Oil ring for piston and piston assembly

Abstract

An oil ring for an internal combustion engine is provided. The oil ring includes an oil ring body having an outer side and an inner side. The oil ring also includes an expansion ring having an outer surface. The outer surface of the expansion ring makes contact with the inner side of the oil ring body so that the oil ring body can be expanded. And an oil discharge gap is formed between the outer surface of the expansion ring and the inner side of the oil ring main body.

Description

[0001] Cross-references to related applications This application claims priority and filing date interest in U.S. Provisional Application Serial No. 63 / 509,084, filed June 20, 2023, which is incorporated herein by reference. Technical Field

[0002] This application relates generally to internal combustion engines, and more specifically, to an oil ring for use with pistons and piston assemblies of internal combustion engines. Background Technology

[0003] A piston used in an internal combustion engine moves up and down or left and right within a cylinder during engine operation. The piston includes one or more piston rings that extend around the piston between the piston and the cylinder bore wall. Pistons used in internal combustion engines typically include one or two piston rings per piston, and at least one oil ring for scraping excess oil from the cylinder bore wall. The excess oil flows over the oil rings and down the piston to the crankcase.

[0004] An oil ring typically comprises two components, such as, for example, the oil ring body and a helical expansion ring. The helical expansion ring is configured to offset the oil ring body radially outward from the piston groove that houses the oil ring, against the cylinder bore wall. Forcing the oil ring body against the cylinder bore wall facilitates the scraping of excess oil.

[0005] The oil ring body includes an upper protrusion and a lower protrusion, with a central portion defining the outer periphery of the oil ring body adjacent to the cylinder bore wall. During piston reciprocating motion, the outer surfaces of the upper and lower protrusions slide against the cylinder bore wall. The central portion of the oil ring body also includes multiple drain holes extending from the outer periphery to the inner periphery. These drain holes allow scraped oil to enter the space behind the oil ring from the outer periphery, where it drains from the grooves housing the oil ring to the oil pan below the piston.

[0006] Current industry trends suggest utilizing oil rings with thinner axial heights to achieve lower friction and improved oil consumption. However, assembling these thinner (i.e., lower tension) oil rings presents challenges in maintaining production cycles due to assembly capability issues. Currently available alternative designs that facilitate assembly risk excess oil clogging and may lead to high oil consumption. Therefore, further improvements in this technological area are needed to address these and other issues.

[0007] Publication of example implementation schemes For the sake of clarity, conciseness, and accuracy, in the description of the example embodiments of the application, the manner and process of making and using them, and to enable others skilled in the art to practice, make, and use these example embodiments, reference will now be made to certain example embodiments, including those illustrated in the accompanying drawings, and specific language will be used to describe the same. It will, however, be understood that no limitation of the scope of the application is intended by this specification of example embodiments. The application includes and protects such alterations, modifications, and further applications of the example embodiments as would occur to one skilled in the art. SUMMARY

[0008] The present application includes an oil ring for a piston of an internal combustion engine. In one embodiment, the oil ring includes an oil ring body having a cross-section viewed in an axial direction. The cross-section has an outer side and an inner side, and the oil ring body includes at least one oil drain hole extending between the outer side and the inner side. The oil ring also includes a flare having an outer surface. The outer surface of the flare contacts the inner side of the oil ring body at a first contact location and a second contact location to expand the oil ring body. An oil drain gap is formed between the outer side of the flare and the inner side of the oil ring body extending from the first contact location to the second contact location. The oil drain gap includes a slot portion intersecting the at least one oil drain hole.

[0009] In one embodiment, a piston assembly for an internal combustion engine is provided. The piston assembly includes a piston including a plurality of annular grooves. At least one piston ring is located in one of the plurality of grooves, and an oil ring is disposed in another one of the plurality of grooves. The oil ring includes an oil ring body having an inner side facing the piston and an opposite outer side. The oil ring body includes at least one oil drain hole extending between the inner side and the outer side. A flare includes an outer surface in contact with the inner side of the oil ring body to radially expand the oil ring body. The outer surface of the flare contacts the oil ring body at a first contact location and a second contact location, and the second contact location is circumferentially spaced apart from the first contact location. An oil drain gap is formed between the inner side of the oil ring body and the outer surface of the flare. The oil drain gap extends from the first contact location to the second contact location. The inner side of the oil ring body includes a channel around the inner side that intersects the at least one oil drain hole and forms a slot portion of the oil drain gap.

[0010] This summary is neither intended nor should it be construed to identify any key or essential features of the claimed subject matter, nor is it used to determine the scope of the claimed subject matter. Further embodiments, forms, aspects, features, objects, benefits, and advantages of the claimed subject matter will become apparent from the following description and drawings. BRIEF DESCRIPTION OF DRAWINGS

[0011] The description herein makes reference to the accompanying drawings, which, in multiple views, show the following: Figure 1is a schematic view of an internal combustion engine cylinder housing a piston assembly according to the present application.

[0012] Figure 2 is a schematic view of an oil ring according to an example embodiment of the present application.

[0013] Figure 3 is a cross-sectional view of a portion of a piston, wherein Figure 2 the oil ring of is positioned in its groove.

[0014] Figure 4 is an axial cross-sectional view of the oil ring of Figure 2 .

[0015] Figure 5 is a detailed view of the oil drain gap of the oil ring of Figure 4 .

[0016] Figure 6 is a detailed view of one of the contact locations between the expander ring and the oil ring body of the oil ring of Figure 4 . DETAILED DESCRIPTION

[0017] Referring to Figure 1 , the internal combustion engine 10 includes a cylinder 12 and a crankcase 14 located below the cylinder 12. The cylinder 12 includes a combustion chamber 16. A cylinder bore wall 18 extends around the combustion chamber 16. The cylinder 12 includes a piston assembly 20 having a piston 22. The piston 22 is housed in the combustion chamber 16. The piston 22 includes a plurality of annular grooves 57, 58. During operation of the internal combustion engine 10, the piston 22 reciprocates in the combustion chamber 16 along the bore wall 18 to rotate a crankshaft (not shown).

[0018] The piston assembly 20 can include one or more piston rings 24 and an oil ring 26. The piston rings 24 project outwardly from the piston 22 toward the bore wall 18. Each piston ring 24 is located in an annular groove 57 of the piston 22. The piston 22 also includes at least one annular groove 58 for the oil ring 26. The oil ring 26 is positioned closest to the crankcase 14 (e.g., a lower ring). The oil ring 26 is configured to wipe excess oil from the bore wall 18 as the piston 22 travels up and down in the cylinder 12.

[0019] With further reference to Figures 2-6An embodiment of an oil ring 26 for a piston 22 of an internal combustion engine 10 is disclosed. The oil ring 26 includes an oil ring body 28 having a cross-section viewed in an axial direction. The cross-section of the oil ring body 28 has an outer side 48 and an inner side 32. The oil ring body 28 further includes at least one oil drain hole 50 extending from the outer side 48 to the inner side 32. The oil ring 26 also includes a flinger 30 having an outer surface 80 that contacts the inner side 32 of the oil ring body 28 at a first contact location 82 and a second contact location 84. Between the first contact location 82 and the second contact location 84, an oil drain gap 54 is formed by the inner side 32 of the oil ring body 28 and the outer surface 80 of the flinger 30. The oil drain gap 54 includes a slot portion 56 that intersects the at least one oil drain hole 50.

[0020] In one embodiment, a piston assembly 20 for an internal combustion engine 10 is provided. The piston assembly 20 includes a piston 22 including a plurality of annular grooves 58. At least one piston ring 24 is located in one of the plurality of grooves 58 and an oil ring 26 is disposed in another one of the plurality of grooves 58. The oil ring 26 includes an oil ring body 28 having an inner side 32 facing the piston 22 and an opposite outer side 48. A flinger 30 includes an outer surface 80 that contacts the inner side 32 of the oil ring body 28 to radially expand the oil ring body 28. The outer surface 80 of the flinger 30 contacts the oil ring body 28 at a first contact location 82 and a second contact location 84, and the second contact location 84 is circumferentially spaced apart from the first contact location 82. An oil drain gap 54 is formed between the inner side 32 of the oil ring body 28 and the outer surface 80 of the flinger 30. The oil drain gap 54 extends from the first contact location 82 to the second contact location 84. The inner side 32 of the oil ring body 28 includes a channel 60 around the inner side 32 that intersects the at least one oil drain hole 50 and forms a slot portion 56 of the oil drain gap 54.

[0021] Reference is made to Figure 2 FIG. 1 shows an oil ring 26 according to an example embodiment of the present application. In Figure 2 FIG. 1, the oil ring 26 is C-shaped and includes an oil ring body 28 having a flinger 30 disposed along an inner side 32 of the oil ring body 28. In one embodiment, "C-shaped" is a generally circular shape having a gap or opening on one side between adjacent opposite ends of the oil ring body 28. In one embodiment, the flinger 30 is a spiral flinger having a spring-like body that expands radially outward when released from a radially collapsed configuration, although other types of flingers are contemplated in the present application.

[0022] Further reference is made to Figure 2The oil ring body 28 includes a first rail 34 at an upper side 36 of the oil ring body 28 and a second rail 38 at a lower side 40 of the oil ring body 28. The first rail 34 and the second rail 38 are integrally connected by an intermediate portion 42 and extend in a circumferential C-shaped direction of the oil ring body 28. The oil ring body 28 forms a gap 29 between adjacent ends of the oil ring body 28 that facilitates radial expansion of the oil ring body 28 and expulsion of excess oil from behind the oil ring 26 in the recess 58.

[0023] The first rail 34 includes a first protrusion 44 and the second rail 38 includes a second protrusion 46. The first protrusion 44 and the second protrusion 46 are configured to contact the bore wall 18 of the cylinder 12. The first protrusion 44, the second protrusion 46, and the intermediate portion 42 define an outer side 48 of the oil ring body 28 facing the bore wall 18. The first protrusion 44 and the second protrusion 46 slide against the bore wall 18 as the piston 22 reciprocates up and down in the cylinder 12. The intermediate portion 42 includes a plurality of oil drain holes 50 arranged around a periphery of the oil ring body 28 in the intermediate portion 42.

[0024] Referring to Figure 3 a cross-sectional view of the oil ring 26 including the oil ring body 28 and the expander ring 30 is shown. In particular, Figure 3 a cross-sectional view of the piston 22 including the oil ring 26 positioned in the lower piston recess 58 is shown, as well as the piston 22 positioned along the combustion bowl surface of the bore wall 18 of the cylinder 12. The outer side 48 of the oil ring body 28 includes the first protrusion 44, the second protrusion 46, and the intermediate portion 42 extending between the first protrusion 44 and the second protrusion 46 and facing the bore wall 18. The oil drain holes 50 are disposed in the intermediate portion 42 and extend from the outer side 48 to the inner side 32 to drain excess oil collected between the protrusions 44, 46.

[0025] Further referring to Figures 4-6 The inner side 32 of the oil ring body 28 includes a concave surface portion 52 having an oil drain gap 54 fluidly coupled with the oil drain holes 50. The concave surface portion 52 receives the expander ring 30. A trough portion 56 of the oil drain gap 54 is formed by a channel 60 extending in the inner side 32 in the concave surface portion 52. The channel 60 extends continuously between and intersects each of the oil drain holes 50. It is understood that the expander ring 30 can be deformed to fit into the concave surface portion 52 and, once positioned in the concave surface portion 52, the expander ring 30 is biased radially outwardly such that an outer surface 80 of the expander ring 30 is pressed against the oil ring body 28 at a first contact location 82 and a second contact location 84.

[0026] In one embodiment, the oil drainage gap 54 includes a first side 62 along the outer surface 80 of the expander ring 30 extending from a first contact location 82 to a second contact location 84. The oil drainage gap 54 also includes a second side 64 along the concave surface portion 52 of the inner side 32 of the oil ring body 28. The second side 64 extends from the first contact location 82 to the slot portion 56. The oil ring gap 54 also includes a third side 66 along the concave surface portion 52 of the inner side 32 of the oil ring body 28. The third side 66 extends from the second contact location 84 to the slot portion 56, and the slot portion 56 connects the second side 64 and the third side 66.

[0027] In one embodiment, the second side 64 and the third side 66 of the oil drainage gap 54 each have a first curvature formed by the concave surface portion 52 along the inner side 32 of the oil ring body 28. The slot portion 56 has a second curvature formed by the inner side 32 of the oil ring body 28. The first curvature is greater than the second curvature such that the slot portion 56 provides the oil drainage gap 54 with a passage 60 having additional volume at a location bisecting or intermediate the oil drainage hole 50.

[0028] In one embodiment, the concave surface portion 52 of the oil ring body 28 includes a first concave curvature along the first contact location 82 and the second contact location 84. The first concave curvature is defined by a first radius or radius of curvature Rl. The concave curvature of the concave surface portion 52 along the slot portion 56 is defined by a second radius or radius of curvature R2. The second radius of curvature R2 is greater than the first radius of curvature Rl such that the slot portion 56 is more strongly curved than the second side 64 and the third side 66. In one embodiment, the first radius of curvature Rl extends from each of the first contact location 82 and the second contact location 84 to an opposite side of the slot portion 56 along the oil drainage gap 54.

[0029] In one embodiment, the outer surface 80 of the expander ring 30 includes a circumference. The first contact location 82 and the second contact location 84 each define a contact length L around the expander ring 80. In one embodiment, it has been found that a total contact length (2L) provided by the first contact location 82 and the second contact location 84 is between 30% and 40% of the circumference to provide a contact length that fits the expander ring 30 to the oil ring body 28 that improves retention during handling of the oil ring 26 and assembly with the piston 22 while providing sufficient clearance between the expander ring 30 and the body 28 to allow oil to pass in the oil drainage gap 54. In another embodiment, the total contact length 2L is between 33% and 37% of the circumference to improve retention during handling of the oil ring 26 and assembly with the piston 22 while providing sufficient clearance to form the oil drainage gap 54. In yet another embodiment, the total contact length 2L is 35% of the circumference to improve retention during handling of the oil ring 26 and assembly with the piston 22 while providing sufficient clearance to form the oil drainage gap 54.

[0030] In one embodiment, the oil drain gap 54 has a width W along the outer surface 80 of the expander ring 30. The width W extends from a first contact location 82 to a second contact location 84. In one embodiment, the width W is in the range of 10% to 20% of the circumference of the expander ring 30 to provide sufficient clearance between the expander ring 30 and the oil ring body 28 for oil drainage to prevent oil clogging issues. In another embodiment, the width W is 15% of the circumference of the expander ring 30 to provide sufficient clearance between the expander ring 30 and the oil ring body 28 for oil drainage to prevent oil clogging issues.

[0031] The oil drain hole 50 allows any excess oil to drain into the oil drain gap 54, and then into the recess 58, and then from the recess 58 back to an oil sump (not shown) located below the piston 22. Failure to maintain a gap for excess oil can cause oil clogging issues that can result in an increased oil consumption rate. In the example embodiment, the slot 56 of the oil drain gap 54 provides additional flow volume around the oil ring 26 that is in communication with the oil drain hole 50 for receiving oil and enabling excess oil to drain without causing oil clogging.

[0032] Further, the configuration of the first contact location 82 and the second contact location 84 provides improved support and expander ring retention to adequately retain the expander ring 30 in engagement with the inner side 32 of the oil ring body 28, particularly during handling and assembly. According to the example embodiment described above, there is improved helical expander ring retention that also provides assembly capability benefits by enabling the expander ring 30 to fit and be retained against the oil ring body 28. Additionally, the oil drain gap 54 enhances the drainage of excess oil from the outer side 48 of the oil ring body 28.

[0033] A further written description of the various example embodiments will now be provided. In one embodiment, an oil ring for a piston of an internal combustion engine is provided. The oil ring includes an oil ring body and an expander ring. The oil ring body has a cross-section viewed along an axial direction, and the cross-section has an outer side and an inner side. The oil ring body further includes at least one oil drain hole extending from the inner side to the outer side of the cross-section. The expander ring has an outer surface, and the outer surface contacts the inner side of the oil ring body at a first contact location and a second contact location. Between the first contact location and the second contact location, an oil drain gap is formed by the inner surface of the oil ring body and the outer surface of the expander ring, and the oil drain gap includes a slot portion that intersects the at least one oil drain hole.

[0034] In a further embodiment, the inner side of the oil ring body includes a concave surface along the first contact location and along the second contact location, and the outer surface of the expander ring includes a convex surface along the first contact location and along the second contact location.

[0035] In yet another embodiment, the outer surface of the expander ring includes a circumference, and the first contact location and the second contact location together define a contact length around the expander ring, the contact length being between 30% and 40% of the circumference of the expander ring. In yet another embodiment, the contact length is between 33% and 37% of the circumference of the expander ring. In yet another embodiment, the contact length is 35% of the circumference of the expander ring.

[0036] In yet another embodiment, the oil drainage gap has a width along the outer surface of the expander ring from the first contact location to the second contact location, the width being in a range of 10% to 20% of the circumference of the expander ring. In yet another embodiment, the width is 15% of the circumference.

[0037] In a further embodiment, the oil drainage gap includes a first side extending along the outer surface of the expander ring from the first contact location to the second contact location, a second side along the inner side of the oil ring body extending from the first contact location to the slot portion, and a third side along the inner side of the oil ring body extending from the second contact location to the slot portion. The slot portion connects the second side and the third side.

[0038] In yet another embodiment, the second side and the third side of the oil drainage gap each have a first radius of curvature formed by the inner side of the oil ring body, the slot portion has a second radius of curvature formed by the inner side of the oil ring body, and the second radius of curvature is greater than the first radius of curvature. In yet another embodiment, the slot portion bisects the at least one oil drainage hole.

[0039] In a further embodiment, the inner side of the oil ring body includes a first concave curvature along the first contact location and the second contact location, a second concave curvature along the slot portion, and the second concave curvature is greater than the first concave curvature such that the slot portion is more strongly curved than the inner side of the oil ring body along the first contact location and the second contact location. In yet another embodiment, the inner side having the first concave curvature extends along the oil drainage gap from each of the first contact location and the second contact location to the slot portion.

[0040] In a further embodiment, the oil ring body includes a first protrusion and a second protrusion extending from the outer side, and the at least one oil drainage hole is located between the first protrusion and the second protrusion.

[0041] According to another embodiment, a piston assembly for an internal combustion engine is provided. The piston assembly includes a piston having a plurality of annular grooves; at least one piston ring in one of the grooves; and an oil ring in another of the grooves. The oil ring includes an oil ring body having an inner side facing the piston and an opposing outer side. The oil ring body includes at least one drain hole extending between the inner and outer sides. The oil ring also includes an expansion ring having an outer surface that contacts the inner side of the oil ring body to radially expand the oil ring body. The outer surface of the expansion ring contacts the oil ring body at a first contact position and a second contact position circumferentially spaced from the first contact position. A drain gap is formed between the inner side of the oil ring body and the outer surface of the expansion ring. The drain gap extends from the first contact position to the second contact position, and the inner side of the oil ring body includes a channel extending around the inner side, the channel intersecting the at least one drain hole and forming a groove portion of the drain gap.

[0042] In a further embodiment, at least one drain hole includes a plurality of drain holes surrounding the oil ring body, and the channel intersects with each of the plurality of drain holes.

[0043] In a further embodiment, the inner surface has a first concave curvature along the first contact position and the second contact position, and the first concave curvature extends from each of the first and second contact positions along the oil drain gap to the channel. In yet another embodiment, the channel includes a second concave curvature, and the second concave curvature is more strongly curved than the first concave curvature. In yet another embodiment, the outer surface of the expansion ring includes a convex curvature extending along the oil drain gap, along the first contact position, and along the second contact position.

[0044] In a further embodiment, the outer surface of the expansion ring includes a circumference, and the first contact position and the second contact position together define a contact length around the expansion ring, which is between 30% and 40% of the circumference.

[0045] In a further embodiment, the oil drain gap has a width along the outer surface of the expansion ring from the first contact position to the second contact position, the width being in the range of 10% to 20% of the circumference of the expansion ring.

[0046] While exemplary embodiments of the invention have been shown and described in detail in the accompanying drawings and the foregoing description, their nature should be considered illustrative rather than restrictive. It should be understood that only certain exemplary embodiments have been shown and described, and protection is intended for all variations and modifications falling within the spirit of the claimed invention. It should be understood that while terms such as “preferred,” “preferred,” “most preferred,” or “more preferred” are used in the foregoing description to indicate that a feature so described may be more desirable, it may not be essential, and embodiments lacking such features may be conceived as within the scope of the invention, defined by the following claims. When reading the claims, it is intended that when terms such as “a,” “at least one,” or “at least a portion” are used, the claims are not intended to be limited to a single item unless expressly stated otherwise in the claims. When the language “at least a portion” and / or “a portion” is used, the item may include a portion and / or the entire item unless expressly stated otherwise.

Claims

1. An oil ring for a piston in an internal combustion engine, the oil ring comprising: An oil ring body having a cross-section viewed in an axial direction, the cross-section having an outer side and an inner side, the oil ring body further including at least one oil drain hole extending from the inner side to the outer side of the cross-section; and An expansion ring, the expansion ring having an outer surface, the outer surface contacting the inner side of the oil ring body at a first contact position and a second contact position. Between the first contact position and the second contact position, an oil discharge gap is formed by the inner surface of the oil ring body and the outer surface of the expansion ring, and the oil discharge gap includes a groove portion that intersects with the at least one oil discharge hole.

2. The oil ring according to claim 1, wherein: The inner side of the oil ring body includes a concave surface along the first contact position and along the second contact position; and The outer surface of the expansion ring includes a convex surface along the first contact position and along the second contact position.

3. The oil ring of claim 2, wherein the outer surface of the expansion ring includes a circumference, and the first contact position and the second contact position together define a contact length around the expansion ring, the contact length being between 30% and 40% of the circumference of the expansion ring.

4. The oil ring according to claim 3, wherein the contact length is between 33% and 37% of the circumference of the expansion ring.

5. The oil ring according to claim 3, wherein the contact length is 35% of the circumference of the expansion ring.

6. The oil ring according to claim 3, wherein the oil discharge gap has a width along the outer surface of the expansion ring from the first contact position to the second contact position, the width being in the range of 10% to 20% of the circumference of the expansion ring.

7. The oil ring according to claim 6, wherein the width is 15% of the circumference.

8. The oil ring according to claim 1, wherein the oil discharge gap comprises: The expansion ring extends along its outer surface from the first contact position to the first side of the second contact position; Along the second side of the inner side of the oil ring body, the second side extends from the first contact position to the groove portion; as well as Along the inner third side of the oil ring body, the third side extends from the second contact position to the groove portion, wherein the groove portion connects the second side and the third side.

9. The oil ring according to claim 8, wherein: The second and third sides of the oil discharge gap each have a first radius of curvature formed by the inner side of the oil ring body; The groove portion has a second radius of curvature formed by the inner side of the oil ring body; and The second radius of curvature is greater than the first radius of curvature.

10. The oil ring according to claim 9, wherein the groove portion bisects the at least one oil drain hole.

11. The oil ring according to claim 1, wherein the inner side of the oil ring body comprises: The first concave curvature along the first contact position and the second contact position; The second concave curvature along the groove portion; and The second concave curvature is greater than the first concave curvature, causing the groove portion to bend more strongly along the first contact position and the second contact position than the inner side of the oil ring body.

12. The oil ring of claim 11, wherein the inner side having the first concave curvature extends along the oil drain gap from each of the first contact position and the second contact position to the groove portion.

13. The oil ring according to claim 1, wherein the oil ring body includes a first protrusion and a second protrusion extending from the outer side, and the at least one oil drain hole is located between the first protrusion and the second protrusion.

14. A piston assembly for an internal combustion engine, the piston assembly comprising: The piston includes a plurality of annular grooves; At least one piston ring in one of the plurality of grooves; as well as An oil ring in another groove of the plurality of grooves, the oil ring comprising: An oil ring body having an inner side facing the piston and an opposite outer side, the oil ring body including at least one drain hole extending between the inner side and the outer side; and An expansion ring includes an outer surface that contacts the inner side of the oil ring body to cause the oil ring body to expand radially. The outer surface of the expansion ring contacts the oil ring body at a first contact position and a second contact position circumferentially spaced from the first contact position. An oil drain gap is formed between the inner side of the oil ring body and the outer surface of the expansion ring, the oil drain gap extending from the first contact position to the second contact position. The inner side of the oil ring body includes a channel extending around the inner side, the channel intersecting the at least one oil drain hole and forming a groove portion of the oil drain gap.

15. The piston assembly of claim 14, wherein: The at least one oil drain hole includes a plurality of oil drain holes surrounding the oil ring body; and The channel intersects with each of the plurality of oil drain holes.

16. The piston assembly of claim 14, wherein the inner surface has a first concave curvature along the first contact position and the second contact position, and the first concave curvature extends from each of the first contact position and the second contact position along the oil drain gap to the channel.

17. The piston assembly of claim 16, wherein the channel includes a second concave curvature, and the second concave curvature is more strongly curved than the first concave curvature.

18. The piston assembly of claim 16, wherein the outer surface of the expansion ring includes a convex curvature extending along the oil discharge gap, along the first contact position, and along the second contact position.

19. The piston assembly of claim 14, wherein the outer surface of the expansion ring includes a circumference, and the first contact position and the second contact position together define a contact length around the expansion ring, the contact length being between 30% and 40% of the circumference.

20. The piston assembly of claim 14, wherein the oil discharge gap has a width along the outer surface of the expansion ring from the first contact position to the second contact position, the width being in the range of 10% to 20% of the circumference of the expansion ring.

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