Abstract:
The invention relates to an internal combustion engine comprising a valve drive which is arranged in the region of the cylinder head and is used to actuate a valve. Said internal combustion engine comprises a first drive means that can be rotated about a first rotational axis, a connecting rod that is connected to the first drive means in an articulated manner by means of a first connecting rod articulation, and a guiding element which is used to guide the connecting rod, can be pivoted about a guiding axis, and is connected to a second articulation of the connecting rod in an articulated manner. The position of the first rotational axis can be modified in relation to the cylinder head.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a combustion engine, in particular to a combustion engine with a valvetrain. In particular, the present invention relates to a combustion engine with a valvetrain with variable valve control. 
     Typically, in combustion engines, valvetrains are provided that generate a valve lift of fixed lift height and duration using a camshaft. The inflow by volume of the air fuel mixture is controlled by a throttle valve. Such a valvetrain, however, has disadvantages regarding an optimal adjustment to different load regimes of the combustion engine. Such disadvantages can at least partially be avoided by a valvetrain of variable valve lift. 
     2. Description of the Related Art 
     Valvetrains with variable valve lift are known in the art. The Valvetronic system of BMW, for example, is a valvetrain with variable valve lift, which is being produced in series. It allows a variable adjustment of valve lift and duration of valve opening in response to a number of drive parameters, such as rotational speed and the position of the throttle control. Further, valvetrains with variable valve lift are known for motors of low rotational speed with cam in block. Cam-in-block motors, however, generally have a number of disadvantages, in particular in the regime of high rotational speed. For example, relatively large masses generally need to be moved when opening and closing the valve. 
     Further, EP 1 350 928 describes an apparatus for smoothly varying the valve lift in combustion engines. Therein, a linear cam is pushed more or less deeply between two rollers by an actuating member, wherein one of the rollers is movable in the sense of the valve lift. Thereby, a lift curve on a roller, mounted in a rocker level or another actuating member towards the valve, actuates the valve lift. 
     A further valvetrain for a combustion engine is described In U.S. Pat. No. 5,357,915. The valvetrain described therein allows modifying the valve synchronization and the valve lift of the intake and exhaust valves, which are driven by rocker arms and overhead camshafts. 
     PCT 2002/053881 describes a variably adjustable mechanical valve gear for a gas-reversing valve with a closing spring on a piston engine, in particular a piston-type internal combustion engine, comprising a driving mechanism for generating a lifting movement that is effective counter to the force of the closing spring on the gas-reversing valve and a stroke transfer means arranged between the driving mechanism and the gas-reversing valve, which acts upon the gas-reversing valve in the direction of its movement axis and for which the stroke distance can be adjusted in the direction of the movement axis via an adjustable guide element and which is realized with a pivoting element that is connected with its end facing away from the gas-reversing valve to the driving mechanism and is guided so as to pivot back and forth on the guide element designed as control curve. 
     Further, FR 1 284 700 describes a valvetrain for combustion engines, which allows to adjust automatically the duration of the valve lift and the lifting height of the valves, in order to adapt them to the rotational speed. 
     A valvetrain for a motor according to EP 1 515 008 is composed as follows: An intermediate rocking element is interposed between a pivoting earn surface and a rocking surface of a rocking arm. By shifting of contact spots an opening duration and an opening height of the valve can be adjusted continuously. 
     DE 23 35 634 relates to a valve adjustment for combustion engines, comprising a driving shaft, an arrangement for transforming the rotational movement to a pivotal movement actuating the valve, and an arrangement for modifying the valve lift in response to the rotational speed and the load of the combustion engine. 
     U.S. Pat. No. 6,684,832 describes a valvetrain of a combustion engine, which allows modifying the lift and the opening and closing angle. To this purpose, the camshaft is not mounted fixedly on the cylinder head, but it oscillates about an axis, and by this oscillation the valve lift and duration of valve opening are modified. 
     The valvetrains mentioned above, however, are deficient in several respects, for example regarding their space requirements, their weight, susceptibility to wear, and limitations of the variability of the valve control. 
     The present invention attempts to reduce at least some of the above-mentioned problems. Further, the present invention attempts to provide a combustion engine, which has good properties with respect to motor performance, wear, and/or other aspects. The object is solved by a combustion engine according to independent claim  1 . Further advantages, features, aspects and details of the invention as well as preferred embodiments and particular aspects of the invention are evident from the dependent claims, the description and the drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     The Combustion engine has a valvetrain for actuating a valve, the valvetrain being arranged in the cylinder head portion, wherein the valvetrain comprises a first driving member, which is rotatable about a first rotation axis; a connecting rod with a first joint and a second joint, and a guiding member for guiding the connecting rod, the guiding member being pivotable about a guiding member axis. The connecting rod is joined with its first joint to the first driving member, and is joined with its second joint to the guiding member. The position of the first rotation axis is adjustable in relation to the cylinder head. Preferably, the connecting rod and the guiding element are members of a gear or transmission, which is coupled to the first driving member in order to transmit a rotational movement of the first driving member into a lifting movement for actuating the valve. 
     The connecting rod allows transforming the rotational movement of the first driving member into a lifting movement, by which the valve is actuated. The guiding member allows avoiding an unwanted pivoting of the connecting rod about the first connecting rod joint, whereby a directed transmission of the force from the connecting rod to the valve can be achieved. Further, thereby the susceptibility to wear of the valvetrain can be reduced. Further, by having the position of the first rotation axis adjustable, an additional degree of freedom of the valvetrain is created, which can be used for adjusting various properties of the valvetrain. 
     According to a further preferred aspect of the invention, the valvetrain further comprises a second driving member for driving the first driving member, wherein the second driving member is preferably arranged in the cylinder head portion. The second driving member is rotatable about a second rotation axis. 
     According to a further preferred aspect, the valvetrain comprises a pushing member for transmitting a lifting movement of the connecting rod for actuating the valve, wherein the pushing member is fastened or joined to the connecting rod or to the guiding member. Preferably, the pushing member is joined rotatably or pivotably. According to a particularly preferred aspect, the pushing member is a roller. Thus, energy loss or wear due to sliding friction can be reduced. 
     According to a further preferred aspect, a transmission member is in mechanical and preferably in releasable mechanical contact with the pushing member. The transmission member allows a force exerted by the pushing member to be transmitted for actuating the valve. Further, a desired valve lifting curve can be approximated by a suitable design of the transmission member. 
     According to a further preferred aspect, the transmission member is biased by a forcing member, e.g. by a spring or a spring-like member, towards the valve. Thus, a non-positive connection between the transmission member and the valve can be promoted by the forcing member, hence an unwanted mechanical play of the transmission member can be limited. 
     According to a further preferred aspect, a hard stop defines a maximum displacement of the transmission member. The maximum displacement defined in this manner is preferably directed away from the valve. It is further preferred that the fixed stop is stationarily arranged in the cylinder head. It is further preferred that the stop is adjustable. By means of the stop, an unwanted mechanical play of the transmission member can be limited. 
     According to a further preferred aspect, the transmission member is a lever, which is pivotable about a lever axis; it is particularly preferred that the lever is a one-arm lever. Thus, an advantageous force transmission between the pushing member, the transmission member and the valve can be achieved, and a stable mounting of the transmission member can be provided. 
     According to a further preferred aspect, the transmission member has a valve pushing surface for actuating the valve and/or for mechanical contact of the valve. The valve pushing surface allows transmitting to the valve a pushing force generated by the pushing member. 
     According to a further preferred aspect, a movement of the pushing member toward the lever axis and/or a sliding or rolling of the pushing member along a pushing receiving surface of the lever toward the lever axis causes the valve to open. This arrangement allows for an advantageous load of the mounting of the lever about the lever axis. 
     According to a further preferred aspect, the valve is an intake valve. According to a further preferred aspect, the combustion engine comprises a second intake valve, wherein preferably both intake valves are associated to the same piston chamber. The valvetrain is preferably adapted such that both valves are actuated in the same or similar manner, such that e.g. a same or similar valve lift behaviour and/or a same or similar valve lift curve is generated. By the second intake valve, e.g. an improved supply of air-fuel-mixture and hence an increase of motor power can be achieved. 
     According to a further preferred aspect, a valve lifting curve and/or a quantity characterizing the valve lifting behaviour is adjustable by the adjustment of the position of the first rotation axis. Thereby, e.g. a magnitude of valve lift and/or a duration of the valve opening is adjustable. Herein, the duration of the valve opening is in terms of the motor cycle and is, hence, defined e.g. in terms of the rotational angle of the crankshaft. Thus, a variable valve actuation control e.g. depending on the load can be achieved. 
     According to a further preferred aspect, the first driving member is rotatable synchronously to a motor cycle of the combustion engine, such that there is a phase relation between the rotational angle of the first driving member and a phase angle of the engine cycle. By the adjustment of the position of the first rotation axis, the phase relation is adjustable. Hence, different aspects of the valve actuation can be designed variably, in particular a phase relation of the valve actuation in relation to the motor cycle. 
     According to a further preferred aspect, a phase of the valve lifting behaviour relative to the motor cycle of the combustion engine is adjustable by the adjustment of the position of the first rotation axis. According to a further preferred aspect, there is a phase relation between the rotational angle of the first driving member and half of the rotational angle of the crankshaft of the combustion engine, and by the adjustment of the position of the first rotation axis, the phase relation between the rotational angle of the first driving member and the half rotational angle of the crankshaft is adjustable. 
     According to a further preferred aspect, the pushing member is guided on a guided path, and preferably the guided path of the pushing member is adjustable by the adjustment of the position of the first rotation axis. This allows a variation of the valve lifting behaviour. 
     According to a further preferred aspect, the adjusting of the position of the first rotation axis is a pivoting of the first rotation axis about a pivoting axis. Preferably, the pivoting axis is also the second rotation axis. Thereby, the distance between the first rotation axis and the second rotation axis can be held constant during pivoting, which may result in advantages with respect to the interaction of first and second driving member, in particular if the interaction is by means of two meshing gearwheels. 
     According to a further preferred aspect, the valvetrain comprises a pivoting drive for pivoting the first driving member. The pivoting drive comprises a pivoting drive gearwheel, which is rotatable about a third rotation axis, and a pivoting drive gear segment, which is in meshing connection with the pivoting drive gearwheel. Preferably, the pivoting drive gear segment is fixedly connected to a pivoting member, in which the first driving means is mounted. Thereby, a stable mounting of the first driving means can be achieved. 
     According to a further preferred aspect, the valvetrain comprises a worm wheel or worm gear, which is in meshing connection with the pivoting drive gearwheel for driving the pivoting drive gearwheel. Thereby, e.g. the position of the first rotation axis can be held stably. 
     The third rotation axis is also the lever axis of the lever. This arrangement allows a compact structure of the valvetrain. 
     According to a further preferred aspect, the connecting rod and the guiding member are members of a linkage, preferably members of a pinned, and preferably planar linkage, particularly preferably a pinned linkage of four links. This allows an advantageous guiding of the connecting rod. 
     According to a further preferred aspect, the valve is an intake valve, and the second driving member also actuates an exhaust valve. This promotes a coordinated actuation of intake and exhaust valve. 
     According to a further preferred aspect, a maximum valve lift of the valve is at least 5 mm, preferably at least 7 mm, further preferably at least 10 mm, particularly preferably at least 12 mm, and especially particularly preferably at least 15 mm. This allows achieving a high power output of the combustion engine. 
     The combustion engine can be used particularly advantageously in devices or vehicles with high rotational speeds of the engine, for example in motorcycles. It can further be used in cars, trucks, airplanes, or watercrafts. 
     According to a preferred aspect of the invention, the valvetrain can be subdivided into an active subsystem, which is connected positively to the first driving member, and into a passive subsystem, which is connected only non-positively to parts of the active subsystem. Preferably, the first driving member, the connecting rod, and the guiding member and, if applicable, the pushing member, are associated to the active subsystem. Further, preferably the valve and, if applicable, the transmission member are associated to the passive subsystem. The non-positive connection allows designing the valvetrain such that a certain valve clearance is possible, and that manufacturing tolerances or thermal expansion can be compensated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention are shown in the Figs. and are described in more detail in the following. 
         FIG. 1  shows a cross-sectional site view of a first embodiment of a valvetrain according the invention; 
         FIG. 1   b  shows a schematic view of the valve crank of the valvetrain of  FIG. 1 ; 
         FIG. 2  shows an example valve lift diagram of a valvetrain according to the invention; 
         FIG. 3  shows a cross-sectional side view of an embodiment of a valvetrain according to the invention including a pivoting frame; 
         FIG. 4  shows an enlarged portion of a part of  FIG. 3 ; 
         FIG. 5  shows a perspective view of the valvetrain of  FIG. 3 ; 
         FIG. 6  shows a cross-sectional side view of a further embodiment of a valvetrain according to the invention; 
         FIG. 7  shows a cross-sectional front view of the valvetrain of  FIG. 6 ; 
         FIG. 8  shows a cross-sectional side view of a further embodiment of a valvetrain according to the invention; 
         FIG. 9  shows a perspective view of the valvetrain of  FIG. 8 ; 
         FIG. 10   a  shows a further cross-sectional side view of the valvetrain of  FIG. 9 ; 
         FIG. 10   b  shows an enlarged portion from  FIG. 10   a;    
         FIG. 11  shows a further cross-sectional side view of the valvetrain of  FIG. 9 ; 
         FIG. 12  shows a perspective view of the valvetrain of  FIG. 9 , which is adjusted for small valve lift; 
         FIG. 13   a  shows a side/cross-sectional view of the valvetrain of  FIG. 12 ; 
         FIG. 13   b  shows an enlarged portion from  FIG. 13   a;    
         FIG. 14  shows a side view of a further embodiment of a valvetrain according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of a combustion engine  1  according to the invention with a valvetrain  2  is shown, in a cross-sectional side view, in  FIG. 1 . 
     Herein, the shaded area  3  represents the cylinder head; also, the structures  3   a ,  3   b  mounted fixedly thereon represent parts of the cylinder head. Not shown in  FIG. 1  are further parts of the combustion engine  1 , which are mounted below the cylinder head  3 , such as combustion chamber, piston and crank shaft, which are arranged in a usual manner. 
     The valvetrain  2  is shown in the upper portion of  FIG. 1 . The valvetrain  2  comprises a driving system  10  and a transmission unit or gear unit  4 . The driving system  10  provides a rotational movement. The rotational movement is preferably synchronous to the motor cycle of the combustion engine, so that one full rotation corresponds to one full motor cycle, and it is particularly preferred that the rotational movement is driven by the crank shaft of the combustion engine  1 . The transmission unit  4  transmits the rotational movement of the driving system into a lifting movement for actuating the valve  70 . An actuation of the valve is herein understood to be a lifting movement of the valve  70 , which opens and/or closes the valve  70 , preferably synchronous to the motor cycle. 
     The driving system  10  comprises a driving gearwheel  22 , a valve crank gearwheel  12 , and a valve crank  16 . The driving gearwheel  22  is mounted stationarily in the cylinder head  3   b  and rotatably about a driving axis  24 . The valve crank gearwheel  12  is fixedly connected to the valve crank  16 . The valve crank  16  and the valve crank gearwheel  12  are rotatably mounted about a valve crank axis  14 . Here and in the following, the term “axis” means a geometrical axis and/or a rotational axis. The bearing of the valve crank  16  is not shown in  FIG. 1 . 
     Although the detailed driving mechanism of the driving gearwheel  22  is not shown in  FIG. 1 , any driving mechanism suitable for a camshaft is also suited for the driving gearwheel  22 , for example a gear unit, a chain drive, sprocket, drive belt, gearwheel, or spur gear. The driving gearwheel  22  is driven by a crank shaft of the combustion engine  1 . The driving is synchronous to the motor cycle, i.e. a full rotation of the driving gearwheel  22  corresponds to a motor cycle. In a four stroke engine, this is the case if the transmission between crank shaft and driving gearwheel is 2:1. 
     The driving gearwheel  22  is in meshing connection with the valve crank gearwheel  12 . The transmission ratio between driving gearwheel  22  and valve crank gearwheel  12  is 1:1. Thereby, also the valve crank gearwheel is driven synchronously to the motor cycle. Here and in some of the following Figs., the meshing connection of this and of other gearwheels is partially shown in a displaced manner. Nevertheless, the Figs. are to be understood in that the teeth of one of the gearwheels mesh into the notches of the other gearwheel. 
       FIG. 1   b  is a schematic top view of the valve crank  16  shown in  FIG. 1 . As shown there, the valve crank  16  comprises a bearing pin  16   a , which is arranged along and is rotatable about the valve crank axis  14 , a lifting pin  16   b , which is arranged parallel and eccentric to the valve crank axis  14 , and a radial element  16   c , which connects the bearing pin  16   a  and the lifting pin  16   b  to each other. Further, the valve crank gearwheel  12  is mounted fixedly on the bearing pin  16   a  of the valve crank  16 . The valve crank  16  is mounted, by means of the bearing pin  16   a , in a rotational bearing (represented by boxes at the upper and lower end of the valve crank  16 ), such as to be rotatable about the valve crank axis  14 . 
     A connecting rod is joined to the lifting pin  16   b  of the valve crank  16 , i.e. connected via a joint  44  to the valve crank  16 . Consequently, the connecting rod  30  is pivotable or rotatable around the lifting pin  16   b  about a rotational axis defined by the joint  34 . This rotational axis is parallel and eccentric to the valve crank axis  14 . By means of the arrangement shown here, a preferred general aspect of the invention is illustrated, namely that the connecting rod is joined with its joint  34  to the valve crank  16  eccentrically to the valve crank axis  14 , and/or that the connecting rod joints  34  and/or  36  are swivel joints. 
     Further, compensating mass elements  18  are arranged on the side of the valve crank  16  opposing the lift pin  16   b  with respect to the valve crank axis  14 . The compensating mass elements  18  generally serve the purpose of partially compensating for an imbalance of the valve crank  16 , which can be caused by a force transmitted from the connecting rod  30  to the valve crank  16 . They are arranged, relative to the first rotational axis, opposite of the connecting rod  3 D, and serve the purpose of reducing an imbalance of the rotation of the valve crank caused by the connecting rod  30 . 
     Also in other embodiments, such a compensating mass element or compensating mass elements can be arranged on the valve crank or on the driving member  16  opposite the connecting rod. Preferably, they have a mass for reducing the imbalance of the driving member  16  caused by the connecting rod  30  by about 40%, 50%, 70% or 100%, or for reducing the imbalance by a percentage, which lies in an interval between two of these numbers. The arrangement and the purpose of the compensating mass elements  18  are analogous to the arrangement and purpose of compensating mass elements of the crank shaft, which are known in the state of the art. In particular, it is desired to arrange two compensating mass elements  18  symmetrically about the connecting rod, in order to avoid an unwanted free moment or torsional moment. In FIG.  1   b , the compensating mass elements  18  are mounted on the lifting pin  16   a  of the valve crank  16 . Alternatively, the compensating mass elements can also be mounted in another manner on the valve crank  16  or on the valve crank gearwheel  12 . For example, a compensating mass element can be mounted on the valve crank gearwheel  12 . 
     As is further shown in  FIG. 1 , the connecting rod  30  comprises, besides the first joint or connecting rod end  34  described above, an additional second joint or connecting rod end  36  as well as a rod body, which fixedly connects the first joint  34  and the second joint  36 . 
     The connecting rod  40  has generally (i.e. independent of the embodiment described herein) a short length, i.e. a length of less than 10 cm, preferably of less than 5 cm. A length of the connecting rod is hereby understood to be the distance between the first and the second joint  34 ,  36  and/or between an axis defined by the first joint  34  and an axis defined by the second joint  36 . The short length of the connecting rod  30  allows an efficient use of the available space and, in particular, a small headroom of the valvetrain  2  and a advantageous force transmission of the gear  4 . 
     In order to guide the connecting rod, i.e. for example in order to reduce or disallow a free pivoting of the connecting rod  30  about the first joint  34 , the connecting rod is joined, with its second joint  36 , to a guiding member  60 . Consequently, the connecting rod  30  is connected, pivotably about a joint axis defined by the joint  36 , to the guiding member  60 . Further, the guiding member  60  is mounted pivotably about a guiding axis  66 . The bearing of the guiding member  60  is fixedly arranged in the cylinder head  3   a . Thereby, the connecting rod  30  is guided: By the position of the second connecting rod joint  36  being restricted to a radius about the guiding axis, a free pivoting of the connecting rod  30  about the first connecting rod joint  34  is restricted and/or prevented. 
     It is a general aspect of the invention that the valvetrain  2  comprises a planar linkage with four links, and/or a pinned linkage of four links. Herein, the joints preferably comprise the driving axis  24 , the guiding axis  66 , the first connecting rod joint  34  and the second connecting rod joint  36 . Further, the pinned linkage comprises the following links: firstly, the link between the guiding axis  66  and the driving axis  24  (by the cylinder head  3 ); secondly, the link between the driving axis  24  and the bearing of the connecting rod  30  on its first joint  34  (by the valve crank  16  or its lifting pin  16   c ); thirdly, the link between the first joint  34  and the second joint  36  of the connecting rod  30  (by the connecting rod  30 ); and fourthly, the link between the second joint  36  of connecting rod  30  and the guiding axis  66  (by the guiding element  60 ). 
     All elements of the above described linkage are positively coupled to each other, i.e. no independent movement of the elements with respect to each other is possible, or no such movement is possible that substantially influences the valve lift. In particular, because the guiding axis  66  is fixedly mounted in the cylinder head, and at a predeterminded position of the valve crank axis  14 , the rotational angle of the valve crank  16  is the only substantial degree of freedom of motion of the pinned linkage. Hence, in particular the movement and/or the geometrical arrangement of connecting rod  30  and guiding element  60  are determined by the rotational angle of the valve crank  16 . 
     Further,  FIG. 1  shows a roller  40 . The roller is rotatably attached to the joint connection of the second connecting rod joint  36  with the guiding element  60 . The roller  40 , connecting rod  30  and guiding element  60  are connected by a transmission bolt, which is fixedly connected to the guiding element  60 , and on which the connecting rod  30  as well as the roller  40  are mounted such as to be rotatable and/or pivotable about the axis defined by the connecting rod joint  36 . 
     By means of the pinned linkage described above, the position and/or the movement of roller  40  (except for a rotational motion of the roller  40  about its rolling axis) are determined by the rotational angle of the valve crank  16 . This results in the following general aspect of the invention, i.e. preferred aspect independent of the described embodiment, which is for example also illustrated in  FIG. 4 : The roller or pushing element  40  is moved along a guided path  68  by the rotational movement of the valve crank or the driving member  16 . The guided path preferably defines a position of the pushing element  40  depending on the rotational angle of driving member  16 . Particularly preferably, the guided path  68  is determined by the shape and the geometrical arrangement of valve crank  16 , connecting rod  30  and guiding element  60 . In the valvetrain of  FIG. 1  and in the valvetrain of  FIG. 4 , the guided path  68  is for example along a segment of a circle about the guiding axis  66 , which is limited by an upper and a lower turning point (dashed circles at the upper and lower ends of the guided path  68 ). 
     The valvetrain of  FIG. 1  further comprises a transmission lever or rocker lever  50 , which is mounted pivotably about a rocker lever axis  52 . On the rocker lever  50 , a rolling surface  55  is provided, along which the roller  40  can roll. Herein, the term “rolling” is to be understood to potentially also comprise a combination of rolling and sliding, i.e. generally the roller  40  will rotate, during its motion along the rolling surface  55 , about its rolling axis; the rotation, however, can be such that also a partial sliding motion of the roller  40  along the rolling surface  54  occurs. Thereby, losses due to friction can be minimized. In particular, this is possible by dispensing elements having sliding friction, and by providing elements having rolling friction instead. Also, the requirements regarding the lubrication of the valvetrain are less critical. 
     In  FIG. 1 , the rocker lever  50  is pushed against the roller  40 , so that a non-positive connection between rocker lever  50  and roller  40  is established. However, a maximum displacement of the rocker lever  50  towards the roller  40  is defined by the stopping element  57 , so that the roller  40  can lift off from the rocker lever  50  if the roller  40  is moved further away from the rocker lever  50  than it would correspond to this maximum displacement. 
     By means of the non-positive connection between roller  40  and rocker lever  50 , a position of the roller  40  predetermines a pivotal position of the rocker lever  50 . Hence, the pivotal position of the rocker lever  50  is ultimately determined by the rotational angle of the valve crank  16 . The exact relation between the rotational angle of the valve crank  16  and the pivotal position of the rocker lever  50  depends on the one hand on the form of the guided path  68  of roller  40  and, on the other hand, on the contour of the rolling surface  54  of the rocker lever  50 . 
     Further, a valve  70  is shown in  FIG. 1 . The valve  70  comprises a cylindrical valve stem and a valve disc. The valve  70  is seated on a valve seat  76  in the cylinder head, and is thus shown in closed position. The valve  70  is connected via a spring seat  74  to a valve spring  72 . The valve spring  72  is mounted in the cylinder head and pushes the valve  70  in a closed direction (i.e. upwardly in  FIG. 1 ). The valve  70  is actuated as follows: it is pushed downwardly by a lifting movement against the force of the valve spring  72  along the valve axis (dashed line), and thus opened, and thereafter it is closed again by a lowering motion along the valve axis. 
     The valve contacts with its valve shaft the rocker lever  50  via a adjusting element  71 . The rocker lever  50  is arranged such that it can open the valve  70 , i.e. push it in an opening direction. Conversely, as long as the valve is opened, it is pushed by the force of the valve spring  72  against the rocker lever  50 . Thus, a non-positive connection is established between the valve  70  and the rocker lever  50 , and also between the rocker lever  50  and the roller  40 . In contrast, when the valve is closed and seated in the valve seat  76 , the valve stream  72  is not able to generate a non-positive connection between valve  70  and rocker lever  50  and/or between rocker lever  50  and roller  40 . 
     The stopping element  57  is arranged such that it defines the maximum displacement of the rocker lever  50  to be about the displacement that corresponds to the closing of the valve  70 . Thus, the rocker lever  50  and/or the adjusting element  71  is not able to lift off from the valve stem, even when the valve  70  is closed and no non-positive connection between valve  70  and rocker lever  50  is present. 
     In order to ensure the above described effect of the stopping element  57  in spite of typical manufacturing tolerances, the height of the adjusting element  71  can be adjusted. To that purpose, the adjusting element  71  is selected from a selection of elements of different height. The adjusting element  71  is inserted in the rocker lever  50  such as to be easily replaceable. 
     The height of the adjusting element  71  should herein still allow a certain play of the valve, which is desired or necessary in order to compensate a thermal expansion and/or manufacturing tolerances of the parts. The adjusting element  71  can be realized by different further elements, in particular by a screw element on the valve stern or by a hydraulic element (hydraulic tappet). 
     It is a general aspect of the invention that the valvetrain  2  is provided in a cylinder head  3  portion, as is exemplified in  FIG. 1 . A arrangement in the cylinder head portion  3  is to be understood as follows: The valve crank  16  is generally (i.e. in at least one possible position of the rotational axis  14  and/or in at least one pivotal position of a pivoting frame  80 , as shown for example in  FIG. 3 ), mounted on a cylinder head side relative to a dividing surface between motor block and cylinder head. Even if a cylinder head and a motor block should not be clearly distinguishable from one another in the combustion engine, such a dividing surface can be defined, for example, by a surface defined by the piston head, wherein the piston is in the top dead center position. According to this characterization, the valvetrain  2  corresponds to an overhead camshaft valvetrain, wherein the valve crank  16  corresponds to the camshaft. 
     The operation of the valvetrain shown in  FIG. 1  is as follows: The valve crank  16  is turned by means of the driving gearwheel  22  and the valve crank gearwheel  12  synchronously to the engine stroke. 
     A rotational movement of the valve crank  16  about the axis  14  causes a lifting movement of the connecting rod  30 . The lifting movement of the connecting rod  30  in turn causes a pivoting movement of the guiding element  60  about the guiding axis  66 . Together with the lifting movement of the connecting rod  30  and/or with the pivoting movement of the guiding element  60 , the roller  40  is moved periodically back and forth along the guided path  68  (see  FIG. 4 ). 
     As long as the rocker lever  50  does not rest on the stopper  57 , the roller  40  is in non-positive contact with the roller surface  54  of the rocker lever  50  and rolls on the roller surface  54 . During the movement of the roller  40  along the roller surface  54  in direction of the rocker lever axis  52 , the roller  40  pushes the rocker lever  50  downwards and forces thereby a pivotal motion of the rocker lever  50  towards the valve  70 . 
     The path of the roller  40  is determined to be along the guided path  68 . By the non-positive connection between roller  40  and rocker lever  50 , each position of the roller  40  on its guided path  68  is assigned to a particular displacement of the rocker lever  50 . This assignment results from the contour shape of the roller surface  54  in relation to the guided path  68 . 
     The rocker lever  50  transmits the pushing force or displacement received from the roller  40  to the valve  70  and thereby pushes the valve  70  in an opening direction. A counterforce to this force is generated by the valve spring  72 . The valvetrain  2  or the driving system  10  of the valvetrain performs work against this force. 
     During the opposite movement, i.e. the backwards movement of the roller  40  away from the rocker lever axis  52 , the roller  40  allows a pivotal movement of the rocker lever  50  away from the valve  70 . Thus, the valve spring  72  can close the valve  70  again. 
     By the described mechanism, the valvetrain  2  assigns to a given point in time of the motor cycle a rotational angle of the valve crank  16 ; which in turn determines a position of the roller  40  along its guided path  68 ; which in turn determines a pivotal position of the rocker lever  50 ; which in turn determines an associated valve lift of the valve  70 . By the described chain of action, the valvetrain  2  assigns to each point in time of the motor cycle a valve lift. 
     The valvetrain  2  can be sub-divided according to the above description into an active sub-system and a passive subsystem. The active sub-system can be characterized as follows: The motional state of the active sub-system is substantially determined by the motional state of the valve crank  16 , i.e. by a rotational angle of the valve crank  16  and by the position of the valve crank axis  14 . The passive sub-system, in contrast, is characterized as follows: The motional state of the passive sub-system has, besides the motional state of the valve crank  16 , further substantial degrees of freedom, which can influence the valve lift. The division into an active and, if applicable, a passive sub-system is independently of the shown embodiment a preferred general aspect of the invention, wherein it is particularly preferred that the valve crank  16  or the first driving means, the connecting rod  30 , and the guiding element  60  are associated to the active sub-system. It is further preferred that the roller or pushing element  40  is associated to the active sub-system. The rotational movement of the roller  40  may represent a degree of freedom that is independent of the motional state of the valve crank  16 , however this degree of freedom is not substantial for the valvetrain in the sense that it does not substantially influence the valve lift. Further, the valve  70  and, where applicable, the rocker lever or the transmission element  50  are preferably associated to the passive subsystem, since these elements are only non-positively connected to the active sub-system. For that reason, they principally have further own degrees of freedom of motion, which could, for example at extremely high rotational speeds, lead to the non-positive connection to be released. 
     Again independently of the described embodiment, it is, however, generally desired that the passive sub-system is provided or arranged such that the non-positive connection is largely conserved at the rotational speeds for which the combustion engine  1  is designed. Thereby, valve float can be largely avoided. To this purpose, it is a preferred aspect of the invention that the masses moved or accelerated by the valve spring  72 , and/or the masses of the passive sub-system, are less than 200 gram, preferred less than 100 gram. Depending on the design of the valvetrain and on the materials used, these masses can be reduced down to 90 gram, down to 60 gram, or even down to 50 gram. 
     A reduction of the weight down to the stated lower weight limit is, for example, possible if titanium or (sheet) steel are used for the valve body, if aluminum or steel are used for the spring seat, and/or if a pneumatic spring is used as a valve spring. A further reduction in weight can be achieved by implementing the valve as a hollow valve stem valve. 
     Further, in various preferred embodiments, the mass that is to be moved by the force of the valve spring can be limited, such as to include only the mass of the valve  70 , of the valve spring  72  (or a portion—generally half—of the mass of the valve spring  72 ), of the spring seat  74 , and of the rocker lever  50 . 
     The lifting movement of the valve is typically divided into several different phases. These phases can be understood with the aid of the side view of the engine in  FIG. 1  and  FIG. 4  in connection with the valve lift curve  90  (solid line) shown in  FIG. 2 . Therein, the valve lift is shown in dependence of a point in time or a phase of the motor cycle. Here, the valve lift is the lifting displacement of the valve  70  along the valve axis in comparison to the closed position of the valve  70  in the valve seat  76 . It should be noted that the valve lift diagram of  FIG. 2  is shown by means of example and was not calculated from the embodiment shown In  FIG. 1 . Nevertheless, it has the same qualitative features as the valve lift diagram of the valvetrain of  FIG. 1 . 
     In a closed valve phase  93 , the valve crank  16  is turned about the axis  14  in such a way that the roller  40  is located away from the rocker lever  50 , i.e. that the roller  40  is not in contact with the roller surface  54 . Hence, the valve  70  is closed, i.e. it is being pushed by the valve spring  72  into the valve seat  76 , such that there is no valve lift. 
     Now, when the valve crank  60  is turned, then the roller  40  is moved by the connecting rod  30  along its guided path  68  towards the rocker lever  50  (i.e. downwards in  FIG. 1 ), such that the roller  40  gets into contact with the rocker lever and rolls along the rolling surface  54 , whereby it pushes onto the roller surface  54 . Thereby, the rocker lever  50  is pushed and displaced towards the valve  70 . The rocker lever  50 , in turn, pushes the valve  70  against the force of the valve spring  72  and thereby forces an opening of the valve  70 . 
     The further actuation of the valve could be subdivided into the following phases shown in the valve lift diagram of  FIG. 2 : after a point in time of opening of the valve  94 , there follows an opening phase  95 , which consists of an opening phase  95   a  of a little valve lift and a subsequent opening phase  95   b  of fast increasing of the valve lift. Subsequently, there follows the point in time  96  of maximum valve lift  90   a  or H akt . Subsequently, there follows a valve closing phase, which is subdivided in analogy to the opening phase into a closing phase  97   b  of fast reduction of lift and a subsequent closing phase  97   a  of little valve lift. Subsequently, a point in time of closing of the valve  98  follows. Subsequently thereto, the valve closing phase  93  is repeated again. The opening duration is the period between opening  94  and closing  98  of the valve. 
     The phases  95   a  and  95   b , or  97   a  and  97   b , can be delimited from each other for example in that the phases  95   a  and  97   a  comprise a respective domain, in which the valve lift is less than a particular percentage (e.g. less than 50%, 66%, or 26%) of the maximally attainable lift height  92  or H MAX , and that the phases  95   b  and  97   b  comprise respective domains, in which the valve lift is above these values. In accordance with the subdivision of the valve lift diagram into phases  93  and  95  or  97 , the guided path  68  of the roller  40  can be subdivided into different portions: A closing portion, in which the roller  40  lifts from the roller surface  54 , and an opening portion, in which the roller  40  contacts the roller surface  54 . In accordance with the further subdivision of the phases  95  or  97 , the opening portion of the guided path  68  or the roller surface  54  can be subdivided into still further portions, which are associated to the phases  95   a  and  97   a  or  95   b  and  97   b . According to this subdivision, the phases  95   a  and  97   a  correspond to a first portion and the phases  95   b  and  97   b  to a second portion within the opening portion of the guided path  68  or the roller surface  54 . Further, the part of the roller surface, on which the roller  40  hits the roller surface  54  at the opening point in time  94 , is denoted in  FIG. 4  as the hitting portion  54   a .  FIGS. 1 and 4  show the motional state of the valvetrain when the roller hits the roller surface  54 , i.e. at the point of time of opening  94  (or—depending on the rotational direction of the valve crank—at the point of time of closing  98 ). 
     The shape of the valve lift diagram shown in  FIG. 2  is essentially defined by the guided path  68  of the roller  40  and, in relation thereto, by the design of the profile of the roller surface  54 . For a given guided path  68 , a desired shape of the valve lift diagram can therefore be obtained or approximated by a corresponding design of the roller surface  54 . 
     Regarding the shape of the valve lift diagram, it is on the one hand advantageous to have a fast opening of the valve, e.g. in order to secure a sufficient supply of air fuel mixture into the combustion chamber of the motor and hence to secure high motor power in the upper rotational speed regime. On the other hand, it is advantageous to have a smooth touchdown of the valve on the valve seat, because thereby a hard hitting of the valve  70  on the valve seat and a consequent mechanical strain of the parts can be reduced. 
     Therefore, it is advantageous if on the one hand the portion of the roller surface associated to the phases  95   b  and  97   b  is designed such that a fast opening of the valve occurs, i.e. that the gradient of the valve lift is as high as possible; and if on the other hand the portion of the roller surface, which is associated to the phases  95   a  and  97   a , is designed such that a smooth closing of a valve occurs, i.e. that the gradient of the valve lift at closing of the valve in phase  97   a , in particular near the point of time of closing  98 , is as small as possible. It is further advantageous if the hitting surface  54   a  of the roller surface  54 , on which the roller  40  hits at the time  94  of opening of the valve, is designed such that even under certain manufacturing tolerances, a smooth hitting of the roller onto the roller surface  54   a  is guaranteed. 
     According to the invention, in the valvetrain shown in  FIG. 1  the position of the valve crank  14  can be adjusted. The detailed mechanism for this is shown in the valvetrain of  FIG. 3 . Therein, in addition to the elements shown in  FIG. 1 , a pivoting frame  80  is visible. The pivoting frame  80  consists of three parts that are fixedly connected with one another, however a different number is equally possible. It is mounted on the cylinder head  3  pivotally about the pivoting axis, wherein the pivoting axis is identical to the driving axis  24  shown in  FIG. 1 . Further, the valve crank  16  is mounted in the pivoting frame  80 , so that the pivoting of the pivoting frame  80  causes a pivoting of the valve crank axis  14 , i.e. a change of the position of the valve crank axis  14  along a circular path about the pivoting axis  24 . 
     By the pivoting axis  24  and the driving axis being identical, it is guaranteed that the position of the valve crank axis  14  remains, in every pivoting position of the pivoting frame  80 , on a circular segment about the driving axis  24 . Thereby it is made sure that the valve crank gearwheel mounted rotatably about the valve crank axis  14  and the driving gearwheel  22  remain in meshing connection regardless of the pivoting position of the pivoting frame  80 . 
     By means of the pivoting drive  84  the pivoting frame can be held in a fixed position or pivoted. In  FIG. 3 , the pivoting drive  84  is shown symbolically as a rod, which is connected to the pivoting frame  80 . The pivoting drive  84  should guarantee a sufficient stability so that the pivoting frame  80  can be securely held in a fixed pivoting position, and such that an unwanted pivoting of the pivoting frame  80  can be inhibited. 
     In particular, the pivoting drive  84  should guarantee that the bearing of the valve crank  16  can be stably held in a fixed position relative to the cylinder head, in spite of the forces acting thereon. Also, the pivoting frame  80  is preferably constructed such that it has a high degree of stiffness. These measures promote a good transmission of force between the valve crank  16  and the valve  70 , a low tolerance of the valvetrain, as well as a high wear resistance of the valvetrain. 
     The pivoting drive  84  can also be realized in other ways than the rod shown in  FIGS. 3 and 5 . An alternative pivoting drive could, for example, comprise a hydraulic automatic drive fixedly connected with a pivoting frame, or a gearwheel drive, which meshes in a gear segment fixedly mounted on the pivoting frame  80 . The gearwheel segment cant hereby, be mounted on an outer side or an inner side of the pivoting frame. 
     By the change in position of the axis  14 , the guided path  68  of the roller  40  is changed. Herein, as it is shown by means of example in the valvetrain of  FIG. 4 , the guided path  68  is principally restricted by the guiding element  60  to a circular segment about the guiding axis  66 . However, by changing the position of the axis  14 , the guided path of the roller  40  can be changed in the sense that a) the portion of the circular segment that the roller  40  covers during its motion is changed, e.g. by a change in the upper and lower turning point of the guided path  68  shown in  FIG. 4 , and b) by changing the position on this circular segment, at which the roller  40  is provided during a given point in time of the motor cycle. 
     The motor cycle of a four stroke engine corresponds hereby to a revolution by 720 degrees of the motor crankshaft, and the given point in time of the motor cycle corresponds to a associated phase angle, which runs through the interval of 0 degrees to 360 degrees during the motor cycle, wherein a fixed value of the phase angle is associated to a respective fixed point of the motor cycle, for example the upper dead centre or the lower dead centre of the piston. Such a phase angle can for example be given in a four stroke engine by a rotational angle of the crankshaft of the motor divided by two. Because the driving gearwheel  22  is driven in sync to the motor cycle, a given point in time or a given phase angle of the motor cycle can in particular also be represented by a corresponding rotational angle of the driving gearwheel  22 , for example by an angle of a mark provided on the driving gearwheel  22  in relation to the horizontal. By a change of the guided path  68 , various properties of the valve lift behaviour  90  shown in  FIG. 2  can be influenced. The individual changes are shown in the following. Independent of the described embodiment, the valvetrain according to the invention allows one or more of these changes by a change of the position of the valve crank axis or rotational axis  14 . 
     Firstly, a point of maximum valve lift can be changed. This point is the outermost point of the roller  40  on its guided path in direction of valve opening, which the roller can achieve by turning of the valve crank  16  about the valve crank axis  14 ; this point corresponds in the valve lifting diagram of  FIG. 2  to the point in time  96  of the maximum valve lift. Hence, a lifting height  90   a  (see  FIG. 4 ) of the valve can be changed. The change of the point of maximum valve lift is a accompanied by a change of the portion of the roller surface  54 , along which the roller  40  is rolling. 
     Secondly, the duration (in terms of a motor cycle, i.e. the magnitude of a corresponding phase angle interval) of the opening of the valve can be changed. Preferably, this change goes hand in hand with a change of the duration during which the roller  40  is in contact with the roller surface  54 . 
     Thirdly, a rotational angle of the valve crank  16  relatively to the motor cycle can be changed. Thereby, a phase of the valve opening is changed relatively to the motor cycle, i.e. the valve lifting curve of  FIG. 2  is shifted along the x-axis, and/or a given point in time of the valve lifting curve is changed in relation to the motor cycle. Such a given point in time of the valve lifting curve can, for example, be defined by the point in time  96  of the maximum valve lifting or by a point in time that is associated with a particular rotational angle of the valve crank  16 . 
     In the valvetrain shown in  FIG. 1 , the phase of valve opening is changed in the following manner: as described above, the position of the valve crank axis  14  is changed along the circular segment about the driving axis  24 . The angle, about which the valve crank axis  14  is moved along the circular segment, is designated as α. 
     Because during the movement of the valve crank axis  14  the driving gearwheel  22  and the valve crank gearwheel  12  are continuously in meshing connection with each other, and because the meshing connection has a transmission ratio of 1:1, during this motion a rotational angle of the valve crank gearwheel  12  (for example the angle between a mark on the valve crank gearwheel  12  and the horizontal) relative to a rotational angle of the driving gearwheel  22  {for example an angle between a mark on the driving gearwheel  22  and the horizontal) is changed by the value α. Thus, also a rotational phase of the valve crank  16 , i.e. a rotational angle of the valve crank  16 , is changed in relation to the motor cycle by the value α. Hence, also a phase of the valve opening relative to the motor cycle is changed by the value α. 
     The valve lift diagram of  FIG. 2  shows, as dashed line, a possible change of the valve control caused by a change in position of the valve crank axis  14  away from the rocker lever  50 . Herein, it can be seen how the lifting height  90   a ′ and a point in time of opening of the valve  96 ′ are changed simultaneously, whereby the point in time of opening of the valve  96 ′ is changed with respect to the point in time  96  by the value α. Further, a point in time of opening  94 ′ and of closing  98 ′ and, hence, a duration of opening of the valve are changed. 
     The solid line of  FIG. 2 , in which the lifting height  90   a  corresponds to the maximum lifting height  92 , could for example correspond to a valve control of an intake valve under full load, and the dashed line could correspond to the valve control of an intake valve under partial load. Under full load, the lifting height  90   a , the duration  35  and the point in time  96  of the valve opening are chosen such that the valve is opened widely and during a long time, such that a high amount of air-fuel mixture is introduced to the combustion chamber; herein, a high overlapping of an open intake and outtake valve are excepted or desired. At partial load, the lifting height  90   a ′ and the duration  35 ′ and point of time  96 ′ of the valve opening are chosen such that a little amount of air-fuel mixture is introduced to the combustion chamber, here, the overlap is reduced or avoided. Thereby, fuel consumption, noise generation, power and further characteristics of the motor can be adapted dynamically to the respective needs. 
     Further, the change of phase  90   c  or of the control times of the valve lift behaviour are coupled to the change of lifting height and the valve opening duration in such a way that these quantities are adapted to the above mentioned needs under full or partial load. For example, at high rotational speed or load, a high power per motor capacity can be achieved. Simultaneously, the driveability can be improved at low rotational speed and partial load: for example, bucking of the motor can be reduced and/or the response behaviour of the motor can be improved. 
     Hence, a more effective combustion is possible by an efficiently adapted use of the motor power. This can also lead to a reduction of pollution and fuel consumption. Further, the torque speed characteristic of the motor, the exhaust characteristics and the noise emission can be optimized over the entire range of rotational speed. 
     The change of valve control can be performed electronically, i.e. by an electronically controlled mechanism for changing the valve crank axis  14 , the mechanism being coupled to the pivoting drive  84 . The electronic control can be effected in response to various relevant data, for example to a displacement of the throttle control or accelerator or to a position of the throttle control or accelerator, to a rotational speed of the motor, or to a driving speed. Further, the electronic control can be influenced by a traction control system, a system for acoustic control, or a system for controlling emissions. Further, by the electronic control of the valvetrain, a traction control can be easily implemented without accessing the break system. 
     Thus, in the shown valvetrain, there is no need for a throttle valve, since the fuel intake into the combustion chambers is controlled by the valvetrain. Since losses at the throttle valve can thus be avoided, a better motor performance can be achieved in particular in the high load regime. 
     Further, the valve control can be adapted with respect to the motor performance in the upper performance regime as well as in respect to exhaust, to fuel consumption or to acoustical compatibility of the motor. On the one hand, high peak performances can be achieved; to this purpose, high valve lifts, long valve opening durations and high overlaps between a simultaneous opening of intake and outtake valves are advantageous. On the other hand, disadvantages in the partial load regime and at low rotational speeds can be avoided, by avoiding overly high valve overlaps. 
     It is a preferred aspect, independently of the shown embodiments, that the valvetrain  2  allows the lifting height of the valve lifting behaviour or the valve lifting curve  90  to be varied. The lifting height can preferably be varied in a range of lifting heights, which comprises the interval from 0 mm to 5 mm, preferably the interval from 0 mm to 7 mm, further preferred the interval from 0 mm to 10 mm, and particularly preferred the interval from 0 mm to 12 mm. 
     It is a further preferred aspect of the invention, independently of the shown embodiments, that the valvetrain  2  allows a variation of control times or of the phase  90   c  of the valve lifting curve  90  over a interval of phases or angles, which comprises an interval of 10 degrees (i.e. 10/360 of a motor cycle or 20 degrees of crankshaft angle of a crankshaft of a four stroke engine) and preferably an interval of 15 degrees. 
     The phase  90   c  can be adjusted without a separate phase adjusting element. This allows advantages with respect to manufacturing cost, space requirements, maintenance requirements and weight. To this purpose, it is advantageous to choose the construction of the valvetrain  2  and in particular the size of the driving gearwheel  22  and of the valve crank gearwheel  24  such that the phase, the lifting height and the opening duration of the valve are adapted to respective needs, for example in respect to different load regimes, simultaneously. 
       FIG. 5  shows the valvetrain of  FIG. 3  in an perspective view. Therein, the three-dimensional arrangement of the elements shown in  FIG. 3  is visible. In addition to the elements of  FIG. 3 , herein a sprocket wheel  26  is shown, which is connected to the driving gearwheel  22  for driving the driving gearwheel  22  by means of a chain, which in turn is driven by the crankshaft of the motor. The transmission ratio of this drive is chosen such that the driving gearwheel  22  is rotatable synchronously to the motor cycle of the combustion engine, i.e. in a four stroke engine the transmission ratio between motor crankshaft and driving gearwheel is 2:1. 
     Further, it can be seen that the valvetrain provides for the simultaneous operation of two valves. To this purpose, a common transmission rod is connected, via a common connecting element  38 , with two rollers  40  (wherein only one of the rollers is visible); the rollers  40  are guided by a respective own guiding element  60 ,  60 ′. Further, two rocker levers  50 ,  50 ′ are visible, which actuate a respective valve, wherein only one of the valves is visible. 
     The guiding elements  60 ,  60 ′ are connected to the connecting element  38 , either by a bearing, for example a rolling bearing, or by a fixed connection. On an outer side of the connecting element  38 , the rollers  40  are rotatably mounted; on an inner side, the connecting rod  30  is mounted rotatably on the connecting element  38 . This ensures that both valves are actuated synchronously. 
     The valvetrains shown in  FIGS. 1 to 5  can be varied in various aspects. In the following, embodiments are described that result for example as variations of the valvetrains shown in  FIGS. 1 to 5 . These variations are, however, independent of the embodiments shown therein, insofar as that they could equally be implemented in combination with other embodiments, for example with the embodiments shown in  FIGS. 6 to 14  or in still further embodiments. 
     In an embodiment, the transmission ratio between valve crank gear wheel  12  and driving gear wheel  22  is not 1:1 but is generally l:x. It is a preferred aspect of the invention that the valve crank  16  or a corresponding rotatable driving means is rotated synchronously to the motor cycle, i.e. that one motor cycle corresponds to a full revolution of the valve crank  16 . The driving gear wheel  22  should therefore be driven with such a transmission ratio to the motor crankshaft that this condition is fulfilled. In a four stroke engine, the transmission ration between driving gear wheel and motor crankshaft should therefore be x:2. 
     In a further embodiment the valve crank  16  can also be driven by a different mechanism than that of  FIG. 1 . For example, the valve crank  16  can also be driven by any other driving means that is common for a crankshaft. Herein, it is preferred that when the position of the valve crank axis  14  is changed, there is also a change of the rotational angle or a phase of the valve crank  16 , and hence of the phase of the valve lifting curve. 
     For example, the valve crank  16  can be driven by a chain, which is a meshing connection with a sprocket wheel connected to the valve crank  16 . Herein it should be made sure that the transmission ratio is such that a full revolution of the valve crank corresponds to a motor cycle. By changing the position of the valve crank axis  14 , it may be possible changing the position of the sprocket wheel along the circumference of the chain. If 1 designates the distance, by which the sprocket wheel travels along the circumference of the chain, and if L represents the length, by which the chain advances during a motor cycle, then the resulting change of rotational angle of the valve crank  16  is given by the angle
 
α=(1 /L )×360 degrees
 
     Consequently, the rotational angle or phase of the valve crank  16  and hence the phase of the valve lifting curve relative to the motor cycle changes by the value α. 
     In a further embodiment, further elements which are common for a valvetrain can be added. For example it is possible to adjust the phase of the driving means  16  independently of the valve lift by means of an optional phase adjustment element. 
     In a further embodiment, elements of the valvetrain can be replaced by elements of respective equivalent function. For example, the valve crank  16 , which is shown to be a crank shaft, can also be realized as an eccentric shaft. Further, for example the valve spring shown as a spiral spring can also be realized as a pneumatic spring. 
     In a further embodiment, an alternative design of the rocker lever  50  is possible, in which the roller  22  touches the roller surface  55  or the rocker lever  50  also during the valve closing phase  39 , and thus provides a fixed stop for the rocker lever  50 . In this case, the roller surface  54  is divided into a closing portion, which corresponds to the closing phase  39  of the valve; and into an opening portion, which corresponds to an opening of the valve, i.e. to the phases  95   a ,  95 ,  97  and  97   a . In this embodiment, the holding element  57  can be disposed with; and the valve clearance of the valve needs to be adjusted via the valve adjusting element  71 . 
     In a further embodiment the rocker lever  50  can be pushed by a rocker lever spring towards the valve  70 . Thereby, preferably a non-positive contact between rocker lever  50  and valve  70  can also be achieved when the roller  40  does not push onto the rocker lever  50 . For example, a torsion spring, a spiral spring, a hydraulic, pneumatic or any other spring can be used as rocker lever spring. Further, instead of a spring, any other means that achieves pushing of the rocker lever towards the valve  70  can be used as well. In this case, there is no necessity for the holding element shown in  FIG. 1 , since its function of providing a contact between valve  70  and rocker lever  50  can be achieved by the rocker lever spring. In this case, also the adjusting element  71  can be disposed with, if a non-positive connection between rocker lever  50  and spring  70  ensures sufficient valve clearance independent of small manufacturing tolerances. In this case a simple adjustment of the valve clearance can be achieved, which is tolerant with respect to changes of the valvetrain or of the valve seat due to wear. 
     In an further embodiment, the roller  40  can be replaced by an alternative pushing element, for example a sliding block, which pushes onto the pushing receiving surface  54  of the rocker lever  50 . 
     In a further embodiment, the roller or pushing element  40  can be mounted at any place of the pushing rod  30  or the guiding element  60 . The kind of mounting can further be in any suitable manner. If the roller  40  is mounted on the connecting rod  30 , then—differently from FIG.  4 —the guided path  68  of the roller  40  can also comprise a backwards motion that is different from the forward motion of the roller  40 . As long as a pivoting motion  60  of the guiding element or a lifting motion of the connecting rod  30  is transmitted to the roller  40  in such way that a rotation of the valve crank  16  moves the roller  40  along a periodic path, the embodiment of  FIGS. 1 to 5  can be adapted in a simple manner to the changed mounting of the roller or the pushing element  40 . To this purpose, mainly the arrangement of the rocker lever  50  and the contour of the pushing receiving surface  54  need to be adjusted. 
     In a further embodiment, the pushing element pushes directly onto the valve stem of the valve  70 . To this purpose, the connecting rod  30  and the guiding element  40  are preferably designed in such a range that the result is a suitable guided path  68  of the pushing element  40 , i.e. a guided path that is approximately in direction of the valve axis of the valve  70 . This embodiment does in particular not necessitate a rocker lever. 
     In a further embodiment, when the position of the valve crank axis  14  is changed, the position of the guiding axis  66  is changed as well. This embodiment can for example be realized by mounting not only the valve crank  16  at its axis  14 , but also the guiding element  60  at its guiding axis  66  within the pivotable frame  80 . In this embodiment, a pivoting of the pivoting frame  80  does not only change the guided path  68  of the roller  40  along the circular segment about the guiding axis  66 ; instead, in contrast to the embodiment shown in  FIG. 1 , also the circular segment itself, i.e. its centre  66 , is changed. 
       FIG. 6  shows a further embodiment of a valvetrain according to the invention in a cross-sectional side view. This valvetrain corresponds in its functional features to the valvetrain of  FIG. 1 , so that the description of  FIG. 1  can substantially also be used for  FIG. 6 ; same or similar components are referred to using identical reference numbers. 
     In comparison to the valvetrain shown in  FIG. 1 , individual elements of the valvetrain of  FIG. 6  are arranged differently. In particular, the motion of the roller  40  along the roller surface  55  is rather in horizontal direction, whereas in  FIG. 1  it is rather in vertical direction. 
     Further, the guiding element  60  is arranged such that depending on the motional state of the valvetrain, more or less—it is almost collinear with the valve axis {dashed line} of the valve  70 , whereas the connection rod  30  is arranged almost perpendicular to the valve axis. By this arrangement, the counterforce for compensating the pushing force generated by the valve spring  72  or the acceleration generated by the lifting movement of the valve  70 , can be provided mainly via the guiding element  60 , which is fixedly mounted at the guiding axis  66  within the cylinder head. 
     In contrast to the valvetrain shown in  FIG. 1 , a spring  58  acts upon the rocker lever, so that a non-positive connection between the rocker lever  50  and the valve  70  is provided even when the roller  40  does not push onto the rocker lever  50 . The spring is shown as a torsional spring, however also a spiral spring, a hydraulic spring, a pneumatic or any other spring can be used. In analogy to  FIG. 3 , the spring can also be replaced by a holding element that provides a fixed stop of the rocker lever  50 . 
     In analogy to  FIG. 1   b , the valvetrain comprises a compensating mass element  18 . The compensating mass element  18  is mounted directly on the valve crank gear wheel  12 . In order to avoid an unwanted mass moment and hence an unbalanced load of the valve crank  16 , a further compensating mass element of equal size is mounted symmetrically to the shown compensating mass element  18  on the valve crank  16 . 
       FIG. 7  shows a cross-sectional front view of the valvetrain of  FIG. 6 . Therein, the spatial arrangement of the various elements shown in  FIG. 6  can be understood in more detail. The valvetrain is designed for driving two valves  70 ,  70 ′. The connecting rod  30  is provided for actuating both valves. The connecting rod is mounted rotatably about the connecting element  38  by a rolling bearing. The connecting element  38  is fixedly connected to the guiding element  60 . This connection defines the rotatable or pivotable joint connection  36  between the connecting rod  30  and the guiding element  60 . 
     Further, two rollers  40 ,  40 ′ are rotatably mounted on the connecting element  38  by a rolling bearing  42 . Beneath each of the rollers  40 ,  40 ′, a respective rocker lever  50 ,  50 ′ is mounted for being actuated by the rollers. Each of the rocker lever  50 ,  50 ′ in turn actuates an own valve  70 ,  70 ′. 
     Independent of the shown embodiment, it is advantageous that the connecting rod is mounted (as is shown here) rotatably about a connecting element  38 , wherein the connecting element  38  is preferably fixedly connected to the guiding element  60 . Likewise, it is preferred that the roller  40  is mounted rotatably about the connecting element  38 . 
     The guiding element  60  comprises at its upper end a frame, which encompasses the connecting rod  30  as well as the rollers  40 ,  40 ′, and whose outer surface is fixedly connected to the outer surface of the connecting element  38 . Hereby, the connecting element  38  is supported on both sides of the rollers  40 ,  40 ′. Hence, the frame allows an improved force transmission via the connecting element  38 , without there being an overly large imbalanced load on the connecting element  38 . 
     The valvetrain of  FIG. 7  is described for the actuation of two valves  70 ,  70 ′. Nevertheless, the valvetrain can be adapted for any other number of valves by means of simple modifications. 
       FIG. 8  shows a cross-sectional side view of a valvetrain according to the invention. In addition to the elements shown in  FIGS. 6 to 7 , also a pivoting frame  80  is shown, which is pivotable about the pivoting axis  24 , wherein the pivoting axis  24  and the rotational axis of the driving gearwheel are again identical. Structure and function of the pivoting frame  80  are essentially analogous to the description of  FIG. 3 , wherein some differences with respect to the pivoting frame are described in the following. The pivoting frame of  FIG. 8  consists of two fixedly connected parts, which are connected to each other by means of a screw connection with screws  86 . 
     On the right side of the pivoting frame  80 , the pivoting drive  84  is shown. It comprises a teeth segment  84   a  which is in fixed connection with the pivoting frame  80 , and which is in meshing connection with a gearwheel  84   b . The pivoting frame  80  can be pivoted by moving the teeth segment  84   a  up and down by turning the gearwheel  84   b . In correspondence to this function, the teeth segment  84   a  is bent along a circular segment about the pivoting axis  24 . 
     Independent of the embodiment described herein, it is generally advantageous that the gearwheel  84   b  is arranged such that it is rotatable about the rocker lever axis  52 . This allows for a compact construction that has advantages with respect to spatial requirements and also with respect to the rigidity of the construction. 
       FIG. 9  shows the valvetrain of  FIG. 8  in a perspective view. In addition, a further pair of valves  78  is shown in  FIG. 9 , which are realized as tappet valves. The valves  78  are driven by a conventional camshaft drive  5 . The camshaft drive  5  is driven together with the drive of the valve crank  16  by the driving gearwheel  22 . The conventionally driven valves  78  of  FIG. 9  are exhaust valves, whereas the valves  70  driven according to the invention are intake valves. 
     Further, a worm gear  84   c  is shown in  FIG. 9 . The worm gear  84   c  is in meshing connection with a gearwheel  84   b  and serves for rotating the latter. Thereby, the pivoting frame  80  can be pivoted as described for  FIG. 8 . The worm gear  84   c  is driven by a electronically controlled motor (not shown) in analogy to the description of the pivoting frame of  FIG. 1 . 
     As an alternative to the worm gear  84   c , the gearwheel  84   b  could also be driven for example by a gear, a sprocket drive, a pair of bevel gearwheels, or in any other manner. Here, a drive is preferred that avoids an unwanted pivoting of the pivoting frame  80 . 
       FIG. 10   a  shows a side view of a valvetrain. The elements shown in  FIG. 10   a  correspond to the respective descriptions of  FIGS. 6 to 9 .  FIG. 10   b  shows an enlarged view of a part of  FIG. 10   a . Therein, the rolling surface  54  can be seen more clearly than in FIG.  10   a . The roller  40  is in this figure located on the hitting portion of the roller surface, i.e. on the portion at which the roller hits the roller surface and hence gets into contact with the roller surface during its motion along its guided path. It can be seen in  FIG. 10   b  that the hitting portion of the roller surface  54  is formed such that it is hit by the roller  40  smoothly, i.e. in a angle that is as flat as possible, thereon (for illustrating this fact, in  FIG. 10   b  the guided path of the hitting portion of the roller  40  is shown as dashed line  40   a ). It can also be seen that there is a ramp portion around the hitting portion, which is slightly sloped with respect to this guided path. The ramping portion allows to provide a suitable hitting portion in spite of possible manufacturing tolerances. Also in other embodiments it is advantageous to provide a ramping portion of the roller surface or of the pushing receiving surface  54 , which comprises a hitting portion of the roller or of the pushing element  40 , and that preferably is at an angle to the guided path of the pushing element that preferably is less than 30 degrees and particularly preferably less than 20 degrees. 
     Further, the roller surface  54  is formed in  FIG. 10   b  such that the valve is opened slowly in the beginning (i.e. at small opening), and that is subsequently opened fast up to the full lift height (corresponding to the phases  95   a  and  95   b  shown in  FIG. 2 ) and/or that has an opposite behaviour at valve closing (corresponding to the phases  97   b  and  97   a  shown in  FIG. 2 ). 
       FIG. 11  is a further side view of the valvetrain of  FIG. 9 . In contrast to  FIG. 10   a , also the further valves  78  of  FIG. 9  and their drive are shown therein. 
     In the valvetrains shown in  FIGS. 6 to 11 , the pivoting frame is adjusted to respective small lifting heights, i.e. it corresponds to the valve lifting diagram shown in  FIG. 2  as dashed line.  FIG. 12  shows a further perspective view of the valvetrain shown in  FIGS. 6 to 11 , in which the pivoting frame  80  is, however, adjusted for large valve lift. A side view of this configuration is further shown in  FIG. 13 . 
     Starting from the state shown in  FIGS. 6 to 11 , in the valvetrain of  FIGS. 12 and 13  the pivoting frame  80  is pivoted in direction of the valve  70 . As described above, the pivoting of the pivoting frame  80  is achieved as follows: If the worm gear  84   c  is rotated, then the gearwheel  84   b  rotates. The gearwheel  84   b  acts on the teeth segment  84   a , such that the teeth segment  84   a  is moved in direction of the valve  70 , whereby the pivoting frame  80  is pivoted. 
     By the pivoting frame  80  being pivoted by an angle α towards the valve  70  when compared to the state shown in  FIGS. 6 to 11 , the position of the valve axis  14  is changed, i.e. rotated by an angle α about the pivoting axis  24 . A first consequence of this change is that the roller  40  rolls back and forth along a changed portion of the roller surface  54 . The changed portion of the roller surface has the following consequences: 
     Firstly, the roller  40  reaches a portion of the roller surface  54  that corresponds to a larger valve lift when compared to the portion reached in  FIGS. 6 to 12 . Hence, the lifting height  90   a , i.e. the maximum valve lift that can be reached in a cycle, is increased. Thereby, in particular the integrated valve lift, i.e. the area beneath the valve lift diagram of  FIG. 2 , increases as well. 
     Secondly, the roller  40  touches and pushes the roller surface  54  across a increased rotational portion of the valve crank  16 . Consequently, the point in time of valve opening  94  shown in  FIG. 2  takes place at a smaller rotational angle of the valve crank  16 , i.e. at an rotational angle that is reached earlier. Correspondingly, the point in time of valve closing  98  takes place at a rotational angle of the valve crank  16  that is reached later. The valve opening duration between these points in time is consequently longer, and the valve closing phase  93  is correspondingly shorter. Herein, as in other parts of the description, the duration is in terms of a motor cycle. 
     A second consequence of the change in position of the valvetrain axis  14  is that the rotational angle of the valve crank  16  is changed by the angle α when compared to a rotational angle of the driving gearwheel  22 . Hereby, the phase of the valve crank rotation, i.e. the phase angle relative to the motor cycle, is changed by a phase angle α. Consequently, the phase of the valve lift diagram is changed by a phase angle a ( 90   c ), i.e. for example the point in time of maximum valve lift  96  is shifted by a phase angle α ( 90   c ). 
     A further valvetrain according to the invention is schematically shown in  FIG. 14 . This valvetrain corresponds essentially to the embodiment shown in  FIG. 1  the same or similar components are referred to with identical numbers. 
     A difference to  FIG. 1  is that the rocker lever  50  is pushed in direction of the valve  70 , by movement of the roller  40  away from the rocker lever axis  52 . Hence, the lever of the rocker lever  50  can be used efficiently when the rocker lever is pushed in direction of the valve  70 . 
     In the valvetrain shown in  FIG. 14 , and in contrast to the embodiments of  FIGS. 1 to 13 , the rocker lever  50  is so long that the roller  40  is always in contact with the rocker lever. Consequently, there is no need anymore to provide for a separate stop  57  or a lever holding spring  58 . 
     The drive of the valve crank  16  is not shown and can be implemented analogously to  FIG. 1 . 
     The position of the valve crank axis  14  can be changed, i.e. adjusted. The mechanism for that is not shown explicitly and can be implemented by means of a pivoting frame, which is analogous to the pivoting frame  80  shown in  FIG. 3 .