Abstract This paper discusses whether it is possible to improve the motor efficiency by a two-speed transmission in an electric bus, and if so, to what extent. Based on the China Bus Urban Cycle, an 8-meter electric bus was studied via simulation in Matlab/Simulink. The comparison of motor efficiency between two different configurations was made: direct drive and drive through a two-speed transmission. In the first part of the simulation, the speed ratios of the two-speed transmission were chosen as 1.5 and 3.5. The motor efficiency was improved by 1.22% for driving and 1.66% for generation. To find out the maximum improvement and corresponding optimal speed ratio combination, scanning experiment of the lower ratio and upper ratio was conducted in the second part. As much as 1.66% improvement of driving efficiency and 2.20% of regenerating efficiency was achieved. Moreover, two 3-D maps (drive and regenerate) were derived, in which X-axle and Y-axle represented upper gear ratio and lower gear ratio respectively and Z-axle for the improvement of efficiency. Moreover, two 3-D maps were generated to show the relationship between efficiency and lower/upper gear ratios. These two maps revealed optimal speed ratio combination and provided reference for choosing ratios. This study shows that two-speed transmission has potential to improve overall operating efficiency of traction motor, and also provided a method to determine the optimal speed ratio combination. Introduction In most mega-cities around the world, electric buses are taking place of conventional buses, for energy-saving and environment-protection. Thanks to the high starting torque of traction motors, the “direct drive” configuration (shown in Figure 1) has been widely used. However, for most of the time, buses travels at low speed through city traffic. The low speed and torque operating conditions cause traction motor to deviate from the high-efficiency region. Figure 1. Direct drive configuration of an electric bus. As shown in Figure 2, a considerable fraction of operating points is below 500rpm, while the high-efficiency region is approximately between 600rpm and 1200rpm. If a two-speed transmission were installed between the traction motor and the main reducer, the low-speed operating region in the left can be shifted rightwards where the motor efficiency is higher.The Study of Operating Efficiency Enhancement of Traction Motor with the Application of a Two-Speed Transmission in an Electric Bus2014-01-2891 Published 10/13/2014 Fuyuan Yang, Lei Du, Changsheng Yao, and Jiaen Du Tsinghua Univ. Ping Yu Jing-Jin Electric Technologies CITATION: Yang, F., Du, L., Yao, C., Du, J. et al., "The Study of Operating Efficiency Enhancement of Traction Motor with the Application of a Two-Speed Transmission in an Electric Bus," SAE Technical Paper 2014-01-2891, 2014, doi:10.4271/2014-01-2891. Copyright © 2014 SAE InternationalDownloaded from SAE International by Univ of Nottingham - Kings Meadow Campus, Thursday, August 09, 2018Figure 2. Operating points of a 12-meter electric bus show the mismatch of high-efficiency region and operating region. (Data acquired from a demonstrator electric bus in Beijing) This paper discusses whether it is possible to improve the motor operating efficiency by a two-speed transmission and if so, to what extent. An 8-meter electric bus is studied in this paper by simulation in Matlab/Simulink. Part 1 of the paper chooses a two-speed transmission with a lower ratio i1 = 1.5 and an upper ratio i2 = 3.5, which comes from engineering experience, to compare the efficiency with “direct drive” configuration. Part 2 conducts parametric study of i1 and i2 to study the maximum improvement and optimal speed ratio. It is the efficiency of traction motor that this paper mostly concerns, therefore, the driving energy and the regenerative braking energy are calculated independently to avoid taking the whole electric path into co