Hydraulic control system using a torque allotment ratio to determine engagement pressures for an engaging and a disengaging frictional element during a shift in an automatic transmission

A hydraulic control system for an automatic transmission mounted in a vehicle and comprising:

• an input shaft for receiving drive force from an engine output shaft;
• an output shaft connected to a wheel of the vehicle;
• a plurality of frictional engagement elements for establishing and changing a torque transfer path between the input shaft and the output shaft;
• hydraulic servos for engaging and disengaging the frictional engagement elements, said hydraulic control system downshifting to a predetermined speed by operation of a first hydraulic servo to release a first frictional engagement element of said plurality of frictional engagement elements and operation of a second hydraulic servo to engage a second frictional engagement element of said plurality of frictional engagement elements;
• hydraulic control means for controlling fluid pressures on said first and second hydraulic servos;
• a plurality of sensors for detecting various vehicle running conditions;
• and
• a control unit for receiving signals from said sensors and for outputting a hydraulic control signal to said hydraulic control means, said control unit comprising:
• torque allotment ratio setting means for selecting, responsive to the hydraulic control signal, one of the following control routines for said downshifting:
• (1) a power-on, high vehicle speed or high torque routine wherein said second frictional engagement element transmits no torque until start of a final-stage control whereat pressure to said second hydraulic servo is increased by a predetermined increment and wherein said first frictional engagement bears all torque transmitted through the torque transfer path up to start of the final-stage control of the second frictional engagement element;
• (2) a power-on, low vehicle speed or low torque routine wherein the torque transmitted through the torque transfer path is shared by said first and second frictional engagement elements during said downshifting;
• (3) a power-off routine wherein pressure to said first hydraulic servo is swept down to zero prior to final phase control of said second frictional engagement element while pressure to said second hydraulic servo is swept up;
• and
• (4) a power-on to power-off transition routine wherein the transmitted torque is shared by said first and second frictional engagement elements and wherein pressure to said second hydraulic servo is increased in dependence upon release of pressure from said first hydraulic servo during a torque allotment control.