Abstract:
A valve drive ( 1 ) for a cam-operated valve ( 2 ) of an internal combustion engine is provided, in which a closing force is applied to the valve ( 2 ) against the opening direction of the valve ( 2 ) by a valve spring ( 3 ). The valve drive includes a hydraulic force application device ( 4 ), with which a force can be applied directly or indirectly against the direction of the closing force onto the valve ( 2 ), and includes a piston ( 5 ) that is moveable in a displacement direction (R) relative to a cylinder ( 6 ) of the force application device ( 4 ) by the introduction of hydraulic fluid into the pressure chamber ( 7 ) formed between the piston ( 5 ) and the cylinder ( 6 ). The piston ( 5 ) can move relative to the cylinder ( 6 ) from a first end position (A) into a second end position (B). In order to achieve improved damping of the piston in the region of the end positions, a braking or damping system ( 8, 9 ) is provided with which the movement of the piston ( 5 ) can be braked relative to the cylinder ( 6 ) when a predetermined relative position between the piston ( 5 ) and cylinder ( 6 ) is reached and until one of the end positions (A, B) is reached.

Description:
BACKGROUND 
   The invention relates to a valve drive for a cam-operated valve of an internal combustion engine, in which a closing force is applied to the valve against the opening direction of the valve by a valve spring, with a hydraulic force application device, with which a force can be applied directly or indirectly onto the valve against the direction of the closing force, in that a piston of the force application device is moved relative to a cylinder of the force application device by introducing hydraulic fluid into the pressure chamber formed by the piston and the cylinder in a displacement direction, wherein the piston can be moved relative to the cylinder from a first end position to a second end position. 
   Valve drives of this type are known in the state of the art, for example, from DE 101 56 309 A1 and from U.S. Pat. No. 4,796,573. They are used to generate additional valve lifting in addition to the opening lift of the valve that is dependent on the shape of the cam of a camshaft. For this purpose, a force application device is pressurized with hydraulic fluid in such a way that the valve lifting is, to a large extent, variable. 
   In DE 102 42 866 A1, which also belongs to this class, such a variable valve drive is provided, such that the valve lifting caused by the cams of the camshaft can be minimized by a control valve by shutting off hydraulic fluid from the control chamber of the force application device, whereby the control chamber can be connected to hydraulic fluid at high pressure. 
   The valve timing device known from EP 0 196 441 B1 has a valve piston, which has a stepped section in the form of an annular radial shoulder on one end. Through a special configuration of the valve piston, during the shut-off process, thus when compressed fluid from the working chamber of the force application device is shut off and therefore when the valve piston returns, an annular gap in a stepped and continuously tapering configuration is produced, whereby a pressure can be established, which generates end position damping of the valve piston. 
   Although an essentially variable influence on the valve lifting is already possible with the known valve drives, wherein damping of the movement of the force application device can also be realized in the end position, the known systems have a few disadvantages. 
   The targeted path-controlled braking of the piston of the force application device is not possible relative to the cylinder for a few solutions. Instead, as, for example, in U.S. Pat. No. 4,796,573, pressurization with hydraulic fluid is necessary for braking the piston, wherein the dynamics of the braking process are produced from the hydraulic behavior of the hydraulic elements used there. 
   Furthermore, in some of the known solutions, there is a relatively slow acceleration of the piston from the damping end position, which is disadvantageous. 
   The stepped pistons also known for targeted braking of the piston cause considerable production problems from time to time or have a complicated overall structure for the force application device as a result, which makes the systems costly. 
   If maximum stroke limiting through hydraulic shutoff is used, such force application devices have the disadvantage that the shutoff is burdened with losses, whereby the efficiency of the device is decreased. 
   SUMMARY 
   Therefore, the present invention is based on the object of improving a valve drive of the type named above, so that the listed disadvantages are prevented. Therefore, the force application device distinguishes itself in that it or its components can be produced easily in large batches economically. Furthermore, the device should enable fast acceleration of the piston of the force application device from the end position, whereby the dynamic response of the system should be high. Furthermore, in terms of an optional hydraulic lash adjustment function, there should be freedom from feedback, i.e., the end position damping or braking should have no effect thereon. 
   This object is met according to the invention in that the movement of the piston relative to the cylinder can be braked when a predetermined relative position is reached between the piston and cylinder and until one of the end positions is reached. 
   Then, when a defined relative displacement of the piston of the force application device to the cylinder of the device is reached, the braking or damping process is triggered, wherein it requires no startup or shutoff from the outside. 
   A preferred configuration of the invention provides that the braking is provided by a braking piston, which is supported so that it can move relative to the piston of the force application device in the displacement direction and can move relative to the cylinder in the displacement direction, wherein an oil chamber is formed between the piston and the braking piston, which is sealed from the pressure chamber formed between the piston and the cylinder, and wherein there are closing means, which open a fluid opening after exceeding a predetermined displacement of the braking piston relative to the cylinder and close this opening again after falling below this displacement, whereby a fluid connection between the pressure chamber formed between the piston and cylinder and the oil chamber can be created or blocked. 
   This end position damping or braking is used preferably for each end position of the force application device, in which it is not pressurized with hydraulic fluid. 
   For this solution, it has proven especially advantageous that the braking piston is supported in a preferably cylindrical recess in the piston. Between the pressure chamber formed between the piston and cylinder and the oil chamber formed between the piston and braking piston, there can be an aperture, which permits an overflow of hydraulic fluid between the oil chamber and pressure chamber, especially an outflow of fluid possibly only in the direction from the oil chamber to the pressure chamber. Here, the aperture can have a constant aperture cross section or else also a varying aperture cross section over the displacement path between the piston and braking piston. 
   An especially precise triggering of the damping or braking process of the piston relative to the cylinder is enabled, if, according to the refinement, the closing element is formed by a pin, which is connected rigidly to the cylinder and which interacts with the fluid opening in the braking piston. The piston, braking piston, and pin can be arranged concentric to a longitudinal axis of the force application device. Furthermore, preferably a spring element is arranged between the piston and braking piston, which presses the braking piston away from the piston. Finally, limiting means, which limit the displacement of the braking piston relative to the piston, have proven advantageous. 
   An alternative possibility for reducing the invention to practice is provided in that the braking of the movement of the piston is provided by a damping plate arranged on the piston, which can move into a damping chamber formed in the cylinder in one of the end positions for the movement of the piston relative to the cylinder. 
   The damping chamber can be in fluid connection with the pressure chamber formed between the piston and cylinder or can be a component of this pressure chamber. 
   For influencing the braking characteristics, the damping chamber can have a radially outer, conical side wall. The damping plate can be pressed against an axial stop on the piston by a spring element. It is especially preferred if, in the position contacting the axial stop, the damping plate opens an overflow channel between the pressure chamber formed between the piston and cylinder and the damping chamber, wherein the damping plate closes the overflow channel in the state pressed away from the axial stop. 
   The force application device is preferably arranged between a cam and the valve; in a preferred configuration, the force application device is part of a valve rocker lever support part for supporting a valve rocker, especially a cam operated finger lever, operating the valve. 
   With the proposed configuration of a valve drive, a force application device that can be produced easily in terms of manufacturing can be created, which can be realized cost-effectively in series production. 
   The force application device enables a precisely controlled damping or braking of the piston relative to the cylinder when a defined relative position of the two components to each other is reached. This also provides maximum lift limiting for the piston movement. 
   Furthermore, the force application device is distinguished by fast acceleration of the piston from the damping end positions. If the system is combined with hydraulic lash adjustment, the force application device has no effects on the compensation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, exemplary embodiments of the invention are shown. They show: 
       FIG. 1  a finger lever drive shown partially in cross section, with the force application device, finger lever, camshaft, and valve; 
       FIG. 2  the same illustration as in  FIG. 1  with an alternative hydraulic controlling of the force application device; 
       FIG. 3  an enlarged illustration of the force application device, shown in cross section; 
       FIG. 4  a further enlarged view of the bottom right area of the force application device according to  FIG. 3 ; 
       FIG. 5  an alternative configuration of the force application device in the illustration according to  FIG. 3  shown in cross section; and 
       FIG. 6  another alternative configuration of the force application device, shown in cross section, wherein only its bottom half is shown. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIGS. 1 and 2 , the basic configuration of the valve drive and its hydraulic control is shown for a finger lever drive. The illustrated embodiment provides a finger lever drive for a finger lever  26 , which is supported so that it can pivot in the cylinder head of an internal combustion engine. On one side, the finger lever  26  presses on a valve  2 , which has a valve seat  28  for sealing. The valve  2  is connected to a valve spring  3 , which biases the valve  2  in the closing direction. A cam  24  of a camshaft operates the finger lever  26 , i.e., the cam  24  applies pressure to a contact point  27  of the finger lever  26 , such that the valve  2  is moved. 
   In order to achieve a targeted movement of the valve in addition to the movement of the valve  2  dependent on the cam shape, a force application device  4  is provided on the other side of the finger lever  26 , namely at the site of the finger lever support part  25 . This is charged with oil at the motor oil pressure p M  (shown schematically by the arrow) and charged with hydraulic fluid (oil) under high pressure p H.    
   For this purpose, in  FIG. 1  a 3/3 port directional control valve  29  is provided. The valve  29  controls the input of hydraulic fluid under high pressure p H  via an oil pressure line  30  into the force application device  4 . Alternatively, in  FIG. 2  it can be seen that the force application device  4  can be pressurized by two 2/2 port directional control valves  31  and  32 . 
   The configuration of the force application device  4  is sketched for three different embodiments in  FIGS. 3 and 4  or  5  or  6 . 
   The force application device  4  has a cylinder  6 , which, in the embodiment according to  FIGS. 3 and 4 , has a guide sleeve  33 , which is connected with a positive fit and pressure-tight to an outer housing  34 ; the guide sleeve  33  has a one-sided collar, which acts as an axial stop for joining the parts  33  and  34 . 
   In the cylinder  6 , there is a piston  5  which can be moved relative to the cylinder  6  in the displacement direction R when the pressurization is performed with high pressure oil (see  FIGS. 1 and 2 ). Here, the high pressure oil is introduced into the pressure chamber  7  formed between the piston  5  and cylinder  6 . 
   Here, the piston  5  can assume two end positions A and B in the cylinder  6 . The first, bottom end position is designated with A and sketched in  FIGS. 1 ,  2 ,  3 ,  4 , and  5 . The second, top end position B is shown in  FIG. 6 . 
   In order to achieve end position damping or braking both in the bottom and also in the top end position A, B, the force application device  4  has a system  8  for braking the movement of the piston  5  in the bottom end position A and a system  9  for braking the movement of the piston  5  in the top end position B. 
   The braking system  8  is formed from a cup-shaped braking piston  10 , which is arranged concentrically in a cylindrical recess  14  in the piston  5 , which is movable in the displacement direction R relative to the piston  5 . An oil chamber  11 , which is sealed from the pressure chamber  7 , is formed between the braking piston  10  and the piston  5 . The fit between the cylindrical recess  14  and the braking piston  10  is selected accordingly. The displacement movement of the braking piston  10  relative to the piston  5  is limited by limiting means  17  (spring ring and groove). A spring element  16  in the shape of a helical spring applies a force on the braking piston  10 , so that this is pressed away from the piston  5 , wherein this movement is limited by the limiting means  17 . 
   In the braking piston  10 , there is a fluid opening  13 , which can be opened or closed by closing element  12  in the form of a pin as a function of the relative position of the braking piston  10  to the cylinder  6 , concentric to the longitudinal axis of the force application device  4 . Here, the pin  12  is anchored rigidly in the cylinder  6 . Optionally the pin  12  can be completely eliminated or formed as a cone or sphere through suitable shaping of the contact surface between the braking piston  10  and the cylinder  6 . 
   As can be seen further in  FIG. 4 , an aperture  15  is provided between the oil chamber  11  and the pressure chamber  7 , which enables hydraulic fluid to flow from the oil chamber  11  into the pressure chamber  7 . 
   If hydraulic fluid is input via the oil pressure line  30  (see  FIGS. 1 and 2 ) into the pressure chamber  7 , the piston  5  moves in the displacement direction R upwards out of the bottom end position A. Here, a negative pressure is produced in the oil chamber  11 , because the braking piston  10  is pulled away from the stationary pin  12 . In order to prevent cavitation due to large negative pressures, an annular gap is provided between the top edge  35  of the braking piston  10  and the piston  5 , whose volume corresponds at least to the volume of the pin  12  pulled from the fluid opening  13 . Therefore, a relative movement between the piston  5  and braking piston  10  is possible. 
   As soon as the pin  12  is pulled completely from the fluid opening  13  of the braking piston  10 , the oil chamber  11  can be expanded by the spring element  16 , in that now oil is fed through the now open fluid opening  13 . This expansion is limited by the limiting means  17 . 
   Through the displacement of the piston  5  directed upwards in the displacement direction R, the valve  2 , independent of the influence of the cam  24 , is opened. To close the valve  2 , the return path  36  is opened by the directional control valve  29  (see  FIG. 1 ) or  32  (see  FIG. 2 ), so that the hydraulic fluid can flow back into a storage tank  37 . Here, the piston  5  moves downwards due to the force acting on the finger lever  26  and stored in the valve spring  3 . 
   In the course of the downwards movement, the pin  12  is inserted into the fluid opening  13  in the floor of the braking piston  10 , whereby the fluid opening is closed. Starting at the time of receiving the contact of the braking piston  10  with the cylinder  6 , the braking piston  10  moves relative to the piston  5 , whereby oil is forced from the oil chamber  11  and fed via the aperture  15  (see  FIG. 4 ) to the pressure chamber  7 . The pressure build-up in the oil chamber  11  brakes the valve  2  and damps the sliding in the valve seat  28 . 
   Thus, the pin  12  replaces an expensive and space-intensive non-return valve of a conventional type, e.g., a spring-loaded ball non-return valve. 
   In the piston  5 , there is an oil passage  38  in order to equalize pressure differences between the volume spaces bordering each other. 
   With the described solution, there is the possibility of setting a defined valve seat speed in the bottom end position A or a desired damping or braking of the movement of the valve  2  when this position is reached. 
   Alternative configurations of the invention are shown in  FIGS. 5 and 6 . For the embodiment according to  FIG. 5 , the braking piston  10  surrounds the piston  5  from the outside. Here, the pin  12  is arranged in the cylinder head  39 . Therefore, it is possible to embody the guide sleeve  33  (see  FIG. 3 ) and the outer housing  34  as a one-piece component  6  (see  FIG. 5 ), whereby the manufacturing costs can be reduced. 
   The aperture  15  (see  FIG. 4 ) has linear damping characteristics due to the fixed aperture cross section. It offers the advantage of damping essentially decoupled from the oil viscosity. If the damping or braking effect is to be freely shaped as a function of the displacement path, an aperture  15 , as shown in  FIG. 6 , can be used, which has a varying throttling cross section over the displacement path. 
   For the pressurization of the pressure chamber  7 , if the piston  5  moves upwards and approaches its top end position B, a top end position damping of the piston  5  is performed by the means  9  shown in  FIGS. 3 ,  4 , and  5 . Thus, damping or braking of the opening movement of the piston  5  is performed when the maximum valve lifting is reached. 
   The damping or braking is performed as soon as a damping plate  18  arranged concentrically around the piston  5  enters a cylindrical and/or conical damping chamber  19  due to the upwards movement of the piston  5 . Here, the damping chamber  19  has a side wall  20 , which has the shown shape. 
   The damping plate  18  is pressed against an axial stop  22  on the piston  5  by a spring element  21 . The spring element  21  is supported against a counter support  40  with a U-shaped cross section. 
   As mentioned, the damping or braking of the movement of the piston  5  begins as soon as the damping plate  18  enters the damping chamber  19  due to the upwards movement of the piston  5 . As soon as the flow resistance rising due to the narrowing throttle gap exceeds the spring force of the spring element  21 , the damping plate  18  is pressed away from the axial stop  22  and against the counter support  40 . The flat surfaces of the two components  18  and  40  seal the damping chamber  19 , in that an overflow channel  23  that is opened when the damping plate  18  contacts the piston  5  is closed. Due to the volume flow reduced by the throttle gap, the lifting of the piston  5  is damped. 
   Instead of a narrowing throttle gap, a damping device with aperture characteristics can also be provided. 
   After reaching the top end position B and opening the return path  36  (see  FIGS. 1 and 2 ) due to corresponding switching of the valves  29 ,  31 ,  32 , the piston  5  is moved downwards by the valve spring  3  acting via the finger lever  26 . 
   In order to achieve acceleration that is as quick as possible and that is free from losses in flow from the top end position B, the spring element  21  moves the damping plate  18  in the course of the upwards movement against the axial stop  22 . In this way, the overflow channel  23  is opened again, so that the hydraulic fluid can flow unhindered into the damping chamber  19 . 
   The top end position damping simultaneously takes over the function of a mechanical maximum stroke limiter. Therefore, flow losses are prevented, like those that occur in conventional system with stroke limiting by hydraulic shut-off. 
   Overall, end-position damping that can be realized easily on both ends of the movement of the piston  5  of the force application device  4  is realized. 
   In the exemplary embodiment, the use of the force application device  4  was explained for a finger lever drive through hydraulic displacement of the finger lever support. It is also possible to use of the inventive concept in a tappet drive or in the support for a rocker arm. 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               List of reference symbols 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
                1 
               Valve drive 
             
             
                 
                2 
               Valve 
             
             
                 
                3 
               Valve spring 
             
             
                 
                4 
               Force application device 
             
             
                 
                5 
               Piston 
             
             
                 
                6 
               Cylinder 
             
             
                 
                7 
               Pressure chamber 
             
             
                 
                8 
               System for braking the movement of the piston 
             
             
                 
                9 
               System for braking the movement of the piston 
             
             
                 
               10 
               Braking piston 
             
             
                 
               11 
               Oil chamber 
             
             
                 
               12 
               Closing means 
             
             
                 
               13 
               Fluid opening 
             
             
                 
               14 
               Cylindrical recess 
             
             
                 
               15 
               Aperture 
             
             
                 
               16 
               Spring element 
             
             
                 
               17 
               Limiting means 
             
             
                 
               18 
               Damping plate 
             
             
                 
               19 
               Damping chamber 
             
             
                 
               20 
               Side wall of the damping chamber 
             
             
                 
               21 
               Spring element 
             
             
                 
               22 
               Axial stop 
             
             
                 
               23 
               Overflow channel 
             
             
                 
               24 
               Cam 
             
             
                 
               25 
               Finger lever support part 
             
             
                 
               26 
               Finger lever 
             
             
                 
               27 
               Active position 
             
             
                 
               28 
               Valve seat 
             
             
                 
               29 
               3/3 port directional control valve 
             
             
                 
               30 
               Oil pressure line 
             
             
                 
               31 
               2/2 port directional control valve 
             
             
                 
               32 
               2/2 port directional control valve 
             
             
                 
               33 
               Guide sleeve 
             
             
                 
               34 
               Outer housing 
             
             
                 
               35 
               Edge 
             
             
                 
               36 
               Return path 
             
             
                 
               37 
               Storage tank 
             
             
                 
               38 
               Oil passage 
             
             
                 
               39 
               Cylinder head 
             
             
                 
               40 
               Counter bearing 
             
             
                 
               R 
               Displacement direction 
             
             
                 
               A 
               First (bottom) end position 
             
             
                 
               B 
               Second (top) end position 
             
             
                 
               p M   
               Motor oil pressure 
             
             
                 
               p H   
               High pressure