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1. WO2010075207 - PISTON ASSEMBLY HAVING A PASSAGE EXTENDING TO THE SECOND RING GROOVE

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PISTON ASSEMBLY HAVING A PASSAGE EXTENDING TO THE SECOND RING GROOVE

Description

Technical Field The present disclosure is directed to a piston assembly, and more particularly, to a piston assembly for an internal combustion engine.

Background

Reciprocating, internal combustion engines generally utilize piston assemblies that reciprocate within a cylinder to convert the energy released during the combustion process into useful rotational energy. To accomplish this conversion, piston assemblies generally cooperate with the cylinder liner (or cylinder in the engine block) and head to form a combustion chamber that is capable of changing its volume. The energy released during the combustion process creates significant pressure within the combustion chamber, which forces the piston assembly to move within the cylinder. The piston assembly is coupled to a crankshaft in such a way that the linear movement of the piston assembly is converted into the rotational movement of the crankshaft. Thus, as the piston assembly is forced to move within the cylinder by the combustion pressures, the piston assembly also forces the crankshaft to rotate. The greater the pressure within the combustion chamber, the more force is applied to the piston and ultimately to the crankshaft. Thus, it is beneficial to minimize any leakage of gases past the piston assembly to ensure that the maximum pressure is acting on the piston assembly. It is also desirable to sufficiently lubricate the piston assembly as it reciprocates within the cylinder to help insure that it functions properly. However, keeping oil from passing the piston assembly and entering the combustion chamber where it will be burned during the combustion process is becoming more important, not only because it minimizes the consumption of oil, but also because it helps to keep regulated emission constituents, like soot and NOx, at their lowest levels. As emissions regulations continue to reduce the permissible levels of certain exhaust constituents, minimizing the amount of oil that is burned during the combustion process, which generally results in higher levels of the regulated exhaust constituents than the combustion of the fuel, is one way to help minimize the production of the regulated exhaust constituents.

In order to reduce the amount of pressurized gas and lubricant that is able to leak or move past the piston assembly, many modern piston assemblies include a set of piston rings. Although the number and configurations of the piston rings may vary, one common configuration is to have three piston rings - a top piston ring, an intermediate piston ring, and a lower piston ring. The primary purpose of the top piston ring is to minimize the leakage of combustion gases past the piston assembly. The primary purpose of the lower piston ring is to scrap off excess oil that may be provided on the cylinder wall. The intermediate piston ring normally serves to backup the top piston ring by providing an additional gas seal, and to backup the lower piston ring by scraping off any remaining oil film on the cylinder wall. In general, the piston rings work together to accomplish the desired sealing action. If one ring does not function properly, then the appropriate sealing action will most likely not be achieved.

At different points within the four-stroke engine cycle, each of the three piston rings may be exposed to different pressures, different portions of a particular piston ring may be exposed to different pressures, and each piston ring may move to different locations within its respective ring groove, both axially and radially. These pressures and locations will be dependent on, among other possible factors, where the piston assembly is within the four-stroke cycle, the acceleration or deceleration of the piston assembly, combustion pressures, and the level of friction between the piston ring and the cylinder wall.

Under some circumstances, the intermediate piston ring may experience a situation where it is pressed against the top of its ring groove (which creates a situation where the face of the intermediate piston ring is exposed to the pressure between the top piston ring and the intermediate piston ring and the back side (or the radially inner side) of the intermediate piston ring is exposed to the pressure between the lower piston ring and the intermediate piston ring)) and the pressure differential between the face of the ring and the back side of the ring is enough to cause the intermediate piston ring to "collapse" or essentially move away from the cylinder wall. When this happens, the intermediate piston ring is not able to perform its oil scraping function, which may allow oil to pass by the intermediate piston ring and be consumed during the combustion process. This not only leads to the consumption of oil, but also tends to increase unwanted engine emissions.

The disclosed piston assembly is directed to overcoming one or more of the problems set forth above or other problems.

Summary

According to one exemplary embodiment, a piston configured to reciprocate within a cylinder of an internal combustion engine comprises a top surface and a cylindrical wall. The cylindrical wall extends away from the top surface and includes a first ring groove, a second ring groove, a third ring groove, a first land, a second land, and a third land. The first ring groove is configured to receive a first ring. The second ring groove is configured to receive a second ring. The third ring groove is configured to receive a third ring. The first land is between the top surface and the first ring groove. The second land is between the first ring groove and the second ring groove. The third land is between the second ring groove and the third ring groove. The at least one passage extends between the second land and the second ring groove. The at least one passage is configured to fluidly couple the second ring groove to a volume surrounding the second land.

-A-

According to another exemplary embodiment, a piston assembly configured to reciprocate within a cylinder of an internal combustion engine comprises a top surface, a cylindrical wall, a first ring, a second ring, and at least one passage. The cylindrical wall extends from the top surface and includes a first ring groove, a second ring groove, and a first land between the first ring groove and the second ring groove. The first ring is received within the first ring groove. The second ring is received within the second ring groove. The second ring includes an outer surface configured to engage the cylinder and an inner surface opposite the outer surface. The at least one passage fluidly couples a first volume to which the outer surface is exposed to a second volume to which the inner surface is exposed.

According to another exemplary embodiment, an internal combustion engine comprises a block, a cylinder head, a crankshaft, and a piston assembly. The block includes at least one cylindrical bore having a first end and a second end. The cylinder head is coupled to the block and substantially encloses the first end of the cylindrical bore. The crankshaft is rotatably received within the block. The piston assembly is received within the cylindrical bore and is configured to reciprocate within the cylindrical bore. The piston assembly is coupled to the crankshaft and is configured to rotate the crankshaft as the piston reciprocates within the cylindrical bore. The piston assembly includes a top surface, a cylindrical wall, a first compression ring, a second compression ring, and an oil ring assembly. The cylindrical wall extends away from the top surface and includes a first ring groove, a second ring groove, a third ring groove, a first land between the first ring groove and the second ring groove, and at least one passage extending between the first land and the second ring groove. The first compression ring is received within the first ring groove. The second compression ring is received within the second ring groove. The oil ring assembly is received within the third ring groove. The at least one passage is confϊgured to fluidly couple the second ring groove to the volume surrounding the first land.

Brief Description of the Drawings

Figure 1 is a partial cross-sectional view of an engine according to one exemplary embodiment.

Figure 2 is a partial cross-sectional view of a piston assembly according to one exemplary embodiment.

Figure 3 is a partial cross-sectional view of a piston assembly according to another exemplary embodiment. Figure 4 is a partial perspective view of the piston assembly of

Figure 3.

Figure 5 is a partial cross-sectional view of a piston assembly according to another exemplary embodiment.

Although the drawings depict exemplary embodiments or features of the present disclosure, the drawings are not necessarily to scale, and certain features may be exaggerated in order to provide better illustration or explanation. The exemplifications set out herein illustrate exemplary embodiments or features, and such exemplifications are not to be construed as limiting the inventive scope in any manner.

Detailed Description

Reference will now be made in detail to specific embodiments or features, examples of which are illustrated in the accompanying drawings. Generally, the same or corresponding reference numbers will be used throughout the drawings to refer to the same or corresponding parts. It should be appreciated that the terms "upper," "lower," "top," "bottom," "up," "down," and other terms related to orientation are being used to solely to facilitate the description of the objects as they are depicted in the figures and should not be viewed as limiting the scope of this description to the orientations associated with each of these terms.

Referring generally to Figure 1, an internal combustion engine 10 is shown according to one exemplary embodiment. Engine 10 is a device that converts chemical energy (from a fuel such as diesel fuel, biodiesel, gasoline, etc.) into rotational mechanical energy and that may be used as a power source in a multitude of different applications, such as in vehicles, locomotives, machines, etc. According to one exemplary embodiment, engine 10 includes a block 12, a head assembly 16, a piston assembly 18, a connecting rod 20, and a crankshaft 22.

Block 12 is a generally rigid structure or housing that is configured to receive and support head assembly 16, piston assembly 18, connecting rod 20, and crankshaft 22. According to one exemplary embodiment, block 12 defines a generally cylindrical chamber 24 that receives crankshaft 22 and one or more cylindrical bores 26 that extend radially outward from chamber 24. Each of cylindrical bores 26 may be configured to receive a piston assembly 18 directly, or to receive a cylinder liner 28, which in turn is configured to directly receive a piston assembly 18. Each cylinder liner 28 is a generally tubular member that is received within a cylindrical bore 26 and that defines a piston bore 34 along which a piston assembly 18 reciprocates. Block 12 also defines a generally flat face 30 located at the distal end of each cylindrical bore 26 that is configured to facilitate the coupling of head assembly 16 to block 12. Head assembly 16 is an assembly of components that cooperates with cylinder liners 28, block 12, and piston assemblies 18 to define substantially enclosed combustion chambers 32 and that includes the mechanisms that control the flow of air and other gases into and out of combustion chambers 32 as well as the injection of fuel into the combustion chambers 32. According to one exemplary embodiment, head assembly 16 includes a housing 35 that forms the general structure of head assembly 16 and that is configured to receive and support the other components of head assembly 16. Housing 35 includes a lower face that mates against flat face 30 of block 12 to form a generally sealed interface between head assembly 16 and block 12. When coupled to block 12, housing 35 serves to cover or enclose each of piston bores 34. Housing 35 also includes intake ports 36 that direct air into combustion chambers 32, exhaust ports 40 that direct exhaust out of combustion chambers 32, and bores 44 (normally one per combustion chamber 32) that are each configured to receive a fuel injector 46 that injects fuel into the corresponding combustion chamber 32. Head assembly 16 also includes intake valves 34 that correspond to intake ports 36 and that serve to control the flow of air into each combustion chamber 32, and exhaust valves 42 that correspond to exhaust ports 40 and that serve to control the flow of exhaust out of each combustion chamber 32. According to various alternative and exemplary embodiments, the head assembly may take one of a variety of different configurations, may include other various components such as various mechanisms to control the actuation of intake valves 38 and exhaust valves 42 (e.g., cams, rocker arms, engine brakes, variable valve actuation systems, etc.), the housing of the head assembly may be provide in one or more individual pieces coupled together, or the head assembly may take one of a variety of other embodiments that are known in the art. Piston assemblies 18, described in more detail below, are received within piston bores 34 (with one piston assembly 18 in each piston bore 34) and generally serve to cooperate with cylinder liners 28 and head assembly 16 to form combustion chambers 32. Each piston assembly 18 is configured to reciprocate within the corresponding piston bore 34 as it completes each engine cycle (e.g, a four-stroke engine cycle or a 2-stroke engine cycle) and continues to repeat the engine cycle for as long as engine 10 operates. Connecting rod or piston rod 20 is a rigid rod or member that couples piston assembly 18 to crankshaft 22. Crankshaft 22 is a shaft that rotates around an axis and that provides the rotational power output of engine 10. Crankshaft 22 includes radially offset "crank pins" to which one end of connecting rods 20 are coupled. As each piston assembly 18 reciprocates within piston bore 34 in a linear fashion, the corresponding connecting rod 20 causes crankshaft 22 to rotate.

Referring now to Figures 1 and 2, piston assembly 18 is a rigid device that slides along, or moves within, piston bore 34 of cylinder liner 28 to enable the volume of combustion chamber 32 to change. According to one exemplary embodiment, piston assembly 18 includes a piston 50, a set of piston rings 52, and a pin 54.

According to one exemplary embodiment, piston 50 is an articulated piston assembly and includes a head or crown 56 and a skirt 58.

According to one exemplary embodiment, head 56 is a generally cylindrical member having a top surface 60, a bowl 62, a cylindrical sidewall or skirt 64, a cooling gallery 66, and pin bosses 68. Top surface 60 is a substantially flat, annular region located in a plane that is perpendicular to the axis of cylinder liner 28. Bowl 62 is a recessed region that extends into the plane defined by top surface 60 and is intended to improve combustion efficiency. According to various exemplary and alternative embodiments, bowl 62 may take one of a variety of different shapes and sizes depending on the particular characteristics affecting combustion (e.g., injection pressure, injection spray angle, cylinder diameter, etc.).

Sidewall 64 extends perpendicularly from the outer edge of top surface 60. According to one exemplary embodiment, sidewall 64 includes, starting at top surface 60 and moving away, a first or top land 70, a first or top ring groove 72, a second or upper intermediate land 74, a second or intermediate ring groove 76, a third or lower intermediate land 78, a third or bottom ring groove 80, and a fourth or bottom land 82. Top ring groove 72 is a generally wedge-shaped or keystone-shaped annular recess that extends radially inwardly into sidewall 64 and is configured to receive one of the piston rings in set 52. Intermediate ring groove 76 is a generally rectangular-shaped annular recess that extends radially inwardly into sidewall 64 and is configured to receive another one of the piston rings in set 52. Intermediate ring groove 76 is defined by two substantially parallel sides 84 and 86 that are substantially parallel to top surface 60 and an inner side 88 that extends between sides 84 and 86. Bottom ring groove 80 is a generally rectangular-shaped annular recess that extends radially inwardly into sidewall 64 and is configured to receive yet another one of the piston rings in set 52. According to various alternative and exemplary embodiments, each of ring grooves 72, 76, and 80 may take one of a variety of different configurations suitable for the engine in which they are used. Cooling recess 66 is a cavity or chamber that is configured to receive a cooling fluid, such as engine oil, to facilitate the transfer of heat away from piston assembly 18. According to one exemplary embodiment, cooling recess 66 is defined by an annular recess that extends axially into a bottom surface 90 of head 56. Cooling recess 66 is configured such that it is spaced radially inwardly from sidewall 64 of head 56 and axially below bowl 62.

According to various alternative and exemplary embodiments, the cooling recess may take one of a variety of different shapes and sizes, and may be provided in different radial and axial locations relative to sidewall 64 and bowl 62, depending on the operational characteristics of a particular piston assembly 18 and engine 10.

Pin bosses 68 are projections or structures that facilitate the coupling of head 56 to pin 54 and skirt 58. According to one exemplary embodiment, two pin bosses 68 extend downwardly from bottom surface 90 of head 54 and are spaced apart such that a portion of connecting rod 20 may be received between them. Each pin boss 68 includes a bore that is configured to receive pin 54.

According to one exemplary embodiment, skirt 58 is a generally tubular member that is coupled to head 56 via pin 54. Skirt 58 includes a bore that extends through skirt 58 in a radial direction. When skirt 58 is assembled to head 56, the bore of skirt 58 is aligned with the bores in pin bosses 68 of head 56 and pin 54 is inserted through all the bores. In this way, pin 54 serves to couple skirt 58 and head 56 together.

Set of piston rings 52 refers to the piston rings that are received within ring grooves 72, 76, and 80 of piston 50. According to one exemplary embodiment, set of piston rings 52 includes a first compression ring 92 configured to be placed within top ring groove 72, a second compression ring 94 configured to be placed within intermediate ring groove 76, and an oil ring assembly 96 configured to be placed within bottom ring groove 80. First compression ring 92 and second compression ring 94 generally serve to reduce, minimize, or substantially eliminate the passage of combustion gases and unburned fuel past piston assembly 18 and into the crankcase (a phenomenon known as "blow-by"). In this function, rings 92 and 94 help to maintain the efficiency of engine 10 by preventing or reducing the leakage of gases past piston assembly 18 that could otherwise be used to help drive piston assembly 18, and therefore crankshaft 22. Rings 92 and 94 also serve, at least partially, to reduce, minimize or substantially eliminate the passage of oil (from the crankcase) into the combustion chamber. In this latter function, reducing or eliminating the passage of oil into the combustion chamber helps to keep emissions down and also helps to prevent or reduce the consumption of oil by engine 10. Oil ring assembly 96, which according to one exemplary embodiment may include an "M" shaped ring or rail that is urged radially outwardly by a coiled expansion spring, generally serves to control the layer of oil on piston bore 34. Each of the three rings 92, 94, and 96 generally work together as a system to reduce blow-by, to reduce the consumption of oil, and to transfer heat from piston 50 to cylinder liner 28. According to various exemplary and alternative embodiments, each of the three rings may take one of a variety of different configurations that are suitable for a particular application, such as, for example, one of the many confϊgurations currently known in the art or other configurations that may be designed in the future.

According to one exemplary embodiment, second compression ring 94 includes a radially outwardly facing surface 100, a radially inwardly facing surface 102, an axially upwardly facing surface 104, and an axially downwardly facing surface 106. Surface 100 is configured to engage piston bore 34 to perform a sealing and wiping function. Axially upwardly facing surface 104 and axially downwardly facing surface 106 are generally parallel to one another and are configured to cooperate with side 84 and side 86 of ring groove 76, respectively. Radially inwardly facing surface 102 includes a generally axial region 107 and a tapered region 108. According to various exemplary and alternative embodiments, the second compression ring may take any one of a variety of different cross-sectional shapes depending at least in part on the performance and operational characteristics of first compression ring 92 and oil ring assembly 96.

Pin 54, also known as a wrist pin or gudgeon pin, is a rigid pin that is configured to extend through the bores in skirt 58, the bores in pin bosses 68 of head 56, and a bore in one end of connecting rod 20 to couple skirt 58 and head 56 together and to coupled piston assembly 18 to connecting rod 20. According to various exemplary and alternative embodiments, the pin may take any one of a variety of different configurations, such as one of those configurations currently known in the art.

According to various exemplary embodiments, sidewall 64 of head 56 of piston 50 also includes a passage (apart from any gap that may be formed between side 84 of second ring groove 76 and axially upwardly facing surface 104 of second compression ring 94 during the operation of engine 10) that serves to fluidly couple a cavity 98 formed between first compression ring 92, second compression ring 94, second land 74, and piston bore 34 with a cavity 99 formed between sides 84, 86, and 88 of second ring groove 76 and radially inwardly facing surface 102 of second compression ring 94. This passage allows pressurized gas from cavity 98 to pass into cavity 99, which then acts against radially inwardly facing surface 102 of second compression ring 94 to apply a force that urges second compression ring 94 radially outwardly against piston bore 34. In general, the appropriate size, shape, configuration, and number of passages may depend, at least in part, on the pressures to which piston assembly 18 is exposed, the size of piston assembly 18, the speed ranges of the engine in which piston assembly 18 is used, and other factors.

According to one exemplary embodiment illustrated in Figure 2, the passage is formed by a cylindrical bore 108 that extends diagonally from second land 74 to a portion of side 84 of ring groove 76 that forms a border of cavity 99. In one embodiment, the diameter of bore 108 may be equal to or greater than about 0.5 millimeters. In another embodiment, the diameter of bore 108 may be equal to or greater than about 0.75 millimeters. In another embodiment, the diameter of bore 108 may be equal to or greater than about 1 millimeter. In another embodiment, the diameter of bore 108 may be equal to or greater than about 2.5 millimeters. In a further embodiment, the diameter of bore 108 may be in the range of about 0.5 to 1.0 millimeters, for example about 0.75 millimeters. According to various exemplary and alternative embodiments, piston 50 may include just one bore 108 or it may include multiple bores 108 spaced apart around the circumference of piston 50. According to other embodiments, the bore may be any type of aperture, such as for example a hole, slot, or slit, and may have a shape other than a cylindrical shape. For example, the bore may be tapered with the diameter gradually increasing or decreasing as it extends from second land 74 to ring groove 76, it may have a champagne glass shape, it may be barrel-shaped, or it may one of a variety of other shapes. According to other exemplary and alternative embodiments, the bore may be formed in piston 50 using any suitable manufacturing process, including drilling, milling, boring, laser drilling, or other suitable processes or techniques.

According to still another exemplary embodiment illustrated in Figures 3 and 4, the passage is formed by a notch 110 that extends axially upwardly into second land 74 and radially inwardly into side 84 of second ring groove 76 so as to fluidly couple cavity 99 and cavity 98. According to various exemplary and alternative embodiments, piston 50 may include just one notch 110 or it may include multiple notches 110 spaced apart around the circumference of piston 50. According to other exemplary and alternative embodiments, each notch may take any one of a variety of different configurations. For example, the notch may be a recess, groove, detent, or any one of a variety of other configurations, and may take the shape of a partial cylinder, sphere, rectangle, or other shapes, or it may be V-shaped, or any other suitable shape or configuration. According to other exemplary and alternative embodiments, the notch may be formed in piston 50 using any suitable manufacturing process, including drilling, milling, boring, laser drilling, or other suitable processes or techniques.

The total area of the passages provided within piston 50 will depend on the shape and configuration of each passage and on the total number of passages provided. In one embodiment, the total area may be equal to or greater than about 0.2 mm2. In another embodiment, the total area may be equal to or greater than about 0.44 mm2. In another embodiment, the total area may be equal to or greater than about 0.79 mm2. In another embodiment, the total area may be equal to or greater than about 4.91 mm2' In a further embodiment, the total area may be in the range of about 0.2 mm2 to about 0.79, for example about 0.44 mm2. According to another alternative embodiment illustrated in Figure

5, the passage may be in the form of a notch 112 provided in axially upwardly facing surface 104 of second compression ring 94 rather than in piston 50. According to various exemplary and alternative embodiments, second compression ring 94 may include just one notch 112 or it may include multiple notches 112 spaced apart around the circumference of second compression ring 94. According to various exemplary and alternative embodiments, each notch may take any one of a variety of different configurations. For example, the notch may be a recess, groove, detent, or any one of a variety of other configurations, and may take the shape of a partial cylinder, sphere, rectangle, or other shapes, or it may be V-shaped, or any other suitable shape or configuration. According to other exemplary and alternative embodiments, the notch may be formed in second compression ring 94 using any suitable manufacturing process, including drilling, milling, boring, laser drilling, or other suitable processes or techniques. Although described above in connection with an articulated piston, the passage coupling cavities 98 and 99 may also be provided in a single piece piston or in other piston configurations. Similarly, although described in connection with a diesel engine, the passage coupling cavities 98 and 99 may be used in connection with pistons for gasoline engines or other types of internal combustion engines (e.g., gaseous fuel engines, etc.)

Industrial Applicability

During a typical four-stroke engine cycle, piston assembly 18 will travel through an expansion or power stroke, an exhaust stroke, an intake stroke, and a compression stroke. As piston assembly 18 reciprocates within piston bore 34, each of first compression ring 92, second compression ring 94, and oil ring assembly 96 may move within first ring groove 72, second ring groove 76, and third ring groove 80, respectively. There are at least three factors that affect the movement of second compression ring 94 within second ring groove 76. These three factors include the friction between second compression ring 94 and piston bore 34, the acceleration or deceleration of piston assembly 18, and the different pressures of the gases acting upon second compression ring 94. These three factors work together or against one another, depending on the situation, to move second compression ring 94 within second ring groove 76.

During the compression stroke, piston assembly 18 will start at a bottom dead center position, and move to a top dead center position. The movement of piston assembly 18 from the bottom dead center position to the top dead center position will take place over 180 degrees of rotation of crankshaft 22. Over this 180 degrees, piston assembly 18 will accelerate for roughly about the first 110 degrees, and will then decelerate the remaining 70 degrees until piston assembly 18 reaches to the top dead center position. The precise location where deceleration begins will depend on the length of connecting rod 20 and the stroke of crankshaft 22, and may vary with different engine configurations. However, for many engine configurations, this location will be roughly 70 degrees before the top dead center position. As piston assembly 18 moves from the bottom dead center position to the top dead center position, it will be causing the volume of combustion chamber 32 to decrease. Because intake valves 38 and exhaust valves 42 are generally closed during the compression stroke, the volume reduction caused by piston assembly 18 will cause the pressure within combustion chamber 32 to increase. Some of this pressure is likely to leak past first compression ring 92 and act upon second compression ring 94.

In the absence of the passages (e.g., bore 108, notch 110, or notch 112) in piston assembly 18 or in second compression ring 94, it is believed that, in some circumstances, a situation can arise during the compression stoke where the friction forces and pressure acting on second compression ring 94 are not enough to outweigh the effect of the deceleration of piston assembly 18 so that when piston assembly 18 reaches the top dead center position, second compression ring 94 is in a position where it is urged against side 84 (the upper side) of second ring groove 76. Then during at least the beginning of the power stroke, the acceleration of piston assembly 18 in combination with the friction forces acting on second compression ring 94 are enough to maintain second compression ring 94 urged against side 84 of second ring groove 76. Although, in this situation, the gas pressure generated by combustion that makes its way past first compression ring 92 is urging second compression ring 94 toward side 86 (the lower side) of second ring groove 76, the pressure forces acting on second compression ring 94 are not enough to move it from its position at the top of second ring groove 76. However, because second compression ring 94 is still pressed against side 84 (the upper side) of second ring groove 76, a pressure differential may be created between radially outwardly facing surface 100, which is exposed to the pressure in cavity 98 above second compression ring 94, and radially inwardly facing surface 102 of second compression ring 94, which is exposed to the pressure in cavity 99 (which is generally equal to the pressure in a cavity 114 below second compression ring 94). In some situations, this pressure differential is believed to cause second compression ring 94 to collapse (or move away from piston bore 34). When second compression ring 94 collapses during the power stroke, second compression ring 94 is no longer able to perform the function of scraping oil off of piston bore 34. More oil is therefore allowed to remain on piston bore 34, and as piston assembly 18 continues to move downward, that remaining oil is burned as part of the combustion process. The burning of the oil not only consumes the oil, but it may also have the effect of increasing certain unwanted exhaust gas constituents, making it more difficult to meet the increasingly stringent emissions regulations. It is believed that the addition of the passages (e.g., bore 108, notch 110, or notch 112) to piston assembly 18 and/or second compression ring 94 may help to reduce or eliminate the collapse of second compression ring 94. The passages are believed to accomplish this by reducing the magnitude of, or eliminating, the pressure differential between radially outwardly facing surface 100 and radially inwardly facing surface 102 of second compression ring 94 by fluidly coupling cavity 98, the pressure within which acts upon radially outwardly facing surface 100, and cavity 99, the pressure within which acts upon radially inwardly facing surface 102. When the pressures acting on the opposing faces of second compression ring 94 are equal, or closer to being equal, the collapse of second compression ring 94 is less likely.

It is important to note that the construction and arrangement of the elements of the piston assembly as shown in the exemplary and other alternative embodiments is illustrative only. Although only a few embodiments of the piston assembly have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, orientations, component locations, etc.) without materially departing from the novel teachings and advantages of the subject matter recited. For example, elements shown as integrally formed may be constructed of multiple parts or elements shown as multiple parts may be integrally formed, the operation of the interfaces may be reversed or otherwise varied, and/or the length, width, or other dimensions of the structures and/or members or connectors or other elements of the system may be varied. It should be noted that the elements and/or assemblies of the piston assembly may be constructed from any of a wide variety of materials that provide sufficient strength or durability, and in any of a wide variety of textures and combinations. It should also be noted that the piston assembly may be used in association with any of a wide variety of engines in any of a wide variety of applications. Accordingly, all such modifications are intended to be included within the scope of the present disclosure. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary and other alternative embodiments without departing from the spirit of the present disclosure.