Abstract:
A lock-up control device controls a lock-up clutch in a torque converter. The lock-up clutch is engaged by supplying a lock-up control pressure. The torque converter transmits torque via oil in a torque converter chamber when the lock-up clutch is disengaged, and transmits torque directly when the lock-up clutch is engaged. In the lock-up state, the oil pressure in the torque converter chamber is decreased by discharging oil in the torque converter chamber. By providing a circuit which leads oil discharged from the torque converter chamber to a lubricating part of the transmission, the balance between the inflowing oil amount and outflowing oil amount is improved, and oil insufficiency is prevented.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a lock-up control device of a torque converter used for an automatic transmission. 
     BACKGROUND OF THE INVENTION 
     A torque converter drives a turbine by oil stirred by an impeller, and transmits power by hydraulic power transmission. Although the torque converter increases torque and absorbs torque fluctuations, slip occurs between the impeller and turbine, so efficiency of the transmission decreases. 
     In a lock-up torque converter, the slip can be eliminated and efficiency of the transmission can be increased by locking the impeller and turbine by engaging a lock-up clutch in a running region where torque increase and absorption of torque fluctuation is unnecessary. 
     The torque converter disclosed by JP-A-H5-79560 published by the Japanese Patent Office in 1993 discloses a torque converter comprising a lock-up control chamber partitioned by a lock-up clutch piston from a converter chamber. When a lock-up control pressure is supplied to this lock-up control chamber, the lock-up clutch piston displaces so that the lock-up clutch is engaged, and the impeller and turbine are locked. This torque converter is a three circuit lock-up torque converter comprising an inlet circuit which supplies oil to the torque-converter chamber, an outlet circuit which discharges oil from the torque-converter chamber, and a lock-up control circuit which supplies lock-up control pressure to the lock-up control chamber. 
     SUMMARY OF THE INVENTION 
     In the three circuit lock-up torque converter, the pressure (converter pressure) in the converter chamber is decreased by making the torque converter outlet circuit communicate with the drain port during lock-up, and the lock up control pressure can thereby be decreased. 
     However, if the torque converter outlet circuit is merely made to communicate with the drain port to decrease the converter pressure, oil balance (balance between the inflowing oil amount and outflowing oil amount) is impaired, and in particular when the pump is driven with the minimum necessary discharge amount to improve fuel cost performance, it may occur that oil is insufficient. 
     It is therefore an object of this invention to make effective use of the oil pressure discharged from a torque converter to decrease the converter pressure, and to prevent impairment of oil balance. 
     It is a further object of this invention to prevent seizure of an automatic transmission even under low temperature conditions when oil does not flow easily to an oil cooler for cooling during non lock-up periods. 
     It is yet a further object of this invention to prevent variation of pressure in the converter chamber during lock-up, and prevent a shock due to variation of the engaging force of a lock-up clutch. 
     In order to achieve above object, this invention provides a lock-up control device for controlling a lock-up clutch in a torque converter, the lock-up clutch being engaged by supplying a lock-up control pressure, and the torque converter transmitting torque via a fluid in a torque converter chamber when the lock-up clutch is disengaged and transmitting torque directly when the lock-up clutch is engaged. The lock-up control device decreases the fluid pressure in the torque converter chamber by discharging fluid in the torque converter chamber during lock-up, and leads the fluid discharged from the torque converter chamber to a lubricating part of the transmission. 
     According to an aspect of the invention, this invention provides a lock-up control device for controlling a lock-up clutch in a torque converter, the lock-up clutch being engaged by supplying a lock-up control pressure, the torque converter transmitting torque via a fluid in a torque converter chamber when the lock-up clutch is disengaged and transmitting torque directly when the lock-up clutch is engaged. The lock-up control device comprises a fluid circuit which decreases the flow pressure in the torque converter chamber by discharging fluid in the converter chamber during lock-up, and a lubricating circuit which leads the fluid discharged from the torque converter chamber to a lubricating part of the transmission. 
     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows an oil pressure circuit of a lock-up controller of torque converter according to this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawing, a three circuit lock-up torque converter  1  comprises an impeller  1   a,  a turbine  1   b  disposed facing the impeller  1   a,  and a stator  1   c.  The impeller  1   a  is joined to a crankshaft  2  of an engine, not shown, and the turbine  1   b  is joined to an input shaft  3  of the automatic transmission, not shown. The stator  1   c  is disposed on a fixed shaft  5  via a one-way clutch  4  so that it cannot rotate in the opposite direction to the rotation direction of the engine. The stator  1   c  functions as a reactor. 
     Oil is supplied from a torque converter inlet circuit  7  to a converter chamber  6  housing the impeller  1   a,  turbine  2   b  and stator  1   c,  and the supplied oil is discharged to a torque converter outlet circuit  8 . The oil in the converter chamber  6  is stirred by the impeller  1   a  driven by the engine, and after it impacts the turbine  1   b,  it is guided by the stator  1   c  to be returned to the impeller  1   a.  In this way, the turbine  1   b  is rotated while increasing the torque. 
     The torque converter  1  is further provided with a lock-up clutch  9  to lock the impeller  1   a  and turbine  1   b.  When a lock-up clutch piston  10  displaces, the lock-up clutch  9  is engaged, and the impeller  1   a  and turbine  1   b  are locked. 
     The lock-up clutch piston  10  partitions a lock-up control chamber  11  which is partitioned from the converter chamber  6 . The lock-up clutch piston  10  displaces due to a force in the right-hand direction of the figure according to a differential pressure between a lock-up control pressure P L  supplied to the lock-up control chamber  11  from the lock-up control circuit  12 , and a converter pressure P C  inside the converter chamber  6 , engages the lock-up clutch  9 , and locks the impeller  1   a  and turbine  1   b.    
     Next, the oil pressure circuit of the lock-up controller will be described. 
     The oil pressure circuit comprises a torque converter control valve  21 , lock-up control valve  22 , lock-up solenoid  23  and oil cooler  24 . A lubricating part  25  is a lubricating part of the automatic transmission. 
     The torque converter control valve  21  supplies oil to the converter chamber  6  of the torque converter  1 , and controls recirculation of oil to the oil cooler  24  and lubricating part  25 . A spool  21   b  is maintained in the normal position shown in the figure by a spring  21   a.  When a signal pressure P S  is supplied to the chamber  21   c,  the spool  21   b  is pushed down from the normal position to a working position against the spring  21   a.    
     When the spool  21   b  is in the normal position, the torque converter control valve  21  causes an input circuit  26  to which a torque converter working pressure P T  is supplied, to communicate with the torque converter inlet circuit  7 , causes parallel circuits  27 ,  28  to communicate with each other, and causes the torque converter outlet circuit  8  to communicate with an oil cooler circuit  29  which connects to the inlet of the oil cooler  24 . Orifices  27   a,    28   a  are respectively interposed in parallel circuits  27   a,    28   a.  The circuit  28  is connected to the circuit  27  at a position nearer the torque converter control valve  21  than the orifice  27   a,  and the circuit  27  is connected to the torque converter inlet circuit  7 . 
     The spool  21   b  is pushed down from the position shown in the figure, and when it is in the working position, the torque converter control  21  causes the input circuit  26  to communicate with the circuit  27 , causes the circuit  28  to communicate with the oil cooler circuit  29 , and causes the torque converter outlet circuit  8  to communicate with a lubricating circuit  30  which connects to the lubricating part  25 . An outlet of the oil cooler  24  is connected to the lubricating circuit  30 , and oil which has flowed through the oil cooler  24  is supplied from the oil cooler circuit  29  to the lubricating part  25 . 
     Here, a bypass valve  31  is provided between the oil cooler circuit  29  and lubricating circuit  30 . The bypass valve  31  is a check valve which allows oil to flow from the circuit  29  to the circuit  30 , but prevents flow in the reverse direction. The bypass valve  31  opens when the viscosity of the oil becomes high at low temperature, oil does not flow easily to the oil cooler  24 , and the inlet pressure of the oil cooler  24  increases above a predetermined value. Oil which has been stopped by the oil cooler  24  then returns to an oil pan via the lubricating part  25 . By providing the bypass valve  31 , seizure of the automatic transmission due to decrease of oil flowing to the lubricating part  25  is prevented even when the viscosity of the oil becomes high at low temperature. 
     Here, the opening pressure of the bypass valve  31  is determined as follows. 
     The decrease of oil flow to the oil cooler  24  due to the high viscosity of the oil, and seizure of the automatic transmission, occur when the oil is at low temperature. Therefore, at the temperature after warm-up when this problem does not occur, the above effect is achieved even if the bypass valve  31  is permanently closed. 
     However, if it is made possible to open the bypass valve  31  even at the temperature after warm-up, the bypass valve  31  can be opened and closed during lock-up at the temperature after warm up. If the bypass valve  31  is opened and closed during lock-up, the converter pressure P C  of the converter chamber  6  varies via the circuit  29 , and the engaging force of the lock-up clutch  9  varies so that a shock occurs. 
     Therefore, the bypass valve  31  is arranged not to open at the temperature after warm-up (e.g., over 40° C.) when lock-up is performed. Specifically, the set load of an internal spring which determines the opening pressure of the bypass valve  31  is set to a value at which the bypass valve  31  does not open due to the pressure generated at the inlet of the oil cooler  24  at the temperature at which lock-up is performed. 
     The lock-up control valve  22  performs lock-up control as to whether or not to perform lock-up of the torque converter  1  by supplying the lock-up control pressure P L  to the lock-up control chamber  11 , and controls the lock-up control pressure P L  during lock up control. The spool  22   b  is supported in the normal position shown in the figure by the spring  22   a.  When the signal pressure P S  is supplied to the chamber  22   c,  the spool  22   b  is pushed down from the normal position against the spring  22   a  and displaces to a working position. 
     When the spool  22   b  is in the normal position, the lock-up control valve  22  causes the lock-up control circuit  12  to communicate with the drain port  22   d,  eliminate the lock-up control pressure P L,  disengage the lock-up clutch  9 , and place the torque converter  1  in the non-lock-up state. 
     When the spool  22   b  is pushed down from the normal position to the working position, the lock-up control valve  22  causes the lock-up control circuit  12  to communicate with the D-range pressure circuit  32  to which a D-range pressure P D  generated when the selector lever of the automatic transmission is in a forward running range (D range) and the vehicle is moving forwards, is supplied. This D-range pressure P D  is taken as a source pressure, the lock-up control pressure P L  is output to the lock-up control circuit  12 , and the torque converter  1  is placed in the lock-up state. 
     In the spool  22   b,  a torque converter inlet pressure in the torque converter inlet circuit  7  and the torque converter outlet pressure in the torque converter outlet circuit  8  are respectively made to act downwards in the figure on step parts  22   e,    22   f,  and the lock-up control pressure P L  which is fed back is made to act upwards in the figure on a plug  22   g.    
     The spool  22   b  does not displace against the spring  22   a  due only to the torque converter inlet pressure and torque converter outlet pressure acting on the steps  22   e,    22   f,  but due to these pressures and the lock-up control pressure P L  which is fed back, the differential pressure between the lock-up control pressure P L  which is the output pressure and the converter pressure P C  in the converter chamber  6 , i.e., the engaging force of the lock-up clutch  9 , can be precisely controlled to a value depending on the signal pressure P S  supplied to the chamber  22   c  even if the D-range pressure P D , which is the source pressure, varies. 
     The signal pressure P S  is controlled by the lock-up solenoid  23 . The lock-up solenoid  23  is a linear solenoid, and outputs the signal pressure P S  which is proportional to the supply current to a signal pressure circuit  33  taking a constant pilot pressure P P  as a source pressure. The signal pressure circuit  33  is connected to the chamber  21   c  of the torque converter control valve  21  and the chamber  22   c  of the lock-up control valve  22 . 
     The current supplied to the lock-up solenoid  23  is determined according to the lock-up control pressure P L  required for the lock-up clutch piston  10  to push the lock-up clutch  9  in opposition to the converter pressure P C  with a force at which the required engaging capacity of the lock-up clutch  9  is obtained. 
     Next, the action of this lock-up controller will be described. 
     When the torque converter  1  is not to be locked up, current is not supplied to the lock-up solenoid  23 , and the signal pressure P S  is not output to the circuit  33 . 
     Therefore, the spool  21   b  of the torque converter control valve  21  is in the normal position shown in the figure, and the torque converter working pressure P T  in the input circuit  26  is supplied to the converter chamber  6  of the torque converter  1  from the torque converter inlet circuit  7 . The torque converter outlet circuit  8  is made to communicate with the oil cooler circuit  29 , and after oil returning from the converter chamber  6  is cooled by the oil cooler  24 , it is supplied for lubricating the lubricating part  25  and is drained to the oil pan. 
     When the signal pressure P S  is not generated when lock-up should not be performed, the spool  22   b  of the lock-up control valve  22  is in the normal position shown in the figure, the lock-up control circuit  12  is made to communicate with the drain port  22   d,  and the lock-up control pressure P L  is eliminated. Therefore, as the piston  10  does not engage the lock-up clutch  9 , and the torque converter  1  performs power transmission in the non lock-up state. 
     When the torque converter  1  is to be engaged, current is supplied to the lock-up solenoid  23 , and the signal pressure P S  proportional to the current value is output to the circuit  33 . The spool  21   b  of the torque control valve  21  is pushed down to the working position from the normal position, and the spool  22   b  of the lock-up control valve  22  is also pushed down to the working position from the normal position. 
     When the spool  21   b  of the torque converter control valve  21  displaces to the working position, the input circuit  26  is made to communicate with the circuit  27 , the circuit  28  is made to communicate with the oil cooler circuit  29 , and the torque converter output circuit  8  is made to communicate with the lubricating circuit  30 . 
     As a result, the torque converter working pressure P T  of the input circuit  26  is introduced to the circuit  27 . On one hand, the torque converter working pressure P T  to the circuit  27  is supplied to the converter chamber  6  of the torque converter  1  from the torque converter inlet circuit  7  via the orifice  27   a.  On the other hand, oil is supplied to the oil cooler  24  from the circuit  29  via the orifice  28   a,  and after cooling, it is supplied for lubrication of the lubricating part  25  and is drained off to the oil pan. Oil returning from the converter chamber  6  is supplied for lubrication of the lubricating part  25  via the lubricating circuit  30 , and then flows down to the oil pan. 
     When the spool  22   b  displaces to the working position described above, the lock-up control valve  22  outputs the lock-up control pressure P L  which is proportional to the signal pressure P S , i.e., a current amount supplied to the lock-up solenoid  23 , to the lock-up control circuit  12  taking the D-range pressure from the D-range pressure circuit  32  as a source pressure. Due to the lock-up control pressure P L , the lock-up clutch piston  10  displaces, the lock-up clutch  9  is engaged by a force according to the lock-up control pressure P L , and torque converter  1  enters the lock-up state. 
     The lock-up control pressure P L  is determined as a value required to push the lock-up clutch piston  10  against the lock-up clutch  9  in opposition to the converter pressure P C  with a force at which the required engaging capacity of the lock-up clutch  9  is obtained. The supply current to the lock-up solenoid  23  is determined corresponding to this lock-up control pressure P L . 
     In this embodiment, as described above, during lock-up, oil returning from the converter chamber  6  to the circuit  8  is discharged to the oil pan via the lubricating circuit  30  and lubricating part  25  without passing through the oil cooler  24  which has a large resistance. Therefore, the converter pressure P C  in the converter chamber  6  can be decreased, and the required engaging capacity of the lock-up clutch  9  can be achieved even if the lock-up control pressure P L  is determined low by a corresponding amount. As the lock-up control pressure P L  can be reduced, the stiffness of the torque converter  1  which must be designed to withstand this pressure can be suppressed low, and the capacity of the oil pump which supplies the oil can also be reduced. This is advantageous from the viewpoint of cost, reduces the drive load of the oil pump, and is largely beneficial from the viewpoint of fuel economy. 
     Moreover, when oil returning from the converter chamber  6  to the circuit  8  is discharged to the oil pan for this purpose, the discharged oil is led to the lubricating part  25  via the lubricating circuit  30 , and is discharged to the oil pan after making effective use of it for lubricating the lubricating part  25 , and impairment of oil balance between the inflowing oil amount and outflowing oil amount is prevented. 
     The bypass valve  31  is interposed between the oil cooler circuit  29  and lubricating circuit  30  so as to permit oil flow from the circuit  29  to the circuit  30 , and prevent oil flow in reverse direction. As a result, when the viscosity of the oil increases at low temperature so that it does not flow easily to the oil cooler  24 , and the inlet pressure of the oil cooler  24  increases above the valve opening pressure, the bypass valve  31  opens, and after oil which is stopped by the oil cooler  24  is directed to the lubricating part  25 , it is returned to the oil pan. 
     Therefore, even if oil does not flow easily to the oil cooler  24  at low temperature and the oil amount flowing into the lubricating part  25  from the oil cooler  24  decreases, the required lubricating oil amount can be maintained by opening the bypass valve  31 , and seizure of the automatic transmission due to poor lubrication can be prevented. 
     Further, the opening pressure of the bypass valve  31  is set so that the bypass valve  31  does not open at the oil cooler inlet pressure (pressure inside the oil cooler circuit  29 ) in the temperature at which lock-up is performed, so the bypass valve  31  can be maintained in the closed state during lock-up. Therefore, the pressure in the converter chamber  6  due to opening and closing of the bypass valve  31  does not vary during lock-up, the engaging force of the lock-up clutch  9  does not vary, and shocks due to the variation of the lock-up engaging force are prevented. 
     The temperature range at which the bypass valve  31  is maintained in the closed state is the temperature range at which lock-up is performed. In this temperature range, oil does not reach such a high viscosity that it does not pass through the oil cooler easily, so the aforesaid advantage due to provision of the bypass valve  31  is still obtained. 
     The entire contents of Japanese Patent Application P11-295441 (filed Oct. 18, 1999) are incorporated herein by reference. 
     Although the invention has been described above by reference to a certain embodiment of the invention, the invention is not limited to the embodiment described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.