Development of a three-speed electric drivetrain

Development of a three-speed electric drivetrain

High Speed Electric Drive with a Three-Speed Gearbox

Development of a weight, volume and efficiency optimized powertrain for a Battery Electric Vehicle (BEV) with high speed transmission.

  • Dipl.-Ing. Uwe Reichert, Scientific Assistant, Research Group Drive Systems, Karlsruhe Institute of Technology, Institute of Product Engineering
  • Dipl.-Ing. Torsten Epskamp, Scientific Assistant, Research Group Hybrid Electric Vehicles, Karlsruhe Institute of Technology, Institute of Electrical Engineering
  • Dipl.-Ing. Aline Radimersky, Scientific Assistant, Research Group Drive Systems, Karlsruhe Institute of Technology, Institute of Product Engineering
  • Dipl.-Ing. Sascha Ott, Managing Director, Karlsruhe Institute of Technology, Institute of Product Engineering

Introduction

Increasingly more stringent regulations of CO2 and fine dust in urban and interurban areas emphasize the need for clean mobility. Vehicles without local emissions provide a serious opportunity to avoid the local production of fine dust and pollution of the environment with exhaust gases.

Therefore, research and development focusses electric vehicles. While most vehicle manufacturers provide some electric vehicles in series production, there are still many aspects to optimize within the electric drive system. The main weakness is the low range of actual electric vehicles combined with high costs of electric cars in comparison to conventional cars. One solution of this matter could be battery cells with a higher ratio of energy per mass. Beside improvement of energy density of the batteries there are also approaches to improve the energy efficiency of the powertrain by raising the power density and reducing the total mass of the powertrain. An approach to increase the power density is to increase the maximum speed of the electric machine. Current machines in electric vehicles have a maximum speed of about 15 000 rpm. By raising the maximum speed at the same maximum power, the space and the mass of electric drives could be significantly reduced. But raising the maximum speed often leads to decreasing the maximum torque because of limitations regarding the maximal possible rotor-diameter at high speed as well as the limited possible current of the power electronics. In order to match all performance-requirements of the vehicle, a multi speed gearbox is needed. It enables the electric machine to operate in an efficient point, leading to new requirements for the electric machine and the gearbox and to more degrees of freedom in the design of the powertrain. Thus giving raise to the question of how to find suitable combinations of electric machine and gearboxes for different vehicles.

Research Project Effect 360°

The Karlsruhe Institute of Technology (KIT) develops a high speed electric machine with a maximum speed of 30 000 rpm in combination with a loss reduced three-speed gearbox without traction interruption. For the overall system integration of the complete drivetrain, Daimler AG and Robert Bosch GmbH are also part of the research project. This article presents the approach implemented in the project as well as the developed drivetrain concept. In this context, special attention is given to the functional interactions in the development of the whole drivetrain. Through the mixed physical and virtual approach of the development and verification of the drivetrain, the fulfilment of the vehicle requirements is continuously under consideration. The comparison in virtual simulations of the high speed drivetrain with a single speed reference drivetrain with lower speed shows a higher power density by competitive overall system efficiency of the three-speed gearbox. Initial simulation (virtual testing) is then gradually changed into physical tests on test rig with remaining system being modelled virtually (mixed physical and virtual tests). New findings of the subsystem experiments enter in the holistic conception and dimensioning of the drivetrain.