Abstract:
A piston assembly is disclosed for use with an engine. The piston assembly may include a first and second piston crown and a first and second connecting rod. The first and second connecting rods may each have a first end pivotally connected to the first and second piston crowns, respectively, and a second end with a circular opening configured to receive a throw of a crankshaft. The second connecting rod may have a running surface defining at least two outer lands and at least one inner land disposed between the at least two outer lands that alternately support a load of the second piston crown. The piston assembly may further include a bearing. Both the inner and outer lands may simultaneously support the load of the second piston crown against the bearing at a point of highest load on the second piston crown.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to a piston assembly and, more particularly, to a piston assembly having an offset connecting rod bearing. 
       BACKGROUND 
       [0002]    Internal combustion engines convert chemical energy in fuel into mechanical energy through a series of explosions within a combustion chamber of the engine. These explosions cause pistons of the engine to reciprocate within enclosed spaces called cylinders. Each piston is typically connected to a crankshaft by a connecting rod, such that movement of the piston results in rotation of the crankshaft. A bearing is disposed between an end of the connecting rod and the crankshaft. In some applications, multiple connecting rods connect to the crankshaft via a single bearing. 
         [0003]    During engine operation, each connecting rod experiences tremendous stress under the load of the corresponding piston as force from the explosion is mechanically transferred to the crankshaft. Generally, this stress intensifies with higher engine speeds and engine firing pressures. Under such tremendous stress, an insufficient lubrication between the connecting rod and its associated bearing can result in elevated friction and wear. The elevated friction and wear can reduce the durability, reliability, and efficiency of the engine. 
         [0004]    One attempt to improve lubrication in a common bearing/multi-rod application is described in “Dynamics of Offset Journal Bearings—Revisited” by S. Boedo and J. F. Booker that published in 2009. In particular, Boedo and Booker describes applications of offset journal bearing designs for diesel engines that improve lubrication between a connecting rod and its associated bearing. Offset journal bearing designs traditionally involve offsetting journal segments within a bearing. In such arrangements, the primary support of the rod load alternates between the segments. Load sharing between segments facilitates lubrication by periodically relieving the load carried by each segment, which helps to maintain a hydrodynamic lubrication in the segments. Boedo and Booker also describes grooved bearing surfaces that enhance lubrication. 
         [0005]    Although the offset bearing of Boedo and Booker may enhance lubrication between connecting rods and their associated bearing, it may be less than optimal. This is because the angular arrangement of the segments of Boedo and Booker does not correspond with the highest loads generated by the associated pistons. As a result, a sufficient oil film may not be generated at the appropriate time during movement and loading of the pistons. Further, while grooved bearings may generally facilitate lubrication distribution, such grooves can also limit a load bearing area available on the bearing. This reduction in load bearing area may reduce the maximum load that can be transmitted through the bearing. 
         [0006]    The piston assembly of the present disclosure solves one or more of the problems set forth above and/or other problems in the art. 
       SUMMARY 
       [0007]    In one aspect, the present disclosure is related to a piston assembly. The piston assembly may include a first piston crown, and a first connecting rod having a first end pivotally connected to the first piston crown and a second end with a circular opening configured to receive a throw of a crankshaft. The piston assembly may also include a second piston crown, and a second connecting rod having a first end pivotally connected to the second piston crown and a second end with a circular opening configured to receive the throw of the crankshaft. The second connecting rod may have a running surface defining at least two outer lands and at least one inner land disposed between the at least two outer lands that alternately support a load of the second piston crown. The engine may further include a bearing disposed within the circular openings of the first and second connecting rods between the second ends of the first and second connecting rods and the throw of the crankshaft. Both the inner and outer lands may simultaneously support the load of the second piston crown against the bearing at a point of highest load on the second piston crown. 
         [0008]    In another aspect, the present disclosure may be related to an engine. The engine may include an engine block at least partially defining a plurality of cylinders, and a crankshaft rotatably disposed within the engine block. The engine may further include a first piston crown disposed within a first of the plurality of cylinders, and a first connecting rod having a first end pivotally connected to the first piston crown and a second end with as circular opening configured to receive a throw of the crankshaft. The engine may also include a second piston crown disposed within a second of the plurality of cylinders, and a second connecting rod having a first end pivotally connected to the second piston crown and a second end with a circular opening configured to receive the throw of the crankshaft. The second connecting rod may have a running surface defining at least two outer lands and at least one inner land disposed between the at least two outer lands that alternately support a load of the second piston crown. The piston assembly may further include a bearing disposed within the circular openings of the first and second connecting rods between the second ends of the first and second connecting rods and the throw of the crankshaft. Both the inner and outer lands may simultaneously support the load of the second piston crown against the bearing at a point of highest load on the second piston crown. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a cross-sectional illustration of an exemplary disclosed engine; 
           [0010]      FIG. 2  is a perspective view illustration of exemplary disclosed connecting rods that may be used in conjunction with the engine of  FIG. 1 ; 
           [0011]      FIG. 3  is an enlarged side view illustration of an end of a connecting rod of  FIG. 2 ; 
           [0012]      FIG. 4  is a top view illustration of an exemplary disclosed bearing that may be used in conjunction with the connecting rods of  FIG. 2 ; 
           [0013]      FIG. 5  is a cross-sectional illustration of the bearing of  FIG. 4 ; and 
           [0014]      FIG. 6  is a pictorial illustration of a connecting rod of  FIG. 2  and the bearing of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  illustrates an exemplary embodiment of an engine  10  that may be, for example, a diesel engine, a gasoline engine, or a gaseous fuel-powered engine. Engine  10 , in this embodiment, is a two-cycle diesel engine of a locomotive. Engine  10  may include, among other things, an assembly of pistons  12 , connecting rods  14 , and a crankshaft  16 . These components may operate together to transform chemical energy in fuel into useful rotational motion of crankshaft  16  through a series of explosions within combustion chambers  18  of engine  10 . These explosions may cause pistons  12  and connecting rods  14  of engine  10  to reciprocate within cylinders  20 . 
         [0016]    Each piston  12  may be connected to crankshaft  16  by a corresponding one of connecting rods  14 , such that movement of piston  12  results in rotation of crankshaft  16 . Connecting rods  14  may include a first end  24  having a piston pin bore  13  and a second end  26  having a crank bore  27 . Piston pin bore  13  may receive a piston pin that pivotally connects each connecting rod  14  to a corresponding piston  12  at a crown  45  of piston  12 . Crank bore  27  may receive a throw  11  that pivotally connects each connecting rod  14  to crankshaft  16 . During operation of engine  10 , connecting rods  14  may move in a tilted reciprocating motion, which may generally be defined by the linear movement of first end  24  and the rotational movement of second end  26 . 
         [0017]      FIG. 2  illustrates a perspective view of an exemplary embodiment of connecting rods  14 . Connecting rods  14  may be arranged in pairs, involving a fork rod  14   a  and a blade rod  14   b.  Each pair of connecting rods  14  may share a common bearing  28 . Bearing  28  may rotate on throw  11  (referring to  FIG. 1 ) of crankshaft  16 . Bearing  28  is described in greater detail below. Blade rod  14   b  may be disposed within tines  15  of fork rod  14   a.    
         [0018]    Fork rod  14   a  may be permanently connected to bearing  28  via a series of dowel pins (not shown; recess  41  that receives the dowel pins is shown in  FIG. 4 ), a cap  17  (shown only in  FIG. 1  and removed from  FIG. 2  for clarity), and one or more fasteners  19 . This arrangement may inhibit movement of fork rod  14   a  relative to bearing  28 , while permitting fork rod  14   a  to rotate with bearing  28  about throw  11 . Because fork rod  14   a  may not rotate relative to bearing  28 , fork rod  14   a  may not require lubrication between its second end  26  and bearing  28 . 
         [0019]    As also shown in  FIG. 2 , second end  26  of blade rod  14   b  may include a blade  21  inter-leaved with tines  15  of fork rod  14   a.  Second end  26  of blade rod  14   b  may include a running surface  34  that abuts an outer surface  30  of bearing  28 . As shown in  FIG. 3 , blade rod  14   b  may include a long toe  38  and a short toe  40  oriented opposite long toe  38 . Blade rod  14   b  may oscillate around bearing  28  within tines  15  of fork rod  14   a.    
         [0020]    As shown in  FIG. 3 , running surface  34  of second end  26  of blade rod  14   b  may include two substantially identical and spaced apart outer lands  29  and one inner land  31  disposed between outer lands  29 . Outer and inner lands  29 ,  31  may have generally cylindrical shapes, although barrel shapes may also be utilized, if desired. Inner land  31  may have a width greater than approximately 50% of the width of running surface  34 . Outer lands  29  may each have a width ranging between approximately 25-50% of the width of inner land  31 , Outer and inner lands  29 ,  31  may each have substantially identical curvatures of radius. 
         [0021]    A centerline of radius -C 1 - of outer lands  29  may be radially offset from a centerline of radius -C 2 - of inner land  31 , such that a radial offset  36  (e.g., a step) may be created between inner land  31  and outer lands  29 . In particular, inner land  31  may be shifted towards long toe  38  and outer lands  29  may be shifted towards short toe  40 . In another embodiment (not shown), offset  36  may be created by shifting inner land  31  towards short toe  40  and shifting outer lands  29  towards long toe  38 , if desired. Offset  36  may range between approximately 0.010-0.030 inches. 
         [0022]      FIGS. 2 ,  4 , and  5  illustrate features of bearing  28  that may be used in conjunction with connecting rods  14 . Bearing  28  may include outer surface  30  and an opposing inner surface  32 . Bearing  28  may be disposed within crank bore  27  of connecting rods  14 , between the second ends  26  of connecting rods  14  and throw  11  of crankshaft  16  (referring to  FIG. 1 ). Inner surface  32  of bearing  28  may engage throw  11  of crankshaft  16 , while outer surface  30  may engage surfaces of connecting rods  14  (e.g., inner and outer lands  29 ,  31  of running surface  34  of blade rod  14   b ). Inner surface  32  of bearing  28  may include a center axis  37 . 
         [0023]    Outer surface  30  of bearing  28  may define two substantially identical and spaced apart outer lands  33 , one inner land  35  disposed between outer lands  33 , and peripheral lands  25  positioned adjacent to outer lands  33 . Outer, inner, and peripheral lands  33 ,  35 ,  25  may have generally cylindrical shapes, although barrel shapes may also be utilized, if desired. Outer and inner lands  33 ,  35  of bearing  28  may correspond to outer and inner lands  29 ,  31  of running surface  34  of blade rod  14   b,  while peripheral lands  25  may correspond with tines  15  of fork rod  14   a.  Inner land  35  may have a width greater than approximately 50% of the width of outer surface  30  of bearing  28 . Outer lands  33  may each have a width ranging between approximately 25-50% of the width of inner land  35 . Outer and inner lands  33 ,  35  of bearing  28  may each have substantially identical curvatures of radius. Inner lands  31  and  35  may have widths that are substantially equal. Similarly, outer lands  29  and  33  may have widths that are substantially equal. 
         [0024]    Like outer and inner lands  29 ,  31  of running surface  34  of blade rod  14   b,  outer and inner lands  33 ,  35  of bearing  28  may also be offset from each other, In particular, a centerline of radius -C 3 - (i.e., a center axis) of outer lands  33  may be shifted a first direction along a parting line  43  of bearing  28  (referring to  FIG. 5 ), while a centerline of radius -C 4 - (i.e., a center axis) of inner land  35  may be shifted in an opposing second direction along parting line  43  of bearing  28 . The offset  39  created by this shift may range between approximately 0.010-0.030 inches. 
         [0025]    The location of outer lands  29 ,  33  and inner lands  31 ,  35  may correspond with a direction of maximum force transmission associated with the movement of blade rod  14   b.  In particular, the location of offsets  36 ,  39  between outer lands  29 ,  33  and inner lands  31 ,  35  may be understood by considering the working cycle of engine  10 . As a two-cycle engine, engine  10  may include two distinct piston strokes that regularly occur in the same order. The first or intake/compression stroke may involve both an intake and compression process. When piston  12  is near bottom dead center (BDC), a position in which piston  12  has reached its nearest point to crankshaft  16 , air may be drawn into cylinder  20  through ports in the wall of cylinder  20 . Piston  12  may then move upward from BDC to top dead center (TDC), a position in which piston  12  has reached its furthest point from crankshaft  16 . At TDC, the angle of rotation of crankshaft  16 , or crank angle, is about 0°. As piston  12  moves upward toward TDC, piston  12  may compress the air, thereby heating it. This process may mark the completion of the first stroke. During this stroke, the force on piston  12  and blade rod  14   b  may be greatest when piston  12  is about 30-0° before top dead center (BTDC). 
         [0026]    The second or power/exhaust/intake stroke may involve a combustion process, an exhaust process, and an intake process. As piston  12  nears TDC, fuel may be injected into combustion chamber  18 . During the second stroke, piston  12  may move downward from TDC to BDC. At BDC, the crank angle is about 180°. The fuel may react with the heated air and ignite, forcing piston  12  downward. As piston  12  nears BDC, an outlet may open and exhaust gases may be released. Afterward, ports in the wall of cylinder  20  may open to allow intake air to enter. During combustion, piston  12  may generally transfer force from the expanding gas in cylinders  20  to crankshaft  16 . In this manner, crankshaft  16  may convert the reciprocating linear motion of piston  12  into rotational motion. The force on piston  12  and blade rod  14   b  during the second stroke may be greatest when piston  12  is about 0-70° after top dead center (ATDC), and peak at about 10° ATDC. 
         [0027]    As shown in  FIG. 6 , two planes may be envisioned during the working cycle of engine  10  to describe the interaction between offsets  39  of bearing  28  and  36  of blade rod  14   b.  In particular, a first plane  22  corresponding to the offset  36  in blade rod  14   b  may be understood to contain the center axis -C 1 - and -C 2 - of outer and inner lands  29 ,  31 . A second plane  23  corresponding to the offset  39  in bearing  28  may be understood to contain the center axis -C 3 - and -C 4 - of outer and inner lands  33 ,  35 . An offset angle α may exist at the intersection of first plane  22  and second plane  23 . Because the offset angle α may vary during different parts of the strokes (due to the relative rotation between blade rod  14   b  and bearing  28 ), the offset angle α may be described in terms of the crank angle, an approach that is adopted below. 
         [0028]    The offsets  36 ,  39  between outer lands  29 ,  33  and inner lands  31 ,  35  may be selected to provide the most support to blade rod  14   b  when the greatest force is being exerted on blade rod  14   b  by piston  12 . In particular, at a crank angle of approximately 67.5° BTDC, which may generally coincide with the start of the compression process of the first stroke, the load of piston  12  may be supported most directly by both inner lands  31 ,  35  and outer lands  29 ,  33 . At this point, the offset angle α may be oriented at approximately 0° (i.e., first plane  22  and second plane  23  may be generally aligned). As engine  10  proceeds toward the second stroke, support for the load of piston  12  may shift to only inner lands  31 ,  35 . At a crank angle of approximately 22.5° ATDC, the offset angle α may be oriented at approximately 10.5°. As engine  10  proceeds through the second stroke, the offset angle α may decrease until the crank angle reaches approximately 112.5° ATDC. At a crank angle of approximately 112.5° ATDC, the offset angle α may return to approximately 0° and support for the load of piston  12  may shift again such that the load may again be supported most directly by inner lands  31 ,  35  and outer lands  29 ,  33 . Between a crank angle of approximately 112.5° ATDC and 67.5° BTDC, support for the load of piston  12  may shift such that the load may be supported by only outer lands  29 ,  33 . 
         [0029]    As the crank angle progresses through the compression process of the first stroke toward the combustion process of the second stroke, the highest loading may occur between a crank angle of approximately 30° BTDC and approximately 70° ATDC. The maximum force transmission may peak at a crank angle of approximately 10° ATDC. At a crank angle of approximately 10° ATDC, the offset angle α may be approximately 10.4°. In this manner, bearing  28  may support the highest load while blade rod  14   b  is situated near an end of its angular travel. 
         [0030]    As blade rod  14   b  pivots relative to bearing  28 , the load may transition from inner lands  31 ,  35  to outer lands  29 ,  33  and vice versa. In particular, outer lands  29 ,  33  may engage each other to support the load of piston  12 , at which point inner lands  31 ,  35  may be away from each other. As inner lands  31 ,  35  are forced away from each other, the divergence may draw oil into the resulting clearance volume and generate a film that supports high loads occurring during the subsequent cycle of engine  10 . Similarly, as inner lands  31 ,  35  engage to support the load of piston  12 , outer lands  29 ,  33  may be forced away from each other to create a divergence that draws oil into the resulting clearance volume, thereby generating an oil film that supports high loads that occur during the subsequent cycle of engine  10 . At a crank angle of approximately 10° ATDC, both inner lands  31 ,  35  and outer lands  29 ,  33  may engage to support the load of piston  12 . Some lubrication may remain during simultaneous engagement of inner lands  31 ,  35  and outer lands  29 ,  33 . 
         [0031]    In addition to the offset design of the present disclosure, bearing  28  may include additional features that facilitate lubrication. In particular, as crankshaft  16  begins to turn, oil from a high-pressure pump (not shown) may be pumped axially into crankshaft  16  and radially outward through bearing  28  via one or more passages  44  (referring to  FIGS. 4 and 5 ). This arrangement may help to maintain lubrication at the interface between outer surface  30  of bearing  28  and running surface  34  of second end  26  of blade rod  14   b.    
         [0032]    Passages  44  may be further defined by annular grooves  42  (referring to  FIG. 4 ) for carrying return oil (referring to  FIG. 4 ), if desired. Grooves  42  may be formed within outer surface  30  (and/or inner surface  32 ) of bearing  28 , between outer and inner lands  33 ,  35 . A plurality of smaller grooves (not shown) may be defined in outer surface  30  of bearing  28  and may be interconnected with grooves  42  defined between outer and inner lands  33 ,  35  of bearing  28 . For example, the smaller grooves may interconnect perpendicularly to grooves  42  defined between outer and inner lands  33 ,  35 . Oil leaving grooves  42  may spread laterally to provide lubrication at the interface between outer surface  30  of bearing  28  and running surface  34  of second end  26  of blade rod  14   b.    
       INDUSTRIAL APPLICABILITY 
       [0033]    The objective of engine lubrication may include the alleviation of friction to thereby reduce heating and wear of the working parts of engine  10 . Maintaining sufficient oil lubrication at the interface between blade rod  14   b  and bearing  28  may be necessary to support the load exerted by piston  12 . This oil lubrication may help to prevent contact from occurring between blade rod  14   b  and bearing  28 , which can reduce the durability, reliability, and efficiency of engine  10 . 
         [0034]    Lubrication of engine  10  may be accomplished, at least in part, by employing a hydrodynamic film of lubricating oil between the surfaces of bearing  28  and blade rod  14   b,  During hydrodynamic lubrication of bearing  28 , a wedge-like film of a may be generated between the load-carrying surfaces of bearing  28  and blade rod  14   b.  This arrangement may function to separate the load-carrying surfaces of bearing  28 . 
         [0035]    Offset bearings may generally be used to enhance film lubrication conditions by dividing a bearing axially into cylindrical segments with offset centerlines. This arrangement may permit load sharing between segments of the hearing, which may periodically relieve the load carried by each segment. Lubrication may occur during these periods of relief. 
         [0036]    The piston assembly having the offset bearing of the present disclosure may provide the greatest support under the highest loads of piston  12 , while still ensuring sufficient lubrication. In particular, lubrication at the interface between second end  26  of blade rod  14   b  and bearing  28  may be provided by generating an oil film on inner lands  31 ,  35  during each cycle of engine  10 . Inner lands  31 ,  35  may be optimized to support the highest loads occurring while blade rod  14   b  is situated near an end of angular travel, and to provide the highest level of lubrication between high loading situations. 
         [0037]    As engine  10  progresses through its working cycle, the oscillatory motion of crankshaft  16  at the interface between blade rod  14   b  and bearing  28  may transfer the load of piston  12  between outer lands  29 ,  33  and inner lands  31 ,  35 . For example, as second end  26  of blade rod  14   b  rocks toward inner lands  31 ,  35 , outer land  29  of running surface  34  of second end  26  of blade rod  14   b  may be forced away from the complementary outer surface  30  of bearing  28 . This divergence may increase the volume defined by outer lands  29 ,  33  and draw oil into this volume. As blade rod  14   b  rocks back toward outer lands  29 ,  33 , running surface  34  of blade rod  14   b  may ride on the oil film generated on outer lands  29 ,  33 . 
         [0038]    Similarly, as blade rod  14   b  rocks toward outer lands  29 ,  33 , inner land  31  of running surface  34  of second end of blade rod  14   b  may be forced away from the complementary outer surface  30  of bearing  28 . This divergence may increase the volume defined by inner lands  31 ,  35  and draw oil into this volume. As blade rod  14   b  rocks back toward inner lands  31 ,  35 , running surface  34  of second end  26  of blade rod  14   b  may ride on the oil film generated on inner lands  31 ,  35 . 
         [0039]    Additionally, benefits may be realized by the piston assembly having offset bearing of the present disclosure. In particular, because an uninterrupted bearing surface may be able to support a greater load than a grooved bearing surface of comparable size, the disclosed piston assembly may have increased load bearing capability due to a reduced network of grooves. 
         [0040]    It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed piston assembly without departing from the scope of the disclosure. Other embodiments of the piston assembly will be apparent to those skilled in the art from consideration of the specification and practice of the piston assembly disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.