Energy Management Strategies for Series-Parallel Hybrid Electric Vehicles Considering Fuel Efficiency and Degradation of Lithium-Ion Batteries

Abstract

Lithium-ion batteries are the most crucial component of hybrid electric vehicles (HEVs) with respect to cost and performance. In this article, a new energy management strategy (EMS) is developed that improves fuel efficiency (FE) and suppresses the degradation of the battery. A hybridized two-layer algorithm that combines multi-objective nonlinear model predictive control (NMPC) with a rule-based (RB) algorithm is proposed as a new EMS that is called RB-NMPC. The RB-NMPC is designed to optimize the torque split between the engine and electric motors while maintaining the maximum and minimum constraints of each component. The proposed EMS is incorporated into control-oriented vehicle models, and their performances are analyzed for different driving cycles by comparing with RB, dynamic programming (DP), and NMPC. In addition, the RB-NMPC algorithm is applied for two different powertrain configurations of HEV, P0P2 and P1P2 configurations for both an Urban Dynamometer Driving Schedule (UDDS) and a Highway Fuel Economy Test (HWFET). For P0P2, the results show that RB-NMPC outperforms other methods for UDDS with an FE that is 4.7% higher than that of RB and is the closest to that of DP, which is an optimal standard that is limited for real-time application due to its complexity among others. The capacity loss of the battery using RB-NMPC is 19.1% less than that using DP when applied to the UDDS. The FE of P1P2 is higher than that of P0P2, but the similar capacity fade is comparable. RB-NMPC shows the lowest capacity loss for both P0P2 and P1P2 configurations. Parallel comparisons are performed for the HWFET. For the HWFET, the FEs of P0P2 and P1P2 are similar. However, the capacity fades by RB-NMPC are 16.3% and 67.0% reduced compared to that by DP for P0P2 and P1P2, respectively. Finally, to verify the effectiveness of the RB-NMPC in reducing battery aging, the currents from DP and RB-NMPC EMSs are applied to pouch-type lithium-ion batteries and tested for multiple UDDSs using a battery test station. The results demonstrate that the RB-NMPC can effectively reduce battery aging.