Gear ratio optimization of one- and two-speed transmissions for fully-electric vehicles

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Chen, Kevin, M.S. in Engineering

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Currently, electric vehicles use a transmission with a single-speed fixed gear ratio optimized for performance. This work features the development of a framework to optimize the gear ratio for a given vehicle design, motor efficiency map, motor noise spectrum, and transmission noise spectrum. The framework provides a methodology for designing the optimization problem to be solved using derivative-free optimization methods. As electric vehicles become increasingly prevalent in the global automotive market, the need to develop multi-speed transmissions to provide a competitive edge against other electric vehicles. This work also explores the optimization of gear ratios for a two-speed transmission and identifies a corresponding switching point to switch between the two ratios. Using the same methodology, the optimization problem is formatted such that derivative-free optimization methods can be used to solve the optimization problem quickly and efficiently. Utilizing the framework developed, the program was able to identify an optimal gear ratio for a single-speed transmission in about half the time it took a brute-force search method to the same. The two-speed optimization process completed about 50 times faster than the brute force search. In both cases, the result returned by the optimization process was compared against the brute force search result to confirm global optimality. The single-speed gear ratio optimization, the two-speed gear ratio optimization, and the two-speed switching point optimization reliably located a global optimum.


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