Abstract:
A multi-part piston for an internal combustion engine has an upper piston part with a piston crown, and a lower piston part, each of the piston parts having an inner and an outer support element that delimit an outer circumferential cooling channel and an inner cooling chamber. The cooling chamber bottom has an opening. A holding element is disposed in the inner cooling chamber and extends from the underside of the piston crown vertically toward the opening. The holding element carries a closure element that closes the opening and has at least one cooling oil opening.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional of U.S. patent application Ser. No. 12/381,838, filed on Mar. 17, 2009, which claims priority under 35 U.S.C. §119 of German Application No. 10 2008 055 909.1 filed Nov. 5, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a multi-part piston for an internal combustion engine, having an upper piston part that has a piston crown, and a lower piston part. Each of the piston parts has an inner and an outer support element, which elements delimit an outer circumferential cooling channel and an inner cooling chamber, whose cooling chamber bottom has an opening. 
     2. The Prior Art 
     A piston of this type is disclosed in European Patent No. EP 1 222 364 B1. The opening in the cooling chamber bottom allows cooling oil to flow away out of the inner cooling chamber in the direction of the piston crown, in order to achieve a cooling effect as a consequence of the oil passage from the outer circumferential cooling channel to the inner cooling chamber, and to lubricate the piston pin. In order to achieve this goal, the opening in the cooling chamber bottom cannot be too large, because then, the cooling oil would no longer flow away in a metered manner, and its cooling effect in the inner cooling chamber would at least be reduced. This means that the cooling chamber bottom is configured essentially as a relatively wide and thin circumferential ring land that extends approximately in the radial direction, in the upper region of the lower piston part. However, such a structure is difficult to produce. In the case of a forged lower piston part, in particular, there is the additional problem that the microstructure of the material is changed in the region of the ring land, as the result of forging, and this results in an increase in stress in the material structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide a piston of the stated type, in such a manner that good cooling of the cooling oil in the interior of the cooling chamber and effective lubrication of the piston pin are guaranteed, and, at the same time, the stability of the lower piston part is not impaired. 
     This object is achieved according to the invention with a multi-part piston for an internal combustion engine, having an upper piston part that has a piston crown, and a lower piston part. The upper piston part and the lower piston part each have an inner and an outer support element, which elements delimit an outer circumferential cooling channel and an inner cooling chamber, whose cooling chamber bottom has an opening. A holding element that extends from the underside of the piston crown vertically toward the opening is provided in the inner cooling chamber, which holding element carries a closure element that closes the opening and has at least one cooling oil opening. 
     The configuration according to the invention makes it possible to provide a very large opening in the cooling chamber bottom, so that the relatively wide and thin circumferential ring land, which extends approximately in the radial direction, is eliminated. Instead, the opening is closed off with a closure element that is fixed in place by way of a holding element that is connected with the underside of the piston crown. As a result, the stability of the lower piston part is maintained even if it is a forged part. The inner cooling chamber is configured as a circumferential inner cooling channel as the result of the introduction of the holding element, so that the cooling oil is distributed more uniformly and its cooling effect is therefore improved. The at least one cooling oil opening in the closure element provided according to the invention also allows significantly better and more precise metering of the cooling oil that flows away in the direction of the piston pin. 
     The closure element preferably has two or more cooling openings, so that a very precisely metered amount of cooling oil can flow away out of the inner cooling chamber, in the direction of the piston crown. 
     The opening in the cooling chamber bottom and the closure element are generally configured to be essentially round. If the opening in the cooling chamber bottom is configured to be oval or an oblong hole, it is practical if the closure element has a shape that corresponds to this, in order to completely cover the opening. 
     A preferred embodiment provides that the holding element is formed onto the underside of the piston crown, in one piece. As an alternative to this, however, the holding element can also be configured as a separate component and can be held on the underside of the piston crown. The selection is at the discretion of the person skilled in the art, and allows flexible adaptation of the piston properties to the requirements in each operation. 
     If the holding element is configured as a separate component, it can be provided with a conical depression, for example. The underside of the piston crown then has a conical elevation that corresponds to this. The holding element is held between the underside of the piston crown and the closure element, with force fit, i.e. in clamped manner, whereby the depression and the elevation engage into one another. This method of construction is particularly easy to implement. 
     However, the separate holding element can also have a journal, for example, which is accommodated in a corresponding dead-end hole on the underside of the piston crown. The shape-fit connection of piston crown and holding element brings about a particularly good seat of the holding element, and therefore particularly great stability of the piston according to the invention. 
     Independent of how the holding element is attached to the underside of the piston crown, the end of the holding element that faces the opening can have a circumferential contact shoulder that lies on the closure element. The shoulder surrounds a projection that engages into a recess provided in the closure element. Another possibility of attaching the holding element to the closure element consists, for example, in the fact that the end of the holding element that faces the opening has a circumferential groove, into which the closure element engages. Here, too, the shape-fit connection of holding element and closure element offers a particularly reliable, stable hold. 
     It is practical if the length of the holding element is dimensioned so that the closure element supports itself on the cooling chamber bottom under resilient bias, and thus no longer has any lateral play. The holding element is thereby fixed in place in a particularly firm manner, above the opening in the cooling chamber bottom. 
     In another preferred embodiment of the piston according to the invention, the holding element is configured as a screw or threaded pin, and the underside of the piston crown has a threaded dead-end hole that corresponds to this, in which the holding element is accommodated. The effect of force on the closure element can therefore take place also on its underside. It is practical if the end of the holding element that faces the opening has a circumferential or interrupted flange that engages underneath the closure element. 
     Preferably, the opening is provided with a circumferential holding collar that is directed radially inward, and the closure element engages underneath the holding collar with its outer edge. This embodiment has the advantage that it can be assembled even after the upper piston part and lower piston part have been connected. 
     The closure element can be made from any desired material. In particular, a spring steel sheet has proven to be well suited. The upper piston part and/or the lower piston part can be cast parts or forged parts, and can be produced, for example, from a steel material, particularly forged steel. Friction welding is a possibility for the joining method. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
       In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
         FIG. 1  shows a section through a first embodiment of a piston according to the invention, whereby the right half of the figure has been rotated by 90° relative to the left half; 
         FIG. 2  shows a section through another embodiment of a piston according to the invention, whereby the right half of the figure has been rotated by 90° relative to the left half; 
         FIG. 3  shows a section through another embodiment of a piston according to the invention, whereby the right half of the figure has been rotated by 90° relative to the left half; and 
         FIG. 4  shows a section through another embodiment of a piston according to the invention, whereby the right half of the figure has been rotated by 90° relative to the left half. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings and, in particular,  FIG. 1  shows a first embodiment of a piston  10  according to the invention, which is forged from a steel material in this embodiment. Piston  10  according to the invention is composed of an upper piston part  11  and a lower piston part  12 . Upper piston part  11  has a piston crown  13  having a combustion bowl  14 , a circumferential top land  15 , and a circumferential ring belt  16 . Lower piston part  12  has a piston skirt  17 , pin bores  18  for accommodating a piston pin, and pin bosses  19 . Upper piston part  11  and the lower piston part  12  form a circumferential outer cooling channel  21  and a central inner cooling chamber  22 . Cooling chamber bottom  23  of cooling chamber  22  is provided with a relatively large opening  24 . 
     Upper piston part  11  has an inner support element  25  and an outer support element  26 . Inner support element  25  is disposed on the underside of upper piston part  11 , circumferentially, in ring shape, and has a joining surface  27 . Inner support element  25  furthermore forms part of the circumferential wall of the inner cooling chamber  22 . Outer support element  26  of the upper piston part  11  is formed below ring belt  16 , and has a joining surface  28 . 
     Lower piston part  12  also has an inner support element  31  and an outer support element  32 . Inner support element  31  is disposed on the top of lower piston part  12 , circumferentially, and has a joining surface  33 . Inner support element  31  furthermore forms part of the circumferential wall of inner cooling chamber  22 . Outer support element  32  is formed as an extension of piston skirt  17  in the embodiment shown, and has a joining surface  34 . One or more cooling oil channels  35  are provided in inner support element  31 , and connect cooling channel  21  with cooling chamber  22 . Cooling oil channel  35  runs at an angle upward, proceeding from cooling channel  21 , in the direction of cooling chamber  22 . 
     Upper piston part  11  and lower piston part  12  were joined, in the embodiment shown, in known manner, by means of friction welding along joining surfaces  27 ,  28  and  33 ,  34 , respectively. 
     Opening  24  in cooling chamber bottom  23  is closed off with a closure element  36 . In the embodiment shown, closure element  36  is produced from a spring sheet metal, approximately 0.8 mm thick, and has multiple cooling oil openings  37 , which allow the cooling oil to flow away from inner cooling chamber  22  in the direction of the piston crown during operation. 
     A holding element  38 , which has approximately the shape of a journal in the embodiment shown, is formed on in one piece on the underside of piston crown  13 , and projects into center axis M of piston  10 , vertically, in the direction of opening  24 . At its free end, holding element  38  has a projection  39  that is surrounded by a circumferential contact shoulder  41 . Projection  39  passes through a central recess  42  provided in closure element  36 , whereby contact shoulder  41  lies on the top of closure element  36 . The length of holding element  38  is dimensioned in such a manner in this embodiment, that closure element  36  supports itself on cooling chamber bottom  23  under spring bias. Closure element  36  is therefore held securely and without play. 
       FIG. 2  shows a second embodiment of a piston  110  according to the invention. Piston  110  has essentially the same construction as piston  10  according to  FIG. 1 , so that the same structures are provided with the same reference symbols, and with regard to these reference symbols, reference is made to the description of  FIG. 1 . 
     A significant difference as compared with piston  10  according to  FIG. 1  consists in the fact that in piston  110 , the holding element  138  is present as a separate component. In the embodiment shown, holding element  138  is provided with a conical depression  143  at its end that faces piston crown  13 . The underside of piston crown  13  has a corresponding conical elevation  144 . Holding element  138  has a projection  139  at its end that faces closure element  36 , which projection is surrounded by a circumferential contact shoulder  141 . Projection  139  passes through a central recess  42  provided in closure element  36 , whereby contact shoulder  141  lies on the top of closure element  36 . The length of holding element  138  is dimensioned in such a way, in the embodiment shown, that closure element  36  supports itself on cooling chamber bottom  23  under resilient bias, and the conical depression  143  and conical elevation  144  engage into one another. Closure element  36  is therefore held securely and without play. 
       FIG. 3  shows a third embodiment of a piston  210  according to the invention. Piston  210  has essentially the same construction as piston  10  according to  FIG. 1 , so that the same structures are provided with the same reference symbols, and with regard to these reference symbols, reference is made to the description of  FIG. 1 . 
     In the case of piston  210 , as well, holding element  238  is configured as a separate component. In contrast to piston  110  according to  FIG. 2 , holding element  238  has a journal  245  at its end that faces piston crown  13 . The underside of piston crown  13  is provided with a corresponding dead-end hole  246 , in which journal  245  is accommodated. Holding element  238  has a circumferential groove  247  at its end that faces closure element  36 , in which groove closure element  36  is held by snapping it in. The length of holding element  238  is dimensioned in such a way, in the embodiment shown, that closure element  36  supports itself on cooling chamber bottom  23  under resilient bias. Closure element  36  is therefore held securely and without play. 
     Of course, closure element  36  in these embodiments can also consist of a non-resilient, preferably metallic material, and be held on cooling chamber bottom  23  with a clamping action, i.e. with force fit. 
     For assembly of these embodiments, holding element  138 ,  238 , as applicable, is attached to upper piston part  11 , and then closure element  36  is attached to holding element  38 ,  138 ,  238 . After upper piston part  11  and lower piston part  12  have been connected, closure element  36  lies firmly on the cooling chamber bottom. 
       FIG. 4  shows a fourth embodiment of a piston  310  according to the invention. Piston  310  has essentially the same construction as piston  10  according to  FIG. 1 , so that the same structures are provided with the same reference symbols, and with regard to these reference symbols, reference is made to the description of  FIG. 1 . 
     The significant difference as compared with all the embodiments described until now consists in the fact that in the embodiment of  FIG. 4 , holding element  338  is configured as a threaded pin. In place of a threaded pin, of course, a screw can also be used. The underside of piston  13  is provided with a corresponding threaded dead-end hole  348 , into which holding element  338  is screwed. The end of holding element  338  that faces opening  24  has a circumferential or interrupted flange  349  (in the case of a screw: a screw head). Holding element  338  passes through the central bore provided in the closure element, from the underside of closure element  36  that faces the piston pin. Thus, closure element  36  is not on cooling chamber bottom  23 , but rather on the underside of cooling chamber bottom  23 , with force fit, if applicable under resilient bias. For this purpose, the edge of opening  24  is provided, in the embodiment shown, with a circumferential holding collar  351  that is directed radially inward, on which collar closure element  36  lies with its outer edge  352  and engages underneath the holding collar  351 . 
     Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.