Patent Publication Number: US-11384661-B2

Title: Valve train for an internal combustion engine of a motor vehicle

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to a valve train for an internal combustion engine of a motor vehicle, in particular a commercial vehicle. 
     From DE 10 2013 019 000 A1, a valve train having at least one camshaft is already known, which has at least one cam group having at least one firing cam and at least one braking cam, having at least one cam follower assigned to the firing cam, which is provided in a firing mode for actuating at least one gas exchange valve, and a cam follower assigned to the braking cam, which is provided in a braking mode for actuating the at least one gas exchange valve, and having a changeover device which is provided for changing over between the firing mode and the braking mode. The changeover device is provided for converting a torque of the camshaft into a force for changing over between the firing mode and the braking mode. 
     The known design of the valve train represents a purely mechanical changing over between two different valve lifts. The cam followers are functionally assigned to a specific valve lift, i.e., one cam follower for the fired lift and another for the brake lift. The mounting of the cam followers occurs on a common rocker arm axis, which is also rotatably mounted in the engine housing. The cam followers themselves are mounted on this axis with a central offset in relation to the axis of rotation of the rocker arm axis, such that it results in a movement of the rocker arm central axes relative to the camshaft axis when the rocker arm axis is rotated. This rotation causes an opposite movement of the rocker arm bearings such that they come into contact with the camshaft alternatively. A changeover between different valve lifts is thereby achieved, which are independent of each other and do not have to have a common base circle phase. 
     The synchronous rotation of the rocker arm axis at a specific control point in time is caused by the cam lift of a cam follower itself, i.e., the bearing force generates a corresponding adjustment torque on the rocker arm axis by which it is rotated accordingly. Since the cam elevation is continuous, the rocker arm shaft is also adjusted harmoniously and continuously. In addition, the adjustment takes place at the same time interval in which a regular valve lift would take place. Changing over between both cam profiles occurs by unlocking the end positions of the rocker arm shaft. In the case of a triggered unlocking, a rotation takes place during the next cam lift, the rotation being carried out until the opposite end position is reached and the locking is automatically arrested again. 
     The triggering of the switching operation, i.e., the unlocking of the rocker arm axis, is ensured by a camshaft-synchronous triggering device. This is constructively designed in such a way that the control point in time for the switching back and forth is shifted along the length of the half cam lift. It is thereby achieved that, when switching from the fired mode, the opening edge of the switching cam of a valve lift used as such causes the rocker arm axis to rotate into the braking mode and, corresponding to the switching-back, the closing edge of the same switching cam of a valve lift used as such causes the rotation back into the fired mode, wherein either the cam lift of the brake profile or a return spring here causes the corresponding torque on the rocker arm axis. 
     The object of the invention is in particular to provide an advantageously flexible and reliable valve train. 
     The invention is based on a valve train having at least one camshaft, which has at least one cam group having at least one firing cam and at least one braking cam, having at least one cam follower assigned to the firing cam, which is provided in a firing mode for actuating at least one gas exchange valve, and a cam follower assigned to the braking cam, which is provided in a braking mode for actuating the at least one gas exchange valve, and having a changeover device, which is provided to changeover between the firing mode and the braking mode. 
     It is provided that the camshaft has a separate switching cam which, in at least one operating condition, is provided to act directly on a switching element of the changeover device, which is provided for a direct changeover between the firing mode and the braking mode. Preferably, the changeover device is provided to convert a torque of the camshaft into a force for changeover between the firing mode and the braking mode. In this way, the torque and/or the rotational movement of the camshaft can be used, whereby an actuator which provides the force for the changeover, for example in the form of hydraulic pressure, can be dispensed with. Preferably, the switching element of the changeover device is provided for a direct switching between the firing mode and the braking mode to adjust a position of the cam followers. Preferably the switching element of the changeover device is provided for a direct changeover between the firing mode and the braking mode for adjusting the positions of the rotation axes of the cam followers. Preferably the switching cam is provided for an adjustment of the switching element and a related changeover between the firing mode and the braking mode for a direct contact of the switching element. Various designs of the switching element which appear sensible to a person skilled in the art are conceivable, such as a gear lever and/or a switching shaft, for example. Due to the design of the valve train according to the invention, a changeover between the firing mode and the braking mode independent of the firing cams and the braking cams can in particular be achieved. Due to the arrangement of the switching cam and the switching element, a kinematic coupling between the camshaft movement and the rotation of the independent changeover between the firing mode and the braking mode can in particular be produced. In particular, a targeted design of the independent changeover between the firing mode and the braking mode can be enabled. A “cam group” is to be understood to mean a group of cams which comprises all the cams provided for a cylinder of the internal combustion engine which the camshaft has. A “firing mode” is to be understood in particular as a control of the gas exchange valves for a fired mode. A “braking mode” is to be understood in particular as a control of the gas exchange valves for a braking mode in which a compression work within the cylinder is used for braking mode. The firing mode and the braking mode differ in particular in the control times for the gas exchange valves. In this context, a “changeover device” is to be understood in particular as a mechanism which is provided for a switching between the firing mode and the braking mode. The term “provided” is to be understood in particular as specially designed and/or equipped. Furthermore, in this context, a “separate switching cam” is to be understood in particular as a switching cam which is different from the firing cam and the braking cam, and is in particular independent. Preferably, the separate switching cam has at least one cam different from the firing cam and the braking cam. 
     It is also provided that the switching element of the changeover device is formed by a switching eccentric shaft. In particular, an advantageously reliable design of the switching element can be achieved. In particular, a switching element can be provided by means of which an advantageously high changeover force can be applied. Preferably, a changeover force can advantageously be set via an eccentricity. In particular, an advantageously versatile switching element can be provided. In this context, a “switching eccentric shaft” is to be understood in particular as a switching shaft which has at least one switching section which is eccentric relative to a mounting axis of the switching shaft. Preferably, the eccentric switching section is designed like a crankshaft. Preferably, the switching eccentric shaft also has at least one switching cam, which is provided for adjusting the switching eccentric shaft. Particularly preferably, the switching eccentric shaft is adjusted via the at least one switching cam, wherein the switching eccentric shaft is in turn provided to further transmit a switching movement via the eccentric switching section. In principle, it would also be conceivable that the switching eccentric shaft has several eccentric switching sections, wherein one of the eccentric switching sections is provided to adjust the switching eccentric shaft. 
     Furthermore, it is provided that the switching cam of the camshaft is designed to be axially moveable relative to the firing cam and the braking cam and forms a separate cam contour. Preferably, the separate cam contour is formed by a contour of the switching cam. Particularly preferably, the at least one switching cam has at least two switching positions, wherein a first switching position is assigned to a firing mode and a second switching position is assigned to a braking mode. One switching position is defined in particular by a defined position relative to the firing cam and the braking cam. In this way, an advantageously quick changeover between the firing mode and the braking mode can in particular be achieved. In particular, an advantageously direct changeover can be achieved. In addition, an advantageously simple changeover can be achieved, in particular if a switching position is assigned to each operating state. 
     It is further suggested that the changeover device has a rocker arm bearing directly coupled to the switching element, which has a first end position assigned to a firing mode and a second end position assigned to a braking mode. Preferably, the switching element, in particular the eccentric switching section, is guided in a groove, in particular in a slot, of the rocker arm bearing. This means that the changeover device can be designed in a particularly simple mechanical manner. By means of such a design, it can be achieved that the end position of the rocker arm bearing determines whether the firing mode or the braking mode is switched, whereby, for switching over, only the rocker arm bearing has to be switched from one end position to the other end position. In this way, the changeover can be easily implemented mechanically without the changeover device needing an additional actuator, whereby a simple and robust changeover device is needed. A “rocker arm bearing” is to be understood here in particular as a bearing for rocker arms for actuating the gas exchange valves, which is provided to absorb and dissipate actuating forces acting on the rocker arms when the gas exchange valves are actuated. 
     It is further provided that the valve train has at least two rocker arms, each of which has one of the cam followers, which can each be swivelled about a rocker arm axis determined by the rocker arm bearing to actuate the at least one gas exchange valve. Preferably the rocker arms are provided for direct actuation of the at least one gas exchange valve. By connecting the rocker arms to the rocker arm bearing, which can be switched between the first end position and the second end position, it can be achieved that, depending on the end position, the one rocker arm or the other rocker arm is in operative connection with the camshaft, whereby it is possible to switch easily between the firing mode and the braking mode. 
     In addition, it is provided that the rocker arm bearing has at least one bearing screw, having a ball head to a bearing that can be pivoted about the rocker arm axis of at least one of the rocker arms. Preferably the at least one bearing screw is formed by a spherical head screw. Preferably a geometrical center of the ball head defines the rocker arm axis. Particularly preferably, the ball head of the bearing screw together with a ball head receiver of the of the assigned rocker arm forms a ball joint by means of which the corresponding rocker arm is mounted on the rocker arm bearing. In particular, the bearing screw is screwed into a base body of the rocker arm bearing, wherein a screw-in depth can be changed. In this way, an advantageously flexible mounting of at least one of the rocker arms can in particular be provided. Preferably, a bearing can in particular be provided which can be used to compensate for advantageous tolerances. Furthermore, this can be used to achieve an advantageously simple assembly. 
     It is further provided that the rocker arm bearing has at least one rotatably mounted bearing element to receive the at least one bearing screw, wherein the bearing screw is designed to be moveable to a limited extent relative to the bearing element for adjusting the valve clearance. Preferably the rotatably mounted bearing element forms a base body of the rocker arm bearing. In particular, the bearing element is mounted so as to be rotatable relative to a housing. Preferably the at least one bearing screw is screwed into the bearing element, in particular into an internal thread of the bearing element. Particularly preferably, a valve clearance can be adjusted by changing a screw-in depth of the bearing bolt. In this way, an advantageously simple and flexible adjustment of the valve clearance can in particular be achieved. In particular, a constructively simple adjustment of the valve clearance can be achieved. 
     Furthermore, it is provided that the at least one bearing element of the rocker arm bearing has a switching gate in which an eccentric switching section of the switching element of the changeover device is guided. Preferably, the switching gate is formed in particular by an elongated recess in which an eccentric shifting section of the shifting element is guided. Preferably, a switching movement, in particular a switching rotation, of the switching element is transmitted to the bearing element via the switching gate. In this way, the changeover device can be designed mechanically in a particularly simple manner. By means of such a design, it can be achieved that the end position of the rocker arm bearing or the bearing element determines whether the firing mode or the braking mode is switched, whereby for changeover, only the rocker arm bearing must be switched from one end position to the other end position. In this way, the changeover can be simply implemented mechanically without the changeover device requiring an additional actuator, whereby a simply and robust changeover device is required. 
     It is further provided that the at least one bearing screw has at least one oil channel, which is provided to provide an engine-side pressure oil supply at the rocker arm depending on a position relative to the respective associated rocker arm. Preferably, the oil channel is provided to provide an engine-side pressure oil supply at oil demand points of the rocker arm, such as the rocker arm roller or the slide shoe contact, depending on a position relative to the respective assigned rocker arm. Preferably, the oil channel ends at different points of the rocker arm depending on a position relative to the respectively assigned rocker arm. Preferably, the oil channel is connected to an oil channel of the rocker arm in at least one position relative to the respective assigned rocker arm. In particular, an advantageously reliable oil supply can be provided in this way. In particular, it can be advantageous to provide an oil supply that meets the requirements, in particular depending on an operating condition. In this context, an “oil channel” is to be understood in particular as a channel which is intended for carrying oil, in particular engine oil. Preferably, it should be understood as a channel through which oil, in particular engine oil, can be passively or actively guided through. 
     It is further provided that at least one of the rocker arms has at least one arresting element which is provided to arrest the rocker arm relative to the rocker arm bearing in at least one position. Preferably, the arresting element is in particular provided for arresting the rocker arm, in particular the rocker arm for a braking mode, in a firing mode. Preferably, the arresting element is provided for arresting the rocker arm to the ball head of the bearing screw. Particularly preferably, the arresting element is formed by a spring-loaded pressure piece, in particular spring-loaded balls. In principle, however, a different design, which would appear to make sense to a person skilled in the art, would also be conceivable. Preferably, the arresting device can be released non-destructively when a defined release force, which exceeds an arresting force, is applied. The arresting element is provided in particular to hold the rocker arm bearing in an end position. In this way, a reliable arresting can in particular be achieved. 
     It is also provided that the at least one arresting element is provided to stop an oil flow of the oil channel in an arrested state. Preferably, the arresting element is provided to directly cover the oil channel in an arrested state. Preferably, the arresting element is provided to engage in the oil channel of the bearing screw in an arrested state. In this way, a reliable disconnection of the oil flow can be achieved. In addition, the arresting element can in particular be used for several purposes. This in turn allows a number of components to be kept to a minimum. 
     Furthermore, the invention is based on a method for operating the valve train. It is provided that, for a direct changeover between the firing mode and the braking mode, a separate switching cam of a camshaft of the valve train acts directly on a switching element of the changeover device. This makes it possible, in particular, to achieve a changeover between the firing mode and the braking mode independent of the firing cam and the braking cam. In particular, the arrangement of the switching cam and the switching element makes it possible to establish a kinematic coupling between the camshaft movement and the rotation of the independent changeover between the firing mode and the braking mode. In particular, a targeted design of the independent changeover between the firing mode and the braking mode can be enabled. 
     Further advantages emerge from the following description of the Figures. In the Figures, an exemplary embodiment of the invention is depicted. The Figures, the description of the Figures and the claims contain numerous features in combination. Expediently, the person skilled in the art will also consider the features individually and combine them into further sensible combinations. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of a motor vehicle having a valve train according to the invention: 
         FIG. 2  is a perspective depiction of the valve train according to the invention in a firing mode from the front; 
         FIG. 3  is a perspective depiction of the valve train according to the invention in a firing mode from behind; 
         FIG. 4  illustrates the valve train in a firing mode in a front view; 
         FIG. 5  is a partial section of the valve train in a firing mode in a cross-section along the intersection V-V; 
         FIG. 6  illustrates the valve train in a firing mode in a cross-section along the intersection VI-VI; 
         FIG. 7  is a perspective depiction of the valve train according to the invention in a braking mode from the front; 
         FIG. 8  is a perspective depiction of the valve train according to the invention in a braking mode from behind; 
         FIG. 9  illustrates the valve train in a braking mode in a front view; 
         FIG. 10  is a partial section of the valve train in a braking mode in a cross-section along the intersection X-X; 
         FIG. 11  illustrates the valve train in a braking mode in a cross-section along the intersection XI-XI; 
         FIG. 12  illustrates a bearing element of a rocker arm bearing of a changeover device of the valve train according to the invention in a perspective depiction; 
         FIG. 13  illustrates a switching element of the changeover device of the valve train according to the invention in a perspective depiction; and 
         FIG. 14  is a schematic flow chart of a method for operating the valve train according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically shows a motor vehicle  33 . The motor vehicle  33  is formed by a commercial vehicle, in particular a heavy goods vehicle (HGV). In principle, however, a different design of the motor vehicle  33  which appears sensible to a person skilled in the art would also be conceivable. The motor vehicle  33  comprises a drive train via which drive wheels  34  of a motor vehicle  33  are driven. The drive train comprises an internal combustion engine  35 . In addition, the motor vehicle  33  has a multi-stage transmission which is not described in more detail. The internal combustion engine  35  has a driven crankshaft which is connected to a transmission input element of the multi-stage transmission. The multi-stage transmission forms part of the drive train of the motor vehicle  33 . The internal combustion engine  35  comprises at least one valve train  36 . Preferably, the internal combustion engine  35  comprises several valve trains  36 . The internal combustion engine  35  is provided to convert chemical energy into kinetic energy, which is used in particular to propel a motor vehicle  33 . 
       FIGS. 2 to 13  show the valve train  36  of the internal combustion engine  35 . The valve train  36  comprises a camshaft  10 , which is provided for a firing mode and a braking mode. The camshaft  10  is designed as an exhaust camshaft. The camshaft  10  is provided to actuate gas exchange valves  15 ,  16  for cylinders of the internal combustion engine  35  that are not depicted in more detail. 
     In the depicted exemplary embodiment, the internal combustion engine  35  comprises two gas exchange valves  15 ,  16  per cylinder, which are designed as exhaust valves. The camshaft  10  comprises at least one cam group having a firing cam  11  and a brake cam  12 . The camshaft  10  comprises one cam group per cylinder for actuating the two gas exchange valves  15 ,  16 . In the exemplary embodiment, only the firing cam  11  and the braking cam  12  of one cylinder are shown. Further cylinders which are not depicted in more detail have similarly designed cams. 
     Furthermore, the internal combustion engine  35  comprises at least one further gas exchange valve per cylinder, which is designed as an intake valve and is operated by an additional valve train. Preferably, two intake valves are provided for each cylinder, which are actuated by an intake camshaft. The further valve train and the intake valves are not depicted in more detail in the Figures. 
     The gas exchange valves  15 ,  16  and the intake valves are accommodated in the usual manner in a cylinder head of the internal combustion engine, which is not depicted in more detail. 
     The firing cam  11  is provided to open the gas exchange valves  15 ,  16  in a firing mode. The braking cam  12  is provided to open at least one of the gas exchange valves  15 ,  16  in a braking mode. The firing cam  11  and the braking cam  12  have different cam curves. The cam curve of the firing cam  11  has a lift which is provided in particular to open the gas exchange valves  15 ,  16 , while a piston in the corresponding cylinder is moved from a bottom dead center to a top dead center in order to expel exhaust gas from the cylinder via the gas exchange valves  15 ,  16  in one expulsion stroke. The cam curve of the brake cam  12  is at least provided to open at least one of the gas exchange valves  15 ,  16 , after the piston of the corresponding cylinder has been moved from the bottom dead center to the top dead center in order to release the compressed air or combustion air in a compression stroke via the at least one gas exchange valve  15 ,  16 . The combustion air is let into the cylinder in an intake stroke via the inlet valves in an intake stroke before the compression stroke. The engine braking effect arises from the release of the compressed combustion air at the end of the compression stroke, which cannot be used in the following work cycle to accelerate a piston from the top dead center to the bottom dead center. By way of example, the cam curve of the brake cam  12  of the engine braking device according to the invention depicted in  FIGS. 2 to 13  has two lifts. The lifts are designed as brake lifts and/or reloading lifts. The valve train  36  depicted in  FIGS. 2 to 14  is executed as a 2-stroke engine brake with recharging. Naturally, the valve train  36  can also be executed as a 4-stroke engine brake with only one brake lift and an optional recharging lift. The function and effect of brake and recharging cams will not be discussed in detail as they are sufficiently known from prior art. 
     The valve train  36  has a cam follower  13  assigned to the firing cam  11 , which is provided for actuating the gas exchange valves  15 ,  16  in a firing mode. Furthermore, the valve train  36  has a cam follower  14  assigned to the braking cam  12 , which is provided for actuating one of the gas exchange valves  15 ,  16  in braking operation. In the shown exemplary embodiment according to  FIGS. 2 to 14 , only the gas exchange valve  16  is actuated in the braking mode. The cam follower  13 , which is provided for the firing mode, is only provided for an active connection with the firing cam  11 . The cam follower  14 , which is provided for braking operation, is only provided for active connection with the braking cam  12 . 
     In addition, the valve train  36  has a changeover device  17 , which is provided to changeover between the firing mode and the braking mode. The changeover device  17  is provided to changeover between an actuation of both gas exchange valves  15 ,  16  by the firing cam  11  and an actuation of the individual gas exchange valve  16  by the braking cam  12 . The changeover device  17  is provided for switching back and forth between a tapping of the cam curve of the firing cam  11  by the assigned cam follower  13  and a tapping of the cam curve of the brake cam  12  by the assigned cam follower  14 . The changeover device  17  is only provided for switching the actuation of the gas exchange valves  15 ,  16  of the one cylinder. For the other cylinders, the valve train  36  can in principle have further, analogously designed changeover devices. 
     The valve train  36  comprises two rocker arms  22 ,  23  assigned to the cylinder. The cam followers  13 ,  14  are assigned to the rocker arms  22 ,  23 . The engine braking device comprises two rocker arms  22 ,  23 , each having one of the cam followers  13 ,  14 . The rocker arms  22 ,  23  can each be pivoted about a rocker arm shaft  24 ,  25  fixed by a rocker arm bearing  21  to actuate the gas exchange valves  15 ,  16 . One rocker arm  22  is provided for the firing mode and has the cam follower  13 , which is provided for the active connection with the firing cam  11 . The other rocker arm  23  is provided for the braking mode and has the cam follower  14 , which is provided for the active connection with the brake cam  12 . The rocker arm  22 , which is provided for the firing mode, acts on both gas exchange valves  15 ,  16 . In the depicted exemplary embodiment, the rocker arm  23 , which is provided for the braking mode, only acts on one gas exchange valve  16 , but can in principle also act on both gas exchange valves  15 ,  16 . In the shown exemplary embodiment according to  FIGS. 1 to 14 , the rocker arm  23  acts on the gas exchange valve  16  in the braking mode via a slide shoe  37 , which can be moved longitudinally and is mounted in the rocker arm  22 . The two rocker arms  22 ,  23  are separated from each other moveably. Depending on whether the firing mode or the braking mode is switched, the camshaft  10  actuates the corresponding rocker arm  22 ,  23 , while the other rocker arm  22 ,  23  is decoupled from the camshaft  10 . 
     The firing cam  11  is positioned substantially centrally between the two gas exchange valves  15 ,  16  in axial extension on the camshaft. Furthermore, the rocker arm  22  is split at its end opposite the cam follower  13 , such that each of the two ends  22 . 1 ,  22 . 2  of the rocker arm  22  can actuate one of the two gas exchange valves  15 ,  16 . The end  22 . 1  of the rocker arm  22  assigned to the gas exchange valve  15  is in direct contact with the gas exchange valve  15 , while the end  22 . 1  of the rocker arm  22  assigned to the gas exchange valve  16  can act on the gas exchange valve  16  via the slide shoe  37 . For this purpose, the end  22 . 1  has a hole  22 . 3  in which the slide shoe  37  is accommodated in a longitudinally moveable manner. The slide shoe  37  has a head  37 . 1  which is connected to a shaft  37 . 3  via a ledge  37 . 2 . The shaft  37 . 3  is accommodated in the hole  22 . 3  in a longitudinally moveable manner, wherein the end  22 . 2  of the rocker arm  22  is supported on the ledge  37 . 2  during a stroke movement of the rocker arm  22  in the firing mode and transmits the stroke of the firing cam  11  to the gas exchange valve  16  via the head  37 . 1 . A cap  54  can be provided between the head  37 . 1  and the gas exchange valve  16 , which is placed on one end  16 . 1  of the gas exchange valve  16 . The slide shoe  37  can have a securing element  37 . 4  on its shaft  37 . 3  opposite the head  37 . 1 , which prevents the slide shoe  37  from falling out of the hole  22 . 3  when the rocker arm  22  is mounted. Advantageously, due to the central arrangement of the firing cam  11  between the two gas exchange valves  15 ,  16 , the bifurcated rocker arm  22  is designed substantially symmetrically in relation to the two ends  22 . 1 ,  22 . 2  of the rocker arm  22  and the firing cam  11 , such that the two ends  22 . 1 ,  22 . 2  substantially have the same distance from the firing cam  11 , whereby an even load of the rocker arm  22  is enabled and a tilting moment along the camshaft  10  is avoided. 
     The braking cam  12  is positioned at a distance from the firing cam  11  on the camshaft  10  substantially in alignment with the gas exchange valve  16  perpendicular to the axis of rotation  40  of the camshaft  10 . In this way, the rocker arm  23  can be designed to be substantially straight, such that an input of lateral forces on the valve train  36  can be avoided, whereby a low-wear operation is possible. In the braking operation, the rocker arm  23  acts as a stroke of the braking cam  12  via a transmission element  55  on the end  23 . 1  of the rocker arm  23  on the slide shoe  37 . The transmission element  55  is firmly connected to the rocker arm  23 . The longitudinally moveable slide shoe  37  slides in the hole  22 . 3  of the rocker arm  22 , which is not actuated in the braking mode, when the stroke of the braking cam  12  is transmitted in the direction of the gas valve  16  and back. 
     The changeover device  17  is provided to convert a torque of the camshaft  10  into a force for switching between the firing mode and the braking mode. For control by means of a control and regulating device which is not depicted in more detail, the changeover device  17  comprises an actuator which is not depicted in more detail, by means of which the changeover can be triggered. Apart from the actuator, which is only provided for triggering the changeover, the changeover device  17  is completely mechanical. 
     The camshaft  10  has a separate switching cam  20 . The switching cam  20  is different from the firing cam  11  and the braking cam  12 . The switching cam  20  is axially moveable relative to the firing cam  11  and the braking cam  12  and forms a separate cam contour  18 . The switching cam  20  rotates with the camshaft  10 . The switching cam  20  is assigned to the changeover device  17 . The switching cam  20  forms an active part of the changeover device  17 . The switching cam  20  is coupled to the actuator which is not further visible and which is provided for triggering the changeover. The actuator is controlled by a not further visible computing unit of the changeover device  17 . The computing unit of the changeover device  17  is formed by a part of the vehicle electronics of the motor vehicle  33 . Via the actuator, the switching cam  20  can be moved axially along the camshaft  10 . For this purpose, the switching cam  20  is guided in a longitudinal groove  10 . 1  of the camshaft  10 . To enable the switching cam  20  to move along the camshaft  10 , the camshaft  10  can be substantially tubular and forms a cavity  10 . 2 . The longitudinal groove  10 . 1  penetrates the camshaft  10 , wherein the switching cam  20  is accommodated in the longitudinal groove  10 . 1  with a positioning element  20 . 1 . In the cavity  10 . 2  of the camshaft  10 , for example, an actuator can act on the positioning element  20 . 1  and move the switching cam  20  in the longitudinal groove  10 . 1 . The switching cam  20  has two switching positions, wherein a first switching position is assigned to a firing mode and a second switching position is assigned to a braking mode. In at least one operating state, the cam contour  18  of the switching cam  20  is provided to act directly on a switching element  19  of the changeover device  17 , which is provided for direct changeover between the firing mode and the braking mode. The changeover device  17  has the switching element  19 . The switching element  19  is rotatably mounted, wherein the valve train  36  is in the firing mode or in the braking mode, depending on a rotational position of the switching element  19 . The switching cam  20  is provided for a direct rotation of the switching element  19 . The switching element  19  is rotated by the rotation of the camshaft  10 . One bearing axis  41  of the switching element  19  extends in parallel to the rotation axis  40  of the camshaft  10 . The bearing axis  41  of the switching element  19  is arranged next to the rotation axis  40  of the camshaft  10 . To rotate the switching element  19 , the switching element  19  is contacted by the switching cam  20  and rotated directly. 
     The switching element  19  of the changeover device  17  is formed by a switching eccentric shaft. The switching element  19  has a switching section  30  which is eccentric in relation to the bearing axis  41 . For this purpose, the switching element  19  is designed like a crankshaft in the region of the eccentric switching section  30 . Furthermore, the switching element  19  has two changeover cams  43 ,  44 , which are provided for adjusting the switching element  19 . The first changeover cam  43  is assigned to the firing mode and the second changeover cam  44  is assigned to the braking mode. The changeover cams  43 ,  44  are arranged directly next to each other. In principle, however, it would also be conceivable that the changeover cams  43 ,  44  are spaced apart. The first changeover cam  43  is located in a plane perpendicular to the rotation axis  40  of the camshaft  10 , in which the switching cam  20  is also located in the first switching position. The second changeover cam  44  is arranged in a plane perpendicular to the rotation axis  40  of the camshaft  10 , in which the switching cam  20  is also located in the second switching position. The switching element  19  is adjusted via the changeover cams  43 ,  44 , wherein the switching element  19  in turn is provided to transmit a switching movement further via the eccentric switching section  30 . If the switching cam  20  is in a first switching position and if the valve train  35  is in a braking mode, the first changeover cam  43  is in an orbit of the switching cam  20  and is contacted during one rotation of the switching cam  20  and pushed out of an orbit of the switching cam  20 . The switching element  19  is brought into a firing mode position. If the switching cam  20  is in a first switching position and the valve drive  36  is in a firing mode, the switching cam  20  and the first changeover cam  43  are contactless. If the switching cam  20  is in a second switching position and the valve train  36  is in a firing mode, the second changeover cam  44  is in an orbit of the switching cam  20  and is contacted during one rotation of the switching cam  20  and pushed out of an orbit of the switching cam  20 . The switching element  19  is brought into a braking mode position. If the switching cam  20  is in a second switching position and the valve train  36  is in a braking mode, the switching cam  20  and the second changeover cam  44  are contactless ( FIGS. 2, 7 and 13 ). 
     Furthermore, the changeover device  17  has a rocker arm bearing  21  directly coupled to the switching element  19 , which has a first end position assigned to the firing mode and a second end position assigned to the braking mode. The rocker arm bearing  21  is provided for the changeover of the active connection between the camshaft  10  and the cam followers  13 ,  14 . The rocker arm bearing  21  serves in particular to bear the rocker arms  22 ,  23  and determines the rocker arm axis  24  for the rocker arm  22  and the rocker arm axis  24 , which is different from the rocker arm axis  25 , for the rocker arm  23  respectively, about which the corresponding rocker arm  22 ,  23  is pivotably mounted. The rocker arm axes  24 ,  25  run parallel to the rotation axis  40  of the camshaft  10  ( FIGS. 2 to 11 ). 
     The rocker arm bearing  21  comprises a pivotably mounted bearing element  28 . The rocker arm bearing  21  comprises a bearing element  28 , on which the rocker arms  22 ,  23  are mounted (see  FIGS. 2, 7 and 12 ). The bearing element  28  itself is pivotably mounted. A bearing axis  38 , about which the bearing element  28  can be pivoted, is arranged in parallel to the rocker arm axes  24 ,  25 . The rocker arm axis  24  and the rocker arm axis  25  are offset at an angle to each other about the bearing axis  38  of the bearing element  28 . The bearing element  28  is mounted in a housing, which is not depicted in more detail, of the valve train  36 . The housing is attached to the cylinder head. The bearing element  28  is executed in the form of a U-shaped bracket, wherein ends  39 ,  39 ′ of the bearing element  28 , which are oriented parallel to the axis of rotation  40  of the camshaft  10 , serve for bearing about the bearing axis  38 , and wherein the rocker arms  22 ,  23  are connected to a part of the bearing element  28  which is substantially parallel to the camshaft  10 . The ends  39 ,  39 ′ of the bearing element  28  are designed in the form of bearing pins and are rotatably accommodated in bearings of the housing which are not further visible. Furthermore, the switching element  19  has two opposite ends  19 . 1 ,  19 . 2 . The ends  19 . 1 ,  19 . 2  of the switching element  19  are also designed in the form of bearing pins and are rotatably accommodated in further bearings of the housing which are not further visible. The camshaft  10  is also rotatably accommodated in other bearings of the housing which are not further visible. It is also conceivable that the camshaft is rotatably mounted in the cylinder head in a known manner and the remaining components of the valve train  36  are accommodated in the housing connected to the cylinder head. 
     The bearing axis  38  of the bearing element  28  is oriented in parallel offset to the rotation axis  40  of the camshaft  10 . In the first end position, the cam follower  13  provided for the firing mode is in constant contact with the firing cam  11 . On the other hand, the cam follower  14  provided for the braking mode is lifted off the brake cam  12 , whereby the braking cam  12  passes under the cam follower  14  without effect ( FIGS. 2 to 6 ). Conversely, in the second end position, the cam follower  14  for the braking mode is in constant contact with the braking cam  12 , while the cam follower  13  for the firing mode is lifted from the firing cam  11 , whereby the firing cam  11  passes under the cam follower  13  without effect. By swivelling the bearing element  28  from one end position to the other end position, the position of the respective rocker arm axes  24 ,  25  relative to the rotation axis  40  of the camshaft  10  is changed. While the one rocker arm axis  24 ,  25  and the associated rocker arm  22 ,  23  is closer to the rotation axis  40  of the camshaft  10  in one end position and ensures contact between one cam follower  13 ,  14  and the respective cam  11 ,  12 , the other rocker arm axis  24 ,  25  and the associated rocker arm  22 ,  23  in the same end position of the bearing element  28  further away from the axis of rotation  40  of the camshaft  10 , whereby there is no contact between the other cam follower  13 ,  14  and the respective cam  11 ,  12  ( FIGS. 7 to 11 ). 
     A swivel movement of the bearing element  28  is limited by the two mechanical stops which define the two end positions of the rocker arm bearing  21 . During a swivel movement of the bearing element  28  from the second end position in the braking mode to the first end position in the firing mode, the stops limit the swivel movement of the bearing element  28 . Accordingly, the stops limit the swivel movement of the bearing element  28  from the first end position in the firing mode to the second end position in the braking mode. 
     To limit the swivel movement of the bearing element  28 , the bearing element  28  of the rocker arm bearing  21  has a switching gate  29 . The switching gate  29  extends perpendicularly to the bearing axis  38  of the bearing element  28 . The shifting gate  29  is formed by a straight, elongated recess. In principle, however, a different design, which would appear sensible to a person skilled in the art, would also be conceivable. The eccentric switching section  30  of the switching element  19  of the changeover device  17  is guided in the switching gate  29 . One position of the bearing element  28  is set via the switching gate  29 . Depending on a rotational position of the switching element  19 , the bearing element  28  is in the first end position or the second end position. Depending on a rotational position of the switching element  19 , the eccentric switching section  30  is accordingly located at a first end of the switching gate  29  or at the second end of the switching gate  29 . When the switching element  19  is rotated, the eccentric switching section  30  slides through the switching gate  29  from one end to the other, wherein the bearing element  28  is rotated from one end position to the other end position (cf.  FIGS. 3, 5, 8, 10 and 12 ). 
     To secure the end positions, the valve drive  36  has a retaining spring  56 . One end of the retaining spring  56 . 1  is attached to the bearing element  28  by means of a retaining element  57 . The retaining element  57  can be designed in a suitable manner, for example, in the form of a head screw. The retaining spring  56  applies a spring tension starting from the bearing element  28  in the direction of the switching section  30 . For this purpose, the retaining spring  56  rests with its other end  56 . 2  in a groove  19 . 3  of the switching section  30 . The groove  19 . 3  is substantially provided between the bearing element and the switching section  30 , wherein, by means of the spring force of the retaining spring  56 , the switching section  30  is retained at the first end of the switching gate  29  or at the second end of the switching gate  29 , depending on the rotational position of the switching element  19 . The retaining spring  56  is designed in the form of a torsion spring, wherein other forms or a retaining spring are also conceivable. 
     The groove  19 . 3  is provided as an at least partially peripheral groove on the end of the switching section  30  opposite the changeover cam  43 . The switching section  30  is substantially designed in the form of a cylinder, wherein the switching section  30  merges at least partially into the changeover cam  43 . 
     The adjustment of the engine brake device is therefore not represented by a locking mechanism and the torque required for the adjustment from the bearing forces and the eccentric offset of the rocker arms  22 ,  23 , but rather by a kinematic coupling of the switching element  19  and the bearing element  28  in such a way that the separate cam contour  18  acts on the switching element  19 , which causes a rotation of the bearing element  28  in the sense of a single step of a Geneva drive. Thus the movement of the switching element  19  is directly coupled to the movement of the bearing element  28 . Furthermore, the presence of a separate switching cam  20  means that the switching time point is no longer coupled to the exhaust cam elevation, but rather can be selected at will, which represents a significant degree of freedom in terms of cam design. The energy required for the switching operation comes from the camshaft  10 , wherein here it is not the firing cam  11  which is applied via the rocker arm  22 , but rather via the switching cam  20  itself. The triggering is represented by an axial movement of the switching cam  20  in the camshaft  10 . In the depicted embodiment of the switching operation of an electro-mechanically operated decompression engine brake, the changeover process is depicted with the separate switching cam  20 , which is located on the camshaft  10 , and the switching element  19 , which acts as a link between the camshaft housing and the rotatably mounted bearing element  28 . By means of the arrangement of the switching cam  20  and the switching element  19 , a kinematic coupling between the rotational camshaft movement, in particular exhaust camshaft movement, and the rotation of the bearing element  28  is produced, and the dependency of the switching movement on the exhaust valve lift is thus eliminated. 
     Furthermore, the rocker arm bearing  21  has two bearing screws  26 ,  26 ′ The bearing screws  26 ,  26 ′ each have a ball head  27 ,  27 ′ for bearing the rocker arms  22 ,  23  which can be pivoted about the rocker arm axis  24 ,  25 . The bearing screws  26 ,  26 ′ are each assigned to one of the rocker arms  22 ,  23  and serve to bear the rocker arms  22 ,  23  on the bearing element  28 . The bearing screws  26 ,  26 ′ are designed identically. In principle, however, an at least partially differentiating design would also be conceivable. The bearing screws  26 ,  26 ′ are each formed by a spherical head screw. A geometrical center of the ball head  27 ,  27 ′ defines the rocker arm axis  24 ,  25  of the respective rocker arm  22 ,  23  in each case. The ball heads  27 ,  27 ′ of the bearing screws  26 ,  26 ′ each form a ball joint together with a ball head receptacle  58 ′ of the associated rocker arm  22 ,  23  via which the corresponding rocker arm  22 ,  23  is mounted on the rocker arm bearing  21 . The bearing screws  26 ,  26 ′ therefore represent the fixed bearing pin of the ball head  27 ,  27 ′, i.e., the outer ball, and the respective rocker arm  22 ,  23  the ball head receptacle  58 ′. The ball head receptacle  58 ′ has a circumferential groove  58 . 1 ′ on its upper edge, in which a circlip  58 . 2 ′ can be inserted. The circlip  58 . 2 ′ prevents the ball head  27 ,  27 ′ from loosening from the ball head receptacle  58 ′. The rotatably mounted bearing element  28  is provided to accommodate the bearing screws  26 ,  26 ′. The bearing screws  26 ,  26 ′ are each designed for limited adjustment of the valve clearance relative to the bearing element  28 . The bearing screws  26 ,  26 ′ are screwed into the bearing element  28 , wherein a screw-in depth can be changed. By means of a screw-in depth, the rocker arm axis  24 ,  25  of the respective rocker arm  22 ,  23  can be changed relative to the bearing element  28  and a valve clearance can thereby be adjusted. To prevent rotation, the bearing screws  26 ,  26 ′ can each be secured or locked by means of a locknut  60 ,  60 ′ against unwanted rotation in the bearing element  28 . In order to carry out the adjustment of the valve clearance, the bearing center of the rocker arm  22 ,  23 , i.e., the rocker arm axis  24 ,  25  is shifted by means of the respective bearing screws  26 ,  26 ′ in such a way that the distance from the respective rocker arm  22 ,  23  to the camshaft  10  and the valve-sided tap varies. 
     The bearing screws  26 ,  26 ′ each have an oil channel  31 . The oil channels  31  are each provided to provide an engine-side pressure oil supply  48  on the respective rocker arm  22 ,  23 , depending on the position of the respective bearing screws  26 ,  26 ′ relative to the respective rocker arm  22 ,  23 . The oil channels  31  are each provided to provide the engine-side pressure oil supply  48  at oil demand points of the respectively associated rocker arm  22 ,  23 , such as a rocker arm roller  46 ,  47  of the cam followers  13 ,  14 , across which the rocker arm  22 ,  23  slides on the firing cam  11  or the braking cam  12 , or the contact point between the slide shoe  37  and the gas exchange valve  16 . The bearing screws  26 ,  26 ′ each produce a connection between the engine-side pressure oil supply  48  and rocker arm-side oil demand points of the rocker arm roller  46 ,  47  and the valve-side slide shoe contact. The bearing screws  26 ,  26 ′ are rotated relative to the rocker arms  22 ,  23  when the operating modes of fired and braked modes are adjusted, which allows the oil flow to be controlled by the design of the oil hole position. This in turn reduces the total oil requirement for the rocker arms  22 ,  23  to the rocker arms  22 ,  23  activated in the respective operating mode. The rocker arms  22 ,  23 , which are therefore not required in the current operating mode, are excluded from an oil supply. This can occur in particular without additional valves. 
       FIGS. 6 and 11  show, by way of example, the oil supply of the rocker arms depending on the operating mode of the internal combustion engine  35 . Both Figures show a sectional view of the rocker arm  23 . In  FIG. 6 , the bearing element  28  is depicted in its end position for the fired mode. Here, the rocker arm  22 , which is not shown, transfers a stroke of the firing cam  11  to the gas exchange valves  15 ,  16 . With its cam follower  14 , the shown rocker arm  23  has no contact with the braking cam  12  and is therefore not moved. The pressure oil supply  48  is still connected to the oil channel  31  in the bearing screw  26 ′. By rotating the bearing screw  26 ′ to the rocker arm  23 , a connection to an oil supply line  59  to the oil demand points is interrupted. The oil supply line  59  is provided in the ball head receptacle  58 ′ substantially opposite the bearing screw  26 ,  26 ′. In  FIG. 11 , the bearing element  28  is depicted in its end position for the braked mode. Here, the rocker arm  22 , which is not shown, does not transmit a stroke of the firing cam  11  to the gas exchange valves  15 ,  16 , while, with its cam follower  14 , the shown rocker arm  23  is in contact with the braking cam  12  and thus transmits a stroke of the braking cam  12  to the gas exchange valve  16 . The pressure oil supply  48  is connected to the oil channel  31  in the bearing screw  26 ′ and the oil channel  31  overlaps the oil supply line  59 . In order to ensure sufficient overlap or alignment of the oil channel  31  and the oil supply line  59  when the rocker arm  23  is moved and thus to enable a constant oil supply during a tilting movement of the rocker arm  23 , the bearing screw  26 ′ on the end of the oil channel  31  has a conical extension  61  of the oil channel  31  opening outwards at its ball head  27 . The rocker arm  22  for the fired mode is designed analogously to the rocker arm  23 . 
     In addition, the rocker arm  23  has an arresting element  32  for the braking mode. The arresting element  32  is provided for arresting the associated rocker arm  23  relative to the rocker arm bearing  21  in at least one position. The arresting element  32  is provided for arresting the rocker  23  for the braking mode relative to the respectively assigned bearing screw  26 ′ in a deactivated or fired mode. The arresting element  32  is provided to arrest the rocker arm  23  in a firing mode ( FIG. 6 ). The arresting element  32  is formed by a spring-loaded ball facing the assigned bearing screws  26 ′. The arresting element  32  is provided next to the oil supply line  59  in the rocker arm  22 ,  23  in the ball head receptacle  58 ′. 
     In principle, however, a different design, which would appear sensible to a person skilled in the art, would also be conceivable. The arresting of the arresting element  32  can be released non-destructively when a defined release force is applied by the switching cam  20 , which exceeds an arresting force. The arresting element  32  is provided to hold the rocker arm bearing  21  in the first end position respectively. The arresting element  32  is directly integrated in the ball joint between the bearing screws  26 ′ and the rocker arm  23  (see  FIGS. 6 and 11 ). 
     The arresting element  32  is also provided to shut off an oil flow of the oil channel  31  of the bearing screw  26 ′ in an arrested condition. One end of the oil channel  31  facing the rocker arm  23  is directly covered by the ball of the arresting element  32 . The arresting element  32  is provided to engage in the oil channel  31  of the bearing screw  26 ′ in an arrested condition. The oil flow is shut off and arrested by the engagement. The conical extension  61  of the oil channel  31 , which opens outwards, forms a corresponding dwell cone on the end facing the rocker arm  23 . The arresting element  32  therefore simultaneously stops the oil flow in the engaged position and releases it again in the activated position (see  FIGS. 6 and 11 ). 
     The rocker arm  22  for the fired mode is designed analogously to the rocker arm  23 . 
       FIG. 14  shows a flow chart of a method for operating the valve train  36 . The method is analogous to the description and functioning of the valve train  36  which has already been explained. In the method, a direct changeover between the firing mode and the braking mode is effected directly on the switching element  19  of the changeover device  17  by means of the separate cam contour  18  of a camshaft  10  of the valve train  36 . In the method, in a first method step  49 , the switching cam  20  is brought into a switching position corresponding to the desired operating state, i.e., a braking mode or a firing mode. If the valve train  36  is already in the corresponding operating state, the switching cam  20  and the switching element  19  are contactless in the further method step  50 . If the valve train  36  is not yet in the corresponding operating state, the switching cam  20  contacts the corresponding changeover cam  43 ,  44  of the switching element  19 , which is located in the same plane, in a second method step  51  during a rotation of the camshaft  10  and rotates the switching element  19  correspondingly. Due to the rotation of the switching element  19 , the bearing element  28  changes the end position in a third method step  52 . Subsequently, by tilting the bearing element  28  in a fourth method step  53 , the rocker arm  22 ,  23 , which previously made contact with the assigned firing cam  11  or the braking cam  12 , is lifted off the assigned firing cam  11  or braking cam  12 , and the rocker arm  22 ,  23 , which was previously lifted off the associated firing cam  11  or braking cam  12 , is pressed against the associated firing cam  11  or braking cam  12 . Subsequently, a changeover between the firing mode and the braking mode has taken place and the first method step  49  can be repeated. 
     LIST OF REFERENCE CHARACTERS 
     
         
         
           
               10  camshaft 
               10 . 1  longitudinal groove 
               10 . 2  cavity 
               11  firing cam 
               12  braking cam 
               13  cam follower 
               14  cam follower 
               15  gas exchange valve 
               16  gas exchange valve 
               16 . 1  end 
               17  changeover device 
               18  cam contour 
               19  switching element 
               19 . 1  end 
               19 . 2  end 
               19 . 3  groove 
               20  switching cam 
               20 . 1  positioning element 
               21  rocker arm bearing 
               22  rocker arm 
               22 . 1  end 
               22 . 2  end 
               22 . 3  hole 
               23  rocker arm 
               23 . 1  end 
               24  rocker arm axis 
               25  rocker arm axis 
               26  bearing screw 
               27  ball head 
               28  bearing element 
               29  switching gate 
               30  switching section 
               31  oil channel 
               32  arresting element 
               33  motor vehicle 
               34  drive wheel 
               35  internal combustion engine 
               36  valve train 
               37  slide shoe 
               37 . 1  head 
               37 . 2  ledge 
               37 . 3  shaft 
               37 . 4  securing element 
               38  bearing axis 
               39  end 
               40  rotation axis 
               41  bearing axis 
               43  changeover cam 
               44  changeover cam 
               45  rocker arm roller 
               46  rocker arm roller 
               47  rocker arm roller 
               48  pressure oil supply 
               49  method step 
               50  method step 
               51  method step 
               52  method step 
               53  method step 
               54  cap 
               55  transmission element 
               56  retaining spring 
               56 . 1  end 
               56 . 2  end 
               57  retaining element 
               58 ′ ball head receptacle 
               58 . 1 ′ groove 
               58 . 2 ′ circlip 
               59  oil supply line 
               60  locknut 
               61  extension