Abstract:
A piston for an internal combustion engine may include a piston head and a piston skirt. The piston head may include a piston crown, an encircling fire land, an encircling ring belt having a plurality of ring grooves and an encircling cooling duct disposed radially inwards from the ring belt. The cooling duct may be open in an axial direction away from the fire land and may be at least partially closed via a closure element. The cooling duct may have a cooling duct base and a cooling duct ceiling. The closure element may be arranged on the piston head to define the cooling duct base in a position above a lowermost ring groove of the plurality of ring grooves.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2013 009 161.6, filed May 31, 2013, and International Patent Application No. PCT/DE2014/000264, filed May 28, 2014, both of which are hereby incorporated by reference in their entirety. 
     TECHNICAL FIELD 
     The present invention relates to a piston for an internal combustion engine, having a piston head and a piston skirt, the piston head having a piston crown, an encircling fire land, an encircling ring belt with ring grooves and, in the region of the ring belt, an encircling cooling duct which is open toward the bottom and is closed by way of a closure element, the cooling duct having a cooling duct base and a cooling duct ceiling. 
     BACKGROUND 
     In modern internal combustion engines, the pistons are subjected to ever higher temperature loading in the region of the piston skirt and of the combustion depression. Inadequate dissipation of heat from the piston head leads, during engine operation, to functional impairments of the piston, in particular to coking or oil carbon formation on the piston. This applies in particular to pistons composed of steel materials, as steel has a low coefficient of thermal conductivity and is thus a poor heat conductor. 
     SUMMARY 
     It is the object of the present invention to develop a piston of the generic type in such a way that optimized heat dissipation from the piston head is realized during engine operation. 
     The object is achieved by virtue of the fact that the closure element is arranged in the piston head in such a way that the cooling duct base is arranged above the lowermost ring groove. 
     In the prior art, the cooling duct extends in the axial direction generally as far as the height of the lowermost ring groove and below, in order to achieve sufficient cooling, in particular of steel pistons, during engine operation with the aid of a cooling duct which is as large as possible. However, on account of the cocktail shaker effect, the cooling oil moves back and forth between the cooling duct ceiling, that is to say a very hot region, and the cooling duct base, that is to say a comparatively cool region. On account of the considerably lower temperatures in the region of the cooling duct base, heat absorption from the piston head into the cooling oil no longer takes place there in practice. Furthermore, owing to the shallow heat gradient in the direction of the ring belt and piston skirt, only a relatively small amount of heat is dissipated from the cooling oil. 
     The piston according to the invention is distinguished from this in that the cooling duct is shortened in the axial direction in relation to the prior art. As a consequence, the cooling oil moves, in particular in the region of the cooling duct base, in closer proximity to the highly thermally loaded cooling duct base and therefore, overall, in hotter regions than in the prior art. Heat absorption from the hot regions of the piston head into the cooling oil therefore takes place in every phase of the piston movement. Considerably improved cooling of the piston head in relation to the prior art is realized in particular if the cooling oil quantity which is known from the prior art is retained and the cooling oil supply is set up in such a way that the cooling oil is exchanged rapidly during engine operation. 
     Advantageous developments will emerge from the subclaims. 
     The cooling duct base is preferably arranged at the level of the second ring groove, particularly preferably between the first ring groove and the second ring groove, in order to further increase the cooling performance by the cooling oil moving in even greater proximity to the hot piston crown during engine operation. 
     One advantageous development provides that the closure element is arranged in the piston head in such a way that an encircling annular gap is formed in the piston crown. This dispenses with the necessity of providing oil outlet openings. 
     A further preferred development provides that the height of the fire land is at most 9% of the nominal diameter of the piston head. In this way, positioning of the cooling duct in relation to the piston crown and the ring belt is realized which is particularly advantageous for the dissipation of heat. 
     In this case, the spacing between the piston crown and the cooling duct base may be between 11% and 17% of the nominal diameter of the piston head. In addition or instead, the height of the cooling duct may be 0.8 times to 1.7 times its width. Furthermore, as an alternative or in addition to this, the spacing between the piston crown and the cooling duct ceiling may be between 3% and 7% of the nominal diameter of the piston head. These dimension rules permit an optimized design and positioning of the cooling duct for all piston sizes. 
     The compression height may be, for example, between 38% and 45% of the nominal diameter of the piston head. 
     A further particularly preferred embodiment consists in that a combustion depression is formed in the piston head, and that the smallest wall thickness in the radial direction between the combustion depression and the cooling duct is between 2.5% and 4.5% of the nominal diameter of the piston head. An improved thermal transfer between the combustion depression and the cooling duct is achieved in this way. 
     The combustion depression may be provided, for example, with an undercut, in order to define the wall thickness between the combustion depression and the cooling duct. 
     In the case of a decoupled piston skirt, the closure element may be formed as a separate component which is fastened to the piston. 
     The piston according to the invention may be formed as a single-piece piston. The cooling duct is then made in a cast or forged blank in a manner known per se by way of machining. It is preferred, however, that the piston is assembled from at least two components which are connected non-releasably to one another. In particular, the piston according to the invention may have a piston main body and a piston ring element. In this case, the closure element may be formed both as a separate component which is fastened to the piston and as a component which is connected in one piece to the piston. In the latter case, the closure element may be connected in one piece either to the piston main body or to the piston ring element. 
     The present invention is suitable in particular for pistons composed of at least one steel material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following text, exemplary embodiments of the present invention will be explained in greater detail on the basis of the appended drawings, in which, in a diagrammatic illustration which is not true to scale: 
         FIG. 1  shows a first exemplary embodiment of a piston according to the invention in section; 
         FIG. 2  shows the piston according to  FIG. 1  in an illustration which has been rotated through 90°; 
         FIG. 3  shows a further exemplary embodiment of a piston according to the invention in section; 
         FIG. 4  shows the piston according to  FIG. 3  in an illustration which has been rotated through 90°; 
         FIG. 5  shows an overall illustration of two further exemplary embodiments in section; 
         FIG. 6  shows an enlarged partial illustration of the piston as per  FIG. 5 , left-hand side, in section; 
         FIG. 7  shows an enlarged partial illustration of a further exemplary embodiment in section; 
         FIG. 8  shows an enlarged partial illustration of the exemplary embodiment as per  FIG. 7 ; 
         FIGS. 9 a , 9 b    show a diagrammatic illustration of the cooling oil movement in a piston according to the present invention, and 
         FIGS. 10 a , 10 b    show a diagrammatic illustration of the cooling oil movement in a piston according to the prior art. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a first exemplary embodiment of a piston  10  according to the invention. As is generally known, the piston  10  may be forged or cast as a single-piece blank, the cooling duct being formed into the blank by way of machining. In the exemplary embodiment, the piston  10  is assembled from a piston main body  31  and a piston ring element  32  which may be cast or forged in a manner known per se and are connected to one another via a welded seam  33 , for example by means of electron beam welding or laser welding. In the exemplary embodiment, the welded seam  33  is arranged at the lowest point of the combustion depression at an acute angle with respect to the piston center axis A. In the exemplary embodiment, the piston  10  is produced from a steel material. 
     The piston  10  has a piston head  11  with a piston crown  12  which has a combustion depression  13 , an encircling fire land  14  and an encircling ring belt  15  with ring grooves  16 ,  17 ,  18  for receiving piston rings (not shown). An encircling cooling duct  19  is provided at the level of the ring belt  15 . 
     Furthermore, the piston  10  has a piston skirt  21  which is thermally decoupled from the piston head  11  and which has piston bosses  22  and boss bores  23  for receiving a piston pin (not shown). The piston bosses  22  are connected via boss attachments  24  to the underside of the piston head  11 . The piston bosses  22  are connected to one another via running faces  25 . 
     The cooling duct  19  is formed so as to be open toward the bottom and is closed by way of a separate closure element  35 , a closure plate in the exemplary embodiment. The closure element  35  is fastened to the piston head  11  in a manner known per se below the ring belt  15  and extends in the direction of the combustion depression  13  in such a way that the annular free end of the closure element  35  forms an encircling annular gap  36  together with the outer wall of the combustion depression  13 . 
     It is self-evidently possible for the annular gap  36  to be dispensed with. Instead, in a manner known per se, the cooling duct  19  may be completely closed off by the closure element  35 , with inlet and outlet openings for cooling oil being provided in the closure element  35 . 
     The closure element  35  is curved in the direction of the piston crown  12  in such a way that a cooling duct base  26  is formed which lies approximately at the level of the second ring groove  17  in the exemplary embodiment. The cooling duct base  26  may also be arranged between the first ring groove  16  and the second ring groove  17 . 
     Furthermore, the cooling duct  19  has a cooling duct ceiling  27 . 
     In the exemplary embodiment, the compression height KH is between 38% and 45% of the nominal diameter DN of the piston head  11 . 
       FIGS. 3 and 4  show a further exemplary embodiment of a piston  110  according to the invention. The piston  110  is constructed in a similar way to the piston  10  according to  FIGS. 1 and 2 . Structural elements which correspond are therefore provided with the same designations, and reference is made in this regard to the description with respect to  FIGS. 1 and 2 . 
     The main difference between the piston according to  FIGS. 3 and 4  and the piston according to  FIGS. 1 and 2  consists in the fact that the inner faces  128  of the running faces  25  of the piston  110  are connected via a connecting wall  129  to the underside of the piston head  11 . 
       FIG. 5  shows, in an illustration as per  FIG. 2 , an overall view of two further exemplary embodiments of pistons  210 ,  310  according to the invention. The illustrations of the respective exemplary embodiments are separated by the center line M. 
     The pistons  210 ,  310  are constructed in a similar way to the piston  10  according to  FIGS. 1 and 2 . Structural elements which correspond are therefore provided with the same designations, and reference is made in this regard to the description with respect to  FIGS. 1 and 2 . 
     The main differences consist firstly in the design of the piston main body  231 ,  331  and of the piston ring element  132 ,  332  and secondly in the fact that the pistons  210 ,  310  have a closure element  235 ,  335  of different design in comparison with the piston  10  according to  FIGS. 1 and 2 . 
     Both exemplary embodiments have in each case one closure element  235 ,  335  in the form of an encircling flange which is connected in one piece to the piston main body  231 ,  331 . Each closure element  235 ,  335  extends in the direction of the ring belt  15  in such a way that the free end of each closure element  235 ,  335  forms an encircling annular gap  236 ,  336  together with the inner wall of the ring belt  15 . 
     The piston  210  (illustration to the right of the center line M) is composed of a piston main body  231  and a piston ring element  232 . In the exemplary embodiment, the piston ring element  232  comprises a part of the depression wall and the depression edge of the of the combustion depression  13  and also the piston crown  12 , the fire land  14  and the ring belt  15 . The piston ring element  232  may be connected to the piston main body  131  in particular by way of a welding process, for example electron beam welding, laser welding or friction welding, wherein the welded seam  233  is arranged in the in the depression wall of the combustion depression  13 . 
     The piston  310  (illustration to the left of the center line M) (cf. also the enlarged partial illustration in  FIG. 6 ) is composed of a piston main body  331  and a piston ring element  332 . In the exemplary embodiment, the piston ring element  332  comprises a part of the piston crown  12 , the fire land  14  and the ring belt  15 . The piston ring element  332  may be connected to the piston main body  331  in particular by way of a welding process, for example electron beam welding or laser welding, wherein the welded seam  333  is arranged in the piston crown. 
       FIG. 7  shows an enlarged partial illustration of a further exemplary embodiment of a piston  410 . The piston  410  is constructed in a similar way to the piston  210  according to  FIG. 5 , right-hand side. Structural elements which correspond are therefore provided with the same designations, and reference is made in this regard to the description with respect to  FIG. 5 . 
     The main difference consists in that the closure element  435  is formed in the manner of an encircling flange which is connected in one piece to the piston ring element  432 . The closure element  435  extends in the direction of the combustion depression  13  in such a way that the free end of the closure element  435  forms an encircling annular gap  436  together with the outer wall of the combustion depression  13 . 
     The piston  410  is likewise composed of a piston main body  431  and a piston ring element  432 . In the exemplary embodiment, the piston ring element  432  comprises a part of the depression wall and the depression edge of the of the combustion depression  13  and also the piston crown  12 , the fire land  14  and the ring belt  15 . In the exemplary embodiment, the piston ring element  432  is connected to the piston main body  431  by way of friction welding, wherein the welded seam  433  is arranged in the in the depression wall of the combustion depression  13 . 
       FIG. 8  shows, by way of example and in an enlarged partial illustration, the cooling duct  19  with cooling duct base  26  and cooling duct ceiling  27  and also the piston crown  12 , a part of the combustion depression  13 , the fire land  14 , the ring belt  15  with the ring grooves  16 ,  17 ,  18 , and also the closure element  435  of the piston  410  according to the invention as per  FIG. 7 . 
     The combustion depression  13  is provided with an undercut  29 , in order to define the wall thickness between the combustion depression  13  and the cooling duct  19  (see below in this regard). 
     It is preferred that the height h of the fire land  14  is at most 9% of the nominal diameter DN of the piston head  11  (see  FIGS. 1 and 2 ). In this way, positioning of the cooling duct  19  in relation to the piston crown  12  and the ring belt  15  is realized which is particularly advantageous for the dissipation of heat. 
     On the basis of this dimension rule for the fire land  14 , it is preferred that the spacing a between the piston crown  12  and the cooling duct base  26  is between 11% and 17% of the nominal diameter DN of the piston head  11  (see  FIGS. 1 and 2 ). In this way, the cooling duct  19  is positioned in optimum proximity to the hot piston crown  12  and in an optimum position relative to the relatively cool ring grooves  16 ,  17 ,  18 . 
     Moreover, it is preferred that the height c of the cooling duct  19  is 0.8 times to 1.7 times its width d. Said dimension rule yields an optimum volume of the cooling duct  19  and an optimum orientation relative to the hot combustion depression  13 , in particular relative to the depression edge, and relative to the hot piston crown  12  and relative to the relatively cool ring grooves  16 ,  17 ,  18 . 
     Finally, it is preferred that the spacing b between the piston crown  12  and the cooling duct ceiling  27  is between 3% and 7% of the nominal diameter DN of the piston head  11  (cf.  FIGS. 1 and 2 ). Said dimension rule also yields optimum positioning of the cooling duct  19  in relation to the hot piston crown  12 . 
     Ultimately, it is preferred that the smallest wall thickness w in the radial direction between the combustion depression  13  and the cooling duct  19  is between 2.5% and 4.5% of the nominal diameter DN of the piston head  11 . An improved thermal transfer between the combustion depression  13  and the cooling duct  19  is achieved in this way. 
       FIGS. 9 a  and 9 b  and 10 a  and 10 b    schematically show the cooling oil movement during engine operation and the temperature zones in the region of the combustion depression, of the piston crown, of the cooling duct and of the ring grooves both for a piston according to the invention ( FIGS. 9 a  and 9 b   ) and for a piston according to the prior art ( FIGS. 10 a  and 10 b   ). 
     In  FIGS. 9 a , 9 b , 10 a , 10 b   , three heat zones are schematically indicated, namely “hot”, “warm” and “cool”. The relative temperature differences in the individual piston regions are intended to be illustrated in this way. 
     According to the present invention ( FIGS. 9 a  and 9 b   ), the cooling duct is shortened in the axial direction in relation to the prior art. As a consequence, the cooling oil moves almost exclusively along the “hot” regions of the piston crown and of the combustion depression. An absorption of heat from the “hot” regions of the piston head into the cooling oil therefore takes place in every phase of the piston movement. The cooling oil quantity known from the prior art should be retained and the engine management should be set up in such a way that the cooling oil is exchanged rapidly during engine operation. 
     In the prior art ( FIGS. 10 a  and 10 b   ), the cooling duct extends in the axial direction generally as far as the level of the lowermost ring groove and below, in order to achieve sufficient cooling during engine operation with the aid of a cooling duct which is as large as possible. On account of the cocktail shaker effect, the cooling oil moves between a “hot” region, namely the piston crown and the depression edge of the combustion depression, and a “cool” region, namely the cooling duct base. On account of the considerably lower temperatures in the region of the cooling duct base, in practice heat absorption from the piston head into the cooling oil no longer takes place there. 
     As a consequence, considerably improved cooling of the piston head in relation to the prior art is realized in the case of the piston according to the invention.