Performance analysis of the system composed of electric motor, variable-displacement pump and constant-displacement motor

In order to enhance the practical applicability of EHST (Electric Hydrostatic Transmission) in the engineering field, the basic characteristics of the vehicle powertrain system mainly composed of electric motor and hydrostatic transmission (variable-displacement pump and constant-displacement motor) were investigated in this paper. The computational expressions for the vehicle speed characteristics and the vehicle torque characteristics were deduced. Regression models for the external characteristics of the electric motor, the efficiency characteristics of the electric motor and the efficiency characteristics of the hydrostatic transmission system were established. The changes of electric motor speed, pump speed, electric motor torque, constant-displacement motor torque, displacement ratio and total system efficiency with the vehicle speed and load torque during the working process of EHST were observed and analyzed by means of contour diagrams. The results show that the EHST has a wide range of high-efficiency operating regions (62.75 % of the regions with ≥80 % maximum efficiency). The pump displacement during operation is generally higher than 80 % of the maximum displacement. The maximum load torque sustained by the vehicle shows a clear segmented linear function with the vehicle speed (3 segments).