Method of manufacturing exhaust pipe of two-passage construction

Abstract: In manufacturing an exhaust pipe of two-passage construction, a metallic plate having a first end and a second end respectively elongated in a longitudinal direction of the exhaust pipe is prepared. The metallic plate is bent into a substantially S shape in cross section. It has a diametrically extending central partition plate (6) and a substantially semicircular peripheral wall (7) on each lateral side of said partition plate (6) in such a manner that a groove (14) of substantially V shape in cross section having a substantially closed end on a radially inner side and an open end on a radially outer side is formed at each diametrically outer portion of the partition plate (6). A target (13) is provided so as to elongate along the groove (14) in a manner to extend substantially radially outward from the closed end. A welding arc is generated between the target (13) and a welding gun (15) disposed on a side of the open end such that the substantially closed end and the target (13) are welded together.

Description: The present invention relates, in general, to a method of manufacturing an exhaust pipe of two-passage construction to be used in an exhaust system of an internal combustion engine according to the preamble of claim 1 (see, for example, US-A-4 633 056). BACKGROUND ART As a method of manufacturing a product such as an exhaust pipe of two-passage construction which is suitable for connection to an exhaust manifold of a multi-cylinder internal combustion engine of a motor vehicle, there is known the following method in, for example, US-A-4 633 056 and Published Unexamined Japanese Patent Application No. 192727/1997 as shown in FIG. 8. Namely, a single piece of metallic plate having a first end and a second end respectively elongated in a longitudinal direction of the exhaust pipe is bent substantially into S shape (i.e., a configuration of an alphabet "S"), as seen in cross section. The product thus obtained has: a partition plate or a partition wall "a" which extends in a diametrical direction and is elongated in a longitudinal direction of the exhaust pipe; and substantially semicircular peripheral wall portions b, b on each lateral side of the partition plate (i.e., on a side at a right angle to the partition plate). In this manner, independent chambers d, d are formed between the partition plate "a" and the peripheral walls b, b for passing therethrough the exhaust gases from the engine. Diametrical end portions, as seen in cross section, of the first end and the second end of the metallic plate are respectively bent radially inward to form connecting pieces c, c which lie along the partition plate "a" in its assembled state. Radially outer sides of the connecting pieces c and the partition plate "a" are welded together from a radially out side to thereby form welding beads e, e. On both longitudinal end portions of this exhaust pipe, flanges are welded for connecting the exhaust pipe to the engine on an upstream side thereof as well as to a catalyst converter on a downstream side thereof. The exhaust pipe of two-passage construction thus formed has, however, the following disadvantages. Namely, a welding bead e which is formed in the longitudinal direction of the exhaust pipe is likely to protrude beyond the outer circumferential surface of the exhaust pipe. It follows that, when the longitudinal end portion of this welded exhaust pipe is to be inserted into the flange for fixing it to the exhaust pipe by means of welding, or when an outer pipe which is used for covering the inner pipe for the purpose of thermal insulation is swaged (or tapered to a smaller diameter) to thereby bring the two members into close contact with each other, the protruded welding bead e is an obstacle. As a solution, the following arrangement may be considered as shown in FIG. 9A. The exhaust pipe of two-passage construction in this example is formed into a substantially S shape in cross section with a single piece of relatively thin metallic plate. It is made up of: a central partition plate "a" which forms, as seen in cross section, a diametrical line of the exhaust pipe and which extends in the longitudinal direction throughout the exhaust pipe; a substantially semicircular peripheral wall b which is formed on each lateral side of the central partition plate "a"; and a connecting piece C1 which is formed on each end of the metallic plate and which is bent so as to partially overlap with the partition plate "a" in close contact with each other at each diametrically outer end of the partition plate "a ". As a result, on each diametrically outer end portion of the partition plate "a", there is provided a groove (or valley) f substantially in the shape of an alphabet V. This groove f is formed by an arched end of the thin metallic plate and that arched end of the semicircular peripheral wall b which transforms (or changes) into the central partition plate "a". Welding is then made at this groove f by means of a welding gun (or a welding torch) g by generating an arc between the welding gun g and a bottom of the groove f in an attempt to integrally weld the central partition plate "a" and the peripheral wall b. In welding, however, electric discharging is likely to take place toward that portion of the mother material (peripheral wall b) which is closer to the welding gun g as shown by arrows g1, g1. As a result of this electric discharging, a lump h of welded metal (welding bead) is likely to be formed as shown in FIG. 9B in a manner not to reach deep into the groove (or the overlapped portion). A penetration bead is sometimes formed to thereby reduce the strength of the mother material. Sometimes, a penetrating hole i which breaks through the peripheral wall b is formed as shown in FIG. 9C. If an attempt is made to enlarge the width of the groove f, the cross section of the exhaust pipe approaches the shape of a flattened one, resulting in a reduction in the area of flow of the exhaust gases. The exhaust pipe of two-passage construction as disclosed in the above-described Published Unexamined Japanese Patent Application No. 192727/1997 has still another disadvantage as explained hereinbelow. Namely, when the exhaust gases from the multi-cylinder internal combustion engine pass through a right side passage (as shown in FIG. 8), the central partition plate "a" tends to deflect toward the left side by the exhaust pressure as shown by dotted lines a1. When the exhaust gases pass through the left side passage d, the central partition plate "a" tends to deflect toward the right side by the exhaust pressure as shown by dotted lines a2. As a result, by the pulsation of the exhaust gases to pass through the two passages d, d, the partition plate "a" vibrates by alternately bending in opposite directions as shown by dotted lines a1, a2. This bending takes place with the following point serving as the point of bending, i.e., each welding portion between the connecting piece "c" and the partition plate "a" or a portion which lies on the radially inside thereof. Stresses are concentrated on these points of bending and fatigue occurs there if the vibrations continue. In view of the above-described points, the present invention has an object of providing a method of manufacturing an exhaust pipe of two-passage construction. Solutions of this object are set forth in the independent claim 1. Advantageous features of the present invention are defined in the corresponding subclaims. According to the method of claim 1, welding arc is generated in a manner directed to the discharge leading tip which tip thus may be regarded as a target. Therefore, an accurate welding is possible, and thus even a relatively thin metallic plate can be welded without giving rise to the problem that the penetrating hole breaks through the metallic plate. Preferably, the partition plate has a larger bending rigidity at a central portion thereof than at both radially outer portions thereof. The central portion of the partition plate may have a larger thickness that the remaining portion of the exhaust pipe. The partition plate may also have a plurality of reinforcing ribs elongated in the diametrical direction of the partition plate, the reinforcing ribs being disposed at a longitudinal distance from each other. The groove preferably satisfies a condition of j/W < 2, wherein j is a depth from a diametrically outer end of the groove to a connecting portion and W is a maximum width of the groove. Otherwise, the groove substantially V shape satisfies a condition of L/D ≧ 0.05, wherein D is an imaginary inner diameter of the exhaust pipe and L is a distance which substracts a distance form a center of an imaginary circle of said semicircular wall to a connecting portion from an inside radium D/2 of said imaginary circle of the semicircular wall. According to this arrangement, there can be obtained an exhaust pipe of two-passage construction which is less subjected to the stresses due the pulsations of the exhaust gases. If the groove is formed to satisfy the above conditions, welding becomes easy with a particular kind of welding gun. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and the attendant advantages of the present invention will become readily apparent by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: FIG. 1 is a perspective view of an exhaust pipe of two-passage construction to which the present invention is applied; FIG. 2A is a sectional view of the exhaust pipe according to one example of the present invention and FIG. 2B is a sectional view of a welded portion in a groove; FIG. 3A is a sectional view of the exhaust pipe with still another modified example of the discharge leading tip of FIG. 2A and FIG. 3B is an enlarged sectional view of a portion to be welded; FIG. 4 is a sectional view of an exhaust pipe in which stresses of a partition plate are reduced; FIG. 5 is a perspective view of an exhaust pipe in which the partition plate has a reinforced construction; FIG. 6 is a modified example of FIG. 5; FIG. 7 is a graph showing the relationship between the stresses ratio of the partition plate and the depth of a groove; FIG. 8 is a sectional view of a conventional exhaust pipe of two-passage construction; and FIGS. 9A through 9C are sectional views of an exhaust pipe of two-passage construction before improvement of the present invention has been made, wherein FIG. 9A is a sectional view showing the occurrence of welding arcs, FIG. 9B is a sectional view of an incomplete welding beam, and FIG. 9C is a sectional view showing a partition plate broken by overpenetration of a welding arc. Figures 4 to 8 describe examples which are not coveved by the scope of the claims and are present for a better understanding of the present invention. First of all, detailed explanation will now be made about preferred embodiments of the present invention of a method of connecting two elongated portions of a metallic plate as exemplified by a method of manufacturing an exhaust pipe of two-passage construction for use in an exhaust system of an internal combustion engine. FIG. 1 shows an exhaust pipe 1 of two-passage construction which is manufactured by a method of the present invention. In the figure, reference numeral 2 denotes an outer pipe which is disposed on the exhaust pipe 1 with a thermally insulating space therebetween. The exhaust pipe 1 and the outer pipe 2 constitute an exhaust pipe assembly 3. One end of the outer pipe 2 is reduced in diameter so as to come into close contact with an outer periphery of the exhaust pipe 1. A flange 4 is fitted onto the outer periphery of the reduced outer pipe 2 for connecting the exhaust pipe assembly 3 to an exhaust manifold (not illustrated) of a multi-cylinder internal combustion engine of a motor vehicle. The other end of the outer pipe 2 has also a flange 5 for further connection to a catalyst container (not illustrated). The exhaust pipe 1 is made up of a single piece of a relatively thin metallic plate which extends in a longitudinal direction of the exhaust pipe 1 and has a width with a free end on each lateral side thereof. The metallic plate whose thickness is about 0.6 through 1.2 mm, for example, is bent so as to have a central partition plate or wall 6 which extends in a diametrical direction and is elongated in a longitudinal direction throughout the exhaust pipe 1, and two passages 8, 8 each of which is formed by the partition plate 6 and a pair of peripheral walls 7, 7 which are substantially semicircular as seen in cross section. As shown in FIG. 2A, each of the semicircular walls 7, 7 starts bending from the neighborhood of the upper and lower end portions, as seen in FIG. 2A, (i.e., diametrically outer end portions) of the partition plate 6. At each of base or end portions of the peripheral walls 7, 7 (i.e., at those portions of the partition plate 6 where the partition plate 6 and the peripheral walls 7, 7 meet each other), there are formed inclined portions 9, 9 and bent portions 10, 10, respectively. On each lateral free end portions of the metallic plate (i.e., on starting end portions of the peripheral walls 7, 7), there are formed other inclined portions 12, 12 and bent portions 11, 11, respectively. The free end portions of the metallic plate is bent substantially backward to thereby form a discharge leading tip 13 or "target", which denotes a portion or member which serves the function of a welding wire or a filler wire in arc welding. The two inclined portions 9, 12 thus form a substantially V-shaped valley or groove 14 on each diametrically opposite ends of the partition plate 6. The V-shaped groove 14 has a closed, or substantially closed, end on the radially inner side and an open end on the radially outer side. The angle "A" of the V-shaped groove 14 is about 40°, and the discharge leading tip 13 extends radially outward from a substantially closed end (i.e., radially inner end of the groove 14) radially outward along a line which evenly divides the groove 14 into half. A welding gun 15 equipped with a welding electrode or a plasma electrode is disposed on the side of the open end of the groove 14 off from (or out of contact with) the discharge leading tip 13 such that the welding gun points to the discharge leading tip 13. The welding gun 15 is moved in the longitudinal direction along the discharge leading tip 13 (i.e., in the longitudinal direction of the exhaust pipe 1) while generating a welding arc between the discharge leading tip 13 and the welding gun 15. The discharge leading tip 13 and the portion which lies in the neighborhood of the discharge leading tip 13 are thus melted together to thereby form welding bead 16 along the bottom of the groove 14 as shown in FIG. 2B. Once the welding of one diametrically outer side is finished, the welding of the other groove on the diametrically opposite side is similarly welded. The discharge leading tip 13 can thus be seen to function as a welding wire. Still further example of the discharge leading tip portion is shown in FIG. 3A. The overall arrangement of the exhaust pipe is substantially the same as that shown in FIG. 2A, the only difference being that a discharge leading tip 13c is arranged to lie along the inclined portion 9 toward the radially outer side. In this example, as shown in FIG. 3B, the welding gun 15 is arranged to generate a welding arc between the tip of the gun and the front end portion of the discharge leading tip 13c to thereby form a welding bead 16'. An explanation has so far been made mainly about the construction of the discharge leading tip portion. Now, an explanation will be made about the exhaust pipe in which stress concentration on diametrically outer welded ends of the partition plate due to pulsations thereof can be prevented, the problem of pulsations being encountered in the exhaust pipe described with reference to FIG. 8. An exhaust pipe of two-passage construction is shown in FIG. 4. The construction is basically the same as that shown in FIG. 2A or the like except for the way of connecting the diametrically outer ends of the partition plate 6 (i.e., the portion at the closed bottom end of each groove 14) and both free ends of the metallic plate (i.e., the radially inner end of each free end). The discharge leading tip which is an essential element in the above-described examples may, or may not, be employed in this particular example. Therefore, no detailed explanation will be made about the construction that is substantially the same as those of the above-described examples, by attaching the same numerals to the same or similar portions. In this example, the bottom of the groove 14 may be connected together by a suitable means, such as by welding, to thereby form a connecting portion 17, 17. The imaginary inner diameter of the exhaust pipe of two-passage construction is D. The depth of the groove 14, i.e., the distance from the line of the imaginary inner diameter D to the connecting portion 17 is defined to be L. The distance between the outer portions 10, 11 of the curves at the diametrically outer ends of the groove 14 are defined to be W. The depth from the diametrically outer ends of the groove 14 to the connecting portion 17 is defined to be j. In this arrangement, when the exhaust gases alternately pass through the two passages 8, 8, the pulsation occurs to thereby cause the changes in the pressure in the passages 8, 8. As a result, due to this pressure difference, the partition plate 6 deflects as shown in imaginary lines 6a, 6b in FIG. 4. Namely, this deflection is repeated by bending the partition plate 6 from the high-pressure side toward the low-pressure side with the neighborhood of the connected portions 16, 16 as points of bending. As compared with the conventional one as shown in FIG. 8, the diametrical length of this partition plate 6 thus becomes smaller in length by an amount of 2 times the amount L which is the distance between the point of contact of the partition plate 6 and the peripheral wall 7 and the radially innermost position of contact. The area of the partition plate 6 of receiving pressure thus becomes smaller and consequently the total pressure to be operated onto the partition plate 6 becomes smaller. The bending in the neighborhood of the connected points of contact becomes smaller and, further, the wall portions in the groove 14 also allows to bend to some degree as shown by imaginary lines in FIG. 4. As a consequence, the bending stresses and stress concentration in the neighborhood of the connected points 17, 17 are alleviated, with the result that the durability of the partition plate 6 is increased. By the way, the inclined portions 9, 9 and bent portions 10, 10, which together form the respective grooves 14, 14, shall preferably be arranged as follows. Namely, the inclined portions 9, 9 are formed into a plane (i.e., having a relatively straight line), and the bent portions 10, 10 are formed into curved portions which do not form the continuation of the inclined and straight portions 9, 9, which is different from continuously formed curved portions 9a, 12a as shown by imaginary lines in FIG. 7. In this arrangement, the cross-sectional area of the exhaust-gas flow can be kept larger. Further, as shown in FIG. 5, the central wall 6 may be formed in a thicker plate than the remaining portion of the exhaust pipe. According to this arrangement, a higher bending rigidity can be secured in the central portion of the partition plate 6. The deflection of the partition plate 6 is thus reduced and the stresses can accordingly be reduced. Alternatively, as shown in FIG. 6, a plurality of reinforcing ribs 18 may be disposed on the partition plate 6 at a longitudinal distance from each other. According to this arrangement, the bending rigidity of the partition plate 6 in the diametrical direction of the partition plate can be increased. As a consequence, the deflection of the partition plate is reduced and the stresses can also be reduced. These reinforcing ribs 18 shall preferably be formed on the relevant portions of the partition plate 6 before the exhaust pipe is subjected to the bending work. FIG. 7 is a graph showing the stresses which occur at the connected points 16, 16 with varying distance L. It can be seen from this graph that a large depth L can reduce the stresses as compared with the example in which the distance L is made substantially zero (as in the conventional example in FIG. 8). If, on the other hand, this distance L or j is too large, the cross-sectional area of the passages 11, 12 becomes small. If the distance W is too small, economical welding work such as MIG (Inert Metal-arc Welding) or TIG (Inert-gas Tungsten-arc Welding), or the like cannot effectively be done from the open end of the groove. As a solution, the groove in this example is formed such that each of the connected points 16, 16 lies radially inward of the imaginary line of an inner circumference of the wall (i.e., inside the line of diameter D) and that the groove satisfies a condition of j/W < 2, most preferably, 0.5 < j/W < 2. Otherwise, the depth of the groove 14 shall preferably be made 5% or more of the imaginary inner diameter D, i.e., is formed to meet the condition of L/D ≥ 0.05. In this arrangement, the stresses in the neighborhood of the connected portions 17, 17 can surely be reduced. As a consequence, the thickness of the metallic plate to be used as the exhaust pipe can be made smaller in thickness, resulting in a smaller weight of the exhaust pipe. Therefore, when the engine is started, the exhaust pipe rapidly rises in temperature and the exhaust gases of high temperature can be sent to the catalyst converter in a shorter period of time. As an example of the metallic plate for manufacturing the exhaust pipe, a stainless steel (defined by JIS, Japanese Industrial Standard, as SUS 430) was used to meet the condition of L/D ≥ 0.05. Then, it has been found that, as compared with the one in which L/D is near zero, the stresses in this example was reduced by 10% or more. Suppose that the time in which the inlet portion of the catalyst converter reaches a catalyst active temperature is "x" seconds with the plate thickness of "t" o Then, the time was found to be 0.9x second with the plate thickness of 0.8t and 0.77x second with the plate thickness of 0.6t. As can be seen from these figures, the catalyst converter can be activated at a shorter time, resulting in an improvement of the exhaust gas purifying capacity. It is readily apparent that the above-described method of manufacturing an exhaust pipe of two-passage construction meets all of the objects mentioned above and also has the advantage of wide commercial utility. It should be understood that the specific form of the invention hereinabove described is intended to be representative only, as certain modifications within the scope of these teachings will be apparent to those skilled in the art. Accordingly, reference should be made to the following claims in determining the full scope of the invention.

Claims: A method of manufacturing an exhaust pipe (1) of two-passage (8) construction, comprising the steps of: preparing a metallic plate having a first end and a second end respectively elongated in a longitudinal direction of the exhaust pipe (1); bending the metallic plate into a substantially S shape in cross section having a diametrically extending central partition plate (6) and a substantially semicircular peripheral wall (7) on each lateral side of said partition plate (6) in such a manner that a groove (14) of substantially V shape in cross section having a closed end on a radially inner side and an open end on a radially outer side is formed at each diametrically outer portion of said partition plate (6); and disposing a welding gun (15) on a side of the open end and generating a welding arc, characterised in providing a discharge leading tip (13) elongated along said groove (14) in a manner to extend radially outward from said closed end by bending the first end and the second end, respectively, of said metallic plate toward the diametrically outer end of said partition plate (6) ; and generating the welding arc between said discharge leading tip (13) and said welding gun (15) such that said closed end and said discharge leading tip (13) are welded together. The method of manufacturing an exhaust pipe (1) of two-passage (8) construction according to claim 1, wherein said groove (14) has two inclined portions (9, 12), the first inclined portion (9) being part of the partition plate (6) and the second inclined portion (12) being part of the substantially semicircular peripheral wall (7), wherein the discharged leading tip (13) lies on the first inclined portion (9). The method of manufacturing an exhaust pipe (1) of two-passage (8) construction according to claim 1 or 2, wherein in the preparing and bending steps the metallic plate comprises a partition plate (6) having a larger bending rigidity at a central portion thereof than at both radially outer portions thereof. The method of manufacturing an exhaust pipe (1) of two-passage (8) construction according to claim 3, wherein said partition plate (6) has a larger thickness than the remaining portion of said exhaust pipe. The method of manufacturing an exhaust pipe (1) of two-passage (8) construction according to claim 3, wherein said partition plate (6) has a plurality of reinforcing ribs (18) elongated in the diametrical direction of said partition plate (6), said reinforcing ribs (18) being disposed at a longitudinal distance from each other.

Inventors: EGUCHI YOSHIHIKO, HOSODA FUNIO, ISHII KAZUO, MUNEMURA TAKESHI, SASAGASE TAKESHI, SATO NORIYUKI, SHIRAI HIROKAZU, SUZUKI AKIHIRO, UEGANE MASAYUKI, YANAGIBAYASHI HIROTO, YOSHITOSHI NOBUYUKI, EGUCHI, YOSHIHIKO, HOSODA, FUNIO, ISHII, KAZUO, MUNEMURA, TAKESHI, SASAGASE, TAKESHI, SATO, NORIYUKI, SHIRAI, HIROKAZU, SUZUKI, AKIHIRO, UEGANE, MASAYUKI, YANAGIBAYASHI, HIROTO, YOSHITOSHI, NOBUYUKI
Applicants: HONDA MOTOR CO LTD, YUTAKA GIKEN CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA, KABUSHIKI KAISHA YUTAKA GIKEN
Application year: 2000
Main classification: B23K33
IPC codes: F16L9, B23K33, B21C37, F01N13