Control of lock-up clutch

1. A lock-up clutch control device which controls a lock-up clutch installed in a torque converter interposed between an engine and a transmission used with a vehicle, comprising:

a differential pressure generating device which engages or disengages the lock-up clutch by adjusting a differential pressure supplied to the lock-up clutch,

a sensor which detects a vehicle speed, and

a controller programmed to:

determine whether or not the vehicle speed is equal to or larger than a first threshold value (V 1 ) corresponding to a lock-up ON vehicle speed;

command the differential pressure generating device to engage the lock-up clutch, when the vehicle speed is equal to or larger than the first threshold value (V 1 );

determine whether or not the vehicle speed has reached a second threshold value (V 2 ) larger than the first threshold value (V 1 );

set a lock-up OFF threshold value (V 3 /V 4 ) which depends on whether the vehicle speed has reached the second threshold value (V 2 ); and

command the differential pressure generating device to disengage the lock-up clutch, when the vehicle speed is equal to or less than the lock-up OFF threshold value.

2. The lock-up clutch control device as defined in claim 1 , wherein the lock-up OFF threshold value (V 3 /V 4 ) is smaller than the second threshold value (V 2 ).

3. The lock-up clutch control device as defined in claim 1 , wherein the controller is further programmed to:

set the lock-up OFF vehicle speed to a third threshold value (V 3 ), when the vehicle speed has not reached the second threshold value (V 2 ); and

set the lock-up OFF vehicle speed to a fourth threshold value (V 4 ) larger than the third threshold value, when the vehicle speed has reached the second threshold value (V 2 ).

4. The lock-up clutch control device as defined in claim 3 , wherein the fourth threshold value (V 4 ) is larger than the first threshold value (V 1 ).

5. The lock-up clutch control device as defined in claim 3 , wherein the third threshold value (V 3 ) is smaller than the first threshold value (V 1 ).

6. The lock-up clutch control device as defined in claim 1 , comprising a sensor which detects an opening (TVO) of a throttle valve which adjusts an air flowrate supplied to the engine,

wherein the controller is programmed to:

when the opening of the throttle valve is larger than a predetermined value near zero, determine whether or not the vehicle speed is equal to or larger than the first threshold value (V 1 ), and determine whether or not the vehicle speed has reached the second threshold value (V 2 ) which is larger than the first threshold value (V 1 ).

7. The lock-up clutch control device as defined in claim 1 , wherein the controller is programmed to:

determine whether or not the vehicle is in an acceleration state based on the detected speed,

and when the vehicle is in an acceleration state and the vehicle speed is equal to or larger than the first threshold value (V 1 ), command the differential pressure generating device to engage the lock-up clutch.

8. The lock-up clutch control device as defined in claim 1 , wherein the first threshold value (V 1 ) is set according to a vehicle speed at which engagement of the lock-up clutch is completed.

9. The lock-up clutch control device as defined in claim 1 , wherein the second threshold value (V 2 ) is larger than the vehicle speed at which booming noise is generated.

10. The lock-up clutch control device as defined in claim 1 , wherein the second threshold value (V 2 ) is a vehicle speed obtained by adding a predetermined vehicle speed to an upper limiting value of vehicle speed at which booming noise is generated.

11. The lock-up clutch control device as defined in claim 1 , wherein the third threshold value (V 3 ) is equal to or larger than the vehicle speed at which engine stall occurs.

12. The lock-up clutch control device as defined in claim 1 , wherein the fourth threshold value (V 4 ) is the upper limiting value of vehicle speed at which booming noise is generated.

13. The lock-up clutch control device as defined in claim 1 , comprising a sensor which detects the opening (TVO) of a throttle valve,

wherein the controller is programmed to set the first threshold value (V 1 ) according to the opening (TVO) of the throttle valve.

14. The lock-up clutch control device as defined in claim 1 , comprising a sensor which detects the opening (TVO) of a throttle valve which adjusts an air flowrate supplied to the engine,

wherein the controller is programmed to set the second threshold value (V 2 ) according to the opening (TVO) of the throttle valve.

15. The lock-up clutch control device as defined in claim 1 , comprising a sensor which detects the opening (TVO) of a throttle valve which adjusts an air flowrate supplied to the engine,

wherein the controller is programmed to set the third threshold value (V 3 ) according to the opening (TVO) of the throttle valve.

16. The lock-up clutch control device as defined in claim 1 , comprising a sensor which detects the opening (TVO) of a throttle valve which adjusts an air flowrate supplied to the engine,

wherein the controller is programmed to set the fourth threshold value (V 4 ) according to the opening (TVO) of the throttle valve.

17. A lock-up clutch control device which controls a lock-up clutch installed in a torque converter interposed between an engine and a transmission used with a vehicle, comprising:

differential pressure generating means for engaging or disengaging the lock-up clutch by adjusting a differential pressure supplied to the lock-up clutch;

detecting means for detecting a vehicle speed;

determining means for determining whether or not the vehicle speed is equal to or larger than a first threshold value (V 1 ) corresponding to a lock-up ON vehicle speed;

commanding means for commanding the differential pressure generating means to engage the lock-up clutch, when the vehicle speed is equal to or larger than the first threshold value (V 1 );

determining means for determining whether or not the vehicle speed has reached a second threshold value (V 2 ) larger than the first threshold value (V 1 );

setting means for setting a lock-up OFF threshold value (V 3 /V 4 ) which depends on whether the vehicle speed has reached the second threshold value (V 2 ); and

commanding means for commanding the differential pressure generating means to disengage the lock-up clutch, when the vehicle speed is equal to or less than the lock-up OFF threshold value.

18. A lock-up clutch control method for controlling a lock-up clutch installed in a torque converter interposed between an engine and a transmission used with a vehicle, comprising:

detecting a vehicle speed;

determining whether or not the vehicle speed is equal to or larger than a first threshold value (V 1 ) corresponding to a lock-up ON vehicle speed;

commanding a differential pressure generating device to engage the lock-up clutch, when the vehicle speed is equal to or larger than the first threshold value (V 1 );

determining whether or not the vehicle speed has reached a second threshold value (V 2 ) larger than the first threshold value (V 1 );

setting a lock-up OFF threshold value (V 3 /V 4 ) which depends on whether the vehicle speed has reached the second threshold value (V 2 ); and

commanding the differential pressure generating device to disengage the lock-up clutch, when the vehicle speed is equal to or less than the lock-up OFF threshold value.