Optimal torque split strategy for BEV powertrain considering thermal effects

Abstract: A common architecture for electric vehicles is to have two electric machines one each on the front and rear axle. Despite the redundancy, this configuration ensures performance. Being energy efficient is equally important for electric vehicles to deliver a sufficiently high range. Hence, operating a single machine at low to medium torque requirement is desirable. A clutch can be implemented on the front axle and its engagement dynamically controlled to reduce the magnetic drag losses in the front machine. With clutch disengaged, the entire torque will be delivered by the rear machine causing it to heat up quickly. As electric machine and inverter losses are also temperature dependent, this work attempts to derive an optimal torque split strategy between the two machines considering thermal effects. An upper-temperature limit for both electric machine and inverter is imposed for component protection. Thermal models for the electric machine, inverter and coolant circuit are simplified using system identification and model order reduction approach. Dynamic optimal torque split is realized by minimizing the energy loss over the entire drive cycle. Dynamic programming is used to investigate the benefits of including thermal losses and to generate a benchmark solution for optimal torque split strategy. Further, two online controllers are developed, one based on non-linear model predictive control and the other being a static controller with added heuristic rules to prevent temperatures of critical components to exceed the limits. A high-fidelity plant model was developed using VSIM as master and GT-Suite thermal model as slave to compare the performance of these controllers. The results show that it is possible to obtain decent thermal performance of electric motor and inverter with one node lumped parameter thermal model and a five-node lumped parameter model for the coolant circuit. Including thermal dynamics in the controller can constraint the temperature within the limits and give an optimal torque split. The benefit of adding temperature-dependent thermal maps is found to be limited to certain operating regions. The static controller with torque split based on instantaneous power loss also performed well for the given configuration. The major contribution to energy saving was obtained by dynamic disengagement of clutch in the form of reduced magnetic drag losses.