Abstract:
A hydraulic system for controlling a double-clutch transmission operating with wet clutches. A pump provides a system pressure in a supply line, and a control valve for applying to individual clutch actuators an actuation pressure that is derived from the system pressure and that is modulated in accordance with the operating conditions. A charging valve directs hydraulic fluid delivered by the pump into a return line when the system pressure is reached, wherein the hydraulic fluid flowing through the return line can be fed to the clutches to cool them.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a hydraulic system for controlling a double-clutch transmission that includes wet clutches. 
         [0003]    2. Description of the Related Art 
         [0004]    Such hydraulic systems include a pump to provide a system pressure in a supply line, and a control valve to individually apply to clutch actuators appropriate operating pressure that is derived from the system pressure and that is modulated in accordance with the operating conditions. A system valve directs hydraulic fluid delivered by the pump into a return line when the system pressure is reached. 
         [0005]    A hydraulic system of the type described above is usable not only for wet clutches, but also for dry clutches, where it has proven itself well. 
         [0006]    An object of the present invention is to further develop such a hydraulic system in such a way as to include a cooling arrangement for providing cooling hydraulic fluid to the clutches as needed, and to provide the cooling necessary for wet clutches to be accomplished in a simple manner. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, the present invention involves using hydraulic fluid that is delivered by the pump to supply pressure to the hydraulic system to cool the clutches by circulating the hydraulic fluid when the system pressure is reached, so that it flows back through a return line without pressure to cool the clutches. 
         [0008]    The invention relates to a hydraulic system for controlling a double-clutch transmission operating with wet clutches, which system includes a pump to provide a system pressure in a supply line. A control valve is provided to individually apply to clutch actuators actuating pressure derived from the system pressure and modulated in accordance with the circumstances, and a system valve which directs hydraulic fluid delivered by the pump into a return line when the system pressure is reached. 
         [0009]    Advantageously, the hydraulic system in accordance with the invention includes a cooling control valve to control the quantity of hydraulic fluid that is supplied to the clutches. 
         [0010]    The pump is driven for example by an internal combustion engine contained in a vehicle power train that includes the double-clutch transmission. 
         [0011]    In a preferred embodiment, the hydraulic system in accordance with the invention additionally includes an auxiliary pump to transport cooling fluid to the clutches as needed. The auxiliary pump is advantageously electrically driven. 
         [0012]    Furthermore, the auxiliary pump is advantageously configured in such a way, and is integrated into the hydraulic system in such a way, that a supply pressure can be produced in the supply line by means of the auxiliary pump. 
         [0013]    The supply line of the hydraulic system is advantageously connected to a pressure accumulator, which ensures a system pressure when it is in the charged state. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The structure, operation, and advantages of the present invention will become further apparent upon consideration of the following description taken in conjunction with the accompanying drawings, in which: 
           [0015]      FIGS. 1 through 4  show hydraulic circuit diagrams of several embodiments of a hydraulic system in accordance with the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    As shown in the hydraulic circuit of  FIG. 1 , a double-clutch transmission that is designated in the aggregate as  10  includes two clutches K 1  and K 2 , by means of which a drive shaft (not shown) driven by an internal combustion engine  40  can be rotatably coupled with input shafts  12  and  14 . Input shaft  12  is hollow, and input shaft  14  extends within input shaft  12 . Input shafts  12  and  14  are connected through shiftable gear sets to a common output shaft (not shown) of the double-clutch transmission, in a rotationally fixed connection. In the illustrated embodiment, input shaft  12  is operatively associated with a part of the transmission in which gears  1 ,  3 ,  5 , and  7  are selectable. Input shaft  14  is operatively associated with a part of the transmission in which gears  2 ,  4 ,  6 , and R are selectable. Transmission actuators for engaging the gears are designated as  16 ,  18 ,  20 , and  22 . Clutch actuators with which clutch K 1  or clutch K 2  can be operated are designated as  24  and  26 , respectively. 
         [0017]    To operate the hydraulically operated actuators, a hydraulic system identified in the aggregate as  30  is provided. Hydraulic system  30  includes a pump  32  to supply hydraulic fluid under pressure. Pump  32  delivers hydraulic fluid from a hydraulic fluid supply  34  through a filter  36  into a supply line  38 . Pump  32  is driven by an internal combustion engine  40 , which also furnishes torque that is transmitted by the double-clutch transmission  10 . Input line  38  is connected through an accumulator charging valve  42  to a supply line  44 , in which a system pressure that is necessary for the hydraulic system to be able to function prevails. Connected to supply line  44  is a pressure accumulator  46 , which is set to a predetermined system pressure and stores a certain volume of hydraulic fluid when that system pressure is reached, so that that system pressure is available even if a certain quantity of hydraulic fluid escapes from the supply line  44 . Supply line  44  is connected through a valve  50  that is operated by a solenoid  48 , to a control valve  52  that controls the clutch actuator  24  of clutch K 1 , to a control valve  54  that controls the clutch actuator  26  of clutch K 2 , and to a control valve  56  that controls the transmission actuators  16 ,  18 ,  20 , and  22  through an intermediate changeover valve  58 . 
         [0018]    Control valves  52 ,  54 , and  56  each include proportional magnets, to which current is applied from outputs  60  of an electronic control unit  62 , to which signals that are relevant for the operation of the double-clutch transmission are supplied at inputs  64 . 
         [0019]    The construction and function of the arrangement described above are known, and therefore will not be described further. 
         [0020]    In a known manner, the function of accumulator charging valve  42  includes switching over when the system pressure is reached in supply line  44  or when pressure accumulator  46  is full, so that the hydraulic fluid delivered by pump  32  then flows into a non-pressurized return line  66  and from there back to the hydraulic fluid supply  34 . 
         [0021]    In accordance with the present invention, return line  66  leads to a cooling control valve  68 , which has as output lines a cooling line  72  that leads to clutch cooling device  70  and a recirculation line  74  that leads into hydraulic fluid supply  34 . 
         [0022]    The function of the described arrangement is as follows: when pressure accumulator  46  is full, that is, when full system pressure is present in supply line  44 , accumulator charging valve  42  switches over, so that the hydraulic fluid delivered by pump  32  is delivered into return line  66 . Cooling control valve  68  is connected to an output of electronic control unit  68  that supplies a signal relating to the cooling need of clutches K 1  and K 2 , which is derived from the operating conditions of the clutches (slippage, torque), for example. Alternatively, cooling control valve  68  is connected to one or more temperature sensors that determine the temperatures of the clutches, whereby exceeding predetermined temperatures indicates a need for cooling. When cooling is needed, cooling control valve  68  connects the return line  66  with the cooling line  72 , so that the clutches can be cooled by means of a known clutch cooling device  70 , which includes in particular a flow of hydraulic fluid through the wet clutches, thereby cooling them. When there is no need for cooling, cooling control valve  68  connects the return line  66  to the recirculation line  74 . The cooling control valve  68  can be so designed that when there is a need for cooling it connects the cooling line  72  completely with the return line and disconnects the recirculation line  74 , or it delivers only the specifically needed flow of hydraulic fluid to the cooling line  72 . 
         [0023]    In a simplified embodiment, the cooling control valve  68  can be omitted, so that cooling fluid that is recirculated at full system pressure is used constantly to cool the clutches. 
         [0024]    A characteristic feature of the described system is that in order to ensure adequate cooling of the clutches, pump  32  must be sized to be larger than is necessary to supply the actuators, namely large enough so that the maximum cooling need can be covered. Another characteristic feature is that at high rotational speeds of internal combustion engine  40 , clearly too much hydraulic fluid is delivered. Thus, in order to keep the flow losses small, large line cross-sections are necessary. 
         [0025]    In order to avoid the above-identified characteristic features, the system can be modified as illustrated in  FIG. 2 . In accordance with  FIG. 2 , an auxiliary pump  78  is provided, which preferably also feeds hydraulic fluid from the hydraulic fluid supply  34 , through a line  80  to cooling control valve  68 . Auxiliary pump  78  is a tandem pump of pump  32 , for example, and thus is likewise driven by internal combustion engine  40 . If pump  32  is a vane pump, the auxiliary pump  78  can be designed as a second flow chamber of the vane pump. The auxiliary pump is adjusted to the quantity of cooling fluid necessary for maximum cooling of the clutches, or is advantageously adjusted to the cooling fluid throughput that is necessary in addition to the hydraulic fluid delivered through return line  66  by pump  32 . 
         [0026]    Auxiliary pump  78  can be suction-throttled if necessary, which is possible without acoustic problems because the auxiliary pump operates only within a small pressure range, for example a maximum of 3 bar. It goes without saying that cooling control valve  68  is designed in such a way that when there is a need for cooling it connects lines  66  and  80  to cooling line  72 . Advantageously, a check valve (not shown) is situated in return line  66 , which prevents cooling fluid delivered by auxiliary pump  78  from flowing in the direction of accumulator charging valve  42 . Cooling control valve  68  can also be omitted in the embodiment shown in  FIG. 2 , in which case overcooling of the clutches is then tolerated if necessary. 
         [0027]      FIG. 3  shows an embodiment of the hydraulic system that is especially well adapted to the particular cooling need, and which operates with low energy consumption. 
         [0028]    In the embodiment in accordance with  FIG. 3 , auxiliary pump  78  is driven by a separate electric motor  81 , which is controlled by the electronic control unit  62 , for example, or by one or more separate temperature sensors for determining the temperatures of the clutches. When the need for cooling is great the electric motor  81  is actuated, so that auxiliary pump  78  delivers additional hydraulic fluid only then, and advantageously in accordance with need. Line  80  in  FIG. 3 , like line  80  in  FIG. 2 , could be connected directly to cooling line  72 , in which case a check valve (not shown) is then advantageously positioned in line  80  to prevent a flow of hydraulic fluid in the direction toward auxiliary pump  78 . 
         [0029]    Whereas in the embodiment shown in  FIG. 1  there is no hydraulic fluid available for cooling for the brief time periods in which pressure accumulator  46  must be recharged, in the embodiments in accordance with  FIGS. 2 and 3  hydraulic fluid is constantly available for cooling. 
         [0030]      FIG. 4  shows an embodiment of the invention that is a modification of the arrangement shown in  FIG. 3 , in that hydraulic fluid is delivered by auxiliary pump  78  through an auxiliary feed line  82  directly into the supply line  38 . In addition, the cooling control valve  68  of  FIG. 3  is replaced by a valve  84  that is preferably likewise connected to electronic control unit  62 . Electric motor  81  and auxiliary pump  78  are advantageously of somewhat higher capacity design than in the embodiment in accordance with  FIG. 3 , so that by means of auxiliary pump  78  double-clutch transmission  10  can be supplied by means of auxiliary pump  78  with the system pressure that is necessary for its functioning. 
         [0031]    The embodiment in accordance with  FIG. 4  is suitable for vehicles with stop-start systems, and for hybrid vehicles in which the internal combustion engine  40  is automatically shut down in certain operating phases, and is quickly switched on again in case of need. When internal combustion engine  40  is not running and the system pressure in supply line  44  is dropping, electric motor  81  is actuated and line  80  is connected through valve  84  to auxiliary feed line  82 , which leads into supply line  38 . Advantageously, a check valve (not shown) is situated between the junction of auxiliary feed line  82  with supply line  38  and pump  32 , which prevents a flow of fluid from auxiliary feed line  82  through pump  32 . Thus, the functional capability of double-clutch transmission  10  continues to be ensured in phases during which internal combustion engine  40  is not running. When the system pressure is present, electric motor  81  is advantageously shut off. However, it can continue to run if cooling is needed for the clutches, so that there is standby hydraulic fluid for cooling the clutches, which is delivered by pump  32  through lines  82 ,  38 , valve  42 , and line  66  to cooling line  72 , or directly from line  80  into cooling line  72 . In other respects the functioning of the hydraulic system in accordance with  FIG. 4  corresponds to that shown in  FIG. 3 . 
         [0032]    Although particular embodiments of the present invention have been illustrated and described, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit of the present invention. It is therefore intended to encompass within the appended claims all such changes and modifications that fall within the scope of the present invention.