Abstract:
The valve drive mechanism is particularly suitable for internal combustion engines of motor vehicles. The mechanism has at least one driven cam element and a valve control member which is moved (translationally or rotationally) by the cam element. The cam element is rotatingly mounted in a flexible surround element which is connected to the valve control member in a plane orthogonal to the axis of rotation of the cam element. The surround element can be reversably extended, such as elastically extended, to enable a variation in the resulting valve lift.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation-in-part of Application No. 10/075,066, filed Feb. 12, 2002, which is a continuation of International Application No. PCT/AT00/00215, filed Aug. 8, 2000, which designated the United States and was not published in English. 
     
    
     
       BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0002]    The invention relates to a valve mechanism, in particular for internal combustion engines of motor vehicles, having at least one driven cam element and having a valve actuator, which can be displaced or pivoted by the cam element, the cam element being arranged rotatably in a flexible surround element which is connected to the valve actuator in such a manner that it can move in a plane which is perpendicular to the axis of rotation of the cam element.  
           [0003]    Valve mechanisms for controlling the valves of internal combustion engines, in particular for motor vehicles, usually have a device (spring, hydraulic element, etc.) which is used to load the valve toward its closed position. In this position, a valve actuator (valve lifter, drag lever, rocker lever or the like) is pressed against a continuous valve control surface, which in part runs eccentrically with respect to the shaft axis. When the valve is closing, it should be ensured that the valve disk does not strike the valve seat too quickly, since otherwise it will rebound. This requires relatively complex matching between the shape of the cam, the masses which are to be moved, the forces which are generated, the materials properties, etc.  
           [0004]    Therefore, there is no lack of proposals concerning forced guidance of the valve actuator on the cam element; various embodiments have been developed, which are each based on two eccentric valve control surfaces instead of the restoring spring. Specific designs are to be found, for example, in British patent specifications GB 19,193(1913) and GB 434,247, wherein the cam element, on at least one end face, has a groove, the two side walls of which form the valve control surfaces. A roller or the like which is arranged at the end of the valve actuator engages in the groove from the side. A cam element which has a web which can be gripped around is known, for example, from European publication EP 429 277 A.  
           [0005]    A further proposal for a desmodromic valve mechanism, wherein a space-saving, lightweight and inexpensive design is achieved, is to be found in published German patent application DE 37 00 715 A which describes the generic type referred to in the introduction. In this design, a surround element is provided, which surrounds the circumference of the cam element without significant play, so that it always matches the shape of the cam, yet the cam element, on account of the nature of the surround element, can rotate inside the latter. Since the surround element connected to the valve actuator cannot rotate with the cam element, the movement of the cam region about the axis of rotation of the cam element is converted into a lifting or reciprocating movement of the valve actuator which is mounted displaceably or pivotably in the cylinder head. The valve actuator does not execute a movement as long as the connecting region of the surround element together with the valve actuator rests against the base circle region of the rotating cam element, is then moved away from the axis of rotation of the cam element in the radial direction and finally is returned again, while the cam region of the cam element moves past the connecting region of the surround element and the valve actuator. The moveable connection of the surround element to the valve actuator allows the pivoting or tilting movement of the surround element in the cam region, so that the required freedom of movement of the valve actuator in its sliding or pivot bearing is preserved. In the first exemplary embodiment, the surround element is formed by two flexible rings, between which needle-shaped rolling bodies are provided in order to reduce the friction. A second embodiment provides a plastic strip having an inner ceramic slip layer.  
           [0006]    Particularly when the valve mechanism is used in internal combustion engines, a surround element is subject to high loads, and it is necessary to rule out temperature- or fatigue-related plastic lengthening of the surround element. An irreversible increase in the size of the gap between the circumference of the cam element and the surround element affects in particular the valve-closing position.  
           [0007]    Furthermore, the term variable valve control has revealed a wide range of different structures which can be used to change the opening and closing time and the lift of the valve, in order to improve the performance, the exhaust emissions, the torque, etc. of an internal combustion engine. Compared to the non-adjustable valve control with fixed values, the filling of a cylinder is improved if the valve is opened later and closed earlier at low rotational speeds and is opened earlier and closed later at higher rotational speeds. It is therefore possible, by means of a speed-dependent adjustment of the valve control, to optimize the exhaust emissions, the torque, the engine performance, etc. All the variable valve control arrangements which have been revealed to date change the position of the actuating surface of the valve actuator relative to the eccentric valve control surface through rotation, linear displacement or enlargement of the cam element. These adjustment mechanisms are relatively complex and, in some cases, also require considerable adjustment forces, since they have to operate counter to the restoring elements of the valves.  
         SUMMARY OF THE INVENTION  
         [0008]    It is accordingly an object of the invention to provide a valve lifting mechanism, particularly for an internal-combustion engine, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which provides for a variable forced valve control.  
           [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a valve mechanism, comprising:  
           [0010]    at least one driven cam element and a valve actuator driven by said cam element;  
           [0011]    a flexible surround element, said cam element being rotatably disposed in said flexible surround element about an axis of rotation and said flexible surround element being movably connected to said valve actuator for movement in a plane perpendicular to said axis of rotation of said cam element,  
           [0012]    and wherein said surround element is configured to be reversibly lengthened for adjusting a valve lift of said valve actuator.  
           [0013]    In other words, the objects of the invention are achieved, in a valve mechanism of the type described in the introduction, in that the surround element is designed so that it can be lengthened reversibly.  
           [0014]    The cam element rotating in the surround element generates tensile forces, which rise as a function of rotational speed, at the connecting point to the valve actuator, so that the surround element, which bears against the circumference of the cam element virtually without play at idling speed, is lifted increasingly further away from the circumference as the rotational speed rises, thus adopting positions which correspond to cam elements with greater circumferential lengths. Since, in this way, the distance between the axis of rotation of the cam element and the connecting point between the surround element and the valve actuator increases, an additional valve lift which is dependent on rotational speed is produced.  
           [0015]    In a first embodiment, the reversible lengthening of the surround element is achieved by elastic stretchability of at least a partial region of the surround element, so that the play which is formed between the cam circumference and the surround element is reduced further as the rotational speed falls. Moreover, it allows advantageous, slight prestressing of the surround element in the at-rest state, in order to ensure that the valve-closing position is reached outside the cam region despite any temperature-related changes in length.  
           [0016]    The surround element may consist of an elastically stretchable material or may be composed of two materials with different properties, at least one of which can stretch elastically. By way of example, a non-stretch strip may be closed with respect to the surround element by an elastically stretchable intermediate piece, wherein case a holder for the valve actuator may be provided either in the nonstretch region or in the elastically stretchable region. If the holder is in the elastically stretchable region, it may itself also consist of an elastically stretchable material and, if appropriate, may also form the elastic region.  
           [0017]    For internal combustion engines of motor vehicles, the elastically stretchable material is preferably designed for an additional valve lift of 10% to 30% of the valve lift at idling speed. In order, in a preferred embodiment, to ensure an upper limit value of the elastic stretching, which can be selected for a permissible maximum rotational speed or a rotational speed above which an additional valve lift is of subordinate importance, a stretch-limiting means can be assigned to the elastically stretchable material by arranging nonstretch filaments or fibers, the length of which corresponds to the length of the elastic material which has been stretched to the limit, in or parallel to the stretchable material.  
           [0018]    In a second embodiment, the surround element has a protuberance which is formed by an elastically resilient constriction, the tensile forces acting on the holder of the valve actuator causing the elastically resilient constriction to widen. The reduction of the constriction lengthens the surround element, which in this embodiment may itself be of nonstretch design. The holder is preferably arranged in the protuberance, with the result that the two regions of the surround element, which, at idling speed, come into contact with one another between the cam element and the holder, move away from one another as the rotational speed increases and move closer to one another as the rotational speed falls.  
           [0019]    In a further embodiment, it is provided that the surround element has a strip comprising a textile-bound sheet material, in particular comprising a woven fabric, the two ends of which are connected to a holder for the valve actuator. When the two ends of the strip penetrate through one another or project from the cam element in contact with one another, the flexibility of the material of the surround element means that a physical axis in the connection to the valve actuator may be unnecessary, since the two ends together can be bent to both sides to the required extent. For connection to the valve actuator, it is preferable for the two ends of the strip to have plug-in openings for a connecting element. The plug-in openings may be formed by winding round and—depending on the material used for the strip—sewing, adhesively bonding or welding the wrapped-around end, or the like. A particularly advantageous embodiment provides for the strip to comprise a continuous loop which is guided backward and forward about the cam element and the reversal points of which form the plug-in openings. The connecting element may also be of elastically resilient design and consist, for example, of spring steel.  
           [0020]    If the surround element consists of two different materials, the textile-bound sheet material may have a nonstretch region, wherein it contains filaments of Kevlar®, glass, carbon or aramid fibers, or the like, substantially constant-length fibers, extending in the circumferential direction of the cam element.  
           [0021]    A surround element which forms a continuous loop may consist in particular of a sheet material which is produced using a textile circular working technique (circular weaving, circular knitting, etc.) and is provided with a holder for the valve actuator.  
           [0022]    The elastic stretching of the loop may be selected to be linear, progressive or degressive, for example by incorporating filaments with different stretching properties, which become active simultaneously or in succession.  
           [0023]    Further possible options provide an elastically stretchable cord or an elastically stretchable ring made from plastic, which is preferably provided with a recess for a bearing pin of the valve actuator. The plastic ring may be fiber-reinforced and/or provided with a slip-reducing metal coating. As an alternative, it is also possible to use a flat belt, in particular a ribbed belt, between the transverse ribs of which there is space for the bearing pin of the valve actuator, which is fixed by an adhesively bonded cover strip or the like. The ribbed belt may also be fitted in such a way that the ribs are internal, which eliminates the need for additional fixing of the bearing pin.  
           [0024]    Materials which are particularly suitable for a surround element which has at least elastically stretchable partial regions have a modulus of elasticity of between 1 and 4000 N/mm 2 . Rubber-like materials have low moduli of elasticity and are preferably provided with a stretch-limiting means.  
           [0025]    Materials such as plastics which have higher moduli of elasticity, in particular between 600 and 2000 N/mm 2 , preferably between 800 and 1200 N/mm 2 , do not generally require a stretch-limiting means, although it is, of course, possible to provide such means.  
           [0026]    A simple possible option for the stretch-limiting means consists in assigning nonstretch filaments of Kevlar®, glass, aramid fibers or the like, which-extend in the circumferential direction and are, for example, woven into a strip, to the surround element or the elastically stretchable region of the surround element. Specifically in this design, it would also be possible to use an elastomeric plastic, which is vulcanized to the strip, for the ring or flat belt.  
           [0027]    For internal combustion engines wherein the cylinders have two intake or discharge valves which operate in parallel, the valve pairs may have different stretching levels, for example one stretch-limited valve under partial load and the other valve without stretch-limiting means or with stretch-limiting means at full load.  
           [0028]    If the surround element consists of a material with a low-friction surface or a surface which has been provided with a low-friction coating, it may be that lubrication of the sliding surfaces, i.e. of the circumferential surface of the cam element and of the inner surface of the surround element which bears against it, will not be required. If lubrication is required or desirable, it is preferable for the cam element to have at least one oil bore which runs radially with respect to the axis of rotation and opens out on the circumference of the cam element, inside the flexible surround element. Since the surround element does not rotate, external supply of oil through the surround element via a flexible line is also conceivable.  
           [0029]    Instead of a film of lubricating oil, it is also possible to build up an air cushion surrounding the cam element by means of compressed air. This may be advantageous in particular in the case of a surround element made from plastic or woven plastic fabric.  
           [0030]    In the valve mechanism according to the invention, the masses which have to be accelerated are reduced by the elimination of the valve spring and spring disk and by a significantly lighter design of the valve lifter or rocker lever. The use of light metals, ceramics or plastics for the valve and/or the valve actuator allows the masses which have to be accelerated and decelerated to be reduced by from 50% to 80% of the value for a valve lifter with restoring spring and hydraulic play compensation. The high values result in particular in the part-load range, since the valve springs have to be designed to be able to withstand full load. Furthermore, the valve may be of shorter design, since the bulky valve spring is eliminated.  
           [0031]    It is also possible for the cam element to be of shorter design. It also becomes possible to form plastic cam elements or camshafts which are produced completely from plastic, for example by injection molding. The use of other lightweight materials for the production of the camshafts or of the cam elements, for example aluminum, also becomes possible. On account of the reduction in mass and the lubrication, fuel savings of 5% and more are to be expected.  
           [0032]    Particularly if valve actuators are actuated together, it is possible to provide a weak spring for acting on each closed valve.  
           [0033]    According to another feature of the invention, the cam element has a circumference variably adjustable in correspondence with a length of the surround element.  
           [0034]    According to yet another feature of the invention, the cam element has a cam circumference surface with a cylindrical region and an eccentric region; the cam element has a first cam part and a second cam part, the first and second cam parts are movable relative to one another; and the first cam part includes the cylindrical region of the cam circumference surface, the second cam part includes the eccentric region of the cam circumference surface.  
           [0035]    According to a further feature of the invention, a camshaft has an inclined camshaft surface, the camshaft is movable along the longitudinal direction in a support shaft; the second cam part is guided in the first cam part and can be pushed radially outwardly; and the second cam part has an inclined surface cooperating with the inclined camshaft surface.  
           [0036]    According to another feature of the invention, a camshaft has a spiral-shaped control surface, the camshaft being rotatable in a support shaft; the second cam part is guided in the first cam part and can be pushed radially outwardly; and the second cam part has a bridge cooperating with the spiral-shaped control surface of the camshaft.  
           [0037]    According to yet another feature of the invention, a support shaft defines an axis of rotation; a camshaft has a crankpin and is rotatable in the support shaft; and the second cam part is disposed at the first cam part and is pivotable about an axis parallel to the axis of rotation, the second cam part has a guide surface cooperating with the crankpin.  
           [0038]    In the previously described exemplary embodiments the lengthening of the surround element takes place exclusively by the tensile forces which increase in dependence on the rotational speed at the junction with the valve actuator, such that it lifts off from the cam element.  
           [0039]    This lifting off which may be disadvantageous with regard to an uninterrupted lubricating film is eliminated when the circumference of the cam element is lengthened to the same extent. A lengthening of the circumference can be achieved in particular in that the cam element is constructed as having at least two parts and in that a part which carries the eccentric valve control surface can actively be adjusted with respect to a distance to the axis of rotation of the cam element for example in that the part which carries the eccentric valve control surface can be radially pushed inward or outward or can be pivoted inward or outward about an axis which is parallel to the axis of rotation of the cam element. Operating mechanisms for adjusting a part which carries the eccentric valve control surface are known from variable valve controls (U.S. Pat. Nos. 2,888,837 and 3,489,032).  
           [0040]    In a first embodiment of the invention, the operating mechanism includes a longitudinally adjustable camshaft with an inclined surface which cooperates with a corresponding inclined surface at the adjustable part of the cam element. In a further embodiment, the control shaft can be rotated relative to the support shaft and can include a crankpin-like element which moves the adjustable part out or in. If the part is pivoted, an asymmetrical adjustment of the cam profile takes place which additionally results in different valve opening and closing characteristics. The active adjustment of the valve lift allows an increased adjustment range due to the fact that a no-stroke or “zero lift” can be chosen as a basis, wherein the parts of the cam element fit into each other within a circular peripheral outline. The no-stroke, for example, is important if it should be possible to turn off individual cylinders in a combustion engine.  
           [0041]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0042]    Although the invention is illustrated and described herein as embodied in a valve mechanism, in particular for internal combustion engines, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.  
           [0043]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    [0044]FIG. 1 shows a graph illustrating a speed-dependent change in length of the surround element;  
         [0045]    [0045]FIG. 2 shows components of a first embodiment of a forcibly controlled variable valve mechanism in perspective;  
         [0046]    FIGS.  3  to  5  show cross-sectional illustrations of various angular positions of the first embodiment of the valve mechanism;  
         [0047]    [0047]FIGS. 6 and 7 show longitudinal sections through the first embodiment, FIG. 6 illustrating the valve actuator at idling speed and FIG. 7 illustrating the valve actuator at a higher speed;  
         [0048]    [0048]FIG. 8 shows components of a second design of a valve mechanism, in a perspective view,  
         [0049]    [0049]FIG. 9 shows a longitudinal section through the third embodiment; and  
         [0050]    [0050]FIG. 10 shows a side view of the third embodiment, in each case at idling speed;  
         [0051]    [0051]FIG. 11 shows components of a fourth embodiment of a valve mechanism, in a perspective view;  
         [0052]    [0052]FIG. 12 shows a side view of the fourth embodiment; and  
         [0053]    [0053]FIG. 13 shows a cross section through the fourth embodiment, in each case at idling speed;  
         [0054]    [0054]FIG. 14 shows a perspective view of components of a fifth embodiment;  
         [0055]    [0055]FIGS. 15 and 16 show cross sections through the fifth embodiment, FIG. 15 showing the valve actuator at idling speed and FIG. 16 showing the valve actuator at a higher speed;  
         [0056]    [0056]FIG. 17 shows a perspective view of components of a sixth embodiment;  
         [0057]    [0057]FIG. 18 shows a perspective view of the sixth embodiment in the closed position;  
         [0058]    [0058]FIG. 19 shows a longitudinal section through the sixth embodiment;  
         [0059]    [0059]FIG. 20 shows a perspective view of components of a seventh embodiment;  
         [0060]    [0060]FIG. 21 shows a cross section through the seventh embodiment;  
         [0061]    [0061]FIG. 22 shows an enlarged detailed view of part of FIG. 21;  
         [0062]    [0062]FIG. 23 shows a perspective view of the seventh embodiment;  
         [0063]    [0063]FIG. 24 shows a perspective view of components of an eighth embodiment;  
         [0064]    [0064]FIG. 25 shows a longitudinal section through the eighth embodiment;  
         [0065]    [0065]FIG. 26 shows a perspective view of components of a ninth embodiment;  
         [0066]    [0066]FIG. 27 shows a cross section through the ninth embodiment;  
         [0067]    [0067]FIG. 28 shows a perspective view of components of a tenth embodiment;  
         [0068]    [0068]FIG. 29 shows a perspective view of the tenth embodiment;  
         [0069]    [0069]FIG. 30 shows a perspective view of components of an eleventh embodiment;  
         [0070]    [0070]FIG. 31 shows a cross section through the eleventh embodiment;  
         [0071]    [0071]FIG. 32 shows an enlarged detailed view of part of FIG. 31;  
         [0072]    [0072]FIG. 33 shows a perspective view of components of a twelfth embodiment;  
         [0073]    [0073]FIG. 34 shows a longitudinal section through the twelfth embodiment;  
         [0074]    [0074]FIG. 35 shows an enlarged detailed illustration of part of the twelfth embodiment;  
         [0075]    [0075]FIG. 36 shows a perspective view of parts of a thirteenth embodiment;  
         [0076]    FIGS.  37  to  39  show longitudinal sections through the thirteenth embodiment, wherein FIG. 37 shows a no-stroke position, FIG. 38 shows a normal stroke position, and FIG. 39 shows an auxiliary lift position;  
         [0077]    [0077]FIGS. 40 and 41 show a no-stroke and an auxiliary lift position of a fourteenth embodiment; and  
         [0078]    [0078]FIGS. 42 and 43 show a no-stroke and an auxiliary lift position of a fifteenth embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0079]    The drawings in each case show a forcibly guided valve mechanism, wherein a valve mechanism used for an internal combustion engine of a motor vehicle has, on the support shaft  1 , the number of cam elements  2  which are required for the valves. A supply of oil in order to build up a film of oil or of air in order to build up an air cushion on the circumferencial surface of the cam element  2  can be effected via a hollow support shaft, radial openings  30  in the support shaft  1  and via bores  3  in the cam element  2 . An arrangement of openings  30  and bores  3  can also, as shown in FIGS.  8  to  10 , be used to secure the cam element  2  on the support shaft  1  when a fixing pin  20  is inserted. The valve lift, the increase wherein as a function of the rotational speed is illustrated in the diagram shown in FIG. 1, can be seen from a comparison of FIGS.  3  to  5 . The diagram shown in FIG. 1 illustrates the change in length of the surround element  4 ; the eccentric range  0  to  1  of the cam element  2 , starting and ending at the base circle region (not shown), wherein the valve-closing position is produced, is plotted on the abscissa. Depending on the selected conditions, the eccentric range extends over an angle of approximately one third to two thirds of the circumference of the cam element  4 , for example over an angle of approximately 150°, as shown in the figures. At a rotational speed of the camshaft of 400 revolutions per minute, which corresponds to the engine idling, the diagram shows a valve lift of 9.7 mm, denoted as 100%. If the rotational speed increases, the lift should become greater, for example reaching an additional valve lift of 1.75 mm at a maximum of 4000 revolutions per minute, corresponding to an increase of approximately 18% in valve lift. The speeds indicated, in this figure and below, always refer to the rotational speeds of the camshaft itself, which in the case of internal combustion engines for motor vehicles are generally half as great as the engine speeds, i.e. in the example indicated the idling speed of the engine is 800 and the maximum speed 8000 revolutions per minute.  
         [0080]    In order, despite forced control, to increase the valve lift as a function of the rotational speed, the cam element  2  is surrounded by a surround element  4 , which can be reversibly lengthened and substantially bears against the circumferential surface, it being possible for the cam element  2  to rotate in the surround element  4 , about its axis of rotation  8 , with continuous pulsed deformation of the surround element  4 . In the figures, the cross-sectional shape of the surround element  4  is in each case illustrated matched to the cam element  2 , since in these figures the valve mechanism is shown in an exploded view. As an individual element, the surround element  4  is only in the form of a ring if the material is sufficiently elastic and thick, while otherwise it forms a collapsed oval or the like. The surround element  4  is prevented from rotating by the connection to a valve actuator  10 , which in the case of the valve lifter is mounted in such a manner that it can be moved in translation in a sliding-contact bearing, while in the case of a rocker or drag lever is mounted so that it can be pivoted in a pivot bearing. This also permits an embodiment wherein a lubricant is supplied through the stationary surround element  4 . The surround element  4  is connected to the valve actuator  10  in such a manner that it can tilt or pivot about an axis  15 , so that, when the cam of the cam element  2  passes through the connecting region of the valve actuator  10 , it is possible for the surround element  4  to pivot relative to the valve actuator  10 . This is necessary since, as illustrated in FIGS.  3  to  5 , the sliding-contact bearing of the valve stem  11  does not allow any lateral deflection, and the valve stem  11  has to be directed radially toward the axis of rotation  8 .  
         [0081]    The higher the rotational speed of the cam element  2 , the greater the tensile forces which are produced in the surround element  4 , these forces, on account of the fact that the surround element  4  can be lengthened reversibly, leading to the distance between the axis of rotation  8  and the axis  15 , at which a valve actuator  10  is articulatedly mounted, increasing. This increase in distance produces an additional valve lift.  
         [0082]    In the first embodiment, shown in FIGS.  2  to  7 , the surround element  4  is formed by a ring made from a flexible, elastically stretchable and, if appropriate, fiber-reinforced plastic, which has only a low resistance to deformation. At one point, the ring contains a window  5 , wherein a bearing pin  14 , which runs parallel to the axis of rotation  8  of the cam element  2  and lies in the axis  15 , passes through the valve stem  11 . On the inner surface of the ring, which surrounds the cam element  2 , there is a continuous thin loop of a friction-reducing strip  22 , wherein the cam element  2  rotates. The strip  22  may likewise be elastically stretchable and may consist, for example, of a low-friction plastic, a woven fabric or the like. As shown in FIG. 6, a small gap  31  remains between the strip  22  and the circumferential surface of the cam element  2 , generally if only for assembly reasons, wherein gap a film of oil can be formed for lubrication purposes. As the rotational speed rises, the elastic stretching of the surround element  4  means that the gap  31  increases in size, as can be seen from a comparison between FIGS. 6 and 7, so that the valve lift is increased.  
         [0083]    FIGS.  8  to  10  show an embodiment wherein a holder  12  is formed in the shape of an “iron”, the bearing pin  18 , in a similar manner to that shown in FIG. 2, being arranged between the elastically stretchable strip  22  and the ring  4  made from elastically stretchable plastic or the like and forming the axis  15 . The end section of the bearing pin  18  widens slightly, in order to prevent axial slipping in the surround element  4 , the opposite region of the holding body  12  being beveled, in order for it to be possible to push the holding body  12  in laterally. The holding body  12  has a threaded bore, into which the valve stem  11 , which has a screw thread  28 , can be screwed and fixed adjustably by a locking nut  27 . As has been mentioned, FIGS.  8  to  10  also show a possible way of fixing the cam element  2  on the support shaft  1  by means of a pin  20 , which is fitted through bores  30  in the shaft  1  and bores  3  in the cam element  2 .  
         [0084]    FIGS.  11  to  13  show an embodiment wherein the surround element  4  is formed by a continuous loop of an elastically stretchable cord, which is arranged slideably in a groove  16  in the circumferential surface of the cam element  2 . The cam element  2  is divided into two cam regions  43  which are spaced apart from one another in the axial direction, the groove  16 , wherein the oil bores  3  of the cam element  2  open out, forming the central region. The valve stem  11  of the valve actuator  10  is provided with an, in particular laterally open, hook-like eyelet  17 , wherein the cord loop is suspended, and is rounded on the top side parallel to the axis  15 , in order to allow the pivoting, as can be seen in particular from FIG. 13. The eyelet  17  may also be of closed design, if a piece of a cord is only closed up to form the cord loop after it has been threaded into the eyelet  17 . In this embodiment, the surround element  4  is slightly larger than the cam circumference, since it is also guided through the eyelet  17 . The elastic stretchability also compensates for the changes in the surround length which result from rotation of the cam element on account of the eyelet  17  holding the cord at a distance from the circumference.  
         [0085]    FIGS.  14  to  16  show an embodiment wherein the surround element  4  is formed from a strip of elastically stretchable sheet structure with textile binding, in particular a woven fabric or the like. For connection to the valve actuator  10 , a protuberance  6  is formed on the surround element  4 , which protuberance can be fitted into a slot  29  in the valve stem  11 . Connection is effected by means of a cotter pin  19 , which passes through the bores  25  in the valve stem  11  and the protuberance  6 . The surround element  4  may be a single, continuous loop or turn with a protuberance  6  which has been pressed flat. The single loop or turn may also be formed by bringing together both ends  13  of a strip, which when in contact with one another form the protuberance  6  and together are inserted into the slot  29 . In this embodiment, the axis  15  is not physically embodied, but rather results from the bending region between the protuberance  6  and that part of the surround element  4  which surrounds the circumferential surface of the cam element  2 . FIG. 15 shows the position of the valve stem  11  in the closed position, and FIG. 16 shows a position at a high rotational speed, wherein the two ends  13  of the strip have been moved away from one another between the upper end of the valve stem  11  and the cam element.  
         [0086]    In the embodiment shown in FIGS.  17  to  19 , the cam element  2  is provided with a circumferential groove  16 , the base of which is concentric with respect to the support shaft  1 . In this way, the cam element  2  is divided into two cam regions  43 , which are connected by means of a material-saving central region. The surround element  4 , which in this embodiment is formed by a continuous loop of an elastically stretchable strip, has at one point an adhesively bonded or sewn tab  45  which defines a plug-in opening  47 . In the central region the loop and the tab  45  are provided with a window  5 . In the securing region, the valve actuator  10  has a bore  46 , so that, after insertion into the window  5 , a connecting element  48 , in the form of a pin or cotter pin, can be pushed through the plug-in opening  47  and the bore  46 . The pin in turn forms the axis  15 , which extends parallel to the support shaft  1 . The free end of the valve stem  11  in this case projects into the circumferential groove  16 , resulting in axial guidance also being provided. The seamless woven strip of the surround element  4 , which is preferably produced using a textile circular working technique (circular weaving, circular knitting or the like) contains carbon, Kevlar® or aramid filaments or fibers or the like to protect against excessive stretching, since this produces a highly constant length and a good thermal stability. The nonstretch filaments have a length which corresponds to the maximum circumferential length and may be the weft filaments, running in the circumferential direction, of the woven-fabric ring or additional filaments which, for example, in the unstretched state are connected to the woven-fabric ring in waves or zigzag form. The woven fabric may also be provided with a low-friction coating.  
         [0087]    FIGS.  20  to  23  show a similar design, wherein, once again, a circular-worked, in particular circular-woven strip is used to produce the surround element  4 . The strip circumference substantially corresponds to twice the circumference of the cam element  2  and is brought together so as to form a double-layer open loop. The reversal points of the strip at the ends  13  of the open loop form the plug-in openings  47  for the hollow connecting element  48 , which in this embodiment is bent into a U shape. Both ends  13  are cut out in the central region  52 , and the two cutouts complement one another to form the window  5  through which the end of the valve stem  11  projects into the circumferential groove  16  of the cam element. As a result, the installation position of the valve actuator  11  can lie laterally offset, parallel to the axial plane, as can be seen from FIG. 21, which may result in advantages with regard to a change in the rolling and contact lines. Of course, the valve stem  11  may also lie in the axial plane, so that the two plug-in openings  47  are not symmetrical. A second part  53  which is bent into a U shape is inserted into the hollow connecting element  48  and is, for example, adhesively bonded, so that the connection between the surround element  4  and the valve actuator  10  is ensured.  
         [0088]    Instead of using the U-shaped connecting element  48 , the two ends  13  of the open loop could also be connected by an element which is similar to a belt buckle and has one or two slots through which the ends  13  are guided and are fixed by pins inserted into their plug-in openings  47 . The belt-buckle-like element forms the holder  12  for the valve actuator, into which it is screwed or latched.  
         [0089]    An elastically stretchable connection of the two ends  13  can also be achieved by connecting the protruding ends of pins which have been inserted into the plug-in openings  47  by two tension springs made from steel.  
         [0090]    In the embodiment shown in FIGS. 24 and 25, a sleeve  55 , which is provided with a pair of connecting tabs  56  and projects inward into the circumferential groove  16 , is inserted into the window  5  in the surround element  4 , which is formed by a continuous loop of a woven strip or the like. The connecting tabs  56  are-adhesively bonded or welded or joined in some other way to the surrounding area of the window  5 . At the free end, the valve stem  11  has a screw thread  28 , and the stem can be screwed into a screw thread in the sleeve  55  to an adjustable depth and can be clamped by means of a mating nut  27 . In this embodiment, the cam element  2  comprises two cam regions  43 , which are not connected to one another, but rather are fixed separately on the support shaft. Instead of the screw connection, it would also be possible to form a latching or snap-action connection between the sleeve  55  and the valve stem  11 , so that rotation about the axis of the valve stem  11  is possible. The axis  15  about which the surround element  4  has to be pivoted backward and forward to a limited extent with respect to the valve actuator  10  runs between the connecting tabs  56 , on account of the flexibility of the material used.  
         [0091]    In the embodiments shown in FIGS.  26  to  32 , the surround element  4  is in each case designed as a continuous loop with a protuberance  6 , which is divided from the cam element by a constriction which is, for example, adhesively bonded, sewn or clamped, and accommodates an insert  54  which serves as holder  12  of the valve actuator  10 . Particularly in these embodiments, the surround element may also be of constant-length design, if the constriction between the holder  12  and the cam element  2  is of elastically resilient design. In this way it is possible, for example, to sew the constriction by means of rubber filaments or the like.  
         [0092]    [0092]FIGS. 26 and 27 show an embodiment wherein the constriction of the surround element  4  is effected by an elastically widenable eyelet  50 , through which the protuberance  6 , which has been pressed flat, is threaded. The insert  54  which has been pushed into the protuberance  6  has a latching or threaded bore  57 , into which the latchable or threaded end  28  of the valve stem  11  can be pushed or screwed. In the latter case, a mating nut  27  is used for adjusting and fixing the length of the valve actuator  10 . The tensile forces, which rise at higher rotational speeds, widen the eyelet, so that the regions which are in contact with one another in the constriction move away from one another, and the constriction is stretched.  
         [0093]    [0093]FIGS. 28 and 29 show a similar connection between the surround element  4  and the valve actuator  10 , wherein the constriction of the protuberance  6  is effected by two clamping jaws  49  which are clamped to one another, in particular resiliently. The two clamping jaws  49  may also be of identical design, so that in each case one connecting screw is inserted into a clamping jaw  49 . If appropriate, the spring prestressing may also be adjustable.  
         [0094]    Instead of the eyelet  50  or the clamping jaws  49  in the embodiments shown in FIGS.  26  to  29 , a latchable, elastically widenable constricting device is also conceivable, for example by clipping together two parts which are of identical design and are provided with latching hooks and latching openings.  
         [0095]    In FIGS.  26  to  29 , the insert  54  may also comprise a rubber or a rubber-sheathed metal or plastic core, which is pinched into an oval shape by the tensile forces, which rise in the surround element  4  at higher rotational speeds, on account of the rising mass forces of the valve. This likewise leads to an elastic increase in the distance between the axis of rotation  8  and the pivot axis  15  of the valve actuator  10 .  
         [0096]    FIGS.  30  to  32  show a possible way of producing a latching connection between the valve stem  11  and the holder  12 , which allows the valve stem  11  to rotate about its axis. The end of the valve stem  11  is provided with a polygonal, conical or rounded annular groove  59 , and the insert  54  is provided with two webs, which are resilient on account of a slot  51  and on which polygonal, conical or rounded ribs  60  are formed. The valve stem is pushed into the bore  57 , so that the insert  54  is widened, until the ribs  60  latch into the annular groove  59  (FIG. 32). The connecting element  48 , which is responsible for the constriction and is in the form of a U-shaped hollow bracket, is then pushed on and secured by the U-shaped mating piece  53 , which is adhesively bonded or pinched in place. In FIGS.  26  to  32 , the constriction in each case forms an elastic, flexible connection, wherein the axis  15  is embodied.  
         [0097]    FIGS.  33  to  35  show an embodiment wherein the surround element  4 , in a similar manner to the embodiment shown in FIGS.  20  to  23 , comprises a continuous strip of an elastically stretchable woven fabric which is laid together so as to form an open, two-layer loop and the reversal points of which once again form plug-in openings  47 . The ends  13  of the open loop are cut out in such a way that they can be fitted into one another. In this embodiment, the holder  12  is assembled from two parts  12 ′ each of which has a pin-like section  48 ′ of the connecting element  48 , a receiving part and a threaded sleeve for a threaded screw  61 . Two ribs  60 , which engage in a circumferential groove  59  in the valve stem  11 , which is once again held rotatably, project into the opening  57 , which is likewise divided. The two pin-like sections  48 ′ engage in the mutually aligned plug-in openings  47  in the mutually engaging ends  18  and come into contact with one another in the center, as can be seen from FIG. 34. In this embodiment, the cam element  2  is not shown as a part which can be mounted individually, but rather the cam shaft is produced as a single part using a conventional process.  
         [0098]    Since the variable forced guidance of the valve actuator allows the valve mechanism to be of very lightweight design, it is also possible for the entire camshaft to be of very lightweight design. Therefore, it can even be produced in a single piece from an optionally reinforced plastic or other lightweight materials.  
         [0099]    FIGS.  36  to  39  illustrate a first embodiment in which the lengthening of the surround element  4  does not lead to an enlargement of a gap between the eccentric region of the cam element  2  and the connection region of the valve actuator  10 , because the cam element is also enlarged, i.e. increased in size.  
         [0100]    The cam element  2  consists of two cam parts  70 ,  71 . The cam part  70  has a cylindrical base shape and a central window  75  including slightly less than half of the circumference, whereby two cam regions  43  with a circular outer outline remain on both sides. A camshaft  64 , which rotates with the support shaft  1 , is axially displaceable in the support shaft  1 . In the region of each cam element  2 , the camshaft  64  includes a control section  66  with an axially increasing inclined surface  67 . The second cam part  71  of the cam element  2  which also has an inclined surface  72  contacts the inclined surface  67  which is exposed in the window  75 , the second cam part being guided in the window  75  between the lateral cam regions  43  and carrying the eccentric valve control surface at the circumference. FIGS.  37  to  39  show different positions. In FIG. 37 the camshaft  64  is shifted so far to the left that the second cam part  71  reaches its lowest position where it lies within the circular outer outline of the first cam part  70 . In this position the rotation of the cam element  2  which does not have an eccentricity also does not result in a valve lift and the valve actuator  10  being guided in a guide sleeve  41  of the cylinder block  80  or the like remains in the closed position and the surround element  4  is not lengthened. The corresponding cylinder of the internal combustion engine is thus turned off.  
         [0101]    When the camshaft  64  is shifted towards the right by an operating mechanism (arrow  81 ), a normal stroke position is provided in a position according to FIG. 38, which is advantageous for example for the idling speed or a lower speed range, whereby the second cam part  71  has been pushed radially outwardly. The surround element  4  is lengthened to a certain degree in part by pushing the cam part  71  outwardly and in part by the tensile forces in the direction of the valve shaft  11  which are in effect even at idling speed, whereby the increase in distance between the axis of rotation  8  and the holder  12  corresponds to the extent of a “push-out” of the cam part  71 . Without the actively effected expansion of the surround element  4  which is effected by shifting the camshaft  64 , the transition from the no-stroke position in FIG. 37 to the normal stroke position according to FIG. 38 is not possible, because the stationary valve actuator  10  does not exert tractive forces on the surround element which result from the rotation of the support shaft. A further shift of the camshaft  64  in the direction of the arrow  81  transfers the cam part  71  into the auxiliary stroke position of FIG. 39 wherein the uppermost region of the inclined surface  67  is reached. The surround element  4  is elongated to its maximum length and contracts when the camshaft  64  is shifted back.  
         [0102]    In the embodiment according to FIGS. 40 and 41, a rotatable camshaft  65  for the radial movement of the cam part  71  is provided in the support shaft  1 , the control region of which is formed by a crankpin-shaped element  68  with an eccentric control surface  69  which inclines eccentrically in a spiral-shaped manner. The course of the adjusting of the cam part  71  can be seen from a comparison of the two FIGS. 40 and 41, when the camshaft  65  is rotated in the direction of the arrow  82 . In the pushed-out position according to FIG. 41, the cam element  2  is supported by the element  68  of the camshaft  65 . When the camshaft  65  is rotated back counter-clockwise in the support shaft, the cam part  71  located at the spiral control surface  69  once again moves inward until the zero-stroke position according to FIG. 40 is reached. In this position, the cam part  71  is located within the cylindrical outer surface of the cam part  70  so that the contracted surround element  4  is located completely around the cam part  70 .  
         [0103]    Due to the elasticity of the surround element  4 , it can be advantageous when the element contains stiffenings in the transverse direction, i.e. axial direction of the support shaft  1 , for example in the shape of reinforcement ribs  63  comprising pins inserted or bonded into the surround element  4 , for example. The transverse stiffenings prevent that non-supported parts of the surround element  4  get pulled into free spaces  73  of the cam element  2  resulting from the engaging of the two cam parts  70 ,  71 .  
         [0104]    The embodiments according to FIGS. 42 and 43 show a second cam part  71  carrying the eccentric valve control surface, being pivotably mounted at the first cam part  70  about an axis of rotation  8  parallel to the axis  77 . The second cam part  71  has a guide surface  78 . The pivoting takes places via a crankpin  76  of the support shaft  1  that pivots the second cam part  71  from the no-stroke position according to FIG. 2 up into the auxiliary stroke position according to FIG. 43 by lengthening the surround element  4  (arrow  83 ). The valve control surface located within the circular peripheral outline of the cylindrical first cam part  70  according to FIG. 42 gives an asymmetrical shape to the cam element  2  so that other opening and closing characteristics of the valve are given for each amount of stroke or lift. The selection of the arch shape of the cam part  71  preferably gives a position in which the cam element  2  is symmetrical.  
         [0105]    If a woven-fabric strip is used for the surround element  4 , its ends can either be sewn, adhesively bonded or welded together to form a continuous loop, or can be wrapped around and sewn, adhesively bonded or welded, in order to form plug-in openings  47  of the open loop. The windows  5  or edge and center cutouts  52  can readily be formed in a woven fabric which has been treated in this manner.  
         [0106]    A holder  12  as shown in FIGS.  8  to  10  or  26  to  35  is preferably formed from inelastic material, so that an elastically stretchable surround element  4  or an elastically widenable constriction of the surround element  4  is provided for the purpose of changing the distance between the axis of rotation  8  of the support shaft  1  and the articulation axis  15  of the valve actuator  10 .  
         [0107]    However, it is also possible for the holder  12  to consist of an elastically stretchable, rubber-like material, which can in particular be permanently adhesively bonded or vulcanized onto a woven-fabric strip or its ends. The rubber-like material, which is preferably of varying thickness according to the stress profile, effects damping of the surrounding squeezing movement in the surround element  4  which is produced by the cam peaks and good transfer, without stress peaks, of the shear forces from the surround element  4  to the valve actuator.  
         [0108]    As has already been mentioned, the surround element may be composed of an elastically stretchable material and a substantially nonstretch material. In this connection, embodiments wherein the elastically stretchable region is provided opposite the holder  12  are also possible, with the result that any weakening in the connecting area between the valve actuator and the surround element  4  is avoided. A design of this type is illustrated in FIG. 24, wherein the region of the surround element  4  which lies opposite the opening  5 , between the dashed lines, can stretch elastically.  
         [0109]    In all embodiments, the valve mechanism is shown with a valve lifter as valve actuator  10 . However, the valve actuator  10  may equally well comprise a pivotably mounted rocker or drag lever, on one end of which the surround element  4  is arranged in such a manner that it can pivot about the axis  15 . A camshaft for use with internal combustion engines usually has a plurality of valve mechanisms of this type, wherein the cam elements are arranged in an angularly offset manner.