Power split control for a hybrid energy storage system with two propulsion machines in electric vehicles

The Majority of electric vehicles (EVs) on the market have only one propulsion machine in their powertrains. To enhance power performance and propulsion efficiency, two propulsion machines, with complimentary torque-speed characteristics, are coupled to the same shaft via a torque coupler could be deployed in the EV powertrain. A potential merit of using two propulsion machines is the improved powertrain efficiency and effective load power split. Another advantage of using two complimentary propulsion machines, instead of a single large machine, is that the space under the hood can be effectively utilized. This is particularly an attractive solution for race cars, with limited space due to their aerodynamic body frame, which in turn will reduce the drag coefficient. Nowadays, batteries are used as the primary energy storage devices in EVs; however, they are usually oversized in order to deliver high power and avoid the unwanted degradation due to frequent acceleration and deceleration. In principle, batteries have relatively high energy density. Ultra-capacitors (UCs), on the other hand, have high power density and excellent instantaneous charging and discharging capabilities. In order to take advantage of these complementary features of the battery and UC, a battery/UC hybrid energy storage system (HESS) is considered. In the HESS, UC pack acts as a power buffer for instantaneous charging/discharging of high peak power. In this way, the stress on the battery is substantially relieved, resulting in battery lifetime extension In this dissertation, a power split control strategy is proposed for an electric vehicle (EV) powertrain with two propulsion machines and a battery/ultra-capacitor (UC) hybrid energy storage system (HESS). The proposed power split control strategy consists of two stages. In the first stage, the load power is split between the two propulsion machines to obtain the highest powertrain efficiency in both propulsion and regenerative braking modes. A real-time implementable power split control strategy is proposed to benefit from complementary operation features of these two propulsion machines. In the second stage, the load power is split between the battery pack and UC in the HESS. To optimize the power split in HESS, a convex optimization problem is formulated to minimize the battery power magnitude and battery current variations to extend the battery lifetime. The powertrain topologies for single propulsion machine, two identical propulsion machines and two complemantary propulsion machines are compared in the simulation cases. The implementation of proposed power split control strategy in the powertrain of an EV with two propulsion machines will potentially result in up to 10% improvement in the powertrain efficiency and up to 82% extension of the battery lifetime. The improvement of propulsion efficiency will in turn lead to up to 27% extension of all-electric driving range (AER). On the other hand, a battery/UC/Fuel Cell (FC) HESS topology is introduced. Remaining ESS total weight same, only battery, battery/UC and battery/UC/FC topologies are compared based on AER and battery health. Due to additional weight of FC and UC, smallest battery size is utilized in the battery/UC/FC topology to keep ESS weight same as other compared topologies and the biggest battery size is utilized in the only battery ESS topology. According to simulation results, the longest battery life time is achived with battery/UC topology which is slightly better than battery/UC/FC topology. The longest AER is achived with battery/UC/FC topology which is 2.4 times better than battery/UC topology. Moreover, optimal ESS sizing is studied for only battery ESS for high power (HP) and high energy (HE) batteries. Six case studies are presented for different powertrain and ESS topologies. The main contributions of this paper are as follows, First, a real-time implementable power split control strategy is presented, which optimally splits power between two propulsion machines to improve the powertrain efficiency. Second, the powertrain topology with two identical propulsion machines and the powertrain topology with two complementary propulsion machines are compared with utilizing single propulsion machine in the powertrain according to weight, efficiency and all-electric-range (AER). Third, Optimal battery sizing is studied for a battery ESS with utilizing HP and HE batteries, Fourth, a convex HESS optimization problem is formulated with the objective function to extend battery life. Fifth, six case studies are investigated based on different powertrain and ESS topologies. Finally, battery/UC/FC HESS topology is discussed and only battery, battery/UC and battery/UC/FC topologies are compared based on AER and battery health.