Load transfer point offset of rocking journal wristpins in uniflow-scavenged, opposed-piston engines with phased crankshafts

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.

Claims

1. A uniflow-scavenged, opposed-piston engine having first and second rotatable crankshafts, one or more cylinders each with a first piston interconnected by a first connecting rod with the first crankshaft and a second piston opposing the first piston and interconnected by a second connecting rod with the second crankshaft, a first rocking journal bearing situated between the first piston and the first connecting rod and including a plurality of sets of bearing surfaces, a second rocking journal bearing situated between the second piston and the second connecting rod and including a plurality of sets of opposed bearing surfaces, each rocking journal bearing having a respective load transfer point at which a compressive load transfer occurs from one set of opposed bearing surfaces to another set of opposed bearing surfaces during successive cycles of engine operation, wherein: the first crankshaft is positioned so as to lead the second crankshaft during engine operation by an angle; the load transfer point of the first rocking journal bearing being selected such that a load transfer from one set of opposed bearing surfaces to another set of opposed bearing surfaces occurs during each cycle following a top center position of the first piston and closely preceding the occurrence of a cyclic peak load; and, the load transfer point of the second rocking journal bearing being selected such that a load transfer from one set of opposed bearing surfaces to the another set of opposed bearing surfaces occurs during each cycle closely preceding the occurrence of the cyclic peak load; wherein, the plurality of sets of bearing surfaces of the first rocking journal includes a plurality of axially-spaced, eccentrically-disposed journal segments formed on a wristpin and a plurality of corresponding axially-spaced, eccentrically-disposed surface segments formed on a segmented bearing surface of a sleeve, and when the first piston is at a top center or a bottom center the wristpin journal segments are rotated by an angular offset φ with respect to the bearing surface segments, in which a first wristpin rocking journal segment has a centerline A and second wristpin rocking journal segments share a centerline B that is offset from the centerline A, and the angular offset φ is measured between a longitudinal axis of the first connecting rod and a line that joins the centerlines A and B.

2. The uniflow-scavenged, opposed-piston engine according to claim 1, wherein the angle by which the first crankshaft leads the second crankshaft has a value x in the range of 4°<x<12°.

3. The uniflow-scavenged, opposed-piston engine according to claim 2, wherein the angular offset φ has a value in the range of 2°<φ<4°.

4. The uniflow-scavenged, opposed-piston engine according to claim 3, wherein the wristpin is mounted to a small end of the first connecting rod for rocking oscillation against the segmented bearing surface of the sleeve.

5. The uniflow-scavenging, opposed-piston engine according to claim 1, wherein the angle is fixed or variable.

6. The uniflow-scavenging, opposed-piston engine according to claim 5, wherein the first crankshaft is an exhaust crankshaft and the second crankshaft is an intake crankshaft.

7. A method of operating the uniflow-scavenging, opposed-piston engine according to claim 1, by: causing the first and second crankshafts to rotate in response to combustion in a combustion chamber formed in the uniflow-scavenging, opposed-piston engine between end surfaces of the first and second pistons; causing rotation of the first crankshaft to lead rotation of the second crankshaft; causing the load transfer point of the first rocking journal to occur at a first crank angle measured with respect to rotation of the first crankshaft; and, causing the load transfer point of the second rocking journal to occur at a second crank angle measured with respect to rotation of the second crankshaft; wherein the first crank angle is greater than the second crank angle.

8. A two-stroke cycle, opposed-piston engine having first and second rotatable crankshafts a cylinder, a first piston disposed in a bore of the cylinder and interconnected by a connecting rod with the first crankshaft, a second piston disposed in the bore in opposition to the first piston and interconnected by a connecting rod with the second crankshaft, a first rocking journal bearing acting between the first piston and its connecting rod and including a plurality of sets of bearing surfaces, a second rocking journal bearing acting between the second piston and its connecting rod and including a plurality of sets of opposed bearing surfaces, each rocking journal bearing having a respective load transfer point at which a compressive load transfer occurs from one set of opposed bearing surfaces to another set of opposed bearing surfaces during successive cycles of engine operation, wherein: the first crankshaft is positioned so as to lead the second crankshaft during engine operation by a fixed or variable angle; the load transfer point of the first rocking journal bearing being selected such that a load transfer from one set of opposed bearing surfaces to another set of opposed bearing surfaces occurs during each cycle after a top center position of the first piston and preceding the occurrence of a cyclic peak load; and, the load transfer point of the second rocking journal bearing being selected such that a load transfer from one set of opposed bearing surfaces to another set of opposed bearing surfaces occurs during each cycle preceding the occurrence of the cyclic peak load; wherein, the plurality of sets of bearing surfaces of the first rocking journal includes a plurality of axially-spaced, eccentrically-disposed journal segments formed on a wristpin and a plurality of corresponding axially-spaced, eccentrically-disposed surface segments formed on a segmented bearing surface of a sleeve, and when the first piston is at a top center or a bottom center the wristpin journal segments are rotated by an angular offset φ with respect to the bearing surface segments, in which a first wristpin rocking journal segment has a centerline A and second wristpin rocking journal segments share a centerline B that is offset from the centerline A, and the angular offset φ is measured between a longitudinal axis of the first connecting rod and a line that joins the centerlines A and B.

9. The two-stroke cycle, opposed-piston engine according to claim 8, wherein the fixed or variable angle by which the first crankshaft leads the second crankshaft has a value x in the range of 4°<x<12°.

10. The two-stroke cycle, opposed-piston engine according to claim 8, wherein angular offset φ has a value in the range of 2°<φ<4°.

11. The two-stroke cycle, opposed-piston engine according to claim 10, wherein the wristpin is mounted to a small end of the first connecting rod for rocking oscillation against the segmented surface of the sleeve.

12. The two-stroke cycle, opposed-piston engine according to claim 11, wherein the first crankshaft is an exhaust crankshaft and the second crankshaft is an intake crankshaft.

13. A method of operating a two-stroke cycle, opposed-piston engine according to claim 8, by: causing the first and second crankshafts to rotate in response to combustion in a combustion chamber formed in the uniflow-scavenging, opposed-piston engine between end surfaces of the first and second pistons; causing rotation of the first crankshaft to lead rotation of the second crankshaft; causing the load transfer point of the first rocking journal to occur at a first crank angle measured with respect to rotation of the first crankshaft; and, causing the load transfer point of the second rocking journal to occur at a second crank angle measured with respect to rotation of the second crankshaft; wherein the first crank angle is greater than the second crank angle.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of a two-stroke cycle, opposed-piston engine, and is properly labeled “Prior Art”.

(2) FIG. 2 is a graph showing a phase offset between two crankshafts of an opposed-piston engine, and is properly labeled “Prior Art”.

(3) FIG. 3 is an exploded perspective view of a piston coupling mechanism including a rocking journal bearing, and is properly labeled “Prior Art”.

(4) FIG. 4 is a schematic diagram illustrating the bearing surfaces of the rocking journal of FIG. 3, and is properly labeled “Prior Art”.

(5) 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”.

(6) 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.

(7) 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.

(8) FIG. 8 is a graph showing forces acting on the intake and exhaust pistons of FIGS. 7A and 7B during the engine operating cycle.

(9) 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.

(10) 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.

(11) 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

(12) Fixed Crankshaft Phasing:

(13) 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'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.sub.1-J.sub.2 wristpin journal segments are rotated by φ with respect to the J.sub.1′-J.sub.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.

(14) The effect of applying the fixed angular offset φ to the wristpin of the exhaust piston'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's rocking journal bearing. The curve 70 shows combustion pressure acting against the end surface 20e of the intake piston 20 versus CA of the intake crankshaft 30; the curve 72 shows combustion pressure acting against the end surface 22e 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'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.

EXAMPLE

(15) 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.sub.1 and J.sub.2 journal segments on intake piston wristpins (MOFT J.sub.1 INT and MOFT J.sub.2 INT) and exhaust piston wristpins (MOFT J.sub.1 EX and MOFT J.sub.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.sub.1 segments loaded during the power stroke, while the J.sub.2 segments are loaded during the compression stroke.

(16) As shown in FIG. 9, for an intake piston rocking bearing journal at 4° exhaust crankshaft lead, the J.sub.1 journal segment has approximately 0.2 μm less MOFT than the J.sub.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.sub.2 segments transition and carry the load from BC at low load until close to TC. The load transfer then occurs to the J.sub.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.sub.2 segments carry the load closer to the PCP, resulting in decreased MOFT and the J.sub.1 segment accepts the load at a point closer to PCP resulting in increased MOFT. At 12° exhaust crankshaft lead the J.sub.2 segments experience the highest peak loads and have the lowest MOFT's and the J.sub.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.sub.1 and J.sub.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.

(17) 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.sub.1 and J.sub.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.sub.2 segments experience decreasing peak load with increased exhaust crankshaft lead and the J.sub.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.sub.2 segments and a decreasing MOFT on the J.sub.1 segment for the exhaust piston wristpin. In order to enhance the MOFT for the J.sub.1 segment and more evenly balance the MOFT between the J.sub.1 and J.sub.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.sub.2 segments to accept higher load resulting in lower MOFT and the J.sub.1 segment to have a shorter increasing pressure ramp resulting in a higher MOFT.

(18) 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.sub.1 segment and a lower MOFT on the J.sub.2 segments across the exhaust crankshaft lead range. With the 2° angular offset on the exhaust piston wristpin the J.sub.1 and J.sub.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.sub.2 segments at 4° lead. Further increases to the load point transfer offset of the exhaust piston pin would result in diminishing the J.sub.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.

(19) Variable Crankshaft Phasing:

(20) 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.

(21) 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.