A hierarchical control strategy for hybrid vehicles.

Abstract

With increasing gasonline prices and the rise of environmental concerns the demand for cleaner, more fuel efficient vehicles rises. The development of an environmentally friendly transportation technology is necessary. Hybrid vehicles were found to have high potential in decreasing fuel consumption in the transportation sector. Fuel efficiency increases with additional flexibility in the drive train due to multiple power sources. With growing complexity in the drive train appropriate control is critical to achieve maximum fuel economy. Research conducted by the Test Bed 3 group in the Center for Compact and Efficient Fluid Power at the University of Minnesota is investigating the potentials of fluid power power trains to drastically decrease fuel consumption in hybrid vehicles. In the scope of the Test Bed 3 project controls algorithms are developed for implementation on the hybrid vehicle. In this thesis a control strategy based on hierarchical control is proposed. A generic three level control strategy that is applicable to any hybrid vehicle configuration is derived. On the example of two power split hybrid drive trains the use of this controls framework will be illustrated in simulation. Case studies were performed using an off-line simulation approach. These investigate optimal sizing of the power train, optimal operation of a power split configurated vehicle and the relationship of hydraulic efficiency and fuel consumption. Three hybrid architectures are compared in fuel economy. Fuel savings through hybridization of a conventional vehicle are discussed. Using an input coupled hydraulic hybrid vehicle the implementation of the proposed strategy in real time is discussed. System level control implementation is shown and results from test drives are presented.