Abstract:
A hydraulic actuator for gas exchange valves of internal combustion engines is disclosed, in which the lifting of the gas exchange valve from the valve seat is effected with great force. The opening motion of the gas exchange valve then ensues at reduced force. Upon closure of the gas exchange valve, the gas exchange valve is braked before striking the valve seat, so that operation of the gas exchange valve with little wear and little noise can be achieved.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 USC 371 application of PCT/DE02/02791 filed on Jul. 30, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a hydraulic actuator for a gas exchange valve for internal combustion engines. 
     2. Description of the Prior Art 
     The opening and closing of the gas exchange valve should be as fast as possible, in order to minimize flow losses from the gas exchange valve either when the combustion air is aspirated or upon expulsion of the exhaust gases from the combustion chamber. 
     The overpressure intermittently prevailing in the combustion chamber of the engine presses the gas exchange valve into the valve seat. Because of this overpressure, opening the gas exchange valves requires an increased expenditure of force for lifting the gas exchange valve, in particular the outlet valve, from the valve seat. Once the gas exchange valve has lifted from the valve seat, the pressure in the combustion chamber drops sharply, so that the force needed to open the gas exchange valve is correspondingly less. 
     Upon closure of the gas exchange valve, it must also be noted that the speed at which the valve plate of the gas exchange valve strikes the valve seat should not be excessive. If that speed is too high, unwanted noise and increased wear occur when the valve plate strikes the valve seat. 
     The object of the invention is to furnish a hydraulic actuator for a gas exchange valve which can exert a strong force at the onset of the opening motion on the gas exchange valve, which enables fast control motions of the gas exchange valve, and in which the gas exchange valve strikes the valve seat at low speed. 
     According to the invention, this object is attained by a hydraulic actuator for a gas exchange valve of an internal combustion engine, having a cylinder bore, having a piston, and having an annular piston, the piston and the annular piston being guided in the cylinder bore, and the piston, annular piston and cylinder bore define a first chamber in the axial direction whose volume increases when the actuator opens the gas exchange valve, and the annular piston and the cylinder bore define a second chamber in the axial direction whose volume decreases when the actuator opens the gas exchange valve, and the piston and the cylinder bore define a third chamber whose volume decreases when the actuator opens the gas exchange valve, and having a device for limiting the volumetric decrease of the second chamber. 
     SUMMARY AND ADVANTAGES OF THE INVENTION 
     In the hydraulic actuator of the invention, at the onset of the opening motion of the gas exchange valve, a strong hydraulic force is transmitted by the actuator to the gas exchange valve, so that despite the contrary pressure on the valve plate of the gas exchange valve from the combustion chamber, the gas exchange valve can be lifted securely and quickly from the valve seat. As soon as the force needed to actuate the gas exchange valve has decreased, for instance because there is no longer any substantial contrary pressure in the combustion chamber, the annular piston is no longer moved onward, and consequently only a lesser hydraulic force is now exerted on the piston of the actuator, and this lesser force is transmitted in turn to the gas exchange valve. With the reduction in the hydraulic force, the energy required to adjust the actuator piston is also reduced, so that the overall energy required for valve control of the engine drops. Simultaneously with the reduction in this force, the adjusting speed of the gas exchange valve also varies. Finally, upon closure of the gas exchange valve, braking of the gas exchange valve by the hydraulic actuator of the invention can be achieved before the gas exchange valve strikes the valve seat of the engine. This reduces the wear to the valve seat and gas exchange valve and also lessens the noise produced by the valve control of the engine. 
     The onset of the braking operation of the gas exchange valve upon its closure is moreover independent of production tolerances in the gas exchange valve and of the temperature-caused changes in length that always exist in internal combustion engines because of thermal expansion. With the actuator of the invention, highly stable operation of the engine can therefore be achieved and is affected by neither temperature expansions nor production tolerances. 
     In a variant of the invention, it is provided that the piston has a plunge cut; that the annular piston has a stepped center bore with one larger diameter and one smaller diameter; and that the annular piston can be slipped by the larger diameter of the center bore onto the piston, so that the ratio of the actuating forces of the actuator upon opening of the gas exchange valve and during the remaining adjusting motion is adjustable in a simple way. 
     This effect can be further enhanced by providing that the diameters of the piston on both sides of the plunge cut are different; and that the annular piston can be slipped onto the larger diameter. 
     In a further feature of the invention it is provided that the device for limiting the volumetric reduction of the second chamber is a pressure reservoir that is in communication with the second chamber and that has a piston; and that the travel of the piston is limitable, so that the annular piston can be arrested in a simple way by hydraulic means. Since the pressure reservoir does reach the high temperatures of the gas exchange valve and the cylinder head of the engine, the position in which the annular piston is arrested after the gas exchange valve has opened is independent of the thermal expansions of the gas exchange valve and of the cylinder head. 
     Further features of the invention provide that the pressure reservoir is a spring reservoir or a gas reservoir, and/or that the travel of the piston is limitable by a stop, in particular an adjustable stop, so that the actuator of the invention can be adjusted simply. 
     Further features of the invention provide that the first chamber can be made to communicate with a pump via a first switching valve; that the second chamber can be made to communicate with an oil pump via a second switching valve; and that the third chamber is acted upon by the feed pressure of the pump, so that by the actuation of two switching valves, the gas exchange valve can either be opened or closed by the hydraulic actuator of the invention, and the increased force upon liftoff of the gas exchange valve from the valve seat and the slowing down of the gas exchange valve before it strikes the valve seat can be realized automatically by the hydraulic actuator of the invention. 
     Separate triggering of the hydraulic actuator for that purpose is unnecessary. This makes the work of the control unit required for triggering the actuator easier, and makes the hydraulic actuator of the invention robust and insensitive to external factors. 
     The action according to the invention of the actuator is further reinforced by the provision that the first chamber and the second chamber are hydraulically in communication with one another via a throttle, in particular an adjustable throttle, and/or that a check valve is provided between the second chamber and the first chamber and blocks the hydraulic communication from the first chamber to the second chamber. The throttle has a definitive influence on the braking of the gas exchange valve before it strikes the valve seat. 
     In an advantageous embodiment of the invention, the device for limiting the volumetric decrease in the second chamber has a shutoff valve which is in communication with an opening in the second chamber and which in one switching position closes the opening and in its other switching position opens it to allow fluid to flow out. With the closure of the shutoff valve, the annular piston is fixed, so that the instant of closure of the shutoff valve defines the stroke length of the annular piston. The instant of onset of the braking action upon closure of the gas exchange valve is in turn dependent on the stroke length of the annular piston; this braking action ensues earlier with a longer stroke of the annular piston and later with a shorter stroke. Thus by means of the shutoff valve, the onset of braking can be adjusted independently of production tolerances or material expansions caused by temperature fluctuations. 
     In an advantageous embodiment of the invention, the shutoff valve is not used as an additional component unit; instead, its function is allocated to the second switching valve, which is required anyway to initiate the closing operation of the gas exchange valve. With the omission of the shutoff valve and by dispensing with the above-described pressure reservoir for the device for limiting the volumetric decrease in the second chamber, the construction costs for valve control are reduced. 
     In an advantageous embodiment of the invention, between the first chamber and the throttle disposed between the two chambers for varying the braking behavior of the actuator piston and thus of the gas exchange valve, a flow-controlled valve is provided which is embodied such that it is closable by the fluid flowing to the first chamber. This has the advantage that in the initial phase of the stroke of the actuator piston, in which both switching valves are open, fluid from the first switching valve cannot flow directly via the throttle into the hydraulic relief chamber or oil sump. It is true that if the throttling action of the throttle is strong, this flow-controlled valve can be dispensed with, since only slight quantities of fluid flow out via the throttle; however, if there is a relatively large throttle opening for the sake of attaining an only slight braking action at the gas exchange valve, then the flow-controlled valve is indispensable for blocking off the throttle, if major leakage is to be avoided. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is explained in further detail in the ensuing description of exemplary embodiments, taken in conjunction with the drawings, in which: 
     FIG. 1 is a schematic illustration of a longitudinal section through a hydraulic actuator of the invention, with its hydraulic connection; 
     FIG. 2, a longitudinal section through the actuator of FIG. 1 in three different positions; 
     FIGS. 3 and 4, respective fragmentary longitudinal sections through the actuator of FIG. 1 with a variously modified hydraulic connection; and 
     FIGS. 5 and 6, respective longitudinal sections through a flow-controlled valve of FIG. 4, in the open state (FIG. 5) and in the closed state (FIG.  6 ). 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows an exemplary embodiment of a hydraulic actuator with a housing  1  in longitudinal section. The housing  1  has a stepped cylinder bore  3 . To simplify production, a sleeve  5  is press-fitted into the housing  1 , and its inner bore defines part of the stepped cylinder bore  3 . In the region of the sleeve  5 , an annular piston  7  and a piston  9  are guided in the cylinder bore  3 . In the position of the piston  9  as shown in FIG. 1, the gas exchange valve, not shown, is closed. 
     The cylinder bore  3 , piston  9  and annular piston  7  define a first chamber  13  in the direction of a longitudinal axis  11  of the piston  9 . So that no liquid or fluid can escape between the cylinder bore  3  and the piston  9 , a first sealing ring  15  is disposed on the left-hand end, in terms of FIG. 1, of the first chamber  13 . 
     The piston  9  has a plunge cut  17 . The diameters of the piston  9  on opposed sides of the plunge cut  17  are of different sizes. On the side toward the sealing ring  15 , the piston  9  has a smaller diameter d 1 , and on the other end of the plunge cut  17 , the piston  9  has a larger diameter d 2 . 
     The annular piston  7  is disposed between the sleeve  5  and the piston  9 . The annular piston  7  is fitted into the cylinder bore  3  in such a way that on the one hand it is displaceable in the axial direction, and on the other, a good sealing action is attained between the cylinder bore  3  and the annular piston  7 . The annular piston  7  has a stepped center bore  19 , with one smaller diameter d 3  and one larger diameter that is the same size as d 2 . The fit between the annular piston  7  and the larger diameter d 2  of the piston  9  is likewise selected such that the annular piston  7  and the piston  9  are movable relative to one another in the axial direction, yet nevertheless a good sealing action is achieved. 
     On the right-hand side, in FIG. 1, of the annular piston  7 , the cylinder bore  3  and the annular piston  7  define a second chamber  27 . In this region, the cylinder bore  3  has a diameter d 4 , which is equal to the outer diameter of the annular piston  7 . The piston  9 , on its right-hand end in terms of FIG. 1, has a shoulder with the diameter d 5 . 
     On the right-hand end, in FIG. 1, of the cylinder bore  3 , the annular gap between the cylinder bore  3  and the piston  9  is bridged by a second sealing ring  21  and is sealed off from the environment. On this end of the piston  9 , the shaft  23  of a gas exchange valve, shown only in part, is connected by positive engagement to the piston  9 . 
     Between the shoulder of the piston  9  having the diameter d 5  and the cylinder bore  3  having the diameter d 2 , there is a third chamber  25 , which is sealed off from the environment by the sealing ring  21 . The annular piston  7 , the part of the cylinder bore  3  having the diameter d 4 , and the piston  9  define the second chamber  27 . The first chamber  13  can be made to communicate hydraulically with a pump  31  via a first switching valve  29 . The first switching valve  29  can be embodied for example as an electrically actuated magnet valve. 
     The pump  31  permanently subjects the third chamber  25  to the feed pressure that it generates. 
     By means of a second switching valve  33 , embodied for example as an electrically actuated magnet valve, a hydraulic communication can be established between the second chamber  27  and a relief chamber or oil sump  35 . A check valve  39  is disposed in a line  37  that connects the second chamber  27  and the second switching valve  33 . A hydraulic reservoir  41  is connected between the check valve  39  and the second chamber  27 . The hydraulic reservoir  41  has a piston  43 , which moves counter to the force of a spring  45  when the pressure exerted on the face end of the piston  43  remote from the spring  45  is high enough. This pressure is equal to the pressure in the line  37 . The travel of the piston  43  counter to the force of the spring  45  is limited by a stop  47 , which may also be embodied adjustably. Between the first chamber  13  and the second chamber  27 , a hydraulic communication is provided in which an adjustable throttle  49  is disposed. 
     When the first chamber  13  and the third chamber  25  are acted upon by the feed pressure of the pump  31 , which is the case when the first switching valve  29  is open, then various hydraulic forces, which will now be described, act on the piston  9 : 
     The diameter d 4  of the cylinder bore  3 , the annular piston  7 , and the right-hand side, in FIG. 1, of the plunge cut  17  form a first annular face A 1  with an outer diameter d 4  and an inner diameter d 6 , the latter being equivalent to the inner diameter of the plunge cut  17 . The pressure of the hydraulic fluid, located in the first chamber  13  and acting on the first annular face A 1 , seeks to move the piston  9  to the right. The resultant force is responsible for the opening of the gas exchange valve, not shown. 
     The shoulder on the right-hand side, in FIG. 1, of the plunge cut  17 , which is defined by the diameters d 2  and d 6 , will hereinafter also be called the second annular face A 2 . 
     The hydraulic force exerted on the first annular face A 1  is reduced by the hydraulic forces acting on a third annular face A 3  and a fourth annular face A 4 . 
     The third annular face A 3  is defined by the shoulder in the piston  9  that is formed by the diameter d 1  of the piston  9  and by the diameter d 6  of the plunge cut  17 . The hydraulic fluid located in the first chamber  13  exerts a force toward the left in FIG. 1 on the third annular face A 3 . 
     The fourth annular face A 4  is defined by a shoulder  51  of the piston  9  in the region of the third chamber  25 . The shoulder  51  is formed by the diameter d 2  and the diameter d 5  of the piston  9 . The fourth annular face A 4  always exerts a force acting counter to the opening direction on the piston  9 , since as already noted, the third chamber  25  is always subjected to the feed pressure of the pump  31 . 
     Since the first annular face A 1  is larger than the third annular face A 3  and the fourth annular face A 4 , the piston  9  moves to the right when the first chamber  13  is subjected to the feed pressure of the pump  31 . The annular piston  7  transmits the hydraulic force exerted upon it to the piston  9 , via the shoulder of the stepped center bore  19  of the annular piston. The motion of the piston  9  to the right in FIG. 1 results in the opening of the gas exchange valve, not shown. 
     When the annular piston  7  and the piston  9  move to the right in terms of FIG. 1, the volume of the second chamber  27  decreases. Since the second switching valve  33  is closed, the fluid positively displaced to the right from the second chamber  25  by the motion of the annular piston  7  and piston  9  can flow only into the hydraulic reservoir  41 . The hydraulic fluid that flows into the hydraulic reservoir  41  moves the piston  43  counter to the spring  45 , until the piston  43  rests on the stop  47 . 
     Once the piston  43  rests on the stop  47 , no further hydraulic fluid can flow out of the second chamber  27  into the hydraulic reservoir  41 , and the result is that the volume of the second chamber  27  remains constant. This means nothing more than that the annular piston  7  can no longer move farther to the right. As a consequence, the hydraulic force that moves the piston  9  to the right decreases, since now only the hydraulic force acting on the second annular face A 2  is available for opening to the gas exchange valve, not shown. 
     The hydraulic forces described above, acting on the third annular face A 3  and the fourth annular face A 4  and which seek to move the piston to the left, that is, counter to the opening motion, remain unchanged. As a result, the opening force acting on the gas exchange valve, not shown, decreases once the gas exchange valve has lifted from the valve seat, not shown. 
     In FIGS. 2 a ,  2   b  and  2   c , various stages in the opening motion and closing motion are shown, which are intended to illustrate what has been said above. In order not to overcomplicate the drawing, not all the reference numerals of FIG. 1 have been repeated in FIG.  2 . 
     In FIG. 2 a , the actuator is shown in a position in which the gas exchange valve is closed, and the full opening force is available. 
     In FIG. 2 b , the state is shown in which the volume of the second chamber  27  no longer decreases, since the pressure reservoir  41 , not shown in FIG. 2, does not receive any further fluid. As a consequence, the annular piston  7  no longer moves. When the gas exchange valve is opened again, the piston  9 , with its diameter d 2 , moves out of the stepped center bore  19  of the annular piston  7 . From that position on, a direct hydraulic communication exists between the first chamber  13  and the second chamber  27 . This does not change the opening force at all. 
     In FIG. 2 c , the hydraulic actuator is shown in a position in which the gas exchange valve is fully open, and the piston  9  has moved to the right out of the annular piston  7 . 
     For closing the gas exchange valve, the piston  9  must be moved to the left in terms of FIGS. 1 and 2. This is accomplished by closing the first switching valve  29  and opening the second switching valve  33 . This position of the switching valves  29  and  33  is shown in FIG.  1 . The hydraulic force exerted on the shoulder  51  of the piston  9  by the fluid located in the third chamber  25  at the feed pressure of the pump  31  moves the piston  9  to the left. Hydraulic fluid is now pumped out of the first chamber  13  and second chamber  25  into the oil sump  35  via the check valve  39  and the second switching valve  33 . In addition, the spring  45  of the hydraulic reservoir  41  is capable of lifting the piston  43  from the stop  47  and moving the piston  43  onward into its outset position. 
     As soon as the piston  9  plunges with its diameter d 2  into the stepped bore  19  of the annular piston  7 , the hydraulic fluid located in the first chamber  13  can no longer reach the oil sump  35  directly via the second chamber  27  and the line  37  but must instead flow into the oil sump  35  via the throttle  49 . As a result, a certain overpressure builds up in the first chamber  13  and the motion of the piston  9  is braked. As soon as the annular piston  7  rests with its stepped inner bore  19  on the piston  9 , the annular piston  7  and the piston  9  move together. As a result, a greater oil volume is pumped through the throttle  49 , which leads to a boosting of the braking action. 
     The position beyond which the desired braking of the gas exchange valve ensues before the gas exchange valve strikes the valve seat, not shown, is dependent on the stroke of the reservoir piston  43  and is thus not dependent on the thermal expansion that the hydraulic actuator is exposed to. Nor do production tolerances of the actuator affect this position. As a result of the suitable choice of the diameters d 1  through d 6 , the ratios of the opening force upon liftoff of the gas exchange valve from the valve seat and the reduced opening force upon further opening of the gas exchange valve and the closing force upon closure of the gas exchange valve can be adapted to one another, in order to attain an optimal operating performance of the hydraulic actuator. 
     In FIG. 3, the actuator of FIG. 1 is shown in fragmentary form, only to the extent of interest below, with the housing  1 , first chamber  13 , second chamber  27  and third chamber  25 , and with its hydraulic connection to the hydraulic pump  31  with the first switching valve  29 , embodied for instance as a 2/2-way magnet valve, and the hydraulic communication between the first chamber  13  and second chamber  27  via the throttle  49 . The hydraulic relief chamber or oil sump is identified, as before, by reference numeral  35 , and the line connecting the second chamber  27  with the second switching valve  33 , embodied for instance as a 2/2-way magnet valve, is identified by reference numeral  37 . The hydraulic actuator has been modified to the extent that the device for limiting the volumetric decrease of the second chamber  27 , which in FIG. 1 is embodied as a hydraulic spring reservoir  41 , is now replaced with a shutoff valve  50 , which is in communication with an opening in the second chamber  27 , for instance being connected to the line  37 , and in one switching position it closes the opening in the second chamber  27 , or the connection to the line  37 , while in its other switching position it opens it so that fluid can flow out to the oil sump  35 . The function of this shutoff valve  50 , represented only symbolically in FIG. 3, is, however, assigned to the second switching valve  33 , which to enable fluid to flow out of the second chamber  27  is in the basic position shown in FIG.  3  and which is switched over to its other switching position in order to block off the second chamber  27 . The switchover valve  33  furthermore maintains its function, already described in conjunction with FIG. 1, for the closure of the gas exchange valve without modification. 
     As described above, to open the gas exchange valve the first switching valve  29  must be opened. Fluid now flows at the feed pressure into the chamber  13 , so that the piston  9  of the actuator is displaced together with the annular piston  7  as shown in FIG. 2 b . If at an arbitrary instant during the displacement of the annular piston  7  the second switching valve is switched over to its blocking position, then fluid cannot flow out of the second chamber  27 , and the annular piston  7  is blocked. The stroke of the annular piston  7  is accordingly defined by the instant of switchover of the second switchover valve  33 , which at the onset of the opening motion of the actuator is open. 
     As described above, to close the gas exchange valve, the annular piston  7  is displaced back again by the pressure in the third valve chamber  25 , as soon as the first switching valve  29  is blocked again and the second switching valve  33  is opened again. In the process, the pressure in the first chamber  13  decreases via the throttle  49 . After a stroke travel, the piston  9  strikes and carries the annular piston  7  along with it in its further stroke course. As a result, a high volumetric current and a pronounced pressure increase in the first chamber  13  are caused, so that the piston  9  is braked sharply. The braking action begins at the instant when the annular piston  7  moves jointly with the piston  9 , so that the instant of onset of the braking operation is defined by the stroke travel of the annular piston  7 , which is established in the opening process of the gas exchange valve. Thus by means of the instant of switchover of the second switching valve  33  into its blocking position upon opening of the gas exchange valve, the instant of onset of the braking event upon closure of the gas exchange valve can be defined. 
     The exemplary embodiment, shown in fragmentary form in FIG. 4, of the actuator with hydraulic connection is modified compared to FIG. 3 only to the extent that between the first chamber  13  in the housing  1  and the throttle  49  in the connecting line to the second chamber  27  in the housing  1 , a flow-controlled valve  51  has been incorporated, which is embodied such that it is closable by the fluid flowing to the first chamber  13 . This flow-controlled valve  51  prevents fluid, in the initial phase for opening the gas exchange valve, in which phase both the first switching valve  29  and the second switching valve  33  are open, from flowing directly from the first switching valve  29  out to the oil sump  35  via the second switching valve  33 ; this is because the leakage flowing via the throttle  49  increases the energy requirement for valve control, if it increases unacceptably. That is the case particularly whenever the braking action upon the closure of the gas exchange valve is to be lowered moderately by means of a wider opening of the throttle  49 . If the first switching valve  29  is opened, then as a result of the fluid flowing from the hydraulic pump  31  into the first chamber  13 , the valve  51  is closed, and the communication with the throttle  49  is thus blocked. If the first switching valve  29  is closed, or in other words has been returned to the switching position shown in FIG. 4, then the valve  51  opens, and the requisite communication for expelling the fluid from the first chamber  13  via the throttle  49  upon the closing of the gas exchange valve is reestablished. 
     The layout of the flow-controlled valve  51  is shown schematically in FIGS. 5 and 6; FIG. 5 shows the valve open, and FIG. 6 shows the valve closed. The flow-controlled valve  51  has a housing  52 , with a first valve connection  53  communicating with the chamber  13  of the actuator, a second valve connection  54  connected to the throttle  49 , and a third valve connection  55  communicating with the outlet of the first switching valve  29 . The first valve connection  53  communicates with a lower valve chamber  56 , the third valve connection  55  communicates with an upper valve chamber  57 , and the second valve connection  54  communicates with an annular chamber  58  located between the lower and upper valve chambers  56 ,  57 . Between the lower valve chamber  56  and the annular chamber  58 , a valve opening  60  surrounded by a valve seat  59  is embodied in the housing  52 . A guide sleeve  61  is inserted into the upper valve chamber  57 , and a valve member  62  embodied as a valve displacement piston is guided displaceably in this guide sleeve. The valve member  62  cooperates with the valve seat  59  to close and open the valve opening  60 , so that the annular chamber  58  is blocked off from the lower valve chamber  56  when the valve member  62  is seated on the valve seat  59  (FIG.  6 ), and communicates with the lower valve chamber  56  when the valve member  62  has lifted from the valve seat  59  (FIG.  5 ). A valve opening spring  63  is placed in the lower valve chamber  56 ; it is embodied as a compression spring and braced on one end on a shoulder  64  embodied in the lower valve chamber  56  and on the other end on the valve member  62 . The valve opening spring  63  presses the valve member  62  against a stop  65  embodied in the guide sleeve  61 . 
     The valve member  62  is provided with a central through opening  66 , which permanently connects the upper valve chamber  57  with the lower valve chamber  56 . The through opening  66  is embodied as a throttle, and for that purpose its inner contour  67  has a design such that the fluid flowing from the upper valve chamber  57  to the lower valve chamber  56  causes a pressure drop in the through opening  66 . In the exemplary embodiment of FIGS. 5 and 6, the through opening  66  has the form of a double truncated cone for this purpose, in which two truncated cones are placed on one another with their smaller bases. 
     If the first switching valve  29  is opened for the sake of opening the gas exchange valve, fluid flows from the outlet of the pump  31  through the through opening  66  in the valve member  62 , and because of the inner contour  67 , a pressure drop occurs between the upper and lower valve chambers  57 ,  56 . Thus the pressure in the upper valve chamber  57  is greater than in the lower valve chamber  56 , and at the valve member there is a resultant displacement force, which counter to the spring force of the valve opening spring  63  seats the valve member  62  on the valve seat  59  and thus closes the valve opening  60 , as a result of which the communication with the throttle  49  is blocked. 
     If the first switching valve  29  is opened again, then no further fluid flows via the through opening  66 . No pressure drop occurs at the inner contour  67 , and so the pressures in the lower valve chamber  56  and in the upper valve chamber  57  are equal. The force acting on the valve member  62  is zero, and by means of the spring force of the valve opening spring  63 , the valve member  62  is pressed against the stop  65  in the guide sleeve  61 . The valve member  62  is thus lifted from the valve seat  59 , and the first chamber  13  of the actuator now communicates with the throttle  49 . Upon closure of the gas exchange valve, the fluid volume positively displaced from the first chamber  13  as a result of the displacement motion of the pistons  9  and  7  can now flow out into the oil sump  35 , via the throttle  49  and the opened second switching valve  33 . 
     The foregoing relates to preferred exemplary embodiments of the invention, it being understood that other variants and embodiments thereof are possible within the spirit and scope of the invention, the latter being defined by the appended claims.