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The performance of the transmission has a great influence on the power, economy and handling stability of vehicles, and the planetary gear automatic transmission is widely used in passenger cars because of its uniform force and compact structure. Therefore, in order to break the patent barriers, large enterprises are actively developing multi-speed transmissions, and the optimization of new topological structure has become the key of research and development. In this paper, the structural characteristics of four typical four-speed automatic transmissions composed of double-row planetary gears are analyzed, and their lever diagrams and shift schedules are deduced. Then, based on the lever diagrams, a new type of SC-CR transmission that uses only three clutches and two brakes to achieve four forward gears and one reverse gear is designed. What’s more, the specific topological structure is devised and the transmission parameters are optimized. The research shows that the equivalent lever speed line diagram and shift schedules of the four double-row four-speed planetary gear transmissions are basically the same. The transmission ratio of the new SC-CR four-speed automatic transmission is increased by 9.43% to 25.43%, the transmission ratio on gear D1 is increased by 8.86% to 18.51%, and the transmission ratio on gear D4 is reduced by 0.53% to 7.83%, which better realizes the optimization of transmission system. What’s more, through the series-parallel method, it can provide a better theoretical basis for the design of the new multi-speed planetary gear automatic transmission.
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