Study on Two-Speed Transmissions for Battery Electric Vehicles
- Publication Type:
- Thesis
- Issue Date:
- 2020
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Due to the shortage of fossil fuel and environment degradation, developing battery electric vehicles (BEVs) has been the irreversible trend in the automotive industry recently. Despite the long-term benefit of BEVs to customers and the environment, the initial cost and limited driving range present the significant barriers for widespread commercialization. At present, BEVs usually adopt a single driving motor with a fixed-ratio transmission in order to simplify the powertrain system; however, the performance requirements of the power battery pack and driving motor are quite high. In order to improve the efficiency of an electric driving system while meeting the requirements of vehicle driveability and reducing manufacturing costs, BEVs can be equipped with multispeed transmissions. A two-speed transmission system appears to be suitable for BEVs in consideration of dynamic performance, energy efficiency, and cost-saving. It can improve the grade ability at low-speed and efficiency at high-speed, thereby reducing the performance requirements of the power battery pack and driving motor, which reduces the manufacturing costs to a degree.
In this thesis, two different types of two-speed transmissions are proposed. The first is a transmission that is comprised of dual-stage planet gear sets and can achieve a gear shifting without the torque interruption, called Uninterrupted Planet-gear Automatic Transmission (UPAT). The other is made up of a single planet gear set, called Planetary Automated Manual Transmission (PAMT). The proposed 2-speed UPAT takes advantage of its mechanical layout to achieve power-on gearshifts by controlling a band brake to block or unblock the one-way-clutch, which makes the gearshift control easier than similar transmissions. Although 2-speed PAMT cannot achieve an uninterrupted torque gearshift, it has a simpler structure and an easier gearshift control system than 2-speed UPAT, and a more compact structure than traditional two-speed automated manual transmission (AMT). This thesis focus on the following six research topics: 1) mathematical modeling of 2-speed UPAT; 2) gearshift control strategies’ design of 2-speed UPAT; 3) rig development and gearshift strategies’ validation of two-speed UPAT; 4) optimal gear shifting control for 2-speed UPAT; 5) torque observer design for 2-speed UPAT; 6) mathematical modeling and gearshift control of 2-speed PAMT.
For two-speed UPAT study, firstly, a mathematical model is developed, including the electric motor, the proposed two-speed UPAT, the vehicle, etc. Secondly, model-based alternative power-on gearshift strategies are developed, and a torque-based gearshift closed-loop controller is proposed. The vehicle jerk and the friction work are taken as the foremost metrics to evaluate the gearshift quality. The simulation results demonstrate that all strategies can achieve power-on gearshifts. The disadvantages and advantages of these strategies are exhibited clearly, which provides beneficial knowledge and reference to the researchers engaged in the development of the transmission controller. Thirdly, to validate the simulation results, a testing rig is developed. The simulation results are well+-validated by the experimental results. Fourth, to comprehensively improve the gearshift quality, optimal gearshift control tactics for the torque phase and the inertia phase are proposed to reduce the vehicle jerk and friction work within the fixed gearshift duration. And, the simulation results demonstrate that the proposed multi-objective optimal tactic for the torque phase effectively reduces the vehicle jerk and friction work, and the optimal coordinating tactic for the inertia phase decreases the friction work to a high degree. Meanwhile, to provide unmeasurable torque information for executing the proposed optimal strategies, sliding mode theory is employed to design the torque observers which are capable of estimating the torque information sufficiently and accurately.
For two-speed PAMT study, to present the transient behaviors during the gear shifting, a detailed and original dynamical model of the electrified powertrain is developed, including an electric machine (EM), a two-speed PAMT, synchronizer mechanisms, driveline, and vehicle, etc. Afterward, the gear shifting control system is devised, and the gearshift process is orderly divided into five stages based on the proposed control strategy. Next, three alternative planning torque trajectories, i.e., the third-degree polynomial (TDP), the fifth-degree polynomial (FDP), and the seventh-degree polynomial (SDP), are proposed to control the EM torque at the first and the fifth stages of the gear shifting respectively. Then, a series of control group simulations are performed to validate which candidate trajectory can obtain the optimal gearshift quality. Simulation results demonstrate that TDP based torque trajectory is the optimal trajectory which is the capacity of not only suppressing the gear shifting jerk but also reducing shift durations. This study will also provide beneficial references for gear shifting control of clutchless automated manual transmission (CLAMT) which is widely adopted in battery electric vehicles.
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