Abstract:
A piston assembly including a piston body pivotally connected to a connecting rod by a wrist pin. The wrist pin includes a sealed cavity therein partially filled with a heat transfer medium. The heat transfer medium may be composed of a fusible alloy. The sealed cavity may be comprised of discrete compartments. The wrist pin may be integrated with the connecting rod and share a conjoined sealed cavity therewith so that heat transfer medium can draw heat effectively away from the piston body.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to pistons for internal combustion engines, and more particularly to a wrist pin as used in piston assemblies to interconnect the piston body to a connecting rod. 
     2. Related Art 
     Piston assemblies as used in internal combustion engines are subject in use to extremely high temperatures, forces and inertia. Optimal operation of such engines requires careful management of the temperature of the components of a piston assembly which are typically manufactured from strong, lightweight metal alloys. 
     A practice long observed in diesel engine applications is to circulate lubricating oil through passages in the piston assembly to serve as both lubricant for the various joints and as a cooling medium to draw heat away from the piston crown. U.S. Pat. No. 2,742,883 to Smith, issued Apr. 24, 1956, disclose one such piston assembly wherein cooling oil is ported through a hollow wrist pin and then moved into an upper gallery formed just under the crown of the piston head. 
     In another example, U.S. Pat. No. 3,702,092 to Zollner, issued Nov. 7, 1972, discloses a lightweight wrist pin assembly formed with a plurality of open air channels or passages extending completely therethrough. The passages may be circular or have other shapes and range in number from two to six. In certain examples, the passages do not extend completely through but are internally stopped and open only at one end of the wrist pin. The stopped holes or through passages in the wrist pin are created in this case for the purpose of weight reduction. 
     In yet another example, U.S. Pat. No. 4,005,686 to Wizemann et al., issued Feb. 1, 1997 discloses a piston assembly including a wrist pin having a hollow passage therein open to receive pressurized cooling liquid (i.e., lubricating oil) from the connecting rod. The oil is routed through the wrist pin and into the piston head or crown area. 
     It has been observed that, at least in diesel applications, pistons operating under a thermal load equal to or higher than 0.4 kW/cm 2  generally require forced piston cooling in order to maintain temperatures below 270° C. At least 2 kW is thus required in a 6-cylinder heavy duty truck engine and considerably more as the size and/or specific power (kW/liter) of the engine increases. This forced piston cooling represents a parasitic loss. That is, fuel is burned in order to move this cooling liquid without any corresponding useful power being developed. 
     There is therefore a need in the art to provide more effective and efficient means by which to rid the crown of a piston of absorbed heat, for the purpose of reducing or eliminating entirely the losses associated with forced cooling. Improved methods are thus sought after with the goal of recovering parasitic losses inherent in prior art engine systems. 
     SUMMARY OF THE INVENTION 
     A wrist pin is provided for a piston assembly of the type used in an internal combustion engine. The wrist pin comprises a generally cylindrical outer surface having closed ends. A sealed cavity is formed internal to the outer surface in the closed ends. A fixed quantity of heat transfer medium is trapped inside the cavity. 
     The heat transfer medium trapped inside the sealed cavity of the wrist pin serves as an additional heat sink to help manage the thermal load in a piston assembly and thereby reduce parasitic losses otherwise required to force cooling liquid through the piston assembly. In other words, the sealed cavity in the wrist pin functions as a heat exchanger that absorbs heat directly from the pin boss structure of a piston body to which it is coupled, thus enabling conductive heat migration away from the piston crown. 
     According to another aspect of this invention, a piston assembly is provided for use in an internal combustion engine. The piston assembly comprises a piston body including a pair of pin bosses. Each pin boss has a pin bore. The pin bores lie along a common pin bore axis and each has a circumferentially extending pin bore surface. The two pin bore surfaces are spaced from one another by an intervening gap or space. A connecting rod has an upper end disposed in the intervening space between the pin bosses. A wrist pin is disposed within the pin bores of the piston body and is operatively connected to the upper end of the connecting rod to pivotally interconnect the piston body and the connecting rod. The wrist pin includes a sealed cavity therein. A fixed quantity of heat transfer medium is trapped inside the sealed cavity. 
     The subject invention offers a unique and relatively inexpensive solution to use the wrist pin as an additional heat sink, thereby bringing engine technology one step closer to recovering parasitic losses inherent in prior art systems which require forced piston cooling in order to maintain temperatures below a threshold temperature limit. 
    
    
     
       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 exploded perspective view of a piston assembly showing an exemplary piston body in partial cross-section; 
         FIG. 2  is a cross-sectional view of another exemplary piston assembly according to an embodiment of the invention; 
         FIG. 3  is a cross-sectional view of a wrist pin taken generally along lines  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a cross-sectional view as in  FIG. 3  but showing an alternative embodiment of the sealed cavity; 
         FIG. 5  is an exploded view of a piston assembly according to another alternative embodiment wherein the wrist pin and connecting rod are formed as an integral unit; and 
         FIG. 6  is a cross-sectional view taken generally along lines  6 - 6  in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the figures wherein like numerals indicate like or corresponding parts throughout the several views, a piston assembly constructed in accordance with preferred embodiments of the invention is generally shown at  10  in  FIGS. 1 and 2 . The piston assembly  10  includes a piston body, generally indicated at  12 . The piston body may be of the single or multi-piece type, and formed with open or closed galleries or other configurations. Those designs shown in  FIGS. 1 and 2  are intended for use in diesel engines which is a preferred implementation of this invention, however not the exclusive means by which the novel concepts of this invention may be practiced. As a consequence, the invention herein is applicable to gasoline, natural gas and other types of internal combustion engines in addition to the depicted diesel types. The piston body  12  is formed with a crown  14  at its upper end having, in these examples, a combustion crater or bowl  16 . A generally cylindrical skirt  18  extends downwardly from the crown  14 . A ring belt may be formed between the crown  14  and skirt  18  so as to contain one or more ring grooves  20  intended to contain piston rings in the customary fashion. A pair of pin bosses  22  extend downwardly from the underside of the crown  14  and are disposed generally within the boundary of the skirt  18 . Each pin boss  22  has a pin bore  24  formed therein. The pin bores  24  are disposed along a common pin bore axis A and each have circumferentially extending pin bore surfaces which are spaced from one another by an intervening space or gap. Typically, the pin bores  24  extend through the skirt  18 . 
     A connecting rod is generally indicated at  26 . The connecting rod  26  may be any suitable type used in an internal combustion engine to operatively interconnect a rotating crankshaft (not shown) to the reciprocating piston body  12 . In particular, the connecting rod  26  in the example shown has an upper end  28 , also referred to commonly as the small end. The connecting rod  26  also includes a lower end  30  and a shank  32  extending between the upper  28  and lower  30  ends. Typically, the entire connecting rod  26  is formed as a monolithic structure made in a casting or forging operation or other suitable manufacturing process. The lower end  30  of the connecting rod  26  is designed to couple with the crankshaft in any of the customary fashions. 
     Within the piston assembly  10 , the upper end  28  of the connecting rod  26  is disposed in the intervening space between the pin bosses  22  of the piston assembly  12 . In the most typical and preferred embodiments, the upper end  28  is formed with a small end bore  34  which may be sized with a diameter equal to the diameter of the pin bores  24 . The small end bore  34  is also aligned along the pin bore axis A so that it is centered together with the pin bores  24 . 
     A wrist pin according to the subject invention is generally indicated at  36 . The wrist pin is, preferably, of generally cylindrical configuration having closed ends  38 . The wrist pin  36  is sized in terms of both diameter and length to snugly fit within the pin bores  24  and small end bore  34  and to be retained in position there by spring clips seated in clip grooves  40  in the respective pin bores  24 . In this manner, the wrist pin  36  pivotally interconnects the piston body  12  and the connecting rod  26  so that the piston body  12  can reciprocate linearly in a cylinder bore (not shown) while the connecting rod  26  is subjected to general plane motion via its lower end  30  connection to a rotating crankshaft (not shown). 
     As perhaps best shown in  FIG. 3 , the wrist pin  36  includes a sealed cavity, generally indicated at  42 . A fixed quantity of heat transfer medium  44  is trapped inside the sealed cavity  42 . Preferably, the heat transfer medium  44  only partially fills the available space inside the sealed cavity  42  so that heat extraction from the inner walls of the cavity  42  can be accomplished with a “cocktail shaker” action during reciprocating motion of the piston body  12 . Any suitable material may be used for the heat transfer medium  44  including liquids, pelletized solid materials, and low temperature fusible metals and alloys. A fusible alloy is a metal alloy capable of being easily fused, i.e., easily meltable, at relatively low temperatures. Fusible alloys are commonly, but not necessarily, eutectic alloys. It is known that melted fusible alloys can be used as coolants as they are stable under heating and can give much higher thermal conductivity than many other coolants. Particularly, this may be the case with alloys made with a high thermal conductivity such as indium or sodium. Other suitable fusible metal alloys include lithium based materials and liquids such as sodium-potassium. Other suitable cooling materials may include silicone oil laced with copper or aluminum. The volume of the coolant inside the sealed cavity  42  can vary. Preferably the capacity of the sealed cavity  42  will be filled from 10-50% in order to promote the so-called “cocktail shaker” effect. As shown in  FIG. 3 , the sealed cavity  42  may be composed of a plurality of distinct compartments  46  rifle-drilled into the wrist pin  36  and sealed with the closed ends  38 . In this example, the compartments  46  are isolated from one another so that the heat transfer medium  44  in one compartment  46  cannot migrate into another compartment  46 . 
       FIG. 4  is a cross-sectional view as in  FIG. 3  but depicting an alternative embodiment wherein the sealed cavity  42 ′ can be formed as a cartouche formed by any suitable method including casting, sintering or machining from a blank. In this example, prime designations are used to indicate like or corresponding parts with like reference numerals introduced in the preceding embodiment. The sealed cavity  42 ′ is filled partially with the coolant of choice  44 ′ and in the volume range already specified. Internal contours in the cavity  42 ′ form compartments  46 ′ which are not isolated from one another, but in fact open to one another so that the heat transfer medium  44 ′ in one compartment  46 ′ can migrate to another compartment  46 ′. The internal contours of the sealed cavity  42 ′ are shown here only as one example, with those of skill in the art appreciating that shapes and contours can be varied to suit the process. This embodiment also demonstrates a concept whereby a cartouche is press fit or shrink-fit into a thin steel shell  48 ′ which in turn provides a durable outer surface or sleeve with which to interconnect the connecting rod and piston body. In other words, the shell  48 ′ will act as the bearing element against the pin bores. The cartouche will confer the steel shell  48 ′ with structural stiffness necessary to carry the loads. 
     Turning now to  FIGS. 5 and 6 , a still further embodiment of this invention is shown wherein the wrist pin  136  is designed to bear against a saddle  124  machined into the piston undercrown. In the embodiment of  FIGS. 5 and 6 , like or corresponding reference numerals to those introduced above are offset by 100. In this case, the connecting rod  126  and wrist pin  136  are integrally connected to one another as a unitary structure. Attachment between the two components can be via any suitable technique including bolting, welding, and the like. Cap bearings  150  are attached by nuts  152  to studs  154  depending from the pin bosses  122 . Caps and upper bearing surfaces might also be coated with tribological coatings like Manganese Phosphate, DLC (Diamond Like Coating), coatings derived from PVD processes and RoC™ (Robust Coating) comprised by a Chrome matrix and nanodiamonds. In this manner, the wrist pin  136  is captured along the pin bore axis for functional articulation in the well known manner. 
     As shown in  FIG. 6 , the wrist pin  136  and connecting rod  126  may be formed with matched rifle drilling that penetrates some into the wrist pin  136  and most of the connecting rod shank  132  length. The sealed cavity in the shank  132  is indicated by reference numeral  156 . This cavity  156  is also filled with the coolant of choice and in the same volume range as specified previously. A particular advantage of this construction is that heat load coming from the piston body  112  is dissipated into the wrist pin  136 /connecting rod  126  pair and effectively transferred to the connecting rod shank  132  via the heat transfer medium  144 . The connecting rod  126  then transfers this heat to the oil film (not shown) flowing through the crank pin (not shown) as well as to the oil mist and air inside the crank case (not shown) of the internal combustion engine in which the assembly  110  is deployed. As will be appreciated by those of skill in this art, the heat transfer path as described is very effective due to the typical dynamics of the components in this assembly  110  and the very high relative speeds in relation to crank case environment. 
     The foregoing invention has been described in accordance with the relevant legal standards, thus the description is exemplary rather than limiting in nature. Variations and modifications to the disclosed embodiment may become apparent to those skilled in the art and fall within the scope of the invention.