Abstract:
The invention relates to a valve drive, in particular for internal combustion engines of motor vehicles, comprising at least one cam element ( 2 ) that is located on a driven shaft ( 1 ) and at least one lifting valve ( 10 ), which has a valve stem ( 11 ) and can be displaced by the cam element ( 2 ). The cam element ( 2 ) is pivotally mounted inside a flexible encapsulation element ( 4 ), which is connected to one end of the valve stem ( 11 ). The end of the valve stem ( 11 ) is guided in the displacement direction of the valve ( 10 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation of my copending International Application PCT/AT01/00405, dated Dec. 27, 2001, which designated the United States and which was published in a language other than English. 

   BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a valve drive, especially for internal combustion engines of motor-powered devices, motor vehicles, or the like, having at least one cam element disposed on a driven shaft and having at least one lift valve which is displaceable by the cam element and has a valve stem, the cam element being arranged rotatably within a flexible enclosing element connected to one end of the valve stem, and further relates to a cylinder head for such a valve drive. 
   A valve drive of this type can be derived, for example, from WO-01/12958-A. In  FIG. 7 , the cam element is arranged alongside the valve and the cylinder head (not shown) can be built somewhat lower compared with a construction represented in  FIG. 5  or  6  of WO-01/12958-A. 
   Critical to the height of the cylinder head is the length of the slideway of the lift valve, which must not fall below a specific measure and is also partly determined by the diameter of the valve stem, since the forces acting upon the valve in the opening motion contain a lateral component. 
   The desmodromic valve control system dispenses with heavy valve springs and allows a lighter construction of the camshaft and valve drives, so that even the height of the cylinder head might further be reduced. However, the minimum length of the slideway precludes this. The above considerations apply generally to all internal combustion engines, since a lighter construction, for example, reduces fuel consumption. Special importance is given to the height of the cylinder head and, hence, the height of the entire engine, particularly in motor racing, where a lighter construction which economizes on structural height places the center of gravity lower down and impacts critically upon roadholding and vehicle handling. 
   SUMMARY OF THE INVENTION 
   The invention set out therefore to create a valve drive of the type stated in the introduction with improved guidance for the lift valves and achieves this by virtue of the fact that that end of the valve stem which is connected to the enclosing element is guided in the direction of displacement of the valve. The entire upper part of the valve stem is thereby able to be incorporated into the guide length dimension. It has been shown that, if the cylinder head height remains constant, more than double the guide length is attainable compared with the known valve drives. The height of the cylinder head can therefore be reduced, so that the arrangement and accommodation of the inlet and outlet duct to be operated by the lift valve emerges as the critical criterion initially to the length of the guide. 
   In a first preferred embodiment, it is envisaged that a holder is configured between the enclosing element and the valve stem, which holder has sliding surfaces which can be guided on cylinder-head-fixed guide surfaces. 
   Depending on the configuration of the holder as the connecting point between the enclosing element and the valve stem, sliding surfaces can be provided on different parts of the holder or of the valve itself. A first embodiment envisages that the holder projects over the cam element in the axial direction of the shaft and the sliding surfaces are provided on the projecting region of the holder. Even if the valve arrangement is central and well aligned, the guide of the holder is in itself sufficient to produce, axially next to the cam element, a substantial shortening of the structural height. 
   In a second embodiment it is envisaged that the cam element has two axially spaced cam regions and, between these, a groove disposed in extension of the sliding surfaces of the holder, the enclosing element, in the holding region for the valve stem, having a slot corresponding with the groove. In this embodiment, the cam element and guide elements provided on the cylinder head penetrate each other, the width of which guide elements maximally corresponds to the width of the groove, so that the guide of the holder and of the valve stem can also approach close to the carrier shaft. 
   In a first preferred embodiment, the holder provided with the sliding surfaces comprises a bearing sleeve in the enclosing element and a hinge pin connected to the valve stem, which hinge pin is rotatably mounted in the bearing sleeve. The sliding surfaces can be configured on the hinge pin. 
   For the connection between the hinge pin and the end of the valve stem, the hinge pin can be assigned a connecting part, which is connected to the valve stem and is provided with the sliding surfaces. The hinge pin and the connecting part can be arranged in L-shape or in T-shape, the valve stem, for example, being screwed, or the like, into the connecting part protruding from the hinge pin. The T-shape of the holder is especially usable in those embodiments in which the cam element has a groove. 
   The connecting part can also be of fork-shaped configuration or can be assembled from two L-shaped parts connected to the hinge pin. In this embodiment, a transverse part or two transverse members additionally connected to the valve stem extend parallel to the hinge pin in order to increase the strength of the connection. 
   In a further embodiment it is envisaged that the valve stem is offset in relation to the cam element in the axial direction of the shaft. The axially projecting region of the holder can then be fastened to the upper part of the valve stem and can have for this purpose a bore, the axis of which lies in the axis of the valve stem. The upper end of the valve stem can be provided with a threaded bore, in which a fastening screw passing through the bore of the holder engages. In order to make the fastening screw accessible, in this embodiment the driven shaft of the valve drive running thereabove is preferably provided with a bore through which a helical spring or the like can be brought up to the fastening screw of the valve stem. Insofar as the carrier shaft is hollow and is used for the supply of oil to that peripheral surface of the cam element which is covered by the enclosing element, a core barrel is drawn through the driven shaft following the fastening and adjustment of all valve stems, which core barrel covers from inside the access bores for the fastening screws. 
   In a further preferred embodiment, the bore, in the axially projecting region of the holder provided with the sliding surfaces, is a threaded bore, and the upper end of the valve stem has a thread which is screwed into the holder. Here, too, the valve stem can be adjusted and fixed through a corresponding bore of the carrier shaft, for example using a counter screw inserted from above. In place of the screw connection, other connection options are also conceivable, for example pressing, squeezing, clamping, connection by means of a transverse pin, etc. 
   An especially simple, holderless embodiment provides for a direct mounting of the valve stem in the enclosing element, in that an upper end is formed in a cranked or T-shape and is inserted in at least one bearing sleeve, connected to the enclosing element, or insertion opening configured there. The sliding surfaces can be provided in the upper part of the valve stem, which can also there be thickened, for example. 
   If the sliding surfaces are configured at the upper end of the valve stem, yet other options are obtained in terms of design particulars. Thus, at the upper end of the valve stem, a bearing eye can be configured, the outer contour of which is provided with the sliding surfaces and in which the hinge pin of the holder engages, which hinge pin, in this embodiment, can be fixedly connected to the enclosing element. 
   For the mounting of this valve drive in the cylinder head, the lower end of the valve stem is preferably provided with a thread and screwed into the valve disk. The valve drive can therefore be inserted into the cylinder head from above, the valve preferably being set to maximum opening, whereupon the valve disk is fixed. The parts of the valve can therefore also consist of different materials, for example of ceramic, steel, etc. The thread can here also have the function of an expansion bolt. Depending on the arrangement and configuration of the inlet or outlet duct, it is also herein conceivable for the valve disk to extend obliquely to the valve stem. If the camshaft is built out of individual elements, the cylinder head can also be configured in one piece and have bush-type bearing openings. 
   Despite the forced guidance through the enclosing element, the valve, too, can assume a slant and, in at least one principal direction, deviate from the right angle to the rotation axis of the shaft if the valve stem is arranged such that it is displaceable, relative to the cam element, parallel to the shaft. This is possible if the hinge pin can slide either in the bearing sleeve of the enclosing element or in the bearing eye of the valve stem. The displacement travel depends on the slant of the valve stem and generally amounts to just a few millimeters. 
   In a further preferred embodiment, two valves can be actuated jointly. For this purpose, it is envisaged, for example, that the cam element is provided on both sides with a holder for a valve guided at the upper end next to the cam element. In a second embodiment, the two valves can be disposed between two equidirectional cam elements, the two holders having a common hinge pin disposed in both enclosing elements. 
   An arrangement in which the axis of the valve stem of the parallel-running axial plane of the shaft is laterally offset is also possible as a result of the guide of the valve stem, which guide is drawn right up into the holding region, in which embodiment altered opening and closing characteristics of the valve are obtained. 
   The lateral arrangement of the valve stems next to the cam elements and their guide, drawn up practically as far as the carrier shaft, can give rise, as already mentioned, to especially low cylinder heads, this lateral arrangement likewise promoting the guidance of the inlet and outlet ducts. The duct can in fact be guided next to the relatively large bearing recess, necessary in the cylinder head, for the cam element, in which case, in combination with a corresponding slant, cross-sectional configuration and valve seat configuration, for example appropriate to the oblique valve disk, the cylinder head height can be so far reduced that, even though its basic measure is dependent, in turn, on the minimum guide length of the valve stem, this guide length lies substantially closer to the driven shaft and is preferably also divided into two mutually spaced portions. Especially in the embodiment in which the two valves are provided on a common hinge pin between two cam elements, the valves can be distanced sufficiently far away from the cam elements that a problem-free arrangement of the ducts is possible. The hinge pin can in this case also be cranked in the style of a stirrup, so that its middle portion runs closer to the shaft. 
   A first preferred embodiment of a cylinder head has a semicircular bearing recess for the shaft and a semicircular bearing recess for each cam element, in the region of a bore for the reception of the valve stem guide surfaces being provided for that end of the valve stem which is connected to the enclosing element, which guide surfaces extend in the direction of displacement of the valve. In particular, a guide sleeve made from an appropriate bearing material and whose upper end has a slot is pressed into each bore of the cylinder head, the guide surfaces being provided in the region of the slot. The slot serves the passage of the hinge pin to the connecting point with the enclosing element, which connecting point lies alongside the guide sleeve. The guide surfaces can also be provided on rollers, rolling elements or the like. 
   In a second, particularly material-saving embodiment of the cylinder head, it is envisaged that it has a base element having a bearing web for the shaft and having a guide web for the valve, which guide web is disposed in the region of the bore for the reception of the valve stem, the guide web being assigned guide surfaces for that end of the valve stem which is connected to the enclosing element. If the cam element has a groove, the guide web can be configured in two parts in extension of the groove and the thickness of the two parts of the guide web corresponds maximally to the width of the groove. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in greater detail below with reference to the figures of the appended drawings, without being restricted thereto. 
       FIGS. 1 to 3  show a first embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 1  showing a section perpendicular to the driven shaft,  FIG. 2  a longitudinal section and  FIG. 3  the detail A from  FIG. 2  in enlarged representation; 
       FIGS. 4 to 7  show a second embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 4  showing an exploded representation in oblique view,  FIG. 5  a longitudinal section,  FIG. 6  the detail A of  FIG. 5  in enlarged representation and  FIG. 7  an enlarged section along the line VII—VII of  FIG. 5 ; 
       FIGS. 8 to 10  show a third embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 8  showing an exploded representation in oblique view,  FIG. 9  a longitudinal section and  FIG. 10  the detail A of  FIG. 9  in enlarged representation; 
       FIGS. 11 to 13  show a fourth embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 11  showing a top view of the empty cylinder block,  FIG. 12  a longitudinal section and  FIG. 13  the detail A of  FIG. 12  in enlarged representation; 
       FIGS. 14 to 16  show a fifth embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 14  showing a longitudinal section,  FIG. 15  a section perpendicular to the shaft and  FIG. 16  a cut oblique view; 
       FIGS. 17 to 20  show details of a sixth embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 17  showing an oblique view of a holder,  FIG. 18  a section through the holding region,  FIG. 19  an oblique view of the guided holding region and  FIG. 20  a side view of the guided side region; 
       FIGS. 21 to 24  show a seventh embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 21  showing an oblique view,  FIG. 22  a holder in oblique view,  FIG. 23  a section through the holding region and  FIG. 24  a section through the holding region along the line XXIV of  FIG. 23 ; 
       FIGS. 25 to 30  show an eighth embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 25  showing an exploded representation in oblique view,  FIG. 26  a carrier shaft portion having a cam element,  FIG. 27  a longitudinal section in oblique view,  FIG. 28  the longitudinal section in top view and  FIGS. 29 and 30  details of the holding region of a valve in oblique view and in section; 
       FIG. 31  shows a variant of the holder with a hinge pin; 
       FIGS. 32 to 40  show a ninth embodiment of a cylinder head having a valve drive comprising at least one valve in three different positions during a revolution of the carrier shaft,  FIG. 32  showing a longitudinal section and  FIG. 33  a cross section through the cylinder head and  FIG. 34  an oblique view of the holding region, respectively in the valve-closing setting.  FIG. 35  represents a cross section and  FIG. 36  a front view of the holding region, respectively in a part-opened valve setting. Further,  FIG. 37  shows a cross section,  FIG. 38  a longitudinal section and  FIG. 39  an oblique view through the cylinder head, and  FIG. 40  a section through the holding region, respectively in valve-open setting; 
       FIGS. 41 to 47  show a tenth embodiment of a cylinder head having a valve drive comprising at least one valve,  FIG. 41  showing an oblique view,  FIG. 42  a longitudinal section,  FIG. 43  the holding region in front view,  FIG. 44  the holding region in section,  FIG. 45  a carrier shaft portion in oblique view,  FIG. 46  a section along the line XLVI of  FIG. 42  and  FIG. 47  an oblique view of a detail of the guide; 
       FIG. 48  shows an oblique view of a guide sleeve and 
       FIG. 49  shows a cross section through an eleventh embodiment of a cylinder head having a valve drive comprising at least one valve. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A valve drive comprises, in all embodiments, a driven carrier shaft  1 , on which at least one cam element  2  is fixed in a manner which is not described in greater detail. The cam element  2  is surrounded by an enclosing element  4 , which consists especially of high-tensile, low-friction fibers, such as Kevlar, aramid, glass or carbon fibers, which, for example, are made up into a fabric produced in a textile circular-working method or, through helical winding, are made up into a closed loop, of a high-tensile plastics or metal band, or the like. The enclosing element  4  has a holding region  6  having a insertion opening  7 , in which holding region it is hinge-connected to a valve  10  by a holder  12 . As a result, the enclosing element  4  cannot rotate jointly with the cam element  2 , but can translate the latter&#39;s rotary motion into an oscillating motion which imparts an opening and closing motion to the valve  10  disposed in a slideway. The valve disk  69  thereby lifts off from the valve seat  70 , or closes it, so that the inlet or outlet duct  89  in the cylinder head  20 ,  80  is opened or reclosed. The cam element  2  can have a radial bore  3 , via which, from the hollow shaft  1 , oil can be introduced into the region between the cam element  2  and the enclosing element  4 . 
   The enclosing element  4  is connected to the valve stem  11  of the valve in several different ways, which are described in greater detail below. The valve stem  11  is guided in the cylinder head  20 ,  80  through a bore  88 , in which is inserted a guide sleeve  81 , the lower region of which is closed and the upper end region of which is provided with a slot  82 . In the embodiments according to  FIGS. 1 to 16 , the cylinder head  80  has a semicircular bearing recess  91  for the shaft  1  and a semicircular bearing recess  86  for each cam element  2 , which latter bearing recess is provided with a central indentation  92  to create room for the connection between the enclosing element  4  and the holder  12 . The bore  88  emerges upward into the bearing recess  91  for the shaft  1  and opens laterally into the bearing recess  86  for the cam element  2 . The inner sides of the slotted region of the inserted guide sleeve  81  form guide surfaces  85  for the holder  12  of the valve stem  11  or its upper end, which guide surfaces ascend almost up to the shaft  1 . The high-drawn guide allows the height of the cylinder head  80  to be considerably reduced without having to dispense with the necessary characteristics (good heat dissipation, high power take-up, etc.). 
   In the embodiments according to  FIGS. 17 to 49 , the cylinder head  80  is reduced in weight and comprises a base plate  20 , from which at least one bearing web  21  rises up, in which the shaft  1  is mounted. At least one guide web  22  rises up, laterally offset, in the region of the bore  88  for each valve  10 , on which guide web the guide surfaces  85  are directly configured or in which guide web a guide sleeve  81  having the guide surfaces  85  and the slot  82  is inserted. The bearing web  21  and the guide web  22  can be screw-connected, plug-connected or otherwise connected to the cylinder head  20 ,  80 ; they can also, however, be configured in one piece with the cylinder head  20 ,  80 . 
   In the embodiments according to  FIGS. 1 to 24 , the holder  12  projects laterally over the cam element  2 , and the valve  10  and high-drawn guide lie respectively alongside the cam element  2 . 
   In the embodiments according to  FIGS. 25 to 49 , the usual arrangement, on the other hand, is maintained, i.e. the holder  12  does not lie offset within the envelope of the rotary cam element. In these embodiments, the cam element  2  has a central peripheral groove  31 , which is provided in extension of the guide web  22 . In the axial direction of the shaft  1 , the guide web  22  is no wider than the peripheral groove  31 , so that, when the cam element  2  rotates, the guide web  22  can penetrate into the cam element  2 . The enclosing element  4  has in the holding region  6  a slot  5 , which leaves the groove  31  uncovered and extends approximately over half of the periphery of the enclosing element  4 . 
   In the embodiment according to  FIGS. 1 to 3 , the insertion opening  7  of the enclosing element  4  is provided with a bearing sleeve  68 , in which, from both sides, a hinge pin  14  of a holder  12  is rotatably inserted. That region  61  of the holder  12  which projects axially over the cam element  2  is provided with a bore  62 . The upper end of the valve stem  11  has a threaded bore, in which is inserted a fastening screw  63  which passes through the bore  62  and fixes the valve  10  on the holder  12 . In order to facilitate access to the screw  63 , the above-lying shaft  1  contains a bore  30  through which a tool can access the screw  63 . The hinge pin  14  in this case passes through the slot  82  of the guide sleeve  81 , the outer face of the region  61  bearing the bore  62  and that portion of the hinge pin  14  which is guided in the slot  82  forming sliding surfaces  65  of the holder  12 , which slide up and down on the guide surfaces  85  of the guide sleeve  81 . 
   In the very similar embodiment according to  FIGS. 4 to 7 , the holder  12  is provided, in turn, with a hinge pin  14 , which is rotatably mounted in a bearing sleeve  68  and the axially projecting region  61  of which has a threaded bore and sliding surfaces  65 . The projecting region  61  is mounted displaceably in the guide sleeve  81 , the hinge pin  14  being guided outward through the slot  82 . The upper end of the valve stem  11  is provided with a thread and screwed into the bore  62  of the holder  12 . Here, too, an adjustment of the valve stem and the placement of a fixing counter-screw, via a bore  30 , in the above-running shaft  1  is possible.  FIGS. 4 and 5  also show a core barrel  38  (not shown in the embodiment according to  FIGS. 1 to 3 ), which, following mounting of the valve drive, is pushed into the shaft  1  and covers the bore  30  from inside. From the enlarged representations of  FIGS. 6 and 7 , the guide for the valve stem  11 , which guide approaches close to the shaft  1 , can be especially well seen. The guide is divided into two mutually distanced regions, between which there is disposed a seal  83  for the valve seat  11 . 
   In the embodiment according to  FIGS. 8 to 10 , the valves  10  are arranged at an inclination relative to the right angle to the rotation axis  8  of the shaft. The bearing sleeve  68  of the enclosing element  4  is passed through by a hinge pin  14 , which has regions  61  which project axially on both sides and engage in a respective bearing eye  78 , which bearing eyes are configured at the upper ends of two valve stems  11  and are provided with the sliding surfaces  65 . The bearing eyes  78  allow the slanting of the valve stems  11 , which, upon the lifting motion, move slightly to and fro on the hinge pin  14 . The valve stems  11  are provided on the lower ends with a threaded portion, which is screwed into the corresponding threaded bore of the valve disk  69 . The hinge pin  14  is again guided in the slots  82  of the two guide sleeves  81 . 
   In the embodiment according to  FIGS. 11 to 13 , the two valves  10  are disposed on a hinge pin  14  connecting two equidirectional cam elements  2  and are inserted in a respective bearing sleeve  68  of an enclosing element  4 . The two valve stems  11  are slightly inclined, so that the ducts  89  can be guided in the region between the cam elements  2 , as is clearly evident from the top view of  FIG. 11 . The two bearing recesses  86  for the cam elements  2 , inclusive of their central indentations  92  for the connecting regions, containing the bearing sleeves  68 , between the enclosing elements  4  and the hinge pin  14  are connected by a central slot  87 , in which the up-and-down moving hinge pin  14  is guided. The middle region of the latter can further be cranked in the style of a stirrup, so that it is proximate to the shaft  1 , whereby the middle region of the slot  87  can be less deep. Emerging into the slot  87  are the two oblique bores  88 , in which are inserted the guide sleeves  81  which, in the upper regions, are slotted for the passage of the hinge pin  14 . The bearing eyes  78  have the sliding surfaces  65 , which slide on the inner guide surfaces  85  of the guide sleeves  81 . As a result of the oblique inclination of the valve stems  11 , the bearing eyes  78  move slightly left and right. 
     FIGS. 14 to 16  show a similar embodiment, in which the height of the cylinder head  80 , despite sufficient guide length for the valves  10 , is once again reduced, since, in the ducts  89 , diagonal valve seats  70  for the valve disks  69  are configured, which, for their part, are again fastened obliquely to the valve stems  11 , for example by the thread  77 , but which could equally be replaced by a press-fastening or another kind of fastening. In the oblique view of  FIG. 16 , the bearing shells  93  for the shaft  1  are also visible, which are mounted on the top side of the cylinder head  80 . 
   In  FIGS. 17 to 24 , two embodiments are shown, in which only the guide, but not the valve  10 , is laterally offset in relation to the cam element  2 . According to  FIGS. 17 to 20 , the holder  21  has a hinge pin  14  which is inserted in the insertion opening  7  of the enclosing element  4  and projects on both sides. In its projecting regions  61 , a fork-shaped connecting part  18 , provided with two eyes, is pivoted, on which the end of the valve stem  11  is centrally fixed. The connecting part  18  has a blind bore  25 , the floor of which is a spherical surface and into which a receiving bore  26  for the upper end of the valve stem  11  emerges, which end, in this embodiment, has an offset spherical head. Inserted in the blind bore  25  is a screw  27 , the front side of which likewise has a spherical surface and fixes the spherical head of the valve stem  11 . If the screw  27  has an end stop, then the spherical head is held not clamped but rotatably. The lateral members  19  of the fork-shaped connecting part  18 , which members are provided with the eyes  78 , are provided on the outer side with the sliding surfaces  65 , which are guided on the guide surfaces  85 . As can be seen from  FIG. 19 , the guide surfaces  85  are configured on guide webs  22  or inserts made from bearing material, which on both sides of the cam element  2  approach close to the shaft  1 . 
   According to  FIGS. 21 to 24 , the guide webs  22  form cylindrical elements and the connecting part  18  of the holder  12  has a circular outer contour. The lateral members  19  of the connecting part  18 , which members are provided with the eyes  78 , constitute cylinder segments, which on the outer side have sliding surfaces  65  and are connected by a transverse part  28  and the distance apart of which corresponds to the width of the cam element  2 . The connecting part  18  has in its lateral members  19  the two eyes  78 , which are mounted rotatably on the hinge pin  14  projecting from the insertion opening  7  of the enclosing element  4  on both sides ( FIG. 23 ). Self-evidently, the hinge pin  14  can also be mounted rotatably in the insertion opening  7 , or a bearing sleeve  68  provided there, and can be fixed in the eyes  78 . In the transverse part  28  of the holder  18 , the blind bores  25  and the bottom-side receiving opening  26  are provided, through which the upper end of the valve stem  11 , provided with an offset spherical head, is inserted. A screw  27  inserted in the blind bore  25  holds the valve stem  11 . The cylindrical guide web  22  has a slot  82  in the width of the cam element  2 , so that the cam region has the necessary passage clearance. The holder  12  connected to the enclosing element  4  is thus guided up and down in the cylindrical guide web  22  in a piston-like manner. 
     FIGS. 25 to 30  show a first embodiment having a cam element  2  provided with a central groove  31  and having an enclosing element  4  provided in the holding region  6  with a central slot  5 . The holder  12  used in this embodiment has a hinge pin  14  inserted in the insertion opening  7  of the enclosing element  4 , said insertion opening being provided, where appropriate, with a bearing sleeve  68 , which hinge pin is provided with a front-sided blind bore  25  and a therein emerging receiving bore  26  for the upper end of the valve stem  11 . The upper end of the valve stem  11  is provided with at least one peripheral channel, in which a rib in the floor of the blind bore  25  and a rib of a fitting piece  16  engage, which fitting piece is held in the blind bore  25  by a screw  17  ( FIG. 30 ). The hinge pin  14  has a flattening on both sides of the receiving bore  26  and the two flattenings form mutually parallel sliding surfaces  65  ( FIG. 25 ). On both sides of the valve stem  11 , which is mounted displaceably in the bore  88  of the cylinder head or of the cylinder head base plate  20 , a guide web  22  rising up in the bearing recess  86  of the cylinder head  80  or from the base plate  20 , extends respectively close to the carrier shaft  1  of the cam element  2 , the mutually facing surfaces of the guide webs  22  forming the cylinder-head-fixed guide surfaces  85 , on which the sliding surfaces  65  of the hinge pin  14  are guided in sliding motion. The upper regions of the guide webs  22 , when the cam element  2  rotates, enter through the slot  5  into the groove  31 , which extends at least over the cam region of the cam element  2 . 
     FIG. 31  shows a variant in which the holder  12 , similar to the embodiment according to  FIGS. 17 to 20 , comprises a hinge pin  14 , in which the upper end of the valve stem  11 , which end has a spherical head, is held directly by a screw  27 . The screw  27  preferably does not clamp the spherical head, but holds it swivel-mounted. The sliding surfaces  65  are formed, in turn, by flattenings of the hinge pin  14 . 
     FIGS. 32 to 40  show a further embodiment having grooved cam elements  2 , the enclosing elements  4  of which, in turn, have slots  5  in the holding regions  6 . This embodiment differs from the previous embodiment by the configuration of a reinforced holder  12 . This comprises a connecting part  18  formed from two L-shaped elements, each of which has a side part  19 , having an eye  78 , and a transverse member  29 , having a bore  34 . The two L-shaped elements are fixed on the projecting ends of the hinge pin  14 . The upper end of the valve stem  11  is provided with two or more mutually parallel bores, the hinge pin  14  being put through the upper bore and the cotter pin  33  being put through the lower bore. This connection is primarily suitable for very thin valve stems  11 , which, where appropriate, might be too much weakened by a single bore for the hinge pin  14 , or the hinge pin  14  of which has too small a cross section. In this embodiment, three different settings of the valve  10  are shown, which are also similar in the other embodiments.  FIGS. 32 to 34  show a basic setting with two valves  10 , which close the inlet and outlet ducts  89 . The guide webs  22  rising vertically from the cylinder head base plate  20 , as can be seen, above all, from  FIG. 33 , approach close to the carrier shaft  1  of the cam elements  2 . The bore for each valve stem  11  is configured within a guide sleeve  81  ( FIG. 48 ), which, in the region inserted in the cylinder head base plate  20 , is closed and in the region situated in the guide web  22  has the slot  82 , which is passed through by the jutting transverse elements of the holder  12 . The width of those wall parts of the guide sleeve  81  which remain on both sides of the slot  82  and on which the guide surfaces  85 , rising up almost to the carrier shaft  1 , are provided corresponds to the thickness of the guide web  22  and the width of the groove  31 , which, in its extension, is configured all the way round in the cam element  2 . 
   As is clearly discernible in the comparison with the oblique view according to  FIG. 34 , in the section according to  FIG. 32  the remaining wall parts of the guide sleeve  81  and the guide web  22  are thus situated exactly behind the valve stem  11  and hence are not, however, fully visible there in the section perpendicular thereto according to  FIG. 33 . 
     FIGS. 35 and 36  are details of the 120°-twisted setting of the cam element  2 , in which cam element the valve disk  69  has been lifted from the valve seat  70 . The holder  12  is displaced downward in the guide sleeve  81  and the guide web  22  has entered the groove  31  through the slot  5  present in the holding region  6  of the enclosing element  4 , i.e. the two cam regions of the cam element  2  move past on both sides of the guide web.  FIG. 36  also shows the twisting of the holder  12  relative to the enclosing element  4  about the axis  15  of the hinge pin  14 , since the valve stem  11  does not extend perpendicular to the tangent to the cam element  2 , as is the case in the basic setting according to  FIG. 33  and in the open setting according to  FIG. 37 . In the open setting, the holder  12  is pushed downward in the guide sleeve  81  over the full height of the slot  82  and bears almost against the surface of the cylinder head base plate  20 . In  FIG. 40 , the wall part of the guide sleeve  81  with the guide surface  85  is therefore visible in the groove  31  of the cam element  2 . 
   In this embodiment, the sliding surfaces  65  are provided on the valve stem  11 , the free ends of the transverse members  29  also, where appropriate, being able to be flattened and guided along the margins of the slot  82  of the guide sleeve  81 . 
   In the embodiment according to  FIGS. 41 to 47 , a further variant having grooved cam elements  2  is shown, the holder  12  having a T-shape ( FIGS. 44 ,  47 ), the transverse part of which forms the hinge pin  14  and the central longitudinal part of which either forms the connecting part  18  to the valve stem  11  or the valve stem  11  itself. In the former case, the connecting part  18  is suitably connected to the valve stem  11 , for example by a screw connection, if one of the two elements has a thread and the other a threaded bore (similar to  FIG. 7 ). In the second case, the valve stem  11 , as in the embodiment according to  FIGS. 8 to 16 , is provided at the lower end with a thread  77  and screwed into the valve disk  69 , which, on the bottom side, can have tool engagement elements  72 , for example. As  FIG. 44  shows, the two side portions of the hinge pin  14  are inserted in the insertion opening  7  which is divided by the slot  5  in the enclosing element  4  ( FIG. 25 ) and in which, where appropriate, bearing eyes  68  are disposed. The slot  5  is sufficiently large for the two parts of the insertion opening  7  in the enclosing element  4  made of flexible material to be moved so far apart that the hinge pin  14  can be inserted from the slot  5  bilaterally into the insertion opening  7 . The further design construction of this embodiment largely corresponds to that of the embodiment according to  FIGS. 31 to 40 . The sliding surfaces  65  are configured on the valve stem  11  or the connecting part  18 , which is guided in the slotted guide sleeve  81  along the guide surfaces  85 . The diameter of the hinge pin  14  is less than the diameter of the valve stem  11  or of the connecting part  18 , as is evident from the section through the hinge pin  14  shown in  FIG. 46 . The slot  15  in the enclosing element  4  must exceed in height at least the lift of the valve. Alternatively, it is also possible to extend the slot over the whole of the periphery of the cam element  2 , so that the enclosing element  4  is divided into two narrow loops, which are connected by the hinge pin  14  only in the holding region  6 . For the axial securement of the enclosing element  4 , it is advantageous if the cam element  2  has at the periphery an indentation, laterally delimited by the marginal webs  9 , the height of which indentation maximally corresponds to the thickness of the enclosing element  4 . In the case of a division into two loops, the margins which delimit the groove  31  are preferably provided with marginal webs  9 . In this embodiment, the groove  31  is provided only over the cam region, but, as in the embodiment according to  FIGS. 32 to 40 , can equally be configured all the way round on the cam element  2 . 
   A further variant is shown in  FIG. 49 . In this embodiment, the axis  71  of the valve stem  11  does not intersect the axis  8  of the carrier shaft  1 , but runs past at a distance therefrom. The valve drive is thus asymmetrical, so that changes in the opening and closing time, as well as in the length of opening, can be obtained by displacement of the rolling and contact lines. The other construction of this embodiment corresponds to that of the embodiments already described above. The guide web  22  engages in the circumferential groove  31  of the cam element  2  and the valve stem  11  is guided through the guide sleeve  81  into the holding region  6  of the enclosing element  4 . The connection of the valve stem  11  and the enclosing element  4  is indicated by the cut hinge pin  14 . An asymmetrical arrangement and guidance of the valve is possible in all the embodiments previously described. In addition, it also allows a steeper arrangement of the inlet and outlet ducts  89 , if the lateral offsetting of the carrier shaft is effected in the direction shown in  FIG. 49 , i.e. toward the side facing away from the ducts  89 .