Shift control device for automatic transmission and control method thereof

1. An automatic transmission comprising:

a shift mechanism that executes a shift from a current gear position to a target gear position by engaging or disengaging a plurality of frictional elements selectively;

a current thermal load calculating unit which calculates a current thermal load state of the frictional element;

a heat generation amount predicting unit which predicts, prior to the start of the shift, a heat generation amount of the frictional element during the shift;

a thermal load predicting unit which predicts a thermal load state of the frictional element upon shift completion on the basis of the current thermal load state of the frictional element and the heat generation amount predicted by the heat generation amount predicting unit; and

a shift control unit which, when the thermal load state upon shift completion predicted by the thermal load predicting unit is inside a predetermined region, either performs the shift after modifying a shift mode such that the heat generation amount of the frictional element is smaller than that of a case in which the predicted thermal load state upon shift completion is outside the predetermined region, or prohibits the shift,

wherein the predetermined region is set at a different region depending on whether the shift is an upshift or a downshift.

2. The automatic transmission as defined in claim 1 , wherein the predetermined region is set at a first predetermined region when the shift is an upshift, and at a second predetermined region when the shift is a downshift, and

the second predetermined region is set at a region which includes a thermal load state in the lower side of the first predetermined region and which is larger than the first predetermined region.

3. The automatic transmission as defined in claim 2 , further comprising a time estimating unit which estimates, prior to the start of the shift, a time remaining to an upshift following a downshift on the basis of a current traveling condition when the shift is a downshift,

wherein the second predetermined region is set on the basis of the time estimated by the time estimating unit.

4. The automatic transmission as defined in claim 2 , wherein the second predetermined region is set at a region which includes a thermal load state in the lower side of the first predetermined region and which is larger than the first predetermined region by a thermal load state corresponding to a maximum heat generation amount generated by the frictional element when an upshift is performed in the modified shift mode in order to reduce the heat generation amount of the frictional element.

5. The automatic transmission as defined in claim 2 , wherein the shift control unit which, when the shift is an upshift and the thermal load state upon shift completion predicted by the thermal load predicting unit is inside the first predetermined region, performs the shift after modifying the shift mode such that the heat generation amount of the frictional element is smaller than that of a case in which the predicted thermal load state upon shift completion is outside the first predetermined region.

6. The automatic transmission as defined in claim 2 , wherein the heat generation amount predicting unit which, when the shift is a downshift and the thermal load state upon shift completion predicted by the thermal load predicting unit is inside the second predetermined region, predicts the heat generation amount of the frictional element if the shift is performed in the shift mode in which the heat generation amount of the frictional element is smaller than that of a case in which the predicted thermal load state upon shift completion is outside the second predetermined region,

the thermal load predicting unit predicts the thermal load state of the frictional element upon shift completion on the basis of the newly predicted heat generation amount and the current thermal load state of the frictional element, and

the shift control unit performs the shift in the modified shift mode when the newly predicted thermal load state upon shift completion is outside the second predetermined region.

7. The automatic transmission as defined in claim 1 , further comprising an oil temperature detecting unit for detecting an oil temperature of the automatic transmission,

wherein the current thermal load calculating unit calculates the current thermal load state of the frictional element on the basis of a decrease gradient of the thermal load state of the frictional element and an elapsed time following shift completion, and

the decrease gradient is a first decrease gradient set on the basis of the thermal load state upon shift completion and the oil temperature from shift completion to the elapse of a first predetermined time period, and is a constant second decrease gradient regardless of the thermal load state upon shift completion and the oil temperature once the first predetermined time period has elapsed following shift completion.

8. The automatic transmission as defined in claim 7 , wherein the first decrease gradient is set at a larger gradient as the thermal load state upon shift completion increases.

9. The automatic transmission as defined in claim 7 , wherein the second decrease gradient is smaller than the first decrease gradient.

10. The automatic transmission as defined in claim 7 , wherein the thermal load state is a temperature, and

the current thermal load calculating unit halts calculation of the thermal load state of the frictional element and sets the current thermal load state of the frictional element at current oil temperature when a second predetermined time period, which is longer than the first predetermined time period, elapses following shift completion.

11. The automatic transmission as defined in claim 7 , wherein the thermal load state is a temperature, and

the current thermal load calculating unit halts calculation of the thermal load state of the frictional element and sets the current thermal load state of the frictional element at current oil temperature when the current thermal load state of the frictional element calculated by the current thermal load calculating unit falls to or below the oil temperature.

12. The automatic transmission as defined in claim 1 , wherein the heat generation amount predicting unit predicts the heat generation amount of the frictional element on the basis of an average value of a transmission torque of the frictional element and an average value of a relative rotation speed of the frictional element during the shift.

13. The automatic transmission as defined in claim 12 , further comprising:

a vehicle speed predicting unit which predicts a vehicle speed at the start of a torque phase and a vehicle speed at the start of an inertia phase on the basis of an acceleration prior to the start of the shift;

a turbine torque predicting unit which predicts a turbine torque at the start of the torque phase and a turbine torque at the start of the inertia phase on the basis of the vehicle speed at the start of the torque phase and the vehicle speed at the start of the inertia phase;

a transmission torque predicting unit which predicts a transmission torque of the frictional element at the start of the torque phase and a transmission torque of the frictional element at the start of the inertia phase on the basis of the turbine torque at the start of the torque phase and the turbine torque at the start of the inertia phase; and

a relative rotation speed predicting unit which predicts a relative rotation speed of the frictional element at the start of the torque phase and a relative rotation speed of the frictional element at the start of the inertia phase on the basis of the vehicle speed at the start of the torque phase and the vehicle speed at the start of the inertia phase,

wherein the average value of the transmission torque of the frictional element during the shift is calculated on the basis of the transmission torque of the frictional element at the start of the torque phase and the transmission torque of the frictional element at the start of the inertia phase, and

the average value of the relative rotation speed of the frictional element is calculated on the basis of the relative rotation speed of the frictional element at the start of the torque phase and the relative rotation speed of the frictional element at the start of the inertia phase.

14. The automatic transmission as defined in claim 12 , wherein, when the shift is an upshift, the average value of the transmission torque of the inertia phase is calculated on the basis of an incline of oil pressure supplied to the frictional element at the start of the inertia phase and a target time period of the inertia phase.

15. A shift control method for an automatic transmission that executes a shift from a current gear position to a target gear position by engaging or disengaging a plurality of frictional elements selectively,

wherein the method comprises:

calculating a current thermal load state of the frictional element;

predicting, prior to the start of the shift, a heat generation amount of the frictional element during the shift;

predicting a thermal load state of the frictional element upon shift completion on the basis of the current thermal load state of the frictional element and the predicted heat generation amount; and

when the predicted thermal load state upon shift completion is inside a predetermined region, either performing the shift after modifying a shift mode such that the heat generation amount of the frictional element is smaller than that of a case in which the predicted thermal load state upon shift completion is outside the predetermined region, or prohibiting the shift, and

the predetermined region is set at a different region depending on whether the shift is an upshift or a downshift.

16. An automatic transmission comprising:

a shift mechanism that executes a shift from a current gear position to a target gear position by engaging or disengaging a plurality of frictional elements selectively;

current thermal load calculating means for calculating a current thermal load state of the frictional element;

heat generation amount predicting means for predicting, prior to the start of the shift, a heat generation amount of the frictional element during the shift;

thermal load predicting means for predicting a thermal load state of the frictional element upon shift completion on the basis of the current thermal load state of the frictional element and the heat generation amount predicted by the heat generation amount predicting means; and

shift control means for, when the thermal load state upon shift completion predicted by the thermal load predicting means is inside a predetermined region, either performing the shift after modifying a shift mode such that the heat generation amount of the frictional element is smaller than that of a case in which the predicted thermal load state upon shift completion is outside the predetermined region, or prohibiting the shift,

wherein the predetermined region is set at a different region depending on whether the shift is an upshift or a downshift.