The control strategy in a hydraulic hybrid vehicle is very important for taking advantage of its full potential. In a hydraulic hybrid vehicle, the hydrostatic transmission can be controlled to optimize the entire system. A hydrostatic transmission is composed basically of a hydraulic motor and a hydraulic pump. For an electric hybrid vehicle, an electric motor is the prime mover. In this study, the control variables are the displacements of the hydraulic devices and the objective is to optimize the system efficiency. All possible combinations of displacements were tested through simulation and the most efficient combination was recorded. Compared with a fixed displacement hydrostatic transmission, around 4.5% of the energy can be saved using the most efficient configuration of the hydrostatic transmission. Fuzzy logic theory was used to simulate the driver input and it was used as a control strategy for the hydrostatic transmission.

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