Wang, H., Song, X., Saltsman, B., and Hu, H., "Comparative Studies of Drivetrain Systems for Electric Vehicles," SAE Technical Paper 2013-01-2467, 2013, doi:10.4271/2013-01-2467.
Vehicle electrification is being actively expanded into coming generations of passenger and commercial vehicles. This technology trend is helping vehicles to become more energy efficient. For electric vehicle (EV) city bus application, the system designers have been experimenting with a number of options including direct drive and multi-speed gearbox architectures. Direct drive scenario offers simplified drivetrain system, however requires a large and powerful electric motor. Multi-speed transmission system provides an opportunity to reduce motor size and optimize its operating points, but increases complexity from the architecture and controls point of view. This paper provides an overview of several common system layouts and examines their advantages and disadvantages. Vehicle simulation results are presented to compare direct drive vs. multi-speed technology from the gradeability, acceleration and energy consumption points of view. These results demonstrate the advantages of EV (electric vehicle) drivetrain system employing a multi-speed transmission over direct drive approach for demanding city-bus duty cycles.