Patent Publication Number: US-2011073061-A1

Title: Pistons with a rough surface

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/246,250, filed on Sep. 28, 2009, hereby incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a piston for an internal combustion engine having a rough interior surface. 
     BACKGROUND 
     A piston assembly for an internal combustion engine generally becomes very hot during use and is subjected to relatively severe thermal stresses as compared to other engine components, especially on the top wall or crown portion of the piston, which is directly exposed to the heat of the gases in the combustion chamber that is partly defined by the piston. The issue of piston assembly crown temperature has become more of an issue with modern internal combustion engines, due to increases in thermal loading arising from increases in engine power output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial cross sectional view of a piston including a piston skirt and a piston crown; 
         FIG. 2  is a bottom view of the interior of the piston of  FIG. 1 ; 
         FIG. 3  is a cross sectional view of a portion of the undercrown of the piston of  FIG. 1  including a region of increased surface area; 
         FIG. 4  is a top view of a casting tool used in forming the undercrown of  FIG. 3 ; 
         FIG. 5  is a top view of the undercrown of the piston of  FIG. 1  including a plurality of regions of increased surface area; 
         FIG. 6  is a top view of an alternate casting tool used in forming the plurality of regions of increased surface area of  FIG. 5 ; 
         FIG. 7  is a process flow chart of an exemplary piston casting process; 
         FIG. 8  is a top view of an alternate illustration of the casting tool of  FIG. 4 ; 
         FIG. 9A  is a top view of the undercrown of the piston of  FIG. 1  including an alternate region of increased surface area; 
         FIG. 9B  is an enlarged view of the cross-hatching of  FIG. 9A ; 
         FIG. 10  is a top view of the interior of the piston including the undercrown of  FIG. 9A , including the alternate region of increased surface area; and 
         FIG. 11  is a top view of a casting tool used in forming the region of increased surface area of  FIG. 9A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the discussion that follows and also to the drawings, illustrative approaches to the disclosed systems and methods are shown in detail. Although the drawings represent some possible approaches, the drawings are not necessarily to scale and certain features may be exaggerated, removed, or partially sectioned to better illustrate and explain the present disclosure. Further, the descriptions set forth herein are not intended to be exhaustive or otherwise limit or restrict the claims to the precise forms and configurations shown in the drawings and disclosed in the following detailed description. 
     Moreover, a number of constants may be introduced in the discussion that follows. In some cases, illustrative values of the constants are provided. In other cases, no specific values are given. The values of the constants will depend on characteristics of the associated hardware and the interrelationship of such characteristics with one another, as well as environmental conditions and the operational conditions associated with the disclosed system. 
     Turning now to the drawings and in particular to  FIG. 1 , an exemplary piston  20  for an internal combustion engine is disclosed. Piston  20  includes a piston crown  30  and a piston skirt  32 . Piston crown  30  includes a combustion bowl  40  and a ring belt portion  42 . Ring belt portion  42  includes a plurality of ring grooves  50 ,  52 ,  54  for receiving a plurality of piston rings (not shown). In particular, the ring belt portion  42  may include a first ring groove  50  closest to the piston crown  30 , a second ring groove  52 , and a third ring groove  54 , the third ring groove spaced the furthest distance away from the piston crown. First ring groove  50  and second ring groove  52  may have compression rings (not shown) disposed therein, while the third ring groove  54  may have an oil control ring (not shown) disposed therein. 
     A cooling gallery  44  may be located within the piston  20 , and may include a cooling gallery surface  60 , where the cooling gallery surface  60  is defined at least in part by an inner wall  62  of the piston crown  30  and an inner wall  64  of the piston skirt  32 . Cooling gallery  44  may also include one or more fluid inlet apertures  70 , and one or more fluid outlet apertures  72  to facilitate fluid flow through the gallery  44 . However, it should be known that a cooling gallery  44  may or may not be present depending on the particular application. 
     As shown in  FIGS. 1 and 2 , piston skirt  32  includes a pair of opposing wrist bores  74  through piston walls  75 ,  77  having inner surfaces  76 ,  78  for receiving a wrist pin (not shown) to rotatively connect the piston  20  to a connecting rod (not shown). Piston skirt  32  defines a generally cup-shaped interior space  79 . Piston skirt  32  further includes a pair of opposing side walls  82 ,  84  having inner surfaces  83 ,  85  and top portions  86 ,  87  which are integral with an undercrown  80 . Undercrown  80  is formed on the lower surface of the piston crown  30  and is generally cup-shaped. Undercrown  80  may be located beneath the highest portion of the piston  20 . 
     As shown in  FIG. 3 , undercrown  80  includes at least one region of increased surface area  88 , in which the surface of undercrown  80  is rough. The roughness in the region of increased surface area  88  is created by a plurality of indentations  90 . Indentations  90  may be formed in the undercrown  80  during the casting of piston  20 , but in a manner intentionally increasing the roughness as compared to more traditional casting techniques where some degree of roughness exists merely by virtue of undertaking a casting. More specifically, indentations  90  may extend to a depth in the range of approximately 0.1 mm to 1.0 mm below the surface of undercrown  80 . Tighter tolerances may be desirable under some circumstances. Thus, each indentation  90  increases the surface area of the undercrown  80 . 
     In this manner, the indentations  90  form regions of increased surface area  88  in the undercrown  80 . In some exemplary approaches the regions are contiguous, and in other approaches the regions are positioned only where more significant heat dissipation is necessary. The regions of increased surface area  88  of undercrown  80  improves the heat dissipation of piston  20  as the larger surface area allows for a greater amount of heat to be dissipated. As the piston  20  is repeatedly subjected to severe thermal stresses and temperatures during operation, such improvements to the heat dissipation of the piston  20  may enhance the performance of the piston  20 . Forming the at least one region of increased surface area  88  in the undercrown  80  is generally useful since the undercrown  80  is located beneath the highest portion of the piston  20 , which is also the hottest area of the piston  20  during operation of the piston  20 . Thus, forming the at least one region of increased surface area  88  in the undercrown  80  has potentially the most significant impact on the heat dissipation of the piston  20 . 
     Increasing the size of the regions of increased surface area  88  also increases the amount of heat that can be dissipated by piston  20 . The regions of increased surface area  88  may encompass any portion of the undercrown  80  of the piston  20 . Preferably, the regions of increased surface area  88  encompass as much of the surface of the undercrown  80  as the casting technology allows. Additionally, as shown in  FIG. 2 , the regions of increased surface area  88  may extend beyond the undercrown  80  and onto additional surfaces in the interior of the piston  20 , including one or both of the inner surfaces  83 ,  85  of side walls  82 ,  84  of piston skirt  32 . 
     As noted above, the regions of increased surface area  88  may be formed in the undercrown  80  during the casting of the piston  20 . Piston  20  may be formed by traditional casting methods, such as, but not limited to, sand, lost foam, investment or die-casting methods. The casting methods may be altered to promote the number and increased depth of indentations  90 . Merely by way of example, the piston  20  may be formed by die-casting in a mold including a first upper mold member (cope, not shown) and a second mold member (drag, not shown). The mold members may include a plurality of cores or casting tools  92 , which create the near net shape of the piston  20  and regions of increased surface area  88 , as shown in  FIG. 4 . Casting tool  92  includes a curved upper portion  94 , which is used during the casting process to form the undercrown  80  and the generally cup-shaped interior space  79  defined by piston skirt  32 . 
     The outer surface of the curved upper portion  94  of casting tool  92  may include a surface roughener  96 . As shown in  FIG. 4 , the surface roughener  96  includes a plurality of casting indentations  90 , which are formed into the curved upper portion  94  of casting tool  92 . Casting indentations  90  may be engraved into the casting tool  92  by any suitable means, including, but not limited to spark erosion or grinding. The plurality of casting indentations  90  creates a roughened surface having peaks and valleys on the undercrown  80  of the piston  20  (shown in  FIG. 3 ) by forming indentations  90  on the undercrown  80  corresponding to the casting indentations  90  during the casting process. These indentations  90  on the undercrown  80  form the regions of increased surface area  88 . As the casting indentations  90  engraved into the casting tool  92  may have varying depths, the indentations  90  formed on the undercrown  80  may have corresponding varying depths. Thus, a pattern of indentations  90  of varying depths may be formed in the region of increased surface area  88  on the undercrown  80 , as shown in  FIG. 3 . 
     The size of the regions of increased surface area  88  formed in the undercrown  80  is determined by the amount of surface area of the curved upper portion  94  of the casting tool  92  in which the casting indentations  90  are engraved. The greater the surface area of the curved upper portion  94  of the casting tool  92  in which the casting indentations  90  are formed, the greater the size of the regions of increased surface area  88  formed in the piston  20  during casting. Indeed, increasing the surface area of the curved upper portion  94  of the casting tool  92  in which casting indentations  90  are engraved can result in the regions of increased surface area  88 , illustrated in  FIGS. 2 and 3 , being formed on other surfaces in addition to the undercrown  80 , including the interior surfaces  83 ,  85  of the side walls  82 ,  84  of the piston skirt  32 . 
     Additionally, as shown in  FIG. 5  and noted above, more than one region of increased surface area  88  may be formed on undercrown  80  during casting. This may be accomplished by engraving casting indentations  90  in the curved upper portion  94  of the casting tool  92  in discrete surface roughener segments  97  that are separated from one another as shown in  FIG. 6  by way of boundary lines  99 . Each of the discrete surface roughener segments  97  will form a region of increased surface area  88  in the undercrown  80  of the piston  20  during casting. 
     Turning now to  FIG. 7 , a process  100  for forming piston  20  is illustrated. Process  100  may begin at step  102 , where a surface roughener  96  in the form of a plurality of casting indentations  90  are engraved into a casting tool  92 . The casting indentations  90  may be engraved into any portion of the casting tool  92  that will allow for modification, such as, but not limited to the curved upper portion  94 . 
     In step  104 , the mold for casting the piston  20  is assembled, including the casting tool  92 . The mold may include a plurality of casting tools (cores)  92  and may be positioned in a variety of ways. Merely by way of example, a five piece casting tool  92  may be used where there is at least a middle section that is inserted and removed first with exterior cores surrounding the middle section. It should be known that voids between the casting tools  92  create the final shape of the piston  20 . In step  106 , the piston  20  is cast in the mold by injecting or pouring molten metal through a gating system and into the voids surrounding the casting tools  92 . The surface roughener  96  formed in the curved upper portion  94  of the casting tool  92  forms a corresponding rough surface in the undercrown  80  of the piston  20  during the casting process. This rough surface defines at least one region of increased surface area  88  which improves the heat dissipation properties of the piston  20 . The casting tool  92  is then separated from the mold in step  108  by removing the middle casting tool first and the exterior casting tools  92  second. The piston  20  is then removed from the mold in step  110  and allowed to cool. Upon removal from the mold, the piston  20  may be allowed to cool naturally or may be submitted to a heat treating process, such as, but not limited to annealing, case hardening, precipitation strengthening, tempering and quenching. The heat treating may alter the physical or chemical properties of the materials used and may impart a particular hardness to the piston  20 . 
     Alternatively, as shown in  FIG. 8 , the surface roughener  96  used to form the indentations  90  in the undercrown  80  may be a coating (not shown), such as, but not limited to ceramic or other heat resistant coating, which is applied to the curved upper portion  94  of the casting tool  92 . When applied to the undercrown  80  of the piston  20 , the coating may form at least a portion of a roughened surface on the undercrown  80  by forming the indentations  90  on the undercrown  80  during the casting process. These indentations  90  on the undercrown  80  form a region of increased surface area  88 . Due to the nature of the coatings applied to the casting tool  92 , the indentations  90  formed on the undercrown  80  may have varying depths. Thus, a random pattern of indentations  90  of varying depths may be formed in the regions of increased surface area  88 . 
     As noted above, the surface roughener coating (not shown) applied to the curved upper portion  94  of the casting tool  92  will affect the surface characteristics of the undercrown  80  by creating a roughened surface of indentations  90  on the undercrown  80 . A variety of coatings may be applied to the casting tool  92  to create the roughened surface on undercrown  80 , including, but not limited to, a rough metallic coating, a ceramic coating and a black wash coating. The depth of the indentations  90  can be controlled by the selection of the specific coating to be applied to the curved upper portion  94  of the casting tool  92  and the thickness of the coating applied to the curved upper portion  94  of the casting tool  92 . 
     Additionally, the size of the regions of increased surface area  88  may be determined by the amount of coating applied to the curved upper portion  94  of the casting tool  92 . The greater the surface area of the coating applied to the curved upper portion  94 , the greater the size of the region of increased surface area  88  formed on the undercrown  80  during casting. Indeed, increasing the surface area of the coating applied to the curved upper portion  94  may result in the region of increased surface area  88  being formed on other surfaces in addition to the undercrown  80 , including the inner surfaces  83 ,  85  of the side walls  82 ,  84  of piston skirt  32 . 
     Turning now to  FIG. 9A , an alternate illustration of the region of increased surface area  188  formed on the surface of undercrown  80  of the piston  20  is shown. Piston  20  includes an exemplary region of increased surface area  188  that is defined by a cross-hatching pattern  190  integrated onto the surface of the undercrown  80  of the piston during casting of the piston  20 . A close-up view of cross-hatching  190  is shown in  FIG. 9B . Cross-hatching  190  may include two groups of generally uniform, parallel ridges  192 ,  194  formed on the surface of the undercrown  80 . Each group of the plurality of ridges  192 ,  194  intersect each other, forming a plurality of cavities  195  between the ridges  192 ,  194 . Ridges  192 ,  194  may be formed at an angle α with respect to the horizontal axis A of the piston  20 . Preferably, the angle α of the ridges  192 ,  194  is approximately 45 degrees. Providing the ridges  192 ,  194  at such an angle α with respect to the horizontal axis A creates a uniform square-shaped waffle-like pattern on the surface of the undercrown  80  containing a plurality of generally diamond shaped cavities  195 . Angling the ridges  192 ,  194  with respect to the horizontal axis A permits the length of the ridges  192 ,  194  to be longer than the width of the cross-hatching  190 . The ends of ridges  192 ,  194  at the edges of cross-hatching  190  may taper to the surface of the undercrown  80 . 
     The distance between adjacent ridges  192  and adjacent ridges  194  may be in the range of approximately 0.5 mm to 1.0 mm, and the ridges  192 ,  194  may have a height from the surface of the undercrown  80  in the range of approximately 0.4 mm to 1.0 mm. Tighter tolerances may be desirable under some circumstances. Reducing the distance between adjacent ridges  192 ,  194  allows for a greater number of ridges  192 ,  194  to be formed on the undercrown  80 . Maximizing the number of ridges  192 ,  194  on the surface of the undercrown  80 , and the height and width of the ridges  192 ,  194 , maximizes the amount of surface area that is created in the region of increased surface area  188  on undercrown  80 . Depending on the environment of use, it may be desirable to increase the surface area as much as possible as the larger surface area allows for greater heat dissipation by the piston  20 , while still taking into account the dynamic and static force requirements of the load bearing surface between the wrist pin and its mating piston surface. 
     The exemplary region of increased surface area  188  defined by the cross-hatching  190  may be formed on a substantial portion of the surface of the undercrown  80 . Preferably, the cross-hatching  190  is formed on a minimum of approximately forty percent (40%) of the surface of the undercrown  80 . Additionally, as shown in  FIG. 10 , the cross-hatching  190  may extend beyond the undercrown  80  and onto additional surfaces in the interior of the piston  20 , forming additional regions of increased surface area  188 , including one or both of the inner surfaces  83 ,  85  of side walls  82 ,  84  of piston skirt  32 , and/or onto one or both of the inner surfaces  76 ,  78  of the piston walls  75 ,  77  of the piston skirt  32 . 
     Turning now to  FIG. 11 , an illustration of the casting tool  200  is shown which forms the cross-hatching  190  on the surface of the undercrown  80  of the piston  20  during the casting process. Alternatively, a machining operation may be undertaken once the casting is completed. When cast, however, the surface roughener  96  of the casting tool  200  includes a plurality of grooves  202 ,  204  which are formed into the surface of the curved upper portion  201  of casting tool  200 . Grooves  202 ,  204  may be formed into the surface of the curved upper portion  201  of casting tool  200  by any suitable means, including, but not limited to, spark erosion or a milling operation. The plurality of grooves  202 ,  204  creates a roughened surface on the surface of the undercrown  80  by forming ridges  192 ,  194  on the surface of the undercrown  80  corresponding to the grooves  202 ,  204  during the casting process. The ridges  192 ,  194  on the surface of the undercrown  80  form the cross-hatching  190  which defines the region of increased surface area  188 . 
     Alternatively, the cross-hatching pattern formed on the surface of the undercrown may include a plurality of generally uniform, parallel grooves formed into the surface of the undercrown. To form such grooves in the surface of the undercrown, the casting tool includes a plurality of generally uniform, parallel ridges formed onto the curved upper portion of the casting tool. 
     The present disclosure has been particularly shown and described with reference to the foregoing illustrations, which are merely illustrative of the best modes for carrying out the disclosure. It should be understood by those skilled in the art that various alternatives to the illustrations of the disclosure described herein may be employed in practicing the disclosure without departing from the spirit and scope of the disclosure as defined in the following claims. It is intended that the following claims define the scope of the disclosure and that the method and apparatus within the scope of these claims and their equivalents be covered thereby. This description of the disclosure should be understood to include all novel and non-obvious combinations of elements described herein, and claims may be presented in this or a later application to any novel and non-obvious combination of these elements. Moreover, the foregoing illustrations are illustrative, and no single feature or element is essential to all possible combinations that may be claimed in this or a later application.