Abstract:
A multi-part piston for an internal combustion engine has a piston upper part and a piston lower part. The piston upper part comprises a piston head, a continuous fire land and a continuous ring part. The piston upper part and the piston lower part are connected together by securing means and form a continuous cooling channel. According to the invention, the connecting means connecting the piston upper part and the piston lower part are embodied as cooling elements that are arranged in the cooling channel and that are made of heating-conducting material.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and Applicants claim priority under 35 U.S.C. §§120 and 121 of U.S. application Ser. No. 12/734,241 filed on Sep. 2, 2010, which application is a national stage application under 35 U.S.C. 371 of PCT Application No. PCT/DE2008/001696 filed on Oct. 17, 2008, which claims priority under 35 U.S.C. §119 from German Patent Application No. 10 2007 050 213.5 filed on Oct. 20, 2007, the disclosures of each of which are hereby incorporated by reference. A certified copy of priority German Patent Application No. 10 2007 050 213.5 is contained in parent U.S. application Ser. No. 12/734,241. The International Application under PCT article 21(2) was not published in English. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a multi-part piston for an internal combustion engine, having an upper piston part and a lower piston part, whereby the upper piston part has a piston crown, a circumferential top land, as well as a circumferential ring belt, whereby the upper piston part and the lower piston part are connected with one another by means of attachment means and form a circumferential cooling channel. 
     In the case of such known multi-part cooling channel pistons, the problem exists of reliably connecting the upper piston part and the lower piston part with one another, while avoiding stresses. Furthermore, optimization of the cooling effect of the cooling oil contained in the cooling channel is aimed at. The cooling oil circulates in the cooling channel and is moved as a result of the shaker effect that is brought about by the piston movement. In DE 102 44 512 A1, it is proposed to provide the circumferential cooling channel with bores that are directed toward the piston crown, in order to achieve a better distribution of the cooling oil. However, in the case of pistons that are subject to great stress, in particular, the heat dissipation brought about by the movement of the cooling oil is not sufficient. 
     The present invention is therefore based on the task of creating a multi-part piston of the type indicated, having an improved cooling effect of the cooling oil that circulates in a cooling channel, in which piston the upper piston part and the lower piston part are reliably connected with one another. 
     SUMMARY OF THE INVENTION 
     To accomplish this task, the present invention comprises a multi-part piston having attachment means that connect the upper piston part and the lower piston part. The attachment means are configured as cooling elements made of a heat-conductive material and disposed in the cooling channel. 
     The piston according to the invention is characterized by an improved cooling effect, which has several causes. The cooling elements represent an additional cooling surface in the cooling space, by way of which the heat transported from the piston crown to the cooling elements is given off to the cooling oil that flows around the cooling elements. Furthermore, a directed flow of heat from the piston crown to the cooling oil, by way of the cooling elements, is guaranteed, and this, in particular, quickly and reliably reduces the heat stress on the piston crown, which faces the combustion chamber. In addition, heat dissipation from the cooling element into the lower piston part takes place. In the case of pistons whose piston crown is provided with a combustion chamber bowl, the bowl edge, in particular, is relieved of stress in particularly effective manner in this way. 
     The attachment means, which are configured as cooling elements, are uniformly distributed in a radially outer region of the piston, i.e. in the immediate vicinity of the ring belt, namely in the circumferential cooling channel, so that a particularly low-stress and reliable connection of upper piston part and lower piston part is achieved. Furthermore, the attachment means, which are configured as cooling elements, bring about an improvement in the shape stability of the piston according to the invention. For this reason, it is possible to configure the piston according to the invention, despite the cooling elements that are provided, in such a manner that a weight increase in comparison with conventional pistons is avoided. This can be brought about in that on the one hand, a suitable material, having as low a density as possible, is selected for the cooling elements, and that on the other hand, the wall thicknesses, particularly between the circumferential cooling channel and the adjacent structural elements, can be reduced because of the improved shape stability. In the case of a suitable method of construction, a weight reduction in comparison with conventional pistons is actually possible. 
     The embodiment according to the invention is suitable for all types of multi-part pistons, and allows a plurality of variants with regard to material selection and design. The piston according to the invention can consist, for example, of steel, cast iron, light metal, as well as a combination of these materials, and it can be configured with or without oil injection, for example. 
     Every cooling element, i.e. every attachment means configured as a cooling element, is accommodated in two recesses, one of which is provided in the upper piston part, and one in the lower piston part. The recesses provided in the lower piston part can be configured, for example, as dead-end bores or as passage bores. Therefore the cooling elements can be inserted into the upper piston part or the lower piston part, for example, in simple manner, if necessary oriented in known manner, using a centering ring, and the missing piston part can be pushed onto the cooling elements, so that these engage into the corresponding bores. Attachment of the cooling elements preferably takes place by means of press fit or shrink fit. 
     The recesses provided in the upper piston part in this embodiment bring about a further reduction in material thickness toward the piston crown. In a particularly advantageous embodiment, the material thickness toward the bowl edge is reduced if the piston crown is provided with a combustion chamber bowl. This brings about a further improved and accelerated transport of heat to the cooling elements. The cooling elements simultaneously ensure sufficient shape stability of the piston according to the invention, in that the cooling elements balance out possible stability losses brought about by the recesses. This has the result that the diameter of a combustion chamber bowl provided in an individual case can be further increased as compared with the state of the art. This in turn once again improves the heat dissipation in the direction of the cooling elements, and from there to the cooling oil. The number of cooling elements can be selected to be particularly large, because of the improved shape stability, so that the effective cooling surface is optimized. In addition, the heat taken up by the piston crown is prevented from penetrating into the region of the ring belt. 
     Another suitable embodiment provides for ring-shaped cooling elements, which can be provided with passage openings for the cooling oil, if necessary, in order to optimize mixing of the cooling oil. A particularly preferred embodiment of the piston according to the invention consists in that pin-shaped cooling elements are provided. This configuration is accompanied by the greatest possible increase in the cooling surface, so that particularly effective heat dissipation from the cooling elements to the cooling oil takes place. 
     The cooling elements are preferably configured as solid metal cooling elements, and consist, for example, of copper, aluminum, or their alloys. 
     In a preferred embodiment, the upper piston part and the lower piston part additionally form an inner cooling chamber, which is separated from the circumferential cooling channel by a circumferential partition, to support the cooling effect. The upper piston part and the lower piston part can have additional connection elements in the region of the partition, which support the connection of upper piston part and lower piston part. These connection elements also can be configured as cooling elements, to further improve the cooling effect. 
     In an advantageous manner, the partition can have at least two overflow openings for cooling oil, which connect the circumferential cooling channel and the inner cooling chamber with one another, in order to optimize mixing of the cooling oil. 
     The materials for the upper piston part and the lower piston part can be selected and combined with one another as desired. For example, steel materials and light-metal materials are suitable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be described in the following, using the attached drawing. The drawing shows, in a schematic representation, not true to scale: 
         FIG. 1  a section through a first exemplary embodiment of a piston according to the invention; 
         FIG. 2  the piston according to  FIG. 1  in a top view, partly in section; 
         FIG. 3  an enlarged detail of a second exemplary embodiment of a piston according to the invention, in section; 
         FIG. 4  an enlarged detail of an upper piston part for a third exemplary embodiment of a piston according to the invention, in section. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIGS. 1 and 2  show a first exemplary embodiment of a piston  10  according to the invention, whereby the representation according to  FIG. 1  is rotated by 90° in the left half, as compared with the representation in the right half. 
     The piston  10  according to the invention is composed of an upper piston part  11  and a lower piston part  12 . The upper piston part  11  has a piston crown  13  having a combustion chamber bowl  14  as well as a side wall having a circumferential top land  15  and a circumferential ring belt  16  for accommodating piston rings (not shown). The lower piston part  12  has a piston skirt  17 , pin bosses  18  having pin bores  18   a  for accommodating a piston pin (not shown), and pin boss supports  19  that are connected with the piston skirt  17 . The upper piston part  11  and the lower piston part  12  form a circumferential outer cooling channel  21  and an inner cooling chamber  22 , which are separated from one another by means of a partition  29 . In the exemplary embodiment, overflow channels  27  are provided in the partition  29 , which connect the cooling channel  21  and the cooling chamber  22  with one another. 
     The upper piston part  11  has an outer contact surface  23  that follows the ring belt  16 , and a ring-shaped, circumferential inner support surface  24  on its underside. The lower piston part  12  also has an outer contact surface  25  on its top, as well as a ring-shaped, circumferential inner support surface  26 . In the assembled state, the upper piston part  11  and the lower piston part  12  are oriented, relative to one another, in such a manner that the two support surfaces  24 ,  26  as well as the two contact surfaces  23 ,  25  lie against one another. The partition  29  is formed in the region of the support surfaces  24 ,  26 , in the assembled state. 
     The materials of the upper piston part  11  and the lower piston part  12  can be selected and combined with one another as desired, for example hot steel, AFP steel, or light-metal alloys, particularly aluminum alloys. For example, the upper piston part  11  can be forged from hot steel, and the lower piston part  12  can be forged from AFP steel; however, the upper piston part  11  can also be forged from AFP steel, and the lower piston part  12  can be cast from an aluminum alloy. However, the upper piston part  11  can also be forged from an aluminum alloy, and the lower piston part can be cast from an aluminum alloy. 
     In the exemplary embodiment, a plurality of pin-shaped attachment means configured as cooling elements  28  are disposed in the cooling channel  21  of the piston  10 . The cooling elements  28  consist of a material that conducts heat well, preferably having a low density. Metallic materials such as aluminum, copper, or their alloys, for example, are suitable. The free ends of the cooling elements  28  are accommodated in recesses  31 ,  32  configured as bores. The recess  31 , configured as a dead-end bore, is disposed in a wall section of the cooling channel  21  formed by the upper piston part  11 , and is directed toward the piston crown  13 . The recess  32 , configured as a passage bore, is disposed in a wall section of the cooling channel  21  formed by the lower piston part  12 . The cooling elements  28  are attached in the upper piston part  11  and in the lower piston part  12  by means of a press fit or shrink fit, in the exemplary embodiment. 
     The cooling elements  28  are therefore simultaneously attachment means, by means of which the upper piston part  11  and the lower piston part  12  are connected with one another essentially without any tension. This is attributable to the fact that the attachment means are disposed, uniformly distributed, in a radially outer region of the piston  10 , i.e. in the immediate vicinity of the ring belt  15 , namely in the circumferential cooling channel  21 . Furthermore, the attachment means, which are configured as cooling elements  28 , bring about an improvement in the shape stability of the piston  10  according to the invention. For this reason, the wall thickness between the combustion chamber bowl  14  and the recesses  31  is configured to be particularly small. This is accompanied by a reduction in material and weight. Furthermore, the heat from the combustion chamber bowl  14  is passed off to the cooling elements  28  particularly quickly. 
     The surfaces of the numerous cooling elements  28  act as an additional large cooling surface in the cooling channel  21 . By way of this cooling surface, the heat transport from the piston crown  13  to the cooling elements  28  is given off to the cooling oil that flows around the cooling elements  28  particularly quickly. Furthermore, a direct heat flow from the piston crown  13  to the cooling channel  21 , and from there both to the cooling oil and to the lower piston part  12 , is guaranteed, and this reduces the heat stress on the piston crown  13  and on the bowl edge of the combustion chamber bowl  14 , in particular, in particularly effective manner. 
     In the exemplary embodiment shown in  FIG. 1 , pin-shaped connection elements  30  are furthermore provided in the partition  29 , which elements, comparable to the attachment means configured as cooling elements  28 , are accommodated in recesses provided in the upper piston part  11  and the lower piston part  12 , respectively, for example by means of press fit or shrink fit. The connection elements  30  support the connection between the upper piston part  11  and the lower piston part  12  brought about by the attachment elements configured as cooling elements  28 . The connection elements  30  can also be configured as cooling elements, in order to support the heat dissipation from the piston crown  13  to the lower piston part  12 . 
       FIG. 3  shows, as a detail, another exemplary embodiment of a multi-part piston  110 , whereby the same reference symbols were used for the same components. In  FIG. 3 , only part of the upper piston part  11  and part of the lower piston part  12 , as well as the cooling channel  21  and a cooling element  128  are shown. The piston  110  has the same structure as the piston  10  shown in  FIG. 1 . The only difference consists in that the lower free ends of the cooling elements  128  are accommodated in recesses  132  provided in the lower piston part  12  and configured as dead-end bores. The upper, free ends of the cooling elements  128  are accommodated in recesses  31  configured as dead-end bores, which are provided in the upper piston part  11 , just as in the piston  10 . In this exemplary embodiment, too, the cooling elements  128  can be attached by means of press fit or shrink fit. The cooling elements  128  have the same effects and advantages as those described for the cooling elements  28  according to  FIG. 1 . 
       FIG. 4  shows, as a detail, another exemplary embodiment of an upper piston part  11  of a multi-part piston  210 , which is the same as the piston  10  and  110  shown in  FIGS. 1 and 3 , respectively. It can be seen that connection elements  30  are disposed in the region of the partition  29 . The only difference consists in that in place of pin-shaped cooling elements, cooling elements  228  configured in the shape of ring segments are provided. The upper free ends of the cooling elements  228  are accommodated, in comparable manner, in recesses (not shown), which are disposed in a wall section of the cooling channel  21  formed by the upper piston part  11 . The cooling elements  228  are attached in suitable manner, as described above, for example by means of shrink fit or press fit. In the assembled state, the lower free ends of the cooling elements  228  are accommodated and attached in recesses (not shown) provided in the lower piston part, in comparable manner. The cooling elements  228  have passage openings  239 , in order to guarantee optimal mixing of the cooling oil accommodated in the cooling channel  21 . The cooling elements  228  demonstrate the same effects and advantages as described for the cooling elements  28  according to  FIG. 1 .