AUTOMATIC TRANSMISSION EQUIPPED WITH FRICTION ELEMENT HAVING LOCKING MECHANISM ATTACHED THERETO, AND CONTROL METHOD THEREFOR

A transmission controller determines whether a vehicle is going to stop based on a driving condition of the vehicle. When it is determined that the vehicle is going to stop, a lock mechanism is placed in an unlocked state through a supply of an OFF pressure to an OFF pressure piston chamber even if a D mode is selected by a select switch.

TECHNICAL FIELD

The present invention relates to control of an automatic transmission equipped with a friction element having a lock mechanism attached thereto.

BACKGROUND ART

A friction element operated with a hydraulic pressure is used as a clutch and a brake of an automatic transmission so as to join two coaxial members to each other (in the case of the clutch, the two coaxial members are both rotational elements, whereas in the case of the brake, one of the two coaxial members is a rotational element and the other is a non-rotational element) (JP 7-12221A).

In such a friction element, for example, the two members each have a plurality of friction plates attached thereto such that the friction plates are slidable in an axial direction, and the friction plates of the two members alternate. When the friction plates of the two members are pressed against one another by a hydraulic piston, the two members are joined to each other via the friction plates.

SUMMARY OF INVENTION

In order to maintain an engaged state of the aforementioned friction element, it is necessary to keep supplying a hydraulic pressure to the hydraulic piston by driving a hydraulic pump with an engine. This undesirably degrades the fuel economy of a vehicle equipped with an automatic transmission.

In view of this, it is possible to supply the hydraulic pressure until the friction element is engaged and, after the friction element is engaged, restrict a movement of the hydraulic piston using a lock mechanism so that the engaged state of the friction element can be maintained even with a reduction in the hydraulic pressure. This configuration makes it possible to reduce the load of the hydraulic pump that generates the hydraulic pressure, and to improve the fuel economy of the vehicle. It is preferred that activation and deactivation of such a lock mechanism be performed with the hydraulic pressure.

However, in the case of the configuration in which activation and deactivation of the lock mechanism is performed with the hydraulic pressure, there is a possibility that deactivation of the lock mechanism is delayed in a low vehicle speed range regardless of an attempt to release the friction element by deactivating the lock mechanism when a mode selected by a select switch is changed from a D mode (driving mode) to an N mode (non-driving mode). This is because, in the low vehicle speed range, an ejection pressure of the hydraulic pump is lowered, and the rise of the hydraulic pressure used in deactivation of the lock mechanism is delayed.

A delay in deactivation of the lock mechanism leads to a delay in release of the friction element, and hence to continuation of a state where the power of the engine is transmitted via the friction element. If the vehicle stops in this state, a problem could possibly arise in which the power of the engine is transmitted even though the N mode is selected and the vehicle is stopped.

It is an object of the present invention to prevent a situation in which the power of an engine is transmitted while a non-driving mode is selected and a vehicle is stopped due to a delay in deactivation of a lock mechanism in a low vehicle speed range.

According to an aspect of the present invention, provided is an automatic transmission including a friction element, a select switch, a control device, and a vehicle stop determination unit. The friction element is arranged on a power transmission path such that a lock mechanism is placed in a locked state through a supply of an ON pressure to an engagement-side oil chamber and placed in an unlocked state through a supply of an OFF pressure to a release-side oil chamber. Once the lock mechanism has been placed in the locked state, the friction element is maintained in an engaged state, even with a reduction in a hydraulic pressure in the engagement-side oil chamber. Once the lock mechanism has been placed in the unlocked state, the friction element is maintained in a released state, even with a reduction in a hydraulic pressure in the release-side oil chamber. The select switch is capable of selecting a driving mode or a non-driving mode as a mode of the automatic transmission. The control device performs engagement control and release control when the driving mode and the non-driving mode are selected by the select switch, respectively. The engagement control places the lock mechanism in the locked state through the supply of the ON pressure to the engagement-side oil chamber and lowers the hydraulic pressure in the engagement-side oil chamber. The release control places the lock mechanism in the unlocked state through the supply of the OFF pressure to the release-side oil chamber and lowers the hydraulic pressure in the release-side oil chamber. The vehicle stop determination unit determines whether a vehicle is going to stop based on a driving condition of the vehicle. When the vehicle stop determination unit determines that the vehicle is going to stop, the control device places the lock mechanism in the unlocked state through the supply of the OFF pressure to the release-side oil chamber even if the driving mode is selected by the select switch. Furthermore, a corresponding control method is also provided.

According to the above-described aspect, upon selection of the driving mode by the select switch, normal engagement control is performed, and the friction element is maintained in the engaged state by placing the lock mechanism in the locked state. If it is determined that the vehicle is going to stop based on the driving condition of the vehicle, the OFF pressure is supplied to the release-side oil chamber, and the lock mechanism is deactivated.

At the time the vehicle stops, the lock mechanism is already deactivated. Accordingly, even if the driving mode is switched to the non-driving mode immediately before or alter the vehicle stops, the friction element can be released promptly by draining the hydraulic pressure in the engagement-side oil chamber. It is hence possible to prevent transmission of the power of an engine caused by a delay in deactivating the lock mechanism.

DESCRIPTION OF EMBODIMENTS

The following describes an embodiment of the present invention with reference to the attached drawings.

FIG. 1shows a schematic configuration of a vehicle equipped with an automatic transmission according to the embodiment of the present invention. The vehicle includes an engine1, a torque converter2, and a transmission3. An output rotation of the engine1is transmitted to unillustrated drive wheels via the torque converter2, the transmission3, and an unillustrated differential gear unit.

The transmission3is a step or continuously variable automatic transmission. The transmission3includes a reverse brake4and a forward clutch5. In a state where the reverse brake4is engaged, the transmission3outputs the rotation of the engine1in reverse. In a state where the forward clutch5is engaged, the transmission3outputs the rotation of the engine1while maintaining its rotational direction.

The reverse brake4is a traditional friction element which is engaged through a supply of an engagement pressure, and which requires a continuous supply of the engagement pressure to maintain the engaged state thereof. The reverse brake4can be released by stopping the supply of the engagement pressure.

As will be described later, the forward clutch5is a friction, element having a lock mechanism BL attached thereto. Once the lock mechanism BL has been placed in a locked state through a supply of an ON pressure to the forward clutch5, the forward clutch5can be maintained in the engaged state even if the supply of the ON pressure is stopped. The forward clutch5can be released by placing the lock mechanism BL in a deactivated state through a supply of an OFF pressure to the forward clutch5. Once the lock mechanism BL has been placed in the deactivated state, the forward clutch5can be maintained in the released state even if the supply of the OFF pressure is stopped. The configuration of the forward clutch5will be described later in detail.

Should both of the reverse brake4and the forward clutch5be engaged simultaneously, the transmission3will be placed in an interlocked state where an input/output shaft of the transmission3cannot rotate. Therefore, the reverse brake4and the forward clutch5are engaged in an alternating manner.

A hydraulic control circuit7includes a regulator valve, a solenoid valve, a hydraulic pump8, and a hydraulic passage. The regulator valve adjusts a hydraulic pressure from the hydraulic pump8, which is driven by the engine1, to a line pressure. Using the line pressure as a source pressure, the solenoid valve adjusts a hydraulic pressure supplied to friction elements, including the forward clutch5(and also to constituent elements of a continuously variable transmission mechanism in the case where the transmission3is a continuously variable transmission). The valves and friction elements are connected to one another by the hydraulic passage.

Each valve of the hydraulic control circuit7is controlled based on a control signal from a transmission controller9. The transmission controller9is composed of a CPU, a ROM, a RAM, an input/output interface, and the like. The transmission controller9determines a driving condition of the vehicle based on various types of signals input from various types of sensors and an engine controller, and outputs instructions to the hydraulic control circuit7so as to achieve a gear position (a speed ratio in the case where the transmission3is a continuously variable transmission) appropriate for the driving condition.

For example, the transmission controller9receives, as input, signals from rotational speed sensors101,102,103, a mode detection switch104, an accelerator pedal opening sensor105, a brake switch106, and the like. The rotational speed sensor101detects a rotational speed Ne of the engine1. The rotational speed sensor102detects an input rotational speed of the transmission3. The rotational speed sensor103detects an output rotational speed of the transmission3. The mode detection switch104detects a mode of the transmission3selected by a select switch11. The accelerator pedal opening sensor105detects an amount of operation on an accelerator pedal (hereinafter referred to as “accelerator pedal opening, APO”). The brake switch106detects ON/OFF of a brake.

The select switch11is lever-operated or button-operated. Operating a lever or buttons of the select switch11enables selection of one of the following modes as a mode of the transmission3: a parking mode (hereinafter “P model”), a reverse mode (hereinafter “R mode”), a neutral mode (hereinafter “N mode”), and a driving mode (hereinafter “D mode”).

The transmission controller9causes each of the reverse brake4and the forward clutch5to be engaged or released in accordance with a mode selected by the select switch11. Specifically, in the D mode, the reverse brake4is released and the forward clutch5is engaged. In the R mode, the reverse brake4is engaged and the forward clutch5is released. In the P mode and the N mode, the reverse brake4and the forward clutch5are released.

A description is now given of a detailed configuration of the forward clutch5.

FIG. 2is a cross-sectional view of the forward clutch5and a clutch operation pack6that causes the forward clutch5to operate. The configurations thereof will now be described.

The forward clutch5includes a clutch drum51, a clutch hub52, driven plates53, drive plates54, and a retainer plate55.

The clutch drum51and the clutch hub52are arranged coaxially. Unillustrated rotational elements (a shaft, a gear, etc.) are joined to the clutch drum51. Other unillustrated rotational elements (a shaft, a gear, etc.) are joined to the clutch hub52.

The driven plates53are attached to the clutch drum51by means of a spline joint such that the driven plates53are slidable in an axial direction. The drive plates54are attached to the clutch hub52by means of a spline joint such that the drive plates54are slidable in the axial direction. Four driven plates53and four drive plates54are arranged alternately. The drive plates54have friction surfaces on both sides, and clutch facings are attached to the friction surfaces.

The clutch drum51transmits, to the clutch hub52, the rotation input from the rotational elements joined to the clutch drum51via the driven plates53and the drive plates54.

The retainer plate55is interposed between a retaining ring64and a drive plate54that is arranged at the end opposite to a hydraulic piston61. The retaining ring64is fixed to a groove on the inner periphery of the clutch drum51. The retainer plate55has a friction surface on one side. Furthermore, the retainer plate55has a larger thickness than the driven plates53in the axial direction, and prevents the driven plates53and the drive plates54from falling.

The clutch operation pack6includes the hydraulic piston61, an ON pressure piston chamber62, an OFF pressure piston chamber63, the retaining ring64, diaphragm springs65, a partition plate66, and the lock mechanism BL.

The hydraulic piston61is arranged such that it is displaceable in the axial direction relative to the forward clutch5. The hydraulic piston61has an ON pressure receiving surface61aon one side, and an OFF pressure receiving surface6lb on the other side.

The ON pressure piston chamber62is defined between the clutch drum51and the hydraulic piston61so as to cause the ON pressure to act on the ON pressure receiving surface61a.of the hydraulic piston61.

The OFF pressure piston chamber63is defined between the partition plate66, which is fixed to the clutch drum51, and the hydraulic piston61so as to cause the OFF pressure to act on the OFF pressure receiving surface61bof the hydraulic piston61.

The retaining ring64is arranged in a position opposite to the hydraulic piston61with the forward clutch5therebetween, and supports the forward clutch5in the axial direction.

The diaphragm springs65are interposed between a clutch-side end surface61cof the hydraulic piston61and a piston-side end surface5aof the forward clutch5. Two diaphragm springs65are arranged in such a manner that they are stacked in the axial direction. The diaphragm springs65exert an engaging force acting on the forward clutch5as a result of moving the hydraulic piston61in an engaging direction toward the retaining ring64.

The lock mechanism BL is built in the clutch drum51, and is composed of the hydraulic piston61, a ball holding piston67, and a ball68.

The hydraulic piston61is arranged such that it is displaceable in the axial direction relative to the forward clutch5. The hydraulic piston61has a housing part61dand a tapered surface61e.The housing part bid houses the ball68when a movement of the hydraulic piston61in a releasing direction is restricted. The tapered surface61eis formed continuously with the housing part61d.At the time of a stroke movement of hydraulic piston61in the releasing direction, the tapered surface61epushes the ball68inward.

The ball holding piston67is arranged in a cylindrical space defined by a cylindrical inner periphery part51aof the clutch drum51that covers the hydraulic piston61and by a cylindrical partition wall part51bthat projects from the clutch drum51in the axial direction. The ball holding piston67moves in the axial direction with the action of the ON pressure or the OFF pressure. A seal ring84seals between the outer peripheral surface of the ball holding piston67and the cylindrical partition wall part51b. A seal ring85seals between the inner peripheral surface of the ball holding piston67and the cylindrical inner periphery part51a.A seal ring86seals between the inner peripheral surface of the hydraulic piston61and the cylindrical partition wall part51b. In this way, the ON pressure piston chamber62and the OFF pressure piston chamber63are defined on both sides of the hydraulic piston61.

An ON pressure port51d,which opens into the clutch drum51, and the ON pressure piston chamber62communicate with each other via an ON pressure communication groove67aformed in the ball holding piston67and via an ON pressure communication hole51e,which is an opening through the cylindrical, partition wall part51b.An OFF pressure port51f, which opens into the clutch drum51, and the OFF pressure piston chamber63communicate with each other via an OFF pressure communication groove67bformed in the ball holding piston67and via an OFF pressure communication clearance secured between an end portion of the cylindrical partition wall part51band the partition plate66.

The ball holding piston67has a housing part67c,a tapered surface67d,and a lock surface67e.The housing part67chouses the ball68when a movement of the hydraulic piston61in the releasing direction is permitted. The tapered surface67dand the lock surface67eare formed continuously with the housing part67c.At the time of a stroke movement of the ball holding piston67in a direction toward the forward clutch5, the tapered surface67dpushes the bail68outward, and the lock surface67elocks the pushed ball68at a resultant position thereof.

The ball68is placed in a ball hole51c,which is an opening through the cylindrical partition wall part51b. The tapered surfaces61c,67dof the hydraulic piston61and the ball holding piston67apply a force to the ball68as the pistons61,67move in the axial direction with the action of the ON pressure or the OFF pressure. Accordingly, the ball68moves in a radial direction between a lock position and a lock deactivation position.

According to the above-described configuration, the supply of the ON pressure to the ON pressure piston chamber62causes the hydraulic piston61to move in the engaging direction toward the forward clutch5, thereby placing the forward clutch5in the engaged state with a pushing force of the diaphragm springs65that have been pushed and compressed. Upon the movement of the hydraulic piston61in the engaging direction, the ball68moves in a direction toward an outer diameter due to a rotation-induced centrifugal force and the hydraulic pressure. Consequently, the ball68is housed in the housing part61d.As the ON pressure acts on the ball holding piston67, the ball holding piston67moves in the axial direction (the direction toward the forward clutch5), and the lock surface67eholds the ball68held in the housing part67c.

As a result, the lock mechanism BL is placed in the locked state. Accordingly, a movement of the hydraulic piston61in the releasing direction is restricted, and the engaged state of the forward clutch5is maintained even if the ON pressure is drained, The ON pressure is supplied to the ON pressure piston chamber62only during an engagement operation. It is not necessary to supply the ON pressure to maintain the engaged state of the forward clutch5.

A lower limit value of the ON pressure with which the lock mechanism BL can be placed in the locked state is referred to as a lock pressure, and is determined uniquely based on a spring constant of the diaphragm springs65, the area of the ON pressure receiving surface61aof the hydraulic piston61, and the like.

Furthermore, by supplying a hydraulic pressure lower than the lock pressure to the ON pressure piston chamber62, the forward clutch5can be placed in the engaged state without placing the lock mechanism BL in the locked state. In this case, as the lock mechanism BL cannot restrict a movement of the hydraulic piston61in the releasing direction, it is necessary to keep supplying the hydraulic pressure to the ON pressure piston chamber62to maintain the forward clutch5in the engaged state. Additionally, the transmission capacity of the forward clutch5at this time depends on the hydraulic pressure supplied to the ON pressure piston chamber62.

The above-described control by the transmission controller9to maintain the forward clutch5in the engaged state by placing the lock mechanism BL in the locked state through the supply and discharge of the ON pressure is referred to as normal engagement control. In contrast, the above-described control by the transmission controller9to maintain the forward clutch5in the engaged state without placing the lock mechanism BL in the locked state by continuously supplying a hydraulic pressure lower than the lock pressure to the ON pressure piston chamber62is referred to as vehicle stop engagement control.

When the D mode is selected by the select switch11, the transmission controller9performs the normal engagement control. However, when it is determined that the vehicle is going to stop based on the driving condition of the vehicle while the D mode is selected, the transmission controller9places the lock mechanism BL in an unlocked state by supplying the OFF pressure to the OFF pressure piston chamber63, and performs the vehicle stop engagement control. This prevents a situation in which the power of the engine1is transmitted while the N mode is selected and the vehicle is stopped due to a delay in deactivating the lock mechanism EL in a low vehicle speed range. The substance of control by the transmission controller9will be described later in detail.

Meanwhile, the supply of the OFF pressure to the OFF pressure piston chamber63causes the ball holding piston67to move in the axial direction (a direction away from the forward clutch5), from the position in which the ball68is held by the lock surface67eto the position in which holding of the ball68by the lock surface67eis cancelled. A combination of a force of the OFF pressure and a reaction force opposing the engaging force exerted by the diaphragm springs65acts on the hydraulic piston61. This results in a stroke movement of the hydraulic piston61in a return direction, thereby pushing the ball68back in a lock deactivation direction. Once the ball68has moved to the lock deactivation position, the lock mechanism BL is placed in the unlocked state, and the forward clutch5is released.

When the forward clutch5is released, the ON pressure is zero. This maintains a state where the ball68is housed in the housing part67cof the ball holding piston67even if the OFF pressure is drained. The OFF pressure is supplied to the OFF pressure piston chamber63only during a release operation. It is not necessary to supply the OFF pressure to maintain the released state of the forward clutch5.

It should be noted that, when the lock mechanism EL is not locked due to the vehicle stop engagement control, the above-described supply and discharge of the OFF pressure are not necessary to release the forward clutch5, and the forward clutch5can be released by draining the hydraulic pressure in the ON pressure piston chamber62.

The above-described control by the transmission controller9to maintain the forward clutch5in the released state by placing the lock mechanism BL in the unlocked state through the supply and discharge of the OFF pressure is referred to as normal release control. In contrast, control by the transmission controller9to release the forward clutch5by draining the hydraulic pressure in the ON pressure piston chamber62is referred to as unlocked state release control. When a mode other than the D mode is selected by the select switch11, the transmission controller9performs the normal release control or the unlocked state release control depending on whether the lock mechanism BL is in the locked state. The substance of control by the transmission controller9will be described later in detail.

A description is now given of the substance of control by the transmission controller9with reference toFIGS. 3 to 5.

FIG. 3is a flowchart showing the substance of processing executed by the transmission controller9to cause the forward clutch5to be engaged and released.

According to this flowchart, the transmission controller9compares a previous value and a current value of a mode of the transmission3selected by the select switch11so as to determine whether the selected mode of the transmission3is the D mode or a mode other than the D mode (P mode, R mode, or N mode) (S11, S12).

When the D mode has been selected, the processing proceeds to S13and the transmission controller9performs the normal engagement control. As a result, the ON pressure is supplied to the ON pressure piston chamber62, and once the lock mechanism BL has been placed in the locked state, the supply of the ON pressure is stopped.

On the other hand, when a mode other than the D mode has been selected, the processing proceeds to S14and the transmission controller9determines whether the lock mechanism BL is in the locked state.

Whether the lock mechanism BL is in the locked state can be determined based on, for example, whether the OFF pressure has been supplied in later-described S25after performing the normal engagement control in S13or later-described S28. It is determined that the lock mechanism BL is in the locked state if the vehicle stop engagement control has not been performed, and that the lock mechanism BL is in the unlocked state if the vehicle stop engagement control has been performed.

Alternatively, whether the lock mechanism BL is in the locked state may be determined based on a change in the ON pressure when performing the normal engagement control and a change in the OFF pressure when supplying the OFF pressure in release control or later-described S25(these changes both represent a change in the hydraulic pressure caused by a stroke movement of the ball holding piston67).

If it is determined that the lock mechanism BL is in the locked state, the processing proceeds to S15and the transmission controller9performs the normal release control. As a result, the OFF pressure is supplied to the OFF pressure piston chamber63, and after the lock mechanism BL is placed in the unlocked state, the supply of the OFF pressure is stopped, and the forward clutch5is released.

On the other hand, if it is determined that the lock mechanism BL is in the unlocked state, the processing proceeds to S16and the transmission controller9performs the unlocked state release control. As a result, the hydraulic pressure in the ON pressure piston chamber62is drained, and the forward clutch5is released.

FIG. 4is a flowchart showing the substance of processing executed by the transmission controller9to switch engagement control for the forward clutch5. After the normal engagement control (S12) is performed in the engagement and release processing ofFIG. 3, the processing ofFIG. 4is executed repeatedly while the D mode is selected by the select switch11.

According to this flowchart, the transmission controller9first determines whether the lock mechanism BL is in the locked state.

If it is determined that the lock mechanism BL is in the locked state, the processing proceeds to S22.

In S22to S24, the transmission controller9determines whether the vehicle is going to stop based on the driving condition of the vehicle. Specifically, it is determined that the vehicle is going to stop if the accelerator pedal is not depressed (S22: YES), a brake pedal is depressed (S23: YES), and a vehicle speed is lower than a stop determination vehicle speed (S24: YES). The value of the stop determination vehicle speed is obtained by referring to a table shown inFIG. 5. The larger the deceleration of the vehicle, the larger the value of the stop determination vehicle speed. The reason why a higher stop determination vehicle speed is set for a larger vehicle deceleration is because the lock mechanism BL cannot be deactivated by the time the vehicle stops unless lock deactivation for the lock mechanism BL is started earlier, that is to say, at a higher vehicle speed in the case of a larger vehicle deceleration.

If it is determined that the vehicle is going to stop in the processes of S22to S24, the processing proceeds to S25. Otherwise, the processing ends.

In S25, the transmission controller9places the lock mechanism BL in the unlocked state by supplying the OFF pressure to the OFF pressure piston chamber63.

In S26, the transmission controller9performs the vehicle stop engagement control. A hydraulic pressure lower than the lock pressure is supplied to the ON pressure piston chamber62, thereby placing the forward clutch5in the engaged state without placing the lock mechanism BL in the locked state. Thereafter, for the duration of the D mode, this hydraulic pressure lower than the lock pressure is continuously supplied to the ON pressure piston chamber62, thereby maintaining the forward clutch5in the engaged state, except when the normal engagement control is performed again in later-described S28.

On the other hand, if it is determined in S21that the lock mechanism BL is not in the locked state, that is to say, the lock mechanism BL is in the unlocked state, the processing proceeds to S27.

In S27and S28, the transmission controller9determines whether the vehicle continues to run based on the driving condition of the vehicle. Specifically, it is determined that the vehicle continues to run if the accelerator pedal is depressed and a vehicle speed is higher than a running determination vehicle speed. The value of the running determination vehicle speed is obtained by referring to the table shown inFIG. 5. At the same deceleration, the running determination vehicle speed is set to be higher than the stop determination vehicle speed by several km/h. The value of the running determination vehicle speed is set in this way so as to prevent hunting in control.

If it is determined that the vehicle continues to run in the processes of S27and S28, the processing proceeds to S29and the normal engagement control is performed again. As a result, the ON pressure is supplied to the ON pressure piston chamber62, and after the lock mechanism BL is placed in the locked state, the supply of the ON pressure is stopped.

If it is determined in the processes of S27and S28that the vehicle does not continue to run, that is to say, the vehicle is going to stop, the processing proceeds to S26and the vehicle stop engagement control is continued.

Operational effects of the present embodiment will now be described.

According to the above-described embodiment, when the D mode (corresponding to the driving mode) is selected by the select switch11, the normal engagement control is performed in which the lock mechanism BL is placed in the locked state, thereby maintaining the forward clutch5in the engaged state (S13). If it is determined that the vehicle is going to stop based on the driving condition of the vehicle, the OFF pressure is supplied to the OFF pressure piston chamber63(S25).

Therefore, at the time when the vehicle stops, the lock mechanism BL is already deactivated. Accordingly, even if the D mode is switched to the N mode (corresponding to the non-driving mode) immediately before or after the vehicle stops, the forward clutch5can be released promptly by a pushing force exerted by the diaphragm spring65upon drainage of the hydraulic pressure in the ON pressure piston chamber62(S16). It is hence possible to prevent transmission of the power of the engine1caused by a delay in deactivating the lock mechanism BL.

Furthermore, after the lock mechanism BL is deactivated as a result of determining that the vehicle is going to stop based on the driving condition of the vehicle, a hydraulic pressure lower than the lock pressure is continuously supplied to the ON pressure piston chamber62so that the forward clutch5is maintained in the engaged state (S26). In this way, the power of the engine1can be transmitted until the D mode is switched to the N mode. It is hence possible to avoid a situation in which the power of the engine1is not transmitted despite being in the D mode.

Moreover, even if the lock mechanism BL has been deactivated as a result of determining that the vehicle is going to stop based on the driving condition of the vehicle, when it is determined that the vehicle continues to run, the ON pressure is supplied to the ON pressure piston chamber62again, the lock mechanism BL is placed in the locked state, and then the supply of the ON pressure is stopped (S29). This makes it possible to prevent degradation of the fuel economy caused by a continuous supply of the hydraulic pressure to the ON pressure piston chamber62owing to continuation of the vehicle stop engagement control despite continuous running of the vehicle.

This concludes the description of the embodiment of the present invention. It should be noted that the above-described embodiment merely represents a part of application examples of the present invention, and is not intended to limit a technical scope of the present invention to specific configurations of the above-described embodiment.

For example, in the above-described embodiment, the supply of the hydraulic pressure to the ON pressure piston chamber62is stopped after the lock mechanism BL is placed in the locked state. However, it is possible to adopt a configuration in which the hydraulic pressure in the ON pressure piston chamber62is lowered without stopping the supply. Similarly, in the above-described embodiment, the supply of the hydraulic pressure to the OFF pressure piston chamber63is stopped after the lock mechanism BL is placed in the unlocked state. However, it is possible to adopt a configuration in which the hydraulic pressure in the OFF pressure piston chamber63is lowered without stopping the supply.

Furthermore, for example, the specific configuration of the forward clutch5shown inFIG. 2is an example of a friction element having a lock mechanism attached thereto. The present invention is also applicable to a friction element having a lock mechanism attached thereto and having a configuration other than this specific configuration.

Moreover, while the forward clutch5is a friction element having a lock mechanism attached thereto in the above-described embodiment, the reverse brake4may be a friction element having a lock mechanism attached thereto. In this case, control similar to those of the above-described embodiment can be performed by supplying the OFF pressure to a release-side oil chamber of the reverse brake4when it is determined that the vehicle is going to stop while running in the R mode. Here, the R mode corresponds to the driving mode.

The present application claims the benefit of priority from Japanese Patent Application No. 2012-212188, filed in the Japan Patent Office on Sep. 26, 2012, the disclosure of which is incorporated herein by reference in its entirety.