Power-split architecture is, in theory, an excellent drive train in city bus applications because it combines the advantages of series (control flexibility) and parallel (high-efficiency power transfer) architectures. However, the power split also increases the complexity of the drive train with more component choices and control degrees of freedom that, if not properly optimized, would result in significant power loss. The hydraulic hybrid drive train has the disadvantages of the low speed limit of the hydraulic pump/motor unit and the low energy density of the hydraulic accumulator. An extensive optimization study has been done to ensure the hydraulic hybrid city bus drive train runs at optimal conditions. This study analyzes all twelve basic power-split configurations before any component or control strategy optimization is done. One input coupled configuration is found suitable for use in a power-split hydraulic hybrid city bus.

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