Abstract:
In a hydraulic system for motor vehicles in particular, including a master cylinder, a slave cylinder, and a pressure medium line connecting them, an inexpensive pressure boost is achieved by the fact that the hydraulic system includes a pump having an inlet and an outlet and a valve which are situated between the master cylinder and the slave cylinder in the pressure medium line. The valve has a first valve position in which the master cylinder and the slave cylinder are directly interconnected and a second valve position in which the master cylinder and the slave cylinder are interconnected via a pump.

Description:
This claims benefit of German Patent Application No. 10 2004 043 958.3, which is hereby incorporated by reference herein. 
   BACKGROUND 
   The present invention relates to a hydraulic system, in particular for motor vehicles, including a master cylinder, a slave cylinder, and a pressure medium line connecting these two, as well as a hydraulic valve, in particular for use in a hydraulic system for motor vehicles. 
   It is known that a servo-supported hydraulic system may be used for clutch operation to reduce the pedal force exerted by the driver of a motor vehicle. For example, an electric pump in the hydraulic system increases the pressure between the master cylinder and the slave cylinder. The pump may be electronically regulated, for example. To do so, at least one pressure sensor is required in the hydraulic system because the pump pressure is pressure dependent. One disadvantage of such systems is that the cost is relatively high (e.g., due to the pressure sensor and the electronic regulation), the hysteresis of the clutch characteristic is reproduced and sometimes even increased in the pedal force curve, in particular at high pedal speeds, and there may be additional pulsation of the pedal. 
   European Patent Application No. 00 412 711 (DE 690 09 345) describes a hydraulic system for a continuously variable transmission (CVT) in which multiple control valves are provided in the regulating circuits for the transmission adjustment and clutch control. The control valves are each triggered by their own electronically regulated hydraulic control circuits. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a hydraulic system which will eliminate the aforementioned disadvantages. In particular electronic regulation of the servo support during pedal operation may be omitted. 
   The present invention provides a hydraulic system, in particular for motor vehicles, including a master cylinder, a slave cylinder, and a pressure medium line connecting the two, wherein the hydraulic system includes a pump having an inlet, an outlet, and a valve which are situated in the pressure medium line between the master cylinder and the slave cylinder, the valve having a first valve position in which the master cylinder and the slave cylinder are connected to one another and a second valve position in which the master cylinder and the slave cylinder are connected via the pump. The valve preferably has a third valve position in which the master cylinder and the slave cylinder are connected via the pump, the outlet and the inlet of the pump being connected via a bypass. The volume flow through the bypass is preferably regulable by the valve with the valve position of the valve preferably being a function of the pressure prevailing in the hydraulic system. The valve may switch the hydraulic connection between the master cylinder and the slave cylinder from a direct connection to a connection via the pump, there being a transitional position between the two end positions in which the pump is only partially active due to a bypass short-circuiting the inlet and outlet of the pump. In this third valve position, the pump thus may be activated gradually, i.e., depending on the pressure and the hydraulic system, and thus indirectly depending on the clutch pedal path. The pump at first has no effect, idling so to speak through the fully active bypass. With a further increase in pressure, the bypass is gradually closed; when the bypass is completely closed, the pressure increase between the master cylinder and slave cylinder due to the pump is at its maximum. With the bypass completely open, i.e., with the clutch engaged, the pump may also be turned off. A limit switch connected to the clutch pedal, for example, is the (only) electric control means, which is understood here not as electronic regulation as defined in the statement of the object of the present invention. 
   In a refinement of the hydraulic system, the valve includes at least one connection on the master cylinder end and one connection on the slave cylinder end as well as a piston which is axially displaceable in a housing, the piston having an end face which is hydraulically linked to the connection on the master cylinder end, and a back face opposite the end face, a spring being situated on the back face, exerting a force on the piston in the direction of the end face, both the piston and the housing having connecting channels which form the valves which open and close with the axial displacement of the piston. The connecting channels may be of any type, e.g., bores, recesses and the like, but they are preferably ring grooves on the outside surface of the piston and the inside surface of the housing. The piston is preferably a turned part having essentially circular cross sections as the outside surface; the bore to receive the piston is shaped accordingly. 
   In a refinement of the hydraulic system, it includes a first valve which is formed by an inlet ring groove of the housing and a piston ring groove of the piston. In addition, it preferably includes a second valve which is formed by an outlet ring groove of the housing and a piston ring groove of the piston and also preferably a third valve which is formed by an inlet ring groove of the housing and the piston ring groove of the piston. These ring grooves run preferably concentrically around the piston, with the different names being used merely to facilitate identification. 
   In a refinement of the hydraulic system, in a starting position of the piston, the first valve is open, the second is closed and the third valve is open. When the piston moves axially, the first valve closes first; when it moves further, the second valve opens and when it moves further, the third valve closes. In addition, it may be provided that during an axial movement of the piston in which the first valve closes, the second valve opens and the third valve is open. In addition, it may be provided that during an axial movement of the piston in which the third valve closes, the second valve is open and the first valve is closed. 
   In a refinement of the hydraulic system, the piston has on its back face a piston face which together with a stepped bore forms a piston/cylinder system which exerts a force on the piston in the direction of the spring force when pressure acts upon it. This forms a type of “counterpiston” to the end face and reduces the effect of its force. Likewise the stepped bore may be hydraulically connected to the connection on the slave cylinder end. This yields on the whole a closed system with the piston operating, for example, against atmospheric pressure; a complex gasket or an oil leakage drain, e.g., to the equalizing container of the hydraulic system, would have to be provided. 
   To limit both the pressure level of the ring piston in particular and the volume flow with the pump running in idling, it is possible to provide for a throttle valve to be situated upstream from the inlet to the pump. The inlet to the pump is connected to a ring piston space formed by the journal and the outside circumference of the piston. 
   The present invention also provides a hydraulic valve for use in a hydraulic system for motor vehicles in which at least one feature or a combination of features of the preceding description directed at a hydraulic valve is implemented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention are explained below on the basis of the accompanying drawings. 
       FIG. 1  shows a schematic diagram of a hydraulic system on the basis of an exemplary embodiment of a clutch release device; 
       FIG. 2  shows a sectional diagram of a valve according to the present invention; 
       FIG. 3  shows a schematic diagram of the hydraulic system having the valve according to the present invention with the clutch not being operated; 
       FIG. 4  shows a schematic diagram of the hydraulic system having the valve according to the present invention with the clutch being operated; 
       FIG. 5  shows an exemplary embodiment of a pump having a valve control according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a schematic diagram of a possible embodiment of a hydraulic system on the basis of a clutch release device  3  having a master cylinder  4  and a slave cylinder  5 . The master cylinder includes a housing  55  and a piston  56 , which is axially movable in the housing and borders a pressure chamber  57  filled with hydraulic fluid; on operation of master cylinder  4 , piston  56  is axially moved by a piston rod  13  acting on the piston, thereby applying pressure to the hydraulic fluid. The functioning of slave cylinder  5  is identical in principle. A valve block  2  is installed in a pressure medium line  15  connecting the master cylinder  4  and the slave cylinder  5  in the exemplary embodiment shown here and dividing this pressure medium line into a first line segment  11  and a second line segment  12 . It is self-evident that in other exemplary embodiments, valve block  2  may be situated on master cylinder  4  or on slave cylinder  5 . In addition, the function of a vibration filter may also be integrated into valve block  2  at the same time, e.g., as an anti-vibration unit. 
   Clutch release system  3  operates clutch  7  hydraulically by acting upon master cylinder  4  via a clutch pedal  14  which may be a foot pedal, an actuator, e.g., an electric actuator, or the like. Pressure is therefore built up in master cylinder  4  via a mechanical transmission  13 , building up a pressure in slave cylinder  5  via pressure medium line  15 , i.e. second line segment  12 , valve block  2 , and first line segment  11 . Slave cylinder  5  may be concentric with transmission input shaft  10  and supported axially on a transmission housing, applying the required clutch release force via a clutch release bearing on clutch  7 , e.g., on its release elements such as disk springs. To facilitate an understanding of how this functions,  FIG. 1  shows a slave cylinder  5  which operates a disengaging device via a clutch release mechanism  6  and is situated outside of the transmission bell housing, this disengaging device acting upon the clutch release mechanism via a piston situated in the slave cylinder housing and hydraulically connected to the master cylinder. To apply the disengagement force, the slave cylinder is fixedly mounted on the transmission housing or on some other part mounted on the housing. When clutch  7  is engaged, transmission input shaft  10  transmits the torque of internal combustion engine  8  to a transmission and then to the drive wheels of a motor vehicle. 
     FIG. 2  shows a diagram of valve block  2  in a sectional drawing. Valve block  2  includes a pump  16 , driven by an electric motor, for example, and a valve  17 . As shown in  FIG. 1 , valve block  2  is situated between the master cylinder  4  and the slave cylinder  5  in pressure medium line  15 .  FIG. 2  shows first line segment  11  which is connected to the slave cylinder  5  and second line segment  12  which is connected to master cylinder  4 . Pump  16  includes an inlet  18  and an outlet  19  connected to first line segment  11 . The pumping direction of pump  16  is from inlet  18  to outlet  19 , indicated by an arrow in  FIG. 2 . 
   Valve  17  includes a housing  20  having a bore  21 . Inside bore  21  there is an axially movable piston  22 , which is movable axially along axis  23 . Housing  20  is essentially a one-piece block sealed by a cover  24 , for example; alternatively, housing  20  may also be divided axially. A stop  44  in the form of a disk-shaped elevation is provided on the side of piston  22  facing away from spring  25 . Spring  25  of piston  22  is supported on a side of housing  20  provided with a stepped bore  26 . Stepped bore  26  has a smaller diameter than bore  21 ; a journal  27  having approximately the same outside diameter as the inside diameter of stepped bore  26  (play fit) may be inserted into stepped bore  26  against the pressure of spring  25 . Stepped bore  26  and journal  27  form a rear end pressure cylinder having journal  27  as the piston and stepped bore  26  as the cylinder bore. Multiple inlet lines and radial grooves belonging to the inlet lines are provided in housing  20 . A first inlet line  28  develops into a peripheral first inlet ring groove  29 . First inlet line  28  and first ring groove  29  are provided on the side of piston  22  facing journal  27 . A second inlet line  30  develops into a second inlet ring groove  31 . A first outlet line  32  develops into an outlet line ring groove  33 . A third inlet line  34  is connected to stepped bore  26 . First outlet line  32  and third inlet line  34  are joined by a connecting line  35 . First outlet line  32  is also directly connected to first line segment  11  at the intersection of first line segment  11  with connecting line  35 . Thus there is always a connection between first outlet line  32  and first line segment  11 . A third outlet line  36  is situated between first outlet line  32  and third inlet line  34 . Third outlet line  36  is connected by a throttle valve  37  and a hydraulic line  38  to the inlet  18  of pump  16 . A fourth inlet line  41  which is provided between third outlet line  36  and third inlet line  34  is connected to ring piston space  42  remaining between journal  27  and bore  21  and is also connected to inlet  18  of pump  16  via another hydraulic line  43 . 
   A first piston ring groove  39  and a second piston ring groove  40  are provided on piston  22 . First and second piston ring grooves  39 ,  40  connect different inlet lines  28 ,  30 ,  34 ,  41  to different outlet lines  32 ,  36 , depending on the axial position of piston  22 . To facilitate understanding, first and second inlet lines  28 ,  30  in  FIG. 2  are also labeled as G (referring to the inlet line on the master cylinder end), while first outlet line  32  is also labeled as N (referring to the connection on the slave cylinder end) and the third outlet line is labeled as P (referring to the connection on the pump end). 
   In the following discussion, the phrase “end facing away from the journal” in referring to  FIG. 2  is understood to refer to the side in the axial direction of piston  22  which is further away from journal  27  and thus closer to stop  44 . Accordingly, the “side facing the journal” is understood to be the side closer to journal  27  in the axial direction. 
   In  FIG. 2 , X 1  denotes the distance between groove side  45  of second piston ring groove  40  facing away from the journal and groove side  46  of second inlet line ring groove  31  facing the journal. This is also referred to below as control edge G-N. Similarly, X 2  denotes the distance between groove side  47  of first piston ring groove  39  facing away from the journal and groove side  48  of outlet line ring groove  33  facing the journal, hereinafter also referred to as control edge N-P. X 3  denotes the distance between groove side  50  of first inlet ring groove  29  facing away from the journal and groove  49  of first piston ring  39  facing the journal, hereinafter also referred to as control edge G-P. A 1  denotes the diameter of piston  22 , A 2  denotes the diameter of journal  27  and/or the inside diameter of stepped bore  26 , and X 4  denotes the axial path or distance of piston  22 . 
   The piston  22  includes an end face  51 , which is always hydraulically connected to second inlet line  30 , so the hydraulic pressure on the side of master cylinder  4  is always acting on this surface. A back face  58  which is opposite end face  51  is in the present exemplary embodiment designed to be stepped due to journal  27 . The terms “end face” and “back face” are understood here to refer to all conceivable surface forms oriented radially to the longitudinal axis of piston  22 . 
     FIG. 2  shows a neutral position of piston  22  in which piston  22  is pressed by spring  25  against stop  44 , and X 4  assumes its highest value. For quantities X 1 , X 2  and X 3  it holds that: X 3 &lt;X 1 &lt;X 2  or X 3 =X 1 &lt;X 2 . When X 1 &lt;X 2 &lt;X 3 , piston  22  is in an axial position itself to the hydraulic transmission path; this is not harmful but is not preferred. In other words, if X 4  is reduced, then first X 3  becomes zero and thus a connection is established between first inlet line  28  via first inlet ring groove  29  and first piston ring groove  39  to third outlet line  36 . If X 1 =X 3 , then at the same time X 1  becomes zero so that the connection between second inlet line  30  via second inlet ring groove  31  and piston ring groove  40  as well as outlet ring groove  33  to first outlet line  32  is interrupted. Thus the second line segment is switched from second inlet line  30  to first inlet line  28 . If X 3 &lt;X 1 , then both the first and second inlet lines are open over a small axial moving path of piston  22 . With a further reduction in X 4 , X 2  becomes zero and the connection between first outlet line  32  and third outlet line  36  is interrupted. 
   In this way, the inlet lines and outlet lines together with the ring grooves in housing  20  and piston  22  form three valves, a first valve GN, a second valve GP, and a third valve NP. The notation of the valves is derived from the connections that are blockable—first valve GN opens and closes hydraulic connection G-N, second valve GP opens and closes connection G-P, and third valve NP opens and closes connection N-P. If X 1  becomes zero, the connection between second inlet line  30  (thus G) and stepped bore  26  is interrupted. As long as this valve GN is open, the hydraulic pressure in second line segment  12  generates a force in the direction of spring  25  which is determined by cross section A 2  of journal  27 . If first valve GN is closed, this force which is additionally exerted on piston  22  is no longer exerted in the active direction of spring  25 . Third valve NP is formed by first piston ring groove  39  and outlet line ring groove  33 . This valve NP is closed as soon as X 2  becomes zero. This interrupts the connection between inlet  18  and outlet  19  of pump  16 . This connection forms a bypass B (N→P) for pump  16  when third valve NP is open. Second valve GP is formed by first piston ring groove  39  and first inlet line ring groove  29 . As soon as X 3  becomes less than zero, this third valve is opened so that a connection is established between second line segment  12  via first inlet line  28  to third outlet line  36  of pump  16 , so connection G to P is opened. Throttle valve  37  ensures a low pressure level in ring piston space  42 . Alternatively, ring piston space  42  could be connected, e.g., with an equalizing tank of the hydraulic system, through known blow valves to the piston space of master cylinder  4 . 
   The opening cross sections of the three valves formed by the ring grooves change continuously due to the movement of piston  22  so that depending on the position of piston  22  different effective flow-through areas of the connections between second inlet line  30  and stepped bore  26 , between first outlet line  32  and stepped bore  26 , and between first inlet line  28  and third outlet line  36  are achieved. 
   If slave cylinder pressure PN ( FIG. 3 ) is less than a threshold PSCH, then piston  22  is at stop  44 . The master cylinder  4  and the slave cylinder  5  are directly connected through first valve GN and slave cylinder pressure PN is equal to master cylinder pressure PG. At the same time, pump  16  operates via the bypass opened by third valve NP in a circulation system including first outlet line  32 , second valve GP, third outlet line  36 , hydraulic line  38 , inlet  18 , pump  16 , and outlet  19 . Connecting line  35  branching off between throttle valve  37  and inlet  18  exerts an additional force in the effective direction of spring  25  on piston  22  via the pressure in ring piston space  42  which is lower than slave cylinder pressure PN. Contrary to the active direction of spring  25 , a compressive force is exerted by master cylinder pressure PG on end face  51  (having area A 1 ). The condition described previously remains constant with an increasing slave cylinder pressure up to the moment when
 
 A   1   P   G   =A   2   P   N   +F   spring   (1)
 
   where A 1  and A 2  are the end faces of the piston, F spring  is the spring force of spring  25 . In this state P N  is equal to P G , so the following holds for pressure threshold P SCH : 
   
     
       
         
           
             
               
                 
                   P 
                   SCH 
                 
                 = 
                 
                   
                     F 
                     spring 
                   
                   
                     
                       A 
                       1 
                     
                     - 
                     
                       A 
                       2 
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   If the force of the spring is replaced in equation (1) with the help of equation (2), this yields 
   
     
       
         
           
             
               
                 
                   P 
                   G 
                 
                 = 
                 
                   
                     
                       
                         A 
                         2 
                       
                       
                         A 
                         1 
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           P 
                           N 
                         
                         - 
                         
                           P 
                           SCH 
                         
                       
                       ) 
                     
                   
                   + 
                   
                     P 
                     SCH 
                   
                 
               
             
             
               
                 ( 
                 3 
                 ) 
               
             
           
         
       
     
   
   These equations are always valid when the piston  22  is not in contact with the stop  44 , in which case the effective piston area A 1  is reduced by the area of stop  44 . At a given pedal path, PN is exactly as large as would be the case without servo support because this pressure is determined by the clutch characteristic. Equation (3) is equivalent to the threshold of the pedal force usually defined as
 
 F   2   −F   SCH   =k ( F   1   −F   SCH )  (4)
 
   where F 2  is the pedal force with servo support, F 1  is the pedal force without servo support, F SCH  is the threshold of the support and k&lt;1 is a reduction factor. In equation (4), k is equivalent to A 2 /A 1 . This means that the hydraulic system according to the present invention statically ensures the required servo support characteristic. 
   Instead of electronic regulation of the pump volume flow, in the hydraulic system and valve according to the present invention, a constant or variable (not controlled electronically) pump volume flow is used, divided between the pedal volume flow and the circulation volume flow of the bypass by automatic positioning of piston  22 . The automatic division is achieved due to the equilibrium of the piston between different pressure surfaces, namely surfaces A 1  and A 2 , and the spring force exerted by spring  25 . Therefore, the linear relationship according to equation (4) between the master cylinder pressure and the slave cylinder pressure is ensured. 
     FIGS. 3 and 4  illustrate the mechanism of operation of the hydraulic system according to the present invention, using the valve according to the present invention, on the basis of simplified schematic diagrams. Each figure shows master cylinder  4 , slave cylinder  5 , clutch pedal  14 , and equalizing tank  53 , first line segment  11 , second line segment  12 , pump  16 , spring  25 , a switchable power supply  54  for pump  16 , and valve  17 . Connections G, N and bypass B as well as surfaces A 1  and A 2  correspond to those in the exemplary embodiment of  FIG. 2 . For the sake of simplicity, valve  17  here is depicted as an open system on sides of spring  25 , which is indicated by a gasket  52  for sealing the hydraulic system with respect to the surroundings. Leaking hydraulic fluid may enter equalization tank  53 , for example. Here again, P G  and P N  denote the pressure on the master cylinder end and the slave cylinder end, respectively. 
     FIG. 3  shows the hydraulic system with the clutch not operated. The pump is short circuited via valve GN and bypass B and may thus be shut down by a switchable power supply  54 , for example, or may continue to operate continuously. The hydraulic fluid flows, i.e., the pressure, is transmitted past pump  16  through connection G, bypass B to connection N. 
     FIG. 4  shows the hydraulic system with the clutch operated. Due to the build-up of pressure in the system, the piston  22  is moved out of the resting position and valve GN is closed. Therefore, bypass B is closed and hydraulic fluid is transmitted only through pump  16  from master cylinder  4  to slave cylinder  5 . In the intermediate positions of the piston between the diagrams in  FIG. 3  and in  FIG. 4 , bypass B is always progressively closed so that the build-up of pressure due to pump  16  always has a progressively greater effect. 
     FIG. 5  shows an exemplary embodiment of a pump having valve control. This figure contains the two diagrams from  FIGS. 3 and 4 . 
   When the clutch is not operated, the pressure is transmitted from the master cylinder to the slave cylinder through bypass B. When the clutch is operated, bypass B is closed and the hydraulic fluid is transferred from master cylinder  4  to slave cylinder  5  via pump  16 . 
   LIST OF REFERENCE NUMERALS 
   
       
         1  hydraulic system 
         2  valve block 
         3  clutch release device 
         4  master cylinder 
         5  slave cylinder 
         6  clutch release mechanism 
         7  clutch 
         8  internal combustion engine 
         9  crankshaft 
         10  transmission input shaft 
         11  first line segment 
         12  second line segment 
         13  mechanical transmission 
         14  clutch pedal 
         15  pressure medium line 
         16  pump 
         17  valve 
         18  inlet 
         19  outlet 
         20  housing 
         21  bore 
         22  piston 
         23  axis 
         24  cover 
         25  spring 
         26  stepped bore 
         27  journal 
         28  first inlet line 
         29  first inlet line ring groove 
         30  second inlet line 
         31  second inlet line ring groove 
         32  first outlet line 
         33  outlet line ring groove 
         34  third inlet line 
         35  connecting line 
         36  third outlet line 
         37  throttle valve 
         38  hydraulic line 
         39  first piston ring groove 
         40  second piston ring groove 
         41  fourth inlet line 
         42  ring piston space 
         43  additional hydraulic line 
         44  stop 
         45  groove side of second piston ring groove  40  facing away from journal 
         46  groove side of second inlet line ring groove  31  facing journal 
         47  groove side of first piston ring groove  39  facing away from journal 
         48  groove side of outlet line ring groove  33  facing journal 
         49  groove side of first piston ring groove  39  facing journal 
         50  groove side of first inlet ring groove  29  facing away from journal 
         51  end face 
         52  sealing element 
         53  equalizing tank 
         54  switchable power supply for pump  16   
         55  housing 
         56  piston 
         57  pressure space 
         58  back side 
         59  pressure cylinder on back side 
         60  engine 
       GN first valve position 
       GP second valve position 
       NP third valve position 
       B bypass