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, a circumferential fire land, a circumferential ring belt having a plurality of ring grooves, and a circumferential cooling duct. The cooling duct may be open in a direction away from the fire land and may be at least partially closed by a closure element. The cooling duct may include a cooling duct bottom and a cooling duct ceiling. The piston skirt may have at least two piston bosses connected to one another via at least two running faces. At least one running face may have an inner face connected via a connecting land to an underside of the piston head.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2013 009 164.0, filed May 31, 2013, and International Patent Application No. PCT/DE2014/000263, 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, a circumferential fire land, a circumferential ring belt with ring grooves and, in the region of the ring belt, a circumferential cooling duct which is open toward the bottom and is closed by way of a closure element, the cooling duct having a cooling duct bottom and a cooling duct ceiling, and the piston skirt having two piston bosses which are connected to one another via two running faces. 
     BACKGROUND 
     In modern internal combustion engines, the pistons are subjected to ever higher mechanical and thermal loads in the region of the piston crown and the combustion bowl. In addition to optimization of the piston cooling, it is therefore necessary to provide the piston firstly with the necessary stability, in order to withstand the mechanical loads which occur, and secondly to design the piston to be so flexible that damage, in particular cracks, are avoided which might be caused by way of said mechanical loads. 
     SUMMARY 
     It is the object of the present invention to develop a piston of the generic type in such a way that an optimized balance between stability and flexibility is achieved and at the same time the cooling is improved. 
     The object is achieved by virtue of the fact that the inner face of exclusively one running face of the piston is connected via a connecting land to the underside of the piston head. 
     The piston according to the invention is therefore of asymmetrical construction. One of its running faces is attached to the two piston bosses. The other running face is additionally attached to the underside of the piston head. This construction ensures both satisfactory stability (additional attachment of one running face to the underside of the piston head), but secondly also a certain flexibility (attachment of one running face merely to the piston bosses). It is unimportant here whether the additional attachment of one running face to the underside of the piston head is provided on the pressure side or on the counter pressure side of the piston. Furthermore, the connecting land which connects one of the running faces to the underside of the piston head can be used to direct an oil jet onto the surface of the connecting land in a targeted manner during engine operation, in such a way that the underside of the piston head is cooled in a targeted manner. In this way, the cooling of the piston according to the invention is also improved. 
     Advantageous developments result from the subclaims. 
     The compression height can be, for example, between 38% and 45% of the nominal diameter of the piston head. 
     One advantageous development provides that the closure element is arranged in the piston head in such a way that a circumferential annular gap is configured in the piston crown. This dispenses with the necessity of providing oil outlet openings. 
     If the piston skirt is decoupled, the closure element can be configured as a separate component which is fastened to the piston. 
     The piston according to the invention can be configured 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 can have a main piston body and a piston ring element. In this case, the closure element can be configured 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 can be connected in one piece either to the main piston body or to the piston ring element. 
     The cooling duct can extend in the axial direction as a rule 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 to and fro between the cooling duct ceiling, that is to say a very hot region, and the cooling duct bottom, that is to say a comparatively cool region. On account of the considerably lower temperatures in the region of the cooling duct bottom, in practice heat absorption from the piston head into the cooling oil no longer takes place there. 
     Particularly effective cooling is therefore preferably achieved by virtue of the fact that the cooling duct is shortened in the axial direction. As a consequence, the cooling oil moves, in particular in the region of the cooling duct bottom, in closer proximity to the highly thermally loaded cooling duct bottom and therefore overall in hotter regions than in a cooling duct which extends as far as the lowermost ring groove or below. Heat absorption from the hot regions of the piston head into the cooling oil therefore takes place in every phase of the piston movement. Particularly effective cooling of the piston head results, 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. 
     The cooling duct bottom 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. 
     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 which is particularly advantageous for the dissipation of heat is brought about. 
     In this case, the spacing between the piston crown and the cooling duct bottom can be between 11% and 17% of the nominal diameter of the piston head. In addition or instead, the height of the cooling duct can be from 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 can 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. 
     A further particularly preferred embodiment consists in that a combustion bowl is configured in the piston head, and that the smallest wall thickness in the radial direction between the combustion bowl 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 bowl and the cooling duct is achieved in this way. 
     The combustion bowl can be provided, for example, with an undercut, in order to define the wall thickness between the combustion bowl and the cooling duct. 
     The present invention is suitable both for pistons made from at least one steel material and for pistons made from at least one light metal alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following text, exemplary embodiments of the present invention will be explained in greater detail using 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 by 90°, 
         FIG. 3  shows a further exemplary embodiment of a piston according to the invention in section, 
         FIG. 4  shows a further exemplary embodiment of a piston according to the invention in section, 
         FIG. 5  shows a further exemplary embodiment of a piston according to the invention in section, 
         FIG. 6  shows an enlarged partial illustration of a further exemplary embodiment in section, 
         FIGS. 7 a , 7 b    show a diagrammatic illustration of the cooling oil movement in a piston according to the present invention, and 
         FIGS. 8 a , 8 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  can be forged or cast as a single-piece blank, the cooling duct being introduced into the blank by way of machining. In the exemplary embodiment, the piston  10  is assembled from a main piston body  31  and a piston ring element  32  which can 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 bowl 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. However, it can also be produced from a light metal material or a combination of both materials. 
     The piston  10  has a piston head  11  with a piston crown  12  which has a combustion bowl  13 , a circumferential fire land  14  and a circumferential ring belt  15  with ring grooves  16 ,  17 ,  18  for receiving piston rings (not shown). A circumferential cooling duct  19  is provided at the level of the ring belt  15 . 
     Furthermore, the piston  10  has a piston skirt  21  which is decoupled thermally from the piston head  11  with piston bosses  22  and boss bores  23  for receiving a gudgeon pin (not shown). The piston bosses  22  are connected via boss attachments  24  to the underside  11   a  of the piston head  11 . The piston bosses  22  are connected to one another via running faces  25   a ,  25   b.    
     The cooling duct  19  is configured such that it is open at 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 bowl  13  in such a way that the annular free end of the closure element  35  forms a circumferential annular gap  36  together with the outer wall of the combustion bowl  13 . 
     According to the invention, the inner face  37  of exclusively one running face, namely the running face  25   a  of the piston  10 , is connected via a connecting land  38  to the underside  11   a  of the piston head  11 . 
     During engine operation, a cooling oil jet can be directed along the inner face  37  of the running face  25   a  in the direction of the surface of the connecting land  38 , in order to improve the cooling of the underside  11   a  of the piston head  11 , as indicated by the arrow P. 
     For further improvement of the cooling of the piston  10 , the closure element  35  is curved in the direction of the piston crown  12  in such a way that a cooling duct bottom  26  is formed which lies approximately at the level of the second ring groove  17  in the exemplary embodiment. The cooling duct bottom  26  can 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 . 
       FIG. 3  shows 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 coincide 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 essential difference between the piston  110  according to  FIG. 3  and the piston  10  according to  FIGS. 1 and 2  consists in the fact that the closure element  135  is configured as an annular disk which completely closes the cooling duct  119 . In this case, inlet and outlet openings for cooling oil are provided in the closure element  135 . The cooling duct bottom  126  of the resulting cooling duct  119  therefore lies approximately at the level of the lowermost ring groove  18 . 
       FIG. 4  shows a further exemplary embodiment of a piston  210  according to the invention. The piston  210  is constructed in a similar way to the piston  10  according to  FIGS. 1 and 2 . Structural elements which coincide 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 essential differences consist firstly in the design of the main piston body  231  and the piston ring element  232  and secondly in the fact that the piston  210  has a closure element  235  of different design in comparison with the piston  10  according to  FIGS. 1 and 2 . 
     The piston  210  has a closure element  235  in the form of a circumferential flange which is connected in one piece to the main piston body  231 . The closure element  235  extends in the direction of the ring belt  15  in such a way that its free end forms a circumferential annular gap  236  together with the inner wall of the ring belt  15 . The closure element  235  forms the cooling duct bottom  226 . In the exemplary embodiment, the cooling duct bottom  226  lies approximately between the first ring groove  16  and the second ring groove  17 . Furthermore, the cooling duct  219  has a cooling duct ceiling  227 . 
     In the exemplary embodiment, the piston ring element  232  of the piston  210  comprises a part of the piston crown  12 , the fire land  14  and the ring belt  15 . The piston ring element  232  can be connected to the main piston body  231 , in particular, by way of a welding method, for example electron beam welding or laser welding, the welded seam  233  being arranged in the piston crown. 
       FIG. 5  shows a further exemplary embodiment of a piston  310  according to the invention. The piston  310  is constructed in a similar way to the piston  210  according to  FIG. 4 . Structural elements which coincide are therefore provided with the same designations, and reference is made in this regard to the description with respect to  FIG. 4 . 
     The essential difference between the piston  310  according to  FIG. 5  and the piston  210  according to  FIG. 4  consists in the fact that the closure element  335  is connected in one piece to the main piston body  331  in such a way that the cooling duct bottom  326  of the resulting cooling duct  319  lies approximately at the level of the lowermost ring groove  18 . The closure element  335  extends in the direction of the ring belt  15  which is formed by the piston ring element  332 , in such a way that the free end of said closure element  335  forms a circumferential annular gap  336  together with the inner wall of the ring belt  15 . 
       FIG. 6  shows an enlarged partial illustration of a further exemplary embodiment of a piston  410 , in which the closure element  435  is configured in the form of a circumferential flange which is connected in one piece to the piston ring element  432 . The closure element  435  extends in the direction of the combustion bowl  13  which is formed by the main piston body  431 , in such a way that the free end of the closure element  435  forms a circumferential annular gap  436  together with the outer wall of the combustion bowl  13 . 
     The combustion bowl  13  is provided with an undercut  429 , in order to determine the wall thickness between the combustion bowl  13  and the cooling duct  419  (see below in this regard). 
     The following details apply to pistons  10 ,  210 ,  410  according to  FIGS. 1, 2, 4 and 6 . 
     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  419  in relation to the piston crown  12  and the ring belt  15  which is particularly advantageous for the dissipation of heat is brought about. 
     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 bottom  426  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  419  is positioned in optimum proximity to the hot piston crown  12  and in an optimum position relative to the cooler ring grooves  16 ,  17 ,  18 . 
     Moreover, it is preferred that the height c of the cooling duct  419  is from 0.8 times to 1.7 times its width d. Said dimension rule brings about an optimum volume of the cooling duct  419  and an optimum orientation relative to the hot combustion bowl  13 , in particular to the bowl edge, and to the hot piston crown  12  and to the cooler ring grooves  16 ,  17 ,  18 . 
     Finally, it is preferred that the spacing b between the piston crown  12  and the cooling duct ceiling  427  is between 3% and 7% of the nominal diameter DN of the piston head  11  (cf.  FIGS. 1 and 2 ). Said dimension rule also brings about optimum positioning of the cooling duct  419  in relation to the hot piston crown  12 . 
     Ultimately, it is preferred that the lowest wall thickness w in the radial direction between the combustion bowl  13  and the cooling duct  419  is between 2.5% and 4.5% of the nominal diameter DN of the piston head  11 . An improved thermal transfer between the combustion bowl  13  and the cooling duct  419  is achieved in this way. 
       FIGS. 7 a  and 7 b  and 8 a  and 8 b    diagrammatically show the cooling oil movement during engine operation and the temperature zones in the region of the combustion bowl, the piston crown, the cooling duct and the ring grooves both for a piston according to the invention with an axially shortened cooling duct ( FIGS. 7 a  and 7 b   ) and for a piston with a cooling duct which extends over all three ring grooves ( FIGS. 8 a  and 8 b   ). 
     In  FIGS. 7 a , 7 b , 8 a , 8 b   , three heat zones are indicated diagrammatically, namely “hot”, “warm” and “cool”. The relative temperature differences in the individual piston regions are intended to be illustrated in this way. 
     According to  FIGS. 7 a  and 7 b   , the cooling duct is shortened in the axial direction. As a consequence, the cooling oil moves almost exclusively along the “hot” regions of the piston crown and the combustion bowl. 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 usual cooling oil quantity 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. 
     According to  FIGS. 8 a  and 8 b   , the cooling duct extends in the axial direction approximately as far as the level of the lowermost ring groove or else under this, 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 bowl edge of the combustion bowl, and a “cool” region, namely the cooling duct bottom. On account of the considerably lower temperatures in the region of the cooling duct bottom, in practice heat absorption from the piston head into the cooling oil no longer takes place there. 
     As a consequence, further improved cooling of the piston head results in the case of pistons with an axially shortened cooling duct.