Development of Compact Two-Speed Transmission for Electric Vehicles

Electric vehicles are rapidly increasing in the global market. Most electric vehicles have adopted a single-speed gearbox, which is a double reduction gear set. Although the traction motor of an electric vehicle has a wide rotational speed range, the single-speed gearbox has the disadvantages of lowering the efficiency of the motor operating region and limiting the maximum vehicle speed. To overcome the shortcomings of the single-speed gearbox, it is necessary to apply a 2-speed transmission to electric vehicles. Based on the patent of system configuration, a unique 2-speed transmission with a planetary gear set and two synchronizers set developed and applied to electric vehicle. The advantage of this 2-speed transmission is that traction force increases by more than 40% at first gear and top speed increases by more than 30% at second gear. The system configuration was designed with the idea of making compact, efficient and cost-effective product. The gearbox, shift mechanism, shift logic and TCU (Transmission Control Unit) were developed. The TCU controls the two shift motors to engage and disengage the two synchronizers during gear shifting and connects to the MCU (Motor Control Unit) to synchronize the traction motor speed and vehicle speed immediately before gear shifting is complete. A 2-speed transmission prototype was tested and calibrated on a transmission dynamometer to verify the gear train, shift actuators, and shift logic performance. The optimum ranges of several control parameters of the traction motor and shift motors obtained through the transmission dyno test. An electric vehicle equipped with a prototype of a 2-speed transmission tested at a driving test site. Several control parameters were calibrated for the performance of shift quality and shift time. The 2-speed transmission proved effective way to improve significantly acceleration performance and top speed compared to an electric vehicle with single-speed gearbox through the transmission dyno test and the actual driving test for the prototype.