1. Field of the Invention
The invention relates to a shift control apparatus and shift control method for a vehicular automatic transmission, which enables suppression of a drop in rotational speed of an input shaft which occurs during a clutch-to-clutch downshift executed while a vehicle is decelerating.
2. Description of the Related Art
A shift control apparatus for a vehicular automatic transmission is known which, when executing a clutch-to-clutch downshift, executes shift hydraulic pressure control so as to reduce an apply pressure of a hydraulic friction device to be released, which was applied in order to achieve a predetermined speed before the downshift, while increasing an apply pressure of a hydraulic friction device to be applied in order to achieve a predetermined speed after the downshift. According to JP(A) 11-287318, for example, during the clutch-to-clutch downshift, feedback control is performed on the apply pressure of the hydraulic friction device to be applied so that a transmitted torque capacity of the hydraulic friction device to be applied becomes constant, i.e., so that a rotational speed of an input shaft of the automatic transmission increases at a constant rate.
In the aforementioned shift control apparatus for a vehicular automatic transmission, the engine speed drops during the clutch-to-clutch downshift when the vehicle is decelerating, and then increases again when the hydraulic friction device to be applied is applied. This combination of a drop followed by an increase in engine speed results in shift shock or a delay in the shift time. Also, fuel efficiency may be reduced if the drop in engine speed is large enough to require that the fuel supply be restarted.
In comparison, it is conceivable to automatically suppress the drop in engine speed during the clutch-to-clutch downshift while the vehicle is decelerating, and appropriately reduce or eliminate shift shock or a delay in shift time caused by that drop. It is also possible to appropriately reduce the adverse effect on fuel efficiency caused by fuel being supplied to the engine again due to a further drop in engine speed. However, doing so may result in shift shock occurring when there is little or no drop in engine speed.