Patent Publication Number: US-2018038265-A1

Title: Piston for an internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to International Patent Application No. PCT/EP2016/054104 filed on Feb. 26, 2016, and German Patent Application No. 10 2015 002 322.5 filed on Feb. 26, 2015, the contents of which are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a piston for an internal combustion engine, having a piston head and a piston skirt, having a circumferential ring belt arranged in the piston head and comprising ring grooves for receiving piston rings, and a circumferential cooling duct in the piston head which is on a level with the ring belt and which has inlet openings and outlet openings for cooling oil. 
     BACKGROUND 
     Pistons of the generic type are known. A characteristic feature of modern pistons is their small overall height, giving them a small compression distance. For this reason an effective cooling of the piston by means of cooling oil during engine operation is ever more difficult to achieve, since less and less space is available for a functional cooling duct. 
     SUMMARY 
     The object of the present invention is to develop a piston of the generic type so as to achieve improved cooling by means of cooling oil. 
     This is achieved in that a cavity for receiving cooling oil, which opens into the cooling duct, is provided at least in the area of an inlet opening for cooling oil. 
     A characterizing feature of the piston according to the invention is that an additional shaker effect is obtained in the area of at least one inlet opening for cooling oil, since the cooling oil is free to move in the cavity provided according to the invention and during engine operation is moved up and down in the cavity due to the reciprocating movement of the piston. This results in an improved heat transfer from the piston head towards the piston skirt, where the heat is dissipated over a large area. 
     Advantageous developments emerge from the dependent claims. The height (H) of the cavity and the height (h) of the cooling duct are preferably designed with a ratio of 5:2 to one another, in order to obtain an optimum improvement in the heat transfer. 
     In a further advantageous development the cross sectional area (Q) of the cavity and the cross sectional area (q) of the cooling duct are designed with a ratio of 4:1.2 to 4:1.5 to one another. This measure also serves to achieve an optimum heat transfer. 
     The piston according to the invention advantageously has a major thrust face (DS) and a minor thrust face (GDS), wherein an inlet opening for cooling oil is provided in the area of the major thrust face (DS) and in the area of the minor thrust face (GDS). If both inlet openings are provided with a cavity according to the invention, the quantity of heat produced during engine operation is dissipated even more efficiently. 
     The cooling duct may comprise additional bored holes and/or fins for controlling the retention time of the cooling oil received in the cooling duct. 
     The compression distance KH of the piston according to the invention is preferably 30 to 35 mm, preferably 32 mm, making such a piston suitable for use in automotive racing. 
     For weight-saving reasons the piston according to the invention is preferably embodied as a full slipper skirt piston. In this case the piston bosses provided in the piston skirt and equipped with center bores are provided with reinforcing ribs, in order to increase the inherent rigidity of the piston. 
     The piston according to the invention is preferably made from an aluminum-based material, in order to further reduce the moving mass of the piston during engine operation. 
     The piston according to the invention may be made up of a piston upper part and a piston lower part, in order to optimize the choice of materials suited to the piston loads in engine operation. In this case the piston upper part and the piston lower part are preferably firmly connected together by means of a high temperature-resistant epoxide- or ceramic-based adhesive. The joining seam between the piston upper part and the piston lower part is preferably arranged between the first ring groove and the second ring groove of the ring belt. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention is described in more detail below, referring to the drawings attached. In a schematic representation, not to scale: 
         FIG. 1  shows an exemplary embodiment of a piston according to the invention, in section along the line I-I in  FIG. 4 ; 
         FIG. 2  shows a section along the line II-II in  FIG. 1 ; 
         FIG. 3  shows a section along the line in  FIG. 1 ; 
         FIG. 4  shows a top view of the piston according to  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 to 4  show an exemplary embodiment of a piston  10  according to the invention. The piston  10  in the exemplary embodiment is embodied as a full slipper skirt piston. The piston  10  comprises a piston head  11  having a piston crown  12 , a combustion recess  13 , a circumferential piston top land  14  and a circumferential ring belt  15  having ring grooves  16   a ,  16   b ,  16   c  for receiving piston rings (not shown). A circumferential cooling duct  17  is provided on a level with the ring belt. 
     The piston  10  further comprises a piston skirt  18 , which in a manner known in the art is provided with piston bosses  19 , into which center bores  21  are introduced for receiving a piston pin (not shown). The piston bosses  19  are connected to one another via bearing surfaces  22   a ,  22   b.    
     In its interior space  10   a  the exemplary embodiment representative of the piston  10  according to the invention has a cavity  24   a ,  24   b  on each face, on its major thrust face DS and on its minor thrust face GDS, which are defined by its bearing surfaces  22   a ,  22   b . Each cavity  24   a ,  24   b  is provided with an inlet opening  23   a ,  23   b  for cooling oil (cf.  FIG. 1 ). Each cavity  24   a ,  24   b  extends into the interior space  10   a  of the piston  10  in the direction of the piston bosses  19  and opens into the circumferential cooling duct  17  in the area of the piston head  11 . The outside wall of each cavity  24   a ,  24   b  and the inside wall of the bearing surface  22   a ,  22   b  associated therewith form an inlet funnel  25   a ,  25   b  for cooling oil, which opens into the respective cavity  24   a ,  24   b  via the corresponding inlet opening  23   a ,  23   b  (cf.  FIG. 1 ). 
     According to the invention the cavities  24   a ,  24   b  act as shaker cavities, in which the cooling oil, in a manner known in the art, is moved up and down during engine operation (so-called “cocktail-shaker effect”). According to the invention this results in improved heat removal from the region of the piston head  11  towards the piston skirt  18 , where the heat is dissipated via the bearing surfaces  22   a ,  22   b.    
     In the exemplary embodiment represented the height H of each cavity  24   a ,  24   b  and the height h of the cooling duct  17  are preferably designed with a ratio of 5:2 to one another, for example. In the exemplary embodiment the cross sectional area (Q) of each cavity  24   a ,  24   b  and the cross sectional area (q) of the cooling duct  17  are designed with a ratio of 4:1.35, for example. 
     In a manner known in the art, the cooling duct  17  in the exemplary embodiment representative of a piston according to the invention comprises outlet openings  26  for cooling oil, through which the cooling oil is fed in the direction of the interior space  10   a  of the piston  10 . This serves on the one hand for additional cooling of the underside  12   a  of the piston crown  12  and on the other for additional lubrication of the center bores  21  and of the piston pin received therein during engine operation. 
     In a manner known in the art, the cooling duct  17  in the exemplary embodiment representative of a piston  10  according to the invention further comprises additional bored holes  27  and fins  28  for controlling the retention time of the cooling oil received in the cooling duct  17  during engine operation. 
     The exemplary embodiment of the piston  10  according to the invention represented in the figures is formed from a piston upper part  31  and a piston lower part  32 , which are preferably made from an aluminum-based material and are firmly connected together by means of a high temperature-resistant (up to 300° C.) epoxide resin- or ceramic-based adhesive. In the exemplary embodiment the joining seam  34  between the piston upper part  31  and the piston lower part  32  is arranged between the first ring groove  16   a  and the second ring groove  16   b.    
     In the exemplary embodiment the piston  10  has a compression distance KH of 32 mm and in this design is suitable for use in automotive racing. 
     For additional stabilization and reinforcement of the piston  10 , the piston bosses  19  comprise reinforcing ribs  33  in the area of the piston bosses  21 . The reinforcing ribs  33  brace the piston head  11  relative to the piston bosses  19  in the area below its ring belt  15 , in such a way that the risk of deformations of the piston head  11  in the area of ring belt  15  is at least reduced during engine operation.