Combination of piston rings and combined structure of piston and piston rings

Abstract

The combination of piston rings assembled to a piston comprises a first compression ring, a second compression ring, a third compression ring, and an oil ring. When the first compression ring has an axial width of h1(1), the second compression ring has an axial width of h1(2), the third compression ring has an axial width of h1(3), and the oil ring has an axial width of h1(4), h1(1)=h1(2) and h1(1)=h1(3) are established. When h1(TOTAL)=h1(1)+h1(2)+h1(3)+h1(4), h1(TOTAL)=3.1 mm is established.

Description

[Technical Field]

[0001] The present invention relates to a combinationof piston rings assembled to a piston in a spark ignitionengine, and a combination structure of a piston andpiston rings.[Background Art]

[0002] In an internal combustion engine mounted in ageneral automobile, a configuration is adopted in which acombination of piston rings including a compression ring(pressure ring) and an oil ring is fitted in a ringgroove formed in a piston. In an axial direction of thepiston, the compression ring is provided on a combustionchamber side, and the oil ring is provided on a crankchamber side. These rings exert abilities by sliding onan inner wall surface of a cylinder. The oil ringfarthest from the combustion chamber has an oil sealfunction of inhibiting outflow of oil (oil rise) to thecombustion chamber side by scraping off, to the crankchamber side, excess engine oil (lubricating oil) adheredon the inner wall surface of the cylinder, and a functionof preventing the compression ring and piston from beingburned in an operation of the internal combustion engineby adjusting an amount of oil to appropriately hold alubricating oil film on the cylinder inner wall surface.The compression ring has a gas seal function ofinhibiting outflow (blow-by) of combustion gas from thecombustion chamber side to the crank chamber side byholding airtightness, and an oil seal function ofsuppressing oil rise by scraping off excess oil thatcannot be completely scraped off with the oil ring. Aspark ignition engine illustrated in a gasoline enginegenerally uses two compression rings and one oil ring.

[0003] In this connection, Patent Document 1 disclosesthat an internal combustion engine, including a pluralityof piston rings assembled to a piston, includes astructure where a width of a closed gap in a piston ringis maximized in the piston ring of a top stage andreduced toward the piston ring of a lower stage.Accordingly, a force with which each ring is pressedagainst a cylinder liner is equal, and an amount of wearon each ring can be about the same.[Citation List][Patent Document]

[0004] [Patent Document 1] Japanese Utility Model Laid-OpenNo. 62-063460[Patent Document 2] Japanese Patent Publication No.5-025024[Summary of the Invention][Problems to be solved by the Invention]

[0005] On the other hand, with increase in output ofan internal combustion engine in recent years, measuresin terms of environmental performance have become anurgent task. In particular, in a spark ignition engineexemplified by a gasoline engine with high cylinder innerpressure, which typically aims to increase output,reduction of blow-by gas is a critical issue, but it isnecessary to reduce a weight of a piston, and hence it isdifficult to consider increasing the number ofcompression rings.On the other hand, in a compression ignition engineillustrated in a diesel engine, particularly in a largediesel engine, although a purpose is different, use ofthree or more compression rings enables maintaining afunction of each compression ring for a long time even ifwear or the like occurs on any ring, and maintenancefrequency is reduced. However, in order to assemble thethree or more compression rings to the piston, it isnecessary to increase an axial length of the piston, sothat the weight of the piston increases. In addition,the increase in the number of the compression rings foruse also increases friction.

[0006] An object of the present invention, which hasbeen made to solve such problems, is to provide atechnology capable of reducing blow-by gas whilesuppressing an increase in friction and an increase inpiston weight in a spark ignition engine illustrated in agasoline engine.[Means for solving the Problems]

[0007] In order to solve the above problems, thepresent invention has a configuration in which threecompression rings and one oil ring are assembled to apiston, but in order to suppress increase in friction andincrease in piston weight in the configuration, afunction of each compression ring, particularly a thirdring (third compression ring) is focused. Specifically,blow-by gas can be reduced while suppressing the increasein friction by reducing a width of each compression ringwhile ensuring strength of the compression ring and byconsidering appropriate combination of axial widths andadditionally appropriate distribution of tension.Furthermore, considering of volume distribution of a landspace in addition to an axial length of each piston landportion enables reducing the blow-by gas whilesuppressing the increase in piston weight.

[0008] More specifically, the present invention is acombination of a plurality of piston rings assembled to apiston fitted in a cylinder in a spark ignition engine,the combination including a first compression ringassembled at a position closest to a combustion chamber,a second compression ring assembled at a position closeto the combustion chamber next to the first compressionring, an oil ring assembled at a position farthest fromthe combustion chamber, and a third compression ringassembled at a position between the second compressionring and the oil ring, wherein when an axial width of thefirst compression ring is h1(1), an axial width of thesecond compression ring is h1(2), an axial width of thethird compression ring is h1(3), and an axial width ofthe oil ring is h1(4), h1(1) ≥ h1(2) and h1(1) ≥ h1(3),and when h1(TOTAL) = h1(1) + h1(2) + h1(3) + h1(4),h1(TOTAL) ≥ 3.1 mm.

[0009] In the present invention, it may be providedthat when a tension of the first compression ring isFt(1), a tension of the second compression ring is Ft(2),a tension of the third compression ring is Ft(3), atension of the oil ring is Ft(4), and a diameter of acylinder bore of the spark ignition engine is d1, Ft(1) >Ft(3), and when Ft(TOTAL) = Ft(1) + Ft(2) + Ft(3) +Ft(4), 0.68 N / mm ≥ Ft(TOTAL) / d1.

[0010] In the present invention, a torsional angle ofthe third compression ring may be 20'± 40' in a statewhere the third compression ring is assembled to thepiston and the piston is fitted in the cylinder.

[0011] In the present invention, the third compressionring may have an outer peripheral surface formed in atapered shape or a tapered undercut shape.

[0012] In the present invention, it may be providedthat 1.8 mm ≥ h1(1), 1.2 mm ≥ h1(2), and 1.0 mm ≥ h1(3).

[0013] In the present invention, the oil ring mayinclude a pair of segments that are formed annularlyalong a circumferential direction of the oil ring andthat are provided independently of each other and side byside in an axial direction of the oil ring, and a spacerexpander that is disposed between the pair of segments.

[0014] Moreover, the present invention may be acombination structure of a piston and piston rings in aspark ignition engine, including the piston and thecombination of the piston rings, the piston having anouter peripheral surface that is formed with a first ringgroove in which the first compression ring is fitted, asecond ring groove in which the second compression ringis fitted, a third ring groove in which the thirdcompression ring is fitted, and an oil ring groove inwhich the oil ring is fitted, wherein when a length froman end face of the first ring groove on a combustionchamber side to an end face of the oil ring groove on acrank chamber side in an axial direction of the piston isLp, Lp ≥ 5. 1 mm, and when a length, in the axialdirection of the piston, of a second land portion that isa region defined by the first ring groove and the secondring groove in the outer peripheral surface of the pistonis Lp2, and a length, in the axial direction of thepiston, of a fourth land that is a region defined by thethird ring groove and the oil ring groove is Lp4, Lp2 >Lp4.

[0015] In the present invention, it may be providedthat when a length, in the axial direction of the piston,of a third land portion that is a region defined by thesecond ring groove and the third ring groove in the outerperipheral surface of the piston is Lp3, Lp3 ≥ Lp4, Lp3 ≥0.5 mm, and Lp4 ≥ 0.5 mm.

[0016] In the present invention, it may be providedthat when a volume of a second space that is a spacesurrounded by the piston, the cylinder, the firstcompression ring and the second compression ring is V2,and a volume of a fourth space that is a space surroundedby the piston, the cylinder, the third compression ringand the oil ring is V4, V2 > V4.

[0017] In the present invention, it may be providedthat when a diameter of a cylinder bore of the sparkignition engine is d1 and Vp = (d1 / 2)2 x π x Lp, V4 / Vp ≥0.00027.[Effects of the invention]

[0018] According to the present invention, in a sparkignition engine, it is possible to reduce blow-by gaswhile suppressing increase in friction and increase inpiston weight.[Brief Description of the Drawings]

[0019] Fig. 1 is a view illustrating a part of a sparkignition engine including a piston structure according toan embodiment.Fig. 2 is a partially cross-sectional view of thespark engine including the piston structure according tothe embodiment.Fig. 3 is a partially cross-sectional view of apiston according to the embodiment.Fig. 4 is a partially cross-sectional view of aspark ignition engine including a piston structureaccording to a comparative example.Fig. 5 is a graph comparing a blow-by amount of apiston structure according to an example with that of thepiston structure according to the comparative example.Fig. 6 is a graph illustrating a relation between atorsional angle of a third ring and the blow-by amount.Fig. 7 is a graph illustrating a relation betweenthe torsional angle of the third ring and oilconsumption.[Mode for Carrying Out the Invention]

[0020] Hereinafter, a preferable embodiment of thepresent invention will be described with reference to thedrawings. The embodiment described below is anapplication of a combination structure of a piston andpiston rings according to the present invention to aspark ignition engine illustrated in a gasoline engine.The configuration described in the following embodimentis not intended to limit the technical scope of theinvention to the embodiment unless otherwise described.

[0021] [Overall Configuration]Fig. 1 is a view illustrating a part of a sparkignition engine 100 including a combination structure(hereinafter, the piston structure) 110 of a piston andpiston rings according to an embodiment. Fig. 2 is apartially cross-sectional view of the spark ignitionengine 100 including the piston structure 110 accordingto the embodiment. Fig. 3 is a partially cross-sectionalview of the piston 20 according to the embodiment. Figs.2 and 3 illustrate a cross section along a central axisof the piston. As illustrated in Fig. 1, the sparkignition engine 100 according to the embodiment includesa cylinder 10, the piston 20 fitted in the cylinder 10,and a piston ring combination 120 including a pluralityof piston rings assembled to the piston 20. Asillustrated in Fig. 2, in the spark ignition engine 100,a predetermined separation distance D1 is acquiredbetween an outer peripheral surface 20a of the piston 20and an inner wall surface 10a of the cylinder 10, so thata piston gap PC1 is formed. In the spark ignition engine100, a combustion chamber side denoted with referencenumeral 30 is an upper side, and a crank chamber sidedenoted with reference numeral 40 is a lower side. Inthe spark ignition engine 100, a configuration includingthe piston 20 and the piston ring combination 120 is thepiston structure 110. Hereinafter, the piston structure110 will be described.

[0022] [Piston]As illustrated in Fig. 3, in the outer peripheralsurface 20a of the piston 20, a first ring groove 201, asecond ring groove 202, a third ring groove 203 and afourth ring groove 204 are formed at predeterminedintervals in an axial direction of the piston 20 in orderfrom the upper side (combustion chamber 30 side).Hereinafter, when the first ring groove 201, the secondring groove 202, the third ring groove 203 and the fourthring groove 204 are described without beingdistinguished, each of the grooves will be referred tosimply as "the ring groove".

[0023] The ring groove is formed on an entirecircumference of the outer peripheral surface 20a as thegroove that extends annularly around an axis of thepiston 20. As illustrated in Fig. 2, each ring groove isformed including a pair of groove walls (inner walls)arranged facing each other in an up-down direction. Inthe pair of groove walls, an upper groove wall isreferred to as an upper wall W1, and a lower groove wallis referred to as a lower wall W2. Also, a groove wallconnecting an inner peripheral edge of the upper wall W1and an inner peripheral edge of the lower wall W2 in eachring groove is referred to as a bottom wall W3.Additionally, in the bottom wall W3 of the fourth ringgroove 204, a drain hole H1 is formed for discharging, tothe crank chamber 40, oil flowing into the fourth ringgroove 204. However, the drain hole H1 does not have tobe formed in the fourth ring groove 204.

[0024] As illustrated in Fig. 3, the ring grooves areformed in the piston 20, and accordingly in the piston20, a first land portion L1, a second land portion L2, athird land portion L3, a fourth land portion L4 and askirt portion PS1 are defined in order from thecombustion chamber side. The first land portion L1 is aportion closer to the combustion chamber 30 side than thefirst ring groove 201. The second land portion L2 is aportion between the first ring groove 201 and the secondring groove 202. The third land portion L3 is a portionbetween the second ring groove 202 and the third ringgroove 203. The fourth land portion L4 is a portionbetween the third ring groove 203 and the fourth ringgroove 204. The skirt portion PS1 is a portion closer tothe crank chamber 40 side than the fourth ring groove204. Hereinafter, a length (axial length) of the secondland portion L2 in the axial direction of the piston 20is Lp2, an axial length of the third land portion L3 isLp3, and an axial length of the fourth land portion L4 isLp4. Further, a diameter of the piston 20 in the secondland portion L2 is φ2, a diameter of the piston 20 in thethird land portion L3 is φ3, and a diameter of the piston20 in the fourth land portion L4 is φ4. Then, asillustrated in Fig. 3, in the piston 20, a region fromthe upper wall W1 that is an end face of the first ringgroove 201 on the combustion chamber 30 side to the lowerwall W2 that is an end face of the fourth ring groove 204on the crank chamber 40 side is a ring fitting region20b. Also, an axial length of the ring fitting region20b is Lp.

[0025] [Piston Ring]As illustrated in Fig. 2, in the piston structure110 according to the embodiment, the combination 120 offour piston rings in total including three compressionrings (pressure rings) of a top ring 1, a second ring 2and a third ring 3 and one oil ring 4 is assembled to thepiston 20. In the present description, when the top ring1, the second ring 2, the third ring 3 and the oil ring 4are described without being distinguished, each of therings will be referred to simply as "the piston ring".The piston ring is a sliding member that is assembled tothe piston fitted in a cylinder in an internal combustionengine and that slides on an inner wall surface of thecylinder with reciprocating motion of the piston. In theembodiment, the top ring 1 is fitted in the first ringgroove 201, the second ring 2 is fitted in the secondring groove 202, the third ring 3 is fitted in the thirdring groove 203, and the oil ring 4 is fitted in thefourth ring groove 204. Hereinafter, as illustrated inFig. 2, a state where each piston ring is assembled tothe piston 20 and the piston 20 is fitted in the cylinder10 will be referred to as "a use state". Further, asillustrated in Fig. 2, a direction along a central axisof the piston ring (axial direction) is defined as "theup-down direction" of the piston ring. Further, in theaxial direction of the piston ring, the combustionchamber 30 side (upper side in Fig. 2) in the sparkignition engine 100 is defined as "the upper side", andthe opposite side, that is, the crank chamber side (lowerside in Fig. 2) is defined as "the lower side". Also, inthe present description, "a barrel shape" refers to ashape of an outer peripheral surface curved so as to beradially outward convex including a top having a maximumdiameter in the piston ring, and includes a symmetricbarrel shape with a top located in a center in the updowndirection, and an eccentric barrel shape with a topoffset either upward or downward from a center in the updowndirection.

[0026] The top ring 1 is a compression ring assembledat a position closest to the combustion chamber 30 amongthe plurality of piston rings constituting the pistonring combination 120. The top ring 1 corresponds to anexample of "a first compression ring" according to thepresent invention.

[0027] In the present example, a cross-sectional shapeof the top ring 1 is a rectangular shape. The top ring 1includes an outer peripheral surface 11, an innerperipheral surface 12, an upper surface 13 and a lowersurface 14. The upper surface 13 and the lower surface14 define a width of the top ring 1 in the axialdirection. The outer peripheral surface 11 is formed inthe barrel shape. The top ring 1 is assembled to thepiston 20 in the spark ignition engine 100 so that whenthe upper surface 13, which is one of opposite end facesin the axial direction, faces the upper side and theother lower surface 14 faces the lower side, the outerperipheral surface 11 slides in contact with the innerwall surface 10a of the cylinder 10. Note that the shapeof the first compression ring according to the presentinvention is not limited to the above. As the firstcompression ring, compression rings having various shapescan be adopted. For example, the first compression ringmay have a straight shape or a tapered shape on an outerperipheral surface. The first compression ring may havea bevel shape, a keystone shape, or a half keystone shapeas a cross-sectional shape.

[0028] The second ring 2 is a compression ringassembled at a position close to the combustion chamber30 next to the top ring 1 among the plurality of pistonrings constituting the piston ring combination 120. Thesecond ring 2 corresponds to an example of "a secondcompression ring" according to the present invention.

[0029] In the present example, a cross-sectional shapeof the second ring 2 is a rectangular shape similar tothe top ring 1. The second ring 2 includes an outerperipheral surface 21, an inner peripheral surface 22, anupper surface 23 and a lower surface 24. The uppersurface 23 and the lower surface 24 define a width of thesecond ring 2 in the axial direction. The outerperipheral surface 21 is formed in a tapered shape thatis inclined so as to widen as being toward the lowerside.

[0030] The third ring 3 is a compression ringassembled at a position between the second ring 2 and theoil ring 4. The third ring 3 corresponds to an exampleof "a third compression ring" according to the presentinvention. The third ring 3 includes an outer peripheralsurface 31, an inner peripheral surface 32, an uppersurface 33 and a lower surface 34. The upper surface 33and the lower surface 34 define a width of the third ring3 in the axial direction. The third ring 3 for use inthis example has the same shape as in the second ring 2.That is, the third ring 3 in this example has arectangular shape as a cross-sectional shape, and theouter peripheral surface 31 has a tapered shape.

[0031] Note that the shapes of the second compressionring and the third compression ring according to thepresent invention are not limited to the above. As thesecond compression ring and the third compression ring,compression rings having various shapes can be adopted.For example, the outer peripheral surface may have abarrel shape or a tapered shape. The cross-sectionalshape may be a bevel shape, a keystone shape, a halfkeystone shape, or a scraper (step) shape. Further, theshapes of the respective compression rings may bedifferent.

[0032] The top ring 1, second ring 2 and third ring 3have self-tension to press the inner wall surface 10a ofthe cylinder 10 in the use state with each outerperipheral surface. Thereby, a gas seal function and anoil seal function can be obtained.

[0033] In Fig. 1, reference symbol G1 indicates aclosed gap formed in the top ring 1, reference symbol G2indicates a closed gap formed in the second ring 2, andreference symbol G3 indicates a closed gap formed in thethird ring 3. A size of the closed gap G1 of the topring 1 is C1, a size of the closed gap G2 of the secondring 2 is C2, and a size of the closed gap G1 of thethird ring 3 is C3.

[0034] The oil ring 4 is a piston ring assembled at aposition farthest from the combustion chamber 30 amongthe plurality of piston rings constituting the pistonring combination 120. The oil ring 4 in this example isa so-called three-piece combined oil ring, and includes apair of segments 5, 5 and a spacer expander 6, asillustrated in Fig. 2.

[0035] The pair of segments 5, 5 are formed annularlyalong a circumferential direction of the oil ring 4 andare provided independently of each other and side by sidein the axial direction. The oil ring 4 according to thisexample includes the pair of segments 5, 5 with the sameshape. Hereinafter, when the pair of segments 5, 5 arereferred to distinctively, the segment 5 provided on theupper side (combustion chamber 30 side) is referred to asan upper segment 5U, and the segment 5 provided on thelower side (crank chamber 40 side) is referred to as alower segment 5L. These segments, which are notdistinguished, are referred to simply as the segments 5.As illustrated in Fig. 2, each segment 5 includes anouter peripheral surface 51, an inner peripheral surface52, an upper surface 53 and a lower surface 54. Theupper surface 53 and the lower surface 54 define a widthof the segment 5 in the axial direction. The outerperipheral surface 51 of the segment 5 is formed in abarrel shape (symmetric barrel). The segment 5 isassembled to the piston 20 in the spark ignition engine100 so that when the upper surface 53, which is one ofopposite end faces in the axial direction, faces theupper side and the other lower surface 54 faces the lowerside, the outer peripheral surface 51 slides in contactwith the inner wall surface 10a of the cylinder 10. Thespacer expander 6 is provided between the pair ofsegments 5, 5 and has self-tension to expand the diameterin the use state. Thereby, the upper segment 5U and thelower segment 5L are biased radially outward by thespacer expander 6, and the outer peripheral surface 51 ispressed against the inner wall surface 10a of thecylinder 10. This can obtain an oil seal function.

[0036] Note that the shape of the oil ring accordingto the present invention is not limited to the above. Inthe embodiment, the pair of segments 5, 5 have the sameshape, but in the present invention, the pair of segmentsmay have different shapes. Further, a shape of an outerperipheral end face of the segment is not limited to asymmetric barrel shape, and may be an eccentric barrelshape, a tapered shape, or the like. Further, the pairof segments may have different outer peripheral shapes.The oil ring may be, for example, a so-called two-piecetype oil ring. The oil ring may be in a form that doesnot include the spacer expander or a coil expander andfunctions as a single member.

[0037] Here, as illustrated in Fig. 2, an axial widthof the top ring 1 is h1(1), an axial width of the secondring 2 is h1(2), an axial width of the third ring 3 ish1(3), and an axial width of the oil ring 4 is h1(4). Atotal of the axial widths of the compression rings ish1(COMP), and a total of the axial widths of the pistonrings is h1(TOTAL). That is, h1(COMP) = h1(1) + h1(2) +h1(3), and h1(TOTAL) = h1(1) + h1(2) + h1(3) + h1(4).

[0038] [Land Space]As illustrated in Fig. 2, a space between the innerwall surface 10a of the cylinder 10 and the outerperipheral surface 20a of the piston 20 is partitioned bythe piston ring, so that a second land space 50a, a thirdland space 50b and a fourth land space 50c are formed.

[0039] The second land space 50a is a space surroundedby the cylinder 10, the piston 20, the top ring 1 and thesecond ring 2. In more detail, the second land space 50ais defined by the inner wall surface 10a of the cylinder10, the outer peripheral surface 20a of the piston 20 inthe second land portion L2, the lower surface 14 of thetop ring 1 and the upper surface 23 of the second ring 2.

[0040] The third land space 50b is a space surroundedby the cylinder 10, the piston 20, the second ring 2 andthe third ring 3. In more detail, the third land space50b is defined by the inner wall surface 10a of thecylinder 10, the outer peripheral surface 20a of thepiston 20 in the third land portion L3, the lower surface24 of the second ring 2 and the upper surface 33 of thethird ring 3.

[0041] The fourth land space 50c is a space surroundedby the cylinder 10, the piston 20, the third ring 3 andthe oil ring 4. In more detail, the fourth land space50c is defined by the inner wall surface 10a of thecylinder 10, the outer peripheral surface 20a of thepiston 20 in the fourth land portion L4, the lowersurface 34 of the third ring 3 and the upper surface 53of the upper segment 5U of the oil ring 4.

[0042] Here, a volume of the second land space 50a isV2, a volume of the third land space 50b is V3, and avolume of the fourth land space 50c is V4. The volume ofeach land space is derived based on an inner diameter ofthe cylinder 10, an outer diameter of the piston 20, adistance between adjacent ring grooves (axial length ofeach land portion), and the like. Specifically, when adiameter of a cylinder bore (inner diameter of thecylinder 10) is d1 as illustrated in Fig. 1, V2 =((d1 / 2)2 - (φ2 / 2)2) x π x Lp2, V3 = ((d1 / 2)2 - (φ3 / 2)2) xπ x Lp3 and V4 = ((d1 / 2)2 - (φ4 / 2)2) x π x Lp 4 can berepresented. In addition, a cutout portion such as arecess or a chamfered portion may be formed in each landportion. Thereby, the volume of each land space can beadjusted. The volume of each land space also includes avolume of the cutout portion such as a recess or achamfered portion.

[0043] Fig. 4 is a partially cross-sectional view of aspark ignition engine 200 including a piston structure210 according to a comparative example. The pistonstructure 210 according to the comparative example isdifferent from the piston structure 110 in that a thirdring groove 203 is not formed in a piston 20, and apiston ring combination 220 assembled to the piston 20does not include a third ring 3. Therefore, a third landspace 50b of the piston structure 210 is defined by asecond ring 2 and an oil ring 4. In the piston structure210, a fourth land space 50c is not formed.

[0044] Here, in the spark ignition engine, a pressurein a second land space close to a combustion chambertends to be higher than a pressure in the third landspace, but if a pressure difference is large, gas flowinginto the second land space passes through a closed gap ofthe second ring and easily flows out to the third landspace, which is a factor of increase in blow-by gas. Inparticular, in the spark ignition engine having a highcylinder inner pressure to achieve high output, thetendency is remarkable.

[0045] On the other hand, in the piston structure 110according to the embodiment, the third ring 3 isassembled between the second ring 2 and the oil ring 4,so that the second ring 2 and the third ring 3 define thethird land space 50b. Therefore, in the embodiment,compared to the piston structure 210 according to thecomparative example in which the compression ring is notassembled between the second ring and the oil ring, thethird ring 3 seals the gas, and a pressure in the thirdland space 50b can be accordingly increased. Thereby,compared to the piston structure 210 according to thecomparative example, a pressure difference between thesecond land space 50a and the third land space 50b can bereduced, and outflow of gas from the second land space50a to the third land space 50b can be suppressed. As aresult, the blow-by gas can be reduced. That is, thepiston structure 110 can enhance so-called labyrintheffect, and improve a gas seal performance by increasingthe number of compression rings to be assembled betweenthe top ring 1 and the oil ring 4 from one to two andaccordingly reducing the pressure difference between thesecond land space 50a and the third land space 50b.

[0046] Furthermore, in the piston structure 110according to the embodiment, a thin compression ring isused, and three compression rings can be accordinglyassembled without making the axial length Lp of the ringfitting region 20b longer than that of the comparativeexample. That is, it is not necessary to increase theaxial length of the piston 20. This can reduce theblow-by gas while suppressing increase in weight of thepiston 20.

[0047] [Example]Table 1 illustrates axial widths of respectivepiston rings and axial lengths of respective landportions in combination structures of pistons and thepiston rings according to Examples 1 to 4 of the presentinvention and Comparative Examples 1 to 4.[Table 1]Examples 1 to 4 are configured in the same manner asin the piston structure 110 illustrated in Figs. 1 to 3.Comparative Examples 1 to 4 are configured in the samemanner as in the piston structure 210 illustrated in Fig.4. As illustrated in Table 1, the axial length Lp of thering fitting region 20b is approximated by a total valueof the axial width h1(1) of the top ring 1, the axiallength Lp2 of the second land portion L2, the axial widthh1(2) of the second ring 2, the axial length Lp3 of thethird land portion L3, the axial width h1(3) of the thirdring 3, the axial length Lp4 of the fourth land portionL4 and the axial width h1(4) of the oil ring 4.

[0048] As illustrated in Table 1, in the pistonstructures of Examples 1 to 4, h1(1) ≥ h1(2), h1(1) ≥h1(3), and h1(TOTAL) ≥ 3.1 mm. The top ring 1 isassembled at the position closest to the combustionchamber among the three compression rings, and istherefore the compression ring with the largest load dueto the pressure of combustion gas. In Examples 1 to 4,h1(TOTAL), which is a total of the axial widths of therespective piston rings, is set to 3.1 mm or more, whilethe axial width h1(1) of the top ring 1 is maximizedamong the three compression rings, so that strength ofthe top ring 1 can be acquired. Accordingly, thesewidths can be reduced while acquiring the strength ofeach piston ring. Further, by making the second ring 2and the third ring 3 thinner than the top ring 1, thesecond ring 2 and the third ring 3 can be reduced inweight. In the present invention, h1(1) > h1(2) may beprovided as in Examples 1 to 4. Furthermore, in thepiston structures of Examples 2 to 4, h1(TOTAL) ≥ 4.7 mm,to further increase the strength of each piston ring.However, the present invention is not limited thereto.

[0049] In the piston structures of Examples 1 to 4, 10mm ≥ h1(TOTAL). By making each piston ring thinner sothat h1(TOTAL) is 10 mm or less, it is possible toinhibit the axial length Lp of the ring fitting region20b from being increased. As a result, while suppressingthe increase in weight of the piston 20, it is possibleto reduce blow-by gas by three compression rings.Furthermore, in the piston structures of Examples 1 to 4,8 mm ≥ h1(TOTAL), which further suppresses the increasein weight of the piston 20. Also, in the pistonstructures of Examples 1 to 4, by making each compressionring thinner, 4 mm ≥ h1(COMP), which acquires thesuppression of the increase in weight of the piston 20.In particular, in the piston structures of Examples 1 to3, 3.7 mm ≥ h1(COMP), which further suppresses theincrease in weight of the piston 20. However, thepresent invention is not limited thereto.

[0050] Further, in the piston structures of Examples 1to 4, 1.8 mm ≥ h1(1), 1.2 mm ≥ h1(2), and 1.0 mm ≥ h1(3).By making each compression ring thin in this manner, theaxial length Lp of the ring fitting region 20b can befurther reduced. Further, the third ring 3 is reduced inthickness and weight, which can reduce an inertial forceof the third ring 3. This can reduce lifting of thethird ring 3, and enhance the seal performance of thelower surface 34 of the third ring 3. However, thepresent invention is not limited thereto.

[0051] Additionally, in the piston structures ofExamples 1 to 4, Lp ≥ 5.1 mm and Lp2 > Lp4. The axiallength Lp of the whole ring fitting region 20b is set to5.1 mm or more, and the axial length Lp2 of the secondland portion L2 that is closer to the combustion chamberand requires higher strength is set to be longer than theaxial length Lp4 of the fourth land portion L4, which canacquire the strength of each land portion. Further, bysetting Lp2 > Lp4, V2 > V4 can be set as described later.As in Examples 2 to 4, it may be provided that Lp ≥ 9.9mm. This can increase the strength of each land portion.However, the present invention is not limited thereto.

[0052] Also, in the piston structures of Examples 1 to4, 23.5 mm ≥ Lp. The widths of each compression ring andeach land portion are decreased and Lp is set to 23.5 mmor less, which can suppress the increase in weight of thepiston 20. However, the present invention is not limitedthereto.

[0053] Further, in the piston structures of Examples 1to 4, Lp3 ≥ Lp4, Lp3 ≥ 0.5 mm, and Lp4 ≥ 0.5 mm. Theaxial length Lp3 of the third land portion L3 that iscloser to the combustion chamber than the fourth landportion L4 is set to be larger than the axial length Lp4of the fourth land portion L4, and Lp3 and Lp4 are set to0.5 mm or more, which can suitably acquire strengths ofthe third land portion L3 and the fourth land portion L4.However, the present invention is not limited thereto.

[0054] Additionally, as described above, in the pistonstructures of Examples 1 to 4, Lp2 > Lp4 is set, therebyresulting in V2 > V4. Accordingly, the volume V4 of thefourth land space 50c far from the combustion chamber isset to be smaller than the volume V2 of the second landspace 50a, so that a pressure difference between adjacentland spaces can be reduced compared to a case where V4 islarger than V2. Accordingly, gas flowing from the secondland space 50a to the third land space 50b and gasflowing from the third land space 50b to the fourth landspace 50c can be reduced. However, the present inventionis not limited thereto.

[0055] Fig. 5 is a graph comparing blow-by amounts ofthe piston structures according to Examples 1 to 4 withthose of the piston structures according to ComparativeExamples 1 to 4. In Fig. 5, a horizontal axis indicatesthe axial length Lp of the ring fitting region, and avertical axis indicates a ratio of the blow-by amounts.As illustrated in Fig. 5, the amount of the blow-by gasin the example is smaller than that in the comparativeexample.

[0056] Additionally, in the embodiment, when Vp =(d1 / 2)2 x π x Lp, it is preferable to set V4 / Vp ≥0.00027. However, the present invention is not limitedthereto. Vp indicates a volume of a cylinder bore in thering fitting region 20b. It is set that V4 / Vp ≥ 0.00027,so that a pressure in the fourth land space 50c can beinhibited from rising excessively. This can reduce thelifting of the third ring 3, and enhance the sealperformance of the lower surface 34 of the third ring 3.

[0057] Here, a tension of the top ring 1 is Ft(1), atension of the second ring is Ft(2), a tension of thethird ring is Ft(3), a tension of the oil ring 4 isFt(4), and it is set that Ft(TOTAL) = Ft(1) + Ft(2) +Ft(3) + Ft(4). That is, Ft(TOTAL) is the total of thetensions of the respective piston rings. At this time,from the viewpoint of friction reduction, it ispreferable that Ft(1) > Ft(3) and that 0.68 N / mm ≥Ft(TOTAL) / d1. However, the present invention is notlimited thereto. Accordingly, the tension Ft(1) of thetop ring 1 closest to the combustion chamber 30 is set tobe larger than the tension Ft(3) of the third ring 3, sothat a blow-by gas reduction performance can be suitablykept. Also, with 0.68 N / mm ≥ Ft(TOTAL) / d1, although fourpiston rings are used, the same degree of friction as inthe conventional piston structure where three pistonrings are used can be made. That is, increase infriction can be suppressed. From the viewpoint of thefriction reduction, it is more preferable that 0.57 N / mm≥ Ft(TOTAL) / d1, and it is further preferable that 0.54N / mm ≥ Ft(TOTAL) / d1.

[0058] Further, from the viewpoint of the sealperformance of the third ring 3, it is preferable that atorsional angle of the third ring 3 is 20'± 40' in a usestate where the third ring 3 is assembled to the piston20 and the piston 20 is fitted in the cylinder 10. Thetorsional angle is defined as an inclination angle of anaxial end face (upper / lower surface) of the third ring 3relative to a horizontal plane (plane orthogonal to theaxis of the piston). The angle indicates a positivevalue in a case where the axial end face inclines upward(combustion chamber side) toward outside in a radialdirection, and the angle indicates a negative value in acase where the axial end face inclines downward (crankchamber side) toward outside in the radial direction.Further, for measurement of the torsional angle, forexample, a method may be used including measuring theaxial end face by use of a surface roughness meter or thelike in a state where the third ring 3 with a closedfitting opening is inserted into a measuring ring gaugehaving a diameter equal to the diameter d1 of thecylinder bore, and calculating the torsional angle. Inthe embodiment, the third ring 3 has torsion in the aboverange, so that when the third ring 3 is subjected topressure in the third land space 50b, the sealperformance of the lower surface 34 of the third ring 3can be enhanced. Fig. 6 is a graph illustrating arelation between the torsional angle (torsion amount) ofthe third ring 3 and the blow-by amount. In Fig. 6, ahorizontal axis indicates the torsional angle of thethird ring 3, and a vertical axis indicates a ratio ofthe blow-by amount. As illustrated in FIG. 6, in a rangeof the torsional angle of -20' to 80', the gas sealperformance of the third ring is fully exerted, and theblow-by gas is noticeably reduced. Fig. 7 is a graphillustrating a relation between the torsional angle(torsion amount) of the third ring 3 and oil consumption.In Fig. 7, a horizontal axis indicates the torsionalangle of the third ring 3, and a vertical axis indicatesa ratio of oil consumption. Also, as illustrated in Fig.7, if the torsional angle has a larger minus value than-20' or a larger plus value than 60', the oil sealperformance of the third ring 3 is not fully exerted, andthere is a tendency for oil consumption to worsen. Fromthe above, by setting the torsional angle of the thirdring 3 to 20'± 40', the seal performance of the thirdring 3 can be suitably enhanced, and the blow-by gas andoil consumption can be reduced. Note that the torsionalangle of the third compression ring according to thepresent invention is not limited to 20'± 40'.

[0059] Here, since the third ring 3 is provided, theblow-by gas is reduced by the gas seal performance of thethird ring 3. When the oil rises on the combustionchamber side above the third ring 3, it becomes difficultto blow down oil with an effect of the blow-by gasblowing down the oil, and the oil consumption mightincrease. On the other hand, in the piston structure 110according to the embodiment, a shape of the outerperipheral surface 31 of the third ring 3 is a taperedshape with a high oil scraping performance. Accordingly,since the oil is less likely to rise on the combustionchamber side above the third ring 3, the oil consumptioncan be suppressed. From the viewpoint of improving theoil scraping performance, the outer peripheral surface 31of the third ring 3 may have a tapered undercut shapethat is inclined so as to widen as being toward the lowerside and has a lower portion cut out. Furthermore, ashape of the outer peripheral surface 51 of at least oneof the pair of segments 5, 5 of the oil ring 4 may be aneccentric barrel shape with the high oil scrapingperformance. Thereby, oil is further inhibited fromrising on the combustion chamber side above the thirdring 3, and the oil consumption can be more suitablysuppressed. However, the present invention is notlimited thereto.

[0060] Also, in the piston structure 110 according tothe embodiment, the size C1 of the closed gap G1 of thetop ring 1 and the size C3 of the closed gap G3 of thethird ring 3 may have a relation of C1 ≥ C3. This canenhance the gas seal performance of the third ring 3.

[0061] Further, from the viewpoint of the sealperformance of the lower surface 34 of the third ring 3,a surface roughness Rz of the lower surface 34 may be 8μm or less. Thus, reducing of the roughness of the lowersurface 34 of the third ring 3 can enhance the sealperformance of the lower surface 34. Additionally, Rz isa maximum height prescribed in JIS B 0601. Further, thethird ring 3 may be formed of a resin material. Thethird ring 3 is made of a resin and reduced in weight,which can reduce the inertial force of the third ring 3.This can reduce the lifting of the third ring 3, and canenhance the seal performance of the lower surface 34.

[0062] In the above-described embodiment, the piston20 is integrally molded by casting or forging, but thepiston 20 may be configured so that a separate landportion is assembled. For example, a ring-shaped membermay be assembled to the piston, as the land portion. Themember of the land portion is formed of a high-strengthmaterial, which can decrease an axial length of the landportion.

[0063] The suitable embodiment of the presentinvention has been described above, and various formsdescribed above can be combined as much as possible.[Reference Signs List]

[0064] 100, 200: spark ignition engine110, 210: combination structure of piston and piston rings120, 220: combination of piston rings10: cylinder20: piston30: combustion chamber40: crank chamber1: top ring (example of first compression ring)2: second ring (example of second compression ring)3: third ring (example of third compression ring)4: oil ring.

Claims

1. A combination structure of a piston and piston rings in a spark ignition engine, comprising the piston fitted in a cylinder in the spark ignition engine and a combination of a plurality of piston rings assembled to the piston, the combination of the plurality of piston rings including a first compression ring assembled at a position closest to a combustion chamber, a second compression ring assembled at a position close to the combustion chamber next to the first compression ring, an oil ring assembled at a position farthest from the combustion chamber, and a third compression ring assembled at a position between the second compression ring and the oil ring, wherein when an axial width of the first compression ring is h1(1), an axial width of the second compression ring is h1(2), an axial width of the third compression ring is h1(3), and an axial width of the oil ring is h1(4), h1(1) ≥ h1(2) and h1(1) ≥ h1(3), when h1(TOTAL) = h1(1) + h1(2) + h1(3) + h1(4), 10 mm ≥ h1(TOTAL) ≥ 3.1 mm, the piston has an outer peripheral surface that is formed with a first ring groove in which the first compression ring is fitted, a second ring groove in which the second compression ring is fitted, a third ring groove in which the third compression ring is fitted, and an oil ring groove in which the oil ring is fitted, when a length from an end face of the first ring groove on a combustion chamber side to an end face of the oil ring groove on a crank chamber side in an axial direction of the piston is Lp, Lp ≥ 5. 1 mm, and when a length, in the axial direction of the piston, of a second land portion that is a region defined by the first ring groove and the second ring groove in the outer peripheral surface of the piston is Lp2, and a length, in the axial direction of the piston, of a fourth land portion that is a region defined by the third ring groove and the oil ring groove is Lp4, Lp2 > Lp4.

2. The combination structure of the piston and the piston rings according to claim 1, wherein when a tension of the first compression ring is Ft(1), a tension of the second compression ring is Ft(2), a tension of the third compression ring is Ft(3), a tension of the oil ring is Ft(4), and a diameter of a cylinder bore of the spark ignition engine is d1, Ft(1) > Ft(3), and when Ft(TOTAL) = Ft(1) + Ft(2) + Ft(3) + Ft(4), 0.68 N / mm ≥ Ft(TOTAL) / d1.

3. The combination structure of the piston and the piston rings according to claim 1 or 2, wherein when an inclination angle of an axial end face of the third compression ring relative to a plane orthogonal to an axis of the piston is a torsional angle, the torsional angle indicates a positive value in a case where the axial end face inclines on a combustion chamber side toward outside in a radial direction, and the torsional angle indicates a negative value in a case where the axial end face inclines on a crank chamber side toward outside in the radial direction, the torsional angle of the third compression ring is 20' ± 40' in a state where the third compression ring is assembled to the piston and the piston is fitted in the cylinder.

4. The combination structure of the piston and the piston rings according to any one of claims 1 to 3, wherein the third compression ring has an outer peripheral surface formed in a tapered shape or a tapered undercut shape.

5. The combination structure of the piston and the piston rings according to any one of claims 1 to 4, wherein 1.8 mm ≥ h1(1), 1.2 mm ≥ h1(2), and 1.0 mm ≥ h1(3).

6. The combination structure of the piston and the piston rings according to any one of claims 1 to 5, wherein the oil ring includes a pair of segments that are formed annularly along a circumferential direction of the oil ring and that are provided independently of each other and side by side in an axial direction of the oil ring, and a spacer expander that is disposed between the pair of segments.

7. The combination structure of the piston and the piston rings according to any one of claims 1 to 6, wherein when a length, in the axial direction of the piston, of a third land portion that is a region defined by the second ring groove and the third ring groove in the outer peripheral surface of the piston is Lp3, Lp3 ≥ Lp4, Lp3 ≥ 0.5 mm, and Lp4 ≥ 0.5 mm.

8. The combination structure of the piston and the piston rings according to any one of claims 1 to 7, wherein when a volume of a second space that is a space surrounded by the piston, the cylinder, the first compression ring and the second compression ring is V2, and a volume of a fourth space that is a space surrounded by the piston, the cylinder, the third compression ring and the oil ring is V4, V2 > V4.

9. The combination structure of the piston and the piston rings according to any one of claims 1 to 8, wherein when a volume of a fourth space that is a space surrounded by the piston, the cylinder, the third compression ring and the oil ring is V4,a diameter of a cylinder bore of the spark ignition engine is d1 and Vp = (d1 / 2)2 x π x Lp, V4 / Vp ≥ 0.00027.