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Demands for higher fuel efficiency for off-highway applications motivate manufacturers to replace existing drive transmissions with more complex, high-efficiency transmissions. Increased intelligence and more advanced architectures are, however, more difficult to design and prototype. This leads to longer product development processes and a greater need for early product evaluation. The great variety of existing concepts also requires a methodology to support the choice of architecture. This paper proposes a design methodology for complex hydromechanical transmissions based on optimization. The main objective is to maximize energy efficiency and adapt the design to suit the typical operating behavior of the application. The methodology is also implemented on a multiple mode transmission concept sui for a heavy wheel loader application. It is shown that the design of the gearbox heavily influences the energy consumption and the necessity to use optimization when designing the gearbox.

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This thesis proposes an automated methodology for the design of complex multiple-mode hydromechanical transmissions. High fuel prices and strict emission regulations are today drivers of the development of new fuel-efficient drive transmissions for construction machinery. Hydromechanical transmissions have high energy efficiency and a wide torque/speed conversion range. They are today strong candidates to replace the fuel-thirsty torque converters conventionally used in heavy construction machines. The trend towards more complex transmission architectures increases the need for more sophisticated product development methods. Complex multiple-mode transmissions are difficult to design and prototype and can be realised in a great number of different architectures. This increases the need for reliable concept evaluation in early design stages. The design of the transmission is also strongly coupled to its energy consumption and for a fair comparison between transmission concepts optimal designs are necessary.Design automation and optimisation with detailed simulation models can support the industrial engineer in the design task and increase the available knowledge early in the design process. The proposed methodology uses simulation-based optimisation to design the transmission for a specific vehicle application. Various aspects of the transmission's characteristics may be targeted, although energy efficiency is in great focus in this work. To evaluate the energy efficiency, the transmission designs are simulated using backward-facing simulations with detailed power loss models. The methodology is applicable for designing the drive transmissions of construction machines and other mobile working vehicles such as agricultural machines, forest machines and mobile mining equipment.

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