Automotive Traction Drive Speed Reducer Efficiency Testing


The move from internal combustion engine vehicles to electric vehicles is happening rapidly, which demands a stepwise change in priorities for the vehicle design process with rational consideration of emerging technologies. This paper focuses on the efficiency of a particular form of high fixed-ratio (15:1) traction drive speed reducer. This is suitable for use in conjunction with a high-speed electric motor for automotive applications. A general discussion of the characteristics of other fixed-ratio traction drives is provided followed by an analysis of underlying efficiency issues. The paper presents details of the speed reducer prototype called a "Silk Drive" and the vehicle for which it was designed. Data from laboratory testing of the prototype are presented, and an efficiency map for the transmission is developed. The efficiency map and vehicle parameters are used in a simulation to determine the overall transmission efficiency for the world harmonized light vehicles test cycles (WLTC) class 3b drive cycle. The importance of transmission efficiency at low power levels, in specific input speed and torque regions, is demonstrated using a novel method for identifying those speed torque regions that most strongly affect overall efficiency. The method applies to all drivetrain components and pinpoints those regions that need to be the focus in the optimal design of such components. This paper presents evidence that the efficiency of zero-spin, fixed-ratio traction drives is similar to that of conventional gear drives.

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Internal combustion engine


Continuously variable transmission


Electric vehicle


World harmonized light vehicles test cycles


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Corresponding author

Correspondence to James M. Durack.

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Conflict of Interest

The corresponding author acknowledges that James M Durack and Michael J. Durack are both directors of Ultimate Transmissions who are the developers of the Silk Drive forming the focus of this paper.

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Wang, W., Durack, J.M., Durack, M.J. et al. Automotive Traction Drive Speed Reducer Efficiency Testing. Automot. Innov. (2021).

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  • Electric vehicle
  • Traction drive
  • Zero spin
  • Transmission efficiency
  • EV range