Advanced Fully Automatic Two-Speed Transmission for Electric Automobiles 951885

To meet the specific requirements of electrically driven passenger cars, Steyr-Daimler Puch Fahrzeugtechnik (SFT) has developed an advanced, optimized drive train integrating both functionally and physically the components of electric motor, electronic control, charging system, transmission, and final drive. Subject of this paper is the design and function of the transmission system.

Most of the state of the art EV drive systems are designed with a single transmission ratio. This, however, leads to high peak currents, a heavy electric motor, and inefficient use of the battery energy to achieve sufficient gradeability, traction, and acceleration. To avoid these disadvantages, SFT has developed a transmission system with a low gear for low speed/high traction conditions, and a high gear for normal driving. Gear shift between low and high is fully automatic, and carried out by an electric actuator. Synchronization of gear shifting is achieved electronically by combining the existing electronic management of the induction drive motor with the control of the transmission. Shifting is performed with a smooth transition, including a no-load condition limited to less than 0,5 s for the actual gear change.

The system combines the high efficiency of a mechanical transmission with the comfort of a fully automatic operation, and features functions and driver controls identical with a current automatic transmission. As compared with a single ratio system of equal driving performance, a substantial reduction of weight and cost is achieved.