Abstract:
A piston for diesel engines includes a piston body formed with a closed oil gallery and a pair of pin bosses having axially aligned pin bores whose surfaces are spaced from one another about their circumference across an intervening gap between the pin bosses. The pin bore surfaces are each formed with a recess which extends in the axial direction of the pin bores across the full width of each of the pin bore surfaces. An oil passage leads from the gallery to each of the recesses and feeds the recesses with lubricating oil from the gallery. The recesses act as retention basins or reservoirs which collect and supply the pin bore surfaces with lubricating oil during the fall cycle of the piston.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates generally to pistons for diesel engine applications, and more particularly to those having a closed oil gallery. 
     2. Related Art 
     It is known in diesel engine applications to provide a piston whose piston body is formed with a closed gallery for cooling oil. The oil circulates through the gallery and cools parts of the piston which are susceptible to damage from the heat of combustion. Such cooling galleries are generally annular or ring-shaped and are provided just inside of the ring belt adjacent the top wall of the piston body. The gallery is bounded by the inner wall and closed at the bottom by a bottom wall. One or more inlets are provided for receiving cooling oil into the gallery. Various outlets are also provided for directing portions of the oil from the gallery into other regions of the piston for cooling or lubrication purposes. It is know to provide an oil passage leading from the cooling gallery to the pin bores of the pin bosses for lubricating the wrist pin. The entry point for the lubrication is rather localized and provides a limited supply of oil for lubrication which may not be present during the entire stroke of the piston. 
     It is an object of the present invention to improve the lubrication of the pin bore and closed gallery diesel piston applications. 
     SUMMARY OF THE INVENTION 
     A piston for diesel engines constructed according to a presently preferred embodiment of the invention includes a piston body having an oil gallery and a pair of pin bosses having pin bores disposed about a common pin bore axis and pin bore surfaces which are spaced from one another about their circumference by an intervening space between the pin bosses. According to the invention, at least one recess is formed in at least one of the pin bore surfaces and extends axially across the at least one pin bore surface. At least one associated oil passage extends from the gallery to the recess for feeding oil from the gallery to the recess. 
     The invention has the advantage of providing improved lubrication to the pin bosses of diesel engine pistons having closed oil cooling galleries which serves as a reservoir for lubricating oil for the pin bosses. 
     The invention has the further advantage of providing an axial relief or recess in the pin bore surface which itself acts as a localized oil reservoir for lubricating oil adjacent the pin bore surface. The recess, fed by the oil passage, captures or wicks the oil and then feeds the oil to the pin bore surface during the full stroke of the piston, including at the top of the stroke when less oil is available in the gallery to be fed to the pin bores. At such times, the pocket of oil retained in the pin bore recess is available to feed the pin bore surfaces with lubricating oil. 
     Another advantage of the invention is that it provides a simple solution for greatly improving the lubrication of the pin bore of closed gallery diesel pistons. The recess may be formed in the pin bore surface by a simple machining operation and could be readily adapted to existing manufacturing processes during the machining of the pin bore surfaces. 
     The invention has the further advantage of prolonging the life of pistons, connecting rods and wrist pin by improving the lubrication to their interactive surfaces during operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become more readily appreciated when considered in connection with the following detailed description and appended drawings, wherein: 
     FIG. 1 is an elevational view of a piston constructed according to a presently preferred embodiment of the invention; 
     FIG. 2 is a cross-sectional plan view taken along lines  2 - 2  of FIG. 1; 
     FIG. 3 is a partially sectioned plan view taken generally along lines  3 - 3  of FIG. 2, but of the entire piston assembly; 
     FIG. 4 is a cross-sectional elevation view taken generally along lines  4 - 4  of FIG. 2, but of the entire piston assembly; 
     FIG. 5 is a fragmentary bottom perspective view of the piston of FIG. 1; 
     FIG. 6 is a fragmentary top perspective view, with a top portion of the piston removed, as in FIG. 2; and 
     FIG. 7 is a fragmentary cross-sectional view taken generally along lines  7 - 7  of FIG.  6 . 
    
    
     DETAILED DESCRIPTION 
     A closed gallery piston assembly constructed according to a presently preferred embodiment of the invention is indicated generally at  10  in FIG.  1  and comprises a piston body  12  having an annular top wall  14  with an upper surface  16 . A combustion crater or bowl  18  extends into the top wall  14  from the upper surface  16 . The top wall  14  has a lower or underside surface  22  opposite the upper surface  16 . 
     The piston body  12  has an outer wall or ring belt  24  that is annular and extends downwardly from the top wall  14 . The outer wall  24  has an outer annular peripheral surface  26  formed with a plurality of ring grooves  28 . The outer wall  24  includes an inner annular surface  30  spaced radially inwardly from the outer surface  26 . 
     The piston body  12  includes an inner wall  32  projecting downwardly from the combustion bowl  18  and having a radially outwardly facing surface  34  spaced radially inwardly from the inner surface  30  of the outer wall  24 . 
     The piston body  12  has an annular bottom wall  36  which is spaced from the top wall  14  and extends between and interconnects the outer wall  24  and inner wall  32  adjacent their lower ends. The bottom wall  36  has an upper floor surface  38  and lower surface  40 . 
     Collectively, the walls  14 ,  24 ,  32  and  36  define an interior, annular, ring-like cavity or gallery  42  within the piston body  12  that is closed by the walls. As illustrated in FIGS. 2-4, the gallery  42  extends completely around the piston body  12  and is bounded at the top by the top wall  14 , at the bottom by the bottom  36 , at the outer periphery by the outer wall  24 , and at the inner periphery by the inner wall  32 . By “closed” it is meant that the gallery  42  is closed at the bottom by a structural component of the piston body  12 , namely the bottom wall  36 , which not only extends between but joins the lower ends of the outer wall  24  and inner wall  32 . As will be explained further below, various openings and passages are provided to allow cooling oil to circulate into and out of the gallery  42 , and thus the term “closed” contemplates the provision of such openings and passages to accommodate the flow of cooling oil through the gallery  42 . It will also be appreciated by those skilled in the art that the terms “top”, “bottom”, “inner” and “outer” in describing the walls are intended and should be construed to represent portions of the surrounding wall structure which enclose the gallery  42  and should not be strictly construed based on the illustrated embodiment shown in the drawings since the particular shape and size of the gallery  42  will likely change from piston to piston depending on the particular cooling requirements necessary for a particular application. 
     The piston body  12  is further formed with a pair of pin boss portions  44  that are formed and preferably investment cast as one piece with the inner wall  32  and bottom wall  36  from steel. The pin bosses  44  have outer faces  46  that face away from one another and inner faces  48  that face toward one another. The inner faces  48  are each generally planar and preferably divergent toward the bottom of the pin bosses  44 , and define a space  50  between the inner faces  48  for accommodating a connecting rod  52  (FIG.  4 ). A dome or cavity  54  may extend above the space  50 , as shown, for cooling the combustion bowl  18 . The surfaces which form the cavity  54  extend from, but out of the plane of, the inner faces  48  of the pin bosses  44  and, in the illustrated embodiment, are provided in part by inner surfaces  56  of the inner wall  32 . The pin bosses  44  are formed with axially aligned pin bores  58  having pin bore surfaces  60  which are substantially cylindrical and aligned about a pin bore axis A (FIG.  3 ). The outer and inner faces  46 ,  48  surround the pin bores  58 . The pin bores  58  receive a wrist pin (not shown) which serves to interconnect the piston body  12  with the connection rod  52 . The pin bore surfaces  60  provide support to the wrist pin, preferably without the assistance of any bushings, such that the pin bores  58  are preferably bushingless. Each of the pin bores  58  includes an annular snap ring groove  62  for receiving a snap ring to secure the wrist pin (not shown) within the pin bores  58  in usual manner. 
     The piston body  12  also includes a piston skirt  64 . The piston skirt  64  is preferably cast as a single piece with the pin bosses  44 , thus providing a monobloc piston structure rather than an articulated skirt. The skirt could, however, be formed as a separate structural component from the piston body  12  and joined through the wrist pin (not shown) in articulated manner to the pin bosses  44 , while retaining the closed gallery structure of the piston body  12 , but the monobloc structure is preferred. The piston skirt  64  has an outer surface  66  extending between the pin bosses  44  that is substantially in line and forms a extension of the outer surface  26  of the outer wall  24 . The outer surface  66  is interrupted across the pin bores  58  to provide recessed side faces  68  where the skirt  64  joins the pin bosses  44 . An inner surface  70  of the piston skirt  64  defines a space  72  adjacent the pin bores  58  that is walled off by the skirt  64 . 
     The closed gallery structure of the piston body  12  is preferably achieved by forming the piston body  12  from at least two separate parts which are subsequently joined across a joint or joints  74  to effectively yield a united, one piece body structure once joined. While there are a number of ways to join such separate components, all of which are contemplated by the invention, the preferred approach is to join the separately formed components across a friction weld joint  74 , as illustrated in FIGS. 3 and 4. In such case, a top part  76  above the joint  74  is separately formed from a bottom part  78  on the opposite side of the joint  74 , and the separately formed parts  76 ,  78  are then friction welded together across the joint  74  to yield the united structure as shown in the drawings. Some examples of other joining techniques that are contemplated include other means of welding, bonding, brazing, screw thread joint, and other mechanical and metallurgical means of uniting the separate components together to yield the closed gallery structure of the piston body  12 . 
     According to a further preferred aspect of the invention, at least the bottom part  78  is investment cast from steel, and the top part  76  may likewise be investment cast from steel or formed by other techniques such as forging or other casting techniques. 
     Referring now particularly to FIGS.  2  and  5 - 7 , the bottom wall  36  of the piston body  12  is formed with at least one and preferably two oil access holes  80  which extend from the lower surface  40  of the bottom wall  36  within the space  72  into the oil gallery  42 . The oil holes  80  preferably are entry ports for introducing cooling oil into the gallery  42 . When the piston  10  is installed in a diesel engine, the oil holes  80  communicate with associated oil injection nozzles (not shown) which direct a stream of cooling oil from below up into the space  72  and into the gallery  42  through the holes  80 . Once in the gallery  42 , the cooling oil serves to cool the upper part of the piston body  12 , extracting heat from the walls as the oil is moved about in the gallery  42  with a “cocktail shaker” action during reciprocation of the piston  10 . 
     Because of the closed gallery structure of the piston body  12 , the combustion forces exerted on the top wall  14  which drive the piston  10  downwardly in the cylinder are transferred to the pin bosses  44  not only through the inner wall  32 , but also through the outer wall  24  and interconnecting bottom wall  36 . As such, the outer wall  24  and bottom wall  36  serve as structural load-bearing portions of the piston which must withstand the forces of combustion and transfer such loads to the pin bosses  44  without failure. The oil holes  80  and the bottom wall  36  represent an abrupt discontinuity in the bottom wall structure, and thus a potential site for stress concentration and potential failure. The present invention address this problem by reinforcing the piston body structure in the vicinity of the oil holes  80  to counteract the stress concentration effects caused by the introduction of the oil holes  80  in the bottom wall  36 . According to the invention, the piston body  12  is formed with oil hole bosses  82  bordering the oil holes  80 , which are best shown in FIGS. 2,  6  and  7 . The oil hole bosses  82  are defined by localized thickened portions of the bottom wall  36  which immediately border the oil holes  80  in order to give added structural integrity to the bottom wall  36  in the area surrounding the oil holes  80 . As illustrated most clearly in FIGS. 6 and 7, the oil hole bosses  82  extend above the upper floor surface  38 , such that the thickness of the bottom wall  36  immediately adjacent the oil hole bosses  82  is thinner than that of the portion of the bottom wall  36  making up the oil hole bosses  82 . The oil hole bosses  82  preferably extend into and are formed as one piece with the outer wall  24 , providing added structural integrity to the transition region between the outer wall  24  and bottom wall  36  in the vicinity of the oil holes  80 . It is preferred that all corners of the oil hole bosses  82  are rounded, as illustrated in FIG. 6 and 7 to reduce stress concentration. 
     As shown best in FIGS. 2 and 6, the oil hole bosses, when viewed from above in plan, have a non-circular shape and preferably include generally triangular regions or portions  84  where the oil hole bosses  82  join the outer wall  24 . It will be appreciated that the particular size and shape of the oil hole bosses  82  will be governed in large part by the structure needed to counteract the stress concentration imparted by the presence of the oil holes  80 . One advantage of investment casting the bottom part  78  is that the oil hole bosses  82  can be precisely formed to the net or near net shape needed to provide the desired counteracting structure against stress concentration of the holes  80 . 
     According to another aspect of the invention, at least one and preferably a pair of passages  86  extend from the gallery  42  directly to the inner faces  48  of the pin bosses  44 , so as to provide direct lubrication to the inner faces  48  between the pin bosses  44  and the connection rod  52 . The passages  86  are best shown in FIGS. 4 and 5. In the illustrated embodiment, there are thus four such passages  86 , two servicing each inner face  48  of the pin bosses  44  on opposite side of the pin bore axis A. The passages  86  are spaced from the walls which form the dome  54  and open directly to the inner faces  48  to provide direct lubrication in the gap between the pin bosses  44  and the connecting rod  52 . 
     According to still a further aspect of the invention and as shown best in FIGS. 1 and 3, the pin bores  58  are formed with an axial recess or pocket  88  which extends axially in the direction of the axis A of the pin bores  58  and presents a discontinuity in the cylindrical pin bore surfaces  60 . The recesses  88  are preferably concave and are located at least partly above the center line axis A of the pin bores. The recesses  88  extend axially across the full width of the pin bores  58  and thus are co-extensive with the width of the pin bore surfaces  60  between the outer  46  and inner  48  faces of the pin bosses  44 . The recesses  88  are aligned axially with one another and are interrupted by the space  50  between the inner faces  48  of the pin bosses  44 , as are the pin bore surfaces  60 . The recesses  88  are dome-shaped or concave in cross-section when viewed in the direction of the pin bore axis A. Oil passages  90  extend from the gallery  42  and open directly into each of the recesses  88  so as to feed oil to the recesses  88  during operation of the piston  10  across the full width of the pin bores  58 . The oil passages  90  preferably originate from the lowest part of the gallery  42  so as to provide a constant supply of oil to the pin bores  58  during the full cycle of movement of the piston. The entry of each oil passage  90  into its associated recess  88  is preferably about midway between the outer and inner faces  46 ,  48  of the pin bosses  44  to promote uniform distribution of oil. The recesses  88  serve as reservoirs or holding pockets for oil and continue to feed oil to the pin bore surfaces  60  during the full stroke of the piston  10  to provide full time uniform lubrication. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. The invention is defined by the claims.