PISTON FOR AN INTERNAL COMBUSTION ENGINE

Description

Technical field

[0001] The invention relates to a piston for an internal combustion engine.

[0002] Pistons in internal combustion engines have diametrically opposed skirt walls, which are portions of the cylinder's outer surface that slide along the cylinder bore or a cylinder liner inserted therein during operation. In an inner area, viewed along the piston stroke axis, piston pin hubs are also provided. These hubs accommodate a piston pin, which connects the piston to a connecting rod. The connections between the skirt walls and the piston pin hubs are called skirt ties, connecting walls, or box ties. Typically, four such box ties are provided: two for connecting the skirt wall on the pressure side and two for connecting the skirt wall on the counter-pressure side.

[0003] The cylinder walls on the pressure side are particularly subjected to high mechanical stresses. During the working stroke, the gas pressure on the piston crown and the inclined position of the connecting rod generate a horizontal lateral force that presses the pressure-side cylinder wall against the cylinder wall. This lateral force is transmitted through the cylinder walls from the piston pin hub to the cylinder wall. This results in high compressive stresses on the inner side, i.e., the side of the cylinder walls facing the piston stroke axis.

[0004] Furthermore, the so-called contact change must be considered, namely when the lateral force changes from the back-pressure to the pressure side at top dead center. In this context, the contact between the cylinder bore and the piston skirt walls also changes from the back-pressure to the pressure side. During this contact change, high compressive stresses also develop on the inside of the skirt walls, especially at the lower edge, the so-called open skirt end. Insufficient strength can lead to cracks at the open skirt end, which is one of the most common types of damage in gasoline engine pistons. State of the art

[0005] To counteract this, it is known, for example, from DE 10 2009 032 379 A1, to design the shaft connections, particularly on the inside, to be largely straight and, in a bottom view, to incline them inwards starting from the piston pin hub. In other words, the distance between the cylinder wall and an imaginary plane perpendicular to the piston pin axis decreases starting from the piston pin hub. A similar design is known from DE 10 2012 203 570 A1.

[0006] However, further boundary conditions must be considered when designing the piston pin connections, in particular the length of the piston pin and the position of an oil nozzle for spraying cooling oil into a cooling channel or onto the underside of the piston. Specifically, the inwardly inclined and straight profile of the cylinder walls described above would result in comparatively far-outward piston pin hubs, which increases the piston weight and necessitates modifications to the piston pin. This conflict is resolved, for example, according to DE 10 2013 214 738 A1, DE 41 09 160 A1, US 6,279,456 B1 and JP 56-40706 B2, by using curved piston pin connections. While this creates space for a cooling oil nozzle without requiring the piston pin hubs to be moved outwards, such designs are disadvantageous with regard to strength.This applies equally to similar designs with box walls that initially slope outwards from the piston pin hub, then curve inwards and then slope inwards, as shown in JP H 03-92544 U, JP 2012-122384 A, JP 2016-128663 A and JP H 09-242603 A.

[0007] Finally, DE 10 2009 045 437 A1 reveals a piston with connecting walls not described in detail, with a recess for a cooling oil nozzle. Description of the invention

[0008] Against this background, the invention is based on the objective of creating a piston optimized with regard to the combination of strength and design freedom with regard to the position of a cooling oil nozzle and / or the bolt hubs.

[0009] This problem is solved by the piston described in the patent claim.

[0010] Consequently, this cylinder has at least one cylinder wall on a pressure side, which, at least at its lower edge and at least on its inner side, extends from the piston pin hub, in a first section inclined largely straight outwards, then curved inwards, and then in a second section inclined largely straight inwards towards the cylinder wall. In other words, viewed from below along the piston stroke axis, starting from the piston pin hub, the distance from the inner side of the cylinder wall to an imaginary plane perpendicular to the piston pin hub, which contains the piston stroke axis, initially increases. This is followed by a comparatively short curvature, which connects the described first section tangentally with a second section that inclines inwards, thus reducing the distance to the aforementioned plane.In the latter area, the ideal design from a strength perspective can thus be implemented, resulting in very evenly and homogeneously distributed compressive stresses, so that the risk of cracking is significantly reduced. Only in the area of ​​the described curvature does a slight local stress concentration occur. Furthermore, the straight first section, inclined outwards from the piston pin hub, creates the space required for a cooling oil nozzle without having to disproportionately shift the piston pin hubs outwards.

[0011] With regard to strength, it was found that, compared to a straight, non-inclined shaft connection, which achieves the same distance to an oil nozzle, the strength of the design according to the invention is approximately 16% higher. Improved strength is also achieved compared to an overall curved design of the shaft connections. At the same time, existing manufacturing methods can be used, and the piston weight and shaft compliance, which determine friction losses and noise, are not negatively affected.

[0012] Particularly with regard to strength, good results are expected for an inclination of the second section with respect to a plane perpendicular to the piston pin axis in the range of 10° to 25°, in particular 15° to 20° and especially preferably at about 18°.

[0013] The requirements are further met particularly well by making the second section longer than the first, in particular by 2.0 to 3.0 times, and especially preferably by 2.3 to 2.7 times, and particularly about 2.5 times as long. The beginning of the first section is defined as an edge line of the inner surface of the piston pin bore, located on the side of the considered cylinder wall and projected onto a plane perpendicular to the piston axis. The end of the first section is formed by the beginning of the curvature. The second section extends from the end of the curvature to the outer surface of the cylinder wall.

[0014] Preferred embodiments of the piston according to the invention are described in the further claims.

[0015] Initial simulations have shown that the requirements described above can be met particularly well if the inclination of the first section with respect to a plane perpendicular to the piston axis is less than the inclination of the second section with respect to a plane perpendicular to the piston axis.

[0016] In particular, an angle of less than 15°, especially less than 13° and especially preferably about 9°, is currently preferred for the inclination of the first section outwards to an imaginary plane perpendicular to the piston pin axis.

[0017] To minimize the unavoidable stress peaks, it is preferred that the curvature between the first and second sections be significantly shorter than that of the first section. In particular, good results are expected for a design in which the arc length of the curvature is less than 75%, particularly less than 65%, and most preferably about 64% of the length of the first section.

[0018] Although this has little influence on achieving the advantages of the invention, it is preferred that the outer surface of at least one box wall be designed in accordance with the inner surface, with the possible exception of a widening in the area of ​​the connection to the shaft wall; in other words, that it has a substantially constant thickness and / or also features the two largely straight sections described above with the comparatively short curvature between them. As mentioned, the described design on the inner and outer surfaces applies at least to the lower edge of at least one, preferably both, shaft walls on the pressure side. The shaft walls can be designed accordingly in their further course towards the piston stroke axis and, in particular, can run largely parallel to the piston stroke axis, at least in sections.

[0019] As is already evident from the foregoing, the piston according to the invention offers particular advantages in connection with a cooling oil nozzle provided in the piston housing of an engine, so that it is preferred for the piston that it has at least one cooling channel, although it is also conceivable that the cooling oil nozzle does not have a cooling channel, but merely supplies the underside of the piston with cooling oil.

[0020] Furthermore, the piston according to the invention is preferably used as a piston for a gasoline engine. Brief description of the drawing

[0021] The invention will now be explained in more detail with reference to an embodiment shown in the drawing.

[0022] The figure shows a partial bottom view of a piston according to the invention with a cooling oil nozzle. Detailed description of a preferred embodiment of the invention

[0023] The figure shows a bottom view of approximately one "quarter" of a piston according to the invention, with a piston pin hub 10 and a pressure-side shaft wall 12. A cooling oil nozzle 14 is also indicated. Line 16 represents a line that would be optimal for a homogeneous stress distribution in the connecting or box wall 18 between the piston pin hub 10 and the shaft wall 12. However, as can be seen particularly from an imaginary extension of line 16 in the figure to the left, up to the piston pin hub 10, this would, if sufficient space is provided for the cooling oil nozzle 14, shift the piston pin hub relatively far outwards, i.e., downwards in the figure, which would be disadvantageous with regard to the necessary piston pin length and piston weight.

[0024] According to the invention, this is achieved by the fact that the box wall, as shown by line 20, runs largely straight and at least slightly inclined outwards in a first section. This means that it does not extend perpendicular to the piston pin axis (running from top to bottom in the figure), but rather its distance from a plane perpendicular to the piston pin axis, which would be visible running from left to right in the figure, increases as line 20 moves further away from the piston pin hub 10 and approaches the shaft wall 12. With regard to its preferred dimensional ratio with respect to line 16, as described above, line 20 begins at the projection of the inner surface of the pin hub 10, shown as line 22.

[0025] Following a relatively short, curved, and tangent-continuous transition, particularly visible on the inside of the box wall 18 in the figure, the box wall slopes inwards along the "ideal" line 16, so that its distance to a plane perpendicular to the piston axis and extending towards the shaft wall 12 decreases. Accordingly, the area of ​​line 16 between the end of the curvature and the shaft wall 12 forms the second section described above, and the drawn line 20 forms the first section.

[0026] The figure also shows the measures described above, according to which the inclination of the first section 20 is less than that of the second section 16, and the arc length of the curvature is significantly shorter than that of the first section 20, and the latter is significantly shorter than that of the second section 16. It should be noted that the shape of the connecting wall, with its two largely straight sections and the curvature between them, can also be described as a "boomerang." Furthermore, the figure shows that the outer surface of the connecting or box wall 18 (shown at the bottom of the figure) largely corresponds to the shape of the inner surface, with a widening 22 provided in the area where it connects to the shaft wall 12.

Claims

1. Piston for an internal combustion engine with side reliefs (18) which are in each case formed between shaft walls (12) and piston pin bushings (10), wherein at least one side relief (18) is, on a pressure side, at least at the lower edge and at least on the inner side, starting out from the piston pin bushing (10), inclined outwards in a largely straight line in a first section (20), then curves inwards and then, in a second section (16), runs to the shaft wall (12) inclined inwards in a largely straight line, characterised in that the inclination of the second section (16) with respect to a plane perpendicular to the piston pin axis is 10 to 25°, preferably 15° to 20°, and the second section (16) is longer than the first section (20), in particular 2.0 to 3.0 and especially preferably 2.3 to 2.7 times as long.

2. Piston according to claim 1, characterised in that the inclination of the first section (20) with respect to a plane perpendicular to the piston pin axis is less than the inclination of the second section (16) with respect to a plane perpendicular to the piston pin axis.

3. Piston according to claim 1 or 2, characterised in that the inclination of the first section (20) with respect to a plane perpendicular to the piston pin axis is less than 15°, preferably less than 13°.

4. Piston according to one of the preceding claims, characterised in that the curve is shorter than the first section (20), in particular having an arc length which is less than 75%, preferably less than 65% of the length of the first section (20).

5. Piston according to one one of the preceding claims, characterised in that the outer side of at least one side relief, optionally with the exception of a widening (22) in the region of the connection to the shaft wall (12), runs parallel to the inner side.

6. Piston according to one of the preceding claims, characterised in that it has a cooling channel.

7. Piston according to one of the preceding claims, characterised in that it is a petrol engine piston.

8. Combination of a piston according to one one of the preceding claims with a cooling oil nozzle (14) which is preferably provided in the region of the second section (16), in particular in a middle region along the course thereof.