Abstract:
Abstract: In order to improve the power shift quality of agricultural tractors with continuously variable transmission, the power shift process of a new kind of tractor hydraulic mechanical CVT was studied. Based on bench tests of the power-shift of the CVT and a homemade control system, we conducted a suite of experiments on the oil charge of the clutches, transmission load with single and multiple factors, and shift timing optimization. During the experiments on the oil charge of the clutches, the engine was turned off, and the relationships between the shift time and the oil pressure of the hydraulic system and the flow rate of the clutch were revealed. During the load experiments with single and multiple factors, the relationship between the shift quality and the rotation speed of the engine, the oil pressure of the hydraulic system, the flow rate of the clutch, and the load torque were revealed. During the experiments on shift timing optimization, the optimized connection time of the clutch was studied. The results show that the shift quality is influenced by the oil pressure of the hydraulic system and the flow rate of the clutch, which would change the shift time. The pressure changing rule of the clutches during the shift process was revealed and proved the overlapping shift to be safe and feasible. The shift experiments with a single factor show that the shift quality is positively related to the flow rate of the clutch and the oil pressure of the hydraulic system, but inversely related to the load torque. There is no significant relation between the shift quality and the engine speed. The shift experiments with multiple factors show that the significance of the factors affecting speed drop and speed impact are the flow rate of the clutch, the load torque, the oil pressure of the hydraulic system, and the engine speed. The significance of the factors affecting the dynamic load are the load torque, the flow rate of the clutch (which is in parallel with the oil pressure of the hydraulic system), and the engine speed. In addition, the overlapping shift would evidently improve the shift quality. According to the experiment results, the optimal flow rate of the clutch is 5 L per minute, the optimal oil pressure of the hydraulic system is 4 Mpa, and the optimal shift timing is 120 ms, i.e. one clutch should be energized 120 ms ahead of the other. The conclusion provides an important reference for the design of transmission control units.