Comprehensive Analysis and Optimal Configurations of the EVT Powertrain

This paper investigates the design of a series-parallel hybrid electric powertrain that includes an electric variable transmission (EVT) and two mechanical gears that connect the EVT to the rest of the powertrain. The EVT is implemented as a dual rotor electric machine (EM), which from a design perspective can be studied as two separate EMs. The goal is to optimize the size of the two EMs and the two mechanical gears, while minimizing fuel consumption, subject to performance requirements on top vehicle speed and acceleration time. A practical optimization methodology is proposed that decouples the multi-objective problem into three different control subproblems: a slow dynamic subproblem for fuel economy, a fast dynamic subproblem for acceleration performance and a static optimization subproblem for the top speed performance. Efficient dynamic programming formulations are presented to solve the two dynamic subproblems. Two different scenarios are discussed and analyzed, where the first one presents the influence of the EMs' sizes on the vehicle performance and the second one delivers near optimal configurations of the EVT powertrain for different gear ratios. A case study is also carried out to compare the sizing results between the proposed method and an existing benchmark method, showing that the two EMs could be reduced by 45% and 11.87% respectively, while powertrain performance could still be maintained at the same level.