Pistons for an internal combustion engine and methods for their manufacture

Abstract

Pistons (10, 110, 210) for an internal combustion engine, - with a piston head (16) having a circumferential cooling channel (19, 119) and a combustion bowl (15) with a circumferential bowl wall (15b) which transitions via a bowl edge area (15c) into a piston base (12, 112), - wherein the combustion bowl (15) is formed at least partially from a piston body (11) and the bowl wall (15b) is formed at least partially from an insert (18), - wherein the insert (18) is connected to the piston base body (11) by means of beam welding, - wherein a lower weld (21) is formed in the recess wall (15b) which forms an acute angle (α) with the piston central axis (M), - wherein the lower weld (21) opens into the lower half of the cooling channel (19), - and wherein an upper weld seam (22, 222) extends from the cooling channel ceiling (19b, 119b) to the piston base (12, 112) and is arranged centrally with respect to the clear width (W) of the cooling channel (19), characterized in that - the thickness of the channel floor (112) above the cooling channel (19) increases radially outwards.

Description

[0001] The present invention relates to a piston for an internal combustion engine, comprising a piston head with a circumferential cooling channel and a combustion bowl with a circumferential bowl wall that transitions into a piston crown via a bowl rim region, wherein the combustion bowl is formed at least partially from a piston body and the bowl wall is formed at least partially from an insert. The present invention further relates to a method for manufacturing such a piston.

[0002] Pistons of this type are described in the publications DE 10 2010 052 578 A1 and JP 2007 - 270 813 A.

[0003] Further pistons are known from the publications DE 21 41 054 A, GB 1 501 387 A, JP H02- 301 650 A, WO 00 / 31 399 A1, US 2009 / 0 020 007 A1, DE 10 2009 059 658 A1, DE 699 26 583 T2 and DE 100 47 258 A1.

[0004] Another piston is described in German publication DE 30 32 671 A1. It has a piston body made of extruded steel and an insert made of a highly heat-resistant material, welded or brazed to the piston body. The piston body has a combustion bowl in the area of ​​the piston head, which is bounded by a circumferential bowl wall that transitions into the piston crown via a bowl rim. The piston body and the insert form a circumferential cooling channel.

[0005] The trough wall, and especially the trough edge area, is subject to particularly high mechanical and thermal stresses during operation, which over time lead to material fatigue and, in turn, cracks. Therefore, numerous specially designed inserts have been proposed to reduce or neutralize the stresses acting on the trough edge area.

[0006] Due to advances in engine technology, these proposals can no longer keep pace with the increasing mechanical and thermal stresses on the pistons during engine operation.

[0007] The object of the present invention is to further develop a generic piston and a method for its manufacture in such a way that it can withstand the increased mechanical and thermal stresses.

[0008] The solution consists of connecting the insert to the piston body by beam welding, forming a lower weld seam in the recess wall that forms an acute angle with the piston's central axis and opens into the lower half of the cooling channel, and an upper weld seam running from the cooling channel ceiling to the piston base and positioned centrally with respect to the clear width of the cooling channel. According to the invention, the thickness of the channel base increases radially outwards above the cooling channel.

[0009] The method according to the invention is characterized by the following process steps: a) pre-machining the piston body and the insert at least in the area of ​​the joining surfaces; b) assembling the piston body and insert; c) joining the piston body and insert along their corresponding joining surfaces by means of beam welding; d) finishing the piston.

[0010] The piston according to the invention withstands high thermal and mechanical loads very well, since the connection between the piston body and the insert is particularly strong and durable thanks to beam welding. Furthermore, the welds are arranged in those areas of the piston head where, due to the thermal and mechanical loads during engine operation, only low stresses occur in the material, thus minimizing the risk of cracking in the weld area. The position of the joining surfaces of the piston body and insert is also selected to be easily accessible for beam welding, thereby optimizing the position of the resulting welds.

[0011] Advantageous further training opportunities arise from the sub-requirements.

[0012] The piston base and the insert are preferably joined together by laser welding in order to obtain a particularly strong and reliable connection between these components.

[0013] The lower weld and the upper weld preferably form an acute angle, so that the corresponding joining surfaces are particularly easily accessible before joining the piston base body and insert.

[0014] In a particularly preferred design, the lower weld seam terminates in the cooling channel at the bottom. During engine operation, the lowest stresses occur in the material in this area, thus minimizing the risk of cracking in the lower weld seam.

[0015] The lower weld seam preferably has a length of 3.5% to 5.5% of the piston diameter. The upper weld seam preferably runs parallel to the piston's central axis and preferably has a length of 4.5% to 6.0% of the piston diameter.

[0016] In a preferred embodiment of the present invention, the thickness of the piston crown increases radially outwards above the cooling channel. This promotes heat dissipation via the first piston ring.

[0017] A particularly simple and effective design involves the insert forming the entire cylinder wall. The piston body and the insert are preferably made of different materials. This allows identical piston bodies to be used to create different piston types for a wide variety of internal combustion engines simply by selecting the appropriate material for the insert.

[0018] A further preferred embodiment of the present invention consists in the ratio between the diameter of the piston pin and the diameter of the piston being 0.33 to 0.38. This ratio is particularly advantageous for passenger car pistons, especially those made of steel, because it results in a particularly favorable compression height.

[0019] An additional advantageous improvement provides that the ratio between the length of the piston pin and the diameter of the piston is between 0.57 and 0.63. This ratio is also particularly advantageous for passenger car pistons, which are primarily made of steel, as box-type pistons with these dimensions are especially lightweight yet robust.

[0020] The insert is preferably made of a high-temperature-resistant, corrosion- and heat-resistant steel, in particular a valve steel. In contrast, the piston body can be made, for example, of a heat-treatable steel.

[0021] A further development of the inventive method consists in step a) of cleaning and smoothing the joining surfaces of the piston base and insert in order to obtain particularly strong and reliable welds. Subsequently, in step b), the piston base and the insert can be assembled, for example, by means of a press fit and / or tack welding, in order to fix the components against each other particularly reliably during the actual welding process.

[0022] The piston body and the insert are preferably joined by electron beam welding, and particularly preferably by laser welding. Prior to the welding process, the piston body and / or the insert can be preheated to 400°C to 550°C to reduce the risk of stress in the material due to thermal stress during the welding process.

[0023] Exemplary embodiments of the invention are explained in more detail below with reference to the accompanying drawings. These show, in a schematic representation not to scale: Fig. 1 a first embodiment of a piston not according to the invention in section; Fig. 2 of the pistons according to Fig. 1 in a representation rotated by 90°; Fig. 3 a detailed cross-sectional view of an embodiment of a piston according to the invention; Fig. 4 a detailed view of a further embodiment of a piston not according to the invention in one of the Fig. 3 corresponding representation.

[0024] The Fig. 1 and Fig. Figure 2 shows a first embodiment of a piston 10. The piston 10 has a piston body 11, which is made, for example, of a heat-treatable steel such as 42CrMo4 or an AFP steel or a bainitic AFP steel doped with 0.4 wt.% molybdenum. The piston body 11 has a portion of a piston crown 12, a circumferential fire land 13, and a circumferential ring section 14 with ring grooves for receiving piston rings (not shown). The piston body 11 also has the base 15a of a combustion chamber 15.

[0025] The piston body 11 thus forms an essential part of the piston head 16 of the piston 10. The piston body 11 also forms the piston shaft 17 of the piston 10 in a manner known per se.

[0026] The compression height KH of piston 10 is defined as the height of piston 10 measured from the center of the hub bore to the piston base (see Fig. 2) In this embodiment, the ratio of the compression height KH to the diameter D of the piston 10 is 0.38 to 0.45 (38% to 45%).

[0027] The piston further comprises an insert 18, which, in the illustrated embodiment, forms the entire combustion bowl wall 15b and the bowl edge region 15c of the combustion bowl 15, as well as a portion of the piston crown 12. The insert 18 is preferably made of a particularly resistant material. A high-temperature-resistant, corrosion- and heat-resistant steel is especially suitable for this purpose. Valve steels such as CrSi steel (X45CrSi93), Chromo193 steel (X85CrMoV182), 21-4N steel (X53CrMnNiN219), 21-2 steel (X55CrMnNiN208), Ni-monic80A steel (NiCr20TiAl), ResisTEL steel, or VMS-513 steel are particularly suitable.

[0028] The piston body 11 and the insert 18 form a circumferential outer cooling channel 19. The cooling channel 19 runs at the level of the ring section 14 on one side and at the level of the combustion bowl wall 15b on the other. The cooling channel 19 has a cooling channel base 19a, a cooling channel ceiling 19b, an outer side wall 19c, and an inner side wall 19d. In this embodiment, the cooling channel base 19a and the outer side wall 19c are formed by the piston body 11. The inner side wall 19d is formed by the insert 18, while the cooling channel ceiling 19b is formed jointly by the piston body 11 and the insert 18.

[0029] The insert 18 has a lower circumferential joining surface which, together with a circumferential joining surface enclosing the base 15a of the combustion bowl 15 on the piston body 11, forms a lower weld seam 21. The lower weld seam 21 has a length of 3.5% to 5.5% of the piston diameter D and forms an acute angle α with the piston's central axis M. The lower weld seam 21 thus extends radially outwards and downwards (towards the piston skirt 17) from the bowl wall 15b and terminates in the cooling channel 19 in the area of ​​the cooling channel base 19a. In this area of ​​the piston 10, the lowest stresses occur in the material during engine operation, thus minimizing the risk of cracking in the area of ​​the weld seam 21.

[0030] The insert 18 further has an upper circumferential joining surface which forms an upper weld seam 22 with a circumferential joining surface on the piston body 11 in the area of ​​the fire bridge 13. The upper weld seam 22 has a length of 4.5% to 6.0% of the piston diameter D. In the exemplary embodiment, the upper weld seam 22 runs from the cooling channel ceiling (19b) to the piston base 12 and parallel to the piston central axis M, forming an acute angle β with the lower weld seam 21. In the exemplary embodiment, the upper weld seam 22 is arranged centrally with respect to the clear width W of the cooling channel 19 (see Figure 1). Fig. 3) According to the invention, the upper weld 22 is thus arranged far enough away from the recess edge area 15c, so that the risk of cracking due to thermal and mechanical stresses in the area of ​​the weld 22 is reduced. In this area of ​​the piston crown 12, where the upper weld 22 is located, comparatively low stresses occur in the material during engine operation.

[0031] In the assembled state, a piston pin 24 is received in a pin bore 23 in the piston shaft 17 in a manner known per se, as shown in the Fig. 1 and Fig. 2 is indicated by a dashed line. In this embodiment, the ratio between the diameter BD of the piston pin and the diameter D of the piston is 0.33 to 0.38 (33% to 38%), i.e., BD / D = 0.33 - 0.38. Furthermore, in this embodiment, the ratio between the length BL of the piston pin and the diameter D of the piston is 0.57 to 0.63 (57% to 63%), i.e., BL / D = 0.57 to 0.63. Naturally, the respective dimensions do not necessarily have to be fulfilled simultaneously.

[0032] Fig. Figure 3 shows a detailed embodiment of a piston 110 according to the invention. The piston 110 differs from the piston 10 according to the Fig. 1 and Fig. 2 simply by the fact that the thickness of the piston crown 112 increases radially outwards in the area of ​​the cooling channel 119. The cooling channel roof 119b of the cooling channel 119 thus extends radially outwards and downwards (in the direction of the piston skirt 17).

[0033] Fig. 4 shows in one of the Fig. Figure 3 shows a further embodiment of a piston 210 not according to the invention. The piston 210 differs from the piston 10 according to the Fig. 1 and Fig. 2 only by the fact that the upper weld 222 is located in the area of ​​the fire bridge 13 and runs obliquely, namely radially inwards and downwards from the outer edge of the piston crown 12 (in the direction of the piston skirt 17). The upper weld 222 opens into the cooling channel 19 between the cooling channel roof 19b and the outer side wall 19c. The upper weld 222 and the lower weld 21 form an acute angle γ.

[0034] The piston body 11 and the insert 18 are joined together in a manner known per se by beam welding, particularly preferably by laser welding. Typically, the piston body 11 and the insert 18 are pre-machined in a manner known per se. In particular, the joining surfaces are cleaned and smoothed. Then the piston body 11 and the insert 18 are joined together, e.g., by press fit. In a next step, the welds 21, 22, 222 can be tacked at specific points or around the circumference with a shallow weld penetration. Then the piston 10, 110, 210 is heated to a temperature of 400°C to 550°C. Subsequently, the welds 21, 22, 222 are fully welded. Finally, after cooling, the piston 10, 110, 210 is finished in a manner known per se.

Claims

[1] Pistons (10, 110, 210) for an internal combustion engine, - with a piston head (16) having a circumferential cooling channel (19, 119) and a combustion bowl (15) with a circumferential bowl wall (15b) which transitions via a bowl edge area (15c) into a piston base (12, 112), - wherein the combustion bowl (15) is formed at least partially from a piston body (11) and the bowl wall (15b) is formed at least partially from an insert (18), - wherein the insert (18) is connected to the piston base body (11) by means of beam welding, - wherein a lower weld (21) is formed in the recess wall (15b) which forms an acute angle (α) with the piston central axis (M), - wherein the lower weld (21) opens into the lower half of the cooling channel (19), - and wherein an upper weld (22, 222) extends from the cooling channel ceiling (19b, 119b) to the piston base (12, 112) and is centrally located with respect to the clear width (W) of the cooling channel (19), characterized by , that - the thickness of the channel floor (112) above the cooling channel (19) increases radially outwards. [2] Piston according to claim 1, characterized by , that the piston base body (11) and the insert (18) are joined together by laser welding, or a ratio of the compression height (KH) of the piston (10, 110, 210) to the diameter (D) of the piston (10, 110, 210) is 0.38 to 0.

45. [3] Piston according to claim 1, characterized by , that the lower weld (21) and the upper weld (22, 222) enclose an acute angle (β, γ). [4] Piston according to claim 1, characterized by , that the lower weld (21) opens into the cooling channel (19, 119) in the area of ​​the cooling channel floor (19a). [5] Piston according to claim 1, characterized by , that the lower weld (21) has a length of 3.5% to 5.5% of the piston diameter (D). [6] Piston according to claim 1, characterized by , that the upper weld seam (22, 222) runs parallel to the piston center axis (M). [7] Piston according to claim 1, characterized by , that the upper weld (22, 222) has a length of 4.5% to 6.0% of the piston diameter (D). [8] Piston according to claim 1, characterized by , that the entire trough wall (15b) is formed from the insert (18). [9] Piston according to claim 1, characterized by , that the ratio between the diameter (BD) of the piston pin (24) and the diameter (D) of the piston (10, 110, 210) is 0.33 to 0.

38. [10] Piston according to claim 1, characterized by , that the ratio between the length (BL) of the piston pin (24) and the diameter (D) of the piston (10, 110, 210) is 0.57 to 0.

63. [11] Piston according to claim 1, characterized by , that the piston body (11) and the insert (18) are made of different materials. [12] Piston according to claim 1, characterized by , that the insert (18) is made of a high-temperature resistant, corrosion- and heat-resistant steel, in particular a valve steel. [13] Method for manufacturing a piston (10, 110, 210) according to any one of the preceding claims 1 to 12, - with a piston head (16) having a circumferential cooling channel (19, 119) and a combustion bowl (15) with a circumferential bowl wall (15b) which transitions via a bowl edge region (15c) into a piston base (12, 112), wherein the combustion bowl (15) is formed at least partially from a piston body (11) and the bowl wall (15b) is formed at least partially from an insert (18), - characterized by the following procedural steps: a) Pre-machining of the piston body (11) and the insert (18) at least in the area of ​​the joining surfaces; b) Assembling the piston body (11) and insert (18); c) Joining the piston base body (11) and insert (18) along their corresponding joining surfaces by means of beam welding, d) Finishing the piston. [14] Method according to claim 13, characterized by , that in step a) the joining surfaces of piston base body (11) and insert (18) are cleaned and smoothed. [15] Method according to claim 13, characterized by , that in step b) the piston body (11) and the insert (18) are assembled by means of a press fit and / or by means of tack welding. [16] Method according to claim 13, characterized by , that in step c) the piston base body (11) and the insert (18) are joined by electron beam welding or laser welding. [17] Method according to claim 15, characterized by, that before step c) the piston body (11) and / or the insert (18) are preheated to 400°C to 550°C.

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