In this study, a new sub-shift schedule was proposed for a hydro-mechanical transmission. To develop the sub-shift schedule, a network analysis was performed by considering the hydrostatic unit loss and mechanical component losses. In the new sub-shift schedule, the sub-shift gear can be selected with respect to the demanded wheel torque and vehicle speed, which provides improved system efficiency for the given vehicle operating condition. Since the sub-shift can only be carried out at a speed ratio where the off-going and on-coming clutch speeds are synchronized in the existing subshift control, a sub-shift control algorithm without the clutch speed synchronization was proposed to apply the new subshift schedule using the forward clutch pressure and hydrostatic unit stroke control. The performance of the sub-shift control algorithm without the clutch speed synchronization was evaluated by the simulation and experiment. It was found from the simulation and experimental results that the sub-shift can be achieved, showing an acceptable peak-topeak torque variation in the driveshaft.

A sub-shift control algorithm was proposed for an agricultural tractor with hydro-mechanical transmission. The hydromechanical transmission investigated in this study consists of a hydro-static unit and planetary gear sets which provide a continuously variable transmission function and four sub-shift gears. The sub-shift needs to be carried out at the hydrostatic unit stroke where the speed of the off-going and oncoming clutches is synchronized. To reduce the hydro-static unit stroke error due to the nonlinear characteristic, the hysteresis characteristic was investigated in the test bench and a hysteresis compensator was developed. Using the compensator in the feedforward loop, a hydro-static unit stroke control algorithm was proposed and validated via experiment. To prevent the torque interruption during the sub-shift, a time delay of the off-going clutch was proposed. In addition, a hydro-static unit stroke duration which maintains the target stroke during the sub-shift was suggested to reduce the speed difference between the clutch plates. The sub-shift control algorithm including the hysteresis compensation, time delay of the off-going clutch, and hydro-static unit stroke duration was evaluated via experiment. It was found from the experimental results that the sub-shift was performed smoothly without the torque interruption, even in the acceleration state when large inertial torque change occurred.