Abstract:
A piston for diesel engine applications has a piston body cast entirely of one piece of steel and includes a piston head with a combustion bowl, a ring belt and an oil cooling gallery. A pair of pin bosses and a piston skirt are cast as one piece with the piston head out of the same steel material.

Description:
[0001]     This application claims priority to U.S. Provisional Patent Application No. 60/589,779, filed Jul. 21, 2004. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates generally to heavy duty pistons for diesel engines, and more particularly to monobloc pistons manufactured with an integrated skirt and an oil cooling gallery in the head of the piston.  
       RELATED ART  
       [0003]     Monobloc pistons for heavy duty piston applications are known to the industry and characteristically include an upper piston head portion formed with an outer ring belt region surrounding a recessed combustion bowl region and formed with an annular oil cooling gallery between the ring belt and combustion bowl in which cooling oil is fed to cool the upper portion of the piston during operation. Such pistons are further formed with a pair of laterally spaced pin bosses featuring aligned bores for receiving a wrist pin to couple the piston to a connecting rod. The pin bosses are provided in the lower portion of the piston beneath the head. Monobloc pistons further include a piston skirt region which is formed as one-piece with the pin bosses so as to be immovable relative to the pin bosses, as opposed to an articulated style piston in which the skirt is separately formed and coupled for articulated movement to the pin bosses through the wrist pin.  
         [0004]     Monobloc pistons are traditionally made as either a one-piece casting of aluminum or cast iron, or as a two or more piece construction from various materials including iron and steel which are cast and/or forged and subsequently united to provide a one-piece joined structure through various means including bolting, brazing, or welding. The intricacy of the various passages and recesses, and in particular the cooling gallery regions, has restricted the choice of materials to aluminum or cast iron. The multipiece joined structure has the advantage of dividing the piston structure into discrete parts which can be individually manufactured and then joined to unite the parts. The typical multipart monobloc piston is divided across a parting line that passes through the oil cooling chamber. In this way, part of the cooling chamber is formed in the upper head or “upper crown” section, and the remaining part of the cooling chamber is formed in the lower pin boss or “lower crown” section. The upper crown is often cast of steel and is united across the parting line to the lower crown which is sometimes forged of iron. Steel has a higher modulus of elasticity than that of iron and thus has advantages for use in the upper crown section which is subjected to heat and cyclic loading of combustion. There has been little motivation to form the lower crown from other than forged cast iron since the lower crown is not exposed to the level of heat and loading as that of the upper crown. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     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:  
         [0006]      FIG. 1  is a perspective view, shown partly broken away, of a piston constructed according to a presently preferred embodiment of the invention;  
         [0007]      FIG. 2  is a perspective view of the piston of  FIG. 1 , shown from another angle and partly broken away;  
         [0008]      FIG. 3  is a perspective view of the piston of  FIG. 1  from a different angle;  
         [0009]      FIG. 4  is a plan view of the piston;  
         [0010]      FIG. 5  is a cross-sectional view taken along lines  5 - 5  of  FIG. 4 ;  
         [0011]      FIG. 6  is a cross-sectional view taken along lines  6 - 6  of  FIG. 5 ; and  
         [0012]      FIG. 7  is a cross-sectional view taken along lines  7 - 7  of  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     A piston constructed according to an embodiment of the invention is shown generally at  10  in the drawings. The piston is of a monobloc construction and cast entirely of one piece of steel, and preferably of SAE 4140H steel. The piston has an upper head portion  12  formed with an upper wall  14  that is generally planar and includes a combustion bowl  16  recessed into the upper wall  14  and bounded by a contoured combustion bowl wall  18  that includes an undercut corner region  20  that extends radially outwardly of an upper lip  22  of the combustion bowl  16  to provide a reentrant structure to the combustion bowl  16 . Inward of the undercut region  20 , the combustion bowl wall  18  is dome-shaped, with the center of the dome-shaped wall  18  rising above the under cut region  20  toward the upper wall  14 , but terminating below the lip  22 .  
         [0014]     The head portion  12  further includes an outer annular ring belt wall  24  that extends downwardly from the upper wall  14  and is formed with a plurality of ring grooves  26  that are either cast into the ring belt and then machined, or formed entirely by machining following casting. The ring grooves  26  accommodate a corresponding plurality of piston rings (not shown) as is conventional.  
         [0015]     The head portion  12  is formed with an as-cast oil cooling gallery  28  inward of the ring belt  24  and below the combustion bowl  16 . The oil cooling gallery  28  has an outer annular wall defined by the ring belt  24  and, an upper wall defined by the undercut region  20  of the combustion bowl wall  18 . An inner annular wall  30  of the gallery  28  is spaced radially inwardly of the ring belt  24  and extends downwardly from the combustion bowl wall  18  at a location radially inwardly of the undercut region  20 . The ring belt  24  is relatively thicker than that of the inner annular wall  30 , and the inner annular wall is, in turn, relatively thicker than that of the combustionable bowl  18 .  
         [0016]     The oil cooling gallery  28  includes a bottom wall or floor  32  which extends between the ring belt  24  and inner annular wall  30  to partially close the oil cooling gallery  28  to the bottom, as will be described in further detail below.  
         [0017]     The piston further includes a pair of laterally spaced pin bosses  34  that are cast as one piece with the head portion  12  and which project downwardly from the bottom wall  32  of the head portion. The pin bosses  34  are cast with a set of pin bores  36  aligned along a pin axis  38  for receiving a wrist pin (not shown) for connection of the piston  10  to a connecting rod (not shown) in the usual manner.  
         [0018]     The piston  10  is further formed with a piston skirt  40  which is cast as one piece with the head portion  12  and pin bosses  34 . This skirt  40  is connected to both the ring belt  24  and the pin bosses  34  and is otherwise unsupported. The skirt  40  is formed with a set of windows or openings  42  that are cast into the skirt  40  on laterally opposite sides of each of the pin bosses  34 , for a total of four such windows  42 . The windows  42  eliminate material mass and thus reduce the overall weight of the piston in areas where the skirt is not needed.  
         [0019]     Turning back to the oil cooling gallery  28 , it will be seen that the relatively thin-sectioned inner annular wall  30  and undercut region  20  of the combustion bowl wall  18  are formed with reinforcement ribs  44  to provide locally thickened wall regions to enhance the structural rigidity of the wall portions to withstand the forces of combustion while minimizing the wall thickness in the adjacent unribbed regions to account for an overall reduction in weight of the piston. It will be seen that the ribs  44  extend only partially into the oil gallery  28  and thus do not close off the gallery in the circumferential direction such that the gallery remains open and continuous in the circumferential direction. The size of the ribs  44  vary, with the thickest of the ribs lying over the pin bosses  34  in line with the pin axis  38 .  
         [0020]     Ribs  46  are also provided on the underside of the combustion bowl wall  18  radially inward of the inner wall  30  to strengthen the otherwise thin wall structure of the combustion bowl wall  18  in the dome region.  
         [0021]     The pin bosses  34  are formed with hollowed regions or pockets  48  forming a generally saddle-shaped chamber extending below the bottom wall  32  as an extension of the oil cooling gallery  28  in order to reduce material mass and allow cooling oil to drain from the oil cooling gallery  28  into the hollowed regions  28  of the pin bosses  34 . The hollowed regions  48  extend down into the pin bosses  34  on either side of the pin bores  36  and terminate short of the pin axis  38 . The bottom wall  32  is absent in the hollowed region areas  48  such that there is direct open communication with the oil cooling gallery  28 .  
         [0022]     In the regions between the pin bosses  34 , the bottom wall  32  is preferably formed with at least one and preferably a plurality of openings  50 . The openings  50  allow the oil cooling gallery and the various associated hollowed regions and ribs inside the gallery to be formed during casting by means of a casting core which, following casting, can be removed completely through the openings  50 . In addition, the openings  50  contribute to a reduction in overall mass of the piston  10 . As shown, there are preferably four such openings, each pair of openings being separated by an intervening ridge section of the bottom wall  32 , although the invention contemplates elimination of the bridge  52  from one or both of the sets of openings, if desired. The openings  50  further serve to provide access to the oil cooling gallery  28  for feeding cooling oil into the piston during operation and to allow, at least in part, for the escape of oil from the gallery. The hollowed regions  48  may also include oil escape holes (not shown) for providing lubrication to the pin bores  36  and/or to the inner faces of the pin bosses  34  to enhance lubrication with the wrist pin and connecting rod interface.  
         [0023]     The piston is preferably cast from 4140H steel. Although not limited to a particular process, the piston may be cast using slow-fill counter gravity casting techniques which enables steel, which is otherwise prone to solidification, shrinkage and porosity, to be cast in complex thin-walled sections with intricate features, as are presenting the piston  10 , without forming unacceptable levels of porosity and solidification defects in the resultant casting.  
         [0024]     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.