Patent Publication Number: US-5829400-A

Title: Valve gear for an internal combustion engine

Description:
FIELD OF THE INVENTION 
     The invention concerns a valve drive of an internal combustion engine comprising at least two equally acting gas exchange valves loaded in lifting direction by at least one cam of a camshaft, said gas exchange valves extending at a small, defined angle to each other. 
     BACKGROUND OF THE INVENTION 
     Multi-valve drives of the aforesaid type are being increasingly used in engine construction. The technical journal MTZ--Motortechnische Zeitschrift--No. 2, 1995, for instance, describes a four-cylinder engine with a five-valve technique i.e., with three intake and two exhaust valves per cylinder. Therefore, five cup-shaped tappets are required for each cylinder as transmitting elements. The intake valves are arranged on two different planes (two valves being arranged on a common plane), because the outer surface of the combustion chamber is domed and the available valve cross-section is limited. The loading of the intake valves disposed at a slant to one another necessitates a cam produced by complicated grinding for each intake valve. 
     OBJECT OF THE INVENTION 
     It is therefore an object of the invention to create a valve drive of the pre-cited type in which the aforesaid drawbacks are substantially eliminated and particularly to provide a simultaneous loading of at least two equally acting gas exchange valves using fewer parts and less complicated manufacturing procedures. 
     SUMMARY OF THE INVENTION 
     The valve drive (1) of the invention of an internal combustion engine comprising at least two equally acting gas exchange valves (2) loaded in lifting direction by at least one cam of a camshaft (3), said gas exchange valves (2) extending at a small, defined angle to each other, is in that a cam follower is arranged in driving relationship between the cam(s) and the gas exchange valves, the cam follower comprising a bridge which is guided for linear displacement relative to a cylinder head of the external combustion engine by guide means, and a longitudinal axis extending through the guide means of the bridge is at least approximately parallel to a bisector of the angle, or the longitudinal axis extending through the guide means is approximately parallel to a longitudinal axis situated between the longitudinal axes of the gas exchange valves, along which longitudinal axis, a sum of the reaction forces acting from the bridge on the guide means is zero or minimal. 
     The bridge described herein which, for example, can have a girder-like configuration, enables an equally acting loading of at least two gas exchange valves. Longitudinal axes of the gas exchange valves can extend, for example, so that their point of intersection is situated on the side of camshaft-proximate ends of the gas exchange valves (see description of figures). Thus only at least one cam is required for each cylinder and number of equally acting valves. Another minimal requirement consists in that only one transmitting element is required in the bridge for transmission to the gas exchange valve. At the same time, the otherwise complicated manufacturing of cams for each gas exchange valve is simplified. 
     An essential basic idea of the invention is to arrange the bridge so that the ends of the valve stems can only migrate to a small extent on the transmitting elements on the under surface of the bridge. This is achieved by the previously described arrangement of the guide means of the bridge parallel to a bisector between the planes of the gas exchange valves. Since, however, as is the case in the example of valve configuration, two same function gas exchange valves are situated on one common plane, a much larger reaction force is transmitted to the guide means by these two same function gas exchange valves than by the third gas exchange valve. According to the invention therefore, the guide means of the bridge is not necessarily arranged parallel to the bisector but parallel to a longitudinal axis along which the sum of the reaction forces acting from the bridge on the guide means is minimal. 
     However, the configuration and arrangement of the bridge of the invention does not apply only to the two and three valve technique (for same function gas exchange valves per cylinder) specifically described herein. If one considers, for instance, a valve drive with any desired number of same function gas exchange valves per cylinder, and these gas exchange valves are arranged, for example, at a slant to one another, or at a slant to one another and only partly along longitudinal axes which intersect the axis of the camshaft, the arrangement of the guide means of the bridge of the invention must be such that it assured either that the total migration of the gas exchange valves during valve lift on the transmitting elements provided in the bridge is as small as possible, or that the sum of the aforesaid reaction forces mentioned above is minimal. 
     Advantageously, one hydraulic clearance compensation element per valve is arranged as transmitting means in the bridge for transmission to the gas exchange valve. It is also conceivable, however, to use known mechanical adjusting devices or their combinations. 
     A particularly favorable embodiment of contacting elements for the cams in the bridge from the tribological point of view likewise forms a subject matter of the invention. Such contacting elements are configured in the form of rotating rollers or pins. 
     According to a further proposition of the invention, when the bridge is used for three equally acting gas exchange valves, the bridge is loaded by two equally acting cams each of which cooperates with one contacting means in the bridge. 
     Further, the counter surface of the transmitting element can be made slightly spherical or cylindrical so as to provide a sufficiently large contact surface for the gas exchange valve during its migration relative to the transmitting element during cam lift. It is equally conceivable to provide the transmitting element (clearance compensation element) with pivoting slide shoes known, per se, from the prior art. 
     It is advantageous for the oscillating masses in the valve drive, to make the bridge of the invention by a lightweight technique or of a light material such as aluminium. A construction out of a plastic is likewise suitable. However, it is also possible to make the bridge by a conventional method out of steel. Further, the configuration of the guide means for the bridge is not limited to the example of embodiment represented in the attached drawings. It is conceivable to use any guide means known from the state of the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of a prior art valve drive, 
     FIG. 2 shows a top view of a cam follower of the invention, and 
     FIG. 3 is a partial cross-sectional view of the cam follower of the invention in the region of its guide means. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a valve drive 1 of a generic type. This valve drive comprises three equally acting gas exchange valves 2 (intake valves). In this particular embodiment described here by way of example, two outer gas exchange valves 2a have longitudinal axes situated at least approximately on one common plane. In the longitudinal direction of a camshaft 3 whose cams load the gas exchange valves 2 in lifting direction, there is arranged between the gas exchange valves 2a, a further gas exchange valve 2b extending at an angle to the plane of the gas exchange valves 2a. The longitudinal axes of the gas exchange valves 2 roughly intersect the longitudinal axis of the camshaft 3. 
     As can be seen from FIGS. 2 and 3, a bridge 4 is arranged between the cams of the camshaft 3 and the gas exchange valves 2. This bridge 4 is guided for linear displacement relative to a cylinder head 6 of an internal combustion engine by guide means 5 needing no further description here. On its side facing the cams, the bridge 4 comprises two rollers 7 serving as direct contacting elements for the cams of the camshaft 3. These rollers 7 permit a particularly low-friction transmission of the cam lift to the bridge 4. On its side facing the valves, the bridge 4 comprises transmitting elements in the form of one hydraulic clearance compensation element 8 for each gas exchange valve 2. 
     As can be seen from FIG. 3, a longitudinal axis 9 of the guide means 5 is arranged parallel to a bisector 10 between the plane extending through the longitudinal axes of the equally acting gas exchange valves 2a and the longitudinal axis of the gas exchange valve 2b. To form a sufficiently large contact surface for the end of the respective valve stem on the counter surface of the clearance compensation element 8, this counter surface (not shown) can be given a spherical or cylindrical shape. It is also conceivable, however, to have the axis of symmetry of each transmitting element extend approximately or exactly along the longitudinal axis of the respective gas exchange valve.