Exhaust emission control device

Abstract

To suppress a crack on an exhaust emission control catalyst by reducing a thermal impact on the exhaust emission control catalyst.SOLUTION: An exhaust emission control device 3 includes a catalyst converter 30 which purifies the exhaust gas from an exhaust manifold 29. The first catalyst converter 30 includes: a porous body 34 made of a metal material through which the exhaust gas from the exhaust manifold 29 passes; and an exhaust emission control catalyst 32 for purifying the exhaust gas that has passed through the porous body 34. The exhaust emission control catalyst 32 is an electric heating catalyst in which a pair of electrodes 35, 35 are attached to a catalyst body 31. The porous body 34 has a plurality of through holes formed along the exhaust gas from upstream to downstream side. A metal material is exposed from a surface of the porous body 34. Under a temperature of 25°C, a thermal capacity ratio of the porous body 34 with respect to the catalyst body 31 is 8% or more and 23% or less.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an exhaust gas purification device. [Background technology]

[0002] An exhaust gas purification device is connected to the exhaust manifold to purify the exhaust gas emitted from the engine. The exhaust gas purification device is equipped with an exhaust gas purification catalyst that purifies the exhaust gas from the exhaust manifold. The exhaust gas purification catalyst is composed of a metal catalyst that purifies the exhaust gas and a carrier (catalyst carrier) that supports the metal catalyst.

[0003] For example, Patent Document 1 proposes a catalytic converter as such an exhaust gas purification device. The catalytic converter includes a first catalyst that purifies exhaust gas from the exhaust manifold and a second catalyst that purifies exhaust gas that has passed through the first catalyst. The first catalyst has a smaller heat capacity than the second catalyst. This catalytic converter allows the first catalyst to heat up quickly, improving exhaust performance at engine start-up. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-019817 Summary of the Invention [Problem to be solved by the invention]

[0005] However, for example, during sudden acceleration of the vehicle, the flow rate of exhaust gas increases, and the temperature of the exhaust gas passing through the first catalyst rises rapidly due to the activation of the catalyst. When the rapidly heated exhaust gas reaches the second catalyst, the second catalyst, which is the exhaust gas purification catalyst, may be suddenly heated. As a result, a thermal shock acts on the exhaust gas purification catalyst, and it is thought that this thermal shock may cause cracks in the exhaust gas purification catalyst.

[0006] The present invention has been made in consideration of these points, and its purpose is to provide a catalytic purification device that can suppress the occurrence of cracks in the exhaust gas purification catalyst by reducing the thermal shock that acts on the exhaust gas purification catalyst. [Means for solving the problem]

[0007] In view of the above problems, an exhaust gas purification device according to the present invention includes a catalytic converter that purifies exhaust gas from an exhaust manifold. The catalytic converter includes a porous body made of a metal material through which exhaust gas from the exhaust manifold passes, and an exhaust gas purification catalyst that purifies the exhaust gas that has passed through the porous body. The exhaust gas purification catalyst is an electrically heated catalyst in which a pair of electrodes are attached to a catalyst body. The porous body has a plurality of through holes formed along the exhaust gas flow from the upstream side to the downstream side. The surface of the porous body is a surface on which the metal material is exposed. In a temperature environment of 25°C, the heat capacity ratio of the porous body to the catalyst body is 8% or more and 23% or less. [Effects of the Invention]

[0008] According to the present invention, by reducing the thermal shock acting on the exhaust gas purification catalyst, it is possible to suppress the occurrence of cracks in the exhaust gas purification catalyst. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic conceptual diagram for explaining an exhaust gas purification device according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic perspective view of a first catalytic converter of the exhaust gas purification device shown in FIG. [Figure 3] (a) is a schematic plan view of the porous body shown in Fig. 2. (b) is a modified example of the porous body shown in (a). [Figure 4] 1 is a graph showing the relationship between the bed temperature, the temperature of exhaust gas passing through the first catalytic converter, and cracks occurring in the exhaust gas purification catalyst when the first catalytic converters according to Examples 1 and 2 and Comparative Examples 1 and 2 are used. [Figure 5] 1(a) is a graph showing the relationship between the heat capacity ratio and the temperature rise rate reduction amount of the first catalytic converter according to Examples 1 and 2 and Comparative Example 2. FIG. 1(b) is a graph showing the relationship between the heat capacity ratio and the pressure drop increase rate of the first catalytic converter according to Examples 1 and 2 and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] [First embodiment] An exhaust gas purification device according to an embodiment of the present invention will be described below with reference to Figs. 1 to 5. Fig. 1 is a schematic conceptual diagram for explaining an exhaust gas purification device 3 according to an embodiment of the present invention, and Fig. 2 is a schematic perspective view of a first catalytic converter 30 of the exhaust gas purification device 3 shown in Fig. 1. In Fig. 2, the housing 33 is shown in half to show the interior of the first catalytic converter 30.

[0011] As shown in Fig. 1, an exhaust gas purification device 3 according to this embodiment is attached downstream of an engine 2, and purifies exhaust gas after combustion in the engine 2. The engine 2 may be either a gasoline engine or a diesel engine, and in this embodiment, a gasoline direct injection engine is shown as an example in Fig. 1.

[0012] In the engine 2, air taken in through the intake valve 25 flows into a combustion chamber formed by the cylinder block 21 and the piston 22, and is mixed with fuel (gasoline) injected by the fuel injection valve 28. The mixed air-fuel mixture is ignited by the spark plug 27 in the combustion chamber and burns, and the exhaust gas after combustion is discharged from the exhaust manifold 29 via the exhaust valve 26.

[0013] Exhaust gas discharged through exhaust manifold 29 is purified by exhaust gas purification device 3. Specifically, exhaust gas purification device 3 includes a first catalytic converter 30 connected to exhaust manifold 29, and a second catalytic converter 37 connected to first catalytic converter 30 downstream of first catalytic converter 30 via an exhaust pipe 36. First catalytic converter 30 is disposed, for example, in an engine compartment (not shown) of the vehicle, and second catalytic converter 37 is disposed, for example, under the floor (not shown) of the vehicle.

[0014] The first catalytic converter 30 comprises a porous body 34 made of a metallic material through which exhaust gas from the exhaust manifold 29 passes, an exhaust gas purification catalyst 32 that purifies the exhaust gas from the exhaust manifold 29, and a housing 33 that houses the exhaust gas purification catalyst 32. Similarly, the second catalytic converter 37 comprises an exhaust gas purification catalyst 38 that further purifies exhaust gas that could not be completely purified by the first catalytic converter 30, and a housing 39 that houses the exhaust gas purification catalyst 38. The porous body 34 and the housings 33, 39 are made of a metallic material such as stainless steel, carbon steel, or aluminum.

[0015] As shown in FIG. 2, the exhaust gas purification catalyst 32 is an electrically heated catalyst in which a pair of electrodes 35, 35 are attached to a catalyst body 31. The catalyst body 31 has a cylindrical shape with a honeycomb structure. Each electrode 35 is comb-shaped and has multiple wiring portions 35a extending in the circumferential direction of the catalyst body 31. The multiple wiring portions 35a are fixed by fixing portions 35c at intervals in the axial direction of the catalyst body 31. The multiple wiring portions 35a of each electrode 35 are connected by connecting portions 35b. By passing a current through the pair of electrodes 35, the catalyst body 31 can generate resistance heat, and the metal catalyst of the exhaust gas purification catalyst 32 can be activated.

[0016] The housing 33 of the first catalytic converter 30 is formed with an inlet cone portion 33a, a body portion 33b, and an outlet cone portion 33c. The inlet cone portion 33a receives exhaust gas from the exhaust manifold 29 and has a cone shape in which the cross section of the exhaust gas passage expands from upstream to downstream in the exhaust gas flow direction. The body portion 33b is formed continuous with the inlet cone portion 33a on the upstream side of the exhaust gas flow and has a cylindrical shape in which the cross section of the exhaust gas passage is constant. The outlet cone portion 33c is formed continuous with the body portion 33b on the upstream side of the exhaust gas flow and has a cone shape in which the cross section of the exhaust gas passage contracts from upstream to downstream in the exhaust gas flow direction. In this embodiment, the porous body 34 is disposed within the inlet cone portion 33a, and the exhaust gas purification catalyst 32 is disposed within the body portion 33b. This allows the axis of the disc-shaped porous body 34 to be aligned with the axis of the exhaust gas purification catalyst 32. As a result, the exhaust gas that has passed through the porous body 34 and absorbed heat can be made to flow into the exhaust gas purification catalyst 32 uniformly.

[0017] In this embodiment, since the engine 2 is a gasoline engine, the exhaust gas purification catalyst 32 is a three-way catalyst that purifies hydrocarbons (HC), carbon monoxide (CO), and nitride oxides (NOx) from the exhaust gas of the gasoline engine. On the other hand, if the internal combustion engine is a diesel engine, the exhaust gas purification catalyst 32 is an oxidation catalyst that removes carbon monoxide (CO), hydrocarbons (HC), etc. Note that the exhaust gas purification catalyst 38 housed in the second catalytic converter 37 is also provided with a catalyst similar to the exhaust gas purification catalyst 32 depending on the type of internal combustion engine.

[0018] The catalyst body 31 of the exhaust gas purification catalyst 32 is a carrier (catalyst carrier) carrying a metal catalyst that purifies exhaust gas. The carrier 31a is a honeycomb-shaped carrier. A plurality of through holes extending along the axis of the carrier 31a are formed. The carrier 31a is made of a ceramic material, such as a porous ceramic material containing one of alumina, zirconia, cordierite, titania, silicon carbide, and silicon nitride as its main component. The carrier of the exhaust gas purification catalyst 38 is similar.

[0019] In this embodiment, the carrier 31a is made of a SiC-based ceramic material. A SiC-based ceramic material is a material primarily composed of SiC. As long as the conductivity of the carrier 31a can be ensured, other ceramic materials may also be contained. In this embodiment, since the carrier 31a is made of a SiC-based ceramic material, the catalyst body 31 can be electrically connected via an electrode 35. A metal catalyst containing at least one of platinum, rhodium, and palladium is supported on the wall surfaces forming the honeycomb structure of the carrier 31a. The metal catalyst can be supported on the carrier by coating the carrier with a slurry containing the above-mentioned ceramic material and the metal catalyst and firing the coating.

[0020] As shown in Fig. 3(a), the carrier of the porous body 34 is a disk-shaped structure having a ring-shaped metal frame (periphery) 34a with a plurality of through-holes formed therein through which exhaust gas passes. Specifically, a corrugated metal strip 34b and a plate-shaped metal strip 34c are wound around the metal frame 34a in an overlapping state. As a result, a plurality of through-holes 34h are formed in the porous body 34 from the upstream side to the downstream side of the exhaust gas.

[0021] In this embodiment, the heat capacity of the porous body 34 is smaller than that of the catalyst body 31. Specifically, in a temperature environment of 25°C, the heat capacity ratio of the porous body 34 to the catalyst body 31 is 8% or more and 23% or less. For example, the heat capacity of the catalyst body 31 is in the range of 184 to 322 J / K in a temperature environment of 25°C. The heat capacity of the porous body 34 is in the range of 14.7 to 74.1 J / K in a temperature environment of 25°C. This heat capacity range is the heat capacity of catalysts that are applied according to the engine displacement of commonly available vehicles, and it is possible to keep the heat capacity within this range by appropriately selecting the above-mentioned materials, etc.

[0022] According to this embodiment, exhaust gas that flows into the first catalytic converter 30 from the exhaust manifold 29 passes through the porous body 34, then reaches the exhaust gas purification catalyst 32, and is purified by the exhaust gas purification catalyst 32. Since the exhaust gas purification catalyst 32 is an electrically heated catalyst, the exhaust gas purification catalyst 32 can be heated by passing an electric current between the pair of electrodes 35 when the engine 2 is started. As a result, even if relatively low-temperature exhaust gas reaches the exhaust gas purification catalyst 32 when the engine 2 is started, the heated exhaust gas purification catalyst 32 is activated quickly. As a result, the efficiency of purifying exhaust gas can be improved from the time the engine 2 is started.

[0023] On the other hand, when the vehicle suddenly accelerates, the rotation speed of the engine 2 increases, and relatively high-temperature exhaust gas flows from the exhaust manifold 29 into the first catalytic converter 30. Furthermore, since the amount of exhaust gas flowing into the exhaust manifold 29 increases, the activation of the exhaust gas purification catalyst 32 also increases, making it more susceptible to a rapid temperature rise.

[0024] Even in this state, in this embodiment, since the surface of the porous body 34 has an exposed metal material, the porous body 34 can absorb the heat of the relatively high-temperature exhaust gas that has flowed into the first catalytic converter 30. This prevents excessive activation of the exhaust gas purification catalyst 32 located downstream of the exhaust gas. As a result, the application of thermal shock to the exhaust gas purification catalyst 32 is reduced, thereby preventing cracks from occurring in the exhaust gas purification catalyst 32. To achieve this effect, the inventors' experiments, which will be described later, have shown that the heat capacity ratio of the porous body 34 to the catalyst body 31 needs to be 8% or more in a temperature environment of 25°C. Furthermore, since an increase in the heat capacity ratio increases the pressure loss of the exhaust gas passing through the porous body 34, the heat capacity ratio of the porous body 34 to the catalyst body 31 needs to be 23% or less. The appropriate ranges for these heat capacity ratios will be explained in the following examples.

[0025] Furthermore, as shown in FIG. 3, when the porous body 34A is viewed in a plan view, the heat capacity per unit area of ​​the circular central region 34s of the porous body 34A is greater than the heat capacity per unit area of ​​the donut-shaped peripheral region 34t surrounding the central region 34s. The "heat capacity per unit area" is the value obtained by dividing the heat capacity of the entire region at a temperature of 25°C by the area of ​​the region in a plan view. The flow velocity of exhaust gas toward the central region 34s is higher than that toward the peripheral region 34t, and the exhaust gas flow rate is also higher. Therefore, by increasing the heat capacity of the central region 34s, the heat of the exhaust gas passing through the central region 34s can be efficiently absorbed. This suppresses the temperature rise in the center of the exhaust gas purification catalyst 32 in a plan view, thereby reducing thermal shock to the catalyst body 31.

[0026] More specifically, the carrier of the porous body 34A is formed by winding a corrugated metal band 34bt (34bs) and a plate-shaped metal band 34ct (34cs) in a stacked state inside a circular ring-shaped metal frame 34a. The wave pitch of the metal band 34bs in the central region 34s is larger than that of the metal band 34bt in the peripheral region 34t. As a result, in a planar view of the porous body 34, the opening area of ​​the through holes 34hs formed in the central region 34s is smaller than the opening area of ​​the through holes 34ht formed in the peripheral region 34t. Furthermore, the number of through holes 34hs per unit area in the central region 34s is greater than the number of through holes 34ht per unit area in the peripheral region 34t. In this way, in addition to the effects described above, by reducing the opening area of ​​the through holes 34hs in the central region 34s and increasing their number, it is possible to reduce the pressure loss of the exhaust gas while maintaining the heat capacity of the central region 34s. [Example]

[0027] Examples of the present invention will be described below.

[0028] Example 1 The first catalytic converter 30 shown in FIG. 2 was fabricated as follows. A circular stainless steel porous body with a diameter of 70 mm and a length of 10 mm was fabricated as the porous body 34. The ring-shaped metal frame 34a had a thickness of 1.0 mm. The corrugated metal strip 34b and the plate-shaped metal strip 34c had thicknesses of 30 μm. The porous body 34 had 600 cells (through-holes) per square inch. The mass of the porous body 34 was 41.0 g. The heat capacity of the porous body 34 was 18.9 K / J in a temperature environment of 25°C. Next, a ceria-zirconia slurry containing a predetermined proportion of rhodium particles as a metal catalyst was coated on a SiC substrate (support), dried, and then calcined. The resulting catalyst body 31 had a mass of 244 g. The heat capacity of the catalyst body 31 is 174 K / J in a temperature environment of 25° C. In a temperature environment of 25° C., the heat capacity ratio of the porous body 34 to the catalyst body 31 is 11%.

[0029] <Example 2> The first catalytic converter 30 was produced in the same manner as in Example 1. The difference from Example 1 is that the length of the porous body 34 was set to 15 mm. The mass of the porous body 34 was 62.0 g. The heat capacity of the porous body 34 was 28.6 K / J in a temperature environment of 25°C.

[0030] <Comparative Example 1> A first catalytic converter was produced in the same manner as in Example 1. The difference from Example 1 is that the porous body 34 was not provided.

[0031] <Comparative Example 2> A first catalytic converter was produced in the same manner as in Example 1. The difference from Example 1 is that the length of the porous body 34 was set to 5 mm. The mass of the porous body 34 was 21.0 g. The heat capacity of the porous body 34 was 9.7 K / J in a temperature environment of 25°C.

[0032] [Evaluation test] The first catalytic converters of Examples 1 and 2 and Comparative Examples 1 and 2 were each connected to an exhaust pipe with a bypass path from a 2.5-liter engine. Next, a test was conducted under rapid acceleration conditions of 500 cycles, with one cycle being the process of changing the engine speed from 1,000 rpm to 3,000 rpm. During this test, the presence or absence of cracks in the catalyst body was confirmed. Furthermore, the bed temperature of these catalyst bodies and the temperature change (maximum temperature rise rate) at a position 5 mm downstream from the porous body were measured. The results are shown in FIG. 4 and Table 1. Next, the reduction in the temperature rise rate of Examples 1 and 2 and Comparative Example 2 was measured using the maximum temperature rise rate of Comparative Example 1 as the reference. The results are shown in Table 1 below and FIG. 5(a). The pressure drop increase rate of Examples 1 and 2 and Comparative Example 2 was measured using the exhaust gas pressure drop of Comparative Example 1 as the reference. The results are shown in Table 1 below and FIG. 5(b). Furthermore, as a reference example, a test was conducted on the catalytic converter of Comparative Example 1 under sudden acceleration conditions of 500 cycles, with one cycle being the process of changing the engine speed from 1000 rpm to 2900 rpm. The results are shown in Table 1.

[0033] [Table 1]

[0034] [Results and Discussion] No cracks were generated in the catalyst bodies of Examples 1 and 2, but cracks were generated in the catalyst bodies of Comparative Examples 1 and 2. This is thought to be because, under rapid acceleration conditions, the porous body in Examples 1 and 2 sufficiently absorbed the heat of the exhaust gas, preventing cracks from occurring in the catalyst bodies of Examples 1 and 2. Furthermore, when the engine speed was changed from 1000 rpm to 2900 rpm as in the Reference Example, the exhaust gas temperature and flow rate were low, so cracks were not generated in the catalyst body even when the catalytic converter of Comparative Example 1 was used. From this perspective, in order to prevent cracks from occurring in the catalyst body, as shown in FIG. 5(a), the heat capacity ratio of the porous body to the catalyst body is 8% or more, and more preferably 11% or more. Furthermore, as shown in FIG. 5(b), taking into account the pressure loss of the porous body, the heat capacity ratio of the porous body to the catalyst body is 23% or less, and more preferably 16% or less. [Explanation of symbols]

[0035] 2: engine, 29: exhaust manifold, 3: exhaust gas purification device, 30: first catalytic converter (catalytic converter), 31: catalyst body, 32: exhaust gas purification catalyst, 33: housing, 33a: inlet cone portion, 33b: body portion, 33c: outlet cone portion, 34, 34A: porous body, 34h, 34hs, 34ht: through holes, 34s: central region, 34t: peripheral region, 35: electrode

Claims

1. An exhaust gas purification device equipped with a catalytic converter that purifies exhaust gas from an exhaust manifold, the catalytic converter comprises a porous body made of a metal material through which exhaust gas from the exhaust manifold passes, and an exhaust gas purification catalyst that purifies the exhaust gas that has passed through the porous body, The exhaust gas purification catalyst is an electrically heated catalyst having a pair of electrodes attached to a catalyst body, the porous body has a plurality of through holes formed along the exhaust gas flow from the upstream side to the downstream side, and a surface of the porous body is a surface on which a metal material is exposed, An exhaust gas purification device, characterized in that, in a temperature environment of 25°C, the heat capacity ratio of the porous body to the catalyst body is 8% or more and 23% or less.

2. the catalytic converter includes a metal housing that accommodates the porous body and the exhaust gas purification catalyst therein; the housing includes an inlet cone portion into which exhaust gas from the exhaust manifold flows, and in which a cross section of the exhaust gas flow path expands from upstream to downstream of the exhaust gas; a body portion that is continuous with the inlet cone portion and has a constant flow path cross section for exhaust gas; an outlet cone portion is formed continuous to the body portion, and the cross section of the exhaust gas flow path is reduced from the upstream side to the downstream side of the exhaust gas, the porous body is disposed within the inlet cone section, 2. The exhaust gas purification device according to claim 1, wherein the exhaust gas purification catalyst is disposed inside the body portion.

3. The porous body is disk-shaped, The exhaust gas purification device according to claim 2, characterized in that, when the porous body is viewed in a plane, the heat capacity per unit area in the central region of the porous body is greater than the heat capacity per unit area in the peripheral region surrounding the central region.

4. In a plan view of the porous body, an opening area of ​​the through-hole formed in the central region is smaller than an opening area of ​​the through-hole formed in the peripheral region, 4. The exhaust gas purification device according to claim 3, wherein the number of through holes per unit area in the central region is greater than the number of through holes per unit area in the peripheral region.

5. 2. The exhaust gas purification device according to claim 1, wherein the catalyst body has a honeycomb-shaped carrier made of a SiC-based ceramic material.

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