PISTON AND METHOD FOR PRODUCING SAME

Abstract

The present invention relates to pistons (10) for use in internal combustion engines, having: a piston crown (12) which is adjoined by a piston skirt (14), wherein the piston skirt (14) has surfaces (18) which abut the cylinder wall during use of the piston, wherein the surfaces (16) have one or more friction-reducing coatings (18) and wherein depressions (20) are provided in the coating, which are arranged such that the spacing S between each two adjacent depressions (20) along the axial direction (A) of the piston and the width L of the depressions (20) along the axial direction (A) of the piston satisfy the formula S>2L.

Claims

1. The piston for use in internal combustion engines, comprising: a piston crown, which is adjoined by a piston skirt, wherein the piston skirt has surfaces which abut a cylinder wall during use of the piston, wherein the surfaces have one or more friction-reducing coatings and wherein a plurality of depressions are provided in the coating, which are arranged in such a way that a spacing S between each two adjacent depressions along the axial direction (A) of the piston and the width L of the depressions along the axial direction (A) of the piston satisfy the formula S>2L, wherein one or more of the depressions are enclosed by the coating in which they are formed, wherein the plurality of the depressions is provided in alignment along the circumference of the piston in one of the one or more coatings.

2. The piston according to claim 1, wherein the depressions extend through the coating such that they reach the material of the piston skirt.

3. The piston according to claim 1, wherein boundary surfaces of the depressions extend at an angle with respect to the coating and the material of the surfaces they adjoin.

4. The piston according to claim 1, wherein a length of extension E of at least some of the depressions along the circumferential direction is shorter than a length of extension E′ along the circumferential direction of the coating in which these are formed.

5. The piston according to claim 4, wherein the depressions that are adjacent to one another in the axial direction are offset relative to one another in the circumferential direction.

6. The piston according to claim 1, wherein the depressions have a substantially rectangular form.

7. The piston according to claim 1, wherein the depressions have a width L of less than 2 mm.

8. A method for producing a piston according to claim 1, wherein the coating including the depressions is applied by screen-printing.

9. The piston according to claim 1, wherein all of the depressions are enclosed by the coating.

10. The piston according to claim 4, wherein the length of extension (E) of all of the depressions in the circumferential direction is shorter than the length of extension (F) in the circumferential direction.

11. The piston according to claim 7, wherein the width L is in the range of 0.6 to 0.8 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 shows a piston according to the invention according to one embodiment.

[0026] FIG. 2 shows a detailed view of coatings of the piston according to the invention.

[0027] FIG. 3 shows a cross-sectional view of a depression of the piston according to the invention.

[0028] FIGS. 4a) and 4b) show different variants of a piston according to the invention.

[0029] FIGS. 4c) and 4d) show a piston which is not in accordance with the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 shows a view of a piston 10 according to the invention according to the first embodiment of the invention. The piston has a cylindrical shape with a cylinder axis A. A piston crown 12 of the piston having annular grooves 13 is adjoined by a piston skirt 14. The piston skirt 14 has convex surfaces 16, which constitute skirt walls, and box walls 17.

[0031] The cylindrical surfaces 16 abut the cylinder wall of the internal combustion engine during use of the piston and have a friction-reducing coating 18 of a polymer material containing graphite. The coating 18 is only provided on one part of the convex surface 16. The convex surface 16 adjoins the annular region with the annular grooves 13 of the piston. The coating 18 extends along the circumference of the piston over a length E′.

[0032] Rectangular depressions 20 are provided in the coating 18, which extend through the coating material such that the material of the piston is exposed through the depressions 20. The ends of the rectangular depressions may be rounded. A plurality of depressions 20 is lined up along the circumferential direction. There is a plurality of such rows of lined-up depressions 20, which are offset relative to one another along the circumferential direction such that the axially adjacent depressions 20 are offset relative to one another along the circumferential direction.

[0033] The depressions 20 are shown in more detail in FIG. 2. A plurality of depressions 20, which are offset relative to one another along the circumferential direction of the piston 10, is shown here in two rows. In plan view, the depressions 20 have a rectangular form and a length E along the circumferential direction and a width L in the axial direction A. The spacing between two adjacent depressions 20 in the axial direction is defined as S. For the spacing S, S>2L applies.

[0034] FIG. 3 shows a cross section through a depression 20 along the circumferential direction. As can be seen in this drawing, the boundary surfaces 17 of the depression are inclined with respect to the material of the convex surfaces 16 and also form an angle with the material of the coating 18. The depression 20 is closed at the bottom, i.e. the underside of the depression 20 is formed by the material of the piston 16 and is closed, such that it prevents oil from draining away downwards.

[0035] It has been ascertained that a piston 10 designed as shown in FIGS. 1 to 3 has friction that is reduced by up to 20%. This is due to the fact that the depressions 20 have comparatively good oil retention properties. In this respect, oil accumulates therein and then reduces the friction. In contrast to depressions which extend over the entire width of the coating, oil retention properties are improved, since the oil cannot drain away. It is important for oil retention behaviour that the slots are formed so as to be sufficiently narrow and deep. Calculations that make this behaviour plausible are described in the paper by M. Scholle entitled “Hydrodynamical modelling of lubricant friction between rough surfaces”, in Tribology International 40 (2007), pp. 1004-1011, for example.

[0036] FIG. 4 shows in drawings a) and b) pistons according to the invention. FIG. 4b) corresponds to the piston already shown in FIG. 1. Thus, this piston will not be addressed in detail.

[0037] FIG. 4a) shows a piston 10′ according to a second embodiment of the invention, having a coating 18′ in which depressions 20′ are arranged such that adjacent depressions 20′ are lined up along the axial direction of the piston. Moreover, an area 19′ of the coating in which no depressions 20′ are provided is located on the side of the coating 18′ opposite the piston crown 12′. Even though such an arrangement of the depressions 20′ is less advantageous compared to the first embodiment in terms of oil retention capability, a corresponding piston is improved as compared to the prior art in terms of oil retention properties, and therefore has less friction.

[0038] FIG. 4c) shows a further piston 10″, which is not part of the invention. This piston 10″ also has a coating 18″ comprising depressions 20″. However, these depressions 20″ extend along the circumferential direction of the piston 10″ over the entire width of the coating 18″ and are therefore not enclosed, as is stipulated by the invention. In the previous embodiments in FIG. 4a) and FIG. 4b), the depressions 20, 20′ each extend only over a portion of the coating 18, 18′ along the circumferential direction, and not over the entire width of the coating 18, 18′. The fact that the depressions 20″ extend over the entire width of the coating 18″ means that oil can drain away therefrom. This then results in increased friction compared to pistons in which the depressions are enclosed.

[0039] FIG. 4d) shows a piston 10 III which is not in accordance with the invention. Here, substantially circular depressions 20 III are provided in a coating 18 III, which are arranged in a plurality of rows along the circumferential direction of the piston 10 III and which are offset relative to one another along the circumferential direction. The depressions 20 III shown herein do not meet the requirement that S>2L; rather, they are arranged too closely together. Moreover, E>L does not apply here.

[0040] The coatings 18 to 18 III shown in FIGS. 4a) to 4d) can be produced by means of a screen-printing process.