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The Design of Advanced Transmissions
Published October 22, 2006 by Society of Automotive Engineers of Japan in Japan
This paper describes the creation of novel transmission mechanisms, both conventional as well as hybrid, using algebraic design techniques. The objective is to create novel transmissions comprised of one or more planetary gear sets, a pair of motor-generators and several torque-transmitting mechanisms (clutches and brakes). The algebraic design procedure represents the planetary gear sets, fixed interconnections, clutches/brakes, and motor-generator sets as algebraic constraints. Appropriate subsets of constraint equations are solved to identify viable transmission mechanism designs.
The steps of the design methodology may be summarized as follows. After deciding on the number of planetary gear sets, clutches, brakes, and motor/generators to be used in the proposed transmission, we enumerate by computer, all possible kinematic combinations of these elements to create an exhaustive candidate pool. The transmission governing equations are then used to identify viable candidates. These constraint equations include requirements on torque ratio values, ratio steps, ratio spread, shift complexity, and element speeds. In the case of hybrid transmissions, we also investigate the level of electrical energy required and the ability to execute mode-shifts. The design process uses graph theory to handle issues related to mechanism planarity and isomorphism. Following the identification of attractive transmission candidates at the lever diagram level, we prepare layouts and detailed stick diagrams, taking into account the packaging of bearings, hydraulic circuitry, supporting shafts and structures.
We have used the above design approach to create several novel candidate multi-speed transmissions as well as EVT concepts. The main benefit of the algebraic design procedure is that it allows the designer to generate and assess novel designs without relying on intuition and prior experience. Another benefit of the above design procedure is its ability to identify minimum-content designs, wherein the emphasis is on achieving the maximum level of functionality with the fewest components.