an energy management strategy must be developed to determine the optimal strategy for splitting the power between the engine and the hydraulics.Three different methods are used to develop the energy management strategy - a rule-based strategy based on dynamic programming results, stochastic dynamic programming, and model predictive control. An experimental hardware-in-the-loop setup is used to replicate a series hybrid in which the different energy management strategies are tried. Through simulation and experimentation, it was found that not one strategy works best in all scenarios, and variables such as knowledge of duty cycle and energy storage must be taken into account when developing the strategy.An input-coupled power-split hybrid was also studied, which combines the mechanical efficiency of the parallel hybrid with the engine management of a series hybrid. Through a series of simulations, a strategy that declutched the engine from the drivetrain while the vehicle is stopped gave a significant reduction in fuel consumption. Another advantage of the power-split architecture is the ability to operate the vehicle in different modes by declutching the engine and removing hydraulic units by the use of valves. By using this strategy, the fuel economy can be almost doubled over a baseline strategy which operates only in power-split mode. Finally, the size of the accumulator can have an effect on the fuel consumption, with a smaller accumulator leading to less fuel consumed; however, if the accumulator is too small, the performance starts to degrade with a downsized engine.The results of this research can be used to develop a toolbox that can be used for developing energy management strategies by having the user enter a model, objective function, and duty cycle for a system. By using other information, such as knowledge of duty cycle, the toolbox can determine the best method of developing the control strategy, reducing the amount of time and resources for developing an optimal control strategy.