Abstract:
A piston for a combustion engine has a basic element and a ring carrier element held on the basic element with a top land all around its circumference and a ring belt all around its circumference. The ring carrier element grips around the basic element at least partially and is held on the basic element by a press fit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This is a Continuation-In-Part of U.S. patent application Ser. No. 11/451,257, filed on Jun. 12, 2006. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to a piston for a combustion engine having a basic element and a ring carrier element held on the basic element with a top land all around its circumference and a ring belt all around its circumference.  
         [0003]     A generic piston is known from DE 103 40 292 A1. This piston comprises a basic element and a ring carrier element bolted to the basic element, said elements forming a cooling channel. A weld is provided as an additional safeguard between the basic element and the ring carrier element. This piston is also known by the name “Monotherm® ”piston.  
         [0004]     In diesel engines in particular, pistons are subject to high thermal and mechanical loads. It is therefore desirable to provide the piston with a good cooling system, i.e. with a cooling channel of the largest possible volume, and with a connection between the basic element and the ring carrier element that can withstand particularly high mechanical loads. At the same time, the piston should be built lightweight and compact. In particular, the compression height should not be too great, in order to keep the cylinder block of the diesel engine as low as possible.  
         [0005]     The bolted connection in the generic piston requires however that the ring carrier element has a considerable minimum height and minimum thickness so that it can accommodate a thread with a number of sufficiently deep turns necessary for a strong bolted connection. The generic piston is therefore comparatively heavy with a relatively high compression height. Since the ring carrier element needs a considerable minimum thickness, the volume of the cooling channel is also comparatively low.  
       SUMMARY OF THE INVENTION  
       [0006]     The object of the present invention is to provide a piston having a basic element and a ring carrier element held on the basic element, said piston being distinguished by a stable and strong connection between the basic element and the ring carrier element with a reduced compression height and reduced weight.  
         [0007]     The solution comprises a piston in which the ring carrier element grips around the basic element at least partially and is held on the basic element by means of a press fit.  
         [0008]     With the piston according to invention, therefore, the bolted connection between basic element and ring carrier element is dispensed with. The ring carrier element can therefore be designed with lower height and thickness. The stable connection between the basic element and the ring carrier element is assured by a press fit that withstands even the increased loads in a diesel engine. Since the ring carrier element is designed considerably less solid than previously, the weight of the piston in accordance with the invention is reduced. In addition, the surface and the volume of the cooling channel can be considerably greater than before, so that the cooling of the piston in accordance with the invention is considerably improved. Due to the lower design height of the ring carrier element, the compression height of the piston in accordance with the invention is reduced, so that the cylinder block of the associated combustion engine can be correspondingly reduced in height. The piston according to the invention can be manufactured more economically and at lower cost overall.  
         [0009]     In a first embodiment of the piston according to the invention, the piston has a piston crown with an inner area formed by the basic element and having an all-round shoulder and with an outer area formed by the ring carrier element and annularly surrounding said inner area, and in that the ring carrier element grips around the all-around shoulder in the radial direction. In this area, in which the ring carrier element grips around the all-around shoulder, i.e. the basic element, the ring carrier element is held on the basic element with a press fit. Since the press fit represents a particularly strong connection between the basic element and the ring carrier element, the latter can be designed particularly low and thin-walled, without weakening the connection between basic element and ring carrier element.  
         [0010]     It is of course also possible to additionally provide a weld between the all-round shoulder and the ring carrier element.  
         [0011]     The piston crown can be provided in a manner known per se with a combustion bowl designed to suit the engine design.  
         [0012]     In a second embodiment of the piston according to the invention, the piston has a piston crown formed by the ring carrier element, and the basic element is provided with an annular collar aligned axially and facing the ring carrier element, and the ring carrier element grips around the annular collar. This embodiment has, in addition to the advantages already set forth, the additional advantage that the ring belt of the ring carrier element is additionally supported by the collar. As a result, the ring belt is additionally stabilized against the loads occurring during engine operation. In particular, both the deformation of the annular grooves and the deformation of the ring belt are considerably reduced overall.  
         [0013]     If an additional strengthening of the connection between ring carrier element and basic element is required, the ring carrier element can be provided with a recess facing the basic element, and the basic element with a raised area facing the ring carrier element. During assembly of the ring carrier element and basic element, the recess and the raised area engage and are likewise held by a press fit.  
         [0014]     A further additional strengthening of the connection between ring carrier element and basic element is provided by an axial oriented thread arranged between the ring carrier element and the annular collar of the basic element.  
         [0015]     If required, the ring carrier element can have at least one all-around contact surface facing the basic element, and the basic element can have at least one all-round support surface corresponding to the at least one contact surface and facing the ring carrier element, so that the ring carrier element is supported on the basic element after assembly.  
         [0016]     In this embodiment too, the piston crown can be provided in a known manner with a combustion bowl designed to suit the engine design.  
         [0017]     The basic element and the ring carrier element can be manufactured from different materials depending on the requirements of the particular application. The basic element can for example be made from a light metal material or from a steel material, and the ring carrier element from a steel material which is particularly thermally stable. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Embodiments of the invention are described in detail below on the basis of the enclosed drawings. The drawings are schematic and not true to scale.  
         [0019]      FIG. 1  shows a first embodiment of a piston in accordance with the invention in section;  
         [0020]      FIG. 2  shows a section along the line II-II in  FIG. 1 ;  
         [0021]      FIG. 3  shows a further embodiment of a piston in accordance with the invention in section;  
         [0022]      FIG. 4  shows a section along the line IV-IV in  FIG. 3 ;  
         [0023]      FIG. 5  shows a further embodiment of a piston in accordance with the invention in section.  
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0024]      FIGS. 1 and 2  show a first embodiment of a piston  10  in accordance with the invention. The piston  10  has a basic element  11  made of a forged or sintered metallic material and a ring carrier element  12  made of a metallic material, for example a temperature-resistant steel. The basic element  11  has a piston skirt  20  and the central or inner area  13  of a piston crown  14  that is provided in a manner known per se with a combustion bowl  15 . Boss connections are provided underneath the piston crown  14  and merge into bosses  17  running parallel to a longitudinal axis A of the piston  10 . The bosses  17  are provided with boss holes  18  for admitting a piston pin, not shown.  
         [0025]     In the area of the boss connections  16 , an all-around flange  19  projecting radially outwards is provided. The combustion bowl  15  is limited by an all-round annular shoulder  21 . The shoulder  21  has a lateral surface  22  parallel to the longitudinal axis A of the piston  10  and an end face  23  at right angles to it.  
         [0026]     The ring carrier element  12  has an all-around and substantially cylindrical ring element  24  with an inner wall  32  which is provided in a manner known per se on its lateral surface. It has a top land  25  and a ring belt  26  with several annular grooves  27  for receiving piston rings, not shown. The lower free end of the ring element  24  forms a contact surface  28  resting on the flange  19  of the basic element  11 .  
         [0027]     The ring element  24  has in the area of the shoulder of the basic element  11  an all-around edge  29  extending radially inwards of which the surface forms the outer annular area  31  of the piston crown  14 , which surrounds the central area  13  of the piston crown  14  formed by the basic element  11 . The ring element  24  and the edge  29  enclose the lateral surface  22  or the end face  23  of the shoulder  21  of the basic element  11  and are so dimensioned that a press fit of the ring carrier element  12  on the basic element  11  is constituted. For assembly, the metallic ring carrier element  12  is therefore heated and mounted on the basic element  11  in the manner shown in  FIGS. 1 and 2 . When it cools, the ring carrier element  12  contracts, forming the press fit as described.  
         [0028]     As an additional safeguard, a weld  33  can also be provided between the edge  29  of the ring carrier element  12  and the end face  23  of the shoulder  21  of the basic element  11 , as shown by example in  FIGS. 1 and 2 .  
         [0029]     The inner wall  32  of the ring element  24  and an all-around recess  34  provided underneath the combustion bowl  15  in the basic element  11  form a cooling channel  35 . Thanks to the slender design of the ring carrier element  12 , the cooling channel  35  has a particularly large volume and extends from the shoulder  21  to the flange  19  of the basic element  11 .  
         [0030]      FIGS. 3 and 4  show a further embodiment of a piston  110  in accordance with the invention. The piston  110  has a basic element  111  made of a forged or sintered metallic material and a ring carrier element  112  made of a metallic material, for example a temperature-resistant steel. The basic element  111  has a piston skirt  120  and boss connections  116  merging into bosses  117  running parallel to a longitudinal axis A of the piston  110 . The bosses are provided with boss holes  118  for admitting a piston pin, not shown. In the area of the boss connections  116 , an all-around flange  119  projecting radially outwards is provided. An axially aligned annular collar  136  is provided in the area of the flange  119 .  
         [0031]     The ring carrier element  112  has a piston crown  114  provided in a manner known per se with a combustion bowl  115 . The piston crown  114  is limited by an all-around and substantially cylindrical ring element  124  with an inner wall  132 . The ring element  124  is provided on its lateral surface in a manner known per se with a top land  125  and a ring belt  126  with several annular grooves  127  for receiving piston rings, not shown. The lower free end of the ring element  124  forms a contact surface  128  resting on the flange  119  of the basic element  111 .  
         [0032]     The ring carrier element  112  has underneath the combustion bowl an annular contact surface  137  facing the basic element  111  and provided in some areas with a recess  138 . The basic element  111  has accordingly a support plate  139  facing the ring carrier element  112  and provided with an annular support surface  141  corresponding to the contact surface  137 . The support surface  141  has in some areas a raised area  142  corresponding to the recess  138 .  
         [0033]     In the assembled state, the inner wall  132  of the ring element  124  encloses the annular collar  136  of the basic element  111 , and the raised area  142  of the basic element  111  engages in the recess  138  of the ring carrier element  112 , as shown in  FIG. 3 . The inner wall  132 , the collar  136 , the raised area  142  and the recess  138  are dimensioned such that a press fit of the ring carrier element  112  on the basic element  111  is constituted.  
         [0034]     For assembly, the metallic ring carrier element  112  is therefore heated and mounted on the basic element  111  in the manner shown in  FIGS. 3 and 4 . When it cools, the ring carrier element  112  contracts, forming the press fit as described.  
         [0035]     The inner wall  132  of the ring element  124 , the collar  136 , an all-around recess  134  provided underneath the combustion bowl  115  in the ring carrier element  112  and the support plate  139  form a cooling channel  135 . Thanks to the slender design of the ring carrier element  112 , the cooling channel  135  has a particularly large volume and extends from the underside of the piston crown  114  to the flange  119  of the basic element  111 .  
         [0036]      FIG. 5  shows a further embodiment of a piston  210  in accordance with the invention. The piston  210  has a basic element  211  made of a forged or sintered metallic material and a ring carrier element  212  made of a metallic material, for example a temperature-resistant steel. The basic element  211  has a piston skirt  220  and boss connections  216  merging into bosses running parallel to a longitudinal axis A of the piston  210 , not shown. The bosses are provided with boss holes  218  for admitting a piston pin, not shown. In the area of the boss connections  216 , an all-around flange  219  projecting radially outwards is provided. An axially aligned annular collar  236  is provided in the area of the flange  219 .  
         [0037]     The ring carrier element  212  has a piston crown  214  provided in a manner known per se with a combustion bowl  215 . The piston crown  214  is limited by an all-around and substantially cylindrical ring element  224  with an inner wall  232 . The ring element  224  is provided on its lateral surface in a manner known per se with a top land  225  and a ring belt  226  with several annular grooves  227  for receiving piston rings, not shown. The lower free end of the ring element  224  forms a contact surface  228  resting on the flange  219  of the basic element  211 .  
         [0038]     An axial oriented thread  250  is arranged between the ring carrier element  212  and the annular collar  236  of the basic element  211 . In the embodiment shown in  FIG. 5 , the thread  250  is arranged between the inner wall  232  of the ring element  224  and an outer surface of the annular collar  236 .  
         [0039]     The ring carrier element  212  has underneath the combustion bowl  215  an annular contact surface  237  facing the basic element  211 . The annular contact surface  237  may be provided in some areas with a recess, not shown. The basic element  211  is provided accordingly with an annular support surface  241  facing the ring carrier element  212  and corresponding to the contact surface  237 . The support surface  241  may have in some areas a raised area corresponding to said recess, not shown.  
         [0040]     In the assembled state, the inner wall  232  of the ring element  224  encloses the annular collar  236  of the basic element  211 . The contact surface  228  of the ring element  224  is in close contact to the surface of the flange  219  of the basic element  211 , and the contact surface  237  of the ring carrier element  212  is in close contact to the support surface  241  of the basic element  211 . The inner wall  232 , the collar  236 , the flange  219  and the surfaces  228 ,  237 ,  241  are dimensioned such that a press fit of the ring carrier element  212  on the basic element  211  is constituted. The press fit on one hand and the close contact between the contact surface  228  and the surface of the flange  219  as well as the close contact between the contact surface  237  and the support surface  241  is improved and supported by the thread  250 . Particularly, an additional, axially effective press fit is constituted by the thread  250 . Said axially effective press fit especially increases the close contact between the contact surface  228  and the surface of the flange  219  as well as the close contact between the contact surface  237  and the support surface  241 .  
         [0041]     For assembly, the metallic ring carrier element  212  is therefore heated and mounted on the basic element  211  in the manner shown in  FIG. 5 . When it cools, the ring carrier element  212  contracts, forming the press fit as described.  
         [0042]     The inner wall  232  of the ring element  224 , the collar  236 , an all-around recess  234  provided underneath the combustion bowl  215  in the ring carrier element  212  form a cooling channel  235 . Thanks to the slender design of the ring carrier element  212 , the cooling channel  235  has a particularly large volume and extends from the underside of the piston crown  214  to the flange  219  of the basic element  211 .  
         [0043]     Accordingly, while only a few embodiments 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.