1. Field of the Invention
The present invention relates in general to a hydraulic control apparatus for a vehicular automatic transmission, and more particularly to techniques associated with pressure regulation of non-used ones of linear solenoid valves provided for hydraulically operated frictional coupling devices of the automatic transmission.
2. Discussion of Related Art
JP-2001-248718A discloses an example of a hydraulic control apparatus for a vehicular automatic transmission including (a) a plurality of hydraulically operated frictional coupling devices which are selectively engaged and released to selectively establish a plurality of gear positions having respective different speed ratios, and (b) a plurality of linear solenoid valves operable to regulate fluid pressures of the respective hydraulically operated frictional coupling devices. The hydraulic control apparatus disclosed in the above-identified publication is adapted to control a vehicular automatic transmission of a planetary gear type.
Each of the linear solenoid valves indicated above includes a spool and a solenoid, and is operable between a pressure-regulating state in which the spool is moved to a position of equilibrium of forces for regulating an output fluid pressure according to an electromagnetic force produced by a solenoid, and a non-pressure-regulating state in which the spool is held at its stroke end at which no output fluid pressure is generated from the valve. The linear solenoid valve placed in the pressure-regulating state is operable to regulate the fluid pressure of the corresponding hydraulically operated frictional coupling device according to the electromagnetic force of the solenoid, when this frictional coupling device is engaged to establish a presently selected gear position of the automatic transmission. However, each non-used linear solenoid valve for the frictional coupling device not to be engaged to establish the currently selected gear position of the automatic transmission is usually placed in either the non-pressure-regulating state or a lowest-pressure-regulating state, irrespective of a present state of a vehicle provided with the automatic transmission. Namely, the non-used linear solenoid valve is always placed in the non-pressure-regulating state or the lowest-pressure-regulating state, irrespective of the present vehicle state. In the lowest-pressure-regulating state, the output fluid pressure is regulated to the lowest level. The non-used linear solenoid valve can be placed in the non-pressure-regulating state, by de-energizing the solenoid where the valve is of a normally-closed type, or by maximizing an electric current applied to the solenoid where the valve is of a normally-open type. The non-used linear solenoid valve can be placed in the lowest-pressure-regulating state, by minimizing the electric current of the solenoid where the valve is of the normally-closed type, or by maximizing the electric current of the solenoid where the valve is of the normally-open type, within a range of the electric current of the solenoid in which the spool is movable to a position of equilibrium of forces.
When the linear solenoid valve is switched from the non-pressure-regulating state to the pressure-regulating state by application of the electric current to the solenoid to regulate the fluid pressure of the corresponding frictional coupling device for establishing the currently selected gear position of the automatic transmission, however, it takes a considerably long time for the spool to be moved to the position of equilibrium of forces, giving rise to a risk of deterioration of a hydraulic shifting response of the automatic transmission due to a slow rise of the fluid pressure of the frictional coupling device. Where the linear solenoid valve is placed in the lowest-pressure-regulating state, the spool is held at the position of equilibrium of forces, permitting a higher hydraulic shifting response of the automatic transmission. In this lowest-pressure-regulating state, however, a pressurized working fluid always flows through the linear solenoid valve, requiring a relatively large amount of delivery of the working fluid from an oil pump, thereby requiring the oil pump to have a relatively large capacity and resulting in a relatively large load on a drive source provided to drive the oil pump, leading to deterioration of energy efficiency of the vehicle due to an increased amount of consumption of a fuel by a vehicle engine functioning as the drive source, for example. Although the pressurized working fluid should be supplied to the linear solenoid valve to hold the valve in its pressure-regulating state in which the spool is held at the position of equilibrium of forces for generating the desired output fluid pressure, the pressurized fluid need not be supplied to the linear solenoid valve placed in the non-pressure-regulating state in which no output fluid pressure is generated.