Patent Publication Number: US-7584694-B2

Title: Composite piston for an internal combustion engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Applicant claims priority under 35 U.S.C. §119 of German Application No. 10 2004 030 218.9 filed Jun. 22, 2004. Applicant also claims priority under 35 U.S.C. §365 of PCT/DE2005/001098 filed Jun. 20, 2005. The international application under PCT article 21(2) was not published in English. 
   The invention relates to a composite piston for an internal combustion engine, in accordance with the preamble of claim  1 . 
   Composite pistons, which consist of an upper part onto which a lower part is screwed by means of a hexagonal head screw, are generally known from the state of the art and are described, for example, in the publication DE 32 49 290 T1. In this connection, the screw head of the hexagonal head screw usually lies on the piston inside, and this brings with it the necessity of providing the upper part with a dead-end bore having an inside thread, so that the upper part and the lower part can be screwed together with one another. This has the disadvantage that the space requirement for the dead-end bore as well as the screw head is relatively great, so that the compression height of the piston also has a relatively great value, and therefore the total piston has a relatively great total axial length. 
   Proceeding from this, the invention is based on the problem of avoiding the stated disadvantages of the state of the art and of creating a composite piston having the lowest possible compression height and the lowest possible total axial length. 
   This problem is solved with the characteristics contained in the characterizing part of the main claim, whereby the additional advantage is obtained that the hexagonal socket screw used according to the invention has a relatively great radial thread diameter, thereby achieving a reduction in the thread pitch and with it an improvement in the strength of the screw connection. Furthermore, this results in an increase in the size of the bearing thread surface, contributing to a further improvement in the strength of the screw connection, since in this way, the risk that the thread flanks both of the screw and of the threaded bore that accommodates this screw will shear off is reduced. 
   A practical embodiment of the invention is the object of the dependent claim, whereby the lower piston part has a region that can be elastically deformed, in the manner of a disk spring, into which the thread for the hexagonal socket screw is worked, so that in this way, a bias is exerted on the hexagonal socket screw, which contributes to a further improvement in the strength of the screw connection. 
   An exemplary embodiment of the invention will be described below, using a drawing. This shows a composite piston consisting of an upper part and a lower part, in a sectional diagram consisting of two halves, which shows two longitudinal sections of the piston, offset by 90°. 

   The FIGURE shows a composite piston  1  in a sectional diagram, the left half of which shows a section through the piston  1  along a longitudinal axis  2  of a pin bore  3 , and the right half of which shows a section through the piston  1  offset by 90° relative to the former. The piston  1  consists of an upper part  4  and a lower part  5 , which are connected with one another by means of a threaded pin  6  disposed in the center, having a hexagonal socket  6 ′ and a smooth head  6 ″ on the side facing away from the hexagonal socket  6 ′, in such a manner that the hexagonal socket  6 ′ of the threaded pin  6  comes to lie on the piston inside. In this connection, the head  6 ″ of the threaded pin  6  has a greater radial diameter than the outside thread  38 . Between the head  6 ″ and the outside thread  38 , the threaded pin  6  has an expansion region  39  having a radial diameter that is reduced relative to the outside thread  38 . 
   The upper part  4  and the lower part  5  of the piston  1  are preferably produced from forged steel. It is also possible, however, to produce the upper part  4  from steel and the lower part  5  from aluminum, or to produce the upper part  4  from forged aluminum and the lower part from cast aluminum. 
   The cylindrically shaped upper part  4  forms the piston crown  7 , into which a combustion bowl  8  having rotation symmetry is worked. The mantle surface of the upper part  4 , which lies radially on the outside, is configured as a ring belt  9  that has three ring grooves  10 ,  11 , and  12  for accommodating piston rings, not shown in the FIGURE. The underside of the upper part  4 , facing away from the piston crown  7 , has a circumferential recess  13  radially on the outside, which forms a ring-shaped, outer cooling channel  15 , together with a corresponding recess  14  of the lower part  5 , on the piston crown side, which channel is delimited radially on the outside by a ring wall  29  formed onto the piston crown  7 . 
   Radially on the inside, the outer cooling channel  15  is delimited partly by a ring flange  16  disposed on the underside of the upper part  4 , and partly by a ring rib  17  disposed on the top of the lower part  5 , whereby the upper part  4  and the lower part  5  of the piston  1  rest on one another by way of the ring flange  16  and the ring rib  17 . In this connection, the ring flange  16  has a first contact surfaces  18  and the ring rib  17  has a second contact surface  19 , by way of which contact surfaces  18 ,  19  of the ring flange  16  and the ring rib  17  stand in contact with one another. 
   Radially within the ring flange  16 , the upper part  4  is provided, on its underside, with another circumferential recess  20 , which forms an inner, ring-shaped cooling channel  22  together with a corresponding other recess  21  formed into the top of the lower part  5 . In this connection, the outer cooling channel  15  is connected with the piston interior  24  by way of an oil run-in opening  23 , and with the inner cooling channel  22  by way of an oil channel  25 . The inner cooling channel  22  is connected with the piston interior  24  by way of an oil run-off opening  26 . To cool the piston  1 , oil is injected into the outer cooling channel  15  by way of an oil run-in opening not shown in the FIGURE; after some time, it gets into the inner cooling channel  22  by way of the oil channel  25 , and runs back into the piston interior  24  by way of the oil run-off openings  23  and  26 . 
   The lower part  5  of the piston  1  consists of two pin bosses  27 ,  27 ′ that lie opposite one another and are trapezoid in section, each having a pin bore  3 ,  3 ′, respectively, which are disposed at such a distance from one another that the upper part of a piston rod (not shown in the FIGURE) finds room between them. Furthermore, the lower part  5  has skirt elements  32 ,  32 ′ that connect the pin bosses  27 ,  27 ′ with one another. On the piston crown side, the lower part  5  has a circumferential collar  28 , rectangular in section, radially on the outside, which collar fits into a recess  30  disposed radially on the inside in a face of the ring wall  29  that faces away from the piston crown, so that during assembly of the piston  1 , the lower part  5  is guided by way of the collar  28  and the recess  30 , and centered relative to the upper part  4 . 
   On the surface of the lower part  5  on the piston crown side, the recess  14  follows the collar  28 , radially on the inside, which recess forms the outer cooling channel  15  together with the recess  13  of the upper part  4 , followed by the ring rib  17  and the other recess  21 , which forms the inner cooling channel  22  together with the other recess  20  of the upper part  4 . In this connection, the recess  21  is worked so far into the lower piston part  5  that a thin-walled region  33  is formed between piston interior  24  and recess  21 , which region is configured elastically, in the manner of a disk spring. 
   Coaxial to the axis  31  of the piston  1 , a continuous bore  34  is made in the piston crown  7 , having a diameter that is slightly greater than the diameter of the outside thread  38  of the threaded pin  6 . The bore  34  has a circumferential recess  35  disposed on the piston crown side, having a radial diameter that is greater, by a slight dimension of tolerance, than the radial diameter of the head  6 ″ of the threaded pin  6 , and the cross-section of which corresponds to the cross-section of the head  6 ″, to such an extent that when the upper part  4  is screwed together with the lower part  5 , the recess  35  can serve to accommodate the head  6 ″ of the threaded pin  6 . 
   A passage bore  36  having an inside thread  37  is worked into the top of the lower part  5 , i.e. into the elastically resilient region  33 , which thread is configured in such a manner that the threaded pin  6  can be screwed into the passage bore  36  by way of its outside thread  38 . 
   To screw them together, the upper part  4  is first set onto the lower part  5  in such a manner that the upper and lower part of the piston  1  are disposed in a certain position relative to one another. Subsequently, the threaded pin  6  is introduced into the bore  34  of the upper part  4  from the top, and screwed tightly into the passage bore  36  provided with the inside thread  37 , by means of a hexagonal wrench. The deformation of the region  33  that occurs in this connection, in the manner of a disk spring, on both sides of the passage bore  36 , increases the bias that acts on the threaded pin  6 , and thereby results in an improvement of the strength of the screw connection. 
   Another improvement in the strength of the screw connection also results from the fact that the outside thread  38  of the threaded pin  6  has a greater axial diameter, because of its design, than a hexagonal head screw having a screw head and a screw shaft that is usually used in this connection. With the number of windings per screw length remaining the same, an increase in the radial thread diameter results in a reduction in the thread pitch. Since the strength of a screw connection is generally all the greater, the lower the pitch of a thread, this results in a further improvement in the strength of the screw connection according to the invention. 
   Furthermore, an increase in the size of the radial thread diameter also means an increase in the size of the bearing thread surfaces, both of the screw and of the threaded bore that accommodates the screw. In this way, the risk that the threaded surfaces will shear off under greater stress on the piston is reduced, making it possible to avoid greater piston and engine damage in this connection. 
   REFERENCE SYMBOL LIST 
   
       
         1  piston 
         2  longitudinal axis 
         3 ,  3 ′ pin bore 
         4  upper part 
         5  lower part 
         6  screw; threaded pin having a 
         6 ′ hexagonal socket and a 
         6 ″ head on the side facing away from the hexagonal socket  6 ′ 
         7  piston crown 
         8  combustion bowl 
         9  ring belt 
         10 ,  11 ,  12  ring groove 
         13 ,  14  recess 
         15  outer cooling channel 
         16  ring flange 
         17  ring rib 
         18  first contact surface 
         19  second contact surface 
         20 ,  21  recess 
         22  inner cooling channel 
         23  oil run-off opening 
         24  piston interior 
         25  oil channel 
         26  oil run-off opening 
         27 ,  27 ′ pin boss 
         28  collar 
         29  ring wall 
         30  recess 
         31  axis of the piston  1   
         32 ,  32 ′ skirt element 
         33  region between the piston interior  24  and the recess  21   
         34  bore 
         35  recess 
         36  passage bore 
         37  inside thread 
         38  outside thread of the threaded pin  6   
         39  expansion region of the threaded pin  6