Patent Publication Number: US-7213562-B2

Title: Piston for a combustion engine and casting method for the production thereof

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Applicant claims priority under 35 U.S.C. 119 of German Patent Application No. 103 25 917.1 filed Jun. 7, 2003. Applicant also claims priority under 35 U.S.C. §365 of PCT/DE2004/001151 filed Jun. 7, 2004. 
   The invention relates to a piston for an internal combustion engine in accordance with the preamble of claim  1 , and a method for its production, in accordance with the preamble of claim  2 . 
   A piston is known from the Offenlegungsschrift DE 199 22 809 A1, having pin bosses molded onto a cylindrically configured upper region of the piston, which are recessed as compared with the edge of the upper region, so that when the piston is cast, recesses can be formed in the underside of the overhang formed thereby, close to the pin bosses. For this purpose, a casting mold is used that contains a pivoting window insert having one casting core per recess, but this can only produce those recesses from which the casting core can easily be pulled out after casting. This has the disadvantage that in this way, the weight of the upper region of the piston cannot be reduced to such an extent that the center of gravity of the piston can be displaced as far as possible into the lower region of the piston, to achieve problem-free engine operation. 
   It is known from Offenlegungsschrift DE 101 42 980 A1 to set a mold body onto a casting tool that can be displaced at a slant to the piston axis, which can be used to produce a recess under the head of the piston, behind its ring field, when the piston is cast, which recess has a nose-shaped undercut that points in the direction of the piston axis. After the piston is cast, the casting tool is pulled away from the piston, towards the bottom, at a slant, whereby the mold body remains in the piston for a short time, before it is washed out. 
   It is a disadvantage in this connection that a very complicated mechanism is required to hold the casting tool precisely in the intended position during casting of the piston, and to move it away from the piston at a downward slant after casting, to pull it out of the mold body. Furthermore, the mold part known from the state of the art does not have any indentations that allow production of ribs in the recess, and this brings with it the additional disadvantage that the piston known from the last prior art reference has only a low mechanical strength. 
   A piston having a combustion chamber bowl in the piston head is known from the European patent application EP 0 364 810 A2, the underside of which is connected with pin bosses by way of ridges. In the region of the piston head and radially outward, the piston furthermore has a recess that is open downward, in the form of a cooling channel, the radially inner region of which is divided by the ridges. However, since the ridges do not reach to the belt that delimits the recess radially on the outside, there is the disadvantage that the mechanical strength of the region on the piston head side of the piston known from the EP application is very low. 
   Proceeding from this, the invention is based on the task of avoiding the disadvantages of the cited prior art. This task is accomplished with the characteristics found in the characterizing parts of the main claim and of the ancillary claim. A practical embodiment of the invention is the object of the dependent claim. 
   In this connection, the advantages of a pivoting window insert with which recesses can be produced in simple manner, in terms of casting technology, for which slides that can be moved in linear manner are unsuitable, are combined with the advantages of a salt core, which offers an unlimited potential of possibilities of configuring cavities in pistons. 

   
     An exemplary embodiment of the invention will be described below, using the drawings. These show: 
       FIG. 1  a piston in partial cross-section, after casting, with the salt mold part inserted, attached to a pivoting window insert, 
       FIG. 2  the piston with the salt mold part inserted, and the window insert, which has come loose from the salt mold part, pivoted out, 
       FIG. 3  the piston in partial cross-section, after the salt mold part has been washed out, and 
       FIG. 4  a view of the piston according to the invention from below. 
   

   In  FIG. 1 , a piston  1  is shown in half cross-section, the left half of which shows a cross-section of the piston lying on the longitudinal axis  2  of a piston pin, not shown, and the right half of which shows a side view of the piston  1 , in which a coated region  3  of a piston skirt  4  can be seen. The piston  1  is configured as a box-type piston, i.e. a lower region  6  that has an approximately box-shaped cross-section is located below an upper region  5  in the shape of a circular cylinder, which accommodates the piston rings; in the former, the bosses  7  and the box walls in the region of the bosses  7  are drawn in towards the central piston axis  8 , and only the skirt wall sections that lie in the region of the major thrust side and the minor thrust side have the shape, in cross-section, of segments of a circle whose diameter corresponds to the piston diameter. 
   The piston  1  has two raised regions  9  and  10 , which are disposed on the two sides of a combustion chamber bowl  11  molded into the piston head. To ensure that these raised regions  9  and  10  do not displace the center of gravity of the piston  1  too far upward in the direction of the combustion chamber bowl  11 , during casting of the piston  1 , recesses  12  that are open towards the lower region  6  of the piston  1  are formed in the combustion chamber bowls  9 ,  10 , and furthermore undercuts  13  are formed in the region between the bosses  7  and the upper region  5 , which are configured to be nose-shaped in the present exemplary embodiment and are oriented towards the central piston axis  8 , in each instance, but which can also have a different shape that is suitable for reducing the weight of the piston. Furthermore, the center of gravity of the piston  1  is moved in the direction of the bosses  7  by means of the material that is saved in this connection. 
   Here, the casting mold for casting the piston  1  has a pivoting window insert  14  for each of the recesses  12  to be produced in the two raised regions  9  and  10 . In order to be able to mold the undercuts  13  in the upper region  5  of the piston  1  with this insert, a pre-finished salt mold part  15  is applied to the window insert  14  before the piston  1  is cast; the shape of this mold part is identical with the shape of the recess  12 , including the undercut  13 . The salt mold part  15  is attached to the window insert  14 , so as not to rotate, by way of two cone-shaped extensions  16  that are disposed on the window insert  14  onto which the salt mold part  15  is set. In  FIGS. 1 and 2 , the window insert  14  is shown in cross-section, and for this reason only one of the two extensions  16  can be seen. The piston blank  1  is cast with the window insert  14  pivoted in and the salt mold part  15  attached to it. 
   Subsequent to the casting process, according to  FIG. 2 , the window insert  14  is pivoted clockwise, at the same time as the rest of the casting mold, not shown in the figures, is opened, whereby the salt mold part  15  is released from the window insert  14  and remains in the piston blank  1 , in order to subsequently be washed out. 
   In this connection, the piston blank  1  shown in  FIG. 3  is formed, which has the recesses  12  with the undercuts  13 , whereby these recesses  12 , as also shown in  FIG. 4 , are divided by ribs  17 ,  17 ′, which serve to improve the shape stability of the walls  18  of the raised regions  9  and  10 . This allows configuring the walls  18  to be very thin, in order to thereby achieve improved cooling of the upper region  5  of the piston  1  by means of spraying it with cooling oil. 
   The view of the piston from below, according to  FIG. 4 , shows the kidney-shaped configuration of the recesses  12 ,  12 ′, with the undercuts  13 ,  13 ′ shown with broken lines, which undercuts are divided by two ribs  17 ,  17 ′, in each instance, in the present exemplary embodiment. In the production of the salt mold part  15  used here, the shape and the arrangement of the ribs  17 ,  17 ′ can be taken into consideration in simple manner, by means of a corresponding configuration of the salt mold part  15 . In the exemplary embodiment shown, the shape of the salt mold part  15  corresponds to the negative mold of the recesses  12 ,  12 ′ and the undercuts  13 ,  13 ′. In this connection,  FIGS. 1 and 2  show cross-sectional representations of the salt mold part  15 , which is configured in one piece and has indentations for forming the ribs  17 ,  17 ′ shown in  FIGS. 3 and 4 , which indentations correspond in their shape and depth to the ribs  17  according to  FIG. 3 , and the arrangement of which corresponds to the arrangement of the ribs  17 ,  17 ′ according to  FIG. 4 . 
   REFERENCE SYMBOL LIST 
   
       
         1  piston, piston blank 
         2  piston axis 
         3  coated region 
         4  piston skirt 
         5  upper region 
         6  lower region 
         7  boss 
         8  central piston axis 
         9  lateral raised region 
         10  lateral raised region 
         11  combustion chamber bowl 
         12 ,  12 ′ recess 
         13 ,  13 ′ undercut 
         14  window insert 
         15  salt mold part 
         16  cone-shaped extension 
         17 ,  17 ′ ribs 
         18  wall