JPH08121158A - Double exhaust pipe for internal combustion engine and its manufacture - Google Patents

Abstract

PURPOSE: To prevent deterioration of durability by forming both ends of an inner pipe as free ends, supporting the inner pipe at the position other than them in respect to an outer pipe, and thereby suppressing generation of stress on the inner pipe. CONSTITUTION: An exhaust manifold 1 has a welded inner pipe 4, an an outer pipe 5 casted so as to surround the inner pipe 4, while a clearance 10 is secured between the inner pipe 4 and the outer pipe 5 as a heat insulative layer. A flange 8 is formed on an end of the outer pipe 5 at its side of a collection part, and is connected to a flange 3a of a catalyst device 3. Both ends 4a, 4b of the inner pipe 4 are not fixed to the outer pipe 5, being free ends. The inner pipe 4 is supported to the outer pipe 5 by means of a support 11 arranged on an intermediate part. The support 11 is composed of a plurality of bosses 12 integrally casted onto an inner peripheral part of the outer pipe 5. Thermal deformation of the inner pipe 4 is distributed to both ends of the support 11 and durability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a dual exhaust pipe of an internal combustion engine having a dual structure for suppressing a temperature decrease of exhaust gas and a method for manufacturing the same.

[0002]

2. Description of the Related Art A double exhaust pipe has been conventionally proposed in order to suppress a temperature decrease of exhaust gas discharged from an internal combustion engine and accelerate a temperature rise of a catalyst device. For example, JP
Japanese Unexamined Patent Publication No. 2-68668 discloses a double exhaust pipe including an inner pipe through which exhaust gas flows and an outer pipe cast so as to surround the inner pipe. In this double exhaust pipe, both ends of the inner pipe are connected and fixed to the outer pipe, and a heat insulating air layer is formed between the inner pipe and the outer pipe in the portion excluding the both ends. As a result, the exhaust gas passing through the inside of the inner pipe is maintained at a high temperature, and the temperature rise of the catalyst device located on the downstream side can be promoted.

Further, Japanese Unexamined Patent Publication No. 63-215809 discloses a double exhaust pipe in which a water jacket through which cooling water flows is formed between an inner pipe and an outer pipe. However, in this structure as well, both ends of the inner pipe are joined to the outer pipe surrounding the inner pipe.

[0004]

As described above, in the conventional dual exhaust pipe of the internal combustion engine, since both ends of the inner pipe are connected and fixed to the outer pipe, the heat of the exhaust gas passing through the inner pipe is reduced. Due to this, when the inner pipe is thermally deformed, a large stress is generated in the inner pipe, and there is a problem that the durability of the inner pipe is reduced. In particular, in an exhaust pipe having a curved middle portion, a temperature difference occurs between the inner side and the outer side of the curve of the inner pipe, and the stress generated by this temperature difference further reduces the durability of the inner pipe. .

[0005]

SUMMARY OF THE INVENTION Therefore, according to the present invention, a cast outer tube having a joining flange portion at its end and a cast outer tube disposed inside the outer tube with a gap therebetween and surrounded by the outer tube. In a double exhaust pipe of an internal combustion engine that has a curved portion in the middle, the both ends of the inner pipe are free ends, and the inner pipe is supported by the outer pipe at a portion other than these both ends. Is characterized by.

According to the second aspect of the invention, among the plurality of supporting portions provided between the inner pipe and the outer pipe, at least one place connects the inner pipe and the outer pipe in a mutually immovable state. It is fixed and the remaining supports are unbonded to allow axial sliding.

According to the third aspect of the invention, in a structure in which a plurality of exhaust pipes of each cylinder connected to the cylinder head are joined together, the end of the exhaust pipe on the cylinder head side is joined with the exhaust from another cylinder. A support portion was provided near the intermediate portion with the portion.

According to the fourth aspect of the present invention, the support portion is provided at the portion of the curved portion of the exhaust pipe where the radius of curvature of the inner pipe is minimum.

According to the fifth aspect of the invention, the support portion is provided in the straight portion of the exhaust pipe.

According to the sixth aspect of the present invention, when the outer pipe is cast, a boss portion is formed from the inner peripheral surface of the outer pipe so as to contact the outer peripheral surface of the inner pipe, and the inner pipe is supported by the boss portion. .

Further, according to the invention of claim 7, a boss portion projecting to the outer peripheral side is formed at a predetermined position of the inner pipe so as to contact the inner peripheral surface of the outer pipe, and the inner pipe is supported by the boss portion. .

Further, in the method for manufacturing a dual exhaust pipe for an internal combustion engine according to the present invention, a steel plate is press-formed into a substantially semicircular cross section, and at the time of this press-forming, a boss portion is formed and fixed at a predetermined position. Forming an inner tube of a predetermined shape by
The inner tube was set in an inner tube core mold provided with a shallow recess into which the tip of the boss was fitted, and core sand was filled between the inner tube and the outer periphery of the inner tube to expose the tip of the boss. The method includes the steps of forming an inner tube core and setting the inner tube core in a mold to cast an outer tube.

According to the ninth aspect of the invention, in the step of forming the inner pipe, the steel plate press-formed into a substantially semicircular cross section is welded to form the inner pipe.

[0014]

In the dual exhaust pipe of the internal combustion engine of the present invention, both ends of the inner pipe are free ends, and the thermal deformation of the inner pipe is distributed to both sides of the support portion, so that stress is not generated in the inner pipe. Suppressed.

Particularly, in the structure of claim 2, when there are a plurality of supporting portions between the inner pipe and the outer pipe, the sliding of the inner pipe is allowed by allowing some of the supporting portions to slide in the axial direction. Deformation is not hindered and stress generation is prevented. Further, it is possible to remove sand after casting without making a hole in the outer tube for removing sand.

According to the third aspect of the present invention, the expansion and contraction deformation of the inner pipe in the exhaust pipe of each cylinder connected to the cylinder head is distributed to both sides of the support portion, and the deterioration of durability due to the expansion and contraction of the inner pipe is prevented. It

Further, in the structure of claim 4, the bending deformation due to the temperature difference between the inside and the outside of the bending of the exhaust pipe bending portion is distributed to both sides of the supporting portion, and deterioration of durability due to this bending deformation is suppressed. To be done.

Further, the inner pipe is also deformed in the radial direction by the heat of the exhaust gas, but in the fifth aspect, the straight portion, which is a portion where the temperature of the inner pipe is low, is hardly supported by the support portion. Therefore, the deterioration of durability due to the radial deformation of the inner pipe is suppressed.

According to the structure of claim 6, a boss portion is provided on the outer pipe side, and the inner pipe is supported by this boss portion. Further, in claim 7, a boss portion is provided on the inner pipe side, and the inner pipe is supported by the outer pipe by the boss portion.

In the method for manufacturing a double exhaust pipe according to the eighth aspect, an inner pipe having a boss portion is formed by press forming a steel plate, and the outer pipe is cast as if the inner pipe is cast. At the time of this casting, the boss portion of the inner pipe is exposed on the surface of the inner pipe core, and by casting the outer pipe, it becomes a support portion that is in contact with the inner peripheral surface of the outer pipe.

According to the invention of claim 9, in the step of forming the inner tube, the steel plates press-formed in a substantially semicircular cross section are welded to each other.

[0022]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below in detail with reference to the drawings.

FIG. 1 shows an embodiment in which the present invention is applied to an exhaust manifold 1 of an internal combustion engine. The exhaust manifold 1 as the double exhaust pipe is arranged between the cylinder head 2 and the catalyst device 3 of the internal combustion engine, and in the illustrated example, includes two branch portions 1a and 1b. The branch portions 1a and 1b are assembled into one and connected to the catalyst device 3.

The exhaust manifold 1 includes an inner pipe 4 formed by press-forming a steel pipe or a steel plate and welding the steel pipe or a steel plate.
An outer pipe 5 is formed outside the inner pipe 4 so as to surround the inner pipe 4, and a gap 10 serving as a heat insulating layer is secured between the inner pipe 4 and the outer pipe 5. . A flange portion 6 is provided at the tip of each branch portion 1a, 1b of the outer tube 5.
Is provided, and the flange portion 6 is attached to the side surface of the cylinder head 2 via a gasket 7. Further, a flange portion 8 is similarly provided at the end portion of the outer tube 5 on the side of the collecting portion, and the flange portion 8 is joined to the flange portion 3 a of the catalyst device 3. Also, both ends 4a of the inner pipe 4,
4b is not fixed to the outer tube 5 and is a free end. The tip 4a of the inner pipe 4 facing the exhaust port 9 of the cylinder head 2 slightly protrudes from the flange portion 6 of the outer pipe 5 and is loosely fitted into the opening of the gasket 7. Thereby, without strongly restraining the tip 4a of the inner tube 4,
The exhaust gas is prevented from flowing into the gap 10 as much as possible. Similarly, the tip 4b side may be loosely fitted in an opening of a gasket (not shown) interposed between the flange portion 8 and the flange portion 3a.

The inner pipe 4 is supported by the outer pipe by a support portion 11 arranged in the middle portion. As shown in FIG. 2, the support portion 11 is composed of a plurality of boss portions 12 integrally cast on the inner peripheral surface of the outer tube 5. Further, in this embodiment, the boss portion 12 is metal-bonded to the outer peripheral surface of the inner pipe 4 at the time of casting, and the inner pipe 4 and the outer pipe 5 are fixedly coupled to each other so that they cannot move. As will be described later, when the support portions 11 are provided at a plurality of places, the tip of the boss 12 of some of the support portions 11 is prevented from being metal-bonded to the inner pipe 4 and the axial direction of the inner pipe 4 is prevented. It is preferable to configure it to allow sliding.

In the structure of the above embodiment, the support portion 11 for connecting and fixing the inner pipe 4 and the outer pipe 5 is provided in the middle portion of the inner pipe 4, and both ends 4a and 4b of the inner pipe 4 are free ends. Therefore, when the inner pipe 4 expands or contracts or bends due to the heat of the exhaust gas, the deformation is distributed to both sides of the support portion 11, and the generation of stress in the inner pipe 4 is avoided. Therefore, it is possible to prevent the durability of the inner pipe 4 from decreasing. Further, since stress generation can be avoided in this way, the inner pipe 4 can be made a thin-walled pipe having a small heat capacity, the temperature drop of exhaust gas immediately after the start of the internal combustion engine can be further reduced, and early activation of the catalyst can be achieved. .

Next, in the embodiment shown in FIG. 3, in the exhaust manifold 1 having a double structure consisting of the inner pipe 4 and the outer pipe 5, each branch portion 1a, 1b is provided with a support portion 11 respectively. Shows. In this embodiment, the support 11
Is arranged in the vicinity of an intermediate portion between the exhaust pipe end portion on the cylinder head 2 side and the merging portion 13 where the exhaust passage of each cylinder merges with the exhaust gas from another cylinder. That is, the inner pipe 4 contracts and deforms in the longitudinal direction in the cooling process after the end of casting, and also expands and deforms due to the heat of the exhaust gas during operation. When such expansion and contraction of the inner pipe 4 is the main cause of the deterioration of the durability of the inner pipe 4, if the support part 11 is arranged as in this embodiment, the expansion and contraction of the inner pipe 4 will be reduced. Is effectively distributed to both sides of the inner tube 4 and the durability of the inner tube 4 is prevented from being lowered.

Next, FIG. 4 shows an embodiment in which the support portion 11 is arranged on the curved portion R of the double exhaust pipe. That is, generally, the exhaust pipe has a complicated shape, and the heat transfer coefficient between the inner pipe 4 and the exhaust gas flowing inside the inner pipe 4 is different in each part, so that the temperature distribution of the inner pipe 4 is uniform. Not like. In the curved portion R of the exhaust pipe, the temperature of the outer portion of the curve tends to be higher than that of the inner portion of the curve. When such a temperature distribution is generated, the inner pipe 4 bends in a direction in which the radius of curvature becomes smaller. However, in the curved portion R, the temperature difference between the inside and the outside of the curve near the portion where the radius of curvature is the minimum. Is maximum, the maximum bending deformation occurs. Therefore, in the case where the bending stress of the inner pipe 4 causes the durability of the inner pipe 4 to be reduced, as shown in FIG. 4, the portion where the largest bending deformation occurs, that is, the radius of curvature of the inner pipe 4 is the smallest. It is advisable to provide the support portion 11 in the region where By disposing the support portion 11 at the portion where the radius of curvature is the smallest in the curved portion R as described above, the bending deformation of the inner pipe 4 described above is distributed to both sides of the support portion 11 and the durability of the inner pipe 4 is reduced. Is prevented.

Further, as shown in FIG. 5, when the radius of curvature of the inner pipe 4 is minimum near the connecting portion of the cylinder head 2, a large bending deformation occurs in the inner pipe 4 near the cylinder head 2. A large stress is generated at the junction with the branch of another cylinder. Therefore, in such a case, as shown in FIG. 5, it is advisable to dispose the position of the support portion 11 at a position closer to the confluence with the branch portion of the other cylinder than the position where the radius of curvature is the minimum.

Further, as shown in FIG. 6, the exhaust manifold 1
In the configuration in which the respective branch portions 1a and 1b are connected in a substantially Y shape, the radius of curvature of the inner pipe 4 becomes the minimum in the vicinity of the confluence portion, and large bending deformation occurs here, so at the tip 4a on the cylinder head 2 side. A deviation occurs between the center position of the inner pipe 4 and the center position of the outer pipe 5. Therefore, in the embodiment of FIG. 6, the support portion 11 is arranged not at the confluence portion where the radius of curvature is the smallest but at a position that is moved to some extent toward the tip 4a on the cylinder head 2 side. This suppresses radial displacement of the tip 4a of the inner tube 4.

Further, the cross-sectional area of the inner pipe 4 expands due to the heat of the exhaust gas, which also deteriorates the durability. Therefore,
In the case where the radial deformation of the inner pipe 4 is the main cause of the deterioration of durability, it is preferable to dispose the support portion 11 on the straight portion of the exhaust pipe as shown in FIG. 7. That is, since the exhaust gas flows smoothly in a straight line in the straight portion of the exhaust pipe, the heat transfer coefficient is small and the temperature of the inner pipe 4 becomes relatively low. Therefore, the inner pipe 4 can be supported without forcibly restraining the thermal expansion of the inner pipe 4 in the radial direction, and a decrease in durability of the inner pipe 4 can be prevented.

FIG. 8 shows the overall structure when the present invention is applied to the exhaust manifold 1 of a V-type 6-cylinder internal combustion engine in consideration of the above-mentioned various elements. This exhaust manifold 1
Is equipped with three branch portions 1a, 1b, 1c so as to join the exhaust gases of the three cylinders, and these are gathered into one exhaust pipe. Three support portions 11a, 11b, 11c are provided as a whole so as to support each of the portions 1a, 1b, 1c. Regarding the branch portion 1b at the center of the figure, since the expansion and contraction deformation and the bending deformation of the inner pipe 4 are relatively small, in consideration of the radial expansion of the inner pipe 4, the support portion 11b is provided on the linear portion with the smallest radial expansion. It is arranged. In this support portion 11b, the tip of the boss portion 12 forming the support portion 11b and the inner pipe 4 are metal-bonded to each other, and the inner pipe 4 and the outer pipe 5 are firmly coupled and fixed in a mutually immovable state. There is.

Further, in the branch portion 1c of the cylinder on the right side of the drawing, since the inner pipe 4 extends long, the expansion and contraction deformation of the inner pipe 4 becomes a problem mainly. Therefore, the support portion 11c is arranged at an intermediate position where the expansion and contraction deformation of the inner pipe 4 can be effectively distributed. Similar to the support portion 11b, the support portion 11c has the inner tube 4 and the boss portion 12 metal-bonded to each other.

Regarding the branch portion 11a of the cylinder on the left side of the drawing, both expansion and contraction deformation and bending deformation of the inner pipe 4 pose a problem, so the expansion and contraction deformation is effectively distributed in the vicinity of the region where the radius of curvature is the minimum. The supporting portion 11a is arranged at a position where it can be formed. In this support portion 11a, the deformation of the inner tube 4 is larger than that of the support portions 11b and 11c at the other two places, so that the boss portion 1
2 The tip and the inner tube 4 are not metal-bonded, and are in a non-bonded state to allow sliding in the axial direction.

FIG. 9 shows an embodiment in which the present invention is applied to the exhaust manifold 1 of an in-line 6-cylinder internal combustion engine. In this embodiment, support portions 11a to 11f are provided in the middle portions of the branch portions 1a to 1f of each cylinder, respectively, and the support portion 1 is provided near the outlet of the exhaust manifold 1.
1g is provided. That is, a total of seven support parts 1
The inner tube 4 is supported by the outer tube 5 by means of 1. here,
The support portions 11a to 11f of the branch portions 1a to 1f are in a state where the boss portion 12 is metal-bonded to the inner pipe 4, but the support portion 11g near the outlet portion is metal-bonded to the inner pipe 4. Instead, the sliding in the axial direction is allowed.

Next, a method of manufacturing the above-mentioned double exhaust pipe will be described with reference to FIGS. In addition, in order to simplify the description, the exhaust pipe is shown as a simple linear pipe in each drawing.

FIG. 10 shows an inner tube core 21 used for casting the outer tube 5 and an inner tube core mold 23 for manufacturing the inner tube core 21. The inner pipe core mold 23 is, for example, divided into four parts in the vertical and horizontal directions as shown in the drawing, and a concave portion 23 corresponding to the required outer shape of the inner pipe core 21 is provided inside thereof.
a is formed, and a plurality of protrusions 24 are provided on the inner peripheral surface thereof in correspondence with the desired formation positions of the support portions 11. The projections 24 serve to position the inner tube 4 when manufacturing the core, and finally form the boss portion 12. The inner pipe 4 formed by press-forming and welding a steel pipe or a steel plate is set in the inner pipe core mold 23, and the surrounding space is filled with the core sand 22, so that the inner pipe as shown in the figure. The core 21 is formed. Then, a coating mold 26 (see FIG. 11) made of an appropriate material is applied to the surface of the inner tube core 21 taken out from the inner tube core mold 23.

FIG. 11 shows a cross-sectional shape of the inner pipe core 21 manufactured as described above. As shown in FIG. 11, the inner tube core 21 formed by covering the inner tube 4 with the core sand 22 is formed with a recess 25 corresponding to the boss portion 12 by the projection 24. Here, the boss portion 12 is connected to the inner pipe 4
In the case of firmly metal-bonding the inner tube 4 to the inner surface of the recess 25 as shown in FIG. 11, the bottom surface of the recess 25 is not coated with the coating mold 26. Further, when the boss portion 12 can be slid without being metal-bonded to the inner pipe 4, the coating mold 26 may be applied to the bottom surface of the recess 25.

Next, the inner pipe core 21 is set in a mold 27 having a mold surface 27a corresponding to the outer shape of the outer pipe 5, as shown in FIG. To do. As a result, the inner pipe 4 is cast, and an exhaust pipe having a double structure is formed. FIG. 13 shows the mold 27 after cooling.
It shows the product taken out from, and the double exhaust pipe is completed by machining unnecessary parts. The core sand 22 inside the outer pipe 5 is taken out from both ends of the exhaust pipe,
At this time, since the support 11 is in the middle and the both ends are open, the core sand 22 can be reliably and easily discharged from the gap between the inner pipe 4 and the outer pipe 5. Therefore, it is not necessary to provide a hole for sand removal in the outer tube 5.

In each of the above-mentioned embodiments, the supporting portion 11
Is constituted by the boss portion 12 cast as a part of the outer pipe 5, but as in the embodiment shown in FIG. 14 and FIG.
The boss portion 31 may be formed on the side. In particular, in this embodiment, the inner tube 4 is a joined product of steel plates formed by press forming, and the boss portion 31 is formed by press forming. However, the bending and deforming portion 32 that is bent in a substantially U-shaped cross section before and after the boss portion 31. Are molded at the same time. By providing the bending deformation portions 32 in front of and behind the boss portion 31 in this way, it is possible to avoid stress concentration in the boss portion 31, particularly in the joint boundary portion 31 a with the outer pipe 5, when the inner pipe 4 is thermally deformed. The durability of the portion 31 is further improved.

Further, as shown in FIG. 14, the outer pipe 5 is in a state where the tip of the boss portion 31 slightly bites into the inner peripheral surface of the outer pipe 5.
Can be cast, and the support portion 11 associated with the dimensional error of the inner pipe 4 is likely to occur when the inner pipe 4 is a press-molded product.
It is possible to avoid the occurrence of gaps in the. That is, as shown in FIG. 10, when the boss portion 12 is integrally formed on the outer pipe 5 side, the outer peripheral surface of the inner pipe 4 must be accurately in contact with the projection 24 of the inner pipe core mold 23. After casting, a gap is generated between the tip of the boss portion 12 and the inner pipe 4. Therefore, the inner pipe 4
When is formed by welding press-formed products, high accuracy control is required during press-forming and welding. On the other hand, if the boss portion 31 is provided on the inner pipe 4 side as shown in FIG. 14 and the outer pipe 5 is cast so as to slightly cover the tip of the boss portion 31, accuracy control of the inner pipe 4 becomes very easy.

In order to obtain the double exhaust pipe shown in FIG. 14, in the inner pipe core used when casting the outer pipe 5, the tip of the boss portion 31 may be slightly projected from the surface of the core sand. .
FIG. 16 shows an inner tube core mold 23 for forming such an inner tube core. The inner tube core mold 23 has a shallow recess 33 into which the tip of a boss 31 formed in the inner tube 4 fits.
Are formed in advance.

In this embodiment, the inner pipe 4 is formed by press-forming steel plates and welded to each other as described above. As shown in FIG. It is set in the tube core mold 23. Then, as in the above-mentioned embodiment, the inner tube core is formed by filling the gap in the inner tube core mold 23 with the core sand. In this inner tube core, the boss portion 31 slightly protrudes from the outer peripheral surface made of core sand and is always exposed. And
If this inner tube core is set in a mold and cast,
As shown in FIG. 4, the boss portion 31 is securely joined to the inner peripheral surface of the outer tube 5.

[0044]

As is apparent from the above description, in the double exhaust pipe of the internal combustion engine according to the present invention, both ends of the inner pipe are not restrained as in the conventional case, and the thermal deformation of the inner pipe is supported. Since it is distributed to both sides of the part, it is possible to prevent the durability of the inner pipe from being lowered.

According to the second aspect of the present invention, when the inner pipe is supported at a plurality of points, the thermal deformation generated between the supporting portions is not forcibly restrained, and the deterioration of the durability of the inner pipe is prevented. it can. Further, it is not necessary to form a hole for sand removal in the outer tube, and sand removal after casting can be easily performed.

According to the third aspect of the present invention, it is possible to prevent the durability of the inner pipe from deteriorating at a location where expansion and contraction of the inner pipe poses a problem.

Further, according to the structure of claim 4, it is possible to prevent the durability of the inner pipe from being deteriorated in the curved portion where the bending deformation of the inner pipe poses a problem.

Further, according to the structure of claim 5, it is possible to prevent the durability of the inner pipe from being deteriorated due to the thermal deformation of the inner pipe in the radial direction.

According to the structure of claim 6, it is possible to easily select whether or not the inner tube and the outer tube are firmly metal-bonded to each other in the support portion.

According to the seventh aspect of the invention, since the support portion is constituted by the boss portion formed on the inner pipe side, the durability of the support portion is further improved.

According to the manufacturing method of the eighth aspect, even if there is some error in the dimension of the boss portion formed on the inner pipe, the tip of the boss portion can be reliably joined to the outer pipe.

Further, according to the manufacturing method of the ninth aspect, the inner pipe can be formed by welding.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a double exhaust pipe according to the present invention.

FIG. 2 is a sectional view taken along the line AA of FIG.

FIG. 3 is a cross-sectional view showing an embodiment in which a support portion is arranged between an end portion on the cylinder head side and a confluence of another cylinder.

FIG. 4 is a cross-sectional view showing an embodiment in which a support portion is arranged in a curved portion.

FIG. 5 is a cross-sectional view showing another arrangement example of the support portion.

FIG. 6 is a cross-sectional view showing still another arrangement example of the support portions.

FIG. 7 is a cross-sectional view showing an example in which a support portion is arranged in a straight portion of an exhaust pipe.

FIG. 8 is a sectional view showing an embodiment in which the present invention is applied to an exhaust manifold for a V-type 6 cylinder internal combustion engine.

FIG. 9 is a sectional view showing an embodiment in which the present invention is applied to an exhaust manifold for an in-line 6-cylinder internal combustion engine.

FIG. 10 is a perspective view showing an inner pipe core and an inner pipe core mold for manufacturing a double exhaust pipe.

FIG. 11 is a sectional view of the inner tube core.

FIG. 12 is a cross-sectional view showing a state where an inner tube core is set in a mold.

FIG. 13 is a cross-sectional view showing a state in which a cast product is taken out from the mold.

FIG. 14 is a cross-sectional view of essential parts showing an embodiment in which a boss portion is provided on the inner pipe side.

15 is a cross-sectional view taken along the line BB of FIG.

FIG. 16 is a cross-sectional view showing an inner pipe core mold used in manufacturing the double exhaust pipe of this embodiment.

[Explanation of symbols]

 1 ... Exhaust manifold 4 ... Inner pipe 5 ... Outer pipe 11 ... Support part 12 ... Boss part

Claims (9)

[Claims] 1. An intermediate portion comprising a cast outer pipe having a joint flange portion at its end, and an inner pipe disposed inside the outer pipe with a gap and being surrounded by the outer pipe. In a dual exhaust pipe of an internal combustion engine having a curved portion at both ends, both ends of the inner pipe are set as free ends, and the inner pipe is supported by the outer pipe at a portion other than the both ends. trachea. 2. A plurality of supporting portions provided between the inner pipe and the outer pipe, at least one portion of which is fixed to the inner pipe and the outer pipe so as to be immovable from each other, and the remaining portions are fixed. 2. The double exhaust pipe for an internal combustion engine according to claim 1, wherein the support portion of the internal combustion engine is in a disengaged state so as to allow sliding in the axial direction. 3. A structure in which a plurality of exhaust pipes of each cylinder connected to a cylinder head are joined together, and an intermediate portion between an end portion of the exhaust pipe on the cylinder head side and a joining portion that joins exhaust gas from another cylinder. The double exhaust pipe of the internal combustion engine according to claim 1 or 2, wherein a support portion is provided in the vicinity thereof. 4. The double exhaust pipe for an internal combustion engine according to claim 1, wherein a support portion is provided at a portion of the curved exhaust pipe portion where the radius of curvature of the inner pipe is minimum. 5. The double exhaust pipe for an internal combustion engine according to claim 1, wherein a support portion is provided on a straight portion of the exhaust pipe. 6. When casting the outer pipe, a boss portion is formed from the inner peripheral surface of the outer pipe so as to contact the outer peripheral surface of the inner pipe, and the inner pipe is supported by the boss portion. A dual exhaust pipe for an internal combustion engine according to any one of claims 1 to 4. 7. A boss portion projecting to the outer peripheral side so as to come into contact with the inner peripheral surface of the outer pipe is formed at a predetermined position of the inner pipe, and the inner pipe is supported by this boss portion. Item 5. A double exhaust pipe for an internal combustion engine according to any one of items 1 to 4. 8. A step of press-forming a steel sheet into a substantially semi-circular cross section, and at the time of this press-forming, forming a boss portion at a predetermined position and fixing it to form an inner pipe having a predetermined shape, and the tip of the boss portion. Set the inner tube in the inner tube core mold with a shallow recess that fits with, and fill the core sand with the outer circumference of the inner tube to expose the boss tip. And a step of setting the inner tube core in a mold and casting an outer tube, the method for producing a double exhaust pipe of an internal combustion engine. 9. The double exhaust pipe for an internal combustion engine according to claim 8, wherein in the inner pipe forming step, a steel plate press-formed into a substantially semicircular cross section is welded to form the inner pipe. Manufacturing method.