JP2003314269A - Exhaust gas purification device - Google Patents

Abstract

(57) [Summary] In an engine exhaust purification system using an HC trap catalyst, HC desorption from the HC trap is started before the three-way catalyst reaches the activation temperature after the cold start of the engine. HC that could easily be discharged later could not be sufficiently purified. SOLUTION: A slit is formed so as to cross the cells of the honeycomb-shaped carrier and to be unevenly distributed in a downstream portion of the carrier,
An HC trap agent is uniformly supported on the carrier, and a catalyst metal is supported so that the amount of the HC trapping agent is large at an upstream portion. Since the temperature of the upstream portion of the carrier easily rises due to contact with the exhaust gas, by supporting a large amount of the catalyst metal in the upstream portion, the time required to reach the activation temperature can be shortened, and the conversion efficiency of the desorbed HC can be increased. Although the carrying amount of the catalyst metal is relatively small in the downstream portion of the carrier, the required HC purification performance can be secured by increasing the conversion efficiency of the catalyst metal by the action of turbulence by the slit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalytic converter mainly applied to engine exhaust purification, and more particularly to a HC converter.
The present invention relates to an improvement of an exhaust emission control device having a trap function.

[0002]

2. Description of the Related Art In an exhaust gas purification system equipped with a three-way catalyst, treatment of HC during cold engine operation has become a problem. This is because HC that is easily discharged during combustion under low temperature conditions cannot be sufficiently purified until the catalyst reaches the activation temperature. As a solution to this problem, JP-A-9-85049 proposes a purification system to which an HC trapping agent is applied. The HC trapping agent has a function of temporarily trapping HC in the exhaust gas, and the trapped HC is desorbed from the trapping agent as the temperature rises. Therefore, in the above system, an HC trapping agent is provided on the upstream side of the three-way catalyst, and H
The emission of C is controlled. However, even if the trapped HC reaches the temperature at which desorption from the trapping agent starts, the three-way catalyst on the downstream side is not yet sufficiently activated at this point, so the effect of suppressing HC emission is not sufficient. Absent.

It is an object of the present invention to provide an exhaust emission control device which can more surely suppress the emission of HC even during cold engine operation.

[0004]

SUMMARY OF THE INVENTION In the present invention, one or more slits are formed so as to traverse cells of a honeycomb-shaped carrier and be unevenly distributed in a downstream portion of the carrier. Further, the carrier is loaded with the HC trapping agent, and catalytic metals such as Pt, Rh, and Pd are loaded so that the loaded amount becomes large in the upstream portion. Since the temperature of the upstream portion of the carrier easily rises due to contact with the exhaust gas, by loading a large amount of catalytic metal on the upstream portion, the time required to reach the activation temperature can be shortened and the conversion efficiency of desorbed HC can be increased.

Although the amount of the catalytic metal carried on the downstream side of the carrier is relatively small, the conversion efficiency of the catalytic metal can be increased by the effect of the turbulent flow due to the slit, so that the required HC purification performance can be secured. . That is, in the conventional exhaust emission control device, since each of a large number of cells formed in a honeycomb shape is continuous from the inlet part to the outlet part of the carrier, the gas flow in the cells is relatively regular and less disturbed. On the other hand, if a slit is formed in the carrier so as to cross the cell, the gas that has entered the cell divided in the flow direction by the slit causes turbulence at the slit portion. This turbulence increases the chances of the exhaust gas coming into contact with the catalyst metal, and thus the HC
Purification performance is improved.

Further, in the exhaust gas purifying device having a continuous cell structure having no slit, the flow rate distribution and the temperature rising property of the gas flow passage are uneven on the cross section of the carrier, in other words, the HC trap performance and the conversion rate are different for each cell. It is difficult to use up the original performance of the exhaust gas purification device because of the unevenness in the exhaust gas. On the other hand, in the case of a carrier provided with slits, the turbulent flow action makes it possible to make the gas flow rate distribution in the cross-sectional direction of the carrier uniform, so that the controllability is improved and the original performance of the exhaust gas purification device can be fully exhibited. Become.

Since the slit formed in the carrier has an effect of blocking heat transfer in the carrier to the downstream side, the temperature rise is delayed by that much to increase the amount of trapped HC and the time until HC is desorbed. Can be long. Further, this allows time for the temperature of the catalytic metal to rise, so that the treatment efficiency at the time of HC desorption is also improved.

Incidentally, as will be described in detail below as an embodiment, the characteristic peculiar to each can be set according to the shape and size of the slit. The same applies to the catalyst metal and the HC trapping agent coated on the carrier, and various preferable characteristics can be given depending on the setting of the layer structure.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of an engine system to which an exhaust emission control device according to the present invention is applied. In the figure, 1 is an engine, 2 is its intake passage, and 3 is an exhaust passage. Reference numeral 4 is an EGR passage for returning a part of the exhaust gas from the exhaust passage 3 to the intake passage 2, and 5 is an EGR control valve for controlling the exhaust gas recirculation amount. Reference numeral 7 is a fuel injection valve, and 8 is a spark plug.

Reference numerals 9 to 11 denote exhaust gas purification devices provided in the exhaust passage 3. These three exhaust emission control devices 9-11 are
Basically, it is a three-way catalyst that has both the oxidation function of CO and HC and the reduction function of NOx, and any or all of them have the function of temporarily capturing HC by an HC trapping agent such as zeolite. It is configured as a provided HC trap catalyst. The upstream side 9 or 10 may be composed of an HC trap device having only an HC trap function.

Reference numerals 12 and 13 are exhaust gas sensors for detecting the air-fuel ratio or oxygen concentration of the exhaust gas upstream and downstream of the uppermost exhaust purification device 9, respectively, and 14 is a temperature for detecting the catalyst temperature of the exhaust purification device 10 in the middle stage. It is a sensor.

Reference numeral 15 is a controller for controlling the air-fuel ratio and ignition timing on the basis of operating state signals such as engine speed and intake air amount, and is composed of a microcomputer including a CPU and its peripheral devices.

An embodiment of the honeycomb carrier 21 housed in the exhaust gas purification devices 9 to 11 is shown in FIG. In the present invention, basically, as shown in FIG. 2, one or more slits 2 are formed so as to cross the gas flow (direction of the arrow) of the carrier 21.
2 is formed so as to be unevenly distributed in the downstream portion of the carrier. in this case,
The slits 22 are arranged in the carrier 21 so that the distribution of the plurality of slits 22 in the gas flow direction becomes dense on the downstream side. When the carrier 21 is made of ceramics, the slits 22 are processed by grinding after firing. In the illustrated example, the slit 22 is formed so as to be orthogonal to the gas flow from one side surface side of the catalyst, and the non-slit portion 23 is left on the other side surface side. Further, the adjacent slits 22 are formed at different angles by 180 degrees around the carrier 21 and at equal intervals in the gas flow direction so that slits and non-slit portions are arranged alternately. On the cell surface of the carrier 21, an HC trap layer containing an HC trapping agent such as zeolite and a catalytic metal layer containing a catalytic metal such as Rh and Pd are formed by coating.

FIG. 3 shows the distribution of the carried amount of the HC trap layer and the catalyst metal layer. In the figure, the hatched portion M shows the portion where the amount of the catalyst metal carried is large. . On the carrier 21, the HC trapping agent is uniformly loaded, while the catalytic metal is loaded so that the loading amount becomes large in the upstream portion as shown in the figure. In the figure, the carrying amount is shown to change stepwise in the upstream and downstream directions, but the present invention is not limited to this, and the carrying amount may be changed continuously. With such a configuration, as described above, the catalytic metal activity is promoted in the upstream part of the carrier, and the turbulent action in the slits in the downstream part achieves high conversion efficiency as the entire carrier. can do. In addition, the slits 2 unevenly distributed in the downstream portion
2 contributes to the improvement of the HC trap performance in the upstream part by suppressing the heat of the catalytic reaction in the downstream part of the carrier from being transferred in the upstream direction.

The structure in which the plurality of slits 22 are vertically formed on the carrier 21 is advantageous in that the strength of the carrier is secured in addition to the fact that the manufacture is relatively easy. It is effective to increase the number of the slits 22 provided from the viewpoint of making the gas flow turbulent and suppressing heat transfer, but from the viewpoint of ensuring the required cell surface area and HC trap performance within a limited carrier volume. From the above, the configuration of the present invention in which the slits 22 are unevenly distributed in the downstream portion of the carrier is advantageous.

The effect of suppressing the heat transfer inside the carrier 21 by the slits 22 can be expected to some extent even if it is a metal carrier, but it becomes more remarkable when a ceramic material having a relatively small thermal conductivity is applied as the carrier. . The ceramic material has a good compatibility with the HC trapping agent such as zeolite and has an advantage that the strength of the coating is high.

If the amount of the HC trapping agent carried is increased in order to increase the amount of trapped HC, the passage area of the cell becomes smaller and the exhaust resistance increases accordingly. In this respect, in the present invention, since the HC trap performance is improved by the effects of the turbulent flow and the suppression of heat conduction described above, it is possible to reduce the cell density and reduce the exhaust resistance. Specifically, the cell density of a general three-way catalyst is 900 or more, whereas in the present invention, it can be 300 or less, or at least 600 or less. The unit of the cell density is the number of cells per square inch of the cross section of the carrier, and is a unit commonly used for transactions by those skilled in the art. The amount of the HC trapping agent loaded is, for example, about 350 when the cell density is 300, and 600 when the cell density is 600.
At that time, it is set to about 250. The unit of the supported amount is grams per cubic foot of the apparent volume of the carrier [g / c
f], which is a commercial unit used by those skilled in the art.

A plurality of exhaust gas purification devices 9 as shown in FIG.
In the exhaust gas purification system including Nos. 11 to 11, it is possible to reduce the exhaust resistance by making the cell density of the exhaust gas purification device 10 or 11 located on the downstream side smaller than that on the upstream side. Although the catalyst on the downstream side has a low temperature rising property and takes time to reach the activation temperature, the configuration of the exhaust gas purification device of the present invention can sufficiently enhance the HC trap performance.
The exhaust resistance can be reduced by making the cell density rough while ensuring the exhaust purification performance.

FIG. 4 shows an example of the support structure of the carrier 21, in which 34 is a catalyst container and 35 is a heat-resistant mat made of ceramic fiber or alumina fiber. The carrier 21 is fixed in the catalyst container 34 via a heat-resistant mat 35 wound around the outer periphery thereof. In this embodiment, the outer periphery of the open portion of the slit 22 is the carrier 2
It is filled with a filler 36 having the same coefficient of linear expansion as 1. By providing the filling material 36 in this way, it is possible to prevent the problem that the mat 35 is wind-eroded and worn by the exhaust gas passing through the slit 22. Also, the carrier 2
By using the filler 36 having the same linear expansion coefficient as that of No. 1, the strength of the carrier 21 can be improved.

FIG. 5 shows another embodiment of the carrier structure. As shown in the figure, the width d of the slits 22 located on the downstream side is increased.

According to this embodiment, it is possible to further promote the turbulent flow of the exhaust gas in the downstream portion where the slit width d is increased, and also the heat transfer suppressing effect is increased. Slit 22 with a large width d
Since the gas turbulence is so large in the portion (2), the gas flow rate distribution in the cross-sectional direction of the carrier on the downstream side becomes uniform, so that both the HC trap performance and the desorbed HC purification performance are improved. This embodiment is suitable when the temperature of the downstream portion of the carrier tends to rise.

6 to 7 are embodiments relating to the layer structure of the HC trapping agent and the catalytic metal when the catalytic metal is applied to the present invention. The HC trapping agent and the catalyst metal may be mixed in the same layer, but since layering each of them inside and outside causes a time difference in temperature rise, purification treatment is performed at the same timing as HC desorption and catalyst activation. It is possible to improve the efficiency of.

In particular, as shown in FIG. 6, in the structure in which the lower layer side of the cell 24 is coated with the HC trap layer 25 containing the HC trapping agent and the upper layer side is coated with the catalytic metal layer 26 containing the catalytic metal, respectively, Since the catalyst metal of the layer 26 is activated early by contact with the exhaust gas, the HC trap layer 25 in contact with the carrier 21 can be kept at a relatively low temperature.
It is possible to satisfy the contradictory requirements of securing the amount of C trap and improving the conversion efficiency by the catalyst.

FIG. 7 shows an embodiment in which a heat insulating layer 29 made of alumina or the like is formed adjacent to the HC trap layer 25 to have a uniform thickness. Since the heat insulating layer 29 can reduce the amount of heat transfer from the catalyst metal layer 26 to the HC trap layer 25, it is possible to increase the amount of HC trap while increasing the temperature of the catalyst metal layer 26 earlier, and to improve the purification performance.

In the structures shown in FIGS. 6 and 7, the HC trap layer 25 or the catalyst metal layer 26 may be composed of a plurality of layers having different characteristics. For example, as for the catalyst metal layer 26, when Pd is applied to the surface side, the Pd-Rh-based or Pt-Rh-based or relatively low-density Pd layer is used on the deep side. It is possible to more appropriately control the HC desorption timing or the catalyst metal activation timing according to the temperature rising property due to the characteristics of the carrier and the HC trapping agent.

FIG. 8 shows an embodiment in which a plurality of slits 22 are formed alternately from both side surfaces of the carrier 21 and inclined toward the upstream side of the gas flow with respect to the cell. When viewed from the gas flow direction, the adjacent slits 22 are alternately arranged so as to partially overlap each other.

According to this embodiment, the inclined slits 22 facilitate the diffusion of the gas from the inside to the outside of the carrier, so that the dead volume part, that is, the gas flow, is small in the outer part of the carrier, and the functions of the HC trapping agent and the catalytic metal are achieved. The dead volume portion can be reduced by applying this configuration when there is a portion that cannot fully utilize.

FIG. 9 shows an embodiment in which a plurality of slits 22 are formed so as to face both sides of the carrier 21 and to be inclined toward the upstream side of the gas flow with respect to the cell. In this embodiment, the adjacent slits 22 when viewed from the gas flow direction
There is no overlapping portion, and the non-slit portion 23 remains in the central portion.

According to this embodiment, the strength of the carrier 21 can be increased by the central non-slit portion 23 while reducing the dead volume part in the outer region of the carrier as in the case of FIG.

FIG. 10 shows an embodiment in which the slit 22 is formed so as to penetrate both side surfaces of the carrier 21.

According to this embodiment, since the non-slit portions 23 are provided on both sides of the slit 22, the strength of the carrier 21 can be increased. Further, in this embodiment, since the side surface shape of the slit 23 is a mountain shape with the top facing the upstream side, it is possible to reduce the dead volume part in the outer area of the carrier as in the case of FIG. 8 or 9.

FIG. 11 shows an embodiment in which the width d of the slits 22 formed alternately from both side surfaces of the carrier 21 is formed in a wedge shape so as to become narrower toward the inside of the carrier. The adjacent slits 22 partially overlap each other when viewed from the gas flow direction.

According to this embodiment, since the slit width d is larger in the outer peripheral region of the carrier, gas turbulence in the outer peripheral region of the carrier can be further promoted and the dead volume portion can be reduced.

FIG. 12 shows a wedge-shaped slit 2 similar to FIG.
2 is an embodiment in which adjacent slits 22 are formed so as not to overlap each other when viewed from the gas flow direction.

According to this embodiment, the strength of the carrier 21 can be increased by the central non-slit portion 23 while reducing the dead volume part in the outer region of the carrier as in the case of FIG.

FIG. 13 shows an embodiment in which a slit 22 is formed so as to penetrate both sides of the carrier 21. Non-slit portions 23 remain on both sides of each slit 22 in the cross section of the carrier. Further, each slit 22 is formed in a polygonal shape so that its width becomes larger toward the central portion.

According to this embodiment, since the gas turbulence in the inner region of the carrier becomes large due to the wide portion in the center of the slit 22, the dead volume portion is generated in the inner region of the carrier by applying this configuration. The volume part can be reduced. Further, since the non-slit portions 23 are provided on both sides of the slit 22, the strength of the carrier 21 is also advantageous.

FIG. 14 shows an embodiment in which a slit 22 is formed in a bent shape so as to penetrate both sides of the carrier 21 and has a plurality of tops in the upstream and downstream directions of the gas flow.

According to this embodiment, the curved slit shape complicates the gas flow in the central part of the carrier and makes the turbulence larger, so that the dead volume part of the carrier can be effectively reduced. Further, the strength of the carrier can be secured by the non-slit portions 23 on both sides of the slit.

FIG. 15 shows an embodiment in which the slits 22 are formed so as to penetrate both side surfaces of the carrier 21 and have a convex curved shape in the upstream direction of the gas flow.

According to this embodiment, in addition to the effect of reducing the dead volume portion of the carrier as in the case of FIG. 17, the slit 22 has a curved shape and has no acute-angled portion, so that the carrier 21 is Stress concentration is less likely to occur when subjected to vibration or impact force, which is advantageous in terms of strength.

Each of the above drawings is a drawing for explaining the formation mode of the slits and the structure of the catalyst carrier, and the width, pitch, etc. of the slits are drawn with dimensions or ratios different from the actual ones for convenience of description. There is.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an engine system to which an exhaust emission control device according to the present invention is applied.

FIG. 2 is an explanatory view of a side shape showing an embodiment of a carrier according to the present invention.

FIG. 3 is an explanatory diagram of an embodiment of an amount distribution of an HC trapping agent or the like on a carrier according to the present invention.

FIG. 4 is a vertical sectional view showing an example of a carrier support structure of an exhaust emission control device according to the present invention.

FIG. 5 is an explanatory view of a side shape showing another embodiment of the carrier according to the present invention.

FIG. 6 is a detailed cross-sectional view of a cell portion of an embodiment relating to a layered structure of an HC trapping agent or the like of a carrier according to the present invention.

FIG. 7 is a detailed cross-sectional view of a cell portion of an embodiment relating to a layered structure of an HC trapping agent or the like of a carrier according to the present invention.

FIG. 8 is a side view of another embodiment of a carrier according to the present invention.

FIG. 9 is a side view of another embodiment of a carrier according to the present invention.

FIG. 10 is a side view of another embodiment of a carrier according to the present invention.

FIG. 11 is a side view of another embodiment of a carrier according to the present invention.

FIG. 12 is a side view of another embodiment of a carrier according to the present invention.

FIG. 13 is a side view of another embodiment of the carrier according to the present invention.

FIG. 14 is a side view of another embodiment of a carrier according to the present invention.

FIG. 15 is a side view of another embodiment of a carrier according to the present invention.

[Explanation of symbols]

1 engine 2 Intake passage 3 exhaust passage 9-11 Exhaust gas purification device 21 carrier 22 slits 23 Non-slit part 24 cells 25 HC trap layer 26 catalytic metal layer 29 Thermal insulation layer 34 Catalyst container

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) F01N 3/08 F01N 3/24 B 3/24 E L B01D 53/36 103Z (72) Inventor Shunichi Mitsuishi Kanagawa 2 Takaracho, Kanagawa-ku, Yokohama, Japan Nissan Motor Co., Ltd. (72) Inventor Yoshinao Ugo, 2nd Takara-cho, Kanagawa-ku, Yokohama, Kanagawa Nissan Motor Co., Ltd. F-term (reference) 3G091 AA02 AA11 AA17 AB03 AB10 BA15 FA02 FB02 FC07 GA06 GA14 GA18 GB05W GB09Y GB17X HA18 HA27 HB05 4D048 AA18 BA10Y BB02 BB13 BB15 BB16 CC41 EA04 4G069 AA03 AA08 CA03 CA15 DA06 EA19 EB04 EC28 EC29

Claims (21)

[Claims] 1. A slit is formed so as to traverse cells of a honeycomb-shaped carrier and be unevenly distributed in a downstream portion of the carrier, the carrier is loaded with an HC trapping agent, and the loading amount is large in the upstream portion. An exhaust emission control device, which carries a catalytic metal so that 2. The exhaust emission control device according to claim 1, wherein a plurality of the slits are arranged in the gas flow direction of the carrier. 3. The exhaust emission control device according to claim 2, wherein the width of the downstream side slit is larger than that of the upstream side slit among the plurality of slits. 4. The exhaust emission control device according to claim 1, wherein the carrier is formed of ceramics. 5. The exhaust emission control device according to claim 1, wherein the HC trapping agent is composed of a plurality of layers having different characteristics. 6. The exhaust emission control device according to claim 1, wherein the catalytic metal is composed of a plurality of layers having different characteristics. 7. The carrier contains HC containing an HC trapping agent.
The exhaust emission control device according to claim 1, wherein the trap layer and the catalytic metal layer containing the catalytic metal are formed by coating, respectively. 8. The exhaust emission control device according to claim 7, wherein the HC trap layer is formed on the lower layer side and the catalytic metal layer is formed on the upper layer side. 9. The exhaust emission control device according to claim 8, wherein a heat insulating layer is formed between the HC trap layer and the catalyst metal layer. 10. The exhaust emission control device according to claim 9, wherein the heat insulating layer is formed to have a uniform thickness. 11. The exhaust emission control device according to claim 2, wherein the plurality of slits are formed so as to face each other from opposite positions on the outer periphery of the carrier and perpendicularly to the gas flow in the cell. 12. The exhaust emission control device according to claim 2, wherein the plurality of slits are formed so as to face each other from opposite positions on the outer periphery of the carrier and be inclined with respect to the gas flow in the cell. 13. The exhaust emission control device according to claim 11 or 12, wherein the opposing slits are formed so as not to overlap each other when viewed in the gas flow direction. 14. The exhaust emission control device according to claim 1, wherein the slit is formed so as to penetrate both side surfaces of the carrier. 15. The exhaust emission control device according to claim 1 or 2, wherein the slit is formed in a shape in which the width is gradually narrowed inward from the outer periphery of the carrier. 16. The exhaust emission control device according to claim 16, wherein a plurality of the slits are formed so as to face each other from opposite positions of the carrier and do not overlap each other when viewed from the gas flow direction. 17. The exhaust emission control device according to claim 14, wherein the penetrating slit is formed in a shape in which the width gradually increases from the outer periphery of the carrier toward the inner side when viewed from the penetrating direction. 18. The exhaust emission control device according to claim 14, wherein the penetrating slit is formed in a bent shape having a plurality of apexes in the upstream and downstream directions when viewed from the penetrating direction. 19. The exhaust emission control device according to claim 14, wherein the penetrating slit is formed in a curved shape convex toward the upstream side when viewed from the penetrating direction. 20. The exhaust emission control device according to claim 1 or 2, wherein the carrier has a cell density of 600 or less. 21. The exhaust emission control device according to claim 1, wherein the coating amount of the HC trapping agent is 250 [g / cf] or more.