JP2007138811A - Exhaust pipe for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust pipe capable of keeping conversion performance of exhaust emission control catalyst high by optimizing flow-in style of an exhaust gas to the exhaust emission control catalyst. <P>SOLUTION: This exhaust pipe 20 for the internal combustion engine establishes communication between an exhaust port 9 of an engine main body and the exhaust emission control catalyst 13 and includes a flow change section in which direction of flow of whole exhaust gas flowing in the exhaust pipe changes. The exhaust pipe 20 includes a restriction part 24 in a right downstream of the flow change section and flow path cross section area of the restriction part is smaller than flow path cross section area of the exhaust pipe in a right upstream of the restriction part. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an exhaust pipe of an internal combustion engine.

In general, the exhaust gas discharged from the engine body is contained harmful components such as hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxides (NO _X). For this reason, in many internal combustion engines, exhaust gas discharged from the engine body is allowed to flow into an exhaust purification catalyst (oxidation catalyst, three-way catalyst, NO _x storage catalyst, etc.) and these harmful components are purified in the exhaust purification catalyst. Exhaust gas is released into the atmosphere.

The purification performance of these harmful components in the exhaust purification catalyst varies depending on various factors. These factors include, for example, the configuration of the exhaust purification catalyst, the degree of deterioration of the exhaust purification catalyst, the air-fuel ratio of the inflowing exhaust gas, the temperature of the exhaust purification catalyst, and the like. For example, when the amount of noble metal supported on the exhaust purification catalyst is small, the oxidation / reduction reaction is hardly promoted on the exhaust purification catalyst, and thus the purification performance of the exhaust purification catalyst is lowered. Further, when the exhaust purification catalyst is poisoned and deteriorated by components such as lead (Pb), phosphorus (P) or sulfur (S), the oxygen storage capacity and NO _X storage capacity of the exhaust purification catalyst are lowered, and the exhaust purification catalyst. When the gas deteriorates due to heat, the oxidation / reduction ability of the noble metal decreases, and the purification performance of the exhaust purification catalyst decreases.

Further, for example, in the device described in Patent Document 1, in order to improve the purification performance of the exhaust purification catalyst, the intake pipe between the reducing agent addition device provided on the exhaust upstream side of the NO _x storage catalyst and the exhaust purification catalyst. Is provided with an aperture. Although the reducing agent adding device is used for adding a reducing agent to exhaust gas flowing into the NO _X storage catalyst in order to disengage the NO _X which is stored in _the NO _X storage catalyst, the reducing agent addition device in the apparatus When the reducing agent is added from the above, vaporization of the reducing agent is promoted when the exhaust gas containing the reducing agent passes through the throttle portion. For this reason, the reducing agent contained in the exhaust gas flowing into the NO _x storage catalyst is sufficiently vaporized, so that NO _x can be efficiently desorbed from the NO _x storage catalyst and reduced. agent is prevented from get through the the NO _X storing catalyst without being used in the NO _X storing catalyst.

JP 2002-213233 A Japanese Patent Laid-Open No. 10-141053

  The purification performance of these harmful components in the exhaust purification catalyst also varies depending on the flow of exhaust gas flowing into the exhaust purification catalyst. For example, if the flow of exhaust gas flowing into the exhaust purification catalyst is biased, that is, if a large amount of exhaust gas flows through only a specific region of the exhaust purification catalyst, the exhaust purification catalyst will greatly deteriorate in that specific region. End up. After that, if a lot of exhaust gas circulates only in a specific region of the exhaust purification catalyst, the exhaust purification catalyst as a whole cannot sufficiently purify harmful components in the exhaust gas even though the degree of deterioration is small. End up.

  The flow rate of the exhaust gas flowing into the exhaust purification catalyst differs depending on the region of the exhaust purification catalyst. If the maximum flow rate (hereinafter referred to as “maximum flow rate”) is too fast, While passing through the exhaust purification catalyst, some of the harmful components in the exhaust gas may not be purified, and therefore the harmful components in the exhaust gas cannot be sufficiently purified.

  In this way, if the flow of exhaust gas flowing into the exhaust purification catalyst is uneven or the maximum flow velocity is too fast, the purification performance of harmful components in the exhaust purification catalyst is lowered. For this reason, in order to maintain the purification performance of the exhaust purification catalyst, it is necessary to uniformly flow the exhaust gas into the exhaust purification catalyst without being biased and to keep the maximum flow rate of the exhaust gas low.

  Therefore, an object of the present invention is to provide an exhaust pipe of an internal combustion engine that can maintain the purification performance of the exhaust purification catalyst high by optimizing the flow form of the exhaust gas into the exhaust purification catalyst.

In order to solve the above-described problem, in the first invention, an exhaust pipe of an internal combustion engine that communicates an exhaust port of an engine body with an exhaust purification catalyst, the flow direction of the entire exhaust gas flowing through the exhaust pipe changes. An exhaust pipe of an internal combustion engine having a flow changing portion having a throttle portion immediately downstream of the flow changing portion, wherein a flow passage cross-sectional area of the throttle portion is a flow passage cross-sectional area of the exhaust pipe immediately upstream of the throttle portion. Smaller than.
According to the first aspect, since the throttle portion is provided immediately downstream of the flow change portion of the exhaust pipe, a swirling flow is generated in the throttle portion. As a result, there is no uneven flow of the exhaust gas downstream of the throttle portion, and the exhaust gas flows relatively uniformly into the exhaust purification catalyst. Further, if the flow passage cross-sectional area of the exhaust pipe downstream of the throttle portion is not smaller than the flow passage cross-sectional area of the exhaust pipe upstream of the throttle portion, the exhaust gas flowing into the exhaust purification catalyst due to the presence of the throttle portion The maximum flow rate can be relatively slow.

  According to a second invention, in the first invention, the exhaust pipe has a plurality of exhaust branch pipes communicating with the exhaust ports of the respective cylinders, and one collective exhaust pipe communicating with all of the exhaust branch pipes at the communicating portion. The throttle portion is provided in the collecting exhaust pipe on the downstream side of the communication portion.

  In the third invention, in the first or second invention, the shape of the throttle portion is such that the maximum flow rate of the exhaust gas flowing into the exhaust purification catalyst disposed downstream of the throttle portion is equal to or lower than a predetermined flow rate. The shape is as follows.

  According to a fourth invention, in any one of the first to third inventions, the throttle portion includes an upstream taper portion whose flow path cross-sectional area gradually decreases toward the downstream in the axial direction of the throttle portion. And a downstream taper portion that is provided on the downstream side of the upstream taper portion and whose flow passage cross-sectional area gradually increases toward the downstream in the axial direction of the throttle portion.

  According to a fifth aspect, in the fourth aspect, the ratio of the volume of the exhaust passage defined by the upstream taper portion and the volume of the exhaust passage defined by the downstream taper portion is approximately 2. : 5.

In order to solve the above-described problem, in the sixth aspect of the invention, in the exhaust pipe of the internal combustion engine that communicates the exhaust port of the engine body and the exhaust purification catalyst, a throttle portion whose flow passage cross-sectional area gradually decreases toward the downstream is provided. And the throttle part is arranged so that the exhaust purification catalyst is located immediately downstream of the throttle part.
According to the sixth aspect of the invention, the exhaust purification catalyst is disposed immediately downstream of the throttle portion where the flow path cross-sectional area gradually decreases toward the downstream. Since the flow of the exhaust gas having a bias also passes through the throttle portion, it becomes a relatively uniform flow, so that the exhaust gas flows into the exhaust purification catalyst relatively uniformly.

  According to the first to fifth inventions, the exhaust gas can flow relatively uniformly into the exhaust purification catalyst, and the maximum flow rate of the exhaust gas flowing into the exhaust purification catalyst can be made relatively slow. As a result, the purification performance of the exhaust purification catalyst can be maintained high.

  According to the sixth aspect of the invention, the exhaust gas flows into the exhaust purification catalyst relatively uniformly, so that the purification performance of the exhaust purification catalyst can be maintained high.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a view showing an entire internal combustion engine provided with an exhaust pipe of the present invention.

  Referring to FIG. 1, 1 is an engine body, 2 is a cylinder block, 3 is a piston that reciprocates in the cylinder block 2, 4 is a cylinder head fixed on the cylinder block 2, and 5 is a piston 3 and a cylinder head 4. A combustion chamber formed therebetween, 6 is an intake valve, 7 is an intake port, 8 is an exhaust valve, and 9 is an exhaust port. As shown in FIG. 1, a spark plug 10 is arranged at the center of the inner wall surface of the cylinder head 4, and a fuel injection valve 11 is arranged around the inner wall surface of the cylinder head 4. A cavity 12 extending from the lower side of the fuel injection valve 11 to the lower side of the spark plug 10 is formed on the top surface of the piston 3.

The exhaust port 9 of each cylinder is connected to an exhaust manifold (exhaust pipe) 20 containing an upstream side exhaust purification catalyst 13, and this exhaust manifold 20 contains a downstream side exhaust purification catalyst 15 via a downstream side exhaust pipe 14. Connected to the casing 16. The upstream side exhaust purification catalyst 13 includes a three-way catalyst supporting noble metal or the like, a NO _x storage agent in addition to the noble metal etc. occluding NO _X in, NO _X storage catalyst such as the air-fuel ratio is carrying approximately components to disengage and the NO _X that occluded into the exhaust gas to the stoichiometric air-fuel ratio or rich) of the inflowing exhaust gas is used .

  2 is a schematic perspective view of the exhaust manifold 20, and FIG. 3 is a cross-sectional view of the exhaust manifold 20 taken along line III-III in FIG. As shown in FIG. 2, the exhaust manifold 20 has an exhaust branch pipe 21 connected to each exhaust port 9 of the engine body 1 and a collective exhaust pipe 22 connected to all the exhaust branch pipes 21. Accordingly, exhaust gas discharged from the corresponding exhaust port 9 flows through each exhaust branch pipe 21, but exhaust gas discharged from all the exhaust ports 9 flows through the collective exhaust pipe 22.

  As shown in FIG. 3, the collective exhaust pipe 22 includes a connecting portion 23 to which the exhaust branch pipe 21 is connected, a throttle portion 24, and a catalyst built-in portion 25 in which the upstream side exhaust purification catalyst 13 is built. Have. The connecting portion 23, the throttle portion 24, and the catalyst built-in portion 25 each have an axis and are arranged so that these axes are aligned (hereinafter, these aligned axes are collectively referred to as “axis A”). The connecting portion 23 is formed in a substantially cylindrical shape around the axis A. Further, a plurality of exhaust branch pipes 21 are connected to the connection part 23 substantially perpendicularly to the axis A of the connection part 23, and the plurality of exhaust branch pipes 21 are spaced apart at predetermined intervals in the circumferential direction. Connected to

  The throttle portion 24 is tapered so that the cross-sectional area (hereinafter simply referred to as “flow-path cross-sectional area”) perpendicular to the axis A through which the exhaust gas can flow is gradually reduced toward the downstream side. Are provided on the upstream side of the exhaust taper 24a (hereinafter simply referred to as “downstream side”) and the cross-sectional area of the flow path gradually increases toward the downstream side. And a downstream taper portion 24b that is tapered in this manner. In the present embodiment, the upstream taper portion 24a has a hollow truncated cone shape whose diameter gradually decreases toward the downstream side, and the downstream taper portion 24b gradually increases in diameter toward the downstream side. It has a hollow truncated cone shape. For this reason, the flow passage cross-sectional area of the throttle portion 24 is the smallest in the connecting portion 24c between the upstream taper portion 24a and the downstream taper portion 24b (hereinafter, this connecting portion is referred to as the “minimum cross-sectional area portion 24c”). ).

In the present embodiment, the diameter D ₁ of the minimum cross-sectional area portion 24c is the diameter of the most upstream portion of the upstream taper portion 24a (hereinafter simply referred to as “upstream side”) (that is, the upstream side of the throttle portion 24). the diameter of the most downstream portion of the diameter) or downstream taper portion 24b of the connecting portion 23 located on (i.e., are approximately half of the catalyst internal portion diameter of 25) D ₂ positioned on the downstream side of the throttle portion 24 . Accordingly, the flow passage cross-sectional area of the minimum cross-sectional area portion 24c is approximately 1 of the flow cross-sectional area of the most upstream portion of the upstream taper portion 24a or the flow cross-sectional area of the most downstream portion of the downstream taper portion 24b. / 4.

Further, the ratio between the length L ₁ in the axial direction of the upstream taper portion 24a and the length L ₂ in the axial direction of the downstream taper portion 24b is approximately 2: 5 (L ₁ : L ₂ = 2: 5). In the present embodiment, the flow passage cross-sectional area of the most upstream portion of the upstream taper portion 24a and the flow passage cross-sectional area of the most downstream portion of the downstream taper portion 24b are substantially the same. For this reason, the volume of the exhaust passage defined by the upstream taper portion 24a and the volume of the exhaust passage defined by the downstream taper portion 24b are approximately 2: 5 as described above.

  The catalyst built-in portion 25 is also formed in a substantially cylindrical shape around the axis A, like the connecting portion 23. An upstream side exhaust purification catalyst 13 is built in the catalyst built-in portion 25. The upstream side exhaust purification catalyst 13 has an axis that is substantially coaxial with the axis A of the catalyst built-in portion 25, and a gap is not formed between the inner surface of the catalyst built-in portion 25 and the outer surface of the upstream side exhaust purification catalyst 13. It is accommodated in the catalyst built-in part 25. An exhaust pipe portion 25 a connected to the downstream exhaust pipe 14 is provided at the downstream end of the catalyst built-in part 25.

  As shown in FIG. 3, the upstream side exhaust purification catalyst 13 has a honeycomb structure, and has a plurality of exhaust flow passages 13a extending in parallel with each other between the upstream end and the downstream end. The exhaust gas flows along these exhaust flow passages 13a with hardly passing through the partition wall 13b between the exhaust flow passages 13a. That is, when the exhaust gas passes through the upstream side exhaust purification catalyst 13, it flows in the direction of the axis A of the upstream side exhaust purification catalyst 13.

  Next, the flow of exhaust gas in the exhaust manifold 20 configured as described above will be described with reference to FIGS. In the following description, the flow of exhaust gas will be described in comparison with the linear exhaust manifold 120 as shown in FIG. FIG. 4 is a schematic perspective view of the straight exhaust manifold 120, which is similar to FIG. The linear exhaust manifold 120 has substantially the same configuration as the exhaust manifold 20 of the above embodiment, but the linear exhaust manifold 120 does not have a throttle portion, and the flow passage cross-sectional area of the collective exhaust pipe 122. Is different from the exhaust manifold 20 of the above embodiment in that it remains constant.

  Exhaust gas discharged from the combustion chamber 5 of each cylinder flows into an exhaust branch pipe 21 corresponding to that cylinder via an exhaust port 9 corresponding to that cylinder. Thereafter, the exhaust gas flows through the exhaust branch pipe 21 to the connecting portion 23 of the collective exhaust pipe 22. Since the connecting portion 23 communicates with all the exhaust branch pipes 21, the exhaust gas discharged from the combustion chambers 5 of all the cylinders flows into the connecting portion 23 through the exhaust branch pipe 21. The exhaust gas flowing into the connecting portion 23 flows to the catalyst built-in portion 25 through the throttle portion 24 and passes through the exhaust flow passage 13a of the exhaust purification catalyst 13 accommodated in the catalyst built-in portion 25. Thereafter, the exhaust gas flows out to the downstream exhaust pipe 14 via the exhaust pipe portion 25a provided at the downstream end of the catalyst built-in portion 25.

  The schematic flow of the exhaust gas from the exhaust port 9 to the downstream exhaust pipe 14 is the same between the exhaust manifold 20 of the present embodiment and the linear exhaust manifold 120 shown in FIG. The flow in the exhaust pipes 22, 122 is different between the exhaust manifolds 20, 120. FIG. 5 is a diagram schematically showing the flow of exhaust gas passing through the linear exhaust manifold 120 and the exhaust manifold 20 of the present embodiment.

  In the straight type exhaust manifold 120, as indicated by arrows in FIG. 5A, the exhaust gas flowing from each exhaust branch pipe 121 into the collective exhaust pipe 122 changes its flow direction from a direction parallel to the exhaust branch pipe 121. The flow rapidly changes in a direction parallel to the collective exhaust pipe 122. In some cases, a tumble flow (exhaust gas flow around an axis perpendicular to the axis A) is formed in the collective exhaust pipe 122 with such a sudden change in the flow direction. When the exhaust gas flows in the collective exhaust pipe 122 in this way, the flow of the exhaust gas that has flowed into the collective exhaust pipe 122 from each cylinder is the upstream end face of the exhaust purification catalyst 13 (upstream of the partition wall 13b of the exhaust purification catalyst 13). (The flat surface on which the side end face is located) is biased, and the flow is not uniform over the entire upstream end face. That is, the exhaust gas flowing into the exhaust purification catalyst 13 through a certain region on the upstream end face of the exhaust purification catalyst 13 has a high flow rate per unit area and a high flow velocity, and passes through other regions. The exhaust gas flowing into the exhaust purification catalyst 13 has a small flow rate per unit area and a slow flow rate.

  6A is a diagram showing the relationship between the crank angle and the flow rate of the exhaust gas flowing into the exhaust purification catalyst 13 when the linear exhaust manifold 120 shown in FIG. 4 is used. The solid line in the figure indicates the exhaust gas flow rate (hereinafter referred to as “maximum flow rate”) having the fastest flow rate among the exhaust gases passing through the upstream end surface of the exhaust purification catalyst 13, and the broken line indicates the upstream end surface of the exhaust purification catalyst 13. The exhaust gas flow rate of the exhaust gas having the slowest flow rate (hereinafter referred to as “minimum flow rate”) of the exhaust gas passing through the exhaust gas is shown by a one-dot chain line as the average flow rate of the entire exhaust gas passing through the upstream end face of the exhaust purification catalyst 13 ( Hereinafter, these are respectively referred to as “average flow velocity”.

  As can be seen from FIG. 6 (a), the maximum flow velocity, the minimum flow velocity, and the average flow velocity increase and decrease as the crank angle advances. This is due to the opening and closing of the exhaust valve of each cylinder. In the example shown in FIG. 6, the case where a four-cylinder internal combustion engine is used is shown, so that it is raised and lowered four times per cycle (crank angle 720 °). ing. Here, in the example shown in FIG. 6A, the maximum flow velocity is extremely large around a crank angle of 180 °. This means that the flow rate of the exhaust gas passing through a part of the upstream end face of the exhaust purification catalyst 13 is extremely high. Thus, if the exhaust gas passing through a partial region of the upstream end face of the exhaust purification catalyst 13, that is, the exhaust gas passing through the exhaust flow passage 13a corresponding to this partial region, has a very high flow rate, such exhaust gas There are cases where harmful components in the exhaust gas are not sufficiently purified while the inside passes through the exhaust flow passage 13a.

  As described above, when the straight exhaust manifold 120 is used, the flow rate per unit area of the exhaust gas flowing into the exhaust purification catalyst 13 varies greatly depending on the region of the upstream end face of the exhaust purification catalyst 13. For this reason, the flow rate of the exhaust gas passing through a certain exhaust flow passage 13a becomes larger than the flow rate of the exhaust gas passing through another exhaust flow passage 13a.

  Here, the deterioration of the exhaust purification catalyst is caused by poisoning due to occlusion and adsorption of components such as lead (Pb), phosphorus (P) and sulfur (S) contained in the exhaust gas on the support of the exhaust purification catalyst. Alternatively, the exhaust purification catalyst carrier is deteriorated by sintering with heat. Therefore, a large amount of components such as lead, phosphorus and sulfur easily flows into the exhaust flow passage 13a where the flow rate of exhaust gas passing therethrough is large, and thus the carrier around the exhaust flow passage 13a is likely to be deteriorated due to poisoning. Further, in such an exhaust flow passage 13a, a large amount of high-temperature exhaust gas flows in, or there are many exothermic reactions that occur on the noble metal that is carried, so that the exhaust flow passage 13a is easily deteriorated by heat. As described above, the carrier around the exhaust flow passage 13a is easily deteriorated, and a large amount of exhaust gas flows after the deterioration, resulting in a reduction in exhaust gas purification efficiency. That is, if the flow rate per unit area of the exhaust gas varies greatly depending on the region of the upstream end face of the exhaust purification catalyst 13, the exhaust gas purification efficiency is lowered.

  On the other hand, in the exhaust manifold 20 of the present embodiment shown in FIG. 2, the exhaust gas flowing from the exhaust branch pipes 21 into the collective exhaust pipes 22 flows as shown by arrows in FIG. The direction suddenly changes from a direction parallel to the exhaust branch pipe 21 to a direction parallel to the collective exhaust pipe 22, and swirl flow (swirl; around the axis A) when passing through the upstream taper part 24a of the throttle part 24. Exhaust gas flow). And exhaust gas passes the downstream taper part 24b of the throttle part 24 with a swirl flow.

  More specifically, as the exhaust gas flowing into the collective exhaust pipe 22 from each exhaust branch pipe 21 hits the wall surface of the connecting portion 23, the flow direction changes from the direction parallel to the exhaust branch pipe 21 to the collective exhaust pipe 22. It changes rapidly in the parallel direction. That is, the connecting portion 23 constitutes a flow changing portion in which the flow direction of the entire exhaust gas flowing therethrough (the flow direction when the exhaust gas flow is viewed macroscopically) changes rapidly. This change in the flow direction of the exhaust gas is not completed in the connecting portion 23 but also occurs in the upstream taper portion 24a of the throttle portion 24 located immediately downstream of the connecting portion 23. In particular, in the upstream taper portion 24a of the throttle portion 24, since the wall surface is inclined toward the axis A, the exhaust gas flows along the wall surface toward the axis A, and thereby the upstream taper portion 24a. Inside, a swirl flow is generated in the exhaust gas. Thereafter, the exhaust gas passes through the minimum cross-sectional area portion 24c and flows in the downstream taper portion 24b with a swirl flow.

  As a result of the swirling flow generated in the throttle portion 24 in this way, the exhaust gas flowing into the upstream side exhaust purification catalyst 13 flows relatively uniformly over the entire upstream end face of the upstream side exhaust purification catalyst 13, and thus It is suppressed that the flow rate and flow velocity per unit area of the exhaust gas passing through each region of the upstream end face of the exhaust purification catalyst 13 are greatly different. Thus, in the exhaust manifold 20 of the present embodiment, a swirl flow is easily generated by providing the throttle portion 24, and accordingly, the flow velocity and flow rate of the exhaust gas passing between the exhaust flow passages 13a are greatly different. It is suppressed.

  FIG. 6B is a diagram showing the relationship between the crank angle and the flow rate of the exhaust gas flowing into the upstream side exhaust purification catalyst 13 when the exhaust manifold 20 of the present embodiment is used. In the figure, the solid line indicates the maximum flow velocity, the broken line indicates the minimum flow velocity, and the alternate long and short dash line indicates the average flow velocity. As can be seen from FIG. 6B, there is no crank angle at which the maximum flow velocity of the exhaust gas flowing into the upstream side exhaust purification catalyst 13 becomes extremely large. That is, the flow rate of the exhaust gas passing through the specific exhaust flow passage 13a is suppressed from becoming extremely fast. For this reason, in all exhaust flow passages 13a, harmful components in the exhaust gas are basically sufficiently purified while passing through the exhaust flow passages 13a.

  As described above, when the exhaust manifold 20 of the present embodiment is used, the flow rate per unit area of the exhaust gas flowing into the upstream side exhaust purification catalyst 13 depends on the region of the upstream end face of the exhaust purification catalyst 13. Large differences are suppressed. For this reason, the flow rate of the exhaust gas passing through the exhaust flow passage 13a is relatively equal for all the exhaust flow passages 13a. Therefore, only the specific exhaust flow passage 13a is not greatly deteriorated, and all the exhaust flow passages 13a are relatively uniformly deteriorated. Therefore, the exhaust gas accompanying the deterioration of the upstream side exhaust purification catalyst 13 is not affected. A reduction in the purification rate can be minimized.

  In the above embodiment, the ratio between the flow path cross-sectional area of the minimum cross-sectional area portion 24c and the flow path cross-sectional area of the connecting portion 23 or the flow path cross-sectional area of the catalyst built-in portion 25 is approximately 1: 4. This ratio is not limited to 1: 4, and any flow path cross-sectional area of the minimum cross-sectional area portion 24c is smaller than the flow path cross-sectional area of the connecting portion 23 or the catalyst built-in portion 25, and the minimum cross-sectional area portion 24c functions as a restriction. It may be a ratio.

  In the above embodiment, the ratio of the volume of the exhaust flow path defined by the upstream taper portion 24a and the volume of the exhaust flow path defined by the downstream taper portion 24b is approximately 2: 5. However, this ratio is not limited to 2: 5, and any ratio can be used as long as the minimum cross-sectional area portion 24c is provided. However, for example, when this ratio is 2: 1, the flow rate per unit area of the exhaust gas flowing into the upstream side exhaust purification catalyst 13 greatly varies depending on the region of the upstream end face of the exhaust purification catalyst 13. And the exhaust gas flows relatively uniformly over the entire upstream end face of the exhaust purification catalyst 13, but the maximum flow velocity of the exhaust gas flowing into the exhaust purification catalyst 13 is not relaxed and is relatively fast. It becomes a thing. On the other hand, when the ratio is about 1: 4 to 1: 1, the exhaust gas flows into the exhaust purification catalyst 13 relatively uniformly and the maximum flow rate is relaxed. Therefore, the ratio is 1: 4. It is preferable to be about ˜1: 1.

  In particular, it is preferable to determine the ratio of the volume and the ratio of the cross-sectional area of the flow path so that the maximum flow velocity is equal to or less than a predetermined limit flow velocity. Here, the predetermined limit flow rate is a flow rate at which the harmful component in the exhaust gas may not be partially purified while the exhaust gas passes through the upstream side exhaust purification catalyst 13 when the maximum flow rate is further increased. is there. By determining the ratio of the volume and the flow path cross-sectional area in this way, the exhaust gas purification rate by the upstream side exhaust purification catalyst 13 can be kept high.

  Further, in the above-described embodiment, the throttle portion 24 is provided immediately downstream of the connecting portion 23, that is, the portion where the exhaust branch pipes 21 gather. However, the position where the throttle portion 24 is disposed is not limited to such a position. However, the throttle portion 24 needs to be arranged immediately downstream of the flow direction changing portion in the exhaust pipe where the flow direction of the entire exhaust gas changes. By arranging the throttle portion 24 at such a position, the change in the flow direction continues in the upstream taper portion 24 a of the throttle portion 24, and as a result, a swirling flow is generated in the throttle portion 24. Because.

  Next, a second embodiment of the exhaust pipe of the present invention will be described with reference to FIG. FIG. 7 is a cross-sectional view of an exhaust manifold 50 similar to FIG. As shown in FIG. 7, the exhaust manifold 50 includes an exhaust branch pipe 51 connected to each exhaust port 9 of the engine body 1 and a collective exhaust pipe 52 communicating with all the exhaust branch pipes 51. Similar to the embodiment shown in FIG. 3, the collective exhaust pipe 52 includes a connecting part 53 to which the exhaust branch pipe 51 is connected, a throttle part 54, and a catalyst built-in part in which the upstream side exhaust purification catalyst 13 is built. 55. The connecting portion 53, the throttle portion 54, and the catalyst built-in portion 55 are arranged so that their axes are aligned.

  The connection part 53 and the catalyst built-in part 55 are configured in the same manner as the connection part 23 and the catalyst built-in part 25 of the first embodiment. However, in the present embodiment, the configuration of the aperture section 54 is different from the configuration of the aperture section 24 of the first embodiment.

  The throttle portion 54 has a tapered portion that is tapered so that the cross-sectional area of the flow path gradually decreases. In this embodiment, the tapered portion is a hollow truncated cone whose diameter gradually decreases toward the downstream side. It has become a shape. However, the throttle portion 54 does not have a portion corresponding to the downstream taper portion 24b in the first embodiment. Therefore, the flow path cross-sectional area is different between the upstream side and the downstream side of the throttle part 54, and the flow path cross-sectional area of the connecting part 53 arranged on the upstream side of the throttle part 54 is arranged on the downstream side of the throttle part 54. It is smaller than the cross-sectional area of the flow path of the catalyst built-in portion 25.

  As described above, by providing the throttle portion 54 having only the tapered portion so that the flow path cross-sectional area gradually decreases immediately upstream of the catalyst built-in portion 25, the exhaust gas flows into the entire upstream side exhaust purification catalyst 13. become.

  That is, when the cross-sectional area of the flow path is large as in the connecting portion 53, the flow rate of the exhaust gas flowing therein is not uniform over the entire cross-section of the flow path, Therefore, the flow rate tends to increase, and the flow rate tends to decrease locally in the other part of the cross section of the flow path. Therefore, for example, as shown in FIG. 4, when the flow passage cross-sectional area is equal from the connecting portion to the catalyst built-in portion without providing the throttle portion 54, it flows into the upstream side exhaust purification catalyst built in the catalyst built-in portion. The exhaust gas flow is biased, and the exhaust gas flow rate in a part of the upstream end face of the upstream side exhaust purification catalyst is high and the exhaust gas flow rate is low in the other part of the region. Therefore, a part of the upstream side exhaust purification catalyst is locally degraded.

  On the other hand, when the throttle portion 54 is provided on the exhaust downstream side of the connecting portion 53 as in the present embodiment, the flow rate of the exhaust gas becomes uniform in the cross section as the flow path cross-sectional area gradually decreases. To go. For this reason, when the exhaust gas passes through the minimum cross-sectional area portion located on the most downstream side of the throttle portion 54, the flow rate of the exhaust gas is relatively uniform over the entire cross-section of the minimum cross-sectional area portion. The upstream side exhaust purification catalyst 13 is provided immediately downstream of the throttle portion 54, so that the exhaust gas flows into the upstream side exhaust purification catalyst 13 relatively uniformly.

  In the above embodiment, the throttle portion 54 is disposed immediately downstream of the connecting portion 53, that is, immediately downstream of the flow direction changing portion of the exhaust gas, but the throttle portion 54 is immediately downstream of the connecting portion 53. It does not need to be arranged, and may be arranged on the downstream side of the linearly extending exhaust pipe. However, even in this case, it is necessary to dispose the catalyst built-in portion 55, that is, the exhaust purification catalyst, immediately downstream of the throttle portion 54. This is considered that the exhaust gas flows most evenly immediately downstream of the throttle portion 54, and when the upstream side exhaust purification catalyst is arranged at a position somewhat away from the throttle portion 54, it is upstream from the throttle portion 54. This is because the exhaust gas flow may become non-uniform again while the exhaust gas flows to the side exhaust purification catalyst.

It is a figure which shows the whole internal combustion engine provided with the exhaust pipe of this invention. It is a schematic perspective view of an exhaust manifold. FIG. 3 is a cross-sectional view of the exhaust manifold taken along line III-III in FIG. 2. 2 is a schematic perspective view of a straight exhaust manifold 120. FIG. It is the figure which showed typically the flow of the exhaust gas which passes through the inside of an exhaust manifold. It is the figure which showed the relationship between a crank angle and the flow velocity of the exhaust gas which flows into an exhaust purification catalyst. It is a schematic perspective view of the exhaust manifold of 2nd embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 13 Upstream side exhaust purification catalyst 14 Downstream side exhaust pipe 15 Downstream side exhaust purification catalyst 16 Casing 20 Exhaust manifold 21 Exhaust branch pipe 22 Collective exhaust pipe 23 Connection part 24 Restriction part 24a Upstream side taper part 24b Downstream side taper part 25 Catalyst built-in part

Claims (6)

In an exhaust pipe of an internal combustion engine that communicates an exhaust port of the engine body and an exhaust purification catalyst, the exhaust pipe of the internal combustion engine having a flow changing portion in which the flow direction of the entire exhaust gas flowing through the exhaust pipe changes.
An exhaust pipe having a throttle portion immediately downstream of the flow changing portion, wherein a flow passage cross-sectional area in the throttle portion is smaller than a flow passage cross-sectional area of the exhaust pipe immediately upstream of the throttle portion.   The exhaust pipe has a plurality of exhaust branch pipes communicating with the exhaust ports of the respective cylinders, and a single collective exhaust pipe communicating with all of the exhaust branch pipes at the communication part, and the throttle part is downstream of the communication part. The exhaust pipe according to claim 1, wherein the exhaust pipe is provided in the collective exhaust pipe.   The shape of the throttle portion is a shape such that a maximum flow rate of exhaust gas flowing into the exhaust purification catalyst disposed on the downstream side of the throttle portion is a predetermined flow rate or less. Exhaust pipe.   The throttle part is provided on the upstream taper part in which the channel cross-sectional area gradually decreases toward the downstream in the axial direction of the throttle part, and on the downstream side of the upstream taper part, and the channel cross-sectional area is The exhaust pipe according to any one of claims 1 to 3, further comprising a downstream tapered portion that gradually increases toward the downstream in the axial direction of the throttle portion.   The exhaust gas according to claim 4, wherein a ratio of a volume of the exhaust flow path defined by the upstream taper portion and a volume of the exhaust flow channel defined by the downstream taper portion is approximately 2: 5. tube.   An exhaust pipe of an internal combustion engine communicating with an exhaust port of an engine body and an exhaust purification catalyst has a throttle portion whose flow passage cross-sectional area gradually decreases toward the downstream in the axial direction of the exhaust pipe, and immediately downstream of the throttle portion An exhaust pipe in which the throttle part is disposed so that the exhaust purification catalyst is located in the exhaust pipe.

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