Catalytic converter

A catalytic converter has a catalyst support that is heated due to supply of electricity, and a pair of electrodes that are disposed so as to contact an outer periphery of the catalyst support as seen in an orthogonal cross-section that is orthogonal to a direction in which exhaust flows. By making a volume resistivity of the electrodes be higher than that of electricity supplying portions of external cables that are connected to the electrodes respectively and are for supplying current to the electrodes, heat generated at the electrodes is provided to the catalyst support, and a generated heat amount of the catalyst support in vicinities of the electrodes is made to be large as compared with a generated heat amount at an inner portion of the catalyst support.

TECHNICAL FIELD

The present invention relates to a catalytic converter that is provided at the exhaust pipe of an internal combustion engine.

BACKGROUND ART

In a catalytic converter that is provided at an exhaust pipe in order to purify the exhaust generated at an internal combustion engine, it is desirable to supply electricity to a catalyst support, that is formed of metal and supports a catalyst, and raise the temperature of the catalyst support so as to obtain a sufficient catalytic effect. For example, Patent Document 1 (Japanese Patent Application Laid-Open No. 04-280086) discloses a honeycomb monolithic heater that has square cells (through-holes) whose cross-sectional shapes are square, and that obtains a uniform heat generating ability due to a pair of electrode plates being disposed such that the angles that are formed with the walls of these through-holes are acute angles.

DISCLOSURE OF INVENTION

Technical Problem

However, even if the distribution of current to the inner portion of the catalyst support is made to be substantially uniform and uniform generation of electricity is carried out, in vicinities of the electrode plates that contact the catalyst support (i.e., directly beneath the electrode plates), there is discharge of electricity from the electrode plates and transfer of heat to cables that are connected to the electrode plates, and therefore, it is easy for the temperature of the catalyst support in vicinities of the electrode plates to fall as compared with at the central portion of the catalyst support between the electrode plates.

In consideration of the above-described circumstances, an object of the present invention is to provide a catalytic converter that reduces non-uniformity of temperature of a catalyst support and that can approach a uniform temperature distribution.

Solution to Problem

A catalytic converter of a first aspect of the present invention comprises: a catalyst support that supports a catalyst for purifying exhaust discharged from an internal combustion engine, and that is heated due to electricity being supplied thereto; and a pair of electrodes that are disposed so as to contact an outer periphery of the catalyst support at opposing positions with the catalyst support therebetween, as seen in an orthogonal cross-section that is orthogonal to a direction in which the exhaust flows; and external cables that are connected to the electrodes respectively and supply current to the electrodes, and that are connected to positions at which a distance between the pair of electrodes is long as seen in the orthogonal cross-section, wherein, by making a volume resistivity of the electrodes higher than that of electricity supplying portions of external cables that are connected to the electrodes respectively and are for supplying current to the electrodes, heat generated at the electrodes is provided to the catalyst support, and a generated heat amount of the catalyst support in vicinities of the electrodes is made to be large as compared with a generated heat amount of an inner portion of the catalyst support, and the pair of electrodes are structured such that the volume resistivity becomes higher, gradually or in a stepped manner, from the positions where the external cables are connected, toward directions in which the distance between the pair of electrodes becomes shorter, as seen in the orthogonal cross-section.

In accordance with the catalytic converter of the first aspect of the present invention, the pair of electrodes are disposed so as to contact the outer periphery of the catalyst support, so as to oppose one another with the catalyst support therebetween. When electricity is supplied to the catalyst support from the external cables that are connected to the pair of electrodes respectively, the catalyst support is heated and the temperature thereof is raised, and therefore, the purifying effect of the exhaust by the supported catalyst is exhibited. At this time, by making the volume resistivity of the electrodes be higher than that of the electricity supplying portions of the external cables, the heat generated at the electrodes is provided to the catalyst support, and the generated heat amount of the catalyst support in vicinities of the electrodes is made to be large as compared with the generated heat amount of the inner portion of the catalyst support. Namely, in the vicinities of the electrodes of the catalyst support, as compared with the inner portion of the catalyst support, the dissipated heat amount is large due to heat dissipation from the electrodes and heat transfer to the external cables. However, by making the volume resistivity of the electrodes high, the generated heat amount of the catalyst support in vicinities of the electrodes is increased due to the heat generated at the electrodes, as compared with the generated heat amount at the inner portion of the catalyst support, and is made to be a generated heat amount that anticipates the dissipated heat amount of the catalyst support (is made to be a generated heat amount that is as if it compensates for the dissipated heat amount). Due thereto, heat generation at the respective regions of the catalyst support is uniformized, non-uniformity of the temperature of the catalyst support is reduced, and the catalyst support can be made to approach a uniform temperature distribution.

Moreover, in accordance with this catalytic converter, as seen in the orthogonal cross-section, there is a structure in which the external cables are connected to positions where the distance between the pair of electrodes is long, and the volume resistivity of the pair of electrodes becomes higher, gradually or in a stepped manner, from the positions where the external cables are connected, toward the directions in which the distance between the pair of electrodes becomes shorter. Due thereto, the flowing of current becomes difficult in accordance with progression from the positions where the external cables are connected, toward the directions in which the distance between the pair of electrodes becomes shorter. Generally, the volume resistivity of the catalyst support is higher than the volume resistivity of the electrodes, and therefore, current attempts to flow more at the regions of the catalyst support where the distance between the pair of electrodes is short, than at the region of the catalyst support where the distance between the pair of electrodes is long. However, due to the structure in which the volume resistivity of the pair of electrodes becomes higher, gradually or in a stepped manner, toward the directions in which the distance between the pair of electrodes becomes shorter, the ease of flowing of the current at the region of the catalyst support where the distance between the pair of electrodes is long, and at the regions of the catalyst support where the distance between the pair of electrodes is short, is made uniform. Due thereto, current can be made to flow more uniformly at the catalyst support, non-uniformity of the temperature of the catalyst support is reduced more effectively, and the catalyst support can be made to approach a uniform temperature distribution.

Advantageous Effects of Invention

In accordance with the catalytic converter relating to the present invention, non-uniformity of temperature of a catalyst support is reduced and a uniform temperature distribution can be approached.

BEST MODES FOR CARRYING OUT THE INVENTION

A first embodiment of a catalytic converter relating to the present invention is described hereinafter by usingFIG. 1AthroughFIG. 4.

A catalytic converter12relating to the present embodiment is shown inFIG. 1A. The catalytic converter12is installed along an exhaust pipe. Exhaust from an engine flows within the exhaust pipe.FIG. 1Billustrates the catalytic converter12in a cross-section in the direction orthogonal to the direction in which this exhaust flows (a cross-section along line2-2ofFIG. 1A).

As shown inFIG. 1AandFIG. 1B, the catalytic converter12has a catalyst support14that is formed of a material that is electrically conductive and rigid. The surface area of the material of the catalyst support14is enlarged by forming the catalyst support14in, for example, a honeycomb shape. A catalyst (platinum, palladium, rhodium, or the like) is supported in a state of adhering to the surfaces of the catalyst support14. The catalyst has the effect of purifying harmful substances within the exhaust that flows within the exhaust pipe (the flowing direction is shown by F1). Note that the structure for increasing the surface area of the catalyst support14is not limited to the aforementioned honeycomb shape, and may be wave-shaped or the like for example.

An electrically conductive ceramic, an electrically conductive resin, a metal or the like can be used as the material that structures the catalyst support14, but, in the present embodiment, in particular, an electrically conductive ceramic is used. If the material that structures the catalyst support14is made to contain at least silicon carbide for example, it is preferable because high strength and heat resistance are obtained. Moreover, if the electrical resistivity is made to be 10 to 200 Ω·cm, it is preferable because the temperature of the catalyst that is supported can be raised efficiently at the time of supplying electricity as will be described later. Making the porosity of the catalyst support be in the range of 30 to 60% is preferable. If the porosity is made to be greater than or equal to 30%, the needed surface area is ensured, and a large amount of the catalyst can be supported. Further, by making the porosity be less than or equal to 60%, the strength required of the catalyst support14can be maintained.

Two electrodes16A,16B are affixed to the catalyst support14, and moreover, terminals18A,18A are connected to the centers of the electrodes16A,16B respectively. External cables30for supplying current are connected to the terminals18A,18B respectively (refer toFIG. 2A). The electrodes16A,16B are disposed so as to contact the catalyst support14at a range having a predetermined expanse along the outer peripheral surface of the catalyst support14. The catalyst support14can be heated due to electricity being supplied to the catalyst support14from the terminals18A,18B through the electrodes16A,16B. Further, by raising the temperature of the catalyst, that is supported by the catalyst support14, due to this heating, the exhaust purifying effect that the catalyst has can be exhibited better.

In the present embodiment, as can be understood fromFIG. 1B, as seen in a cross-section orthogonal to the direction in which the exhaust flows (orthogonal cross-section), the catalyst support14is a so-called track shape in which the both sides in a transverse direction, that is orthogonal to a long axis LA of the oval shape, are formed to be rectilinear and substantially parallel to the long axis LA. Further, the pair of electrodes16A,16B are disposed at opposing positions with the catalyst support14therebetween, such that respective central portions (electrode centers16C) of the electrodes16A,16B are positioned on the long axis LA the catalyst support14.

Here, a central line CL is set as a line segment that connects the electrode centers16C of the electrodes16A,16B, and a width W is defined as the length of the catalyst support14measured in the direction orthogonal to this central line CL. At this time, the central line CL coincides with the long axis LA of the catalyst support14.

The catalyst support14is a shape that has left-right symmetry inFIG. 1Bacross the central line CL (the long axis LA). Moreover, the catalyst support14is a shape that similarly has top-bottom symmetry inFIG. 1Bacross a perpendicular bisector VD of the central line CL. At the catalyst support14, gradually decreasing width portions14D, whose width W in the direction orthogonal to the central line CL gradually decreases toward the electrode center16C, are formed at the regions where the electrodes16A,16B are contactingly disposed. In the present embodiment, the portions where the electrodes16A,16B are affixed are curved surface portions that curve convexly toward the electrode16A or the electrode16B. Further, at the region where the electrodes16A,16B are not disposed contactingly, wide width portions14W, whose outer edges are wider than the regions where the electrodes16A,16B are contactingly disposed (the gradually decreasing width portions14D), are formed at the catalyst support14. The wide width portions14W are formed in rectilinear shapes substantially parallel to the central line CL. The wide width portions14W are a maximum width portion where the width W of the catalyst support14is a maximum. At any arbitrary position, the width W of the catalyst support14is shorter than a length L1of the central line CL (the long axis LA).

In the present embodiment, the wide width portions14W of the catalyst support14are formed in rectilinear shapes substantially parallel to the central line CL. As compared with the vicinities of the electrodes16A,16B, the amount of reduction in the sectional surface area of the flow of current at the wide width portions14W is small, and the decrease in the current density is small. Therefore, uniformizing of the amount of heat generated at the catalyst support14can be devised.

Further, by making the volume resistivity of the electrodes16A,16B higher than that of the electricity supplying portions (the electrical wires) of the external cables30, there is a structure in which the heat generated at the electrodes16A,16B is provided to the catalyst support14, and the generated heat amount of the catalyst support14in the vicinities of the electrodes16A,16B is made to be large as compared with the generated heat amount of the inner portion of the catalyst support14(e.g., a vicinity of a center14C of the catalyst support14between the electrodes16A,16B). Here, volume resistivity means the electrical resistance value (Ω·cm) per unit volume. The resistance value of the material overall is determined by multiplying the length (L) by the volume resistivity and dividing by the sectional surface area (A). Volume resistivity is a value (a physical property value) that is unique to a substance, and, when comparing by using the same dimensions, a substance whose volume resistivity is large also has a large resistance value. In the present embodiment, the volume resistivity is increased by, for example, adjusting the material of the electrodes16A,16B, or the amount of additives that are added to the material.

In order to make the temperature distribution of the catalyst support14at the time that electricity is supplied be substantially uniform, the balance between the generated heat amount and the dissipated heat amount must be made to be substantially the same in the vicinities of the electrodes16A,16B (directly beneath the electrodes16A,16B in the drawings) and at the inner portion of the catalyst support14(e.g., a vicinity of the center14C of the catalyst support14between the electrodes16A,16B).

In the present embodiment, a uniform temperature distribution is realized by controlling the generated heat amount of the catalyst support14in vicinities of the electrodes16A,16B.

Generated heat amount W due to the supply of electricity is expressed by
W=R×I2
Here, W is the generated heat amount, I is the current, and R is the electrical resistance.

Further, the electrical resistance R is expressed by
R=ρ×L/A.
Here, ρ is the volume resistivity of an electrical conductor (the electrodes16A,16B in the present embodiment), L is the length of the electrical conductor (the electrodes16A,16B), and A is the sectional surface area of the electrical conductor (the electrodes16A,16B). From the above formulas, it can be understood that, as a means for controlling the generated heat amount W, the volume resistivity ρ of the electrodes16A,16B is a parameter.

The generated heat amounts in vicinities of the electrodes16A,16B of the catalyst support14and at the central portion of the catalyst support14are schematically shown inFIG. 2A. The dissipated heat amounts in vicinities of the electrodes16A,16B of the catalyst support14and at the central portion of the catalyst support14are schematically shown inFIG. 2B. Further, the temperatures in vicinities of the electrodes16A,16B of the catalyst support14and at the central portion of the catalyst support14are schematically shown inFIG. 2C.

As shown inFIG. 2B, in the vicinities of the electrodes16A,16B of the catalyst support14, the dissipated heat amount is great due to heat dissipation from the electrodes16A,16B and heat transfer to the external cables30. Therefore, as shown inFIG. 2A, the generated heat amount of the catalyst support14in vicinities of the electrodes16A,16B must be made to be larger than the generated heat amount of the central portion of the catalyst support14. Therefore, in order to increase the generated heat amount of the catalyst support14in vicinities of the electrodes16A,16B, the volume resistivity of the electrodes16A,16B is made to be higher than that of the electricity supplying portions of the external cables30. Namely, by making the volume resistivity of the electrodes16A,16B be higher than that of the electricity supplying portions of the external cables30, the heat generated at the electrodes16A,16B is provided to the catalyst support14, and the generated heat amount of the catalyst support14in vicinities of the electrodes16A,16B is made to be large as compared with the generated heat amount of the inner portion of the catalyst support14(e.g., a vicinity of the center14C of the catalyst support14between the electrodes16A,16B).

A holding member24, that is formed in the shape of a tube and of an insulating material, is disposed at the outer periphery of the catalyst support14. Moreover, a case cylinder28, that is molded in a cylindrical shape of a metal such as stainless steel or the like, is disposed at the outer periphery of the holding member24. Namely, the catalyst support14is accommodated at the interior of the case cylinder28that is cylindrical tube shaped, and the catalyst support14is held at the interior of the case cylinder28without a gap by the holding member24that is disposed between the case cylinder28and the catalyst support14. Further, because the holding member24that is insulating is disposed between the catalyst support14and the case cylinder28, flow of current from the catalyst support14toward the case cylinder28is impeded.

Operation and effects of the catalytic converter12of the present embodiment are described next.

The case cylinder28of the catalytic converter12is mounted midway along an exhaust pipe, and exhaust passes through the interior of the catalyst support14in the arrow F1direction. At this time, harmful substances within the exhaust are purified by the catalyst that is supported by the catalyst support14. At the catalytic converter12of the present embodiment, current is supplied from the external cables30, and electricity is supplied to the catalyst support14by the terminals18A,18B and the electrodes16A,16B, and the catalyst support14is heated. At the catalyst support14, the current between the electrodes16A,16B flows like arrows EC. Due to the catalyst support14being heated, the temperature of the catalyst that is supported by the catalyst support14is raised, and the purifying action can be exhibited well. In cases in which the temperature of the exhaust is low, such as immediately after start-up of the engine or the like for example, by supplying electricity to and heating the catalyst support14in advance, the purifying performance of the catalyst in the initial stage of engine start-up can be ensured.

The catalytic converter12of the present embodiment is structured such that the volume resistivity of the pair of electrodes16A,16B, that oppose one another with the catalyst support14therebetween, is higher than that of the electricity supplying portions of the external cables30.

Here, as shown inFIG. 5AthroughFIG. 5C, there is supposed a catalytic converter112of a comparative example that is structured such that the volume resistivity of a pair of electrodes116A,116B is not increased more than that of the electricity supplying portions of external cables130. As shown inFIG. 5B, in vicinities of the electrodes116A,116B of a catalyst support114, the dissipated heat amount is large due to heat dissipation from the electrodes116A,116B and the heat transfer to the external cables130(refer toFIG. 2B). Therefore, as shown inFIG. 5A, even if the current distribution of the inner portion of the catalyst support114is made to be substantially uniform and uniform heat generation is realized, as shown inFIG. 5C, the temperature in the vicinities of the electrodes116A,116B of the catalyst support114decreases more than the temperature of the central portion of the catalyst support114, by an amount corresponding to the amount by which the heat dissipation amount is greater.

In contrast, at the catalytic converter12of the present embodiment, as shown inFIG. 2A, by making the volume resistivity of the electrodes16A,16B be higher than that of the electricity supplying portions of the external cables30, the electrodes16A,16B are made to generate heat, and the generated heat thereof is provided to the catalyst support14, and the generated heat amount in the vicinities of the electrodes16A,16B (directly beneath the electrodes16A,16B) of the catalyst support14is made to be large as compared with the generated heat amount of the central portion of the catalyst support14. Namely, the generated heat amount in the vicinities of the electrodes16A,16B of the catalyst support14is increased as compared with the generated heat amount of the central portion of the catalyst support14(the generated heat amount in the vicinities of the electrodes16A,16B of the catalyst support14is made to be a generated heat amount that anticipates the dissipated heat amount of the catalyst support14), in order to compensate for the dissipated heat amount in the vicinities of the electrodes16A,16B of the catalyst support14. Due thereto, heat generation at the respective regions of the catalyst support14can be uniformized. Therefore, as shown inFIG. 2C, non-uniformity of the temperature of the catalyst support14is reduced, and the catalyst support14can be made to approach a uniform temperature distribution.

A catalytic converter52of a second embodiment of the present invention is described next by usingFIG. 3andFIG. 4. Note that structural portions that are the same as in the above-described first embodiment are denoted by the same numerals, and description thereof is omitted.

As shown inFIG. 3, as seen in a cross-section orthogonal to the direction in which the exhaust flows (orthogonal cross-section), two electrodes56A,56B are affixed, so as to oppose one another with the catalyst support14therebetween, to the gradually decreasing width portions14D of the catalyst support14, and further, the terminals18A,18B are connected respectively to the centers of the electrodes56A,56B. Namely, the external cables30(refer toFIG. 2A) are connected via the terminals18A,18B to positions at which the distance between the pair of electrodes56A,56B is long as seen in the orthogonal cross-section (positions of the central line CL in the present embodiment).

The pair of electrodes56A,56B are structured such that the volume resistivity becomes higher from, as seen in the orthogonal cross-section, the positions at which the terminals18A,18B are provided (the positions at which the external cables30are connected) toward the directions in which the distance between the pair of electrodes56A,56B becomes shorter. In other words, the pair of electrodes56A,56B are structured such that the volume resistivity becomes higher from a position60A where the distance between the pair of electrodes56A,56B is long (the position of the central line CL), toward positions60B where the distance between the pair of electrodes56A,56B becomes shorter (positions at the wide width portion14W sides of the catalyst support14).

The present embodiment is structured such that the volume resistivity becomes higher toward the directions in which the distance between the pair of electrodes56A,56B becomes shorter, by adjusting, for example, the material of the electrodes56A,56B or the amount of additives that are added to the material. The present embodiment may be structured such that the volume resistivity of the electrodes56A,56B becomes higher gradually from the positions at which the terminals18A,18B are provided toward the directions in which the distance between the pair of electrodes56A,56B becomes shorter, or may be structured such that the volume resistivity becomes higher in a stepped manner.

By structuring the catalytic converter52such that the volume resistivity becomes higher from the position where the distance between the pair of electrodes56A,56B is long toward directions in which the distance between the pair of electrodes56A,56B becomes shorter, it becomes difficult for current to flow from the position where the distance between the pair of electrodes56A,56B is long (the position where the terminals18A,18B are provided) toward directions in which the distance between the pair of electrodes56A,56B becomes shorter.

Generally, because the volume resistivity of the catalyst support14is higher than the volume resistivity of the electrodes56A,56B, current attempts to flow at the regions of the catalyst support14where the distance between the pair of electrodes56A,56B is short (vicinities of the wide width portions14W), rather than the region of the catalyst support14where the distance between the pair of electrodes56a,56B is long (a vicinity of the central line CL). In contrast, in the present embodiment, due to the structure in which the volume resistivity of the pair of electrodes56A,56B becomes higher from the position where the distance between the pair of electrodes56A,56B is long toward the directions in which the distance between the pair of electrodes56A,56B becomes shorter, as shown inFIG. 4, there is a structure in which the electrical resistance of the electrodes56A,56B and the catalyst support14, at the position60A where the distance between the pair of electrodes56A,56B is long, and the electrical resistance of the electrodes56A,56B and the catalyst support14, at the positions60B where the distance between the pair of electrodes56A,56B is short, become substantially the same.

Here, as shown inFIG. 3, given that the volume resistivity of the electrodes56A,56B is ρ, the length of the electrodes56A,56B is L, and the sectional surface area of the electrodes56A,56B is A, the electrical resistance R is expressed by
R=ρ×L/A.
Further, the portion that is surrounded by the catalyst support14shown by the two-dot chain line inFIG. 4shows the electrical resistance due to the catalyst support14. As shown inFIG. 4, at the position60A where the distance between the pair of electrodes56A,56B is long, the electrical resistance of the catalyst support14is large as compared with at the positions60B where the distance between the pair of electrodes56A,56B is short. Further, the outer sides of the portion that is surrounded by the catalyst support14at the positions60B where the distance between the pair of electrodes56A,56B is short inFIG. 4, show the electrical resistance due to the electrodes56A,56B. In the present embodiment, the volume resistivity of the electrodes56A,56B is adjusted such that the total electrical resistances become substantially the same at the position60A, where the distance between the pair of electrodes56A,56B is long, and the positions60B where the distance between the pair of electrodes56A,56B is short.

Due thereto, the ease of flowing of current at the regions of the electrodes56A,56B and the catalyst support14at the position60A where the distance between the pair of electrodes56A,56B is long, and at the regions of the electrodes56A,56B and the catalyst support14at the positions60B where the distance between the pair of electrodes56A,56B is short, is uniformized, and current can be made to flow more uniformly at the catalyst support14. Therefore, non-uniformity of the temperature of the catalyst support14is reduced more effectively, and the catalyst support14can be made to approach a substantially uniform temperature distribution.

Note that, in the above-described embodiments, the catalyst support14has, at the regions where the pair of electrodes are contactingly disposed, the gradually decreasing width portions14D, whose width W in the direction orthogonal to the central line CL gradually decreases toward the electrode center, and has, at the region where the pair of electrodes are not disposed contactingly, the wide width portions14W that are rectilinear and are formed substantially parallel to the central line CL. However, the present invention is not limited to this structure, and the shape of the catalyst support can be changed. For example, the shape of the catalyst support can be changed so as to be oval, drum-shaped, circular or the like as seen in the cross-section orthogonal to the direction in which the exhaust flows.