Abstract:
Load transfer point offset of rocking journal bearings in uniflow-scavenged, opposed-piston engines with phased crankshafts includes differing offsets for the load transfer points of opposed pistons. More specifically, under the condition that a first crankshaft leads the second crankshaft, an angular offset of a rocking journal wristpin of a piston coupled to the first crankshaft proportional to an offset of the first crankshaft relative to the second crankshaft is made to ensure adequate oil film thickness to the wristpin when it experiences a peak combustion pressure during a power stroke

Description:
RELATED APPLICATIONS 
       [0001]    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 
       [0002]    The field includes rocking-journal bearings for uniflow-scavenged, opposed-piston engines with phased crankshafts. 
       BACKGROUND OF THE DISCLOSURE 
       [0003]    Construction of an opposed-piston engine is well understood. In  FIG. 1 , the engine  8  illustrates an example of a two-stroke cycle, opposed-piston engine. 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”). In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions in  FIG. 1 . 
         [0004]    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. In uniflow-scavenged, opposed-piston engines with a two or more cylinders, all exhaust pistons are coupled to the exhaust crankshaft and all intake pistons to the intake crankshaft. 
         [0005]    Operation of an 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 liner, and a top center (TC) position where the piston is at its innermost position with respect to the liner. At BC, the piston&#39;s end surface 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 in a combustion chamber formed between the end surfaces of the pistons. Fuel is injected into the combustion chamber. 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. However, one piston may lead the other in phase, in which case the intake and exhaust ports have different opening and closing times. 
         [0006]    One reason for introducing a phase difference in piston movements is to drive the process of uniflow scavenging in which pressurized charge air entering a cylinder through the intake port pushes the products of combustion (exhaust gas) out of the cylinder through the exhaust port. The replacement of exhaust gas by charge air in the cylinder is “scavenging.” The scavenging process is uniflow because gas movement through the cylinder is in one direction: intake-to-exhaust. In order to optimize the uniflow scavenging process, the movement of the exhaust piston  22  is advanced with respect to the movement of the intake piston  20 . In this respect, the exhaust piston is said to “lead” the intake piston. Such phasing causes the exhaust port  16  to begin to open before the intake port  14  opens and to begin closing before the intake port. Thus, exhaust gas flows out of the cylinder before inflow of pressurized charge air begins (this interval is referred to as “blow down”), and pressurized charge air continues to flow into the cylinder after the outflow of exhaust gas ceases. Between these events, both ports are open (this is when scavenging occurs). Scavenging ends when the exhaust port  16  closes. Now, having no exit, the charge air that continues to flow into the cylinder  10  between time of closure of the exhaust port  16  and the time of closure of the intake port  14  is caught in the cylinder  10 , and is retained therein when the intake port  14  closes. This retained portion of charge air retained in the cylinder by the last port closure is referred to as “trapped air”, and it is this trapped air that is compressed during the compression stroke. 
         [0007]    Movement of the pistons in response to combustion is coupled to the crankshafts  30  and  32 , which causes the crankshafts to rotate. The rotational position of a crankshaft with respect to a piston coupled to it is called the crank angle (CA). The crank angle is given as the angle from the position of the crankshaft to the centerline of the bore in which the piston moves; CA=0° when the piston is at TC. Presuming that the opposed-piston engine  8  is constructed for uniflow scavenging, a piston phase difference is established as per  FIG. 2  by advancing the rotational position of the exhaust crankshaft  32  relative to the intake crankshaft  30  by some fixed amount, which is typically expressed as a “phase offset” in degrees of crankshaft rotation. This causes the exhaust piston  22  to lead the intake piston  20  by a corresponding amount throughout the operational cycle. During engine operation, the phase offset is maintained as the crankshafts rotate, and the crankshafts are said to be “phased.” More broadly, the term “phased crankshafts” refers to the two crankshafts of an opposed-piston constructed as per  FIG. 1 , in which the rotational movement of one crankshaft leads the rotational movement of the other crankshaft by a fixed number of degrees throughout the cycle of engine operation. 
         [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 . In some aspects of two-stroke cycle engine operation, due to the nature of the cycle, a load reversal on a journal bearing 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 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. Related U.S. patent application Ser. No. 13/776,656 describes and illustrates a solution to the problem of non-reversing compressive loads that includes a rocking wristpin bearing (also called a “biaxial bearing”), which is incorporated into the engine  8  of  FIG. 1 . In this regard, each coupling mechanism  40  of the engine  8  may be constructed in a manner described in the &#39;656 patent application and illustrated in  FIG. 3 . Referring to  FIGS. 1 and 3 , 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 sleeve  46  is fixed to internal structure of the piston by conventional means. The wristpin  48  is retained on the small end  49  of a connecting rod  50  by threaded fasteners  51  for rocking oscillation on the bearing surface of the sleeve. 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). This structure is preferred, but is not intended to be limiting or to exclude other structures in which the wristpin is fixed and the sleeve is retained on the connecting rod for rocking oscillation on the wristpin. In either case, relative rocking oscillation occurs between the wristpin  48  and sleeve  46 . 
         [0009]    As seen in  FIG. 4 , 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 surface formed in an intermediate portion of the wristpin, between two journal segments J 2 . The two journal segments J 2  comprise annular bearing surfaces formed at 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. As per  FIG. 5 , the centerlines A and B are offset by equal distances from each other on a line  60  that is orthogonal to the longitudinal axis  62  of the connecting rod  50 . As seen in  FIG. 4 , the sleeve  46  is a semi-cylindrically shaped piece with a segmented 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 wristpin, 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 surface segments J 2 ′ share a centerline B′ that is offset from the centerline A′ of surface segment J 1 ′. As per  FIG. 5 , the centerlines A′ and B′ are offset by equal distances from each other on a line  60 ′ that is orthogonal to the longitudinal axis  62  of the connecting rod  50 . The wristpin  48  is mounted to the small end  49  of the connecting rod  50  and the sleeve  46  is mounted to an internal structure of the piston, with bearing surface sets J 1 -J 1 ′ and J 2 -J 2 ′ in opposition. 
         [0010]    In operation, as the piston to which they are mounted reciprocates between TC and BC positions, oscillating rocking motion between the wristpin  48  and the sleeve  46  causes the bearing surface sets J 1 -J 1 ′ and J 2 -J 2 ′ to alternately receive the compressive load. The segments receiving the load come together and the segments being unloaded separate, which enables a film of oil to enter space between the separating segment surfaces. A “load transfer point” occurs during oscillation of the bearing when the bearing surface sets are equally loaded and the direction of oscillation is causing the load to be increasingly transferred from one bearing surface set to another. During one full cycle of the two-stroke engine, this point is traversed twice, once when the piston moves from TC to BC, and again when the piston moves from BC to TC. As per  FIG. 5 , with 0° angular offset between the crankshafts, the load transfer points of the pistons occur at or near crankshaft positions of 0° (when the pistons are at their respective TC locations) and 180° (when the pistons are at their respective BC locations). 
         [0011]    It has been recognized that positioning the load transfer point is important in the operation of traditional two-stroke engines with continuous compressive loads that have a peak cyclic intensity. For example, U.S. Pat. No. 3,762,389 discloses positioning a load transfer point to occur following the cycle peak load point (which occurs just after the piston TC position) so as to avoid minimization of the oil film between the bearing surfaces. However, with a single crankshaft and a single piston in each cylinder, each rocking journal interface is configured to the same load transfer point at the same time in each cycle. 
         [0012]    What the &#39;389 patent fails to consider is that setting all piston rocking journals to the same load transfer point in a two-stroke cycle, opposed-piston engine, with the exhaust crankshaft leading the intake crankshaft, will cause the same wristpin segments in the exhaust pistons to transition to an increasing highly loaded state further into the cycle and then diminish in loading as the pistons approach BC. When compared with the intake wristpin segments, this shift in loading of the exhaust wristpin segments will result in a lower minimum oil film thickness (MOFT) on the wristpin segment (J 1  or J 2 ) affected during the power stroke and higher MOFT on the segment that is loaded during the compression stoke, which is an undesired effect in a rocking journal lubrication scheme. 
       SUMMARY OF THE DISCLOSURE 
       [0013]    Load transfer point offset of rocking journal bearings in uniflow-scavenged, opposed-piston engines with phased crankshafts includes differing offsets for the load transfer points of intake and exhaust pistons. For example, under the condition that the exhaust crankshaft leads the intake crankshaft, an angular offset of the exhaust rocking journal wristpin proportional to an offset of the exhaust crankshaft relative to the intake crankshaft is made to ensure adequate oil film thickness to the interfaces of the bearing journal when it experiences a peak combustion pressure during the power stroke. 
         [0014]    In some instances, the load transfer point offset is given effect by selecting an arcuate position of a rocking journal wristpin with respect to the small end of the piston connecting rod interface that offsets the load transfer point proportionally to a crankshaft lead. With this load transfer point offset, peak loading conditions on the wristpin will occur following TC, and before maximum load, of the piston during a full engine operating cycle. This offset of the load transfer point shifts the loading regimes of the rocking journal bearing to later in the crankshaft cycle so that adequate oil film thickness is provided to the loaded journal segments during power and compression strokes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic representation of a two-stroke cycle, opposed-piston engine, and is properly labeled “Prior Art”. 
           [0016]      FIG. 2  is a graph showing a phase offset between two crankshafts of an opposed-piston engine, and is properly labeled “Prior Art”. 
           [0017]      FIG. 3  is an exploded perspective view of a piston coupling mechanism including a rocking journal bearing, and is properly labeled “Prior Art”. 
           [0018]      FIG. 4  is a schematic diagram illustrating the bearing surfaces of the rocking journal of  FIG. 3 , and is properly labeled “Prior Art”. 
           [0019]      FIG. 5  is an illustration of a piston coupling mechanism comprising a rocking journal with a first load transfer point, and is properly labeled “Prior Art”. 
           [0020]      FIG. 6  is an illustration of a piston coupling mechanism comprising a rocking journal with a second load transfer point offset from the first load transfer point. 
           [0021]      FIGS. 7A and 7B  are schematic drawings showing relative positions of intake and exhaust piston coupling mechanisms at successive points in the engine operating cycle. 
           [0022]      FIG. 8  is a graph showing forces acting on the intake and exhaust pistons of  FIGS. 7A and 7B  during the engine operating cycle. 
           [0023]      FIG. 9  is a graph showing values of minimum oil film thickness (MOFT) on bearing segments of intake and exhaust piston rocking journal wristpins for various exhaust crankshaft leads, with 0° load transfer point offsets to the bearings. 
           [0024]      FIG. 10  is a graph showing values of MOFT on bearing segments of exhaust piston rocking journal wristpins for various exhaust crankshaft leads, with 0° and 2° load transfer point offsets to the bearings. 
           [0025]      FIG. 11  is a graph showing values of MOFT on bearing segments of exhaust piston rocking journal wristpins for various exhaust crankshaft leads, with 0° and 2.5° load transfer point offsets to the bearings. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    Fixed Crankshaft Phasing: Presume that the piston coupling mechanisms for a pair of opposed exhaust and intake pistons of a uniflow-scavenged, opposed-piston engine according to  FIG. 1  are assembled with rocking journal bearings as shown in  FIG. 5 . With 0° angular offset between the exhaust and intake crankshafts, the load transfer points of the exhaust and intake pistons occur approximately at crankshaft positions that are 180° apart (0° and 180°, for example). Presume now that the exhaust crankshaft is advanced in phase by a crank angle of x with respect to the intake crankshaft. In this case as per  FIG. 6 , a fixed angular offset φ is applied to the wristpin of the exhaust piston&#39;s rocking journal bearing, resulting in a delayed load transition point for the exhaust piston. In other words, the load transfer point of the exhaust rocking journal bearing is shifted by the angular offset φ. In this regard, the offset φ is measured between the longitudinal axis of the coupling rod and the line  60  that joins the centerlines A and B of the wristpin. Thus, when the exhaust piston is at TC or at BC, the J 1 -J 2  wristpin journal segments are rotated by φ with respect to the J 1 ′-J 2 ′ bearing surface segments. The offset may be put into effect, for example, by circumferential positioning of the threaded recesses  52  (best seen in  FIG. 3 ) which receive the threaded fasteners  51 . It should be evident that the CCW direction of the angular offset illustrated in  FIG. 6  is not meant to be limiting. 
         [0027]    The effect of applying the fixed angular offset φ to the wristpin of the exhaust piston&#39;s rocking journal bearing is illustrated in  FIGS. 7A and 7B . In these figures, which merely illustrate principle of the angular offset and are not intended to be limiting, the view is from the rear toward the front of the engine and the crankshafts  30  and  32  are both rotating in a clockwise direction. When the exhaust crankshaft  32  is at CA=0°, the exhaust piston  22  is at TC and its wristpin has not yet rotated to the load transfer point. At this time, the intake crankshaft  30  is at CA=(0−x)° and the intake piston  20  is approaching TC and its wristpin has not yet rotated to the load transfer point. Then, when the exhaust crankshaft  32  has advanced to CA=(0+x)°, the exhaust piston  22  is leaving TC and its wristpin has rotated to the load transfer point. At this time, the intake crankshaft  30  is at CA=0° and the intake piston  20  is at TC and its wristpin has rotated to the load transfer point. Presuming combustion occurs a short time after the pistons have moved through their respective TC locations, the cylinder pressure, and the resulting load on the pistons, peaks at the transition from the compression stroke to the power stroke.  FIG. 8  shows the desired result of applying a fixed angular offset φ to the wristpin of the exhaust piston&#39;s rocking journal bearing. The curve  70  shows combustion pressure acting against the end surface  20   e  of the intake piston  20  versus CA of the intake crankshaft  30 ; the curve  72  shows combustion pressure acting against the end surface  22   e  of the exhaust piston  22  versus CA of the exhaust crankshaft  32 . Preferably, compressive load transfer from one set of opposed bearing segments to the other in each of the rocking journals occurs during each cycle closely preceding the occurrence of a cyclic peak load. With respect to the intake piston  20 , this occurs at or very near CA=0° (when the intake piston is at or very near TC). With the exhaust piston leading, cyclic peak load occurs well after TC (CA=0°); thus, without an angular offset, the exhaust piston&#39;s load transfer point occurs well before the exhaust piston experiences cyclic peak load. With an angular offset according to this disclosure, the load transfer point of the exhaust piston occurs at  75  on the curve  72 , which follows TC of the piston but precedes the occurrence of a cyclic peak load to the same degree as the intake piston. 
         [0028]    Example: Presuming that engine specifications indicate a preferred phase difference x between exhaust and intake crankshafts, a preferred angular offset φ may be determined empirically, for example by means of a rocking-journal specific, mass conserving finite element model. According to this example, the phase difference is a fixed value in the range 4°≦x≦12°; that is to say that the exhaust crankshaft  32  leads the intake crankshaft  30  by x.  FIGS. 9-11  illustrate this example, showing how the MOFT may be impacted on the J 1  and J 2  journal segments on intake piston wristpins (MOFT J 1  INT and MOFT J 2  INT) and exhaust piston wristpins (MOFT J 1  EX and MOFT J 2  EX) as exhaust crankshaft lead is varied in this range. In this example, which is not intended to be limiting, the intake and exhaust piston rocking journals are assembled so as to have the J 1  segments loaded during the power stroke, while the J 2  segments are loaded during the compression stroke. 
         [0029]    As shown in  FIG. 9 , for an intake piston rocking bearing journal at 4° exhaust crankshaft lead, the J 1  journal segment has approximately 0.2 μm less MOFT than the J 2  segments. This is because the transition between journal segments occurs close to TC. At 4° exhaust crankshaft lead, the intake piston peak cylinder pressure occurs further in the cycle than at the higher exhaust crankshaft leads. The J 2  segments transition and carry the load from BC at low load until close to TC. The load transfer then occurs to the J 1  segment which sees an initial high load and increasing loading until peak cylinder pressure (PCP). At higher exhaust crankshaft leads the intake piston peak loads occur earlier in the cycle, closer to the transition point. The J 2  segments carry the load closer to the PCP, resulting in decreased MOFT and the J 1  segment accepts the load at a point closer to PCP resulting in increased MOFT. At 12° exhaust crankshaft lead the J 2  segments experience the highest peak loads and have the lowest MOFT&#39;s and the J 1  transition occurs very close to PCP causing high initial squeeze and a slightly lower MOFT than at 8 degrees exhaust crankshaft lead. Overall the intake piston pin MOFT on J 1  and J 2  journals is sufficient and reasonably balanced throughout the range of exhaust crankshaft leads desired for testing. Manipulation of the wristpin initial radial position to alter the transition point is not required or beneficial for the intake piston. 
         [0030]    As is shown in  FIG. 9  the MOFT on the exhaust piston wristpin is not well balanced. As exhaust crankshaft lead is increased, MOFT on the J 1  and J 2  segments diverges. Unlike the intake piston in which PCP occurs earlier in the cycle, as exhaust crankshaft lead is increased the exhaust piston PCP occurs later in the cycle. The exhaust J 2  segments experience decreasing peak load with increased exhaust crankshaft lead and the J 1  segment transitions into a longer positive loading ramp until PCP as the exhaust crankshaft lead increases. The result of the varying load regimes is an increasing MOFT on the J 2  segments and a decreasing MOFT on the J 1  segment for the exhaust piston wristpin. In order to enhance the MOFT for the J 1  segment and more evenly balance the MOFT between the J 1  and J 2  segments on the exhaust piston wristpin, a fixed angular offset is applied to the wristpin, resulting in a delayed transition point forcing the J 2  segments to accept higher load resulting in lower MOFT and the J 1  segment to have a shorter increasing pressure ramp resulting in a higher MOFT. 
         [0031]    The effect of applying a 2° initial piston wristpin angular offset on the exhaust piston pin is shown in  FIG. 10 . As shown in the figure, the effects of applying a 2° initial angular rotation to the exhaust piston wristpin are a higher MOFT on the J 1  segment and a lower MOFT on the J 2  segments across the exhaust crankshaft lead range. With the 2° angular offset on the exhaust piston wristpin the J 1  and J 2  segments MOFT is well balanced at 4° exhaust crankshaft lead, and MOFT diverges as the exhaust crankshaft lead increases. The effect of a larger exhaust piston wristpin initial angular position of 2.5° exhaust crankshaft leads is shown in  FIG. 11 . Increasing the angular offset on the exhaust piston pin from 2° to 2.5° results in a more balanced MOFT at 8° and 12° exhaust crankshaft lead but a slightly lower overall minimum MOFT on the J 2  segments at 4° lead. Further increases to the load point transfer offset of the exhaust piston pin would result in diminishing the J 2  segments MOFT further, which is undesirable. As the example of  FIGS. 9-11  suggests, there is an optimal initial offset of the load transfer point of the exhaust piston wristpin; specifically, the example suggests that the optimal value lies between 2° and 2.5° for exhaust crank leads of 4° through 12°. Of course the ranges and values used in this example may be illustrative, they should not be considered to be limiting. 
         [0032]    Variable crankshaft phasing: In some aspects of dual-crankshaft operation, it may be desirable to equip an opposed-piston engine for dynamically variable crankshaft phasing. In this regard, see, for example, commonly-owned U.S. application Ser. No. 13/858,943, filed Apr. 8, 2013, for “Dual Crankshaft, Opposed-Piston Engines With Variable Crank Phasing”, which has been published as US 2014/0299109 A1 on Oct. 9, 2014. For example, the crank angle of one of the crankshafts may be dynamically positioned or changed with respect to the other crankshaft in order to optimize engine performance in response to variable engine conditions such as engine speed, engine load, charge air flow, charge air composition, or, possibly, other engine conditions. In such instances, the load transfer point of the first rocking journal bearing may be selected so as to be effective over a range of crankshaft lead, for example the range of 4° to 12° illustrated in  FIGS. 9-11 . In such a case, the angular offset of the wristpin will remain fixed at some CA selected according to design and performance requirements within some range of crankshaft lead. Accordingly, the angular offset of the rocking journal elements (the wristpin, for example) can be applied to either fixed crankshaft phasing or dynamic crankshaft phasing over a prescribed CA range. 
         [0033]    Although this disclosure describes particular embodiments for load transfer point offset of rocking journal wristpins in opposed-piston engines with phased crankshafts, 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.