Abstract:
An exhaust purification system includes a selective catalytic reduction (SCR) catalyst disposed at an exhaust system of an engine for using ammonia that is generated from urea water as a reducing agent to reduce NOx contained in exhaust gas, a device that injects urea water to the SCR catalyst, an inlet-side electrode that detects capacitance within the SCR catalyst at least from a vicinity of an inlet of the SCR catalyst to a vicinity of an intermediate section in an exhaust gas flowing direction, an outlet-side electrode that detects the capacitance within the SCR catalyst at least from the vicinity of the intermediate section to an outlet of the SCR catalyst in the exhaust gas flowing direction, and a calculation unit that calculates an ammonia adsorption amount within the SCR catalyst on a basis of the capacitances detected from the inlet-side and the outlet-side electrodes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National Stage Application, which claims the benefit under 35 U.S.C. §371 of PCT International Patent Application No. PCT/JP2014/076963, filed Oct. 8, 2014, which claims the foreign priority benefit under 35 U.S.C. §119 of Japanese Patent Application No. 2013-210701, filed Oct. 8, 2013, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an exhaust purification system, and in particular to an exhaust purification system including an NOx catalyst to reduce and purify nitrogen compounds (NOx) contained in exhaust gas. 
     BACKGROUND ART 
     As an NOx catalyst arranged in an exhaust system of a diesel engine or the like, a selective catalytic reduction (SCR) catalyst is known which selectively reduces and purifies NOx contained in exhaust gas with ammonia (NH 3 ). Ammonia is used as a reducing agent, and is generated by hydrolysis from an aqueous urea solution (urea water). 
     If an excessive amount of aqueous urea solution is injected into the SCR catalyst, and an amount of NH 3  supplied to the SCR catalyst exceeds NH 3  adsorption capacity of the SCR catalyst, then unwanted slip of excess NH 3  occurs, and the excess NH 3  is emitted to the atmosphere. This is not a desirable situation. Accordingly, there is a known technique of estimating the adsorbed amount of NH 3  in the SCR catalyst on the basis of a detection value of an NH 3  sensor provided at an outlet of the SCR catalyst, and correcting as necessary an amount of aqueous urea solution to be injected into the SCR catalyst in accordance with the estimated adsorbed amount of NH 3  (see, for example, Patent Literature Document 1). 
     LISTING OF REFERENCES 
     Patent Literature Document 1: Japanese Patent Application Laid-Open Publication No. 2003-293737 
     In general, an amount of NH 3  adsorbed in the SCR catalyst exhibits an uneven distribution, with a greater amount of NH 3  being adsorbed near an inlet of the SCR catalyst than near an outlet of the SCR catalyst. Because the NH 3  sensor cannot be arranged directly in the SCR catalyst, it is impossible to accurately recognize the actual adsorbed amount of NH 3  in the SCR catalyst with the NH 3  sensor. Therefore, the technique of estimating the adsorbed amount of NH 3  on the basis of the sensor value of the NH 3  sensor may not be able to achieve an optimum correction to the injection amount of the aqueous urea solution in accordance with the actual adsorbed amount of NH 3 . 
     SUMMARY OF THE INVENTION 
     A system disclosed herein is designed to detect an adsorbed amount of NH 3  in the SCR catalyst with high precision. 
     A system disclosed herein includes: at least one selective reduction catalyst arranged in an exhaust system (exhaust gas passage) of an internal combustion engine to reduce and purify nitrogen compounds contained in exhaust gas with ammonia generated from an aqueous urea solution as a reducing agent; an aqueous urea solution injection unit for injecting the aqueous urea solution to the selective reduction catalyst; a first capacitance detecting unit for detecting (measuring) capacitance of the selective reduction catalyst at least from a vicinity of an inlet of the selective reduction catalyst to a vicinity of a middle of the selective reduction catalyst in an exhaust gas flowing direction; a second capacitance detecting unit for detecting the capacitance of the selective reduction catalyst at least from the vicinity of the middle of the selective reduction catalyst to a vicinity of an outlet of the selective reduction catalyst along the exhaust gas flowing direction; and a reducing agent adsorption amount calculation unit for calculating an adsorbed amount of the reducing agent in the selective reduction catalyst on the basis of the capacitance entered from the first capacitance detecting unit and the capacitance entered from the second capacitance detecting unit. 
     According to the system disclosed herein, it is possible to precisely detect an amount of NH 3  adsorbed in the SCR catalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic overall configuration diagram illustrating an exhaust purification system according to an embodiment of the present invention. 
         FIGS. 2A and 2B  illustrate examples of arrangement patterns of electrodes according to embodiments of the present invention. 
         FIG. 3  is a functional block diagram illustrating an ECU according to the embodiment of an present invention. 
         FIG. 4  is a diagram illustrating an example of a capacitance-temperature characteristics map according to an embodiment of the present invention. 
         FIG. 5  is a diagram illustrating an example of a capacitance-NH 3  adsorption amount map according to an embodiment of the present invention. 
         FIG. 6  is a diagram illustrating an example of a maximum adsorbable NH 3  amount map according to the embodiment of an present invention. 
         FIG. 7  is a flowchart illustrating control according to an embodiment of the present invention. 
         FIG. 8  is a schematic overall configuration diagram illustrating an exhaust purification system according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, an exhaust purification system according to embodiments of the present invention will be described with reference to the accompanying drawings. Like parts are designated by like reference numerals, and such like parts have like names and functions. Accordingly, redundant detailed descriptions of such like parts will be omitted. 
     As shown in  FIG. 1 , a diesel engine (hereinafter referred to simply as “engine”)  10  has an intake manifold  10   a  and an exhaust manifold  10   b . An intake passage  11  for introducing fresh air is connected to the intake manifold  10   a , and an exhaust passage  12  for discharging exhaust gas to the atmosphere is connected to the exhaust manifold  10   b.    
     On the intake passage  11 , disposed are an air cleaner  13 , a compressor  15   a  of a turbo charger  15 , an intercooler  17 , and so on in this order from the upstream side in the intake air flowing direction. On the exhaust passage  12 , disposed a turbine  15   b  of the turbo charger  15 , an upstream after-treatment system  20 , a downstream after-treatment system  30 , and so on in this order from the upstream side in the exhaust gas flowing direction. It should be noted in  FIG. 1  that reference numeral “ 18 ” denotes an engine rotation speed sensor, and reference numeral “ 19 ” denotes an accelerator opening degree sensor. 
     The upstream after-treatment system  20  includes a catalyst casing  20   a , an oxidation catalyst (a diesel oxidation catalyst, which will be hereinafter referred to as “DOC”)  21 , and a diesel particulate filter (hereinafter referred to as “DPF”)  22 , with the DOC  21  and the DPF  22  arranged in this order from the upstream side in the gas exhaust flowing direction in the catalyst casing  20   a . In addition, an exhaust pipe injection device (in-pipe injection device)  23  is arranged at a position upstream of the DOC  21 . 
     The exhaust pipe injection device  23  injects unburnt fuel (mainly HC) into the exhaust passage  12  at a position upstream of the DOC  21  in response to an instruction signal entered from an electronic control unit (hereinafter referred to as “ECU”)  50 . It should be noted that in the case where post-injection by means of multi-stage injections of the engine  10  is employed, the exhaust pipe injection device  23  may be omitted. 
     The DOC  21  has a ceramic support having, for example, a cordierite honeycomb structure, and catalytic components supported on a surface of the ceramic support. As the unburnt fuel (HC) is supplied to the DOC  21  by the exhaust pipe injection device  23  or the post-injection, the DOC  21  oxidizes the unburnt fuel to raise the temperature of the exhaust gas. 
     The DPF  22  has, for example, a large number of cells defined by porous partitions and arranged along the exhaust gas flowing direction, with the upstream and downstream sides of the cells being sealed or plugged alternately. The DPF  22  collects particulate matter (PM) in the exhaust gas that collects in pores of the partitions and on surfaces of the partitions. When an estimated amount of accumulated PM reaches a predetermined amount, so-called forced regeneration is carried out, i.e., the accumulated PM is burnt for removal. The forced regeneration is accomplished by supplying the unburnt fuel (HC) into the DOC  21  through the exhaust pipe injection device  23  or the post-injection, and thus increasing the temperature of the exhaust gas flowing into the DPF  22  up to a PM combustion temperature (for example, about 500 to 600 degrees C.). 
     The downstream after-treatment system  30  includes an aqueous urea solution injection device  31 , a casing  30   a , and an SCR catalyst  32  received in the casing  30   a , with the aqueous urea solution injection device  31  being arranged upstream of the SCR catalyst  32  with respect to the gas exhaust flowing direction. 
     The aqueous urea solution injection device  31  is an example of aqueous urea solution injection unit according to the present invention, and injects an aqueous urea solution from an aqueous urea solution tank (not shown) into the exhaust passage  12  at a position upstream of the SCR catalyst  32  in response to an instruction signal introduced from the ECU  50 . The injected aqueous urea solution is hydrolyzed to NH 3  through exhaust gas heat, and NH 3  is supplied to the SCR catalyst  32  on the downstream side as a reducing agent. 
     The SCR catalyst  32  has a ceramic support having, for example, a honeycomb structure, and zeolite or the like supported on a surface of the ceramic support. The SCR catalyst  32  includes a large number of cells defined by porous partitions and arranged along the exhaust gas flowing direction. The SCR catalyst  32  adsorbs NH 3  supplied as the reducing agent, and the adsorbed NH 3  selectively reduces NOx contained in the exhaust gas passing through the SCR catalyst  32  for purification of the exhaust gas. In addition, the SCR catalyst  32  of this embodiment has a plurality of inlet-side electrodes  37  which are arranged opposite to one another with at least one partition placed therebetween to form capacitors, and a plurality of outlet-side electrodes  38  which are arranged opposite to one another with at least one partition placed therebetween to form capacitors. 
     The inlet-side electrodes  37  are inserted in the cells of the SCR catalyst  32  from the inlet (i.e., upstream side) up to the vicinity of a substantial middle (center) of the SCR catalyst  32  in the exhaust gas flowing direction. The outlet-side electrodes  38  are inserted in the cells of the SCR catalyst  32  from the outlet (i.e., downstream side) up to the vicinity of the substantial middle of the SCR catalyst  32  in the exhaust gas flowing direction. An outer surface of each of the inlet-side electrodes  37  and the outlet-side electrodes  38  is coated with a corrosion-resistant insulating layer (not shown). Each of the inlet-side electrodes  37  and the outlet-side electrodes  38  is electrically connected to the ECU  50  through a capacitance detecting circuit (not shown). The inlet-side electrodes  37 , the outlet-side electrodes  38 , and the capacitance detecting circuits (not shown) are preferred examples of capacitance detecting unit according to the present invention. 
     Preferable arrangement patterns of the inlet-side electrodes  37  and the outlet-side electrodes  38  include, for example, a pattern in which two rows each extending in a diametrical direction of the SCR catalyst  32  are arranged in parallel as illustrated in  FIG. 2(A) , and a pattern in which two sets of diametrical rows cross each other as illustrated in  FIG. 2(B) . Such patterns allow effective detection of overall capacitance inside the SCR catalyst  32 . 
     The ECU  50  performs various types of control, such as control over the engine  10 , the aqueous urea solution injection device  31 , and so on, and includes a CPU, a ROM, a RAM, input ports, output ports, and so on which are known in the art. 
     As shown in  FIG. 3 , the ECU  50  includes an intra-SCR temperature calculation unit  51 , an NH 3  adsorption amount calculation unit  52 , an aqueous urea solution injection control unit  53 , and an injection amount correction unit  54  as functional components thereof. It is assumed in the following description that all of these functional components are included in the ECU  50 , which is a single piece of hardware. Alternatively, one or more of these functional components may be included in a separate piece of hardware. 
     The intra-SCR temperature calculation unit  51  is an example of an internal temperature calculation unit according to the present invention, and calculates an internal temperature (inside temperature) of the SCR catalyst  32  on the basis of capacitance between the inlet-side electrodes  37  and capacitance between the outlet-side electrodes  38 . In general, the capacitance C between the electrodes  37 ,  38  is given by Equation 1, where □ is the permittivity of a medium between the electrodes  37 ,  38 , S is the area of the electrodes  37 ,  38 , and d is the distance between the electrodes  37 ,  38 . 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     ɛ 
                     × 
                     
                       S 
                       d 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
     In Equation 1, the area S of the electrodes  37 ,  38  and the distance d are constant, and a change in the permittivity ε effected by the temperature of the exhaust gas causes a change in the capacitance C. Therefore, the internal temperature of the SCR catalyst  32  can be calculated by detecting the capacitance C between the electrodes  37 ,  38 . 
     The ECU  50  stores a capacitance-temperature characteristics map (see, for example,  FIG. 4 ) representing the relationship between the capacitance C and the intra-SCR catalyst temperature T. The map is prepared in advance on the basis of experiments or the like. The intra-SCR catalyst temperature calculation unit  51  reads a value corresponding to the capacitance C between the inlet-side electrodes  37  from the capacitance-temperature characteristics map to calculate an inlet-side internal temperature T SCR IN  of the SCR catalyst  32 . Further, the intra-SCR catalyst temperature calculation unit  51  reads a value corresponding to the capacitance C between the outlet-side electrodes  38  from the capacitance-temperature characteristics map to calculate an outlet-side internal temperature T SCR OUT  of the SCR catalyst  32 . It should be noted that each of the inlet-side internal temperature T SCR IN  and the outlet-side internal temperature T SCR OUT  may not necessarily be calculated using the map. For example, each of the inlet-side internal temperature T SCR IN  and the outlet-side internal temperature T SCR OUT  may be calculated using an approximate equation or the like, which is prepared in advance on the basis of experiments or the like. 
     The NH 3  adsorption amount calculation unit  52  is an example of a reducing agent adsorption amount calculation unit according to the present invention, and calculates an actual amount of NH 3  adsorbed by the SCR catalyst  32  on the basis of the capacitance between the inlet-side electrodes  37  and the capacitance between the outlet-side electrodes  38 . NH 3  has a high permittivity □. As the adsorption of NH 3  by the SCR catalyst  32  progresses, the capacitance C between the electrodes  37 ,  38  increases (see Equation 1). Therefore, the actual amount of NH 3  adsorbed by the SCR catalyst  32  can be calculated by detecting the capacitance C between the electrodes  37 ,  38 . 
     The ECU  50  stores a capacitance-NH 3  adsorption amount map (see, for example,  FIG. 5 ) representing the relationship between the capacitance C and the actual adsorbed amount ST NH3  of NH 3 . The map is prepared in advance on the basis of experiments or the like. The NH 3  adsorption amount calculation unit  52  reads a value corresponding to the capacitance C between the inlet-side electrodes  37  from the capacitance-NH3 adsorption amount map to calculate an actual adsorbed amount ST NH3 IN  of NH 3  on an inlet side of the SCR catalyst  32 . The NH 3  adsorption amount calculation unit  52  also reads a value corresponding to the capacitance C between the outlet-side electrodes  38  from the capacitance-NH3 adsorption amount map to calculate an actual adsorbed amount ST NH3 OUT  of NH 3  on an outlet side of the SCR catalyst  32 . It should be noted that each of the actual adsorbed amount ST NH3 IN  of NH 3  on the inlet side and the actual adsorbed amount ST NH3 OUT  of NH 3  on the outlet side may not necessarily be calculated using the map. For example, each of the actual adsorbed amount ST NH3 IN  of NH 3  and the actual adsorbed amount ST NH3 OUT  of NH 3  may be calculated using an approximate equation or the like, which is prepared in advance on the basis of experiments or the like. 
     The aqueous urea solution injection control unit  53  is an example of an injection control unit according to the present invention, and controls the amount of the aqueous urea solution injected by the aqueous urea solution injection device  31  on the basis of an operating state (running condition) of the engine  10  and/or the like. Specifically, the aqueous urea solution injection control unit  53  calculates the amount of NOx emission from the engine  10  on the basis of an engine rotation (revolution) speed Ne and an accelerator opening degree Q, and sets a standard injection amount INJ U std  of the aqueous urea solution required for this amount of NOx emission. The standard injection amount INJ U std  is corrected as necessary by the injection amount correction unit  54 , which will be described below. 
     The injection amount correction unit  54  is an example of an injection amount correction unit according to the present invention, and corrects the standard injection amount INJ U std  set by the aqueous urea solution injection control unit  53  on the basis of the inlet-side internal temperature T SCR IN  and the outlet-side internal temperature T SCR OUT  , which are entered from the intra-SCR catalyst temperature calculation unit  51 , and the actual adsorbed amount ST NH3 IN  of NH 3  on the inlet side and the actual adsorbed amount ST NH3 OUT  of NH 3  on the outlet side, which are entered from the NH 3  adsorption amount calculation unit  52 . 
     Specifically, the ECU  50  stores a maximum adsorbable NH 3  amount map (see, for example,  FIG. 6 ) representing the relationship between the internal temperature T SCR  of the SCR catalyst  32  and the maximum adsorbable NH 3  amount ST NH3 MAX . The map is prepared in advance on the basis of experiments or the like. 
     The injection amount correction unit  54  reads an inlet-side adsorption amount difference ΔST NH3 IN (=ST NH3 MAX IN −ST NH3 IN ), which is a difference between an inlet-side maximum adsorbable amount ST NH3 MAX IN  corresponding to a current inlet-side internal temperature T SCR IN  and a current actual adsorbed amount ST NH3 IN  of NH 3  on the inlet side, from the maximum adsorbable NH 3  amount map. Further, the injection amount correction unit  54  reads an outlet-side adsorption amount difference ΔST NH3 OUT (=ST NH3 MAX OUT −ST NH3 OUT ), which is a difference between an outlet-side maximum adsorbable amount ST NH3 MAX OUT  corresponding to a current outlet-side internal temperature T SCR OUT  and a current actual adsorbed amount ST NH3 OUT  of NH 3  on the outlet side, from the maximum adsorbable NH 3  amount map. Then, the injection amount correction unit  54  corrects (i.e., makes an addition to or a subtraction from) the standard injection amount INJ U std  on the basis of an injection correction amount ΔINJ corresponding to the sum ΔST NH3 (=ΔST NH3 IN +ΔST NH3 OUT ) of the inlet-side adsorption amount difference ΔST NH3 IN  and the outlet-side adsorption amount difference ΔST NH3 OUT  (i.e., INJ U exh =INJ U std ±ΔINJ). After the correction, the aqueous urea solution is injected with an increased or reduced width of a pulse applied to an injector (not shown) of the aqueous urea solution injection device  31  for each injection, or with an increased or reduced number of injections. 
     Next, a control flow of the exhaust purification system according to this embodiment will now be described below with reference to  FIG. 7 . The control starts upon turning on of an ignition key. 
     At Step (hereinafter “Step” will be denoted simply as “S”)  100 , the standard injection amount INJ U std  of the aqueous urea solution is set in accordance with the amount of NOx emission from the engine  10  calculated on the basis of the engine revolution speed Ne and the accelerator opening degree Q. 
     At S 110 , the inlet-side internal temperature T SCR IN  of the SCR catalyst  32  is calculated on the basis of the capacitance C between the inlet-side electrodes  37 , and the outlet-side internal temperature T SCR OUT  of the SCR catalyst  32  is calculated on the basis of the capacitance C between the outlet-side electrodes  38 . 
     At S 120 , the actual adsorbed amount ST NH3 IN  of NH 3  on the inlet side of the SCR catalyst  32  is calculated on the basis of the capacitance C between the inlet-side electrodes  37 , and the actual adsorbed amount ST NH3 OUT  of NH 3  on the outlet side of the SCR catalyst  32  is calculated on the basis of the capacitance C between the outlet-side electrodes  38 . 
     At S 130 , the inlet-side maximum adsorbable amount ST NH3 MAX IN  corresponding to the inlet-side internal temperature T SCR IN  calculated at S 110 , and the outlet-side maximum adsorbable amount ST NH3 MAX OUT  corresponding to the outlet-side internal temperature T SCR OUT  calculated at S 110  are calculated on the basis of the maximum adsorbable NH 3  amount map ( FIG. 6 ). 
     At S 140 , the inlet-side adsorption amount difference ΔST NH3 IN (=ST NH3 MAX IN −ST NH3 IN ), which is a difference between the inlet-side maximum adsorbable amount ST NH3 MAX IN  calculated at S 130  and the actual adsorbed amount ST NH3 IN  of NH 3  on the inlet side calculated at S 120 , is calculated, and the outlet-side adsorption amount difference ΔST NH3 OUT (=ST NH3 MAX OUT −ST NH3 OUT ), which is a difference between the outlet-side maximum adsorbable amount ST NH3 MAX OUT  calculated at S 130  and the actual adsorbed amount ST NH3 OUT  of NH 3  on the outlet side calculated at S 120 , is calculated. 
     At S 150 , it is determined whether the sum ΔST NH3 (=ΔST NH3 IN +ΔST NH3 OUT ) of the inlet-side adsorption amount difference ΔST NH3 IN  and the outlet-side adsorption amount difference ΔST NH3 OUT  calculated at S 140  is greater than a predetermined threshold value. If the sum ΔST NH3  is greater than the predetermined threshold value (i.e., if Yes), the control proceeds to S 160 , and the standard injection amount INJ U std  is corrected (i.e., an addition or a subtraction is made to or from the standard injection amount INJ U std ) on the basis of the injection correction amount ΔINJ corresponding to the sum ΔST NH3  (i.e., INJ U exh =INJ U std ±ΔINJ). At S 170 , the aqueous urea solution is injected from the aqueous urea solution injection device  31  on the basis of a corrected injection amount INJ U exh . 
     On the other hand, if it is determined at S 150  that the sum ΔST NH3  is not greater than the predetermined threshold value (i.e., if No), the control proceeds to S 180 , and the aqueous urea solution is injected from the aqueous urea solution injection device  31  on the basis of the standard injection amount INJ U std  set at S 100  without a correction being made. Thereafter, S 100  to S 180  are repeatedly performed until the ignition key is turned off. 
     Next, beneficial effects of the exhaust purification system according described above will be described below. 
     As a conventional technique to reduce NH 3  slip from the SCR catalyst, there is a known method of estimating the adsorbed amount of NH 3  in the SCR catalyst on the basis of a detection value of an NH 3  sensor provided at an outlet of the SCR catalyst, and correcting the injection amount of the aqueous urea solution in accordance with the estimated adsorbed amount of NH 3 . However, the method of estimating the adsorbed amount of NH 3  on the basis of the sensor value of the NH 3  sensor may not allow an accurate recognition of the actual adsorbed amount of NH 3  in the SCR catalyst. This may lead to a failure to optimally control the injection amount of the aqueous urea solution. 
     On the contrary, the exhaust purification system described above is configured to directly calculate the adsorbed amount of NH 3  in the SCR catalyst  32  on the basis of the capacitance between the inlet-side electrodes  37  inserted into the SCR catalyst  32  from the inlet up to a middle portion of the SCR catalyst  32 , and the capacitance between the outlet -side electrodes  38  inserted into the SCR catalyst  32  from the outlet up to the middle portion of the SCR catalyst  32 , and to correct the injection amount of the aqueous urea solution as necessary in accordance with a difference between an accurate actual adsorbed amount of NH 3  and a maximum adsorbable NH 3  amount. 
     Accordingly, the exhaust purification system of this embodiment can highly precisely detect an amount of NH 3  adsorbed over the entire region of the SCR catalyst  32  from the inlet to the outlet of the SCR catalyst  32 . Moreover, it is possible to accurately control the injection amount of the aqueous urea solution in accordance with the actual adsorbed amount of NH 3  in the SCR catalyst  32 . This leads to a secure prevention of NH 3  slip from the SCR catalyst  32  and an effective improvement in the NOx reduction and purification. The need to arrange a DOC or the like on the downstream side of the SCR catalyst  32  to oxidize and remove excess NH 3  is eliminated. This leads to effective reductions in cost, weight, size, etc. of the system as a whole. 
     It should be noted that the present invention is not limited to the above-described embodiments, and that modifications can be made to the above-described embodiments as appropriate without departing from the scope and spirit of the present invention. 
     For example, referring to  FIG. 8 , the SCR catalyst  32  may be divided into a plurality of SCR catalyst segments (two SCR catalyst segments in the example of  FIG. 8 ) depending on, for example, the capacity of the SCR catalyst  32 . In this configuration, each of the SCR catalyst segments  32 a and  32   b  has inlet-side electrodes  37  and outlet-side electrodes  38 . It should also be noted that the number of electrodes  37  and the number of electrodes  38  are not limited to the numbers as illustrated in the accompanying drawings, but the number of electrodes  37  ( 38 ) may be two, which forms a pair, or any number greater than two. The engine  10  is not limited to the diesel engine. The present invention can be widely applied to other internal combustion engines, such as, for example, gasoline engines.