Analytical Methodology to Derive a Rule-Based Energy Management System Enabling Fuel-Optimal Operation for a Series Hybrid

Due to the continuous electrification of vehicles, the variety of different hybrid topologies is expected to increase in the future. As the calibration of real-time capable energy management systems (EMS) is still challenging, a development framework for the EMS that is independent of the hybrid topology would simplify the overall development process of hybrid vehicles. In this paper an analytical methodology, which is used to derive a fuel-optimal, rule-based EMS for parallel hybrids, is transferred to a series topology. It is shown that the fundamental correlations can be applied universally to both parallel and series configurations. This enables the possibility to develop a real-time capable, rule-based controller for a series HEV based on maps that ensures a fuel-optimal operation. These maps provide the optimal power threshold for the activation of the auxiliary power unit and the optimal power output dependent on the driver’s power request. The implemented rule-based EMS possesses characteristics of both thermostatic and power following control and is compared to the optimal EMS derived by global optimization. Furthermore, the impact of different powertrain characteristics on the optimal operating strategy of a series hybrid are discussed within this paper. The variations lead to the conclusion that the installed power ratio between the traction motor and the auxiliary power unit leads to a difference in operation. The smaller the installed power of the auxiliary power unit is compared to the traction motor, the more a thermostatic control behavior is adopted. Vice versa, as the power ratio approaches one, a stronger power following strategy is adopted. By transferring the methodology for parallel HEVs to series HEVs, it is successfully demonstrated that a development framework that is not topology-specific can be established.