This research finds the optimal power-split drive train for hybrid hydraulic city bus. The research approaches the optimization problem by studying the characteristics of possible configurations offered by the power-split architecture. The critical speed ratio (S_crit) is introduced in this research to represent power-split configurations and consequently simplifies the optimal configuration search process. This methodology allows the research to find the optimal configurations among the more complicated dual-staged power-split architecture. The search for optimal single-staged and dual-staged power-split configurations is done by tuning the S_crit values until the optimal configuration is reached. The configuration candidates are simulated over three city bus drive cycles and to ensure they operate at optimal condition at each drive cycle, a dynamic programming is used. This study also establishes that there are two kinds of power re-circulation modes and one of them has a usable range with better efficiency than the series hybrid hydraulic architecture. A configuration candidate from single-staged power-split architecture is chosen to be the optimal drive train. The ( S_crit value of the optimal configuration is used to determine how all the gears in the optimal power-split HHB drive train are connected and the planetary gear ratio so that the physical drive train can be built.
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