CATALYTIC CONVERTER FOR EXHAUST GAS PURIFICATION

Abstract

A catalytic converter (11) for exhaust gas purification is formed by disposing a retaining mat (14) between an inner peripheral face of a metal shell (12) and an outer peripheral face of a catalyst support (13), and retaining the catalyst support (13) in an interior of the metal shell (12) by the retaining mat (14). Since a thickness of the retaining mat (14) in a released state after being heated by exhaust gas is 210% or greater of a clearance (α) between the inner peripheral face of the metal shell (12) and the outer peripheral face of the catalyst support (13), it is possible to reliably retain the catalyst support (13) over a long period of time by compensating for a decrease in surface pressure due to deterioration of the retaining mat (14) with an increase in the surface pressure due to sufficient expansion of the retaining mat (14). Moreover, since it is unnecessary to strongly compress the retaining mat (14) when assembling so as to enhance the surface pressure, there is no possibility that the catalyst support (13) will be damaged by excessive surface pressure.

Claims

1. A catalytic converter for exhaust gas purification, in which a retaining mat disposed between an inner peripheral face of a metal shell and an outer peripheral face of a catalyst support retains the catalyst support in an interior of the metal shell, wherein characterized in that a thickness of the retaining mat in a released state after being heated by exhaust gas is 210% or greater of a clearance between the inner peripheral face of the metal shell and the outer peripheral face of the catalyst support.

2. A catalytic converter for exhaust gas purification, in which a retaining mat disposed between an inner peripheral face of a metal shell and an outer peripheral face of a catalyst support retains the catalyst support in an interior of the metal shell, wherein a thickness of the retaining mat in a released state after being heated by exhaust gas is 170% or greater of a thickness of the retaining mat in a released state before being heated by exhaust gas.

3. The catalytic converter for exhaust gas purification according to claim 1, wherein a density of the retaining mat in a released state before being heated by exhaust gas is 100 g/cm.sup.3 or greater.

4. The catalytic converter for exhaust gas purification according to claim 2, wherein a density of the retaining mat in a released state before being heated by exhaust gas is 100 g/cm.sup.3 or greater.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a vertical sectional view of a catalytic converter for exhaust gas purification. (first embodiment)

[0014] FIG. 2 is a diagram for explaining the mechanism via which a retaining mat undergoes initial expansion. (first embodiment)

[0015] FIG. 3 is a graph showing the relationship between the number of cycles of operation of an engine and the surface pressure of the retaining mat. (first embodiment)

[0016] FIG. 4 is a graph showing the relationship between the second expansion rate and the surface pressure of the retaining mat. (first embodiment)

[0017] FIG. 5 is a graph showing the relationship between the product density and the second expansion rate of the retaining mat. (first embodiment)

EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS

[0018] 11 Catalytic converter for exhaust gas purification [0019] 12 Metal shell [0020] 13 Catalyst support [0021] 14 Retaining mat [0022] α Clearance

MODES FOR CARRYING OUT THE INVENTION

[0023] An embodiment of the present invention is explained below by reference to FIG. 1 to FIG. 5.

First Embodiment

[0024] FIG. 1 shows a vertical cross section of a catalytic converter 11 for exhaust gas purification, which is provided in an automobile exhaust pipe. A metal shell 12, made of stainless steel, forming a casing of the catalytic converter 11 for exhaust gas purification is formed by joining a cylindrical main body portion 12a, an upstream side taper portion 12b that is connected to the upstream side, in the direction of flow of exhaust gas, of the main body portion 12a, and a downstream side taper portion 12c that is connected to the downstream side, in the direction of flow of exhaust gas, of the main body portion 12a. A cylindrical catalyst support 13 is retained in the interior of the main body portion 12a of the metal shell 12 via a retaining mat 14.

[0025] The catalyst support 13 is made of a heat resistant ceramic such as cordierite, mullite, alumina, an aluminate of an alkaline earth metal, silicon carbide, silicon nitride, or an analogue thereof, has a large number of pores that extend therethrough in the axial direction and through which exhaust gas can pass, and supports a catalyst for exhaust gas purification formed from platinum, rhodium, palladium, etc. on a surface with which exhaust gas makes contact.

[0026] The retaining mat 14 is formed by collecting alumina fibers into a mat shape having a constant thickness, and has heat resistance and elasticity. A single thickness of the retaining mat 14 cut into a rectangular shape is wrapped around an outer peripheral face of the catalyst support 13 and inserted in the axial direction of the inner peripheral face of the main body portion 12a of the metal shell 12 while compressing the retaining mat 14; due to the elasticity of the retaining mat 14, which attempts to restore its original shape, the outer peripheral face of the catalyst support 13 is retained on the inner peripheral face of the main body portion 12a of the metal shell 12 via the retaining mat 14.

[0027] Such a retaining mat 14 is produced by the following steps. First, discontinuous alumina fibers are charged and stirred in water and fed to a water tank, and the alumina fibers are deposited within the water tank and molded into a sheet shape. Subsequently, after water is removed from the sheet it is hot-pressed so as to have a predetermined thickness, and the sheet is then cut into a predetermined shape, thus completing the retaining mat 14. Since the alumina fibers of the retaining mat 14 thus completed are solidified with a binder, the thickness of the retaining mat 14 can be held constant. However, if the retaining mat 14 is exposed to exhaust gas at for example 600° C. for about one hour, the binder is vaporized by means of heat, and the retaining mat 14 initially expands by virtue of self elasticity.

[0028] When the fiber length of the alumina fibers forming the retaining mat 14 is too short, since the fibers are not tangled with each other, the repulsive force (elasticity) becomes small, when the fiber length of the alumina fibers is too long, since the fibers are easily aligned in one direction, the repulsive force (elasticity) becomes small, and when the fiber length of the alumina fibers is appropriate, the repulsive force (elasticity) becomes large.

[0029] Therefore, it is possible, by adjusting the fiber length and the quantity when producing the retaining mat 14, to freely set the amount of initial expansion when the retaining mat 14 is heated by exhaust gas. Furthermore, as shown in FIG. 2, since many of the alumina fibers of a retaining mat 14 of a Comparative Example are oriented in the lengthwise direction and the width direction of the retaining mat 14, when the binder loses its function, the number of alumina fibers that expand in the thickness direction is small, and the amount of initial expansion is suppressed. However, since many of the alumina fibers of the retaining mat 14 of the present embodiment are oriented in the thickness direction of the retaining mat 14, when the binder loses its function, the alumina fibers can expand in the thickness direction, thereby increasing the amount of initial expansion.

TABLE-US-00001 TABLE 1 Thickness First expansion Second expansion State (mm) rate rate A Before assembly  7-18 — 100% B After assembly 3-7 100% 15%-70% but before heating C After heating 15-  210% 170% in released state

[0030] As shown in Table 1, the thickness of the retaining mat 14 of the present embodiment changes in accordance with the state of the retaining mat 14. That is, the thickness of the retaining mat 14 in an unused state (state before assembling it onto the catalytic converter 11 for exhaust gas purification) is 7 mm to 18 mm (state A); when this is inserted in the axial direction into the inner peripheral face of the main body portion 12a of the metal shell 12 while compressing it, the thickness thereof reduces to 3 mm to 7 mm which is equal to a clearance α (see FIG. 1) between the inner peripheral face of the main body portion 12a of the metal shell 12 and an outer peripheral face of the catalyst support 13 (state B). Although the retaining mat 14 in an unused state undergoes initial expansion when exposed to high temperature exhaust gas because of the binder evaporating, since the retaining mat 14 is actually restrained by being sandwiched between the inner peripheral face of the main body portion 12a of the metal shell 12 and the outer peripheral face of the catalyst support 13, only the surface pressure changes, and the thickness does not change. However, when the retaining mat 14 that has undergone initial expansion is taken out from between the inner peripheral face of the main body portion 12a of the metal shell 12 and the outer peripheral face of the catalyst support 13, the retaining mat 14 that has been released from constraint and attained a released state expands by virtue of self elasticity, and the thickness increases to 15 mm or greater (state C).

[0031] In the present embodiment, the expansion rate of the retaining mat 14 is defined by a first expansion rate and a second expansion rate. The first expansion rate is the expansion rate when the thickness of the retaining mat 14 in state B is defined as 100%, and since in state C the retaining mat 14 is heated and expands, the expansion rate increases to 210% or greater with respect to state B.

[0032] Furthermore, the second expansion rate is the expansion rate when the thickness of the retaining mat 14 in state A is defined as 100%; since in state B the retaining mat 14 is compressed and inserted into the metal shell 12, the expansion rate decreases to 15% to 70% with respect to state A, and since in state C the retaining mat 14 is heated and expands, the expansion rate increases to 170% or greater with respect to state A. Therefore, an expansion rate of 210% as the first expansion rate corresponds to an expansion rate of 170% as the second expansion rate.

[0033] The graph in FIG. 3 shows the relationship between the number of cycles from starting the engine to stopping it, that is, the number of running operations of the automobile, and the surface pressure of the retaining mat 14; the solid line denotes one in which the second expansion rate is 258%, the broken line denotes one in which the second expansion rate is 163%, and the chain line denotes one in which the second expansion rate is 142%.

[0034] While the number of cycles is a few times to a few tens of times, the retaining mat 14 exposed to high temperature exhaust gas is softened due to the binder being evaporated by heat, the surface pressure of the retaining mat 14 against the inner peripheral face of the main body portion 12a of the metal shell 12 and the outer peripheral face of the catalyst support 13 decreases rapidly, and one in which the second expansion rate is the smallest (see chain line) has the maximum decrease in the surface pressure. When the number of cycles increases to a few hundreds of times or a few thousands of times, since the retaining mat 14 deteriorates and gradually loses its elasticity, the surface pressure gradually decreases, but one in which the second expansion rate is the smallest (see chain line) has the maximum rate of decrease in surface pressure.

[0035] On the other hand, it can be understood that one in which the second expansion rate is intermediate (see broken line) has a large decrease in surface pressure during the initial stage but a very suppressed rate of decrease in the surface pressure thereafter, and one in which the second expansion rate is the maximum (see solid line) has the minimum decrease in surface pressure during the initial stage and a very suppressed rate of decrease in surface pressure thereafter.

[0036] In this way, it is possible, by setting the second expansion rate of the retaining mat 14 due to heating at 170% or greater (in other words, the first expansion rate at 210% or greater), to maintain the surface pressure of the retaining mat 14 at a high value for a long period of time, thus reliably retaining the retaining mat 14 in the interior of the metal shell 12. The reason therefor is that even if the surface pressure decreases when the binder is evaporated by heating and the retaining mat 14 is softened, it can be compensated for by an increase in the surface pressure due to expansion of the retaining mat 14, thereby enabling the minimum value for the surface pressure to be maintained at a value necessary to retain the catalyst support 13 or greater.

[0037] Subsequently, the surface pressure of the retaining mat 14 gradually decreases accompanying an increase in the number of cycles, but the retaining mat 14 of the embodiment, which has a high second expansion rate (or first expansion rate), suppresses any decrease in surface pressure due to deterioration of the retaining mat 14 by means of the high expansion rate, thus enabling the necessary surface pressure to be maintained for a long period of time.

[0038] The graph of FIG. 4 shows the relationship between the second expansion rate of the retaining mat 14 and the initial surface pressure when the retaining mat 14 is hot. The initial surface pressure when it is hot referred to here is the surface pressure immediately after the surface pressure has rapidly decreased subsequent to the binder being evaporated by heating and the retaining mat 14 being softened. It can be understood from this graph that the initial surface pressure of the retaining mat 14 gradually increases accompanying an increase in the second expansion rate; when the second expansion rate attains 170% the initial surface pressure attains a maximum value, and even when the second expansion rate increases beyond 170% the initial surface pressure is maintained at the maximum value.

[0039] The graph of FIG. 5 shows the relationship between the product density and the second expansion rate of the retaining mat 14. The product density of the retaining mat 14 is that obtained by dividing the weight of the retaining mat 14 by the expanded volume in the released state of the retaining mat 14 before being heated, that is, the volume of the portion of the retaining mat 14 that is wrapped around the outer peripheral face of the catalyst support 13, and the units are g/cm.sup.3. As is clear from this graph, the product density correlates with the second expansion rate, and if the product density is 100 g/cm.sup.3 this ensures that the second expansion rate will be 170% or greater (or the first expansion rate will be 210% or greater). Since the product density can be measured easily compared with the expansion rate, it is possible, by the use of product density instead of expansion rate, to easily control the expansion rate of the retaining mat 14, thus enhancing the productivity.

[0040] As described above, in accordance with the present embodiment, since the first expansion rate of the retaining mat 14 disposed between the inner peripheral face of the metal shell 12 and the outer peripheral face of the catalyst support 13 of the catalytic converter 11 for exhaust gas purification is set at 210% or greater, in other words the second expansion rate of the retaining mat 14 is set at 170% or greater, it is possible, by allowing the retaining mat 14 to be sufficiently expanded by heating with exhaust gas and compensating for any decrease in surface pressure due to deterioration of the retaining mat 14, to maintain a constant surface pressure over a long period of time, thus enabling the catalyst support 13 to be reliably retained. Moreover, when fitting the catalyst support 13 and the retaining mat 14 into the interior of the metal shell 12, since it is unnecessary to compress the retaining mat 14 excessively in order to enhance the ability to retain the catalyst support 13, there is no possibility that the catalyst support 13 will be damaged by excessive surface pressure.

[0041] An embodiment of the present invention is explained above, but the present invention may be modified in a variety of ways as long as the modifications do not depart from the spirit and scope thereof.

[0042] For example, the fiber of the retaining mat 14 is not limited to an alumina fiber.

[0043] Furthermore, the thickness of the retaining mat 14 is not limited to those described in Table 1.