Patent Application
20160040622
The subject matter disclosed herein relates generally to reciprocating engines, and, more particularly to a piston assembly for a reciprocating engine.
A reciprocating engine (e.g., a reciprocating internal combustion engine) combusts fuel with an oxidant (e.g., air) to generate hot combustion gases, which in turn drive a piston (e.g., a reciprocating piston) within a cylinder. In particular, the hot combustion gases expand and exert a pressure against the piston that linearly moves the piston from a top portion to a bottom portion of the cylinder during an expansion stroke. The piston converts the pressure exerted by the combustion gases and the piston's linear motion into a rotating motion (e.g., via a connecting rod and a crankshaft coupled to the piston) that drives one or more loads, e.g., an electrical generator. The construction of the piston and associated structures (e.g., a piston assembly) can significantly impact exhaust emissions (e.g., unburned hydrocarbons) and engine efficiency, as well as lubricant (e.g., oil) consumption. Furthermore, the construction of the piston assembly can significantly affect friction between components of the reciprocating engine and the operating life of the reciprocating engine. Therefore, it would be desirable to improve the construction of the piston assembly.
Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below.
In one embodiment, a power cylinder system for a reciprocating engine includes a piston configured to move within a cylinder. The system includes a top-most groove extending circumferentially about the piston beneath a top land of the piston and configured to support a top ring having an inner circumferential face. A space is defined between a portion of the top-most groove and the inner circumferential face of the top ring while the top ring is positioned within top-most groove, and one or more channels formed in the top land are configured to enable transfer of combustion gases to the space. The system also includes a bottom-most groove extending circumferentially about the piston and spaced apart from the top-most groove along an axial axis of the piston, and the bottom groove is configured to support a bottom ring and is devoid of oil drains.
In one embodiment, a power cylinder system for a reciprocating engine includes a piston configured to move within a cylinder. The system includes a top-most groove and a bottom-most groove each extending circumferentially about the piston, wherein the top-most groove is positioned beneath a top land. The system includes a top ring disposed in the top-most groove, and one or more channels extend into an axially-facing upper surface of the top-most groove or an upper surface of the top ring to direct combustion gases to a space between an inner circumferential face of the top ring and a portion of the top-most groove. Each of the one or more channels has a first radius. The system includes a bottom ring positioned in the bottom-most groove, and a second radius of each of the oil drains extending along the bottom-most groove is less than the first radius of each of the one or more radial channels.
In one embodiment, a power cylinder system for a reciprocating engine includes a cylinder having an inner wall and surrounding a cavity and a piston disposed within the cylinder and configured to move in a reciprocating manner within the cylinder. The system includes a top ring disposed within a top-most groove of the piston, and one or more channels open to the top-most groove are configured to transfer combustion gases through a first total flow area of the one or more radial channels to an inner circumferential face of the top ring to drive the top ring radially outward toward the inner wall of the cylinder as the piston moves within the cylinder. The system includes a bottom ring disposed within a bottom-most groove of the piston, wherein a second total flow area of oil drains open to the bottom groove is less than the first total flow area.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.
Power cylinder systems for reciprocating engines (e.g., reciprocating internal combustion engines) in accordance with the present disclosure may include one or more pistons each configured to move linearly within a cylinder (e.g., a liner) to convert pressure exerted by combustion gases and the piston's linear motion into a rotating motion to power one or more loads. Each piston may have a top annular groove (e.g., a top ring groove, a top-most ring groove, or a compression ring groove) extending circumferentially about the piston, and a top ring (e.g., a top piston ring or a top compression ring) may be disposed within the top groove. The top ring may be generally configured to block fuel and air, or a fuel-air mixture, from escaping from a combustion chamber and/or to facilitate maintenance of suitable pressure to enable expanding hot combustion gases to cause the reciprocating motion of the piston. Each piston may also have a bottom annular groove (e.g., a bottom ring groove, a bottom-most ring groove, or an oil ring groove) extending circumferentially about the piston, and a bottom ring (e.g., a bottom piston ring or an oil ring) may be disposed within the bottom groove. The oil ring may be generally configured to scrape a lubricant (e.g., oil) that lines an inner wall (e.g., inner circumferential wall) of the cylinder. In some embodiments, one or more additional annular grooves (e.g., additional ring grooves or additional compression ring grooves) may extend circumferentially about the piston, and one or more additional rings (e.g., additional rings or additional compression rings) may be disposed within the one or more additional ring grooves. Together, the top ring, the oil ring, and/or the additional ring form a ring pack and may generally control flow of combustion gases and/or lubricant (e.g., oil) within the engine.
During operation of the reciprocating engine, fuel and air combust in a combustion chamber, causing the piston to move within the cylinder. The combustion gases also exert a pressure against an outer circumferential face of the top ring, driving the top ring radially inward away from the inner wall of the cylinder. The disclosed embodiments may include one or more channels (e.g., radial channels) configured to transfer the combustion gases to a space adjacent to an inner circumferential face of the top ring such that the combustion gases exert a radially outward directed force on the inner circumferential face of the top ring. Advantageously, the one or more channels may also facilitate oil control within the cylinder. For example, the one or more channels may enable the top ring to maintain contact with the inner wall of the cylinder, and therefore, may enable the top ring to scrape oil along the inner wall of the cylinder. By way of another example, without the one or more channels, oil may accumulate in the top groove and may block the flow of the combustion gases to the space. In the disclosed embodiments, the one or more channels may enable oil to escape from the top groove (e.g., into the combustion chamber or along the inner wall of the cylinder). Thus, the one or more channels may facilitate flow of oil out of the top groove (e.g., by reducing adhesion of the oil to the top groove) and/or reduce residence time of oil within the top groove, which may generally improve oil consumption and blowby within the engine.
As discussed in more detail below, present embodiments may be devoid of oil drains or may have relatively small oil drains (e.g., a first radius of the one or more channels is greater than a second radius of the oil drains). Such a configuration may enable reliable, durable operation of the one or more channels. In some engines, large oil drains may be disposed on a bottom surface of the bottom groove to drain oil from the ring pack (e.g., oil scraped from the inner wall of the cylinder) into a sump below the piston. However, in certain engines, including engines with a wet sump, such large oil drains may also enable oil to travel from the sump into the ring pack (e.g., a reverse flow of oil), as well as provide a leak path for blowby. The reverse flow of oil from the sump may travel to the top groove, where the oil may impede the transfer of gases through the one or more channels and limit the reliability and effectiveness with which the one or more channels stabilize the top ring. Additionally, in some cases, oil accumulation within the one or more channels may result in carbon deposits that block the transfer of gases through the one or more channels. Accordingly, in accordance with certain embodiments discussed in detail below, a piston assembly devoid of oil drains or with relatively small oil drains may help to block or reduce the reverse flow of oil from the sump into the ring pack, and thus may enable reliable, effective operation of the one or more channels. The piston assembly having the features disclosed herein may effectively and efficiently control oil, while blocking radial ring collapse, as well as reducing blowby of unburned hydrocarbons, oil consumption, emissions, and/or friction between components, which may result in less wear and scuffing, for example.
Turning to the drawings,
The system 8 disclosed herein may be adapted for use in stationary applications (e.g., in industrial power generating engines) or in mobile applications (e.g., in cars or aircraft). The engine 10 may be a two-stroke engine, three-stroke engine, four-stroke engine, five-stroke engine, or six-stroke engine. The engine 10 may also include any number of combustion chambers 12, pistons 20, and associated cylinders (e.g., 1-24). For example, in certain embodiments, the system 8 may include a large-scale industrial reciprocating engine having 4, 6, 8, 10, 16, 24 or more pistons 20 reciprocating in cylinders. In some such cases, the cylinders and/or the pistons 20 may have a diameter of between approximately 13.5-34 centimeters (cm). In some embodiments, the cylinders and/or the pistons 20 may have a diameter of between approximately 10-40 cm, 15-25 cm, or about 15 cm. In certain embodiments, the piston 20 may be a steel piston or an aluminum piston with a Ni-resist ring insert in a top ring groove of the piston 20. The system 8 may generate power ranging from 10 kW to 10 MW. In some embodiments, the engine 10 may operate at less than approximately 1800 revolutions per minute (RPM). In some embodiments, the engine 10 may operate at less than approximately 2000 RPM, 1900 RPM, 1700 RPM, 1600 RPM, 1500 RPM, 1400 RPM, 1300 RPM, 1200 RPM, 1000 RPM, 900 RPM, or 750 RPM. In some embodiments, the engine 10 may operate between approximately 750-2000 RPM, 900-1800 RPM, or 1000-1600 RPM. In some embodiments, the engine 10 may operate at approximately 1800 RPM, 1500 RPM, 1200 RPM, 1000 RPM, or 900 RPM. Exemplary engines 10 may include General Electric Company's Jenbacher Engines (e.g., Jenbacher Type 2, Type 3, Type 4, Type 6 or J920 FleXtra) or Waukesha Engines (e.g., Waukesha VGF, VHP, APG, 275GL), for example.
The top ring 44 is configured to protrude radially outward from the top groove 42 to contact the inner annular wall 28 of the cylinder 26. The top ring 44 generally blocks the fuel 18 and the air 16, or a fuel-air mixture 82, from escaping from the combustion chamber 12 and/or facilitates maintenance of suitable pressure to enable the expanding hot combustion gases to cause the reciprocating motion of the piston 20. Furthermore, the top ring 44 of the present embodiments may be configured to facilitate scraping of oil, which coats the inner annular wall 28 and which controls heat and/or friction within the engine 10, for example.
As shown, the piston 20 includes a bottom annular groove 46 (e.g., bottom ring groove, bottom-most groove, or oil ring groove) extending circumferentially about the piston 20. A bottom ring 48 (e.g., bottom piston ring or oil ring) is disposed within the bottom groove 46. The oil ring 48 may protrude radially outward from the bottom groove 46 to contact the inner wall 28 of the cylinder 26. The oil ring 48 is generally configured to scrape oil that lines the inner wall 28 of the cylinder 26 and to control oil flow within the cylinder 26.
In some embodiments, one or more additional annular grooves 50 (e.g., additional ring grooves or additional compression ring grooves) may extend circumferentially about the piston 20 between from the top groove 42 and the bottom groove 46. In some embodiments, one or more additional rings 52 (e.g., additional rings or additional compression rings) may be disposed within each of the one or more additional ring grooves 50. The additional rings 52 may be configured to block blowby and/or to scrape oil from the inner annular wall 28 of the cylinder 26.
As shown, the piston 20 is attached to a crankshaft 54 via a connecting rod 56 and a pin 58. The crankshaft 54 translates the reciprocating linear motion of the piston 24 into a rotating motion. As the piston 20 moves, the crankshaft 54 rotates to power the load 24 (shown in
A clearance 78 (e.g., a radial clearance defining an annular space) is provided between the inner annular wall 28 of the cylinder 26 and an outer surface 80 (e.g., an annular surface) of the piston 20. As discussed above, it is desirable to maintain contact between the top ring 44 and the inner annular wall 28 of the cylinder 26 to block blowby as well as to enable the top ring 44 to scrape oil from the inner annular wall 28, for example. However, during operation of the engine 10, the combustion gases from the combustion chamber 12 contact an outer face 90 (e.g., a radially outer face or an outer circumferential face) of the top ring 44 and exert a force that drives the top ring 44 radially inward (e.g., along the radial axis 36) away from the inner wall 28 of the cylinder 26. Accordingly, present embodiments include one or more channels (e.g., passageways, troughs, grooves, or the like), such as one or more radial channels 94, configured to transfer combustion gases to a space (shown in
In the illustrated embodiment, the oil ring 48 is disposed in the bottom groove 46. As shown, the piston assembly 25 includes the oil drains 96 formed at discrete locations about the piston 20 (e.g., discrete locations that are spaced apart circumferentially about the piston 20). The oil drains 96 may be circumferentially aligned with the radial channels 94 as shown, or the oil drains 96 and the radial channels 94 may be circumferentially staggered or offset from one another, for example. In the illustrated embodiment, the oil drains 96 have a curved cross-section (e.g., have a curved wall 105) and are formed into or along an axially-facing bottom surface 106 (e.g., an annular surface) of the bottom groove 46. The oil drains 96 may extend radially inward (e.g., in the radial direction 36) from the outer surface 80 (e.g., an outer annular surface) of the piston 20. As shown, the oil drains 96 are open toward the bottom groove 46.
In some embodiments, a total cross-sectional area of all of the oil drains 96 (e.g., sum of the cross-sectional areas of each oil drain 96 in the piston assembly 25) may be less than a total cross-sectional area of all of the radial channels 94 (e.g., sum of the cross-sectional areas of each radial channel 94 in the piston assembly 25). For example, in certain embodiments, a ratio of the total cross-sectional area of all the oil drains 96 to the total cross-sectional area of all the radial channels 94 may be approximately 0.1 to 0.9, 0.2 to 0.8, 0.3 to 0.7, or 0.4 to 0.5. The cross-sectional area of each oil drain 96 is based on an oil drain radius 109, while the cross-sectional area of each radial channel 94 is based on the radial channel radius 100. More particularly, the total cross-sectional area of all of the oil drains 96 is based on the oil drain radius 109 of each oil drain 96 and the total number of oil drains 96, while the total cross-sectional area of all of the radial channels 94 is based on the radial channel radii 100 of each radial channel 94 and the total number of radial channels 94. Such a configuration may limit reverse flow of oil toward the radial channels 94 through the oil drains 96, while also providing a path for oil that is scraped from the inner wall 26 of the cylinder 28 to drain into the sump 59.
Additionally or alternatively, in certain embodiments, the oil drain radius 109 of each of the oil drains 96 may be less than the radial channel radius 100 of each the radial channels 94. In some such cases, the oil drain radius 109 may be less than approximately 2 millimeters (mm) and the radial channel radius 100 may be greater than approximately 2 mm. In some such cases, the oil drain radius 109 may be less than approximately 0.5 mm, 1 mm, 1.5 mm, 2.5 mm, or 3 mm and the radial channel radius 100 may be greater than approximately 0.5 mm, 1 mm, 1.5 mm, 2.5 mm, or 3 mm, for example. In certain embodiments, a ratio of the oil drain radius 109 to the radial channel radius 100 may be approximately 0.1 to 0.9, 0.2 to 0.8, 0.3 to 0.7, or 0.4 to 0.5.
Additionally or alternatively, the oil drains 96 may have a first total flow volume based on a product of the oil drain radius 109, an oil drain length (shown in
By way of another example, additionally or alternatively, in some embodiments, a total number of the oil drains 96 may be less than a total number of the radial channels 94 in the piston assembly 25. In some such cases, the number of oil drain radius 96 may be less than approximately 10, 9, 8, 7, 6, 5, 4, 3, or 2 and the number of radial channels 94 may be greater than approximately 10, 9, 8, 7, 6, 5, 4, 3, or 2. In certain embodiments, a ratio of the number of the oil drains 96 to the number of the radial channels 94 may be approximately 0.1 to 0.9, 0.2 to 0.8, 0.3 to 0.7, or 0.4 to 0.5. As noted above, the relatively small size or dimensions, such as the smaller radius or total cross-sectional area, of the oil drains 96 may enable the radial channels 94 to reliably, effectively stabilize the top ring 44.
As shown in
Although the radial channels 94 and the oil drains 96 are shown as having a curved cross-section, it should be understood that the radial channels 94 or the oil drains 96 may have any suitable cross-section (e.g., rectangular, triangular, curved with varying curvature, or the like) or configuration that facilitates transfer of the combustion gases in the manner disclosed herein. Furthermore, although multiple radial channels 94 and multiple oil drains 96 are illustrated, it should be understood that any suitable number of radial channels 94 and/or oil drains 96 may be provided, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more. Additionally, the radial channels 94 and the oil drains 96 may be distributed in any suitable manner, including with a uniform circumferential spacing about the piston 20.
Although a gap 120 (e.g., a top groove clearance) is provided between a top face 121 (e.g., an axially upper face) of the top ring 44 and the axially-facing surface 102 of the piston 20 to enable some combustion gases to flow within the top groove 42, the first axial distance 102 across the gap 120 is desirably configured to minimize ring lift and flutter. Thus, oil may accumulate in the relatively small gap 120 and block the flow of combustion gases through the top groove 42, and the gap 120 may not enable efficient, reliable transfer of the combustion gases to an inner face 124 (e.g., a radially inner face or an inner circumferential face) of the top ring 44, if the gap 120 is blocked with oil. Accordingly, without the disclosed radial channels 94, oil may accumulate in the top groove 42 and a large pressure differential may exist across the top ring 44 (e.g., between the outer face 90 and the inner face 124), if the gap 120 is blocked with oil. For example, without the disclosed radial channels 94, oil may adhere to the top groove 42 and block the flow of combustion gases, and thus, the pressure adjacent to the outer face 90 may be greater than the pressure adjacent to the inner face 124. In such cases, the top ring 44 may be susceptible to radial ring collapse, which in turn results in increased oil consumption and blowby, for example.
In the present embodiments, the radial channels 94 may be configured to facilitate transfer of the combustion gases to a space 130 (e.g., an annular space) adjacent to the inner face 124 of the top ring 44 and an inner wall 131 (e.g., an inner annular wall) of the top groove 42, which may provide increased stability of the top ring 44. The combustion gases in the space 130 may exert a radially-outward force 134 to balance or to counter the radially-inward force 108, and the pressure across the top ring 44 may be substantially equal or otherwise controlled to block radial ring collapse and to maintain contact between the top ring 44 and the inner annular wall 28 of the cylinder 26, for example.
Additionally, as shown, the outer surface 90 of the top ring 44 is configured to contact the inner annular wall 28 to form the sealing point 107. Such a configuration may advantageously enable the top ring 44 to scrape oil from the inner annular wall 28 of the cylinder 26 during operation of the engine 10. Additionally, oil is generally a viscous and adhesive liquid that may adhere to the piston 20, including the top groove 42, in certain circumstances. The radial channels 94 provide the larger axial distance 103 and overall larger crevice volume between the top ring 44 and the axially-facing surface 102, as well as lower surface area to volume ratio. Such a configuration may reduce adhesion and generally facilitate flow of the oil out of the top groove 42 (e.g., into the combustion chamber 12 or along the inner annular wall 28 of the cylinder 26) and therefore, may improve oil control and reduce oil consumption within the engine 10.
To limit oil in the radial channels 94 and to enable reliable transfer of combustion gases through the radial channels 94, the oil drains 96 may be sufficiently small (or absent, as shown in
The oil ring 48 may have any suitable shape or configuration. As shown, the oil ring 48 is a two-piece oil ring having a body 150 and a spring 152. The body 150 includes an outer face 154 (e.g., a radially outer face or an outer circumferential face). In the illustrated embodiment, the oil ring 48 has a U-shaped cross-section having a first annular protrusion 156 (e.g., a first rail) and a second annular protrusion 158 (e.g., a second rail) that are configured to engage and to scrape oil from the inner annular wall 28 of the cylinder 26. The first annular protrusion 156 has a first width 160 (e.g., a width configured to contact the inner annular wall 28 of the cylinder 26) and the second annular protrusion 158 has a second width 162 (e.g., a width configured to contact the inner annular wall 28 of the cylinder 26). The first width 160 and the second width 162 may be generally the same or may be different from one another. The spring 152 is coupled to the body 150 of the oil ring 48 and is configured to urge the body 150 radially outward toward the inner annular wall 28 of the cylinder 26 and/or axially outward against the bottom groove 46. The oil ring 48 may be configured to contact the axially-facing bottom surface 106 of the bottom groove 46 and/or an axially-facing upper surface 164 of the bottom groove 46.
As shown, the piston assembly 25 includes one additional ring 52. The additional ring 25 is positioned in the additional groove 50 between the top groove 42 and the bottom groove 46. The additional ring 52 may have any suitable shape or configuration. As shown, the additional ring 52 includes a tapered outer face 140 (e.g., a radially outer face or an outer circumferential face). The additional ring 52 may generally be configured to block blowby, to scrape oil from the inner annular wall 28 of the cylinder 26, and/or to control an amount of oil that reaches the top ring 44. Additionally, in some embodiments having three rings, the bottom groove 48 may not support the oil ring 48 (e.g., having the body 150 and the spring 152), but rather, may support a second compression ring similar in form to the top ring 44 or the additional ring 52. As discussed below, in some embodiments in accordance with the present disclosure, the additional ring 52 may not be provided. Rather, the top ring 44 and the oil ring 48 may together provide adequate oil control without the additional ring 52.
In embodiments having only two grooves, the top groove 42 and the bottom groove 46 may be separated by any suitable distance. Additionally, in some embodiments having only two grooves, the bottom groove 48 may not support the oil ring 48 (e.g., having the body 150 and the spring 152), but rather, may support a second compression ring similar in form to the top ring 44 or the additional ring 52 (shown in
As shown, the radial channels 94 formed in the top ring 44 may extend radially inward (e.g., in the radial direction 36) from the outer face 90 to the inner face 124 of the top ring 44. The radial channels 94 may increase the axial distance 102 between the top face 121 of the top ring 44 and the axially-facing surface 102 of the top groove 42 along the radius 100 coincident with the radial channel 94. Thus, the radial channel 94 may facilitate flow of the combustion gases from the cavity 30 to the space 130 adjacent to the inner face 124, as shown by arrow 172. As discussed above, the transfer of gases to the space 130 may control the pressure differential between the annular outer face 90 and the inner face 124 of the top ring 44, and thus enable the top ring 44 to maintain contact with the inner wall 28 of the cylinder 26.
As set forth above, the radial channels 94 may help to equalize pressures or create pressure differential between the inner face 124 of the top ring 44 and the outer face 90 of the top ring 44, thereby helping to bias the top ring 44 radially-outward against the cylinder 26 to block radial ring collapse and/or blowby, for example. Additionally, the top ring 44 and/or the radial channels 94 may be constructed to block radial ring collapse and blowby, while also providing oil control within the engine 10. Such a configuration may benefit from the use of the relatively small oil drains 96 (e.g., smaller than the radial channels 94, as discussed above), or a lack of a oil drains 96, which advantageously reduce reverse flow of oil from the sump 59 toward the radial channels 94 during operation of the engine 10 and/or reduce blowby, for example.
Additionally, in some embodiments having the radial channels 94 positioned in the top ring 44, the additional groove 50 and the additional ring 52 are not provided (as shown in
Additionally, as shown in
Technical effects of the disclosed embodiments include providing systems for controlling the distribution of combustion gases within the engine 10 via channels, such as radial channels 94. For example, the combustion gases may exert pressure against the outer face 90 of the top ring 44 of the piston assembly. Radial channels 94 formed in the top land 40 or in the top ring 44 may transfer the combustion gases to the space 130 adjacent to the inner surface 124 of the top ring 44, thus controlling a pressure gradient between the outer face 90 and the inner face 124 and enabling the top ring 44 to maintain contact with the inner wall 28 of the cylinder 26. The disclosed embodiments also control oil within the engine 10 through use of relatively small oil drains 96 (e.g., smaller than the radial channels 94) or a lack of oil drains 96. Such configurations may advantageously limit oil in proximity to the radial channels 94, thus providing reliable, durable transfer of combustion gases through the radial channels 94. The disclosed embodiments may advantageously reduce oil consumption, emissions, blowby, radial ring collapse, and/or friction within the engine 10, for example.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
