Towards an Analytical Method for a Combined Energy Management and Shift Strategy in Hybrid Electric Vehicles, and Its Rule-Based Implementation

Abstract

In conventional vehicles the shift strategy has a well-known impact on the system's efficiency. An appropriate gear choice allows the internal combustion engine (ICE) to operate in efficient operating points (OPs) and thus contributes significantly to a reduced fuel consumption. Further efficiency improvements can be achieved by the hybridization of the powertrain. Due to the two propulsion systems, an additional degree of freedom arises, that requires an energy management strategy (EMS). The EMS controls the split of the requested power between the electric machine (EM) and the ICE. Accordingly, the system's overall efficiency in hybrid electric vehicles (HEVs) is highly influenced by the quality of the EMS. This paper proposes to adapt an existing method for deriving fuel-optimal rule-based EMS by including the shift strategy for parallel HEVs. It is shown that fuel-optimal control can be achieved. The analytically derived look-up tables can be used to automatically calibrate in-vehicle EMS and the shift strategy for HEVs. The fuel-optimal shift strategy is characterized by high shift frequency, which hinders a straightforward in-vehicle integration. To further pave the way towards in-vehicle implementation, hysteresis based on energetic characteristics are proposed and a method for implementation in rule-based EMS is deduced. Finally, the benefits of coupling the shift strategy and EMS are depicted by an analysis of charging the battery (BAT) during vehicle operation. This exemplary study proves that the presented strategy allows operation at higher efficiency while improving the electric driving (ED) share. This positive effect is made possible by operating the ICE at higher speeds, which enables a more efficient charging of the BAT and additionally less energetic expenditure for boosting and lowering the ICEs OPs.