Patent Publication Number: US-2010122681-A1

Title: Two-Part piston for an internal combusion engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Applicant claims priority under 35 U.S.C. 119 of German Application No. 10 2008 058 190.9 filed Nov. 20, 2008. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a two-part piston for an internal combustion engine. 
     2. The Prior Art 
     A two-part cooled piston for an internal combustion engine having an upper piston part and a lower piston part is described in German Patent Application No. DE 102 57 022 A1, in which the piston parts are screwed together by way of a threaded pin disposed on the upper piston part and a threaded bore worked into the lower piston part. This piston has the disadvantage that during engine operation, an axially directed force that goes beyond the stretching limit of the threaded pin material can be exerted, which leads to permanent longitudinal stretching of the threaded pin and thus to loosening of the screw connection between the upper part and the lower part of the piston. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to avoid this disadvantage of the state of the art. This object is accomplished according to the invention by a two part piston for an internal combustion engine comprising an upper piston part having a ring wall and a ring belt, and a lower piston part that forms an outer, ring-shaped cooling channel with the upper piston part and an inner, ring-shaped cooling channel. The piston has a box-shaped piston skirt having two pin bosses connected with it. The upper piston part has a threaded pin disposed on the side facing the lower piston part, coaxial to the longitudinal axis of the piston. The lower piston part has a threaded bore disposed on the side facing the upper piston part, coaxial to the longitudinal axis of the piston. The threaded bore has an inside thread that matches the thread of the threaded pin. The threaded pin can be screwed into the threaded bore in order to assemble the upper piston part to the lower piston part. The threaded pin has a stretching limit that is higher in comparison with the rest of the piston. 
     The stretching limit of the threaded pin can be increased by plastic deformation, i.e., stretching or compression, of the threaded pin in the axial direction, prior to assembly of the upper piston part with the lower piston part. This increases the stretching limit to such an extent that at the tensile stress on the threaded pin that is usual during engine operation, there is no risk of plastification of the threaded pin material and thus of loosening of the screw connection between upper piston part and lower piston part. 
     The threaded pin can be plastically deformed by 0.5% to 5% of its length, and preferably by about 1% of its length, in the axial direction, proceeding from the finished upper piston part, in order to increase the stretching limit of the pin. For example, the threaded pin can be stretched in the axial direction toward the lower piston part in order to increase the stretching limit of the threaded pin. The plastic deformation causes a lasting stretching of the piston pin material, so that the stretching limit of the pin is increased as compared with the rest of the piston. 
    
    
     
       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 drawing. It is to be understood, however, that the drawing is designed as an illustration only and not as a definition of the limits of the invention. 
       The drawing shows a sectional diagram of a piston, whose left half represents a half-section of the piston in the pin direction, and whose right half represents a half-section of the piston in the pressure/counter-pressure direction. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The FIGURE shows a two-part cooled piston  1  that consists of an upper piston part  2  that has a combustion bowl  3  and a ring wall  4  having a ring belt  5 , and of a lower piston part  6  that comprises a box-shaped piston skirt  7  and two pin bosses  8 , each having a pin bore  9  for accommodating a piston pin, not shown in the FIGURE, connected with it. Upper piston part  2  and lower piston part  6  delimit an outer ring-shaped cooling channel  10  and an inner cooling channel  11  disposed concentric to the former. Outer cooling channel  10  has at least one inflow opening  12  for introducing cooling oil, and is connected with inner cooling channel  11  by way of at least one overflow channel  13 . Overflow channel  13  can be configured as a bore. For example, two overflow channels  13  that lie opposite one another can be provided. Inner cooling channel  11  has at least one run-off bore  14 , by way of which the cooling oil can exit from inner cooling channel  11 . 
     Upper piston part  2  can be mounted on an upper contact surface of a ring-shaped support rib  17  of lower piston part  6 , by way of a ring-shaped contact surface  15  that is disposed on the side of upper piston part  2  that faces away from combustion bowl  3 , and on an upper cross-sectional surface  19  of a ring-shaped support crosspiece  20  of the lower piston part  6 , by way of a cross-sectional surface  18  situated on the underside of ring wall  4 . In this way, contact surfaces  15  and  16  form an inner contact region  21  disposed in planar and horizontal manner, or configured in the manner of a roof or plate, and cross-sectional surfaces  18  and  19  form an outer contact region  22  disposed coaxial to inner contact region  21  and horizontally, or also configured in the manner of a roof or plate. 
     Support crosspiece  20  is configured in step shape, so that upper piston part  2  can be centered by way of a cylindrical recess  23  worked into the inside of the lower part of ring wall  4 . Thus, when the upper piston part and lower piston part are assembled, the inner wall of cylindrical recess  23  comes into contact with cylindrical face side  24  of support crosspiece  20 , and it is necessary for the inside diameter of cylindrical recess  23  to be larger than the outside diameter of cylindrical face side  24  of support crosspiece  20 , by such a tolerance dimension that problem-free assembly of upper part  2  onto lower part  6  is guaranteed. 
     On the side facing away from combustion bowl  3 , upper piston part  2  has a pin  26  that is disposed centered and coaxial to longitudinal axis  25  of piston  1 , whose end  27  is provided with a thread  28 . Region  31  between ring-shaped support rib  17  of lower piston part  6 , which delimits inner cooling channel  11  together with upper piston part  2 , is configured to have a relatively thin wall, and in its center is provided with a bore  29  that is disposed coaxial to longitudinal axis  25  of the piston  1 , which bore has an inside thread  30  that matches thread  28  of pin  26 . 
     This makes it possible that during assembly of piston  1 , only threaded pin  26  of upper piston part  2  has to be screwed into threaded bore  29  of region  31 . The elasticity of the relatively thin-walled region  31  brings about the result that it deforms like a disk spring when the upper piston part and lower piston part are screwed together, and the inner center of region  31 , provided with threaded bore  29 , domes up in the direction of upper piston part  2 . In addition, the thread-free expansion shaft of threaded pin  26  lengthens during assembly, making it possible to achieve a further improvement in the reliability of the seat of upper piston part  2  on lower piston part  6 . In this way, both contact surfaces  15  and  16  of inner contact region  21  and cross-sectional surfaces  18  and  19  of outer contact region  22  are pressed onto one another, thereby sealing the inner and outer cooling channels  10  and  11 . The strength of the assembly of the upper piston part and lower piston part is therefore increased to such an extent that an additional nut, i.e. a locknut, is not required to achieve a permanent assembly connection. Experiments have shown that a sufficient bias of the two parts of the piston is guaranteed under all possible operating conditions. 
     Upper piston part  2  preferably consists of an oxidation-resistant and heat-resistant material. Lower piston part  6  preferably consists of a ferrite-perlite annealed steel that is precipitation-hardened. 
     In engine operation, there is the risk, in this connection, that a force in the direction of arrow  32  will be exerted on pin  26 , particularly during the intake stroke, which force goes beyond the stretching limit of the pin material, and leads to lasting longitudinal stretching of this material and thus to loosening of the screw connection between upper piston part  2  and lower piston part  6  of piston  1 . For this reason, it is proposed, according to the invention, to increase the stretching limit of the pin material prior to assembly with the lower piston part by subjecting the pin to controlled tensile stress in the cold state, so that it is permanently stretched in the axial direction by approximately 1% of its length, without the ductility of the pin material being impaired. 
     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. 
     REFERENCE SYMBOL LIST 
       1  piston 
       2  upper piston part 
       3  combustion bowl 
       4  ring wall 
       5  ring belt 
       6  lower piston part 
       7  piston skirt 
       8  pin boss 
       9  pin bore 
       10  outer cooling channel 
       11  inner cooling channel 
       12  inflow opening 
       13  overflow channel, bore 
       14  run-off bore 
       15  contact surface of the upper piston part  2   
       16  contact surface of the lower piston part  6   
       17  support ribs 
       18  cross-sectional surface of the upper piston part  2   
       19  cross-sectional surface of the lower piston part  6   
       20  support crosspiece 
       21  inner contact region 
       22  outer contact region 
       23  recess of the ring wall  4   
       24  face side of the support crosspiece  20   
       25  longitudinal axis of the piston  1   
       26  pin, threaded pin 
       27  end of the pin  26   
       28  thread 
       29  bore, threaded bore 
       30  inside thread of the bore  29   
       31  region between the support ribs  17   
       32  arrow