Optimal design of a power-split hybrid hydraulic bus

The hydraulic power-split architecture, or the hydromechanical transmission, can exist in over 1000 configurations. Since a specific hydromechanical transmission configuration has a unique kinematic characteristic, the optimal hydromechanical transmission configuration for one application is different from the optimal hydromechanical transmission configurations for other applications. This study finds the optimal hydromechanical transmission configuration for a hybrid hydraulic city bus. Since a hydromechanical transmission uses at least one planetary gear set, the process of finding a suitable hydromechanical transmission configuration for a hybrid hydraulic city bus is very time consuming, but this study simplifies the procedure by utilizing the critical speed ratio I‰_crit, where I‰_crit is the missing link between the planetary gear set configuration, its gear ratio and its kinematic characteristic. This simplification also allows complex dual-stage hydromechanical transmission architecture configurations to be considered as candidates for the optimal drivetrain of the hybrid hydraulic city bus. The hydromechanical transmission configuration candidates are simulated over three city bus driving cycles and, to ensure that they operate at the optimal conditions for each driving cycle, dynamic programming is used. This study also establishes two power recirculation modes, one of which is useful because of its high efficiency. A single-stage power-split architecture configuration candidate is found to be the optimal drivetrain for a hydromechanical transmission bus.