Cylinder unit and method for forming the sliding surfaces thereof

Abstract: A cylinder unit for an internal combustion engine, comprising a cylinder having a cylinder bore. A crankshaft is connected to a piston fitted into the cylinder bore in the axial direction, so as to freely slide upon the sliding area of the inside circumferential surface thereof. This surface is provided with a plating layer the thickness of which diminishes gradually at the end area of said plating layer adjacent to the crankshaft towards an unplated area.

Description: The invention relates to a cylinder unit for an internal combustion engine, comprising a cylinder having a cylinder bore and a piston connected to crankshaft, said piston being fitted into said cylinder bore in the axial direction, so as to freely slide on an area of sliding contact at the inside circumferential surface of the cylinder bore, wherein said cylinder bore has a plane cylindrical inside circumferential surface over the area of sliding contact, and a plating layer is formed on said inside circumferential surface in said area of sliding contact and covers the whole of this area. The invention relates further to a method of forming the sliding surface of a cylinder unit of an internal combustion engine, comprising the steps of applying a plating layer to the inside circumferential surface of a cylinder, being arranged up-side-down in a plating device, by means of a pipe inserted into said cylinder. Such a method is disclosed in DE 39 37 763 A1. Common cylinder units are provided with a hard plating layer on the inner circumferential surfaces in order to reduce the wear of the sliding areas occurring due to the high speed of the reciprocating piston. However, simply applying a layer of plating to the sliding areas creates the following problems. The first problem is that at the end of the crankshaft side of the sliding areas a step is formed in an area where the plating layer meets the non-plated inner surface of said cylinder unit. During operation of an internal combustion engine stress accumulates at this step so that it is very likely that the plating layer separates from the base stock thereby decreasing the longevity of the cylinder unit and consequently of the engine. The second problem is that usually the piston is fitted with piston rings sliding against the inner wall or siding area of the cylinder unit in order to assure an air-tight seal between the piston and the cylinder. In order to reduce or prevent wear due to this sliding contact, the piston rings are made from a more wear-resistant material than the piston. However, since the outer surface of the piston also comes into sliding contact with the plating layer, the hardness of the plating layer causes accelerated wear of the piston reducing engine longevity. DE 42 38 525 C1 discloses a cylinder unit of the above kind, showing a plating layer within a cut-out rim inside the cylinder wall. The manufacture thereof raises the costs per unit, however. It is therefore an objective of the present invention to provide an improved cylinder unit for engines as indicated above enhancing the longevity of the plating layer as well as of the piston. It is a further objective of the present invention to provide an improved method for forming the sliding surface of a cylinder unit of an internal combustion engine as indicated above enhancing the longevity of the plating layer as well as the longevity of the piston. Accordingly, this first objective is solved in an inventive manner for a cylinder unit as indicated above in that said plating layer comprises an edge area at the end of said plating layer adjacent to said crankshaft, which diminishes gradually in thickness toward the crankshaft side such that there is a smooth continuation from the edge area towards an unplated area of the inside circumferential surface. Furthermore, the second objective is solved by a method for forming the sliding surfaces of a cylinder unit as indicated above in that the top of said cylinder is closed with a plug, said pipe is inserted into said cylinder leaving a predetermined distance between the free end of the pipe and the plug such that the current density applied to an end area of said inside circumferential surface adjacent to said plug is lower than in the remaining part of said inside circumferential surface and wherein the plug is mounted to provide a ring-shaped gap between it and the inside circumference surface, said gap being connected to the inside of the cylinder and subjecting said plating layer to a finishing treatment. According to an embodiment of the invention, the percentage by weight content of the SiC in the plating layer remains relatively constant throughout the ring sliding area 38 of the inside circumferential surface 31 of the cylinder unit 7 where the piston rings 30 slide. Because maintaining the same percentage by weight content of SiC in the foregoing ring sliding area 38 where the piston rings 30 slide against the inside circumferential surface 31 of the cylinder unit 7, the overall resistance to wear in the foregoing ring sliding area 38 has been improved by providing adequate hardness. Further, the percentage by weight of SiC content in the area 39 of the plated layer 33 that lies beyond the foregoing ring sliding area 38 on the crankshaft 4 side is less than that in the foregoing ring sliding area 38. As a result, it is possible to lessen the hardness of the area 39 of the plating layer 33 where only the piston unit 38 makes sliding contact, in order to prevent the unwanted wear of the piston unit 28 due to its sliding contact with this area 39 of the plating layer 33. According to another embodiment of the invention, a positive electrode A made of nickel is positioned inside the cylinder unit 7, with the same cylinder unit 7 serving as the negative electrode B, and a plating solution 49 that contains SiC is caused to flow inside the same cylinder unit 7 to create a plating layer 33 on the inside circumferential surface 31 of that same cylinder 6, the foregoing positive electrode A being situated more distant from the crankshaft 4 side and closer to the cylinder head side 10. As a result, in performing the electroplating, the current density in the plating solution 49 diminishes in the direction toward the crankshaft 4 side along the same inside circumferential surface 31. Here, since the magnitude of the foregoing current density is approximately proportional to the thickness of the plating layer 33 that is formed and to the SiC content in the plating layer 33, if the current density is diminished in the plating solution 49 in the direction toward the crankshaft 4 side, the thickness of the plating layer 33 diminishes toward the edge 35 on the crankshaft 4 side. Further, as described above, when the positive electrode A is situated to be more distant from the crankshaft 4 side the cylinder head 10 side, the plating area is formed as it should be in the foregoing ring sliding area 38 in the axial direction of the cylinder unit 7, and in addition, if it is positioned at the edge of the foregoing ring sliding surface 38 on the crankshaft 4 side of the same inside circumferential surface 31, then on the one hand, the SiC content in the foregoing plating layer 33 throughout the foregoing ring sliding area 38 will be approximately constant, while on the other hand the SiC content in the plating layer 33 in the area 39 that lies beyond the ring sliding area 38 on the crankshaft 4 side will be comparatively lower. Therefore, the formation of the plating layer 33 according to the above-mentioned embodiments of the invention can be performed by the appropriate positioning of the positive electrode A. Other preferred embodiments of the present invention are laid down in further dependent claims. In the following, the present invention is explained in greater detail with respect to several embodiments thereof in conjunction with accompanying drawings, wherein: Figure 1 is a partial sectional view of a cylinder unit. Figure 2 is a graphic representation of the thickness of the plating layer and SiC content in relation to a partial sectional view of a cylinder unit. Figure 3 is an overall side sectional view of the plating device. In Figures 1 and 2, reference number 1 represents a gasoline fueled four-cycle internal combustion engine; for instance this internal combustion engine 1 is mounted in a two-wheeled motorcycle. The foregoing internal combustion engine 1 comprises a crankcase 2 and the crank chamber 3 of this crankcase 2 houses a crankshaft 4 which is supported inside the foregoing crankcase 2 to be freely rotatable around its axis. The cylinder 6 projects from the top surface of the crankcase 2. This cylinder 6 is present in a cylinder unit 7 that is removably attached by fasteners to the top surface of the foregoing crankcase 2. The cylinder bore 9 is formed in the cylinder unit 7 with a vertically oriented axis 8 and a circular cross-sectional shape. A cylinder head 10 is removably fastened to the top end of the foregoing cylinder unit 7 by fasteners, and this cylinder head 10 closes the top end of the foregoing cylinder bore 9. A piston 12 is inserted into the cylinder bore 9 in the foregoing cylinder unit 7 to be freely axially slidable. A connecting rod 14 is attached at one end to the crankshaft 4 and at the other end to the foregoing piston 12 by a piston pin 13. This connecting rod 14 links the movement of the foregoing crankshaft 4 and the piston 12. When the foregoing piston 13 is at its upper dead point (shown by the imaginary lines in Figure 1), a combustion chamber 15 is subtended by the space bounded by the cylinder head 10 and the piston 12 inside the foregoing cylinder bore 9. Air intake ports 17 and exhaust ports 18 are present in the cylinder head 10 and are linked to the combustion chamber 15 and to the outside of the cylinder head 10, and air intake valves 19 and exhaust valves 20 open and close the above ports 17, 18. A spark plug 21 is also situated with its discharge area at the boundary of the foregoing combustion chamber 15. Then, when the foregoing piston 12 descends from its upper dead point to its approximate lower dead point (as shown by the solid lines in Figures 1 and 2), the opening of the air intake valves 19 allows an air/fuel mixture 23 to enter the cylinder bore 9 through the air intake ports 17. Next, this air/fuel mixture 23 is compressed by the rise of the piston 12 from its lower dead point, and the discharge of the foregoing spark plug 21 causes it to be ignited and burned in the combustion chamber 15. The internal combustion engine 1 is thereby driven, and that drive is transmitted by the crankshaft 4. The exhaust 24 created by the foregoing combustion is expelled through the exhaust ports 18 by opening the foregoing exhaust valves 20. A cooling jacket 25 is present in the foregoing cylinder unit 7, and coolant circulating through the cooling jacket 25 prevents the foregoing cylinder unit 7 from becoming overheated by removing the heat generated by the foregoing combustion. The foregoing piston 12 comprises a piston unit 28 positioned in line with the foregoing axis 8, and this piston unit 28 is composed of a disc shaped piston head 28a, that is also centered on the foregoing axis 8, and an integral, downward-extending skirt 28b around its circumference. Three grooves 29 are present in the top outside circumferential surface of the foregoing piston unit 28, and piston rings 30 are fitted into these grooves 29 to attach them to the piston unit 28. The outside circumferential surface of each of the foregoing piston rings 30 is in resilient contact with the inside circumferential surface 31 of the cylinder bore 9 of the foregoing cylinder unit 7, and they may slide vertically in their axial direction. The air-tight seal of the foregoing combustion chamber 15 is principally maintained by the sliding contact of the outside circumferential surfaces of these piston rings 30. The foregoing cylinder unit 7, cylinder head 10, and piston 28 are all made from aluminum alloys. The foregoing inside circumferential surface 31 in the area of sliding contact 32 with the piston 12 comprises a plating layer 33 formed by electroplating that inside circumferential surface 31. This plating layer 33 strongly bonds to the base stock material of the foregoing inside circumferential surface 31. The primary component of this plating layer 33 is nickel (Ni), with SiC distributed in dispersed form inside the nickel. An alternative material for the plating layer is nickel-based with phosphorous and a silicon carbon dispersoid (Ni-P-SiC). The surface of the foregoing plating layer 33 has also been finished by honing. The thickness of the foregoing plating layer 33 gradually diminishes near the edge 35 area on the foregoing crankshaft 4 side in the direction toward that crankshaft side. In this case, the length of this edge area 35 in the axial direction along the foregoing axis 8 is about 2 mm. With the above described configuration, there is a smooth continuation from the edge 35 of the plating layer 33 on the crankshaft 4 side into the inside circumferential surface 31, and the area 36 without a plating layer 33 on the inside circumferential surface 31 of the foregoing cylinder unit 7, thereby preventing a step from being formed at the edge 35 of the plating layer 33, thus preventing any buildup of stress in that area that would be caused by a step. The SiC content in the plating layer 33 is relatively constant throughout the ring sliding area 38 of the inside circumferential surface 31 of the foregoing cylinder unit 7, against which slide the foregoing piston rings 30. Due to this configuration, to achieve an adequate air-tight seal for the foregoing combustion chamber 15, the piston rings 30 remain in sliding contact with the inside circumferential surface 31 of the cylinder unit 7, and inside this foregoing ring sliding area 38, the SiC content as a percentage by weight remains relatively constant throughout, thereby imparting an adequate hardness to the foregoing ring sliding area 38 in order to improve the overall resistance to wear and to assure the required longevity of the foregoing ring sliding area 38. Further, the SiC content as a percentage of weight in the area 39 of the plating layer 33, which lies beyond the foregoing ring sliding area 38 on the crankshaft 4 side, is lower than the content in the foregoing ring sliding area 38. In other words, in the areas against which only the piston unit 28 slides, the hardness of the plating layer 33 has been lessened, thereby preventing unwanted wear or seizing of the piston unit 28 due to its sliding contact with the area 39 of the plated layer 33. In this case, the percent by weight of the SiC content in the area 39 of the foregoing plating layer 33 gradually diminishes the closer toward the crankshaft 4 side. The edge 41 on the cylinder head 10 side of the foregoing inside circumferential surface 31 evinces a rounded, arc-shaped cross section, and the plating layer 33 is also formed on this edge 41 of about the same thickness as the other plating layer 33 areas. With this configuration, the plating layer 33 is tightly bonded to the base stock on the inside circumferential surface 31 at the end edge 41 of the inside circumferential surface 31, assuring that the plating layer 33 does not peel away from the base stock. Also, the radius of curvature of the surface of the plating layer 33 at the foregoing edge 41 is small, preventing carbon deposits adhering to the outside in this area. In Figure 3, reference number 43 represents the electroplating device. The foregoing electroplating device 43 is composed of a holding platform 44 which holds the foregoing cylinder unit 7 upside down, a plug 45 which can be inserted into the top opening of the cylinder bore 9 of the foregoing cylinder unit 7 in order to close it, a pipe 46 which is inserted longitudinally into the cylinder bore 9; a box-shaped screen 47 which encloses, with some space in between, the entire pipe including the upper end of this pipe 46, but which allows liquid to pass through; and a plurality of nickel balls 48 which are contained inside the space between the foregoing pipe 46 and the screen 47. This same electroplating device 43 is further equipped with a tank 50 that holds the plating solution 49 containing SiC, and a pump 51 which pumps this plating solution 49 from the tank 50 through the pipe 46 and into the foregoing cylinder bore 9. A seal material 52 is fitted around the lower outside circumference of the plug 45 fitting into the top opening of the foregoing cylinder bore 9; this seal material 52 helps to maintain an adequate seal between the lower outside circumferential surface of the plug 45 and the upper opening of the foregoing cylinder bore 9. A voltage has been applied to the positive electrode A composed of the foregoing pipe 46 and nickel material 48, and these components 46 and 48 are situated along the foregoing axis 8. On the other hand, the cylinder unit 7 comprises the negative electrode B. Then, as shown by the arrows in Figure 3, the pump 51 pumps the foregoing plating solution 49 into the cylinder bore 9 to create a flow inside the cylinder bore 9, and electroplating is performed on the foregoing inside circumferential surface 31 to create the above mentioned plating layer 33 on this inside circumferential surface 31. In the foregoing case, the top end of the pipe 46 and the nickel balls 48 are situated at some distance from the crankshaft 4 end (the top in Figure 3) of the foregoing cylinder unit 7, being closer to the cylinder head 10 end (the bottom in Figure 3), so that, the closer to the crankshaft 4 end in the above inside circumferential surface 31, the lower the current density. Here, the thickness of the plating layer 33 that is formed, and its amount in terms of percent by weight content of the SiC in this plating layer 33, are approximately proportional to the magnitude of the foregoing current density. Accordingly, the closer to the crankshaft 4 side the lower the current density in the plating solution 49. As a result, the thickness of the plating layer 33 in the end area 35 on the crankshaft 4 side diminishes toward the crankshaft 4 side. Further, as described above, in the case of the positive electrode A being composed of the pipe 46 and the nickel material 48 being set at some distance from the crankshaft 4 side and toward the end of the inside circumferential surface 31 on the cylinder head 10 side, the positive electrode A is positioned in an area that corresponds to that where the above described ring sliding surface 38 is to be formed in the axial direction of the cylinder unit 7, and in addition, it extends to a position that lies between the crankshaft end 4 of the foregoing inside circumferential surface 31 and the foregoing ring sliding surface 38. As a result, the foregoing plating layer 33 containing an approximately constant amount of SiC is formed in the ring sliding area 38, inside the foregoing inside circumferential surface 31 that comes into sliding contact with the piston rings 31. On the other hand, the plating layer 33 in the area 39 that extends from the foregoing ring sliding surface area 38 toward the crankshaft side contains a lower amount of SiC as a percentage by weight than does the foregoing ring sliding area 38. In Figure 3, between the area below the lower outside circumferential surface of the plug 45, which is beneath the inserted seal material 53 that fits around the circumference of the top of side of the foregoing cylinder bore 9, and the upper opening circumference of the foregoing cylinder bore 9, there is a ring-shaped gap 53 which is centered on the foregoing axis 8. This gap 53 connects to the inside the cylinder bore 9. This gap 53 causes the flow of the plating solution 49 therein to become turbulent, and due to the lowered current density as described above, the SiC content in the plating layer 33 formed in that area can be effectively reduced. The overall thickness of the plating layer 33 formed by the foregoing plating device 43 is approximately 100 µm, and this magnitude is reduced to about 50 µm by a subsequent honing process. According to the embodiment shown in the Figures, the crankcase 2 and the cylinder unit 7 are integral, and the internal combustion engine produced in that manner may be mounted in automobiles. According to one embodiment of the invention, the plating layer near the end on the crankshaft side gradually thins toward the crankshaft side. As a result, there is no step in the surface between the plated layer on the end toward crankshaft side and the surface lacking plating; there is smooth merging of the two areas. Accordingly, this feature effectively prevents stress from concentrating on a step area and prevents the resulting peeling of the plating from the base stock around the inside circumferential surface of the foregoing cylinder unit, thereby improving cylinder longevity. According to a further embodiment of the invention, the percentage by weight of SiC content in the plating layer in the area of the inside circumferential surface of the cylinder where the piston rings make sliding contact is held relatively constant throughout that area. The airtight seal of the combustion chamber is largely dependent upon the sliding contact made by the piston rings against this inside circumferential surface of the cylinder, and by having the SiC content approximately constant throughout this area, adequate hardness is assured for the foregoing ring sliding area, and overall, wear properties are improved to secure the requisite longevity for the foregoing ring sliding area. Further, in the plating layer which extends from the foregoing ring sliding area toward the crankshaft side, the percentage by weight of SiC content is less than it is in the foregoing ring sliding area. As a result, it is possible to lessen the hardness of the plating layer in that area which is only in contact with the piston unit, thereby reducing unwanted wear on the piston unit that slides against the plating layer in this area. This feature prevents seizing and, from that perspective, improves the longevity of the cylinder. According to another embodiment of the invention, a positive electrode of nickel material is positioned inside the cylinder, while the cylinder unit itself serves as the negative electrode, and a plating solution containing SiC is caused to flow inside the same cylinder unit, wherein, when creating the layer of plating on inside circumferential surface of the same cylinder unit, the foregoing positive electrode is situated at some distance from the crankshaft side of the cylinder unit, more toward the cylinder head side. Thus, when electroplating, the current density in the plating solution is reduced from around midway in the axial direction of the inside circumferential surface of the cylinder unit down toward the inside circumferential surface on the crankshaft side. Since the thickness of the plating layer that is formed, and its SiC content as a percentage by weight are proportional to the magnitude of the foregoing current density, as was described above, as the current density is reduced approaching the crankshaft side, the thickness of that plating layer will diminish approaching the crankshaft side in the cylinder thereby achieving one embodiment of the invention. Further, when the positive electrode is placed at some distance from the crankshaft end away from the inside circumferential surface and more toward the cylinder head side, in a position that causes the above described ring sliding area to be formed in the same inside circumferential surface in the axial direction, then it is possible to achieve approximately the same percentage by weight of SiC content in the foregoing plating layer throughout the ring sliding area in the foregoing inside circumferential surface that is in contact with the piston rings. On the other hand, in the area extending from the foregoing ring sliding area toward the crankshaft side, the percentage by weight of SiC content in the axial direction in the plating layer is diminished to a level lower than in the foregoing ring sliding area, thereby achieving another embodiment of the invention. To wit, the formation of the plating layer according to embodiments of the inventions described above is achieved merely by the positioning of the positive electrode, making it possible to improve cylinder longevity by a simple layer plating.

Claims: A cylinder unit (7) for an internal combustion engine (1), comprising a cylinder (6) having a cylinder bore (9) and a piston (12) connected to crankshaft (14), said piston (12) being fitted into said cylinder bore (9) in the axial direction, so as to freely slide on an area of sliding contact (32) at the inside circumferential surface (31) of the cylinder bore (9), wherein said cylinder bore has a plane cylindrical inside circumferential surface over the area of sliding contact, and a plating layer is formed on said inside circumferential surface in said area of sliding contact and covers the whole of this area, characterised in that said plating layer (33) comprises an edge area (35) at the end of said plating layer (33) adjacent to said crankshaft (4), which diminishes gradually in thickness toward the crankshaft side such that there is a smooth continuation from the edge area (35) towards an unplated area (36) of the inside circumferential surface (31). A cylinder unit (7) according to claim 1, characterised in that said plating layer (33) is a nickel-based plating coating containing a silicon carbon dispersoid (N 1 -SiC) or phosphorous and a silicon carbon dispersoid (Ni-P-SiC). A cylinder unit (7) according to claim 2, wherein said piston (12) is provided with at least one piston ring (30), characterised in that the content in weight percent of silicon carbon (SiC) in the sliding area (38) of said piston rings (30) is approximately uniform, whereas the content in weight percent of silicon carbon (Sic) in the diminishing portion near said end area (35) is smaller than in the ring sliding area (38). A cylinder unit (7) according to claim 3, characterised in that the content of silicon carbon (SiC) in said diminishing portion near said end area (35) decreases gradually within said end area (35). A cylinder unit (7) according to at least one of the preceding claims 1 to 4, characterised in that the edge (41) of said inside circumferencial surface (31) opposite said crankshaft (4) is rounded and that said plating layer (33) covers said edge (41). A cylinder unit (7) according to at least one of the preceding claims 1 to 5, characterised in that said cylinder (6) is formed of an aluminium alloy. A cylinder unit (7) according to at least one of the preceding claims 1 to 6, characterised in that said plating layer (33) is finished with a honing treatment. A method of forming the sliding surface of a cylinder unit (7) of an internal combustion engine, comprising the steps of applying a plating layer (33) to the inside circumferential surface (31) of a cylinder (6), being arranged up-side-down in a plating device (43), by means of a pipe (46) inserted into said cylinder (6), characterised in that the top of said cylinder (6) is closed with a plug (45), said pipe (46) is inserted into said cylinder (6) leaving a predetermined distance between the free end of the pipe (46) and the plug (45) such that the current density applied to an end area (35) of said inside circumferential surface (31) adjacent to said plug (45) is lower than in the remaining part of said inside circumferential surface (31) and wherein the plug (45) is mounted to provide a ring-shaped gap (53) between it (45) and the inside circumference surface (31), said gap (53) being connected to the inside of the cylinder (6) and subjecting said plating layer (33) to a finishing treatment. A method according to claim 8, characterised by using a plating solution. A method according to claim 9, characterised in that the flow of said plating solution near said end area is turbulent. Method according to claim 9 or 10, characterised by applying a plating layer with an overall thickness of about 100µm, except for said end area, and reducing said thickness to about 50µm by said finishing treatment. Method according to at least one of the preceding claims 8 to 11, characterised in that said finishing treatment is a honing treatment. Method according to at least one of the preceding claims 9 to 12, characterised in that said plating solution flows inside said pipe to the top and is urged to flow along said inside circumferencial surface.

Inventors: IKEGAYA HIROHIKO, MURASE YASUYUKI, IKEGAYA, HIROHIKO, MURASE, YASUYUKI, Ikegaya, Hirohiko, c/o Yamaha Hatsudoki K.K., Murase, Yasuyuki, c/o Yamaha Hatsudoki K.K.
Applicants: YAMAHA MOTOR CO LTD, YAMAHA HATSUDOKI KABUSHIKI KAISHA
Application year: 1995
Main classification: F02B77
IPC codes: F02F1, C25D7, F02B77