Full power shift method of hydro-mechanical transmission and power transition characteristics

Abstract: Hydro-mechanical variable transmission (HMT) is applicable to high-power vehicles, because of its ability to achieve high-power stepless speed changing and high transmission efficiency. However, in general power shift, the target range brakes would be engaged after the current range brakes are disengaged, which would cause immediate output power interruption. In the shift process, there are load reverses in hydraulic transmission unit, power flow reverses in hydraulic branch, and function interchanges in hydraulic component. Simultaneously, high and low pressure circuits in hydraulic circuit interchange, as a result of which the rotational speed of hydraulic component changes abruptly, causing the fluctuation of output speed as well as vibrations and noises. To solve the problems of HMT in power shift, a new full power shift method of HMT is proposed based on the double brakes of the current range and target range overlapping. The process of full power shift is divided into 5 stages: current range, prior stable stage, power transition stage, post stable stage and target range. For the HMT working at current range, when the ideal power shift timing comes, the target range brake would be engaged with zero speed difference; and when the HMT comes to the prior stable stage, its torque and power characteristics remain unchanged. Regulating properly the displacement ratio of variable-displacement hydraulic component can make the high and low pressure circuits of hydraulic circuit interchange. The original low pressure side is pressurized to the target range pressure, the torque moves from current range brake to target range brake, and HMT comes to the post stable stage with its torque and power characteristics remaining identical with those of the target range. The current range brake is disengaged at zero torque, HMT enters the target range, and therefore the power shift process is fulfilled. The interchange between high and low pressure as well as power transition is made when the double brakes overlap. In the power shift process, the output power can be transmitted normally in full power. In this paper, a two-range arithmetic HMT as research object, the model for the speed of HMT in power shift was built based on the double brakes that overlap, taking into consideration the influence of volumetric efficiency of hydraulic transmission unit. The ideal power shift timing was deduced, with the ratio between rotational speed of fixed-displacement hydraulic component and input rotational speed as the criterion. By combining theoretical analysis and experimental studies, the variable law of torque characteristics and power characteristics of HMT with the pressure difference in hydraulic circuit in the full power shift process was studied. In the full power shift process with the double brakes overlapping, by changing the pressures in the hydraulic circuit, the load torque can transit from the current brake to the target brake. And the variable law of the pressure difference in hydraulic circuit with the displacement ratio of variable-displacement hydraulic component was studied. Thus the control method for the full power shift process was obtained. In the full power shift process with the double brakes overlapping, regulating the displacement ratio of variable-hydraulic component is one effective method. Finally, the test bench for HMT was built, and the full power shift was tested. The result shows that in power shift process with the double brakes overlapping, by regulating the displacement ratio of variable-hydraulic component, the interchange between high and low pressure circuits in closed hydraulic circuit can be accomplished, and power transition can be thus realized. Input rotational speed of 1 000 r/min and load torque of 150 N·m or 60 N·m, the output rotational speed had no fluctuation, and the output torque ripple was about 5 N·m. In the full power shift process, torque can transit from the current brake to the target brake, and realize normal full power transmission of output power. This study reveals the torque and power transition principle of HMT in full power shift process, which can provide an important reference for further research of full power shift method and application of HMT.