Patent Publication Number: US-7219491-B2

Title: Exhaust emission control apparatus of internal combustion engine and method thereof

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No.2002-208425 filed on Jul. 17, 2002, including the specification, drawings and abstract are incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an exhaust emission control apparatus and a control method of an internal combustion engine. 
     2. Description of Related Art 
     There is a known internal combustion engine for combustion of fuel at a lean air/fuel ratio having a NO X  catalyst disposed within an exhaust passage. The NO X  catalyst stores NO X  contained in exhaust gas flowing into the NO X  catalyst at a lean air/fuel ratio, and reduces the stored NO X  under the presence of a reducing agent contained in the exhaust gas upon decrease in the air/fuel ratio. The aforementioned internal combustion engine further includes a bypass passage that extends to branch off from the exhaust passage upstream of the NO X  catalyst, and a bypass control valve that serves to adjust a flow rate of the exhaust gas flowing into the bypass passage so as to control the flow rate of the exhaust gas flowing through the NO X  catalyst. A reducing agent supply valve through which the reducing agent is supplied to the NO X  catalyst is disposed within the exhaust passage between the point where the bypass passage is branched and the NO X  catalyst. In the above-structured internal combustion engine, the flow rate of the exhaust gas flowing through the NO X  catalyst is temporarily decreased by the bypass control valve, and at the same time, the reducing agent is supplied from the reducing agent supply valve. 
     The above structure may decrease the quantity of the reduction agent which is required to set the air/fuel ratio of the exhaust gas flowing into the NO X  catalyst to the rich or the theoretical state by decreasing the flow rate of the exhaust gas upon supply of the reducing agent through the reducing agent supply valve. As the space velocity of the exhaust gas within the NO X  catalyst is decreased, the quantity of the reducing agent flowing through the NO X  catalyst without causing reaction can be decreased, resulting in efficient use of the reducing agent. 
     The above-structured internal combustion engine controls the bypass control valve such that the flow rate of the exhaust gas flowing into the NO X  catalyst sequentially changes from the timing when the flow rate begins decreasing until it resumes the originally set value. The reducing agent may be efficiently used at an optimum flow rate of the exhaust gas flowing through the NO X  catalyst upon supply of the reducing agent through the reducing agent supply valve. It is, therefore, preferable to determine the timing at which the flow rate of the exhaust gas flowing through the NO X  catalyst becomes the optimum value for the efficient use of the reducing agent. This makes it possible to supply the reducing agent through the reducing agent supply valve at the determined timing. 
     Each of the bypass control valves, however, widely varies in terms of performance. This may cause the flow rate of the exhaust gas flowing through the NO X  catalyst to become larger or smaller than the optimum value even if the reducing agent is supplied at the determined timing. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an exhaust emission control apparatus of an internal combustion engine, which is capable of appropriately holding the flow rate of the exhaust gas flowing through the NO X  catalyst upon supply of the reducing agent through the reducing agent supply valve. 
     In an exhaust emission control apparatus of an internal combustion engine in which combustion is continuously performed at a lean air/fuel ratio, a NO X  catalyst is provided in an exhaust passage of the internal combustion engine for storing NO X  contained in an exhaust gas at a lean air/fuel ratio flowing into the exhaust passage, and reducing the stored NO X  in the presence of a reducing agent in the exhaust gas when the air/fuel ratio of the exhaust gas is lowered, and a reducing agent supply valve is provided in the exhaust passage upstream of the NO X  catalyst, through which the reducing agent is supplied to the NO X  catalyst. In the exhaust emission control apparatus, the flow rate of the exhaust gas is temporarily decreased while supplying the reducing agent through the reducing agent supply valve so as to execute a control of the flow rate of the exhaust gas flowing through the NO X  catalyst in accordance with a value indicating a state of the exhaust gas flowing through the NO X  catalyst. The value is variable upon supply of the reducing agent through the reducing agent supply valve. 
     According to the embodiment, the value indicating the state of the exhaust gas comprises at least one of an oxygen concentration of the exhaust gas, a temperature of the exhaust gas, a NO X  concentration of the exhaust gas, and a reducing agent concentration of the exhaust gas. 
     According to another embodiment, the flow rate of the exhaust gas that flows through the NO X  catalyst upon the supply of the reducing agent through the reducing agent supply valve is controlled such that the value indicating the state of the exhaust gas accords with a target value. 
     According to another embodiment, the flow rate of the exhaust gas that flows through the NO X  catalyst upon the supply of the reducing agent through the reducing agent supply valve is controlled such that the value indicating the state of the exhaust gas becomes one of a maximum value and a minimum value. 
     According to another embodiment, the flow rate of the exhaust gas that flows through the NO X  catalyst upon the supply of the reducing agent through the reducing agent supply valve is controlled so as to accord a time period elapsing from a predetermined reference timing until the value indicating the state of the exhaust gas reaches a peak upon the supply of the reducing agent through the reducing agent supply valve with a target time period. 
     According to the embodiment, a quantity of the reducing agent supplied through the reducing agent supply valve is controlled on the basis of the value indicating the state of the exhaust gas at one of a timing before and after the execution of the control of the flow rate of the exhaust gas that flows through the NO X  catalyst upon the supply of the reducing agent through the reducing agent supply valve. 
     According to the embodiment, the flow rate of the exhaust gas is continuously changed from a timing when the flow rate of the exhaust gas flowing through the NO X  catalyst is decreased until restoration of the flow rate of the exhaust gas. 
     According to another embodiment, the flow rate of the exhaust gas that flows into the NO X  catalyst is decreased so as to be temporarily held until the flow rate is restored. 
     In the aforementioned embodiments, the ratio of air supplied into the exhaust passage upstream of a certain point thereof, the combustion chamber and the intake passage to the reducing agent, that is, carbon hydride HC and carbon monoxide CO will be designated as the air/fuel ratio of the exhaust gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of an internal combustion engine; 
         FIGS. 2A and 2B  are schematic views each representing a structure of a catalytic converter; 
         FIGS. 3A and 3B  are views each representing a flow of the exhaust gas when a selector valve is in a forward or a reverse flow position; 
         FIG. 4  is a partial enlarged cross sectional view of a partition of a particulate filter; 
         FIG. 5  is a graph representing an output of an O 2  sensor; 
         FIG. 6  is a flowchart representing a control routine for decreasing stored NO X ; 
         FIG. 7  is a timing chart representing the control for decreasing the stored NO X ; 
         FIG. 8  is a flowchart representing a control routine for decreasing stored SO X ; 
         FIG. 9  is a timing chart representing the control for decreasing the stored SO X ; 
         FIG. 10  is a view representing a flow of the exhaust gas when a selector valve locates in a weak forward flow position; 
         FIG. 11  is a graph representing the exhaust gas quantity upon selection of the selector valve; 
         FIG. 12  is a graph representing the exhaust gas quantity upon selection of the selector valve; 
         FIG. 13  is a flowchart representing a routine for initialization; 
         FIG. 14  is a flowchart representing a correction control routine according to a first embodiment; 
         FIG. 15  is a flowchart representing a routine for correcting quantity of the reducing agent according to the first embodiment; 
         FIG. 16  is a flowchart representing a control routine for correcting the exhaust gas quantity according to the first embodiment; 
         FIG. 17  is a flowchart representing a control routine for correcting the exhaust gas quantity according to a second embodiment; 
         FIG. 18  is a flowchart representing a control routine for correcting the exhaust gas quantity according to a third embodiment; 
         FIG. 19  is a flowchart representing a correction control routine according to a fourth embodiment; 
         FIG. 20  is a flowchart representing a control routine for correcting the exhaust gas quantity according to the fourth embodiment; 
         FIG. 21  is a flowchart representing a control routine for correcting the reducing agent quantity according to the fourth embodiment; 
         FIG. 22  is a graph representing the quantity of the exhaust gas detected upon supply of the reducing agent; 
         FIG. 23  is a graph representing the time for which the control for reducing the quantity of the stored SO X  is continued; 
         FIG. 24  is a flowchart representing a control routine for decreasing the stored SO X  according to another embodiment; 
         FIG. 25  is a view representing another type of the internal combustion engine; 
         FIG. 26  is a view that shows the position of the selector valve of the internal combustion engine as shown in  FIG. 25 ; 
         FIG. 27  is a view representing another type of the internal combustion engine; and 
         FIGS. 28A and 28B  are views each representing the position of the selector valve of the internal combustion engine as shown in  FIG. 27 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a compression ignition type internal combustion engine to which the invention is applied. A spark ignition type internal combustion engine, however, may be employed. 
     Referring to  FIG. 1 , the internal combustion engine includes an engine  1 , a cylinder block  2 , a cylinder head  3 , a piston  4 , a combustion chamber  5 , an electrically controlled fuel ignition valve  6 , an intake valve  7 , an intake port  8 , an exhaust valve  9 , and an exhaust port  10 . The intake port  8  is connected to a surge tank  12  via an intake pipe  11 . The surge tank  12  is connected to a compressor  15  of an exhaust turbo charger  14  via an intake duct  13 . A throttle valve  17  driven by a stepping motor  16  is provided within the intake duct  13  around which a cooling device  18  for cooling the admitted air flowing within the intake duct  13  is provided. In an embodiment as shown in  FIG. 1 , an engine coolant is introduced into the cooling device  18  such that the intake air is cooled by the engine coolant. 
     The exhaust port  10  is connected to an exhaust turbine  21  of the exhaust turbo charger  14  via an exhaust manifold  19  and an exhaust pipe  20 . An outlet of the exhaust turbine  21  is connected to a catalytic converter  22  via an exhaust pipe  20   a.    
     Referring to  FIGS. 1 and 2 , the catalytic converter  22  includes a selector valve  61  driven by a stepping motor  60 . An inlet port  62  of the selector valve  61  is connected to an outlet of the exhaust pipe  20   a . An outlet port  63  of the selector valve  61  that faces the inlet port  62  is connected to an exhaust discharge pipe  64  of the catalytic converter  22 . The selector valve  61  includes a pair of inlet/outlet ports  65 ,  66  that face with respect to the longitudinal direction of the catalytic converter from the inlet port  62  and the outlet port  63 . Each of the inlet/outlet ports  65 ,  66  is connected to the respective ends of an annular exhaust pipe  67  of the catalytic converter  22 . An outlet of the exhaust discharge pipe  64  is connected to the exhaust pipe  23 . 
     The annular exhaust pipe  67  extends through the exhaust discharge pipe  64  in which a filter storage space  68  is formed. A particulate filter  69  for trapping particulate matters within the exhaust gas is stored within the filter storage space  68 . Both ends of the particulate filter  69  are designated as  69   a  and  69   b  as shown in  FIGS. 2A and 2B . 
     Referring to  FIG. 2A  showing a partial vertical cross section of the catalytic converter  22  including the one end  69   a  of the particulate filter  69 , and  FIG. 2B  showing a partial transverse cross section of the catalytic converter  22 , the particulate filter  69  has a honeycomb structure including a plurality of exhaust gas passages  70 ,  71  each extending in parallel with each other. The exhaust gas passage  70  has one open end and the other end sealed with a sealing member  72 . The exhaust gas passage  71  has one open end and the other end sealed with a sealing member  73 . A hatched portion of  FIG. 2A  represents the sealing member  73 . The exhaust gas passages  70  and  71  are alternately arranged by disposing a thin partition  74  formed of a porous material such as a cordierite. In other words, the exhaust gas passage  70  is surrounded with four exhaust gas passages  71 , and the exhaust gas passage  71  is surrounded with four exhaust gas passages  70 . 
     A NO X  catalyst  81  is carried on the particulate filter  69  as described later. Meanwhile, a catalytic chamber  75  is provided in a space of the exhaust discharge pipe  64  between the outlet port  63  of the selector valve  61  and the portion where the annular exhaust pipe  67  passes through the exhaust discharge pipe  64 . An auxiliary catalyst  76  with oxidizing ability that is carried on the substrate of the honeycomb structure is contained within the catalytic chamber  75 . 
     An electrically controlled reducing agent supply valve  77  is provided in the annular exhaust pipe  67  between the inlet/outlet port  65  of the selector valve  61  and the particulate filter  69  such that the reducing agent is supplied to the particulate filter  69 . The reducing agent is supplied from a reducing agent pump  78  to the reducing agent supply valve  77 . In this embodiment, the fuel in the internal combustion engine, that is, light oil is employed as the reducing agent. Also, the reducing agent supply valve is not provided in the annular exhaust pipe  67  between the inlet/outlet port  66  and the particulate filter  69 . 
     Referring to  FIG. 1 , the exhaust manifold  19  is connected to the surge tank  12  via an exhaust gas recirculation (hereinafter referred to as EGR) passage  24  having an electric EGR control valve  25  therein. A cooling device  26  is provided around the EGR passage  24  such that the EGR gas flowing through the EGR passage  24  is cooled. In this embodiment as shown in  FIG. 1 , the engine cooling water is introduced into the cooling device  26  so as to cool the EGR gas with the engine cooling water. 
     Each of the fuel injection valves  6  is connected to a fuel reservoir, that is, a common rail  27  via the fuel supply pipe  6   a . The fuel is supplied into the common rail  27  from the fuel pump  28  that is electrically controlled such that the quantity of the supplied fuel is variable. The fuel supplied into the common rail  27  is further supplied to the fuel injection valve  6  via each of the fuel supply pipes  6   a . The common rail  27  has a fuel pressure sensor  29  therein so as to detect the fuel pressure within the common rail  27 . Accordingly the supply quantity of the fuel pump  28  is controlled such that the fuel pressure within the common rail  27  reaches a target fuel pressure in accordance with the output signal of the fuel pressure sensor  29 . 
     An electronic control unit  40  is formed of a digital computer including a ROM (Read Only Memory)  42 , a RAM (Random Access Memory)  43 , a CPU (micro-processor)  44 , an input port  45  and an output port  46 , which are connected with one another via a two-way bus  41 . An output signal of the fuel pressure sensor  29  is sent to the input port  45  via an AD converter  47 . An exhaust sensor  48  is provided at a position opposite to the reducing agent supply valve  77  with respect to the particulate filter  69  in the annular exhaust pipe  67  so as to detect a state of the exhaust gas that flows therethrough in terms of quantity. An output voltage of the exhaust sensor  48  is sent to the input port  45  via the corresponding AD converter  47 . A pressure sensor  49  is provided within the exhaust pipe  20   a  for detecting the pressure therein, that is, the back pressure of the engine. An output voltage of the pressure sensor  49  is sent to the input port  45  via the corresponding AD converter  47 . A load sensor  51  for generating an output voltage in proportional to an amount of depressing an accelerator pedal  50  is connected thereto. An output voltage of the load sensor  51  is sent to the input port  45  via the corresponding AD converter  47 . A crank angle sensor  52  is connected to the input port  45  for generating an output pulse at every moment where the crank shaft rotates at, for example, 30 degrees. 
     The output port  46  is connected to the fuel injection valve  6 , a stepping motor  16  for driving the throttle valve, the EGR control valve  25 , the fuel pump  28 , the stepping motor  60  for driving the selector valve, the reducing agent supply valve  77 , and the reducing agent pump  78  via the corresponding drive circuit  53 . The aforementioned elements are controlled on the basis of output signals from the electronic control unit  40 . 
     Referring to  FIG. 3B , the selector valve  61  is held in the position shown by either the solid line or the dashed line. When the selector valve  61  is held in the position as shown by the solid line in  FIG. 3B , the inlet port  62  is disconnected from the outlet port  63  and the inlet/outlet port  66 , while being communicated with the inlet/outlet port  65 . The outlet port  63  is communicated with the inlet/outlet port  66  via the selector valve  61 . Accordingly as shown by the solid arrow in  FIG. 3B , all the exhaust gas flowing through the exhaust pipe  20   a  flows into the annular exhaust pipe  67  via the inlet port  62 , and then the inlet/outlet port  65  . After the exhaust gas passes through the particulate filter  69 , it is discharged to the exhaust discharge pipe  64  via the inlet/outlet port  66  and the outlet port  63  sequentially. 
     When the selector valve  61  is held in the position as shown by the dashed line in  FIG. 3B , communication between the inlet port  62  and the outlet port  63 , the inlet/outlet port  65  is interrupted, and the inlet port  62  is communicated with the inlet/outlet port  66 . The outlet port  63  is communicated with the inlet/outlet port  65  via the selector valve  61 . Accordingly as shown by the dashed arrow in  FIG. 3B , all the exhaust gas flowing through the exhaust pipe  20   a  flows into the annular exhaust pipe  67  via the inlet port  62  and the inlet/outlet port  66 . After the exhaust gas passes through the particulate filter  69 , it is discharged to the exhaust discharge pipe  64  via the inlet/outlet port  65  and the outlet port  63  sequentially. 
     The direction of the exhaust gas that flows through the annular exhaust pipe  67  may be switched by changing the position of the selector valve  61 . In other words, the direction of the exhaust gas flowing through the NO X  catalyst  81  from one end surface to the other end surface thereof may be reversed. The exhaust gas flow shown by the solid line in  FIG. 3B  will be referred to as a forward flow, and shown by the dashed line will be referred to as a reverse flow, respectively. The position of the selector valve  61  shown by the solid line in  FIG. 3B  will be referred to as a forward flow position, and shown by the dashed line will be referred to as a reverse flow position, respectively. 
     Referring to  FIGS. 3A and 3B , the exhaust gas discharged to the exhaust discharge pipe  64  via the outlet port  66  passes through the catalyst  76 , and further flows along the outer peripheral surface of the annular exhaust pipe  67 . The exhaust gas is finally discharged into the exhaust pipe  23 . 
     In case of the forward flow, the exhaust gas flows into the particulate filter  69  via one end surface  69   a , and is discharged from the particulate filter  69  via the other end surface  69   b . Then the exhaust gas flows into the exhaust gas passage  70  that opens to the end surface  69   a , and is discharged into the adjacent exhaust gas passage  71  through the surrounding partition  74 . Meanwhile, in case of the reverse flow, the exhaust gas flows into the particulate filter  69  via the one end surface  69   b , and is discharged from the particulate filter  69  via the end surface  69   a . Then the exhaust gas flows into the exhaust gas passage  71  that opens to the other end surface  69   b , and is discharged into the adjacent exhaust gas passage  70  through the surrounding partition  74 . 
     As shown in  FIG. 4 , the NO X  catalyst  81  is carried on the partition  74  of the particulate filter  69 , that is, both side surfaces and porous inner wall surface of the partition  74 , for example. The NO X  catalyst  81  includes an alumina substrate which carries at least one element selected from alkaline metal such as kalium K, natrium Na, lithium Li, and cesium Cs, alkaline earth such as barium Ba, and calcium Ca, and rare earths such as lanthanum La, and yttrium Y, and a rare metal such as platinum Pt, palladium Pd, rhodium Rh and irridium Ir. 
     The NO X  catalyst stores NO X  when the mean air/fuel ratio of the introduced exhaust gas is lean, and reduces the stored NO X  such that its quantity is reduced in the presence of the reducing agent of the exhaust gas in response to the decrease in the air/fuel ratio of the exhaust gas. 
     The specific mechanism of the NO X  catalyst function for storing and reducing the NO X  has not been clarified yet. However, such mechanism that has been generally assumed will be briefly described in the case where Pt and Ba are carried on the substrate as below. 
     When the air/fuel ratio of the exhaust gas flowing into the NO X  catalyst becomes considerably leaner than the theoretical value, the oxygen concentration of the exhaust gas is increased to the greater degree, and oxygen O 2  adheres to the surface of Pt in the form of O 2   −  or O 2− . The NO contained in the introduced exhaust gas adheres to the surface of Pt to react with O 2   −  or O 2−  thereon so as to become NO 2  (NO+O 2 →NO 2 +O* where O* is an active oxygen). A part of the generated NO 2  is further oxidized on Pt so as to be absorbed by the NO X  catalyst. The absorbed NO 2  reacts with oxide barium BaO and is diffused within the NO X  catalyst in the form of nitric acid ions NO 3   − . The NO X  is stored in the NO X  catalyst in the aforementioned manner. 
     When the air/fuel ratio of the exhaust gas introduced into the NO X  catalyst has a value indicating the rich or theoretical state, the oxygen concentration of the exhaust gas decreases to reduce the quantity of generated NO 2 . This may reverse the reaction, that is, NO 3   − →NO+2O*, and thus, the nitric acid ions NO 3   −  contained in the NO X  catalyst is released therefrom in the form of NO. The released NO X  is then reduced through reaction with the reducing agent contained in the exhaust gas, for example HC, CO. When the NO X  no longer exists on the surface of Pt, it is released from the NO X  catalyst one after another. As a result, the quantity of the NO X  stored in the NO X  catalyst is gradually decreased. 
     The NO X  catalyst may be structured to store NO X  without forming the nitrate salt, and to reduce the NO X  without being released. It is possible to consider the NO X  catalyst as the catalyst that generates active oxygen upon storage and release of the NO X . 
     The auxiliary catalyst  76  of the embodiment may be formed as a rare metal catalyst including the rare metal such as platinum Pt without employing the alkaline metal, alkaline earth, nor rare earths. The auxiliary catalyst  76 , however, may be formed as the NO X  catalyst as described above. 
     The particulate filter  69  is disposed in substantially the center of the annular exhaust pipe  67 . That is, the distance between the inlet port  62  of the selector valve  61  and the particulate filter  69 , and the distance between the outlet port  63  and the particulate filter  69  hardly change upon setting of the selector valve  61  either in the forward flow position or in the reverse flow position. This shows that a certain state of the particulate filter  69  such as the temperature is hardly influenced by the position of the selector valve  61  either in the forward flow position or in the reverse flow position. Therefore the specific control with respect to the position of the selector valve  61  is not required. 
     In the embodiment of the invention, the exhaust sensor  48  is formed as an oxygen sensor that generates the output voltage in proportional to the concentration COX of oxygen contained in the exhaust gas. When the selector valve  61  is in the forward flow position, the exhaust sensor or the oxygen sensor  48  detects the concentration of oxygen contained in the exhaust gas discharged from the NO X  catalyst  81 . When the selector valve  61  is in the reverse flow position, the oxygen sensor  48  detects the concentration of oxygen contained in the exhaust gas that flows into the NO X  catalyst  81 . An example of outputs OP of the oxygen sensor  48  with respect to the oxygen concentration COX is represented by the graph shown in  FIG. 5 . The outputs OP of the oxygen sensor  48  represent the air/fuel ratio of the exhaust gas that is discharged from the NO X  catalyst  81 . Assuming that the air/fuel ratio of the exhaust gas discharged from the NO X  catalyst  81  is the theoretical air/fuel ratio when the output OP of the oxygen sensor  48  becomes zero, if the OP takes a positive value, the air/fuel ratio of the exhaust gas is considered as being lean. Meanwhile, if the OP takes a negative value, the air/fuel ratio is considered as being rich. 
     The exhaust gas passes through the particulate filter  69  irrespective of the position of the selector valve  61  either in the forward or the reverse flow position. In the internal combustion engine shown in  FIG. 1 , combustion is continuously performed in the fuel lean state. Accordingly the air/fuel ratio of the exhaust gas that passes through the particulate filter  69  is held in the lean state. As a result, the NO X  in the exhaust gas is stored in the NO X  catalyst  81  on the particulate filter  69 . 
     The quantity of the NO X  stored in the NO X  catalyst  81  gradually increases as time passes. The embodiment is structured to perform control for decreasing the stored NO X  when the stored NO X  quantity exceeds an allowable value by temporarily supplying the reducing agent to the NO X  catalyst  81  through the reducing agent supply valve  77  so as to reduce NO X . 
     The control routine for reducing quantity of stored NO X  shown in  FIG. 6  will be described referring to  FIG. 7 . In step  200 , the NO X  quantity QN stored in the NO X  catalyst  81  is calculated. The NO X  quantity QN is obtained using the sum of the quantity of the NO X  that flows into the NO X  catalyst  81  per unit of time at the lean air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81 . Then in step  201 , it is determined whether the calculated NO X  quantity QN is larger than an allowable quantity QN 1 . If NO is obtained in step  201 , that is, the QN is equal to or smaller than the QN 1 , the routine ends. If YES is obtained in step  201 , that is, the QN is larger than the QN 1 , the process proceeds to step  202  where the position of the selector valve  61  is changed from the forward flow position to the reverse flow position or vice versa. The reducing agent is then injected through the reducing agent supply valve  77  only once. 
     If the quantity of NO X  stored in the NO X  catalyst  81  exceeds the allowable quantity, a signal instructing to change the position of the selector valve  61 , for example, from the reverse flow position to the forward flow position is generated at a timing as shown by X in  FIG. 7 . In response to the signal, the position of the selector valve  61  is changed from the reverse to the forward flow position. Upon change in the position of the selector valve  61  from the reverse to the forward flow position, the inlet port  62  is temporarily connected directly to the outlet port  63 . Accordingly upon change in the position of the selector valve  61  from the reverse to the forward flow position, the quantity of the exhaust gas flowing through the NO X  catalyst  81  in the reverse direction gradually decreases as shown in  FIG. 7 . Meanwhile, the quantity of the exhaust gas that bypasses the NO X  catalyst  81  gradually increases. Once the quantity of the exhaust gas flowing through the NO X  catalyst  81  becomes zero, the quantity of the exhaust gas flowing through the NO X  catalyst  81  in the forward direction gradually increases, and the quantity of the exhaust gas that bypasses the NO X  catalyst  81  gradually decreases. That is, change in the position of the selector valve  61  from the forward to the reverse flow position or vice verse, the quantity of the exhaust gas flowing through the NO X  catalyst  81  in the forward direction may be temporarily decreased. Supply of the reducing agent through the reducing agent supply valve  77  at the aforementioned timing makes it possible to decrease the quantity of the reducing agent required to bring the air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81  into rich. The space velocity of the exhaust gas within the NO X  catalyst  81  is decreased at the above moment. As a result, the time period when the reducing agent is stored within the NO X  catalyst  81  is increased. This makes it possible to efficiently use the reducing agent. The reducing agent supplied to the NO X  catalyst  81  is diffused all over thereof in the forward flow direction. 
     In the embodiment, the reducing agent is supplied for the period of tFN upon elapse of tC from a predetermined reference timing such that the exhaust gas flows through the NO X  catalyst  81  by a slight amount, that is, QEXA in the forward direction. The amount QEXA is considered as an optimum flow rate of the exhaust gas for reducing the NO X  as well as decreasing the stored NO X  quantity. The elapsing time tC is preliminarily set such that the flow rate of the exhaust gas flowing through the NO X  catalyst  81  becomes the optimum amount QEXA upon supply of the reducing agent through the reducing agent supply valve  77 . The time tC elapsing when the position of the selector valve  61  is changed from the forward to the reverse flow position is slightly different from the time tC elapsing when the position of the selector valve  61  is changed from the reverse to the forward flow position. However, it is assumed that such time tC is substantially equivalent, and thus, will be collectively utilized hereinafter. 
     The aforementioned predetermined reference time may be determined in an arbitrary manner. In this embodiment, the reference time is set to the one from which the signal is generated to instruct the change in the position of the selector valve  61  from the forward to the reverse flow position or vice versa as shown by the arrow X shown in  FIG. 7 . 
     The exhaust gas contains sulfur in the form of SO X . The NO X  catalyst  81  stores not only NO X  but also SO X . The SO X  is stored within the NO X  catalyst  81  in the same manner as in the case of NO X . Supposing that the catalyst carries Pt and Ba on the substrate, oxygen O 2  adheres to the surface of Pt in the form of O 2   −  or O 2−  at the lean air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81 . The SO 2  contained in the exhaust gas adheres to the surface of Pt on which SO 2  reacts with O 2   −  or O 2−  into SO 3 . The resultant SO 3  is further oxidized on the Pt, and absorbed within the NO X  catalyst  81  so as to be bound with the barium oxide BaO. Accordingly, the resultant SO 4   −  is diffused within the NO X  catalyst  81 . The sulfuric acid ion SO 4   −  is bound with barium ion Ba +  for further forming nitric acid salt BaSO 4    
     The nitric acid salt BaSO 4  is hardly decomposed, and the quantity of the nitric acid salt BaSO 4  within the NO X  catalyst  81  does not decrease even if the air/fuel ratio of the exhaust gas flowing through the NO X  catalyst  81  is brought into the rich state. In this way, the nitric acid salt BaSO 4  within the NO X  catalyst  81  increases as time elapses. As a result, the quantity of the NO X  that can be stored within the NO X  catalyst may be decreased. 
     In the case where the mean air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81  is controlled to the theoretical air/fuel ratio or the rich state while holding the temperature of the NO X  catalyst  81  at 550° C. or higher, the sulfate BaSO 4  within the NO X  catalyst  81  is decomposed and released therefrom in the form of SO 3 . The released SO 3  reacts with HC, CO as the reducing agent contained in the exhaust gas so as to be reduced to SO 2 . The SO X  stored in the NO X  catalyst  81  in the form of the sulfate BaSO 4  is gradually decreased. Accordingly, the SO X  is not released from the NO X  catalyst  81  in the form of SO 3 . 
     In the embodiment, if the quantity of the SO X  stored in the NO X  catalyst  81  exceeds the allowable value, the control of reducing the stored SO X  is executed by holding the temperature of the NO X  catalyst  81  at a lower limit temperature for decreasing the SO X  quantity, for example, 550° C. or higher while holding the mean air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81  at the theoretical air/fuel ratio or in the rich state. 
     A control routine of decreasing the stored SO X  shown in  FIG. 8  will be described referring to  FIGS. 9 and 10 . In step  210 , the quantity of SO X  stored in the NO X  catalyst  81 , that is, QS is calculated. The QS may be obtained on the basis of the sum of the quantity of the fuel supplied through the fuel injection valve, and the reducing agent (fuel) supplied through the reducing agent supply valve  77 . Then in step  211 , it is determined whether the calculated QS is larger than an allowable quantity QS 1 . If NO is obtained, that is, QS≦QS 1 , the control routine ends. If YES is obtained, that is, QS&gt;QS 1 , the process proceeds to step  212 . In step  212 , as shown in  FIG. 9 , the selector valve  61  is set to the weak forward flow position from the forward flow position to be held as shown in  FIG. 10  such that the reducing agent is supplied through the reducing agent supply valve  77 . 
     In the case where the selector valve  61  is held in the weak forward flow position, a part of the exhaust gas flowing through the exhaust valve  20   a  enters into the annular exhaust pipe  67  via the inlet/outlet port  65  as shown by an arrow in  FIG. 10 . The exhaust gas then flows in the forward direction through the NO X  catalyst  81 . The rest of the exhaust gas directly flows into the exhaust discharge pipe  64  from the inlet port  62  via the outlet port  63 , that is, bypasses the NO X  catalyst  81  to flow into the auxiliary catalyst  76 . In the aforementioned case, the reducing agent is supplied to the NO X  catalyst  81  while decreasing the flow rate of the exhaust gas flowing into the NO X  catalyst  81 . 
     Under the control of reducing the stored SO X , the reducing agent is supplied for a time period of tFS. The time period tFS is set as a time period required for holding the temperature of the NO X  catalyst  81  to be equal to or higher than the temperature TNS required for reducing the SO X  quantity while holding the mean air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81  in slightly richer state, for example. 
     Then in step  213  of the flowchart in  FIG. 8 , it is determined whether the time tS has been elapsed from supply of the reducing agent in the state where the selector valve  61  is held in the weak forward flow position. The time tS is set to a predetermined time required to reduce the quantity of SO X  stored in the NO X  catalyst  81  to almost zero. If NO is obtained in step  213 , that is, the time tS has not been elapsed, the process returns to step  212  where the reducing agent is repeatedly supplied while holding the selector valve  61  in the weak forward flow position. If YES is obtained in step  213 , that is, the time tS has been elapsed, the process proceeds to step  214  where the position of the selector valve  61  is changed to, for example, the forward flow position. This indicates that the control of reducing the stored SO X  has been completed. 
     When the selector valve  61  is held in the bypass position as shown in  FIG. 9 , all the exhaust gas flowing through the exhaust pipe  20   a  directly flows into the exhaust discharge pipe  64  from the inlet port  62  via the outlet port  63 . That is, the exhaust gas bypasses the NO X  catalyst  81  and the particulate filter  69  without flowing therethrough. The flow passage of the exhaust gas from the inlet port  62  to the outlet port  63  of the selector valve  61  serves as the bypass passage through which the exhaust gas bypasses the particulate filter  69 . 
     Referring to the exemplary timing chart of  FIG. 9 , the “OPA” represents a mean value of outputs OP of the oxygen sensor  48 . According to the timing chart in  FIG. 9 , the OPA under the control of reducing the stored SO X  takes a negative value. In  FIG. 9 , the “D” represents an opening degree or opening position of the selector valve  61 . If the selector valve  61  is in the bypass position, the D takes zero. As the selector valve  61  approaches the forward flow position, the opening degree D increases. Accordingly the flow rate of the exhaust gas flowing through the NO X  catalyst  81  increases as the opening degree D becomes large. In the embodiment, the opening degree D representing the weak forward flow position is set such that the flow rate of the exhaust gas flowing through the NO X  catalyst  81  is held at the value optimum for executing appropriate control of reducing the stored SO X . 
     In the control of reducing the stored SO X  of the embodiment, the flow rate of the exhaust gas flowing through the NO X  catalyst is decreased to an optimum value which is temporarily held, and further resumed to the original flow rate. Meanwhile in the control of reducing the stored NO X  of the embodiment, the flow rate of the exhaust gas flowing into the NO X  catalyst  81  is decreased, and then continuously adjusted until it resumes the original flow rate. In the control of reducing the stored SO X , the reducing agent may be supplied not only when the position of the selector valve  61  is repeatedly changed from the forward to the reverse flow position or vice versa alternately, but also when the position of the selector valve  61  is changed from the forward to the reverse flow position or vice versa. 
     The particulate matter mainly formed of a carbon contained in the exhaust gas is trapped on the particulate filter  69 . When the exhaust gas flows in the forward direction, the particulate matter is trapped on the side surface and within the pore of the partition  74  at the side of the exhaust gas passage  70 . When the exhaust gas flows in the reverse direction, the particulate matter is trapped on the side surface and within the pore of the partition  74  at the exhaust gas passage  71 . In the internal combustion engine shown in  FIG. 1 , combustion is continuously performed at the lean air/fuel ratio. As the NO X  catalyst  81  has an oxidizing ability, the particulate matter is oxidized on the particulate filter  69  and eliminated if the temperature of the particulate filter  69  is held at the temperature at which the particulate matter can be oxidized, for example, 250° C. or higher. 
     According to the NO X  storage/reducing function of the NO X  catalyst  81 , the active oxygen is generated upon storage and release of the NO X  through the NO X  catalyst  81 . The resultant active oxygen has higher activity compared with oxygen O 2  that functions in quickly oxidizing the particulate matter trapped on the particulate filter  69 . When the NO X  catalyst  81  is carried on the particulate filter  69 , the particulate matter trapped on the particulate filter  69  is oxidized regardless of the state of the air/fuel ratio of the exhaust gas flowing through the particulate filter  69 , i.e., fuel rich or fuel lean. In this way, the particulate matter is continuously oxidized. 
     In the case where the temperature of the particulate filter  69  is no longer held at the temperature for oxidizing the particulate matter, or the quantity of the particulate matter entering into the particulate filter  69  per unit of time becomes substantially large, the quantity of the particulate matter trapped on the particulate filter  69  gradually increases. This may increase the pressure loss of the particulate filter  69 . In the embodiment, if the quantity of the trapped particulate matter exceeds the allowable value, the control of oxidizing particulate matter is executed. In this control, the temperature of the particulate filter  69  is increased to the temperature TNP required to oxidize the particulate matter, for example, 600° C. or higher so as to be held while holding the lean air/fuel ratio of the exhaust gas flowing through the particulate filter  69 . Under the aforementioned control, the particulate matter trapped on the particulate filter  69  is ignited and burnt. The particulate matter, thus, is removed. In the embodiment as shown in  FIG. 1 , if the back pressure of the engine detected by the pressure sensor exceeds the allowable value in the case where the selector valve  61  is held in the forward or reverse flow position, it is determined that the quantity of the trapped particulate matter exceeds the allowable quantity. 
     In the control of reducing the stored NO X  of the embodiment, the reducing agent is supplied through the reducing agent supply valve  77  upon elapse of the time tC from the timing X at which the signal is output for changing the position of the selector valve  61 . Actually, however, each performance of the reducing agent supply valve varies, and the actual elapsing time does not always accord with the normal elapsing time. If the time elapsing from the timing X until supply of the reducing agent is longer than the normal elapsing time, the flow rate of the exhaust gas upon supply of the reducing agent becomes higher than the optimum value QEXA. As a result, the air/fuel ratio of the exhaust gas flowing into the NO X  catalyst  81  cannot be brought into the rich state, failing to sufficiently decrease the space velocity of the exhaust gas within the NO X  catalyst  81 . If the time period elapsing from the timing X until the supply of the reducing agent as shown by Y 2  of the exemplary chart of  FIG. 11  is shorter than the normal elapsing time, the reducing agent will be supplied when the exhaust gas flows in the reverse direction. Accordingly, the reducing agent fails to reach the NO X  catalyst  81 . 
     Actually, each performance of the selector valve  61  or the stepping motor  60  for driving the selector valve  61  varies. This may deviate the actual flow rate of the exhaust gas during supply of the reducing agent from the optimum value QEXA even if the time elapsing from the timing X until the actual supply of the reducing agent is held to the normal elapsing time. If the speed for selecting the position of the selector valve  61  is higher than the normal speed VA as shown by Z 1  in an exemplary chart of  FIG. 12 , the flow rate of the exhaust gas upon supply of the reducing agent becomes higher than the optimum flow rate QEXA as in the case of long elapsing time as described above. If the speed for selecting the position of the selector valve  61  is lower than the normal selection speed VA as shown by Z 2  of the timing chart of  FIG. 12 , the exhaust gas flows in the reverse direction during supply of the reducing agent as in the case of the short elapsing time. 
     The selector valve  61  is held in the weak forward flow position under the control of reducing the stored SO X . In this case, however, the opening degree D of the selector valve  61  at this time may not accord with the normal opening degree. If the actual opening degree of the selector valve  61  is larger than the normal opening degree, the flow rate of the exhaust gas upon supply of the reducing agent becomes higher than the optimum flow rate QEXA. If the actual opening degree is smaller than the normal driving degree, the flow rate of the exhaust gas upon supply of the reducing agent becomes lower than the QEXA. 
     In the embodiment, the control of correcting the flow rate of the exhaust gas is executed so as to hold the flow rate of the exhaust gas flowing through the NO X  catalyst  81  in the forward direction upon supply of the reducing agent at the optimum value. 
     The oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81  varies upon supply of the reducing agent to the NO X  catalyst  81 . In the timing chart shown in  FIG. 7 , when the time tP elapses from the reference timing, for example, the timing X at which the signal for changing the position of the selector valve  61 , the output OP of the oxygen sensor  48  temporarily decreases to reach a peak taking a value PK. Alternatively the output OP temporarily decreases by DLT. In the timing chart of  FIG. 9 , the mean output value OPA is kept at the negative value under the control of reducing the stored SO X . 
     The aforementioned peak value PK, decrease DLT, the time tP elapsing until the peak, and the mean output value OPA are determined in accordance with the reaction state of the reducing agent within the NO X  catalyst  81 . The reaction state of the reducing agent is determined in accordance with the flow rate of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent. The determination as to whether the flow rate of the exhaust gas upon supply of the reducing agent deviates from the optimum value can be made in accordance with the change in the oxygen concentration of the exhaust gas flowing from the NO X  catalyst  81 . 
     The embodiment is structured to detect the oxygen concentration of the exhaust gas from the NO X  catalyst  81 , which is likely to vary with the supply of the reducing agent through the reducing agent supply valve  77 . On the basis of the detected oxygen concentration, the control for correcting the flow rate of the exhaust gas is executed. 
     The actual quantity of the reducing agent through the reducing agent supply valve  77  depends on the time period for supplying the reducing agent. Such supply time period is likely to be influenced by the variation of the individual reducing agent supply valves. This may cause the actual quantity of supplied reducing agent to deviate from the normal quantity. 
     In the aforementioned case, the determination as to whether the actual quantity of supplied reducing agent deviates from the normal quantity can be made on the basis of the change in the oxygen concentration of the exhaust gas from the NO X  catalyst  81 . 
     The embodiment is structured to detect the oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81 , which is likely to vary with the supply of the reducing agent through the reducing agent supply valve  77 . On the basis of the detected oxygen concentration, the control of correcting the quantity of the supplied reducing agent is executed such that the quantity of the reducing agent supplied through the reducing agent supply valve  77  becomes the normal quantity. 
     A first embodiment of the invention will be described hereinafter. In the first embodiment, the control of correcting the quantity of the reducing agent is executed, and upon completion of such control, the control of correcting the flow rate of the exhaust gas is executed. 
     In the control for correcting quantity of the reducing agent according to the first embodiment, a reducing agent quantity correction coefficient KR is calculated for correcting the supply time period tFN under the control of reducing the stored NO X , and the supply time period tFS under the stored SO X  reducing control such that the quantity of the reducing agent supplied through the reducing agent supply valve  77  becomes the normal quantity. That is, the supply time periods tFN and tFS are corrected using the correction coefficient KR (tFN=tFN·KR, tFS=tFS·KR). If the correction coefficient KR increases, both supply time periods tFN and tFS become long. If the correction coefficient KR decreases, both supply time periods tFS and tFS become short. The correction is not required, the correction coefficient KR is held at 1.0. 
     The correction coefficient KR is obtained in the following manner. In the first embodiment, in case of a predetermined engine operating state defined by, for example, the engine speed and the required load, the reducing agent is supplied through the reducing agent supply valve  77  for the time period of tF 0  while fixing the selector valve  61  in the forward flow position. The predetermined engine operating state may be, for example, an idling operating state. The time period tF 0  may be set to the time required for making the outputs OP to become substantially zero, for example. 
     The oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81  reaches a peak upon supply of the reducing agent. If the quantity of actually supplied reducing agent is larger than the normal quantity corresponding to the time period tF 0 , the peak value PK of the output OP of the oxygen sensor  48  becomes smaller than the target peak value PKTP (negative value) corresponding to the normal quantity of the reducing agent. If the quantity of actually supplied reducing agent is smaller than the normal quantity, the peak value PK becomes larger than the target peak value PKTP. The target peak value PKTP is the value predetermined by experiments. 
     The first embodiment is structured to decrease the correction coefficient KR when PK&gt;PKTP, and increase the correction coefficient KR when PK&lt;PKTP. In this way, the correction coefficient KR is updated such that the supply time period tF is changed (tF 0 =tF 0 ·KR). The correction coefficient KR obtained in the condition where PK=PKTP represents the final correction coefficient. 
     Under the control of reducing the stored NO X , the reducing agent is supplied for the supply time period tFN corrected with the correction coefficient KR (=tFN·KR). Under the control of reducing the stored SO X , the reducing agent is supplied for the supply time period tFS corrected with the correction coefficient KR (=tFS·KR). Upon completion of calculation of the correction coefficient KR, the control of correcting quantity of the reducing agent ends. 
     As the control of correcting quantity of the reducing agent is executed in the predetermined engine operating state, the influence of the engine operating state does not have to be considered. As the control is further executed while fixing the selector valve  61  in the forward flow position, the influence of the performance of the selector valve  61  does not have to be considered. If the output OP of the oxygen sensor  48  takes the value around zero, the sensitivity of the oxygen sensor  48  is relatively higher. The reducing agent is then supplied such that the output OP of the oxygen sensor  48  becomes substantially zero. Accordingly the control of correcting quantity of the reducing agent can be executed with higher accuracy. 
     In the control of correcting flow rate of the exhaust gas according to the first embodiment, an exhaust gas flow rate correction coefficient KEX is calculated for correcting an elapsing time period tC such that the flow rate of the exhaust gas flowing through the NO X  catalyst  81  in the forward direction is held at an optimum value upon supply of the reducing agent under the control of reducing the stored NO X . That is, the elapsing time tC is corrected with the correction coefficient KEX under the control of reducing the stored NO X  (tC=tC·KEX). If the correction coefficient KEX increases, the elapsing time tC becomes long. If the correction coefficient KEX decreases, the elapsing time tC becomes short. If the correction is not required, the KEX is held at 1.0 (KEX=1.0). 
     The correction coefficient KEX is calculated in the following manner. In the first embodiment, the peak value PK of the output OP from the oxygen sensor  48  is obtained at every execution of the control of reducing the stored NO X . If the actual elapsing time tC from the timing X until supply of the reducing agent is longer than the normal elapsing time, the quantity of the reducing agent that flows through the NO X  catalyst  81  without being oxidized is increased. Accordingly the peak value PK becomes smaller than the target peak value PKT corresponding to the normal elapsing time. If the actual elapsing time tC is shorter than the normal time period, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is forward, the reducing agent is gradually oxidized. As a result, the peak value PK becomes smaller than the target peak value PKT. If the actual elapsing time tC is shorter than the normal time period, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is reverse, the output OP of the oxygen sensor  48  does not reach the peak. 
     Likewise if the speed for changing the position of the selector valve  61  is higher than the normal speed, the peak value PK becomes smaller than the target peak value PKT. If the speed for changing the position of the selector valve  61  is lower than the normal speed, and the direction of the exhaust gas that flows through the NO X  catalyst  81  upon supply of the reducing agent is forward, the peak value PK becomes smaller than the target peak value PKT. If the speed for changing the position of the selector valve  61  is lower than the normal speed, and the direction of the exhaust gas that flows through the NO X  catalyst  81  upon supply of the reducing agent is reverse, no peak is reached. The target peak value PKT is a predetermined value obtained through experiments. 
     In the first embodiment, if the absolute value of the difference between the peak value PK and the target peak value PKT upon increase in the correction coefficient KEX for correcting the flow rate of the exhaust gas, the correction coefficient KEX is further increased. If the absolute value of the difference is increased, the correction coefficient KEX is decreased. If the absolute value of the difference between the peak value PK and the target peak value PKT is decreased upon decrease in the correction coefficient KEX, the KEX is further decreased. If the absolute value of the difference is increased, the correction coefficient KEX is increased. In this way, the correction coefficient KEX is continuously updated, and the elapsing time tC is changed accordingly (tC=tC·KEX). The correction coefficient KEX in the condition where PK=PKT represents the final value. 
     Under the control of reducing the stored NO X , if the time corrected with the correction coefficient KEX elapses from the timing X (=tC·KEX), the reducing agent is supplied. Accordingly, the control of correcting flow rate of the exhaust gas ends upon completion of calculation of the correction coefficient KEX. 
       FIGS. 13 to 16  represent each control routine for executing the first embodiment of the invention. 
     A flowchart shown in  FIG. 13  represents the routine of initialization executed once upon first start-up of the internal combustion engine. Referring to the flowchart of  FIG. 13 , in step  220 , a flag XR indicating completion of correcting the reducing agent quantity is reset (XR=0) upon completion of the control of correcting the reducing agent quantity. A flag XEX indicating completion of correcting the flow rate of the exhaust gas, which is set upon completion of the control of correcting the flow rate of the exhaust gas is reset (XEX=0). The correction coefficient KR for correcting the reducing agent quantity is set to 1.0, and the correction coefficient KEX for correcting the flow rate of the exhaust gas is set to 1.0. 
     A flowchart of  FIG. 14  represents a routine for controlling the correction to be executed upon interruption at every predetermined time interval. Referring to the flowchart of  FIG. 14 , in step  230 , it is determined whether the flag XR has been reset (XR=0). If YES is obtained in step  230 , that is, the flag XR has been reset, the process proceeds to step  231  where the routine for controlling correction of the reducing agent quantity as shown in the flowchart of  FIG. 15  is executed. If the flag XR is set upon completion of the control of correcting the reducing agent quantity, the process proceeds to step  232  where it is determined whether the flag XEX has been reset (XEX=0). If YES is obtained in step  232 , that is, the flag XEX has been reset, the process proceeds to step  233  where a routine for controlling correction of flow rate of the exhaust gas is executed as shown in  FIG. 16 . 
     The flowchart of  FIG. 15  represents the routine for controlling correction of the reducing agent quantity. Referring to the flowchart, in step  240 , it is determined whether the engine operating state accords with the predetermined state. If YES is obtained in step  240 , that is, the engine operating state accords with the predetermined state, the process proceeds to step  241 . In step  241 , the reducing agent is supplied through the reducing agent supply valve  77  for the supply time period of tF 0  while holding the selector valve  61  in the forward flow position. Then in step  242 , the peak value PK of the output OP of the oxygen sensor  48 , which is generated upon supply of the reducing agent is obtained. In step  243 , it is determined whether the obtained peak value PK is equal to the target peak value PKTP. If NO is obtained in step  243 , that is, PK≠PKTP, the process proceeds to step  244  where the correction coefficient KR and the supply time period tF 0  are updated. If YES is obtained in step  24     3   , that is, PK=PKTP, the process proceeds to step  245  where the flag XR is set (XR=1). 
     A flowchart shown in  FIG. 16  represents a routine for controlling correction of flow rate of the exhaust gas. Referring to the flowchart, in step  250 , it is determined whether the control routine for reducing the stored NO X  that has been described referring to  FIG. 6  is executed, that is, the reducing agent has been supplied through the reducing agent supply valve  77 . If YES is obtained in step  250 , that is, the reducing agent has been supplied, the process proceeds to step  251  where the peak value PK of the output, OP of the oxygen sensor  48 , which is generated upon supply of the reducing agent is obtained. Then in step  252 , it is determined whether the peak value PK is equal to the target peak value PKT. If NO is obtained in step  252 , that is, PK≠PKT, the process proceeds to step  253  where the correction coefficient KEX and the elapsing time tC are updated. If YES is obtained in step  252 , that is, PK=PKT, the process proceeds to step  254  where the flag XEX is set (XEX=1). 
     A second embodiment of the invention will be described hereinafter. The second embodiment is structured to execute the control of correcting flow rate of the exhaust gas after execution of the control of correcting quantity of the reducing agent as in the first embodiment. The control of correcting quantity of the reducing agent in the second embodiment is executed in the same manner as in the first embodiment. However, the control of correcting flow rate of the exhaust gas in the second embodiment is different from that of the first embodiment as described below. 
     In the control of correcting flow rate of the exhaust gas according to the second embodiment, the coefficient KEX for correcting flow rate of the exhaust gas is calculated so as to correct the speed V for selecting the position of the selector valve  61  under the control of reducing the stored NO X . That is, the speed V is corrected with the coefficient KEX (V=V·KEX). If the coefficient KEX increases, the speed V becomes higher. If the coefficient KEX decreases, the speed V becomes lower. As the selector valve  61  is driven by the stepping motor  60 , the speed V for selecting the position of the selector valve  61  is variable. 
     More specifically, the time tP elapsing from the timing X until timing when the output OP of the oxygen sensor  48  reaches a peak is obtained at every execution of the control of reducing the stored NO X  (see  FIG. 7 ). If the actual time period tC elapsing from the timing X until the timing at which the reducing agent is supplied is longer than the normal time period, the space velocity of the exhaust gas upon supply of the reducing agent becomes higher. Accordingly the actual time tP elapsing until the peak is reached becomes shorter than the target elapsing time period tPT corresponding to the normal elapsing time period. If the actual time period tC elapsing until supply of the reducing agent is shorter than the normal time period, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is forward, the actual elapsing time tP becomes longer than the target elapsing time period tPT. If the actual time period tC is shorter than the normal elapsing time, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is reverse, there is no peak in the output OP of the oxygen sensor  48 . 
     If the speed for selecting the position of the selector valve  61  is higher than the normal speed, the elapsing time tP becomes shorter than the target elapsing time tPT. If the speed for selecting the position of the selector valve  61  is lower than the normal speed, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is forward, the elapsing time tP becomes longer than the target elapsing time tPT. If the speed for selecting the position of the selector valve  61  is lower than the normal speed, and the direction of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is reverse, there is no peak in the output of the oxygen sensor. The target elapsing time tPT is predetermined through experiments. 
     In the second embodiment, the coefficient KEX is increased at relatively a lower rate in the condition where tP&gt;tPT. In the case where no peak is formed even if a predetermined time period has elapsed from the timing X, the coefficient KEX is increased at relatively a higher rate. The coefficient KEX is decreased in case of the condition where tP&lt;tPT. In this way, the correction coefficient KEX is continuously updated, and the speed V for selecting the position of the selector valve  61  is changed accordingly (V=V·KEX). In the condition where tP=tPT, the correction coefficient becomes a final correction coefficient. 
     Under the control of reducing the stored NO X , the position of the selector valve  61  is selected from the forward flow position to the reverse flow position or vice versa at the speed V that has been corrected with the correction coefficient KEX. 
     A flowchart of  FIG. 17  represents a control routine for correcting flow rate of the exhaust gas according to the second embodiment. The routine shown in  FIGS. 13 to 15  is executed in the second embodiment. The control routine for correcting flow rate of the exhaust gas shown in  FIG. 17  corresponds to step  233  of the correction control routine shown in  FIG. 14 . 
     Referring to the flowchart of  FIG. 17 , in step  260 , it is determined whether the control routine for reducing the stored NO X  as described referring to  FIG. 6  has been executed, that is, the reducing agent has been supplied through the reducing agent supply valve  77 . If YES is obtained in step  260 , that is, the reducing agent has been supplied, the process proceeds to step  261  where the time tP elapsing from the timing when the signal instructing to select the position of the selector valve  61  until the output OP of the oxygen sensor  48  reaches the peak is obtained. Then in step  262 , it is determined whether the elapsing time tP is equal to the target elapsing time tPT. If NO is obtained in step  262 , that is, tP≠tPT, the process proceeds to step  263 . In step  263 , the correction coefficient KEX and the speed V for selecting the position of the selector valve  61  are updated as described above. If YES is obtained in step  262 , that is, tP=tPT, the process proceeds to step  264  where the flag XEX is set (XEX=1). 
     A third embodiment of the invention will be described hereinafter. The third embodiment is structured to execute the control of correcting flow rate of the exhaust gas after the control of correcting quantity of the reducing agent as in the aforementioned embodiments. The control of correcting quantity of the reducing agent is executed in the same manner as in the first embodiment. The control of correcting flow rate of the exhaust gas in the third embodiment, however, is different from the aforementioned control as described below. 
     According to the third embodiment, the coefficient KEX is calculated for correcting the opening degree D corresponding to the weak forward flow position of the selector valve  61  such that the flow rate of the exhaust gas flowing through the NO X  catalyst in the forward direction upon supply of the reducing agent is held at the optimum value. In the third embodiment, the opening degree D is corrected with the coefficient KEX (D=D·KEX). In this case, if the coefficient KEX increases, the opening degree D becomes large. Meanwhile, if the correction coefficient KEX decreases, the opening degree D becomes small. 
     Under the control of reducing the stored SO X , the mean value OPA of outputs of the oxygen sensor  48  is obtained as shown in  FIG. 9 . If the actual opening degree D is larger than the normal opening degree, the flow rate of the exhaust gas flowing through the NO X  catalyst  81  becomes large. Accordingly, the mean value OPA becomes larger than the target output value OPAT corresponding to the normal opening degree. If the actual opening degree is smaller than the normal opening degree, the flow rate of the exhaust gas flowing through the NO X  catalyst  81  is decreased. Therefore, the mean output value OPA becomes smaller than the target output value OPAT. The target output value OPAT is experimentally predetermined. 
     In the third embodiment, in the condition where OPA&gt;OPAT, the coefficient KEX is decreased, and in the condition where OPA&lt;OPAT, the correction coefficient KEX is increased. In this way, the correction coefficient KEX is continuously updated such that the time for supplying the reducing agent tF 0  is also updated (D=D·KEX). The condition where OPA=OPAT represents that the correction coefficient KEX used herein is the final correction coefficient. 
     Under the control of reducing the stored SO X , the reducing agent is supplied while holding the opening degree D of the selector valve  61 . 
     A flowchart in  FIG. 18  represents the control routine for correcting the flow rate of the exhaust gas according to the third embodiment. The routine of the third embodiment is executed as shown in  FIGS. 13 to 15 . The control routine for correcting the flow rate of the exhaust gas shown in  FIG. 18  is executed in step  233  of the correction control routine shown in  FIG. 14 . 
     Referring to  FIG. 18 , in step  270 , it is determined whether the control routine for reducing the stored NO X  has been executed, that is, the reducing agent has been supplied through the reducing agent supply valve  77  as described referring to  FIG. 6 . If YES is obtained in step  270 , that is, the reducing agent has been supplied, the process proceeds to step  271  where the mean value OPA of the outputs OP of the oxygen sensor  48  is obtained. Then in step  272 , it is determined whether the OPA is equal to a target value OPAT. In NO is obtained in step  272 , that is, OPA≠OPAT, the process proceeds to step  273  where the flag KEX and the opening degree D are updated. If YES is obtained in step  272 , that is, OPA=OPTAT, the flag XEX is set (XEX=1). 
     In the third embodiment, the control of correcting flow rate of the exhaust gas is executed on the basis of the oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81  upon execution of the control of reducing the stored SO X . In the first or the second embodiment, the control of correcting the flow rate of the exhaust gas is executed on the basis of the oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81  upon execution of the control of reducing the stored NO X . 
     A fourth embodiment of the invention will be described. In this embodiment, the control of correcting flow rate of the exhaust gas is executed. Upon completion of the control routine, the control of correcting the quantity of the reducing agent is then executed. 
     Under the control of correcting flow rate of the exhaust gas according to the first embodiment, the elapsing time tC is corrected such that the peak value PK (negative value) of the output OP of the oxygen sensor  48  becomes minimum. Alternatively, the coefficient KEX that makes the peak value minimum is obtained. The peak value PK as the minimum value accords with the target peak value PKT. 
     Under the control of correcting the flow rate of the exhaust gas according to the fourth embodiment, the elapsing time tC is corrected such that the peak value PK becomes minimum, or the coefficient KEX that makes the peak value PK minimum is obtained. More specifically, if the peak value PK decreases upon increase in the correction coefficient KEX, the KEX is decreased. Meanwhile, if the peak value PK increases, the KEX is further increased. If the peak value PK decreases upon decrease in the correction coefficient KEX, the KEX is increased. Meanwhile, if the peak value PK increases, the KEX is further decreased. 
     The minimum peak value is obtained under the control of correcting flow rate of the exhaust gas before execution of the control of correcting quantity of the reducing agent. Accordingly the minimum peak value does not always accord with the target peak value PKT. 
     Under the control of correcting the quantity of the reducing agent according to the fourth embodiment, the time period tF for supplying the reducing agent is corrected such that the minimum peak value obtained under the control of correcting flow rate of the exhaust gas accords with the target peak value PKT. Alternatively, the coefficient KR for correcting quantity of the reducing agent is obtained such that the minimum peak value accords with the target peak value PKT. More specifically, in the condition where PK&gt;PKT, the correction coefficient KR is decreased, and PK&lt;PKT, the correction coefficient KR is increased. 
     The control routine according to the fourth embodiment is shown in  FIGS. 19 to 21 . The routine for initialization as shown in  FIG. 13  is also executed. 
     The correction control routine shown in  FIG. 19  is executed upon every interruption at a predetermined time interval. Referring to  FIG. 19 , in step  280 , it is determined whether the flag XEX indicating completion of the correction of the flow rate of the exhaust gas is reset (XEX=0). If YES is obtained in step  280 , that is, the flag XEX has been reset, the process proceeds to step  281  where the control routine of correcting flow rate of the exhaust gas shown in  FIG. 20  is executed. When the flag XEX is set upon completion of the control of correcting flow rate of the exhaust gas, the process proceeds to step  282  from step  280 . In step  282 , it is determined whether the flag XR indicating completion of correcting quantity of the reducing agent has been reset (XR=0). If YES is obtained in step  282 , that is, the flag XR has been reset, the process proceeds to step  283  where the control routine of correcting quantity of the reducing agent is executed as shown in  FIG. 21 . 
     The control routine of correcting flow rate of the exhaust gas according to the fourth embodiment will be described referring to  FIG. 20 . In step  290  of the flowchart in  FIG. 20 , it is determined whether the control routine for reducing the stored NO X  as described referring to  FIG. 6  has been executed, that is, the reducing agent has been supplied through the reducing agent supply valve  77 . If YES is obtained in step  290 , that is, the reducing agent has been supplied, the process proceeds to step  291  where the peak value PK of the output OP of the oxygen sensor  48  which is generated upon supply of the reducing agent is obtained. Then in step  292 , it is determined whether the peak value PK is a minimum peak value. If NO is obtained in step  292 , that is, the peak value PK is not the minimum peak value, the process proceeds to step  293  where the correction coefficient KEX and the elapsing time tC are updated as described above. If YES is obtained in step  292 , that is, the peak value PK becomes the minimum peak value, the process proceeds to step  294  where the correction flag XEX is set (XEX=1). 
     The flowchart shown in  FIG. 21  represents the control routine for correcting quantity of the reducing agent according to the fourth embodiment. In step  300  of the flowchart in  FIG. 21 , it is determined whether the control routine of reducing the stored NO X  as described referring to  FIG. 6  has been executed, that is, the reducing agent has been supplied through the reducing agent supply valve  77 . If YES is obtained in step  300 , that is, the reducing agent has been supplied, the process proceeds to step  301 . In step  301 , the peak value PK of the output OP of the oxygen sensor  48  which is generated upon supply of the reducing agent is obtained. Then in step  302 , it is determined whether the peak value PK is equal to the target peak value PKT. If NO is obtained in step  302 , that is, PK≠PKT, the process proceeds to step  303  where the correction coefficient KR and the elapsing time tF are updated as described above. If YES is obtained in step  302 , that is, PK=PKT, the process proceeds to step  304  where the correction flag XR is set (XR=1). 
     In the aforementioned embodiments, the exhaust sensor  48  constitutes the oxygen sensor for detecting the oxygen concentration of the exhaust gas discharged from the NO X  catalyst  81  upon supply of the reducing agent. The control of correcting quantity of the reducing agent or correcting flow rate of the exhaust gas may be executed on the basis of the detected oxygen concentration. The aforementioned control may be executed on the basis of other parameters representing the state of the exhaust gas discharged from the NO X  catalyst  81  upon supply of the reducing agent. 
     The timing chart shown in  FIG. 22  represents the change in parameters each indicating the state of the exhaust gas discharged from the NO X  catalyst  81  upon supply of the reducing agent through the reducing agent supply valve  77  while changing the selector valve  61  between the forward and the reverse flow positions. 
     Referring to the timing chart in  FIG. 22 , upon elapse of time tP 1  from the timing X at which the signal instructing to change the position of the selector valve  61  is generated, the temperature T of the exhaust gas temporarily increases to reach a peak of PK 1 . That is, the temperature T increases by DLT 1 . If the quantity of the reducing agent actually supplied to the NO X  catalyst  81  is larger than the normal quantity, the temperature increase DLT 1  becomes large. On the contrary, if the quantity of the reducing agent actually supplied to the NO X  catalyst  81  is smaller than the normal quantity, the temperature increase DLT 1  becomes small. If the flow rate of the exhaust gas flowing through the NO X  catalyst  81  upon supply of the reducing agent is higher than the optimum flow rate, the quantity of the reducing agent that passes through the NO X  catalyst  81  without being oxidized becomes large. Accordingly, the temperature increase DLT 1  becomes small. On the contrary, if the flow rate of the exhaust gas that passes through the NO X  catalyst  81  upon supply of the reducing agent is lower than the optimum flow rate, the reducing agent is gradually oxidized. Accordingly, the temperature increase DLT 1  becomes small. 
     Upon elapse of time tP 2  from the timing X, the NO X  concentration CN of the exhaust gas temporarily increases to reach a peak of PK 2 . That is, the NO X  concentration CN increases by DLT 2 . If the quantity of the reducing agent actually supplied to the NO X  catalyst  81  is larger than the normal quantity, the increase DLT 2  in the NO X  concentration CN becomes small. If the quantity of the reducing agent is smaller than the normal quantity, the increase DLT 2  in the NO X  concentration CN becomes large. If the flow rate of the exhaust gas that flows through the NO X  catalyst  81  upon supply of the reducing agent is higher than the optimum flow rate, the increase DLT 2  becomes large. If the flow rate of the exhaust gas is lower than the optimum flow rate, the increase DLT 2  also becomes small. 
     Upon elapse of the time tP 3  from the timing X, the NO X  concentration CN temporarily decreases to reach a peak of PK 3 . That is, the NO X  concentration CN decreases by DLT 3 . If the quantity of the reducing agent actually supplied to the NO X  catalyst  81  is larger than the normal quantity, the decrease DLT 3  becomes large, for example. If the quantity of the reducing agent is smaller than the normal quantity, the decrease DLT 3  becomes small. If the flow rate of the exhaust gas that flows through the NO X  catalyst  81  upon supply of the reducing agent is higher than the optimum flow rate, the decrease DLT 3  becomes small. If the flow rate of the exhaust gas is lower than the optimum flow rate, the decrease DLT 3  also becomes small. 
     Upon elapse of the time tP 4  from the timing X, the concentration CH of the reducing agent temporarily increases to reach a peak of PK 4 . That is, the concentration CH of the reducing agent increases by DLT 4 . If the quantity of the reducing agent actually supplied to the NO X  catalyst  81  is larger than the normal quantity, the increase DLT 4  becomes large. On the contrary, if the quantity of the reducing agent is smaller than the normal quantity, the increase DLT 4  becomes small. If the flow rate of the exhaust gas that flows through the NO X  catalyst  81  upon supply of the reducing agent is higher than the optimum flow rate, the increase DLT 4  becomes large. On the contrary, if the flow rate of the exhaust gas is lower than the optimum flow rate, the increase DLT 4  becomes small. The state of the exhaust gas may be changed in the same manner as aforementioned if the reducing agent is supplied while holding the selector valve  62  in the forward or the weak forward flow position. 
     In this embodiment, the control of correcting quantity of the reducing agent may be executed such that the increase DLT 1  in the temperature T of the exhaust gas upon supply of the reducing agent while holding the selector valve  61  in the forward flow position reaches the target value. Then the control of correcting flow rate of the exhaust gas may be executed such that the increase DLT 1  in the temperature T of the exhaust gas upon supply of the reducing agent while changing the position of the selector valve  61  between the forward and the reverse flow positions reaches the target value. 
     Assuming that a temperature sensor, a NO X  sensor, or a reducing agent (hydrocarbon) concentration sensor is used as the exhaust sensor  48 , the control of correcting quantity of the reducing agent or correcting flow rate of the exhaust gas may be executed on the basis of the temperature T, the NO X  concentration CN, or the reducing agent concentration CH of the exhaust gas discharged from the NO X  catalyst upon supply of the reducing agent. 
     Alternatively, different types of sensors each detecting different state of the exhaust gas may be employed so as to execute the control of correcting the quantity of the reducing agent or correcting the flow rate of the exhaust gas on the basis of parameters detected by those sensors. The control of correcting quantity of the reducing agent may be executed on the basis of the exhaust gas temperature, and the control of correcting flow rate of the exhaust gas may be executed on the basis of the oxygen concentration of the exhaust gas. 
     Another embodiment of the control of reducing the stored SO X  will be described hereinafter. 
     The actual opening degree D of the selector valve  61  larger than the normal opening degree represents that the flow rate of the exhaust gas flowing through the NO X  catalyst  81  is higher larger than the optimum flow rate. Accordingly the quantity of the reducing agent that effectively functions within the NO X  catalyst  81  is decreased. Meanwhile, the actual opening degree D smaller than the normal opening degree represents that the quantity of the reducing agent that effectively functions within the NO X  catalyst  81  is increased. If the quantity of the effectively functioning reducing agent is decreased, the time required to make the quantity of SO X  stored within the NO X  catalyst  81  substantially zero may become longer. On the contrary, if the quantity of the effectively functioning reducing agent is increased, such time may become shorter. 
     If the quantity of the effectively functioning reducing agent is decreased as described above, the mean value OPA of outputs of the oxygen sensor  48  under the control of reducing the stored SO X  becomes large. Meanwhile, if the quantity of the effectively functioning reducing agent is increased, the mean value OPA becomes small. 
     In this embodiment, under the control of reducing the stored SO X , the time tS for which such control is continued is corrected so as to be longer as the mean value OPA becomes larger. In other words, if the actual opening degree D is larger than the normal opening degree, the time tS is corrected to be longer. If the actual opening degree D is smaller than the normal opening degree, the time tS is corrected to be shorter. The time tS is preliminarily stored in the ROM  43  in the form of a map as shown in  FIG. 23 . 
     The flowchart of  FIG. 24  represents the routine for executing the control of reducing the stored SO X  according to the embodiment. In step  310  of the flowchart, the quantity of SO X  stored in the NO X  catalyst  81 , that is, QS is calculated. Then in step  311 , it is determined whether the QS is larger than the allowable quantity QS 1 . If NO is obtained in step  310 , that is, QS≦QS 1 , the control routine ends. If YES is obtained in step  310 , that is, QS&gt;QS 1 , the process proceeds to step  312 . In step  312 , the reducing agent is intermittently supplied through the reducing agent supply valve  77  while setting the selector valve  61  to be in the weak forward flow position from the forward flow position and holding the selector valve  61  in the selected position. Then in step  313 , the time tS is calculated using the map shown in  FIG. 23 . In step  314 , it is determined whether the time tS has been elapsed from supply of the reducing agent while holding the selector valve  61  in the weak forward flow position. If NO is obtained in step  314 , the process returns to step  312  until the time tS elapses. The reducing agent is repeatedly supplied while holding the selector valve  61  in the weak forward flow position until elapse of the time tS. If YES is obtained in step  314 , that is, the time tS has elapsed, the process proceeds to step  315  where the selector valve  61  is selected to the forward flow position, for example. The control of reducing the stored SO X  is, then, completed. 
     The internal combustion engine as shown in  FIG. 1  has the exhaust sensor  48  provided in the annular exhaust pipe  67  so as to avoid the influence caused by the exhaust gas that bypasses the NO X  catalyst  81  to directly flow from the inlet port  62  to the outlet port  63  of the selector valve  61 . The exhaust sensor  48 , however, may be provided within the exhaust discharge pipe  64  between the outlet port  63  of the selector valve  61  and the auxiliary catalyst  76 . 
     The aforementioned embodiments of the invention are applicable to the internal combustion engines as shown in  FIGS. 25 and 27 . 
     In the internal combustion engine as shown in  FIG. 25 , a casing  168  is connected to an outlet of the exhaust pipe  20   a , and is further connected to a casing  175  via the exhaust pipe  20   c . The casing  175 , then, is connected to the exhaust pipe  23 . The particulate filter  69  that carries the NO X  catalyst  81  and the auxiliary catalyst  76  are provided within the casings  168 ,  175 , respectively. 
     A bypass pipe  185  is branched from the exhaust pipe  20   a , an outlet end of which is opened to the exhaust pipe  20   c . A selector valve  161  controlled by an electronic control unit (not shown) is provided in a point where an inlet end of the bypass pipe  185  is connected with the exhaust pipe  20   a . The reducing agent supply valve  77  is provided in the exhaust pipe  20   a  at a point between the inlet end of the bypass pipe  185  and the particulate filter  69 . The exhaust sensor  48  is provided in the exhaust pipe  20   c  at a position between the particulate filter  69  and the outlet end of the bypass pipe  185 . 
     The selector valve  161  is held in a normal position as shown by a solid line in  FIG. 26 . When the selector valve  161  is held in the normal position, the bypass pipe  185  is blocked such that most of the exhaust gas flowing into the exhaust pipe  20   a  is guided into the particulate filter  69 . Accordingly, the normal position of the selector valve  161  corresponds to the forward flow position or the reverse flow position of the selector valve  61  in the internal combustion engine as shown in  FIG. 1 . 
     When the control of reducing the stored NO X  or stored SO X  is required, the reducing agent is supplied through the reducing agent supply valve  77  while holding the selector valve  161  in the weak flow position as shown by a chain line in  FIG. 26 . When the selector valve is held in the weak flow position, a small part of the exhaust gas flowing into the exhaust pipe  20   a  is guided in the particulate filter  69 , and the rest of the exhaust gas is guided into the bypass pipe  185 . Accordingly the weak flow position of the selector valve  161  shown in  FIG. 26  corresponds to the weak forward flow position of the selector valve  61  in the internal combustion engine as shown in  FIG. 1 . When the selector valve  161  is held in the bypass position as shown by a dashed line in  FIG. 26 , the bypass pipe  185  is unblocked, allowing most of the exhaust gas flowing through the exhaust pipe  20   a  to bypass the particulate filter  69 . Accordingly the bypass position of the selector valve  161  corresponds to the bypass position of the selector valve  61  in the internal combustion engine shown in  FIG. 1 . 
     In an internal combustion engine as shown in  FIG. 27 , the exhaust pipe  20   a  constitutes a Y-like pipe having a pair of branch pipes  91 ′ and  91 ″. Each outlet of the respective branch pipes is connected to casings  68 ′,  68 ″ which are connected to branch pipes  92 ′,  92 ″ of the exhaust pipe  20   c , respectively. They are further connected to a casing  175  via the exhaust pipe  20   c , The casing  175  is connected to the exhaust pipe  23 . Those casings  68 ′,  68 ″ have the first and the second particulate filters  69 ′,  68 ″. The first and the second particulate filters  69 ′,  69 ″ carry the first and the second NO X  catalysts  81 ′,  82 ″, respectively. 
     There are first and second selector valves  61 ′,  61 ″ each driven by a common actuator  160  within the branch pipe of the exhaust pipe  20   c , and the first and the second sensors  48 ′,  48 ″, respectively. The branch pipe of the exhaust pipe  20   a  has the first and the second reducing agent water supply valves  77 ′,  77 ″ in the branch pipe of the exhaust pipe  20   a . The actuator  160  and the reducing agent supply valve  77 ′,  77 ″ are controlled by the electronic control unit (not shown). 
     The selector valves  61 ′,  61 ″ are held in the first normal positions as shown by the solid lines in  FIG. 28A , or in the second normal positions as shown by the dashed lines in  FIG. 28A . When the selector valves  61 ′,  61 ″ are held in the first normal positions, the first selector valve  61 ′ is held in a full open position, and the second selector valve  61 ″ is held in a full close position. As shown by the solid arrow in  FIG. 28A , almost all the exhaust gas flowing into the exhaust pipe  20   a  is guided into the first NO X  catalyst  81 ′. Meanwhile when the selector valves  61 ′,  61 ″ are held in the second normal positions, the first selector valve  61 ′ is held in the full close position, and the second selector valve  61 ″ is held in the full open position. As shown by a dashed arrow in  FIG. 28A , almost all the exhaust gas flowing into the exhaust pipe  20   a  is guided into the second NO X  catalyst  81 ″. The first and the second normal positions of the selector valves  61 ′,  61 ″ correspond to the normal position or the bypass position of the selector valve  161  in the internal combustion engine shown in  FIG. 20 . 
     When the control of reducing NO X  or SO X  stored in the first NO X  catalyst  81 ′ is required, the reducing agent is supplied while holding the selector valves  61 ′,  61 ″ in the first weak flow positions as shown by the solid lines in  FIG. 28B . When the selector valves  61 ′,  61 ″ are held in the first weak flow positions, a small part of the exhaust gas flowing into the exhaust pipe  20   a  is guided into the first NO X  catalyst  81 ′, and the rest of the exhaust gas is guided into the second NO X  catalyst  81 ″. When the control of reducing the NO X  or SO X  stored in the second NO X  catalyst  81 ″ is required, the reducing agent is supplied while setting the selector valves  61 ′,  61 ″ in the second weak flow position so as to be held as shown in the dashed lines in  FIG. 28B . When the selector valves  61 ′,  61 ″ are held in the second weak flow positions, a part of the exhaust gas flowing into the exhaust pipe  20   a  is guided into the second NO X  catalyst  81 ″, and the rest of the exhaust gas is guided into the first NO X  catalyst  81 ′. The weak flow positions of the selector valves  61 ′,  61 ″ correspond to the weak forward flow position of the selector valve  61  in the internal combustion engine as shown in  FIG. 1 . 
     Generally the NO X  catalyst is provided within the exhaust passage, from where the bypass passage is branched to bypass the NO X  catalyst. The selector valve is further provided to control the flow rate of the exhaust gas that flows through the NO X  catalyst by controlling the flow rate of the exhaust gas flowing through the bypass passage. Then the reducing agent supply valve is provided to supply the reducing agent into the exhaust passage between the branch portion of the bypass passage and the NO X  catalyst. 
     The internal combustion engine shown in  FIG. 1  is allowed to select the flow of the exhaust gas into the NO X  catalyst between a direction from one end surface to the other end surface and a direction reverse thereto. 
     In the internal combustion engine shown in  FIG. 27 , the exhaust passage from the branch pipe  91 ″ of the exhaust pipe  20   a  to the branch pipe  92 ″ of the exhaust pipe  20   c  may be regarded as serving as the bypass passage with respect to the exhaust passage from the branch pipe  91 ′ of the exhaust pipe  20   a  to the branch pipe  92 ′ of the exhaust pipe  20   c , In this case, the second reducing agent supply valve  77 ′, the second particulate filter  69 ″, and the second NO X  catalyst  81 ″ may be regarded as additional reducing agent supply valve, particulate filter, and the NO X  catalyst, respectively. 
     The aforementioned embodiments make it possible to hold the flow rate of the exhaust gas that flows through the NO X  catalyst upon supply of the reducing agent through the reducing agent supply valve to an optimum value.