EP2171221B1 - Cam follower for a valve drive of an internal combustion engine - Google Patents

Abstract

A cam follower (2) is proposed for a valve drive (1) of an internal combustion engine with a lifting disc valve (9), to the shaft end face (8) of which at least two different valve strokes can be applied via a contact face (7) of the cam follower (2). The contact face (7) has a profile which is curved convexly in the longitudinal direction of the cam follower (2) and has at least one profile point (P0) from which two contact areas (K1, Kx), having profiles which are different from one another and are respectively adapted to one of the valve strokes, proceed. In this context, at least one of the contact areas (K1, Kx) is intended to have a profile with a continuously changing curvature, wherein the curvatures of the contact areas (K1, Kx) at the profile point (P0) are essentially of the same size.

Description

Field of the invention

The invention relates to a drag lever for a valve train of an internal combustion engine with a Tellerhubventil whose shaft end surface is acted upon via a contact surface of the drag lever with at least two different valve lifts. In this case, the contact surface has a convexly curved in the longitudinal direction of the drag lever profile with at least one profile point, emanating from the two contact areas with mutually different and each adapted to one of the valve lifts profile profiles.

Background of the invention

Such a drag lever is considered from the considered generic DE 10 2004 049 836 A1 known. The rocker arm proposed there has a contact surface for actuating the disk lifting valve whose profile is composed of two circular-arc-shaped contact areas with radii of different sizes. The first large radius contact area is adapted to a conventional valve lift transmitted from the cam of a camshaft to the Tellerhubventil, while the second small radius contact area is adapted to a deactivated valve lift. This deactivated valve lift or Ventilnullhub is generated by the fact that the stroke specified by the cam is not transmitted to the Tellerhubventil, but is received by a the lever arm bearing, switchable support element.

The disclosed in the cited document contact surface with two different radii ranges has the favorable property that the kinematic and hydrodynamic contact relationships between the contact surface of the finger lever and the shaft end surface of the Tellerhubventils largely independent of additional factors resulting from the changed Ventiltriebskinematik with deactivated valve lift, optimally can be adjusted. These influencing variables are, in particular, the contact between the contact surface of the drag lever and the shaft end surface of the disk stroke valve which migrates in the longitudinal direction of the drag lever or, in short, the so-called emigration. Taking into account the sometimes considerable component tolerance chains within the valve train, however, the known contact surface with two different, over the respective contact area but constant radii and correspondingly constant curvatures be disadvantageous to the effect that the profile point as the starting point of different radii / curvature not with the actual contact point the contact surface during the lift-free base circle phase of the cam coincides. Starting from the base circle phase, the profile change between the contact regions as a result of a bending jump would then lead to abruptly changing contact conditions between the drag lever and the Tellerhubventil in the course of the lifting phase of the cam. In addition to undesirable vibration excitations of the valve train, the aforementioned deviation of the profile point from the actual contact point can also be accompanied by undesired contact pressures. This is the case, for example, when at the beginning and at the end of a large valve lift the contact area with a small radius and a correspondingly large curvature is in engagement with the shaft end surface of the plate-lifting valve.

Object of the invention

The present invention is therefore the object of a drag lever of the type mentioned in such a way that the disadvantages mentioned are eliminated with simple means and with respect to the manufacturing costs of the rocker arm substantially cost neutral.

Summary of the invention

The solution of this problem arises from the characterizing features of claim 1, while advantageous developments and refinements of the invention are the dependent claims can be removed. Accordingly, it is provided that at least one of the contact regions has a profile profile with a continuously variable curvature, wherein the curvatures of the contact regions at the profile point are substantially equal, and that the contact region with continuously variable curvature has a logarithmic profile profile.

By such a profiled contact surface can in the profile point as a starting point to the contact areas a kinematic jerk-free, i. significantly smoother profile change can be achieved than is possible in the case of the known contact surface with different, but partially constant radii / curvatures. In other words, the so-called jerk in the profile point, which is unavoidable in a curvature jump due to the coincidence of different radii, can be effectively eliminated.

The advantageous effects of the invention are particularly noticeable with increasing component tolerances of the valve gear according to a relatively large deviation of the profile point from the actual contact point noticeable, since on the one hand the gentle profile change leads to no inadmissibly high vibration excitation in the valve train and on the other in the vicinity of the profile point a small curvature both contact areas in favor of low contact pressures can be provided.

In a development of the invention, it is provided that the contact region with the continuously variable curvature has the logarithmic profile profile K (x) = C ₁ * In [1 / (1-c ₂ * x ^a )]. In this case, c ₁ and c ₂ are constants, x is the profile coordinate originating from the profile point in the longitudinal direction of the drag lever, and the exponent a is a real number. By means of these, with regard to the natural logarithm specific and with regard to the variable parameter generic profile description of the contact area, the expert in the field of valve actuators is given a useful technical and mathematical limits helpful optimization tool for profile design of the contact surface of the finger lever to the hand.

Deviating from the Euler number e as the basis of the natural logarithm, any other basis for the mathematical description of profile curves is conceivable, by means of which mechanically, kinematically and tribologically suitable contact conditions for the shank end surface of the plate-lifting valve can be represented in conjunction with adapted parameters in the sense of the invention.

In addition, it is provided that at least one of the contact areas has a circular arc-shaped profile profile with a constant curvature. In this regard, it can be particularly advantageous if exactly two contact areas are provided, wherein the contact area with constant curvature is adapted to an activated valve lift and the contact area with continuously variable curvature to a deactivated valve lift. Particularly in this case, it is expedient if the contact region with variable curvature has an overall steeper profile than the contact region with constant curvature. As with the kinematic design of valve trains with only one valve lift generally common, the contact surface can then contact the shaft end face also eccentric during the base circle phase of the cam -ideally at Profilpunkt- so that the proven kinematic and tribological contact conditions are maintained with activated valve lift, while at deactivated valve lift then the opposite running emigration of the contact surface on the shaft end surface can be kept very small.

Alternatively to the above-mentioned contact region with constant curvature, however, it is also possible to provide both contact regions with continuously variable curvatures of equal size in the profile point and to adapt these to the activated valve lift or the deactivated valve lift, respectively.

Another possibility consists in a reversed assignment of the contact areas to the different valve lifts. Accordingly, it can also be provided to adapt the contact region with continuously variable curvature to the activated valve lift and the contact region with constant curvature at a comparatively small radius to the deactivated valve lift.

Brief description of the drawings

• Figur 1 einen Ventiltrieb einer Brennkraftmaschine mit einem erfindungsgemäßen Schlepphebel im Längsschnitt;
• Figur 2 ein für den Ventiltrieb gemäß Figur 1 vorgesehenes schaltbares Abstützelement und
• Figur 3 eine Profildarstellung der Kontaktfläche des Schlepphebels gemäß Figur 1.

Further features of the invention will become apparent from the following description and from the drawings, in which the invention is explained with reference to an embodiment. Show it:

• FIG. 1 a valve train of an internal combustion engine with a drag lever according to the invention in longitudinal section;
• FIG. 2 one for the valve train according to FIG. 1 provided switchable support and
• FIG. 3 a profile view of the contact surface of the finger lever according to FIG. 1 ,

Detailed description of the drawings

In FIG. 1 is a valve train 1 of an internal combustion engine with a finger lever 2 disclosed, which is mounted in a known manner at one end on a stationary arranged in the engine support 3 is pivotally mounted and a rotatable roller 4 for tapping a cam 5 of a camshaft. The non-cutting formed here of sheet metal drag lever 2 has at the other end a transverse web 6 with a contact surface 7 for the shaft end surface 8 of a spring-loaded in the closing direction Tellerhubventils 9. As it is already in FIG. 1 is clearly recognizable and based on FIG. 3 will be explained in detail, the contact surface 7 has a convexly curved in the longitudinal direction of the finger lever 2 profile comprising two contact areas with mutually different profile profiles. The design of the contact surface 7 makes it possible, the drag lever 2 according to the common part principle not only in conjunction with the conventional, non-switchable support element 3, but also in conjunction with a switchable support member 10, as known in the prior art in various embodiments and exemplified in FIG. 2 is shown to use. In other words, it may be provided to equip the valve train 1 of the internal combustion engine with the same drag levers 2, which are partially mounted on non-switchable support elements 3 and partially on switchable support elements 10, so that only a portion of the Tellerhubventile 9 is acted upon by a variable valve lift.

In the case of the switchable support element 10, the variable valve lift is an operating point-dependent completely deactivated valve lift or valve zero stroke by interrupting the transmission of the cam lift to the Tellerhubventil 9. In this operating condition, the cam lift is then received by the switchable support element 10 at then pivoting rocker arm 2. The function and structure of the known switchable support element 10 can be summarized at this point to the effect that the switchable support element 10 in a housing 11th against the force of a lost-motion spring 12 longitudinally mounted pivot bearing 13 for the finger lever 2 and a hydraulically actuated locking mechanism 14 includes here, by a form-fitting acting support of the pivot bearing 13 in the outer housing 11 can be produced or released.

The profile of the contact surface 7 is in FIG. 3 shown greatly enlarged in diagram form. It was the in FIG. 1 retained orientation of the contact surface 7, ie the right side of the ordinate y and designated K _x designated contact area facing the roller 4 and the left side of the ordinate y shown and a circular arc profile profile with radius R ₁ having contact area K ₁ facing away from the roller 4. The zero point of the abscissa x corresponds to a profile point P ₀ , from which the two contact areas K _x and K ₁ emanate, where x indicates the distance coordinate of the profile point P ₀ in mm. Compared to a thin line also drawn reference region K ₂ with circular arc profile profile and radius R ₂ according to the cited prior art, the contact area K _{x has} a natural logarithmic profile course K (x). Geometrically, this profile curve K (x) corresponds to the design formula given at the top left in the diagram with the parameters c ₁ , c ₂ and a specified below.

Compared to the reference region K ₂ , whose curvature is by definition 1 / R ₂ , the contact region K _x according to the invention has a significantly smaller curvature in the vicinity of the profile point P ₀ , which at the profile point P ₀ with the curvature 1 / R _{1 of} the contact region K ₁ is essentially identical. This fact is clarified in the additional detailed diagram placed on the ordinate, which highlights the profile profiles of the contact areas K ₁ , K _x and K ₂ at profile point P ₀ according to detail Z. The left side of the ordinate y extending contact area K ₁ radius R ₁ and the contact area K _x logarithmic profile course K (x) have at profile point P _0, the substantially identical curvature 1 / R ₁ , while the curvature 1 / R _{2 of} the reference range K _{2 is} significantly larger and at profile point P ₀ to a jerk due to the curvature jump would lead. Consequently, in the case of the operative rolling-sliding movement of the contact surface 7 on the shaft end surface 8, a gentle, ie kinematically jerk-free transition between the contact regions K ₁ and K _x results, if the profile point P ₀ is not due to component tolerances during the base circle phase of the cam 5, but comes in the course of its lifting phase into engagement with the shaft end surface 8. Depending on the position of the component tolerances, ie the deviation of the profile point P ₀ from the actual contact point during the base circle phase of the cam 5, this situation applies either to the activated valve lift or to the deactivated valve lift.

A further advantage of the contact surface 7 profiled in this way relates to the surface pressures in contact when the valve lift is activated, since the contact region K _{x of} variable curvature at the profile point P ₀ and in the vicinity thereof has a significantly smaller curvature than the reference region K ₂ . Consequently, in the case in which the profile point P ₀ only comes into effective engagement with the shaft end surface 8 during the activated valve stroke, the contact region K _{x would also be} exposed to the then high contact forces due to valve acceleration and valve spring force. However, due to the comparatively low curvature of the contact region K _x in the vicinity of the profile point P ₀ , the corresponding contact pressures can be kept at the same low level which sets in the contact region K ₁ with a circular arc profile profile and comparatively low curvature 1 / R ₁ .

As already explained at the beginning, the contact region K _x with a variable curvature has an overall steeper profile than the circular-arc-shaped contact region K ₁ . The correspondingly small and initially in the direction of the roller 4 running emigration of the contact surface 7 on the shaft end face 8 is thereby kept very small when the valve is deactivated. As is generally customary in the kinematic design of valve trains with only one valve lift, consequently, the profile point P _0, the shaft end face 8 eccentric and here to the roller 4 offset contact during the base circle phase of the cam 5, so that the proven kinematic and tribological contact relationships between the contact surface 7 and the shaft end surface 8 are maintained with activated valve lift.

List of reference numbers

1

valve train

2

cam follower

3

supporting

4

role

5

cam

6

crosspiece

7

contact area

8th

Stem end surface

9

reciprocating poppet

10

switchable support element

11

casing

12

Lost-motion spring

13

pivot bearing

14

locking mechanism

K _x

Contact area with variable curvature

K ₁

Contact area with constant curvature

K ₂

reference range

P ₀

profile point

R ₁

Radius of the contact area with constant curvature

R ₂

Radius of the reference range

Claims (4)

1. Rocker arm (2) for a valve drive (1) of an internal combustion engine, having a lifting disc valve (9) whose shank end surface (8) can be acted on with at least two different valve lifts by means of a contact surface (7) of the rocker arm (2), the contact surface (7) having a profile which is convexly curved in the longitudinal direction of the rocker arm (2) and which has at least one profile point (P₀) from which proceed two contact regions (K₁, K_x) with profile characteristics which differ from one another and which are adapted in each case to one of the valve lifts, characterized in that at least one of the contact regions (K₁, K_x) has a profile characteristic with a continuously variable curvature, the curvatures of the contact regions (K₁, K_x) being substantially identical at the profile point (P₀), and in that the contact region (K_x) with a continuously variable curvature has a logarithmic profile characteristic.
2. Rocker arm (2) according to Claim 1, characterized in that the contact region (K_x) with the continuously variable curvature has the logarithmic profile characteristic K(x) = c₁ * ln [1/(1 - c₂ * x^a)] where c₁ and c₂ are constants, x is the profile coordinate proceeding from the profile point (P₀) in the longitudinal direction of the rocker arm (2), and the exponent a is a real number.
3. Rocker arm (2) according to Claim 1, characterized in that at least one of the contact regions (K₁, K_x) has a profile characteristic which is in the shape of a circular arc and which has a constant curvature.
4. Rocker arm (2) according to Claim 3, characterized in that precisely two contact regions (K₁, K_x) are provided, the contact region (K₁) with constant curvature being adapted to an activated valve lift, and the contact region (K_x) with a continuously variable curvature being adapted to a deactivated valve lift.

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