Patent Publication Number: US-2005115224-A1

Title: Exhaust emission control device and casing structure of the control device

Description:
FIELD OF THE INVENTION  
      The present invention relates to exhaust gas purifying devices for purifying exhaust gas discharged by internal combustion engines including diesel engines and casing structures for the devices.  
     BACKGROUND OF THE INVENTION  
      Conventionally, a number of exhaust gas purifying devices have been proposed. The devices are configured by accommodating a ceramic honeycomb filter for purifying exhaust gas in a metal tubular casing, which is disposed in an exhaust passage of a diesel engine. As the filter is used for a relatively long time, the filter collects soot (diesel particulates) from the exhaust gas. The soot is deposited in the filter and gradually increases the engine load. If this is the case, the filter is heated to the ignition temperature of the soot (600 to 630 degrees Celsius) by a heating means such as a heater or burner. The soot is thus burned and removed such that the filter is regenerated.  
      However, conventional exhaust gas purifying devices have the following problems.  
      That is, when the filter is heated by the heating means, the heat is transmitted to a different component (for example, the casing) that is held in contact with the filter. The heat thus escapes to the exterior of the filter, hampering the heating of the filter. This increases the energy needed to heat the filter to the soot ignition temperature and raises costs. Further, if an electric heater is used as the heating means, an increased electric load is applied to the battery, which accelerates the battery consumption.  
      In addition, a temperature difference is caused between the middle portion of the filter and an outer peripheral portion of the filter. Therefore, if the honeycomb filter is formed of, for example, porous silicon carbide, an increased thermal stress is generated in the filter. In this case, the filter has a tendency to crack, which is damaging to the filter. Moreover, in conventional devices, replacement of the filter is complicated, making it difficult to maintain the devices.  
      The present invention is for solving the above problems. Accordingly, it is an objective of the invention to provide an exhaust gas purifying device that saves costs by decreasing energy loss and suppressing damage to the filter, as well as a casing structure for the device.  
     DISCLOSURE OF THE INVENTION  
      To solve the above problems, the gist of a first embodiment of the present invention is an exhaust gas purifying device comprising a tubular casing disposed in an exhaust passage of an internal combustion engine and a filter accommodated in the casing for collecting, burning, and removing particulates contained in the exhaust gas discharged by the internal combustion engine. The device is characterized in that the casing has a multiple case structure including an inner case supporting an outer peripheral surface of the filter and at least one outer case arranged around the inner case, and that the inner and outer cases are spaced from each other with a clearance defined between the cases.  
      The gist of a second embodiment is an exhaust gas purifying device comprising a tubular casing disposed in an exhaust passage of an internal combustion engine and a filter accommodated in the casing for collecting, burning, and removing particulates contained in the exhaust gas discharged by the internal combustion engine. The device is characterized in that the casing has a double structure including an inner case supporting an outer peripheral surface of the filter and an outer case arranged around the inner case, and that the inner and outer cases are spaced from each other with a clearance defined between the cases.  
      It is desirable that a fluid blocking member is provided at an upstream end portion of the inner case for blocking communication between a space including an upstream end surface of the filter and the clearance.  
      A fluid blocking member may be provided at a downstream side of the inner case and between the inner case and the outer case for blocking communication between a space including a downstream end surface of the filter and the clearance.  
      The fluid blocking member may be a flange projecting from an outer peripheral surface of an end of the inner case. The flange may be secured to an outermost component of the outer case in an attachable or detachable manner.  
      A heater for regenerating the filter may be disposed at an upstream position with respect to the filter. Further, a porous heat reflector may be arranged at a further upstream position with respect to the heater.  
      The gist of a third embodiment is a casing structure of an exhaust gas purifying device having a multiple structure including an inner case supporting an outer peripheral surface of a filter for collecting, burning, and removing particulates contained in the exhaust gas discharged by an internal combustion engine and at least one outer case arranged around the inner case. The casing structure is characterized in that the inner and outer cases are spaced from each other with a clearance defined between the cases.  
      In each of the above-described embodiments, the outer peripheral surface of the filter is held in contact with the inner case. However, the clearance is defined between the inner case and the outer case. Therefore, in other words, a heat insulating air layer is ensured between the inner and outer cases. The heat transmission from the inner case to the outer case is thus hampered. This prevents the heat from escaping to the exterior of the filter such that the temperature of the filter is efficiently raised. That is, the exhaust gas purifying device reduces energy loss and thus saves costs. Further, since the heat is prevented from escaping from the outer peripheral portion of the filter, the temperature difference between the middle portion and the outer peripheral portion of the filter hardly occurs. As a result, a relatively large thermal stress, which leads to cracks damaging the filter, is avoided.  
      If the fluid blocking member is provided at the upstream end portion of the inner case, communication between the space including the upstream end surface of the filter and the clearance is blocked. Therefore, a relatively hot exhaust gas does not flow into the clearance in which the heat insulating air layer is formed. This structure reduces heat energy loss caused by heat transmission from the exhaust gas to the outer case. The costs are thus further saved. Further, the exhaust gas does not bypass the filter or reach the downstream side of the filter without being purified. The purifying efficiency is thus prevented from being lowered.  
      If the fluid blocking member is provided at the downstream end portion of the inner case, communication between the space including the downstream end surface of the filter and the clearance is blocked. This structure further reduces the heat energy loss caused by the heat transmission from the exhaust gas to the outer case. The costs are thus further saved.  
      If the fluid blocking member is configured by the flange, the flange serves also as a portion to which the inner case is secured. This avoids an increase in the number of the parts and complication of the structure. Further, the filter may be removed from the outer case in a state accommodated in the inner case. The replacement of the filter thus becomes relatively easy, as compared to a conventional case. As a result, maintenance is facilitated.  
      If a heater for regenerating the filter and heat reflector are provided, the heat of the heater is reflected by the heat reflector. The heat energy loss of the heater is thus decreased, and the filter is heated efficiently. Further, since the heat reflector is porous, the flow of the exhaust gas to the filter is not hampered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic view showing an example of an exhaust gas purifying device according to a first embodiment of the present invention;  
       FIG. 2  is a cross-sectional view showing the exhaust gas purifying device of  FIG. 1 ;  
       FIG. 3  is an exploded perspective view showing a casing structure of the exhaust gas purifying device of  FIG. 1 ;  
       FIG. 4  is an end view showing a filter employed in the exhaust gas purifying device of  FIG. 1 ;  
       FIG. 5  is a cross-sectional view showing a portion of the filter;  
       FIG. 6  is a cross-sectional view showing an exhaust gas purifying device of a second embodiment;  
       FIG. 7  is a cross-sectional view showing an exhaust gas purifying device of a third embodiment;  
       FIG. 8  is a cross-sectional view showing an exhaust gas purifying device of a fourth embodiment; and  
       FIG. 9  is a cross-sectional view showing an exhaust gas purifying device of a fifth embodiment. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      An exhaust gas purifying device  1  according to a first embodiment of the present invention will hereafter be described with reference to FIGS.  1  to  5 .  
      As illustrated in  FIG. 1 , the exhaust gas purifying device  1  purifies exhaust gas discharged by a diesel engine  2 , which serves as an internal combustion engine. The diesel engine  2  includes a plurality of non-illustrated cylinders. A branch  4  of a metal exhaust manifold  3  is coupled to each of the cylinders. The branches  4  are connected to a manifold body  5 . Thus, the exhaust gas sent from each of the cylinders is concentrated at one point.  
      A first exhaust pipe  6  and a second exhaust pipe  7 , which are formed of metal, are disposed at a downstream side of the exhaust manifold  3 . An upstream end of the first exhaust pipe  6  is coupled to the manifold body  5 . The exhaust gas purifying device  1  is located between the first exhaust pipe  6  and the second exhaust pipe  7 .  
      With reference to  FIG. 2 , the exhaust gas purifying device  1  includes a cylindrical casing  21 . An exhaust gas purifying filter  13  is accommodated in the casing  21 . The filter  13  will hereafter be explained.  
      Since the filter  13  is used for removing diesel particulates, the filter  13  is called also as a diesel particulate filter (DPF). The filter  13  of the first embodiment, as shown in  FIGS. 4 and 5 , is formed by bundling a plurality of sintered honeycomb bodies F as one body. The sintered honeycomb bodies F located in the middle portion of the filter  13  are shaped as square rods. A plurality of sintered honeycomb bodies F that are shaped differently, or unlike the square rods, are arranged around the honeycomb bodies F that are shaped as the square rods. Accordingly, the filter  13  as a whole is shaped as a cylinder.  
      In the first embodiment, each of the sintered honeycomb bodies F is formed from a sintered body of porous silicon carbide (SiC), which is a type of sintered ceramic body. However, other than the sintered body of silicon carbide, a sintered body of silicon nitride or sialon or alumina or cordierite may be selected. A plurality of through holes  11 , each of which has a substantially square cross-sectional shape, are formed in each of the sintered honeycomb bodies F and are arranged regularly in an axial direction. The through holes  11  are spaced from one another by cell walls  12 . An opening of each through hole  11  is sealed by a plug  14  (in this embodiment, a sintered body of porous silicon carbide) at one of end surfaces  13   a,    13   b.  Each of the end surfaces  13   a,    13   b  as a whole thus presents a diced pattern. As a result, a number of cells each having a square cross-sectional shape are formed in each sintered honeycomb body F. Approximately half of the cells open at the upstream end surface  13   a,  while the remaining cells open at the downstream end surface  13   b.    
      As shown in  FIGS. 4 and 5 , the sintered honeycomb bodies F are bound together by an adhesive  15  at the outer peripheral surfaces. The adhesive  15  serves to compensate thermal expansion of the sintered honeycomb bodies F. In other words, the adhesive  15  prevents cracks from being caused by thermal stress. In the first embodiment, a heat resistant ceramic adhesive in which ceramic fibers are dispersed is used as the adhesive  15 . It is preferred that silicon carbide powders, in addition to the ceramic fibers, are dispersed in the adhesive  15 .  
      With reference to  FIGS. 2 and 3 , the casing  21 , which accommodates the filter  13 , is configured by a plurality of metal tubular members. More specifically, in the first embodiment, three members including a heater case  22 , an inner case  23 , and an outer case  24  (all formed of SUS304) configure the casing  21 . The heater case  22 , serving as a heating means accommodating case, forms a portion of a heater unit. The inner case  23  forms a portion of a filter unit.  
      The heater case  22  defines a sealed space for preventing exhaust gas from entering the heater case  22 . The heater case  22  thus provides a heat insulating effect. A mat-shaped heat insulating material  8 , the main component of which is ceramic fibers, is packed in the space of the heater case  22 . This improves the heat insulating effect.  
      Each of the inner case  23  and the outer case  24  has a cylindrical shape. The inner case  23 , which is accommodated in the outer case  22 , is smaller than the outer case  24  (the longitudinal dimension and the diameter of the inner case  23  are smaller than those of the outer case  24 ). Therefore, when the cases are assembled, a clearance C 1  having a constant dimension is defined between the outer peripheral surface of the inner case  23  and the inner peripheral surface of the outer case  24 , which is arranged around the inner case  23 . In the first embodiment, the dimension of the clearance C 1  is approximately 1 to 5 millimeters. A flange  25  of the inner case  23  serves also as a fluid blocking member that blocks communication between the space including the upstream end surface  13   a  of the filter  13  and the clearance C 1 .  
      The flange  25  projects from the outer peripheral surface of an upstream end portion of the inner case  23 . In the same manner, a flange  26  projects from the outer peripheral surface of an upstream end portion of the outer case  24 . The flanges  25 ,  26  are designed to define equal outer diameters. A plurality of bolt holes  28  are formed in the flange  25  and are spaced from adjacent ones at certain intervals. In the same manner, a plurality of bolt holes  29  are formed in the flange  26  and are spaced from adjacent ones at certain intervals. Attachment bolts  27  are passed through the bolt holes  28 ,  29 . The bolt holes  29  of the flange  25  are located at positions corresponding to the bolt holes  28  of the flange  26 . In the same manner, a plurality of bolt holes  31  are formed in a downstream end surface of the heater case  22  at positions corresponding to the bolt holes  28 ,  29 . Thus, if the bolts  27  are passed through the corresponding bolt holes  28 ,  29 ,  31  and fastened as such, the cases  22 ,  23 ,  24  are fixed to each other to form one body, or the casing  21 . If the bolts  27  are removed from the bolt holes  28 ,  29 ,  31 , the casing  21  is separated into three parts. In other words, it may be understood that the flange  25  of the inner case  23  is secured to the flange  26  of the outer case  24  in an attachable or detachable manner.  
      The mat-shaped heat insulating material  17  containing ceramic fibers is wrapped around the filter  13 . In this state, the filter  13  is held in the inner case  23 . That is, the inner case  23  supports the outer peripheral surface of the filter  13  through the heat insulating material  17 . A filter support  30  projects from the inner peripheral surface of the downstream end of the inner case  23  and extends along the entire circumference. The filter support  30  is abutted by an outer peripheral portion of the downstream end surface  13   b  of the filter  13 . This structure prevents the filter  13  from falling from the inner case  23  to the downstream side.  
      A support piece  30   a  is attached to the inner peripheral surface of the upstream end portion of the inner case  23  and extends along the entire circumference. The support piece  30   a  is abutted by an outer peripheral portion of the upstream end surface  13   a  of the filter  13 . This structure prevents the filter  13  from falling from the inner case  23  to the upstream side. Further, the heat insulating material  17  is prevented from being displaced. A plurality of support pieces  30   a  may be deployed as spaced from adjacent ones at predetermined angular intervals.  
      A coupling portion  32  projects from a middle portion of the, upstream end surface of the heater case  22  and is coupled with a downstream end of the first exhaust pipe  6 . Further, a coupling portion  33  projects from a middle portion of the downstream end surface of the outer case  24  and is coupled with an upstream end of the second exhaust pipe  7 .  
      As illustrated in  FIGS. 1 and 2 , the heater case  22 , a component of the casing  21 , accommodates a heater  34  and a temperature detector  35 . The heater  34  serves as an electric heating means for regenerating the filter and is located at an upstream position with respect to the filter  13 . In the first embodiment, an AC heater is used as the heater  34 . The heater  34  is formed by winding a cable spirally. More specifically, the cable includes a conductive core of a nichrome wire covered by a sheath of magnesia, which presents improved insulating performance.  
      The heater  34  is opposed to the upstream end surface  13   a  of the filter  13  and is spaced from the upstream end surface  13   a  at a certain interval. Two end portions  34   a  of the heater  34  extend through an outer peripheral portion of the heater case  22  to the exterior of the casing  21 . The core projecting from each of the end portions  34   a  of the heater  34  is electrically connected to a connector through a glass tube. The connector, as shown in  FIG. 1 , is electrically connected to a driver circuit of a control unit U 1  controlling the regenerating operation of the filter  13 . Thus, when necessary, the control unit U 1  operates to supply power to the heater  34  from an external power source B 1 . This enables the heater  34  to generate heat from the entire portion such that the temperature rises to 800 to 900 degrees Celsius.  
      With reference to  FIG. 2 , a punching plate  38  serving as a porous heat reflector is disposed at an upstream position with respect to the heater  34 . The punching plate  38  is a disk-shaped plate member and is formed of stainless steel (SUS304) in the first embodiment. The outer periphery of the punching plate  38  is bonded with the inner peripheral surface of the heater case  22  through, for example, welding. The punching plate  38  is thus secured to the heater case  22 . Further, the heater  34  is fixed to the punching plate  38  by a fixing tool  37 .  
      A number of through holes  38   a  extend through the punching plate  38 . The through holes  38   a  are arranged regularly to cover substantially the entire area of the punching plate  38 . Therefore, after being discharged from the first exhaust pipe  6 , exhaust gas passes through the through holes  38   a  to reach the filter  13 . Further, the punching plate  38  reflects the heat generated by the heater  34 , preventing the heat from being released to the exterior. In other words, the heat reflected by the punching plate  38  is supplied to the filter  13  as radiation heat. The filter  13  is thus efficiently heated.  
      A ceramic foam reflector  39  serving as another porous heat reflector is disposed at the downstream side of the filter  13 , as illustrated in  FIG. 2 . In the first embodiment, the ceramic foam reflector  39  is supported by a heat reflector support  40 , which projects from the inner peripheral surface of the outer case  24  and extends along the entire circumference. The ceramic foam reflector  39  is a porous body formed of aluminum nitride or the like and presents fluid permeability and heat insulating properties. Therefore, like the upstream side of the filter  13 , a heat insulating effect is ensured at the downstream side of the filter  13 .  
      With reference to  FIG. 2 , the temperature detector  35  is deployed in the vicinity of the heater  34 . The temperature detector  35  is a rod-shaped body, and a temperature detecting portion  35   a  is formed at a distal end of the temperature detector  35 . The temperature detector  35  of this embodiment includes a sheath thermocouple covered by a protecting pipe formed of stainless steel or the like. The temperature detecting portion  35   a  is exposed from a distal end of the covered sheath thermocouple. The temperature detecting portion  35   a  is placed in the space defined by the cable spirally wound in the heater  34 . The temperature detector  35  is electrically connected to the control unit U 1 .  
      The operation of the exhaust gas purifying device  1  configured as described above will hereafter be explained.  
      The casing  21  accommodating the filter  13  is deployed in the exhaust gas passage, or between the first exhaust pipe  6  and the second exhaust pipe  7 . In this state, if the engine  2  is started, exhaust gas is sent to the interior of the casing  21 . That is, after being discharged from the first exhaust pipe  6 , the exhaust gas flows first into the cells opening at the upstream end surface  13   a  of the filter  13 . The gas then permeates the cell walls  12  and reaches the adjacent cells, which open at the downstream end surface  13   b  of the filter  13 . Further, through the openings of the cells, the exhaust gas is discharged from the downstream end surface  13   b  of the filter  13 .  
      However, the soot contained in the exhaust gas is not permitted to permeate the cell walls  12  and is trapped in the cells. As a result, the exhaust gas is purified before being discharged from the downstream end surface  13   b  of the filter  13 . The purified gas further flows in the second exhaust pipe  7  and is eventually released to the atmosphere. Afterwards, the heater  34  is powered to heat the filter  13  and supporting air is supplied such that the soot is burned and removed. More specifically, the soot in the vicinity of the upstream end surface  13   a  of the filter  13  starts to burn. Burning of the soot is gradually spread to the downstream end surface  13   b.  By maintaining the soot burning for a certain time period, the filter  13  is regenerated.  
      Accordingly, the first embodiment has the following effects.  
      (1) The casing  21  of the first embodiment has a double structure formed by the inner case  23  and the outer case  24 . The inner case  23  supports the outer peripheral surface of the filter  13 . The outer case  24  is arranged around the inner case  23 . Further, the cases  23 ,  24  are spaced from each other at an interval corresponding to the clearance C 1 .  
      Although the outer peripheral surface of the filter  13  is held in contact with the inner case  23 , the clearance C 1  is ensured between the inner case  23  and the outer case  24 . In other words, a heat insulating air layer is formed between the cases  23 ,  24 . The heat insulting air layer presents relatively low heat conductivity as compared to metal and hampers the heat transmission from the inner case  23  to the outer case  24 . Further, air convection does not occur readily in the clearance C 1 . This structure stops heat from escaping to the exterior of the filter  13 . The temperature of the filter  13  is thus efficiently raised. In other words, the exhaust gas purifying device  1  has decreased energy loss and thus saves costs. In addition, since the heat energy applied to the filter  13  is also decreased, the power supplied to the heater  34  is reduced and the time required for regenerating the filter  13  is shortened.  
      (2) The casing structure of the first embodiment prevents heat from escaping from the outer peripheral portion of the filter  13 . Thus, a temperature difference is hardly caused between the middle portion of the filter  13  and the outer peripheral portion of the filter  13 . This avoids generation of relatively large thermal stress, which leads to cracks damaging the filter  13 . Therefore, the exhaust gas purifying device  1  presents improved strength and has relatively high reliability.  
      (3) Since the casing structure of the first embodiment stops heat from escaping from the outer peripheral portion of the filter  13 , the temperature of the outer peripheral surface of the outer case  24  is reliably lowered. Therefore, the components attached to the outer surface of the outer case  24  and those disposed around the outer case  24 , for example, do not necessarily have to have high heat resisting performance, as compared to conventional counterparts.  
      (4) In the casing structure of the first embodiment, the flange  25  serving as the fluid blocking member, which is formed at the upstream end portion of the inner case  23 , blocks communication between the space including the upstream end surface of the filter  13  and the clearance C 1 . Thus, relatively hot exhaust gas does not flow into the clearance C 1  in which the heat insulating air layer is located. This prevents the heat from the exhaust gas from being transmitted to the outer case  24 , reducing the heat energy loss otherwise caused by such heat transmission. The costs are thus further saved. In addition, the exhaust gas does not bypass the filter  13  or reach the downstream side without being purified. This structure prevents the purifying efficiency from being lowered.  
      (5) In the casing structure of the first embodiment, the flange  25  serving as the fluid blocking member also functions as a portion to which the inner case  23  is fixed. This structure avoids an increase in the number of the parts and complication of the configuration.  
      Further, this structure allows the filter  13  to be removed from the outer case  24  in a state accommodated in the inner case  23 . It is thus possible to easily replace the filter  13 , as compared to a conventional case. Also, a major portion of the downstream end surface  13   b  of the filter  13  is exposed from the inner case  23 . This makes it relatively easy to clean ashes after the filter  13  is removed from the casing  21 . In this manner, the casing structure of this embodiment facilitates the maintenance.  
      (6) In the casing structure of the first embodiment, the punching plate  38  serving as the heat reflector reflects the heat of the heater  34 . Further, the heat of the filter  13 , which is heated by the heater  34 , is reflected by the ceramic foam reflector  39  also serving as the heat reflector. The heat energy loss of the heater  34  is thus decreased such that the filter  13  is efficiently heated. This structure contributes to shortening of the time required for regenerating the filter  13 . In addition, since the heat reflectors are porous, the exhaust gas flow to and from the filter  13  is not hampered.  
      (7) In the first embodiment, the outer peripheral portion of the upstream end surface  13   a  of the filter  13  is held in contact with the filter support piece  30   a  formed at the upstream end portion of the inner case  23 . This structure prevents the heat insulating material  17  from being corroded. Further, when the filter  13  is removed from the casing  21  and is washed in water for cleaning ashes, the heat insulating material  17  is stopped from being displaced.  
      Other embodiments of the present invention will hereafter be described.  
      In a second embodiment illustrated in  FIG. 6 , a flange  41  projects from the outer peripheral surface of the downstream end portion of the inner case  23  and extends along the entire circumference. In this embodiment, the flange  41  serving as a fluid blocking member blocks communication between the space including the downstream end surface  13   b  of the filter  13  and the clearance C 1 . This structure further reduces the heat energy loss caused by the heat transmission from the exhaust gas to the outer case  24 , as compared to the first embodiment. This effect is brought about by the fact that a heat insulating air layer more preferable than that of the first embodiment is formed in the clearance C 1 . Therefore, costs are further saved. In addition, the filter  13  is further securely fixed as long as the outer periphery of the flange  41  is held in contact with the inner peripheral surface of the outer case  24 , as shown in  FIG. 6 .  
      Further, it is desirable that a clearance is defined between the flange  41  and the inner peripheral surface of the outer case  24 . The clearance makes it possible to remove the inner case  23  smoothly from the outer case  24 , even if the dimensions of the inner case  23  are changed due to thermal expansion.  
      In a third embodiment illustrated in  FIG. 7 , the casing  21  includes an additional outer case  42 , other than the outer case  24 . The casing  21  thus has a triple structure. The additional outer case  42  is arranged between the outer case  24  and the inner case  23 . This structure has two clearances C 1 , each of which serves as a heat insulating air layer. Further, a flange  43  projects from the outer peripheral surface of the upstream end portion of the outer case  42 . The flange  43  is deployed as clamped between the flanges  25 ,  26 .  
      In a fourth embodiment illustrated in  FIG. 8 , a flange  41   a  serving as a fluid blocking member projects from the inner peripheral surface of the outer case  24 . The flange  41   a  is engaged with the filter support  30  of the inner case  23 . The communication between the space including the downstream end surface  13   b  of the filter  13  and the clearance C 1  is blocked by the flange  41   a.  This structure further reduces the heat energy loss caused by the heat transmission from the exhaust gas to the outer case  24 , as compared to a comparative example (which will be described later).  
      Also, the fourth embodiment is not provided with the punching plate  38  such that the configuration becomes simple.  
      In a fifth embodiment illustrated in  FIG. 9 , the support piece  30   a,  which is otherwise formed at the upstream end portion of the inner case  23 , is not provided, as is clear from comparison with the first embodiment. The fifth embodiment thus has the operational effects of the first embodiment, except for that of the support piece  30   a.    
      The exhaust gas purifying devices of the illustrated embodiments were subjected to an operational test. One cycle of the test was defined by sending exhaust gas to each of the devices, heating the filter by the heater, and supplying supporting air for burning the soot. The test included ten cycles. The test results are shown in Table 1. The longitudinal dimension of the filter  13  was 150 millimeters.  
                                                   TABLE 1                               Up-   Down-               Temperature       Insulator               stream   stream       Support   Punching   Difference       Corrosion       Embodiment   Drawing   Blocker   Blocker   Structure   Piece   Plate   (C. °)   Crack   Size                                                                        1       Formed   None   Double   Formed   Formed   30   None   0       2       Formed   Formed   Double   Formed   Formed   20   None   0       3       Formed   None   Triple   Formed   Formed   26   None   0       4       Formed   Formed   Double   Formed   None   40   None   0       5       Formed   None   Double   None   Formed   30   None   15       Comparative   —   None   None   Single   None   None   150   Detected   15                  
 
      In Table 1, the temperature difference indicates the temperature difference between the middle portion and the outer peripheral portion of the filter  13 . The insulator corrosion size indicates the size of the portion of the heat insulating material lost due to corrosion in the test. Detection of cracks was conducted visually after the test was completed. A purifying device prepared as the comparative example did not include any of the inner case, the upstream blocking member, the downstream blocking member, the support piece, and the punching plate. This purifying device was also subjected to the operational test.  
      As indicated clearly in Table 1, in the comparative example, the temperature difference between the middle portion and the outer peripheral portion of the filter was enlarged, causing cracks. In contrast, the temperature difference was relatively small in the first to fifth embodiments. Therefore, no crack was detected, and the durability was improved.  
      Further, as is clear from comparison among the comparative example, the first embodiment, and the fifth embodiment, the support piece  30   a  operated efficiently for preventing the heat insulating material from being corroded.  
      The present invention is not restricted to the illustrated embodiments but may be embodied in the following forms.  
      In the illustrated embodiments, the punching plate  38  and the ceramic foam reflector  39  are employed as the porous heat reflectors. However, the heat reflectors are not limited to those of the embodiments but may be, for example, a body formed of metal meshes or ceramic fibers.  
      The ceramic foam reflector  39 , which is provided at the downstream side of the filter  13 , may be replaced by the punching plate  38 . Further, the ceramic foam reflector  39  may be omitted. In this manner, the number of the components of the exhaust gas purifying device  1  is decreased.  
      The heater  34  does not necessarily have to be an AC heater but may be, for example, a DC heater. Further, the electric heating means such as the electric heater may be replaced by a non-electric heating means such as a burner.  
      The fluid blocking member does not necessarily have to be the flange  25 , which projects from the outer peripheral surface of the end of the inner case  23 . The fluid blocking member may be a different structure provided separately from the inner case  23 .  
      The flange  25  may be fixed to the outer case  24 , which is the outermost layer, through, for example, welding, such that the flange  25  is prohibited from being attached to or detached from the outer case  24 .