Abstract:
A rocking journal bearing is provided in a piston coupling mechanism of a two-stroke cycle engine. The bearing includes a sleeve and a wristpin constructed with two sets of eccentrically-disposed bearing surfaces which alternate in accepting a compressive load during an operational cycle of the bearing. The sleeve includes a network of grooves to transport oil to the bearing surfaces. Lubricating oil flow through the bearing is minimized by limiting provision of pressurized oil from the wristpin to the network of grooves to portions of the cycle when one or the other of the sets of bearing surfaces receives the compressive load.

Description:
PRIORITY 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 14/732,531, filed Jun. 5, 2016, for “Minimizing Oil Leakage From Rocking Journal Bearings Of Two-Stroke Cycle Engines”. 
       RELATED APPLICATIONS 
       [0002]    This application contains subject matter related to the subject matter of commonly-owned U.S. patent application Ser. No. 13/776,656, filed Feb. 25, 2013, titled “Rocking Journal Bearings for Two-Stroke Cycle Engines”. published as US 2014/0238360 A1 on Aug. 28, 2014. 
     
    
     FIELD OF THE DISCLOSURE 
       [0003]    The field is rocking journal bearings. More specifically, the field concerns rocking journal bearings that are incorporated into the piston coupling mechanisms of two-stroke cycle engines, for example, opposed-piston engines. 
       BACKGROUND OF THE DISCLOSURE 
       [0004]    Due to the nature of the two-stroke cycle, a load reversal on a journal bearing of a two-stroke engine such as a wristpin may never occur during the normal speed and load range operation of the engine, or the duration of a load reversal might be relatively short. In these circumstances, it is difficult to replenish the bearings with lubricating oil (“oil”). Furthermore, given limited angular oscillation of the bearing, oil introduced between the bearing surfaces does not completely fill the bearing. Eventually the bearing begins to operate in a boundary layer lubrication mode (also called “boundary lubrication mode”), which leads to excess friction, wear, and then bearing failure. 
         [0005]    A representative two-stroke cycle engine is embodied in the opposed-piston engine  8  of  FIG. 1 . The engine  8  includes one or more cylinders such as the cylinder  10 . The cylinder  10  is constituted of a liner (sometimes called a “sleeve”) retained in a cylinder tunnel formed in a cylinder block. The liner includes a bore  12  and longitudinally displaced intake and exhaust ports  14  and  16 , machined or formed in the liner near respective ends thereof. Each of the intake and exhaust ports includes one or more circumferential arrays of openings in which adjacent openings are separated by a solid portion of the cylinder wall (also called a “bridge”). 
         [0006]    One or more injection nozzles  17  are secured in threaded holes that open through the sidewall of the liner, between the intake and exhaust ports. Two pistons  20 ,  22  are disposed in the bore  12  of the cylinder liner with their end surfaces  20   e,    22   e  in opposition to each other. For convenience, the piston  20  is referred to as the “intake” piston because of its proximity to, and control of, the intake port  14 . Similarly, the piston  22  is referred to as the “exhaust” piston because of its proximity to, and control of, the exhaust port  16 . The engine includes two rotatable crankshafts  30  and  32  that are disposed in a generally parallel relationship and positioned outside of respective intake and exhaust ends of the cylinder. The intake piston  20  is coupled to the crankshaft  30  (referred to as the “intake crankshaft”), which is disposed along an intake end of the engine  8  where cylinder intake ports are positioned; and, the exhaust piston  22  is coupled to the crankshaft  32  (referred to as the “exhaust crankshaft”), which is disposed along an exhaust end of the engine  8  where cylinder exhaust ports are positioned. 
         [0007]    Operation of a two-stroke cycle, opposed-piston engine with one or more cylinders is well understood. Using the engine  8  as an example, each of the pistons  20 ,  22  reciprocates in the bore  12  between a bottom center (BC) position near a respective end of the liner  10  where the piston is at its outermost position with respect to the cylinder, and a top center (TC) position where the piston is at its innermost position with respect to the cylinder. At BC, the piston&#39;s end surface  20   e,    22   e  is positioned between a respective end of the cylinder, and its associated port, which opens the port for the passage of gas. As the piston moves away from BC, toward TC, the port is closed. During a compression stroke each piston moves into the bore  12 , away from BC, toward its TC position. As the pistons approach their TC positions, air is compressed between their end surfaces. Fuel is injected into the compressed air. In response to the pressure and temperature of the compressed air, the fuel ignites and combustion follows, driving the pistons apart in a power stroke. During a power stroke, the opposed pistons move away from their respective TC positions. While moving from TC, the pistons keep their associated ports closed until they approach their respective BC positions. In some instances, the pistons may move in phase so that the intake and exhaust ports  14 ,  16  open and close in unison, Alternatively, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times. 
         [0008]    In  FIG. 1 , the pistons  20  and  22  are connected to the crankshafts  30  and  32  by respective coupling mechanisms  40  including journal bearings  42 . The journal bearings  42  are continuously subjected to non-reversing, compressive loads during operation of the engine  8 . Related U.S. patent application Ser. No. 13/776,656 describes and illustrates a solution to the problem of non-reversing compressive loads for two-stroke cycle, opposed-piston engines. The solution includes a rocking journal bearing (also called a “rocking bearing” or a “biaxial bearing”), which is incorporated into the engine  8  of  FIG. 1 . Each journal bearing  42  of each coupling mechanism  40  of the engine  8  is constructed as a rocking journal bearing. Referring to  FIGS. 1 and 2 , a coupling mechanism  40  supports a piston  20  or  22  by means of a rocking journal bearing  42  including a bearing sleeve  46  having a bearing surface  47 , and a wristpin  48 . The wristpin  48  is retained on the small end  49  of a connecting rod  50  for rocking oscillation on the bearing surface of the sleeve by threaded fasteners  51  received in threaded holes  52 . The large end  53  of the connecting rod  50  is secured to an associated crankpin  54  of a respective one of the crankshafts  30 ,  32  by conventional fasteners (not shown). 
         [0009]    As seen in  FIG. 3 , the wristpin  48  is a cylindrical piece that comprises a plurality of axially-spaced, eccentrically-disposed journal segments. A first journal segment J 1  comprises an annular bearing journal surface formed in an intermediate portion of the wristpin, between two journal segments J 2 . The two journal segments J 2  comprise respective annular bearing journal surfaces formed on opposite ends of the wristpin, on respective sides of the journal segment J 1 . The journal segment J 1  has a centerline A. The journal segments J 2  share a centerline B that is offset from the centerline A of journal segment J 1 . As seen in  FIG. 3 , the sleeve  46  is a semi-cylindrically shaped piece with a bearing surface that includes a plurality of axially-spaced, eccentrically-disposed surface segments. A first surface segment J 1 ′ comprises an arcuately-shaped bearing surface formed in an intermediate portion of the sleeve, between two surface segments J 2 ′. The two surface segments J 2 ′ comprise arcuately-shaped bearing surfaces formed at opposite ends of the sleeve, on respective sides of the surface segment J 1 ′. The surface segment J 1 ′ has a centerline A′. The wristpin  48  is mounted to the small end  49  of the connecting rod  50  and the sleeve is mounted to an internal structure of the piston (not shown), such that corresponding bearing segment sets J 1 -J 1 ′ and J 2 -J 2 ′ are in opposing contact. Thus disposed, the opposing corresponding segment sets J 1 -J 1 ′ and J 2 -J 2 ′ may also be called “bearing interfaces”. 
         [0010]    In operation, as the piston to which they are mounted reciprocates between TC and BC positions, oscillatory rocking motion between the wristpin  48  and the sleeve  46  causes the bearing interfaces J 1 -J 1 ′ and J 2 -J 2 ′ to alternately receive the compressive load. The bearing surface segments receiving the load come together and the bearing surface segments being unloaded separate. Separation enables a film of oil to enter space between the separating bearing surfaces. The point at which the compressive load is shifted from one to the other set of bearing segments is referred to as a “load transfer point.” During one full cycle of the two-stroke cycle engine, this point is traversed twice by each piston, once when the piston moves from TC to BC (that is to say, during the power stroke), and again when the piston moves from BC to TC (during the compression stroke). For illustration and as an aid in visualization, but without limiting the following disclosure, the load transfer points of the pistons may occur at or near crankshaft positions of 0° (when the pistons pass through their respective TC locations) and 180° (when the pistons pass through their respective BC locations). 
         [0011]    With reference to  FIGS. 1 and 2 , the rocking journal bearings are constructed to enable provisioning and distribution of oil at pressures adequate to lubricate the rocking bearing interfaces with a continuous oil film thick and widespread enough to support heavy loading, thereby enhancing the durability of the bearing. The construction of the wristpin  48  includes a gallery  60  which receives and distributes oil for lubricating the bearing interfaces (J 1 -J 1 ′ and J 2 -J 2 ′). The gallery  60  is fed pressurized oil from a pumped oil source. The wristpin  48  includes an oil inlet into, and multiple oil outlets from, the gallery  60 . The gallery  60  receives the pressurized oil through an inlet opening  62  that opens through a portion of the wristpin surface that is out of contact with the sleeve surface segments. The pressurized oil is delivered via a high-pressure oil passage  64  in the connecting rod. Pressurized oil is provided to the bearing interfaces (J 1 -J 1 ′ and J 2 -J 2 ′) from the gallery  60  through outlets that act through a portion of the wristpin surface in contact with the sleeve&#39;s bearing surface during oscillation of the bearing. An influx of pressurized oil into the gallery  60  provides a continuous supply of pressurized oil to the bearing during operation of the engine. 
         [0012]    As seen in  FIG. 4 , oil is circulated to the bearing interfaces via a network of oil grooves formed in the bearing surface  47  of the sleeve  46  for transporting oil to the bearing surface. The network includes circumferential oil grooves  70  for transporting oil in a circumferential direction of the bearing surface. The circumferential oil grooves  70  are formed in the bearing surface at the borders between the central surface segment J 1 ′ and the lateral surface segments J 2 ′. The network further includes circumferentially-spaced, axial oil grooves  72  and  73 , each for transporting oil in an axial direction of the bearing surface. The axial oil grooves are formed in the bearing surface transversely to and intersecting with the circumferential oil grooves  70 . Each of the axial oil grooves  72  and  73  runs across the central surface segment J 1 ′ and extends at least partially into each of the lateral surface segments J 2 ′. Chamfers  74  may be formed along opposing lateral peripheries of the bearing surface  47 .  FIGS. 5A-5C  show a prior art rocking journal wristpin constructed to deliver oil to the sleeve&#39;s bearing surface  47 . 
         [0013]      FIGS. 5A-5C  show a rocking journal wristpin  80  with an outer surface  82  having journal segments J 1  and J 2  that contact surface segments J 1 ′ and J 2 ′ of the sleeve bearing surface  47  during oscillation of the bearing. The journal segments J 1  and J 2  are separated by circumferential grooves  85  in the wristpin outer surface  82 . Outlet passages formed in the wristpin provide pressurized oil to the sets of surface segments during relative oscillatory motion between the sleeve and wristpin. First oil outlet passages  86  for delivering pressurized oil are formed in the contacting portion of the outer surface  82  and extend through the sidewall of the wristpin in the circumferential grooves  85  in a radial direction of the journal segment J 1  and open into an oil gallery  88 . An oil inlet  90  to the oil gallery  88  and the first oil outlet passages  86  are axially spaced, in diametrical opposition. Second oil outlet passages  92  are formed through the sidewall of the wristpin, outside of the circumferential grooves  85 , and open into the oil gallery  88 . The second oil outlet passages  92  are arranged in an axial array such that there is at least one second oil outlet passage located in each journal segment J 1  and J 2 . The wristpin  80  is assembled to the sleeve  46  of  FIG. 4  with the journal segments J 1 -J 2  in opposition to the surface segments J 1 ′-J 2 ′ and the circumferential grooves  85  of the wristpin aligned with the circumferential grooves  70  of the sleeve. As per  FIGS. 4 and 5A , during operation of the engine, the first oil outlet passages  86  continuously supply pressurized oil to the network comprising circumferential oil grooves  70 , which flows to the axial oil grooves  72  and  73  and the oil grooves  74 . As relative oscillation occurs between the wristpin  80  and the sleeve  46 , pressurized oil flows to the space between the separated segments continuously from the oil grooves  70 ,  72 , and  73  and intermittently from the second oil outlet passages  92  as the journal segments in which they are located separate from their opposing surface segments of the sleeve. 
         [0014]    Thus, the prior art wristpin oil delivery construction provides a constant supply of pressurized oil to the oil grooves  70 ,  72  and  73  in the sleeve surface; and, the oil grooves continuously transport oil to the journal segments. However, a continuous supply of pressurized oil results in a high level of oil flow from the ends of the circumferential grooves  70 . This excess oil is detrimental to the performance of the engine for at least two reasons. First, the continuous provision of pressurized oil requires pumping work to supply the oil to the grooves, which reduces the engine&#39;s efficiency. Second, the oil comes in contact with the rotating and reciprocating machinery while returning to an engine oil sump. Extra parasitic drag caused by oil returning to the sump and interacting with a swirling cloud of air in the crankcase of the engine created by the high-speed rotation of the crankshafts, (“windage”), results in frictional losses. At 3,000 RPM, for example, each crankshaft must rotate 50 times per second. As the crankpins and counterweights rotate at such high speeds, they create a swirling cloud of air around them. As a result windage friction losses occur when excess oil is caught up in this turbulent air, drawing energy from the engine to spin the oil mist. Windage may also inhibit the migration of oil into the sump and back to the oil pump, creating lubrication problems. It is therefore desirable to minimize the amount of excess pressurized oil that flows through the rocking bearing journals of an engine. 
       SUMMARY OF THE DISCLOSURE 
       [0015]    Lubricating oil flow through the rocking journal bearing is minimized by limiting provision of pressurized oil from the wristpin to the network of oil grooves in the sleeve to portions of a bearing operating cycle when one or the other of the sets of bearing surfaces receives the compressive load. Excess flow of oil through the rocking journal bearing is minimized by providing pressurized oil to the network intermittently during relative movement between the sleeve and the wristpin. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a schematic representation of a two-stroke cycle, opposed-piston engine, and is properly labeled “Prior Art”. 
           [0017]      FIG. 2  is an exploded perspective view of a piston coupling mechanism including a rocking journal bearing, and is properly labeled “Prior Art”. 
           [0018]      FIG. 3  is a schematic diagram illustrating the bearing surfaces of the rocking journal of  FIG. 2 , and is properly labeled “Prior Art”. 
           [0019]      FIG. 4  is a perspective view showing a bearing surface of a rocking journal sleeve. 
           [0020]      FIGS. 5A-5C  show a rocking journal wristpin constructed to provide oil for lubricating a rocking journal bearing comprising the sleeve of  FIG. 4 , and are properly labeled “Prior Art”. 
           [0021]      FIG. 6  is a perspective view of a rocking journal wris pin according to this disclosure. 
           [0022]      FIG. 7  is an end elevation view of a rocking journal bearing according to this disclosure comprising the sleeve of  FIG. 4  and the wristpin of  FIG. 6 . 
           [0023]      FIG. 8A  is a side elevation view and  FIGS. 8B-8D  are cross-sectional views of the rocking journal bearing of  FIG. 7  showing details of bearing lubrication at a load transfer point of the bearing. 
           [0024]      FIG. 9A  is a side elevation view and  FIGS. 9B-9D  are cross-sectional views of the rocking journal bearing of  FIG. 7  showing details of bearing lubrication at a first loading point of the bearing. 
           [0025]      FIG. 10A  is a side elevation view and  FIGS. 10B-10D  are cross-sectional views of the rocking journal bearing of  FIG. 7  showing details of bearing lubrication at a second loading point of the bearing. 
           [0026]      FIG. 11  is a graph showing an operational cycle of the rocking journal bearing of  FIG. 7 . 
           [0027]      FIGS. 12A-12C  illustrate flow paths of pressurized oil delivered to the rocking journal bearing of  FIG. 7  at respective points in the operational cycle of the bearing shown in  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]      FIG. 6  shows a wristpin  100  according to this disclosure that is combined with the sleeve  46  of  FIG. 4  to form a rocking journal bearing  200  as shown in  FIG. 7  in which the flow of excess pressurized oil from the bearing  200  is reduced throughout its operational cycle. In this regard, pressurized oil is provided intermittently instead of continuously to the network of oil grooves in the bearing surface of the sleeve. The view in  FIG. 6  is toward a contacting portion  102  of the wristpin outer surface  103  that is in contact with the sleeve bearing surface  47  during oscillation of the bearing. The wristpin is constructed with axially-offset surface segments J 1  and J 2  as per  FIG. 3 . As best seen in  FIG. 8C , an oil inlet passage  105  is formed in the non-contacting portion of the J 1  segment of the wristpin. As per  FIG. 6 , at least one oil outlet passage  107  is formed in the contacting portion of the J 1  journal segment. At least one oil outlet passage  109  is formed in the contacting portions of each of the J 2  journal segments. The oil outlet passages  107  and  109  open through the wristpin sidewall to oil gallery space  111  within the wristpin, and are offset along the wristpin&#39;s longitudinal axis  113  relative to the circumferential grooves  115  that separate the segments J 1  and J 2 . There are no oil outlet passages along either of the circumferential grooves  115 . The wristpin  100  is assembled to the sleeve  46  with the journal segments J 1 -J 2  in engagement with the surface segments J 1 ′-J 2 ′ and the circumferential grooves  115  of the wristpin  100  aligned with the circumferential grooves  70  of the bearing surface  47 . As relative oscillation occurs between the wristpin  100  and the sleeve  46 , pressurized oil flows to the space between the separated segments from the oil grooves  70 ,  72 , and  115  and from the outlet passages  107  and  109  located in the separated journal segments of the wristpin. 
         [0029]    With reference to  FIGS. 6 and 8C , to carry out the purposes of a rocking journal bearing construction according to this disclosure, the positioning of the oil outlet passage  107  locates the oil outlet passage in the J 1  journal segment at a first arcuate distance D 1  from one side of a cut plane P containing the longitudinal axis  113  of the wristpin and a radius  117  forming the axis of the oil inlet passage  105 . The positioning of the oil outlet passages  109  locates these oil outlet passages in respective J 2  journal segments at a second arcuate distance D 2  from the opposite side of the cut plane P. Thus, as the rocking journal  200  is viewed as per  FIGS. 8A-8D , in which the rotational position of the wristpin  100  relative to the sleeve  47  is 0°, as would occur when the load transfer point of the bearing  200  is traversed, the oil outlet passages  107  and  109  are positioned between the axial oil grooves  72  and  73  of the sleeve  46 , with the oil outlet passage  107  relatively nearer (for example, adjacent) to the axial oil groove  72  and the oil outlet passages  109  relatively nearer (for example, adjacent) to the axial oil groove  73 . In this relative rotational position, the bearing interfaces J 1 -J 1 ′ and J 2 -J 2 ′ are equally loaded. 
         [0030]    With reference to  FIGS. 9A-9D , presume that the wristpin  100  revolves in the CCW direction from the 0° position relative to the sleeve  46  to a point where the segments J 1 -J 1 ′ are fully loaded, while the segments J 2 -J 2 ′ are separated. As a result of movement in this direction, the oil outlet passage  107  moves across the axial oil groove  72 , which enables a pulse of pressurized oil to enter the oil groove from the oil outlet passage, while the separation between the segments J 2 -J 2 ′ allows the oil outlet passages  109  to deliver pressurized oil to the space therebetween. 
         [0031]    With reference to  FIGS. 10A-10D , presume that the wristpin  100  revolves in the CW direction from the 0° position relative to the sleeve  46  to a point where the segments J 2 -J 2 ′ are fully loaded, while the segments J 1 -J 1 ′ are separated. As a result of movement in this direction, the oil outlet passages  109  cross the axial oil groove  73 , which enables a pulse of pressurized oil to enter the oil groove from each of the oil outlet passages, while the separation between the segments J 1 -J 1 ′ allows the oil outlet passage  107  to deliver pressurized oil to the space therebetween. 
         [0032]      FIG. 11  is a graph showing an exemplary operational cycle of a rocking journal bearing as may be observed when the bearing is incorporated into the piston coupling mechanisms of a two-stroke cycle opposed-piston engine such as the engine  8  of  FIG. 1 . The graph shows wristpin-to-sleeve clearance for the J 1 -J 1 ′ interface and J 2 -J 2 ′ interfaces as a function of the crank angle position (in degrees) of the one of the crankshafts to which the coupling mechanism connects its associated piston. The graph shows a full cycle of crankshaft operation, with the understanding that this represents the operational cycles of each of the two crankshafts seen in  FIG. 1  (with or without a phase difference). Further, the graph is representative of the two-stroke cycle operation of the opposed-piston engine of  FIG. 1 . This graph is based upon load transfer occurring at 0° (TC) and 180° (BC), although this condition should not be considered to be limiting. At a crank angle of 0°, with the piston at TC, the compressive load is about equally divided between the J 1 -J 1 ′ and J 2 -JZ interfaces, as the crank angle advances, the load is increasingly received by the J 1 -J 1 ′ interface while the J 2 -J 2 ′ segments begin to separate. At a crank angle of 90° the compressive load is maximally borne by the J 1 -J 1 ′ interface, while the J 2 -J 2 ′ segments are maximally separated. At this point, the compressive load begins shifting from the J 1 -J 1 ′ interface to the J 2 -J 2 ′ interface and the J 2 -J 2 ′ surface segments begin to close. At 180°, with the piston at BC, the compressive load is about equally divided between the J 1 -J 1 ′ and J 2 -J 2 ′ interfaces. As the crank angle advances the load is increasingly received by the J 2 -J 2 ′ interface while the J 1 -J 1 ′ segments begin to separate. At a crank angle of  270 ° the compressive load is maximally borne by the J 2 -J 2 ′ interface, while the J 1 -J 1 ′ segments are maximally separated. At this point, the compressive load begins shifting to the J 1 -J 1 ′ interface from the J 2 -J 2 ′ interface and the J 1 -J 1 ′ segments begin to close. At 360°, the compressive load is about equally divided between the J 1 -J 1 ′ and J 2 -J 2 ′ interfaces, and the cycle repeats. 
         [0033]      FIGS. 12A-12C  show the pressurized oil flow patterns through the sleeve oil grooves  70 ,  72 , and  73  for load transfer points (0° and 180°), for maximum J 1  loading, and maximum J 2  loading during an engine operating cycle shown in  FIGS. 11 . At 0° and 180° oscillation, no oil outlet passages align with the sleeve axial grooves  72  and  73 , and since all three interfaces are equally loaded, no significant oil is added to the interfaces. At the maximum J 1  loading point (90°), the J 1  oil outlet passage  107  is aligned with the axial oil groove  72  and oil flows freely through the oil grooves  70 ,  72 , and  73  to fill the J 2  lifted segment areas. At the maximum J 2  loading point (90°), the two J 2  oil outlet passages  109  align with the axial oil groove  73  and oil freely flows into the oil grooves  70 ,  72 , and  73  to fill the J 1  lifted segment area. 
         [0034]    The column of oil in the piston connecting rod oil passage  64  applies peak positive and negative pressures to the volume of oil in the wristpin gallery when at TC and BC piston positions, respectively. By using the intermittent alignment system described and illustrated above, and in the absence of oil outlet passages positioned in alignment with the circumferential grooves, the only path for oil to flow through during these peak pressure events is between the equally-loaded J 1 -J 1 ′ and J 2 -J 2 ′ surface segments, which is quite restrictive. As a result, this construction has the additional benefit of reducing the system sensitivity to oil pressure fluctuations in the wristpin gallery. 
         [0035]    Although this disclosure describes particular embodiments for minimizing oil leakage from journal wristpins in two-stroke cycle, opposed-piston engines, these embodiments are set forth merely as examples of underlying principles of this disclosure. Thus, the embodiments are not to be considered in any limiting sense.