Patent Publication Number: US-2021172363-A1

Title: Catalytic converter for exhaust gas purification

Description:
TECHNICAL FIELD 
     The present invention relates to 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. 
     BACKGROUND ART 
     With regard to such a catalytic converter for exhaust gas purification, an arrangement is known from Patent Document 1 below in which the tightening allowance for a retaining mat is minimized by determining the diameter for a catalyst support from the thickness of the retaining mat and the tightening allowance after press fitting the retaining mat so that the bulk density of the retaining mat at the time of assembly is within a predetermined range, thus maximizing the diameter of the catalyst support, and the catalyst support can be retained reliably even after a long period of use by determining the axial length of the retaining mat necessary in order to non-movably retain the catalyst support within a metal shell from the exhaust gas load acting on the catalyst support and the frictional force of the retaining mat. 
     RELATED ART DOCUMENTS 
     Patent Documents 
     Patent Document 1: Japanese Patent Application Laid-open No. 2013-204424 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     Since the retaining mat of a catalytic converter for exhaust gas purification gradually deteriorates over a long period of use and its elasticity is lost, its surface pressure decreases, and there is a possibility that the catalyst support will not be able to be reliably retained. In order to prevent this, when press fitting the retaining mat and the catalyst support into the interior of the metal shell, assembly may be carried out in a state in which a large tightening allowance is set for the retaining mat and it is strongly compressed. However, with this method there is a possibility that the catalyst support, which is made of a brittle ceramic, will be damaged by strong surface pressure when press fitting the retaining mat. 
     Furthermore, even when an attempt is made to increase the retaining force with which the catalyst support is retained by increasing the axial dimension of the retaining mat and increasing the contact area via which the retaining mat makes contact with the catalyst support and the metal shell, since the upper limit of the axial dimension of the retaining mat is restricted by the axial dimension of the catalyst support, this method also has limitations. 
     The present invention has been accomplished in light of the above circumstances, and it is an object thereof to suppress any decrease in surface pressure due to deterioration of a retaining mat of a catalytic converter for exhaust gas purification, thereby enabling a catalyst support to be reliably retained. 
     Means for Solving the Problems 
     In order to attain the above object, according to a first aspect of the present invention, there is provided 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, 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. 
     Further, according to a second aspect of the present invention, there is provided 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, characterized in that 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. 
     Furthermore, according to a third aspect of the present invention, in addition to the first or second aspect, a density of the retaining mat in a released state before being heated by exhaust gas is 100 g/cm 3  or greater. 
     Effects of the Invention 
     In accordance with the first aspect of the present invention, the catalytic converter for exhaust gas purification is formed by retaining the catalyst support in the interior of the metal shell by means of the retaining mat disposed between the inner peripheral face of the metal shell and the outer peripheral face of the catalyst support. Since the thickness of the retaining mat in a released state after being heated by exhaust gas is 210% or greater of the clearance between the inner peripheral face of the metal shell and the outer peripheral face of the catalyst support, it is possible to reliably retain the catalyst support over a long period of time by compensating for a decrease in surface pressure due to deterioration of the retaining mat with an increase in the surface pressure due to sufficient expansion of the retaining mat. Moreover, since it is unnecessary to strongly compress the retaining mat when assembling so as to enhance the surface pressure, there is no possibility that the catalyst support will be damaged by excessive surface pressure. 
     Furthermore, in accordance with the second aspect of the present invention, the catalytic converter for exhaust gas purification is formed by retaining the catalyst support in the interior of the metal shell by means of the retaining mat disposed between the inner peripheral face of the metal shell and the outer peripheral face of the catalyst support. Since the thickness of the retaining mat in a released state after being heated by exhaust gas is 170% or greater of the thickness of the retaining mat in a released state before being heated by exhaust gas, it is possible to reliably retain the catalyst support over a long period of time by compensating for a decrease in surface pressure due to deterioration of the retaining mat with an increase in the surface pressure due to sufficient expansion of the retaining mat. Moreover, since it is unnecessary to strongly compress the retaining mat when assembling so as to enhance the surface pressure, there is no possibility that the catalyst support will be damaged by excessive surface pressure. 
     Furthermore, in accordance with the third aspect of the present invention, since the density of the retaining mat in a released state before being heated by exhaust gas is 100 g/cm 3  or greater, it becomes possible to control the first expansion rate so that it is 210% or greater by using the density, which is easy to measure compared with the expansion rate, thus contributing to an improvement in the productivity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical sectional view of a catalytic converter for exhaust gas purification. (first embodiment) 
         FIG. 2  is a diagram for explaining the mechanism via which a retaining mat undergoes initial expansion. (first embodiment) 
         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) 
         FIG. 4  is a graph showing the relationship between the second expansion rate and the surface pressure of the retaining mat. (first embodiment) 
         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 
       11  Catalytic converter for exhaust gas purification
       12  Metal shell     13  Catalyst support     14  Retaining mat   α Clearance   

     MODES FOR CARRYING OUT THE INVENTION 
     An embodiment of the present invention is explained below by reference to  FIG. 1  to  FIG. 5 . 
     First Embodiment 
       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  12   a,  an upstream side taper portion  12   b  that is connected to the upstream side, in the direction of flow of exhaust gas, of the main body portion  12   a,  and a downstream side taper portion  12   c  that is connected to the downstream side, in the direction of flow of exhaust gas, of the main body portion  12   a.  A cylindrical catalyst support  13  is retained in the interior of the main body portion  12   a  of the metal shell  12  via a retaining mat  14 . 
     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. 
     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  12   a  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  12 a of the metal shell  12  via the retaining mat  14 . 
     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. 
     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. 
     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 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 
               
               
                   
               
            
           
         
       
     
     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  12   a  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  12   a  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  12   a  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  12   a  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). 
     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. 
     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. 
     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%. 
     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  12   a  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. 
     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. 
     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. 
     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. 
     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. 
     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 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 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. 
     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. 
     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. 
     For example, the fiber of the retaining mat  14  is not limited to an alumina fiber. 
     Furthermore, the thickness of the retaining mat  14  is not limited to those described in Table 1.