WO2003104618A1

WO2003104618A1 - Desmodromic valve gear

Info

Publication number: WO2003104618A1
Application number: PCT/EP2003/004896
Authority: WO
Inventors: Mario Illien
Original assignee: Daimlerchrysler Ag
Priority date: 2002-06-07
Filing date: 2003-05-10
Publication date: 2003-12-18
Also published as: DE10225620A1; JP2005529271A; EP1511921A1

Abstract

The invention relates to a valve gear of an internal combustion engine comprising a desmodromic valve gear. A flexible strip is wound round the cam and is rotatably connected to the valve shaft. According to the invention, said strip is connected to the valve shaft by means of a connecting element comprising three parallel axles, two of the axles being connected to the ends of the strip, and the third rotating axle to the valve shaft. Preferably, the radius of curvature of the connecting element between both axles corresponds essentially to the smallest positive radius of curvature of the cam.

Description

Forced valve train

The invention relates to a positively driven valve train, in particular for internal combustion engines of motor vehicles, according to the preamble of patent claim 1.

Forced, so-called desmodromic valve drives are characterized in that a cam element is rotatably arranged in a flexible enclosing element, which in turn is connected to the valve which is movable in a plane perpendicular to the axis of rotation of the cam element. The enclosing element primarily takes over the closing process of the valve.

From WO 01/12958 AI a generic positively driven valve train, in particular for internal combustion engines of motor vehicles, is already known, which has a driven cam element, a valve actuator displaceable or pivotable by the cam element and a flexible enclosing element. The cam element is rotatably arranged in the enclosing element. The enclosing element is also movably connected to the valve actuator in a plane perpendicular to the axis of rotation of the cam element via a joint.

Basically, the following technical problems arise with such a valve train. The distance between the closed containment element and the cam surface should be the same size for any cam angle. If this distance varies, for example due to the type of connection, then depending on the position of the enclosing element relative to the cam there are different loop lengths. The extent to which the loop length varies depends directly on the cam contour, the type of valve connection and the resulting distance from the enclosing element to the cam surface.

Another problem is the connection of the joint to the enclosing element. Depending on the desired transfer function of the cam and the associated valve lifts and accelerations, there is a more or less pronounced back and forth movement of the joint with each cam rotation, so that it is rolled up on one side around the joint. This rolling up naturally means a shortening of the enclosing element. This in turn results in the locking element blocking on the cam as soon as the shortening is greater than the clearance between the cam and the locking element allows.

In addition, a large amount of flexing work is applied in the enclosing element due to the rolling movement, which leads to increased wear.

It is therefore an object of the invention to provide an improved positively driven valve train with which self-locking of the enclosing element can be reliably prevented and at the same time a partial length compensation of the enclosing element can be ensured depending on the respective position relative to the cam element.

This object is achieved by the features specified in the characterizing part of patent claim 1. By using a connecting element between the enclosing element and the valve actuator, on which two axes are provided for fastening the connecting element to the enclosing element in addition to the axis of rotation for the valve actuator, it is possible to use open enclosing elements. Furthermore, during operation, the enclosing element can partially wind up or unwind on the axes. This prevents self-locking of the valve train and enables operation with enclosing elements of high length constancy. The winding or unwinding also leads to a reduction in the variance of the loop lengths. This in turn is the prerequisite for the establishment of a uniform lubrication gap between the enclosing element and the cam element and thus ensures hydrodynamic lubrication.

The structure. of the connecting element consisting of two side parts and three axes arranged between them represents the simplest possible construction.

The adaptation of the radius of curvature of the connecting element to the smallest positive radius of curvature of the cam element ensures that the connecting element nestles as close as possible to the cam element at this point and thus further reduces the variance of the looping length.

By providing slots in the axes of the connecting elements, through which the open ends of the enclosing element are passed and then fixed, a simple and efficient possibility of assembly is realized. The provision of a rounded contour in the entry area of the slots reduces wear on the enclosing element.

By using a polymer drawstring with vulcanized-in tension cords, the elasticity of the ßungses already compensate for the variable enclosure lengths.

Further advantages emerge from the description and the patent claims. Two exemplary embodiments of the invention are shown schematically below. Show:

1 is a perspective view of a first embodiment of a positively driven valve train in an exploded view,

2 shows a top view of a connecting element according to the invention shown in FIG. 1 between a surrounding element and a valve actuator,

3 is a perspective view of an embodiment of one of the axes of the connecting element according to FIG. 1,

4 shows a cross section of the axis according to FIG. 4

Fig. 5 is a schematic diagram of the cam movement

Fig. 6 shows a second embodiment of an enclosing element

FIG. 7 shows an exploded view of a valve train with a enclosing element according to FIG. 6.

In Fig. 1, the positively driven valve train is generally identified with 1. The arrangement of such a valve train 1 in internal combustion engines is generally known to the person skilled in the art and is therefore not described in detail. The valve train 1 has a valve actuator 2 with a valve disk 3 and a valve stem 4. Furthermore, the valve train 1 has a cam element 5, which is provided for actuating the valve actuator 2 and is connected in a conventional manner in a rotationally fixed manner to a camshaft, not shown, of the internal combustion engine. For this purpose, the cam element 5 in its end face 6 has a bore 7 through which the camshaft in installed condition. The cam axis of rotation 8 is also defined by the longitudinal axis of the camshaft. To save weight, the cam element 5 can additionally have one or more recesses 9.

The outer contour 10 of the cam element 5 is surrounded by an open, flexible enclosing element 11. The enclosing element 11 is closed by a connecting element 12, which at the same time also establishes the connection to the valve actuator 2. The exact structure of such a connecting element 12 is shown in a plan view in FIG. 2. The connecting element 12 consists of three axes 13-15 and two side parts 16, 17. The outer contour 18 of the two side parts 16, 17 facing the cam element 5 is preferably adapted to the cam contour in such a way that the cam element 5 with the smallest positive radius of curvature of the curvature the side parts 16, 17 nestles as close as possible. The axis 15 serves as an axis of rotation between the enclosing element 11 and the valve actuator 2. This axis of rotation 15 is preferably arranged in the area between the two axes 13, 14 and is at a greater distance from the outer contour 18 with respect to these axes 13, 14. The two axes 13, 14 are each firmly connected to one end of the open enclosing element 11. Both non-positive and positive types of connection between the ends of the enclosing element 11 and the axes 13, 14 are possible. For example in the form of welding, soldering, clamping, screwing or wrapping. In this way, the connecting element 12 functions as a locking buckle of the enclosing element 11. A prerequisite for a functioning system is the possibility of both ends of the enclosing element 11 being wound or unwound around the respective axes 13, 14 of the connecting element 12 by a few angular degrees.

The valve actuator 2 is rotatably connected to the axis of rotation 15. The axis of rotation 15 is parallel to the cam rotation Axis 8 arranged so that during rotation of the camshaft, regardless of the position of the cam element 5, the valve stem 4 points in the direction of the camshaft axis. An intermediate part 19 is provided for connection, which is fixedly connected to the valve stem 4 and at the same time is rotatably mounted about the axis of rotation 15. In principle, however, it is also possible for a bearing for the axis of rotation 15 to be arranged directly on the valve stem 4.

In the case of internal combustion engines with a relatively long valve lift in particular, it may be necessary to provide additional shear force support. For this purpose, in the exemplary embodiment according to FIG. 1, a guide 20 is arranged on the intermediate part 19 on opposite sides in the direction of the cam rotation axis 8. These guides are guided parallel to the valve stem 4 in rails (not shown) arranged on the rotor side.

The valve drive 1 is shown in FIG. 1b without the cam element 5, so that the guidance of the enclosing element 11 in relation to the connecting element 12 can be seen better. The encircling element 11 is carried out with the respective open ends between the outer contour 10 of the cam element 5 and the respective axis 13, 14, the axes 13, 14 being wrapped in opposite directions by the encircling element 11 on the back at least over a partial circumference. The enclosing element 11 is connected in a rotationally fixed manner to the two axes 13, 14, the axes 13, 14 in turn being connected in a rotationally fixed manner to the side parts 16, 17. Thus, in the event of a relative change in position between the enclosing element 11 and the connecting element 12, the enclosing element 11 is wound up or unwound on the respective axis 13, 14. A preferred fastening method of the enclosing element 11 with the axes 13, 14 and with the side parts 16, 17 is described below with reference to FIGS. 3 and 4. From Fig. Lb continues to can see that the intermediate part 19 encloses the axis of rotation 15 at its end facing the cam element 5.

The structure of the connecting element 12 can be seen from FIGS. 1c and 1d, the valve actuator 2 including the intermediate part 19 not being shown in FIG. 1c, while the side part 17 is still removed in FIG. 1d. Finally, le shows the enclosing element 11.

For example, spring steels with high tensile strength can be used as the enclosing element 11. Due to the high carbon content, however, these are only suitable to a very limited extent for a thermally joined connection between the enclosing element 11 and the connecting element 12. An advantageous solution is a winding technique in which the enclosing element 11 is wound onto the respective axis 13, 14.

As shown in FIGS. 3 and 4, the axes 13, 14 are provided with a central slot 21 through which the open ends of the enclosing element 11 are guided. On the opposite side, the enclosing element 11 is then preferably firmly connected to the respective axis 13, 14 by a thermal joining process, for example soldering, laser or electron beam welding, and is thereby prevented from slipping out. With this type of connection, particular care must be taken to ensure that the enclosing element 11 is aligned exactly at right angles to the respective axis 13, 14. Furthermore, the axes 13, 14 have a strongly rounded contour 22 on their sides opposite the respective joining point 23 at the entry region of the slot 21, so that kinking of the enclosing element 11 is prevented.

In the manufacture of the valve train 11, the enclosing element 11 is first brought to the necessary length and non-positively connected to the two axes 13, 14. Subsequently, the wrapping element 11 is wound uniformly onto each of the two axes 13, 14 in an assembly tool, not shown. For this purpose, the enclosing element 11 with the two axes 13, 14 attached to it, and the two side parts 16, 17 including the axis of rotation 15 are inserted into the tool. The enclosing element 11 is placed over the cam element 5 or preferably over an equivalent base body. When designing the circumference of the base body, particular attention must be paid to the length of the encapsulation that differs depending on the position of the cam element 5 and the largest encapsulation length that occurs plus the desired winding length must be selected as the circumference. The winding itself then takes place until the enclosing element 11 lies evenly on the base body. For the winding process, the axes 13, 14 have circular bearing sections 24 which are rotatably guided in corresponding bores in the side parts 16, 17. Furthermore, the axes 13, 14 for the winding-up process have a profiling 25 at least at one axial end, with the aid of which the axes 13, 14 can be rotated relative to the side parts 16, 17. If the enclosing element 11 is wound up in the desired manner, then a preferably thermal joining process (for example laser welding) takes place, by means of which the axes 13, 14 are non-positively connected to the side parts 16, 17. This prevents unwinding or turning back.

The enclosing / connecting element 11, 12 thus assembled is pushed directly from the assembly tool onto the cam element 5 or for storage on a mandrel of the same circumference. The number of windings on each of the axes 13, 14 is particularly dependent on the tensile force subsequently introduced in the enclosing element 11, the coefficient of friction on the surface of the enclosing element 11 and its width. In order to set the coefficient of friction on the surface of the enclosing element 11 as high as possible, a coating in the Range of winding length possible with appropriate layers. For example, diamond particles in chemical nickel matrix are used for this. Furthermore, the maximum transmissible force is greatly limited by the thickness of the transmission element 11. However, this is also limited by the otherwise too high rigidity. If a higher force is nevertheless required, this can be achieved by a multi-layer structure.

The valve movement is explained in more detail below with reference to FIG. 5. Here, the position of the connecting element 12 relative to the cam axis of rotation 8 is shown schematically in different angular positions of the cam element 5. To simplify the illustration, the valve actuator 2 is not shown here. The possible direction of movement of the valve actuator is defined by the imaginary connecting line between the cam rotation axis 8 and the rotation axis 15. This means that the valve actuator and thus also the axis of rotation 15 can only move in the vertical direction along this imaginary connecting line 26. At the same time, the movement of the cam element 5 is determined by the position of the cam axis of rotation. Since the encircling element 11 is in turn firmly connected to the connecting element 12 and rests on the outer contour of the cam element 5, the cam element 5 thus rotates relative to the encircling or connecting element 11, 12. If the process is viewed from the perspective of an observer rotating with the cam, the connecting element 12 is pulled through the enclosing element 11 like a slide over the outer contour 10 of the cam element 5.

If one considers the rotation of the cam element 5, the force for opening the valve is transmitted from the cam element 5 via the side parts 16, 17 and the axis of rotation 15 into the valve stem 4. The closing process takes place in that the cam element 5 pulls on the two axes 13, 14 via the enclosing element 11 and thereby the closing movement via the axis of rotation 15 of the valve stem 4 initiates. The valve stem 4 is then braked shortly before it is placed in the valve seat (not shown), in turn, directly by the cam element via the two side parts 13, 14 and the axis of rotation 15.

The maximum valve lift h is thus defined via the different positions of the axis of rotation 15 in a position where the connecting element 12 lies against the base circle of the cam element 5 (on the far left in the drawing) or on the cam tip (on the far right in the drawing). At the same time, it can be seen that the connecting element 11 conforms to the outer contour 10 of the cam element 5 to different extents due to the curvature of the side parts 16, 17 depending on the position of the cam element 5. In the flat area of the cam flank (second figure from the left), the connecting element 12 clings to the outer contour 10 the least, while in the area of the greatest curvature (second figure from the right) the connecting element 12 lies directly against the cam element 5. At the same time, the angle of inclination between the connecting element 12 and the valve stem 4 or between the outer contour 10 of the cam element 5 and the connecting element 12 in the area of the axes 13, 14 also changes. This relative rotation between the connecting element 12 and the outer contour 10 of the cam element 5 causes the enclosing element to open the respective axis 13, 14 rolled up or unrolled. This prevents self-locking of the enclosing element 11 despite the change in the enclosing length and a relative rotation. This means that this unwinding or rolling movement only enables the operation of such valve drives 1 with enclosing elements of high length constancy.

At the same time, this device has the advantage that the connecting element simultaneously serves as a closure of the open enclosing element, so that it is possible to manufacture such enclosing elements from finite band material. Furthermore, the reduction in the Variance of the looping length over the cam element created the prerequisites for the establishment of a uniform lubricating gap between the encasing element and the cam element and thus enables improved hydrodynamic lubrication.

A second exemplary embodiment of a valve train according to the invention is shown in FIGS. 6 and 7, the same parts being identified with identical reference numbers. In a departure from the first exemplary embodiment, the enclosing element (11) has bearing sections 27, 28 at the open ends, which are fastened to the axles 13, 14 when the valve train is installed. The encircling element 11 preferably consists of a polymer drawstring, for example neoprene or polyurethane, with vulcanized-in tensile strands 29. The tensile strands used here are, for example, steel or glass fiber strands or also aramid or Kevlar fibers which are wound in an endlessly helical fashion.

The bearing sections 27, 28 serve to introduce the force into the enclosing element 11. These are designed, for example, as steel sleeves, which are wrapped by the tensile strands 29 and subsequently vulcanized. Furthermore, between the bearing sections 27, 28 and the tension cords

29 areas still provided with filling or damping material

30 are provided. In principle, a closed or an open design is possible, the open, finite design having clear advantages in terms of manufacture and assembly in connection with the connecting element 12. The tape is placed around the cam element 5 only during the final assembly and closed by the connecting element 12. For this purpose, the bearing sections 27, 28 are each pushed onto an associated axle 13, 14 and then secured against slipping out by attaching the second side part 17. Closure takes place in the cam position with the minimum enclosure length. A minimum bias of the encircling element 5 must, however also exist in this position. However, the open enclosing element 11 can also be connected to the connecting element 12 by other suitable means. The arrangement of the fastening points (axes 13, 14) relative to the axis of rotation 15 on the connecting element 12 is essential.

In general, the use of a polymer drawstring offers the advantage of a high degree of flexibility and an adjustable elongation within specified limits. Due to the extensibility of the polymer drawstring, the variable wrap length can already be partially compensated.

Claims

claims

1. Valve train (1), in particular for internal combustion engines of motor vehicles, with at least one driven cam element (5), with a valve actuator (2) which can be displaced or pivoted by the cam element (5), the cam element (5) being rotatable in a flexible enclosing element (11) is arranged, with a connecting element (12) which has an axis of rotation (15) arranged parallel to a cam axis of rotation (8), the valve actuator (2) being rotatably connected to the enclosing element (11) via the axis of rotation (15) characterized in that the connecting element (12) has two further axes (13, 14) arranged parallel to the axis of rotation (15), to each of which one end of the openly formed enclosing element (11) is fastened.

2. Valve train according to claim 1, characterized in that the connecting element (12) is formed from two substantially flat, parallel to each other side parts (16, 17) which by the perpendicular to the side parts (16, 17) arranged axis of rotation (15) and the two axes (13, 14) are connected to one another.

3. Valve train according to claim 1, characterized in that the radius of curvature assigned to the cam element (5) the side parts (16, 17) of the connecting element (12) essentially correspond to the smallest positive radius of curvature of the cam element (5).

4. Valve drive according to claim 1, so that the axes (13, 14) are designed such that the ends of the open enclosing element (11) can wind up or unwind on the respective axis (13, 14).

5. Valve train according to claim 1, characterized in that the axis of rotation (15) and the axes (13, 14) are arranged on the connecting element (12) such that in the assembled state the axes (13, 14) have a smaller radial distance from the cam axis of rotation ( 8) have as the axis of rotation (15).

6. Valve train according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the enclosing element (11) is designed as a finite metallic tension band

7. Valve train according to claim 6, so that the axes (13, 14) each have a slot (21) through which the encircling element (11) passes and is non-positively connected to the axis (13, 14).

8. Valve train according to claim 7, characterized in that the axes (13, 14) in the entry region of the slot (21) have a rounded contour.

9. Valve train according to claim 1, d a d u r c h g e k e n n z e i c h n e t that the enclosing element (11) is designed as a polymer drawstring with vulcanized-in tension cords (29).

10. Valve train according to claim 9, d a d u r c h g e k e n n z e i c h n e t that the enclosing element (11) at the open ends bearing portions 27, 28 for attachment to the axes (13, 14) of the connecting element (12).

11. Valve train according to claim 10, so that areas (30) provided with filling or damping material are provided between the bearing sections (27, 28) and the tension cords (29).