Control Strategy Optimization for a Series Hybrid Vehicle

This paper describes the development of an operating strategy for a series hybrid vehicle. The vehicle's powertrain consists of an Engine-Generator Unit (EGU), a buffer for energy storage (such as an electric battery), and a traction motor driving the wheels. The control strategy is developed so that to ensure gradual operation of the EGU along the steady-state Optimal Operating Points Line (OOP-Line) in the engine speed-torque map. The strategy functions as follows. It first determines a set-point for the state of charge of the energy buffer to minimize the probability of either discharging or overcharging it. This probability is estimated based on the statistics derived from the past history of the state of charge. Based on the difference between this set-point and estimated state of charge, the strategy calculates the power demand from the EGU which is then translated into engine torque and engine rotation speed demand so that the engine operation is maintained on the OOP-Line while delivering the requested power. The torque demand is delivered by the engine management system through the control of an engine electronic throttle while the generator voltage is adjusted to control the engine speed to the desired value. A simulation study, based on a control-oriented model of a series hybrid powertrain, is reported to illustrate the basic ideas. We also demonstrate via theoretical analysis that by periodically modulating the power output of the EGU the fuel consumption can be improved in the region where this fuel consumption is a concave function of the EGU power output. The series hybrid powertrain configuration is also relevant for fuel cells applications, and some of our considerations also extend to that case.