Mechanical valve play compensation element for a valve drive on a piston combustion engine

The invention relates to a mechanical valve play compensation element for a valve drive on a piston combustion engine, comprising a first pressure part (1) which is axially displaceable in relation to a second pressure part (2) and which is fixed in such a way that it can turn about the axis of displacement; a torsion spring element (10) which acts between the first pressure part (1) and the second pressure part (2) and is axially flexible at least to a certain extent; and at least one helical surface (9.1) on the first pressure part (1), to which a corresponding helical surface (9.2) on the second pressure part (2) is allocated, these forming a pair of helical surfaces (9). The surfaces of the helical surface pair (9) are configured as a rough surface and are pressed against each other by the torsion spring element (10).

BACKGROUND OF THE INVENTION

The invention relates to a method for determining the reducing agent concentration (NH3) in the exhaust-gas flow of an internal combustion engine.

In piston combustion engines, it is necessary, between the shaft end of the gas exchange valve on the one hand and the valve drive (cam of the camshaft, valve actuating lever, or the like) acting on it on the other, to dispose a valve play compensation element in order to compensate for temperature-caused changes in the length of the valve shaft and changes, caused by wear to the valve seat, in the height of the shaft end when the gas exchange valve is closed, relative to the valve drive. To that end, a hydraulic valve play compensation element is used, which essentially comprises a cup-shaped cylinder and a piston guided in it; the cylinder interior can be subjected to pressurized oil, so that the two parts can be spread apart and can each come into contact without play on the shaft end of the valve on the one hand and the valve drive on the other. Via a throttle restriction, which is for instance provided by means of a defined gap between the cylinder wall and the piston, it is also possible during operation to compensate for a change in the height of the shaft end relative to the valve drive, whether it is caused by thermal expansion or wear to the valve seat, since via the outflow of oil through the throttle restriction, the total length of the valve play compensation element can be shortened. Such hydraulic valve play compensation elements have proven themselves over time and today are used in practically all piston combustion engines.

The disadvantage of the hydraulic valve play compensation element, however, is that an oil supply must be provided especially for it, which necessitates considerable engineering and production effort and expense at the cylinder head.

A further disadvantage is that any change in the viscosity of the oil used definitively affects the function of such a hydraulic valve play compensation, so that it is practically impossible to design one optimal cam shape for all operating states. Another disadvantage is the high oil consumption, with the result that the oil pump must be designed even for critical operating states, such as idling while hot, and hence is designed to be oversized for normal operation.

Mechanical play compensating elements are also known from European Patent Disclosure EP-A 0 032 284, German Patent Disclosure DE-A 36 07 170, and International Patent Disclosure WO 90/10787.

SUMMARY OF THE INVENTION

The object of the invention is to overcome the disadvantages described by means of a mechanical valve play compensation element of simple design and high functional capability.

This object is attained in accordance with the invention by a mechanical valve play compensation element for a valve drive on a piston combustion engine, having a first pressure part, which is axially displaceable relative to a second pressure part and is held rotatably about the displacement axis, and having a torsion spring element, operative between the first pressure part and the second pressure part, that is axially resilient at least to a limited extent, and further having at least one helical surface on the first pressure part, with which surface a corresponding helical surface on the second pressure part is associated, the two forming a pair of helical surfaces, wherein the surfaces of the pair of helical surfaces being embodied as rough surfaces and being pressed against one another by the torsion spring element.

The advantage of this mechanical valve play compensation element is that in the state of response, which is equivalent to the closing position of the gas exchange valve, as a result of the action of the torsion spring element, the two pressure parts are pressed apart to overcome any play, but contact one another with their helical surfaces. The valve drive can be formed directly by the cam of a camshaft, or via valve actuating levers (tilting levers, drag levers or the like). This assures that given the little force exerted between the two pressure parts during the closing time of the valve, any play that may be present is compensated for.

In the ensuing opening stroke, with the greater exertion of force for opening, the rough surface prevents the two pressure parts from rotating against one another, and thus prevents the compensation element from becoming shortened by being screwed together.

In one embodiment of the invention, the rough surface is embodied as a positive-engagement face, for instance in the form of a stair step profile with inclined step surfaces, so that only a compensation of an increasing valve play is possible, since the step edges each prevent reverse rotation of the pressure parts relative to one another in the direction of shortening the compensation element. The “step height” is expediently equivalent to an allowed working play.

A refinement contemplates a mechanical valve play compensation element in which a slide sleeve surrounding the first pressure part is provided, and a bracing spring element operative between the second pressure part and the slide sleeve is disposed, and furthermore on the first pressure part, a further parallel helical surface and a corresponding helical surface, offset in height from one another, are disposed on the slide sleeve and likewise form a pair of helical surfaces, the surfaces of the one pair of helical surfaces being embodied slidably and being pressed against one another by the torsion spring element, and at least one surface of the other pair of helical surfaces is embodied as a rough surface, whose surfaces are at a slight spacing from one another forming a working play and are each brought into contact with one another only during the valve opening event.

To initiate the valve opening, the elements contacting one another via the pair of helical surfaces, which as a rule are the first pressure part and the slide sleeve, are displaced in the direction of the second pressure part, counter to the exertion of force of the bracing spring, so that after a spacing forming a working play is bridged, the helical surfaces of the pair provided with rough surfaces come into contact with one another. The surface roughness of the two rough surfaces, upon touching one another, brings about a positive engagement, so that the two pressure parts, despite the actuation force acting in the opening direction, counter to the closing force of the valve spring, form an intrinsically rigid body, since rotation of the two pressure parts by becoming screwed into one another is not possible.

As soon as the closing position is regained, after the conclusion of the full valve stroke, the two pressure parts are pressed apart via the bracing spring, and via the exertion of force of the bracing spring between the two surfaces of the slidably embodied pair of helical surfaces, the two pressure parts are pressed apart, and by means of a relative rotation to one another, any valve play that may be present and is greater than the predetermined working play is compensated for. The slope of the pairs of helical surfaces extending parallel to one another is chosen such that no self-locking can occur in the pair of helical surfaces embodied slidably.

In an expedient feature of the invention, it is provided that the exertion of force of the bracing spring to the torsion spring element via the slide faces is markedly greater than the restoring force of the torsion spring element. This assures that changes in the valve play in both the positive and the negative direction, that is, spreading or contraction, caused by alternating operating states, for instance thermally, are assured as long as the rough surfaces do not touch another. As a result, even if the valve plays are changing, a greater play up to the predetermined, slight working play, will always be reliably compensated for. The depth of the roughness of the rough surface must be less than the allowed working play.

It is expedient if in a feature of the invention, ventilation bores are provided for the chambers that are surrounded by the pressure parts and/or by one pressure part and the slide sleeve. On the one hand, this prevents air cushions and/or accumulations of oil from being able to build up in these chambers, and on the other, this assures that by way of ventilation, however slight, oil mists can penetrate these chambers, thus lubricating the surfaces, moving relative to one another, of the individual parts.

Further characteristics and advantages of the invention can be learned from the description of the exemplary embodiments and the claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematic illustration of an exemplary embodiment of a mechanical valve play compensation element inFIG. 1shows a first pressure part1, embodied for instance in the form of a die or piston, which is axially displaceable relative to a second pressure part2and is retained rotatably about the displacement axis A and is embodied as cup-shaped or cylindrical, for instance. The pressure part2is braced by its free end14, for instance on a drag lever22to be actuated. The first pressure part1is kept stationary, or is connected to a valve drive, depending on the intended use.

The first pressure part1is provided, on its side oriented toward the second pressure part2, with a helical surface9.1, with which a corresponding helical surface9.2on the second pressure part2is associated. The associated helical surfaces form a pair9of helical surfaces. Of the helical surfaces9.1and9.2of the pair9of helical surfaces, at least one helical surface is embodied as a rough surface, and the coefficient of friction of this rough surface should amount to at least 0.4 μm.

The term “rough surface” includes any surface structure that prevents the surfaces from sliding freely on one another. This can be accomplished by roughening them or shaping them in a targeted way, for instance via a tooth profile, wavelike profile or stepped profile with descending step faces that are oriented at an angle to the helical surface. The profile depth or “roughness depth” amounts to up to several tenths of a millimeter on both helical surfaces9.1and9.2, forming positive-engagement faces that upon engagement reliably prevent rotation of the two pressure parts.

Since the helical surfaces9.1and9.2are in contact with one another over a considerable length, the pressure per unit of surface area operative between them can be reduced markedly, minimizing wear. It is also sufficient if the length of the helical surface is approximately equal to the circumference of the pressure part; the slope must be selected such that with certainty, no self-locking ensues except for locking by way of the roughness.

The cutaway right-hand side of the first pressure part1allows the course of the helical surface9.2on the second pressure part2to be seen.

Between the first pressure part1and the second pressure part2, a torsion spring element10is provided, which is shown here as a spiral spring and which between the two pressure parts brings about a restoring force that is capable of rotating the two pressure parts, if a play exists that is greater than the predetermined roughness depth.

The torsion spring element10is embodied here such that on being screwed into one another in the axial direction, it is either held axially displaceably in its anchor, or is resilient in the axial direction, for instance if a round cross section is chosen for the spiral instead of a rectangular cross section.

The end face11of the first pressure part1rests on a fixed anchor, or as shown inFIG. 2, on the control contour12of a control cam13. The end face14, for instance curved forward convexly, of the second pressure part2rests on the free end of the valve shaft15of a gas exchange valve or actuating element22.

The schematic illustration inFIG. 2shows a further exemplary embodiment for a mechanical valve play compensation element, which has been developed from the embodiment ofFIG. 1. Here the second pressure part2is surrounded on its outside by a slide sleeve3, which is provided on its free end4with an end stop5with which a collar-like extension6on the second pressure part2is associated.

In a free chamber7.1between the slide sleeve3and the second pressure part2, a bracing spring element7, for instance in the form of a helical compression spring, is provided, by which the second pressure part2is pressed by its extension6against the end stop5.

The first pressure part1, on its side toward the second pressure part2and the slide sleeve3, is provided with two parallel-extending helical surfaces8.1and9.1, offset from one another in height, with each of which a corresponding helical surface8.2on the slide sleeve3and a helical surface9.2on the second pressure part2are associated. The associated helical surfaces each form one pair8and9of helical surfaces. The helical surfaces8.1and8.2of the pair8of helical surfaces are smooth and thus embodied slidably; the coefficient of friction should expediently not exceed 0.2 μm. Of the helical surfaces9.1and9.2of the pair of helical surfaces, at least one helical surface is embodied as a rough surface.

The cutaway right-hand side of the first pressure part1makes the course of the helical surface8.2on the slide sleeve3and of the helical surface9.2on the second pressure part2visible.

The torsion spring element10is disposed between the first pressure part1and the second pressure part2and between the two pressure parts effects the restoring force that rotates the two pressure parts slidingly on the pair8of helical surfaces.

AsFIG. 2shows, via the bracing spring7, the slide sleeve3is pressed together with the first pressure part1against the control contour12of the cam13. As a reaction force, correspondingly the second pressure part2is pressed with its end face14against the end of the valve shaft15. This assures that the valve play compensation element will be held without play between the cam13on one side and the valve shaft15on the other. The lateral fixation depends on the particular installation situation, for which exemplary embodiments are provided below.

The torsion spring10that connects the first and second pressure parts to one another is oriented in terms of its exertion of force such that the torsion spring10seeks to screw the two pressure parts into one another, thus assuring a tight contact of the pair of helical surfaces. The two pairs8and9of helical surfaces are disposed at staggered heights from one another, by an amount of a few μm, so that there is a working play AS.

In a direction of rotation of the cam13in the direction of the arrow16, with the onset12.1of the raised area, beginning as a result of the control contour12, above the base circle, the first pressure part1is moved downward together with the slide sleeve3counter to the force of the bracing spring7. Although the two helical surfaces8.1and8.2are embodied as smooth, and thus a rotation of the first pressure part1relative to the second pressure part2is possible, nevertheless the movement is so quick that because of inertia and the effects of friction, no or only slight rotation of the two parts relative to one another takes place, and once the working play AS is overcome in the pair9of helical surfaces, the helical surface9.1takes a seat on the helical surface9.2. Since at least one surface of the pair9of helical surfaces is embodied as a rough surface, the friction is so great that despite the high axial forces in the opening event, rotation of the first pressure part1relative to the second pressure part2is prevented, and thus the entire arrangement acts as a rigid body and is capable of transmitting the opening stroke, predetermined by the control contour12of the cam13, to the valve shaft15.

In the ensuing closing stroke, the entire motion is in the opposite direction, so that immediately after the valve has become seated on its valve seat, is kept in contact via the bracing spring7with the base circle on the control contour12.

By means of a correspondingly predetermined play S between the extension6on the second pressure part2and the end stop5on the slide sleeve3, play-free contact of the entire arrangement between the control contour of the cam13and the valve shaft15is assured.

If because of operating conditions, the spacing between the base circle of the cam13and the free end of the valve shaft15increases, then this increase in spacing is initially compensated for via the compensation for the play S between the extension6and the end stop5via the bracing spring7.

If this play S is then exceeded, then the helical surface8.2is pressed against the helical surface8.1by the bracing spring7via the slide sleeve3, so that by this exertion of force, the first pressure part1is screwed outward compared to the second pressure part2, counter to the force of the torsion spring10. This is possible because the two helical surfaces8are embodied with smooth surfaces and thus are slidable, and the “thread” formed by the contacting helical surfaces is dimensioned such that no self-locking occurs.

Since the two pairs8and9of helical surfaces are disposed extending parallel to one another, the working play AS remains, at a constant magnitude.

The design of the individual springs is intended such that the exertion of force of the bracing spring7via the slide faces8on the torsion spring element10is dimensioned as markedly greater than the restoring force of the torsion spring element10. On the other hand, the force of the bracing spring7must be markedly less than the closing force of the valve spring16.

InFIG. 3, a horizontal section is shown through the embodiment ofFIG. 13, taken along the line III—III inFIG. 1. In the exemplary embodiment shown here, the spiral spring10is made of band material and is held with its outer free end10.1in a corresponding groove10.2in the second pressure part2, so that the longitudinal mobility predetermined by the working play AS is assured. Instead of a band material, it may be expedient to use a round material, so that the torsion spring10, embodied as a spiral spring, can be fastened firmly by both ends, since an adequate deformation in the axial direction is assured by the round material. The horizontal section applies accordingly to the version ofFIG. 1.

In the embodiment ofFIG. 1orFIG. 2, the chamber10.3surrounding the torsion spring element10is ventilated via the groove10.2that is open to the outside. Via a corresponding groove, not shown in detail here, the chamber7.1surrounding the bracing spring7is also ventilated, so that in these chambers, no air cushions can build up during the motion.

In this respect, instead of ventilation via the groove10.2, it may be expedient to provide a separate bore26, so that not only is ventilation assured, but it is also assured that no oil supply can accumulate in the chamber10.3of the second pressure part2. Correspondingly, the chamber7.1should also be ventilated, to avoid an accumulation of oil.

InFIG. 4, a modification of the embodiment ofFIG. 1andFIG. 2is shown, which essentially differs from the embodiment ofFIG. 1only in that the torsion spring10is embodied as a leg spring and is supported on the outside of the slide sleeve3and is fixed by one end to the first pressure part1, while its other end is fixed to the slide sleeve3. The rotary motion for bridging the working gap AS is thus effected between the slide sleeve3and the pressure part1. Accordingly, a means18of securing against relative rotation, shown only schematically here, must be disposed between the second pressure part2and the slide sleeve3, but it must allow an axial motion between the two parts. In this system, once again the “unscrewing” of the pressure part1and pressure part2is again assured via the bracing spring7; on the other hand, by the torsion spring element10, a play-free contact of the pair8of helical surfaces is possible, and upon an exertion of force corresponding to the two arrows P, the contraction of the first pressure part1into the second pressure part2is possible by the amount of the working play AS, and the rough surfaces of the pair9of helical surfaces come into contact with one another, so that the opening and closing strokes can then ensue without any further change in the total length L of the entire arrangement.

In the following drawings, examples of installation for the valve play compensation elements described in conjunction withFIGS. 1,2and3will be shown.

In the arrangement ofFIG. 5, the valve play compensation element VSA is retained in a bearing body19, which is guided displaceably in the cylinder head20. The bearing body19is associated directly with the end of the valve shaft15and its valve spring16, so that the cam13, with its control contour12, can act directly on the end face21of the bearing body19.

InFIG. 6, the disposition of the valve play compensation element VSA of the invention in a tilting lever22is shown; by one end, this lever is in contact with the valve shaft15via the valve play compensation element VSA, and by its other end, it is in contact with the cam13of the camshaft via a roller23.

FIG. 7schematically shows the disposition on a drag lever24, which is braced by one end directly on the valve shaft15and by its other end on a valve play compensation element VSA supported in the cylinder head20. Once again, the drag lever24rests on the control contour12of the cam13, via a roller23.

The control contour12of the cam13can be embodied such that it is provided with a preliminary cam at the beginning12.1of the opening stroke, which assures that the working play AS will rapidly be overcome.

FIG. 8shows a modification of the arrangement ofFIG. 6, with a valve play compensation element ofFIG. 1. Here the tilting lever22is supported tiltably on the valve play compensation element VSA, which in turn is braced on the engine block via a support armature25. The motion is initiated via the cam13or a tappet.