Abstract:
A multi-part piston for an internal combustion engine has an upper piston part 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. The opening is closed off with a separate closure element, which has at least one cooling oil opening

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2008 055 908.3 filed Nov. 5, 2008. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a multi-part piston for an internal combustion engine, having an upper piston part 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. The cooling chamber bottom has an opening. 
         [0004]    2. The Prior Art 
         [0005]    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 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 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. 
       SUMMARY OF THE INVENTION 
       [0006]    It is therefore an object of the invention to provide a piston of the stated type, in such a manner that a good cooling effect 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. 
         [0007]    This object is accomplished according to the invention by a piston for an internal combustion engine, having an upper piston part and a lower piston part, each of the piston parts having an inner and an outer support element, which elements delimit an outer circumferential cooling channel and an inner cooling chamber. The bottom of the cooling chamber has an opening. The opening is closed off with a separate closure element, which has at least one cooling oil opening. 
         [0008]    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, only a narrow circumferential structure for holding the closure element is required. As a result, the stability of the lower piston part is maintained even if it is a forged part. 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. 
         [0009]    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 closure element can be produced from any desired material. For example, a spring steel sheet metal has proven to be well suited. 
         [0010]    The at least one cooling oil opening in the closure element can be configured as a usual round opening, or, for example, also as a slit that extends from the edge of the closure element toward the inside. 
         [0011]    A preferred further development provides that the closure element is held, in clamped manner, in at least one engagement groove provided in the region of the opening of the cooling chamber bottom, by means of at least one spring element, and thus is particularly easy to install. For this purpose, the closure element can have a circumferential clamping flange or at least two spring tongues disposed on the outer edge as a spring element. In the latter case, the slits that delimit the spring tongues can serve as cooling oil openings at the same time. In another variant, however, the closure element can also be welded to the cooling chamber bottom of the lower piston part. 
         [0012]    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. If the closure element is held in a clamped manner, it is sufficient if the closure element has at least two spring elements that lie centered opposite one another. 
         [0013]    The upper piston part and/or the lower piston part can be cast parts or forged parts, and can be produced from a steel material, for example, particularly forged. Friction welding, for example, is a possible joining method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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. 
           [0015]    In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
           [0016]      FIG. 1  shows a section through a first exemplary 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; 
           [0017]      FIG. 2  shows a section through another exemplary 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 
           [0018]      FIG. 3  shows a top view of a lower piston part for another exemplary embodiment of a piston according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    Referring now in detail to the drawings,  FIG. 1  shows a first exemplary embodiment of a piston  10  according to the invention, which is forged from a steel material in the exemplary 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 combustion bowl  13 , a circumferential top land  14 , and a circumferential ring belt  15 . Lower piston part  12  has a piston skirt  16 , pin bores  17  for accommodating a piston pin, and pin bosses  18 . Upper piston part  11  and the lower piston part  12  form a circumferential outer cooling channel  19  and a central inner cooling chamber  21 . Cooling chamber bottom  22  of cooling chamber  21  is provided with a relatively large opening  23 . 
         [0020]    Upper piston part  11  has an inner support element  24  and an outer support element  25 . Inner support element  24  is disposed on the underside of upper piston part  11 , circumferentially, in ring shape, and has a joining surface  26 . Inner support element  24  furthermore forms part of the circumferential wall of inner cooling chamber  21 . Outer support element  25  of upper piston part  11  is formed below ring belt  15 , in the exemplary embodiment, and has a joining surface  27 . 
         [0021]    Lower piston part  12  also has an inner support element  28  and an outer support element  29 . Inner support element  28  is disposed on the top of lower piston part  12 , circumferentially, and has a joining surface  31 . Inner support element  28  furthermore forms part of the circumferential wall of the inner cooling chamber  21 . Outer support element  29  is formed as an extension of piston skirt  16  in the exemplary embodiment, and has a joining surface  32 . A cooling oil channel  43  is provided in the inner support element  28 , and connects cooling channel  19  with cooling chamber  21 . Cooling oil channel  43  runs at an angle upward, proceeding from cooling channel  19 , in the direction of cooling chamber  21 . 
         [0022]    Upper piston part  11  and lower piston part  12  were joined, in the embodiment shown, in known manner, by friction welding along joining surfaces  26 ,  31  and  27 ,  32 , respectively. 
         [0023]    Opening  23  in cooling chamber bottom  22  is closed off with a closure element  33 . In the exemplary embodiment, closure element  33  is produced from a spring sheet metal, approximately 0.8 mm thick, and held in opening  23  in a clamped manner. For this purpose, a circumferential engagement groove  34  is provided in cooling chamber bottom  22  in the inner region of opening  23 . Closure element  33  is provided with slits  35  that extend radially inward, along its edge region, which slits open into a rounded part  36 . Slits  35  and rounded parts  36  serve as cooling oil openings that allow the cooling oil to flow away out of inner cooling chamber  21  in the direction of the piston pin during operation. Slits  35  and rounded parts  36  are punched out of closure element  33  in the embodiment shown. 
         [0024]    The regions delimited by the slits  35  simultaneously represent spring tongues  37  by means of which closure element  33  is held in engagement groove  34 , in a clamped manner. For assembly, closure element  33  is pressed into opening  23  of cooling chamber bottom  22 , coming from the direction of pin bores  17 . In this connection, spring tongues  37  at first give way, and then engage into engagement groove  34 . 
         [0025]      FIG. 2  shows another exemplary 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 . 
         [0026]    A significant difference from the piston  10  according to  FIG. 1  consists in the fact that cooling chamber bottom  22  does not have an engagement groove in the inner region of opening  23 . Furthermore, closure element  133  that closes off opening  23  is provided, in usual manner, round openings  138  for passage of the cooling oil out of cooling chamber  21  in the direction of the piston pin. Closure element  133  consists, in the exemplary embodiment, of a metallic material, and is welded to cooling chamber bottom  22  in the region of opening  23 . For this purpose, closure element  123  has a welding flange  139 . In the region of opening  23 , cooling chamber bottom  22  is provided with a corresponding contact edge  142  for welding flange  139 , which edge runs around opening  23 . 
         [0027]      FIG. 3  shows a top view of a lower piston part  12  for another exemplary embodiment of a piston  210  according to the invention. Piston  210 , i.e. lower piston part  12 , 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 . 
         [0028]    A significant difference from the piston  10  according to  FIG. 1  consists in the fact that cooling chamber bottom  22  in lower piston part  12  has an opening  223  in the approximate shape of an oblong hole opening  223  is closed off with a closure element  233  that is configured to essentially correspond to opening  223 , in order to be able to close this off completely. 
         [0029]    In the exemplary embodiment, closure element  233  is also produced from a spring sheet metal, and held in opening  223  in a clamped manner. For this purpose, two engagement grooves  234  disposed in a centered manner and lying opposite one another are provided in cooling chamber bottom  22 , in the interior region of opening  223 . In the exemplary embodiment, closure element  233  is provided with slits  35  that are disposed centered, lying opposite one another, extending radially inward, which open into a rounded part  36 . In the exemplary embodiment, three slits  35  lie opposite one another. Slits  35  and rounded part  36  serve as cooling oil openings that allow the cooling oil to flow away out of inner cooling chamber  21  in the direction of the piston pin during operation. Slits  35  and rounded parts  36  are punched out of closure element  233  in the exemplary embodiment. 
         [0030]    The regions delimited by slits  35  simultaneously represent spring tongues  37  by means of which closure element  233  is held in the engagement grooves  234 , in a clamped manner. For assembly, closure element  233  is pressed into opening  223  of cooling chamber bottom  22 , coming from the direction of pin bores  17 . In this connection, spring tongues  37  at first give way, and then engage into engagement grooves  234 . Regions  241  of closure element  233  that follow the clamping region make contact below opening  223  of cooling chamber bottom  22 . 
         [0031]    In this representation, in particular, it can easily be seen that the broad, radially circumferential ring lands required in the state of the art have been eliminated. 
         [0032]    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.