Abstract:
An internal combustion engine is configured in such a manner that an exhaust gas purification catalyst ( 13 ) and a hydrocarbon supply valve ( 15 ) are provided in an exhaust gas passage. An exhaust gas purification device selectively uses: a first NOx purification method in which hydrocarbon is sprayed from the hydrocarbon supply valve at predetermined intervals, purifying NOx contained in exhaust gas; and a second NOx purification method in which the air-fuel ratio of exhaust gas which flows into the exhaust gas purification catalyst is made rich at intervals longer than the predetermined intervals, causing the exhaust gas purification catalyst to release occluded NOx and cleaning the NOx. It is determined whether or not the exhaust gas purification catalyst is clogged with the deposition of particulates in the exhaust gas. When it is determined that the exhaust gas purification catalyst is clogged, the second NOx purification method is used.

Description:
TECHNICAL FIELD 
     The present invention relates to an exhaust purification device for an internal combustion engine. 
     BACKGROUND ART 
     Know in the art is an exhaust purification device for an internal combustion engine in which an exhaust purification catalyst is arranged inside an engine exhaust passage and a hydrocarbon feed valve is arranged upstream of the exhaust purification catalyst in the engine exhaust passage, a precious metal catalyst is carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has the property of reducing the NO x  which is contained in the exhaust gas when making the concentration of hydrocarbons which flow into the exhaust purification catalyst vibrate by within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in amount of storage of NO x  which is contained in the exhaust gas if making the vibration period of the hydrocarbon concentration longer than the predetermined range, and a first NO x  removal method in which NO x  contained in the exhaust gas is removed by injecting hydrocarbons from the hydrocarbon feed valve by the predetermined period and, a second NO x  removal method in which stored NO x  is released from the exhaust purification catalyst to remove the NO x  by making the air-fuel ratio of the exhaust gas which flows in to the exhaust purification catalyst rich by a period longer than the predetermined period, are selectively used (for example, see PLT 1). In this exhaust purification device, for example, the NO x  removal action by the first NO x  removal method is performed when the temperature of the exhaust purification catalyst is high, while the NO x  removal action by the second NO x  removal method is performed when the temperature of the exhaust purification catalyst is low. 
     CITATIONS LIST 
     Patent Literature 
     PLT 1: WO2011/114501 
     SUMMARY OF INVENTION 
     Technical Problem 
     In this regard, exhaust gas which is exhausted from an engine contains various particulates, but usually these particulates slip through the exhaust purification catalyst and therefore usually these particulates do not deposit on the upstream side end face of the exhaust purification catalyst or inside the exhaust purification catalyst. In this regard, however, if the NO x  removal action by the first NO x  removal method is performed, not only the particulates which are exhausted from the engine, but also the hydrocarbons which are injected from the hydrocarbon feed valve will flow into the exhaust purification catalyst with a high frequency, so the upstream side end face of the exhaust purification catalyst is liable to gradually increase in buildup of particulates and hydrocarbons. That is, if referring to the particulates which are exhausted from the engine and the hydrocarbons which are injected from the hydrocarbon feed valve as the “particulates in the exhaust gas”, when the NO x  removal action by the first NO x  removal method is performed, particulates in the exhaust gas will deposit on the upstream side end face of the exhaust purification catalyst. Further, while explained in detail later, if particulates in the exhaust gas continue to flow to the upstream side end face of the exhaust purification catalyst, the catalyst will clog due to buildup of the particulates in the exhaust gas. 
     If the upstream side end face of the exhaust purification catalyst clogs due to buildup of particulates, the amount of hydrocarbons which flow into the exhaust purification catalyst will decrease. In this case, the amount of hydrocarbons which is used for producing the reducing intermediate will decrease. Therefore, if the amount of hydrocarbons which flows into the exhaust purification catalyst decreases, the amount of production of the reducing intermediate will decrease and as a result the NO x  removal rate will drop. Therefore, if the NO x  removal action by the first NO x  removal method is performed when the upstream side end face of the exhaust purification catalyst is clogged by buildup of particulates, the NO x  is liable to become unable to be reliably removed. 
     An object of the present invention is to provide an exhaust purification device for an internal combustion engine which can remove NO x  reliably even when the exhaust purification catalyst becomes clogged. 
     Solution to Problem 
     According to the present invention, there is provided an exhaust purification device for an internal combustion engine in which an exhaust purification catalyst is arranged inside an engine exhaust passage and a hydrocarbon feed valve is arranged upstream of the exhaust purification catalyst in the engine exhaust passage, a precious metal catalyst is carried on an exhaust gas flow surface of the exhaust purification catalyst and a basic exhaust gas flow surface part is formed around the precious metal catalyst, the exhaust purification catalyst has the property of reducing the NO x  which is contained in the exhaust gas when making the concentration of hydrocarbons which flow into the exhaust purification catalyst vibrate by within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in amount of storage of NO x  which is contained in the exhaust gas if making the vibration period of the hydrocarbon concentration longer than the predetermined range, a first NO x  removal method in which NO x  contained in the exhaust gas is removed by injecting hydrocarbons from the hydrocarbon feed valve by the predetermined period and, a second N x  removal method in which stored NO x  is released from the exhaust purification catalyst to remove the NO x  by making the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst rich by a period longer than the predetermined period, are selectively used, it is judged whether the exhaust purification catalyst is clogged due to buildup of particulates in the exhaust gas, and the NO x  removal action by the second NO x  removal method is performed when it is judged that the exhaust purification catalyst is clogged due to buildup of particulates in the exhaust gas. 
     Advantageous Effects of Invention 
     It is possible to reliably remove NO x  even when the exhaust purification catalyst becomes clogged. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an overall view of a compression ignition type internal combustion engine. 
         FIG. 2  is a view which schematically shows a surface part of a catalyst carrier. 
         FIG. 3  is a view for explaining an oxidation reaction in an exhaust purification catalyst. 
         FIG. 4  is a view which shows a change of an air-fuel ratio of exhaust gas which flows into an exhaust purification catalyst. 
         FIG. 5  is a view which shows an NO x  removal rate. 
         FIGS. 6A and 6B  are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. 
         FIGS. 7A and 7B  are views for explaining an oxidation reduction reaction in an exhaust purification catalyst. 
         FIG. 8  is a view which shows a change of an air-fuel ratio of exhaust gas which flows into an exhaust purification catalyst. 
         FIG. 9  is a view of an NO x  removal rate. 
         FIG. 10  is a view which shows a relationship between an injection period ΔT of hydrocarbons and an NO x  removal rate. 
         FIG. 11  is a map which shows an injection amount of hydrocarbons. 
         FIG. 12  is a view which shows NO x  release control. 
         FIG. 13  is a view which shows a map of an exhausted NO x  amount NOXA. 
         FIG. 14  is a view which shows a fuel injection timing. 
         FIG. 15  is a view which shows a map of a fuel feed amount WR. 
         FIGS. 16A and 16B  are enlarged views of the area around the exhaust purification catalyst which is shown in  FIG. 1 . 
         FIGS. 17A and 17B  are enlarged views of the area around the exhaust purification catalyst which shows another embodiment. 
         FIG. 18  is a view which shows changes in the pressure inside the exhaust pipe and the differential pressure across a particulate filter. 
         FIG. 19  is a flow chart for performing NO x  removal control. 
         FIG. 20  is a flow chart for judging clogging. 
         FIG. 21  is a flow chart for performing a second NO x  removal method. 
         FIGS. 22A and 22B  are views which show another embodiment of clogging judgment. 
         FIGS. 23A and 23B  are enlarged views of the area around the exhaust purification catalyst in another embodiment according to the present invention. 
         FIGS. 24A and 24B  are enlarged views of the area around the exhaust purification catalyst which shows another embodiment. 
         FIG. 25  is a view which shows changes in an end face blockage rate and a flow rate per unit cross-sectional area. 
         FIGS. 26A and 26B  are views which show changes in the output value of an air-fuel ratio sensor. 
         FIG. 27  is a view which shows changes in the output value of an air-fuel ratio sensor. 
         FIG. 28  is a flow chart for judging clogging. 
         FIG. 29  is a view which shows changes in the output value of an air-fuel ratio sensor. 
         FIG. 30  is a flow chart for detecting clogging. 
         FIG. 31  is a flow chart for detecting clogging. 
         FIG. 32  is a flow chart for NO x  removal control. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is an overall view of a compression ignition type internal combustion engine. Referring to  FIG. 1 , indicates an engine body,  2  a combustion chamber of each cylinder,  3  an electronically controlled fuel injector for injecting fuel into each combustion chamber  2 ,  4  an intake manifold, and  5  an exhaust manifold. The intake manifold  4  is connected through an intake duct  6  to an outlet of a compressor  7   a  of an exhaust turbocharger  7 , while an inlet of the compressor  7   a  is connected through an intake air amount detector  8  to an air cleaner  9 . Inside the intake duct  6 , a throttle valve  10  driven by an actuator is arranged. Around the intake duct  6 , a cooling device  11  is arranged for cooling the intake air which flows through the inside of the intake duct  6 . In the embodiment shown in  FIG. 1 , the engine cooling water is guided to the inside of the cooling device  11  where the engine cooling water is used to cool the intake air. 
     On the other hand, the exhaust manifold  5  is connected to an inlet of an exhaust turbine  7   b  of the exhaust turbocharger  7 . An outlet of the exhaust turbine  7   b  is connected through an exhaust pipe  12   a  to an inlet of the exhaust purification catalyst  13 . In this embodiment according to the present invention, this exhaust purification catalyst  13  is comprised of an NO x  storage catalyst. An outlet of the exhaust purification catalyst  13  is connected through the exhaust pipe  12   b  to the particulate filter  14 . Upstream of the exhaust purification catalyst  13  inside the exhaust pipe  12   a , a hydrocarbon feed valve  15  is arranged for feeding hydrocarbons comprised of diesel oil or other fuel used as fuel for a compression ignition type internal combustion engine. In the embodiment shown in  FIG. 1 , diesel oil is used as the hydrocarbons which are fed from the hydrocarbon feed valve  15 . Note that, the present invention can also be applied to a spark ignition type internal combustion engine in which fuel is burned under a lean air-fuel ratio. In this case, from the hydrocarbon feed valve  1 , hydrocarbons comprised of gasoline or other fuel used as fuel of a spark ignition type internal combustion engine are fed. 
     On the other hand, the exhaust manifold  5  and the intake manifold  4  are connected with each other through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage  16 . Inside the EGR passage  16 , an electronically controlled EGR control valve  17  is arranged. Further, around the EGR passage  16 , a cooling device  18  is arranged for cooling EGR gas which flows through the inside of the EGR passage  16 . In the embodiment shown in  FIG. 1 , the engine cooling water is guided to the inside of the cooling device  18  where the engine cooling water is used to cool the EGR gas. Each fuel injector  3  is connected through a fuel feed tube  19  to a common rail  20 . This common rail  20  is connected through an electronically controlled variable discharge fuel pump  21  to a fuel tank  22 . The fuel which is stored inside of the fuel tank  22  is fed by the fuel pump  21  to the inside of the common rail  20 . The fuel which is fed to the inside of the common rail  20  is fed through each fuel feed tube  19  to the fuel injector  3 . 
     An electronic control unit  30  is comprised of a digital computer provided with a components which are connected with each other by a bidirectional bus  31  such as a ROM (read only memory)  32 , a RAM (random access memory)  33 , a CPU (microprocessor)  34 , an input port  35 , and an output port  36 . Downstream of the exhaust purification catalyst  13 , a temperature sensor  24  for detecting the temperature of the exhaust gas which flows out from the exhaust purification catalyst  13  is attached. The temperature of the exhaust gas which flows out from the exhaust purification catalyst  13  represents the temperature of the exhaust purification catalyst  13 . Further, upstream of the exhaust purification catalyst  13  in the exhaust pipe  12   a , a pressure sensor  25  is attached for detecting the pressure inside of the exhaust pipe  12   a . Further, at the particulate filter  14 , a differential pressure sensor  26  for detecting a differential pressure across the particulate filter  14  is attached. The output signals of these temperature sensor  24 , pressure sensor  25 , and intake air amount detector  8  are input through respectively corresponding AD converters  37  to the input port  35 . Further, an accelerator pedal  40  has a load sensor  41  connected to it which generates an output voltage proportional to the amount of depression L of the accelerator pedal  40 . The output voltage of the load sensor  41  is input through a corresponding AD converter  37  to the input port  35 . Furthermore, at the input port  35 , a crank angle sensor  42  is connected which generates an output pulse every time a crankshaft rotates by, for example, 15°. On the other hand, the output port  36  is connected through corresponding drive circuits  38  to each fuel injector  3 , actuator for driving the throttle valve  10 , hydrocarbon feed valve  15 , EGR control valve  17 , and fuel pump  21 . 
       FIG. 2  schematically shows a surface part of a catalyse carrier which is carried or a substrate of the exhaust purification catalyst  13  which is shown in  FIG. 1 . At this exhaust purification catalyst  13 , as shown in  FIG. 2 , for example, a catalyst carrier  50  which is comprised of alumina carries a precious metal catalyst  51  which is comprised of platinum Pt. Furthermore, on this catalyst carrier  50 , a basic layer  53  is formed which includes at least one element selected from potassium K, sodium Na, cesium Cs, or another such alkali metal, barium Ba, calcium Ca, or another such alkali earth metal, a lanthanoid or another such rare earth and silver Ag, copper Cu, iron Fe, iridium Ir, or another metal, able to deviate electrons to NO x . Inside of this basic layer  53 , ceria CeO 2  is contained. Therefore, the exhaust purification catalyst  13  has an oxygen storing ability. Further, the catalyst carrier  50  of the exhaust purification catalyst  13  can also carry rhodium Rh or palladium Pd in addition to platinum Pt. Note that the exhaust gas flows along the top of the catalyst carrier  50 , so the precious metal catalyst  51  can be said to be carried on the exhaust gas flow surface of the exhaust purification catalyst  13 . Further, the surface of the basic layer  53  exhibits basicity, so the surface of the basic layer  53  is called the “basic exhaust gas flow surface part  54 .” 
     If hydrocarbons are injected from the hydrocarbon feed, valve  15  into the exhaust gas, the hydrocarbons are reformed by the exhaust purification catalyst  13 . In the present invention, at this time, the reformed hydrocarbons are used to remove the NO x  at the exhaust purification catalyst  13 .  FIG. 3  schematically shows the reforming action performed at the exhaust purification catalyst  13  at this time. As shown in  FIG. 3 , the hydrocarbons HC which are injected from the hydrocarbon feed valve  15  become radical hydrocarbons HC with a small carbon number by the precious metal catalyst  51 . 
       FIG. 4  shows the feed timing of hydrocarbons from the hydrocarbon feed valve  15  and the change in the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13 . Note that, the changes in the air-fuel ratio (A/F) in depend on the change in concentration of the hydrocarbons in the exhaust gas which flows into the exhaust purification catalyst  13 , so it can be said that the change in the air-fuel ratio (A/F) in shown in  FIG. 4  represents the change in concentration of the hydrocarbons. However, if the hydrocarbon concentration becomes higher, the air-fuel ratio (A/F) in becomes smaller, so, in  FIG. 4 , the more to the rich side oho air-fuel ratio (A/F) in becomes, the higher the hydrocarbon concentration. 
       FIG. 5  shows the NO x  removal rate by the exhaust purification catalyst  13  with respect to the catalyst temperatures of the exhaust purification catalyst  13  when periodically making the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  change so as to, as shown in  FIG. 4 , make the air-fuel ratio (A/F) in of the exhaust gas which flows to the exhaust purification catalyst  13  change. The inventors engaged in research relating to NO x  purification for a long time. As a result, they learned that if making the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate by within a predetermined range of amplitude and within a predetermined range of period, as shown in  FIG. 3 , an extremely nigh NO x  removal rate is obtained even in a 400° C. or higher nigh temperature region. 
     Furthermore, if is learned that, at this time, a large amount of reducing intermediate containing nitrogen and hydrocarbons continues to be held or adsorbed, on the surface of the basic layer  53 , that is, on the basic exhaust gas flow surface part  54  of the exhaust purification catalyst  13 , and that this reducing, intermediate plays a central role in obtaining a high NO x  removal rate. Next, this will be explained with reference to  FIGS. 6A and 6B . Note that, these  FIGS. 6A and 6B  schematically show the surface part of the catalyst carrier  50  of the exhaust purification catalyst  13 . These  FIGS. 6A and 6B  show the reaction which is presumed to occur when the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is made to vibrate by within a predetermined range of amplitude and within a predetermined range of period. 
       FIG. 6A  shows the case where the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is low, while  FIG. 6B  shows the case where hydrocarbons are fed from the hydrocarbon feed, valve  15  and the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  is made rich, that is, when the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  becomes high. 
     Now, as will be understood from  FIG. 4 , the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  is maintained lean except for an instant, so the exhaust gas which flows into the exhaust purification catalyst  13  normally becomes a state of oxygen excess. At this time, part of the NO which is contained in the exhaust gas deposits on the exhaust purification catalyst  13  and part of the NO which is contained in the exhaust gas, as shown in  FIG. 6A , is oxidized on the platinum  51  and becomes NO 2 . Next, this NO 2  is further oxidized and becomes NO 3 . Further part of the NO 2  becomes NO 2   − . Therefore, on the platinum Pt  51 , NO 2   −  and NO 3  are produced. The NO which are deposited on the exhaust purification catalyst.  13  and the NO 2   −  and NO 3  which are produced on platinum Pt  51  are strong in activity. Therefore, below, these NO, NO 2   −  and NO 3  will be referred to as the active NO x   + . 
     On the other hand, if hydrocarbons are fed from the hydrocarbon feed valve  15  and the air-fuel ratio (A/F in of the exhaust gas which flows into the exhaust purification catalyst.  13  is made rich, the hydrocarbons successively deposit across the exhaust purification catalyst  13  as a whole. The majority of the deposited hydrocarbons is successively reacted with the oxygen and made to burn, while part of the deposited hydrocarbons is successively, as shown in  FIG. 3 , reformed and becomes radicalized inside of the exhaust purification catalyst  13 . Therefore, as shown in  FIG. 6B , the hydrocarbon concentration around the active NO x   +  becomes higher. In this regard, if, after the active NO x   +  is produced, the state of a high oxygen concentration around the active NO x   +  continues for a constant time or more, the active NO x   +  is oxidized and is absorbed in the form of nitrate ions NO 3   −  inside the basic layer  53 . However, if, before this constant time elapses, the hydrocarbon concentration around the active NO x   +  becomes higher, as shown in  FIG. 6B , the active NO x   +  reacts on the platinum  51  with the radical hydrocarbons HC to thereby form the reducing intermediate. This reducing intermediate is adhered or adsorbed on the surface of the basic layer  53 . 
     Note that, at this time, the first produced reducing intermediate is considered to be a nitro compound R—NO 2 . If this nitro compound R—NO 2  is produced, the result becomes a nitrile compound R—CN, but this nitrile compound R—CN can only survive for an instant in this state, so immediately becomes an isocyanate compound R—NCO. This isocyanate compound R—NCO, when hydrolyzed, becomes an amine compound R—NH 2 . However, in this case, what is hydrolyzed is considered to be part of the isocyanate compound R—NCO. Therefore, as shown in  FIG. 6B , the majority of the reducing intermediate which is held or adsorbed on the surface of the basic layer  53  is believed to be the isocyanate compound R—NCO and amine compound R—NH 2 . 
     On the other hand, as shown in  FIG. 6B , if the produced reducing intermediate is surrounded by the hydrocarbons HC, the reducing intermediate is blocked by the hydrocarbons HC and the reaction will not proceed any further. In this case, if the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is lowered and then the hydrocarbons which deposit around the reducing intermediate are oxidized and consumed and, due to this, the oxygen concentration around the reducing intermediate becomes higher, the reducing intermediate reacts with the NO x  in the exhaust gas or the active NO x   +  or reacts with the surrounding oxygen or breaks down on its own. Due to this, the reducing intermediate R—NCO or R—NH 2 , as shown in  FIG. 6A , is converted to N 2 , CO 2 , and H 2 O and therefore NO x  is removed. 
     In this way, in the exhaust purification catalyst  13 , by making the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  higher, a reducing intermediate is produced. The concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is lowered and the oxygen concentration is raised so that the reducing intermediate reacts with the NO x  in the exhaust gas or the active NO x   +  or oxygen or breaks down by itself whereby the NO x   +  is removed. That is, in order for the exhaust purification catalyst  13  to remove the NO x  the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  has to be periodically changed. 
     Of course, in this case, it is necessary to raise the concentration of hydrocarbons to a concentration sufficiently high for producing the reducing intermediate and it is necessary to lower the concentration of hydrocarbons to a concentration sufficiently low for making the produced reducing intermediate react with the NO x  in the exhaust gas or the active NO x   +  and oxygen or for making it break down on its own. That is, it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate by within a predetermined range of amplitude. Note that, in this case, it is necessary to hold these reducing intermediates on the basic layer  53 , that is, the basic exhaust gas flow surface part  54 , until the produced reducing intermediate R—NCO or R—NH 2  reacts with the NO x  in the exhaust gas or the active NO x   +  or oxygen or breaks down on its own. For this reason, the basic exhaust gas flow surface part  54  is provided. 
     On the other hand, if lengthening the feed period of the hydrocarbons, the time period in which the oxygen concentration becomes higher becomes longer in the period after the hydrocarbons are fed until the hydrocarbons are next fed. Therefore, the active NO x   +  is absorbed in the basic layer  53  in the form of nitrates without producing a reducing intermediate. To avoid this, it is necessary to make the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  vibrate by within a predetermined range of period. 
     Therefore, in an embodiment of the present invention, to make the NO x  which is contained in the exhaust gas and the reformed hydrocarbons react and produce the reducing intermediate R—NCO or R—NH 2  containing nitrogen and hydrocarbons, a precious metal catalyst  51  is carried on the exhaust gas flow surface of the exhaust purification catalyst  13 . To hold the produced reducing intermediate R—NCO or R—NH 2  inside the exhaust purification catalyst  13 , a basic exhaust gas flow surface part  54  is formed around the precious metal catalyst  51 . The reducing intermediate P—NCO or P—NH 2  which is held on the basic exhaust gas flow surface part  54  is converted to N 2 , CO 2 , and H 2 O, and the vibration period of the hydrocarbon concentration is made the vibration period required for continuation of the production of the reducing intermediate R—NCO or R—NH 2 . Incidentally, in the example shown in  FIG. 4 , the injection interval is made 3 seconds. 
     If the vibration period of the hydrocarbon concentration, that is, the injection period of the hydrocarbons HC from the hydrocarbon feed valve  15 , is made longer than the above predetermined range of period, the reducing intermediate R—NCO or R—NH 2  disappears from the surface of the basic layer  53 . At this time, the active NO x   +  which is produced on the platinum Pt  53 , as shown in  FIG. 7A , diffuses in the basic layer  53  in the form of nitrate ions NO 3   −  and becomes nitrates. That is, at this time, the NO x  in the exhaust gas is absorbed in the form of nitrates inside of the basic layer  53 . 
     On the other hand,  FIG. 7B  shows the case where the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  is made the stoichiometric air-fuel ratio or rich when the NO x  is absorbed in the form of nitrates inside of the basic layer  53  in this way. In this case, the oxygen concentration in the exhaust gas falls, so the reaction proceeds in the opposite direction (NO 3   − →NO 2 ), and consequently the nitrates absorbed in the basic layer  53  become nitrate ions NO 3   −  one by one and, as shown in  FIG. 7B , are released from the basic layer  53  in the form of NO 2 . Next, the released NO 2  is reduced by the hydrocarbons HC and CO contained in the exhaust gas. 
       FIG. 8  shows the case of making the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  temporarily rich slightly before the NO x  absorption ability of the basic layer  53  becomes saturated. Note that, in the example shown in  FIG. 8 , the time interval of this rich control is 1 minute or more. In this case, the NO x  which was absorbed in the basic layer  53  when the air-fuel ratio (A/F) in of the exhaust gas was lean is released all at once from the basic layer  53  and reduced when the air-fuel ratio (A/F) in of the exhaust gas is made temporarily rich. Therefore, in this case, the basic layer  53  plays the role of an absorbent for temporarily absorbing NO x . 
     Note that, at this time, sometimes the basic layer  53  temporarily adsorbs the NO x . Therefore, if using the term of “storage” as a term including both absorption and adsorption, at this time, the basic layer  53  performs the role of an NO x  storage agent for temporarily storing the NO x . That is, in this case, if the ratio of the air and fuel (hydrocarbons) which are supplied into the engine intake passage, combustion chambers  2 , and exhaust passage upstream of the exhaust purification catalyst  13  is referred to as “the air-fuel ratio of the exhaust gas”, the exhaust purification catalyst  13  functions as an NO x  storage catalyst which stores the NO x  when the air-fuel ratio of the exhaust gas is lean and releases the stored NO x  when the oxygen concentration in the exhaust gas falls. 
     In  FIG. 9 , the solid line shows the NO x  removal rate when making the exhaust purification catalyst  13  function as an NO x  storage catalyst in this way. Note that, in  FIG. 9 , the abscissa shows the catalyst temperature TC of the exhaust purification catalyst  13 . When making the exhaust purification catalyst  13  function as an NO x  storage catalyst in this way, as shown in  FIG. 9  by the solid line, when the catalyst temperature TC is 300° C. to 400° C., an extremely high NO x  removal rate is obtained, but when the catalyst temperature TC becomes a 400° C. or higher high temperature, the NO x  removal rate falls. Note that  FIG. 9  shows the NO x  removal rate which is shown in  FIG. 5  by a broken line. 
     In this way, when the catalyst temperature TC becomes 400° C. or more, the NO x  removal rate falls because if the catalyst temperature TC becomes 400° C. or more, the nitrates break down by heat and are released in the form of NO 2  from the exhaust purification catalyst  13 . That is, so long as storing NO x  in the form of nitrates, when the catalyst temperature TC is high, it is difficult to obtain a high NO x  removal rate. However, in the new NO x  removal method shown from  FIG. 4  to  FIGS. 6A and 6B , as will be understood from  FIGS. 6A and 6B , nitrates are not formed or even if formed are extremely fine in amount, consequently, as shown in  FIG. 5 , even when the catalyst temperature TC is high, a high NO x  removal rate is obtained. 
     In this embodiment of the present invention, to use this new NO x  removal method to remove NO x , a hydrocarbon feed valve  15  for feeding hydrocarbons is arranged inside of an engine exhaust passage, an exhaust purification catalyst  13  is arranged downstream of the hydrocarbon feed valve  15  in the engine exhaust passage, a precious metal catalyst  51  is carried on the exhaust gas flow surface of the exhaust purification catalyst  13 , a basic exhaust gas flow surface part  54  is formed around the precious metal catalyst  51 , the exhaust purification catalyst  13  has the property of reducing the NO x  which is contained in exhaust gas if the concentration of hydrocarbons which flow into the exhaust purification catalyst  13  is made to vibrate within a predetermined range of amplitude and within a predetermined range of period and has the property of being increased in storage amount of NO x  which is contained in exhaust gas if the vibration period of the hydrocarbon concentration is made longer than this predetermined range, and, at the time of engine operation, hydrocarbons are injected by a predetermined period to thereby reduce the NO x  which is contained in the exhaust gas in the exhaust purification catalyst  13 . 
     That is, the NO x  removal method which is shown from  FIG. 4  to  FIGS. 6A and 6B  can be said to be a new NO x , removal method designed to remove NO x , without forming almost any nitrates in the case of using an exhaust purification catalyst which carries a precious metal catalyst and forms a basic layer which can absorb NO x . In actuality, when using this new NO x  removal method, the nitrates which are detected from the basic layer  53  become much smaller in amount compared with the case where making the exhaust purification catalyst  13  function as an NO x  storage catalyst. Note that, this new NO x  removal method will be referred to below as “the first NO x  removal method”. 
     Now, as explained above, if the injection period ΔT of the hydrocarbons from the hydrocarbon feed valve  15  becomes longer, the time period during which the oxygen concentration around the active NO x   +  becomes higher becomes longer in the period after the hydrocarbons are injected to when the hydrocarbons are next injected. In this case, in the embodiment which is shown in  FIG. 1 , if the injection period ΔT of the hydrocarbons becomes longer than about 5 seconds, the active NO x   +  starts to be absorbed in the form of nitrates inside the basic layer  53 . Therefore, as shown in  FIG. 10 , if the vibration period ΔT of the hydrocarbon concentration becomes longer than about 5 seconds, the NO x  removal rate falls. Therefore, in the embodiment which is shown in  FIG. 1 , the injection period ΔT of the hydrocarbons has to be made 5 seconds or less. 
     On the other hand, in this embodiment according to the present invention, if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the fed hydrocarbons start to build up on the exhaust gas flow surface of the exhaust purification catalyst  13 , therefore, as shown in  FIG. 10 , if the injection period ΔT of the hydrocarbons becomes about 0.3 second or less, the NO x  removal rate falls. Therefore, in this embodiment of the present invention, the injection period of the hydrocarbons is made from 0.3 second to 5 seconds. 
     Now, in this embodiment according to the present invention, control is performed to change the amount of hydrocarbon injection from the hydrocarbon feed valve  15  and the injection timing so that the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  and the injection period ΔT become the optimum values in accordance with the operating state of the engine. In this case, in this embodiment according to the present invention, the optimum hydrocarbon injection amount W when the NO x  removal action by the first NO x  removal method is being performed is stored as a function of the amount of depression L of the accelerator pedal  40  and the engine speed N in the form of a map such as shown in  FIG. 11  in advance in the ROM  32 . Further, the optimum injection period ΔT of the hydrocarbons at this time is also stored as a function of the amount of depression L of the accelerator pedal  40  and the engine speed N in the form of a map in advance in the ROM  32 . 
     Next, while referring to  FIG. 12  to  FIG. 15 , the NO x  removal method in the case of making the exhaust purification catalyst  13  function as an NO x  storage catalyst will be specifically explained. The NO x  removal method in the case of making the exhaust purification catalyst  13  function as an NO x  storage catalyst in this way will be referred to below as “the second NO x  removal method”. In this second NO x  removal method, as shown in  FIG. 12 , when the stored NO x  amount ΣNOX which was stored in the basic layer  53  exceeds a predetermined allowable amount MAX, the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  is temporarily made rich. If the air-fuel ratio (A/F) in of the exhaust gas is made rich, when the air-fuel ratio (A/F) in of the exhaust gas is lean, the NO x  which was stored in the basic layer  53  is released all at once from the basic layer  53  and reduced. Due to this, the NO x  is removed. 
     The stored NO x  amount ΣNOX is for example calculated from the amount of NO x  which is exhausted from the engine. In this embodiment according to the present invention, the exhausted NO x  amount NOXA which is exhausted from the engine per unit time is stored as a function of the amount of depression L of the accelerator pedal  4  and the engine speed N in the form of a map such as shown in  FIG. 13  in advance in the ROM  32 . The stored NO x  amount ΣNOX is calculated from this exhausted NO x  amount NOXA. In this case, as explained above, the period by which the air-fuel ratio (A/F) in of the exhaust gas is made rich is usually 1 minute or more. 
     In this second NO x  removal method, as shown in  FIG. 14 . In addition to the combustion use fuel Q from the fuel injector  2 , additional fuel WR is injected into the combustion chamber  2  so that the air-fuel ratio (A/F) in of the exhaust gas which flows into the exhaust purification catalyst  13  is made rich. Note that, in  FIG. 14 , the abscissa shows the crank angle. This additional fuel WR is injected at a timing at which it burns, but does not appear as engine output, that is, slightly before ATDC90° after top dead center of the compression stroke. This fuel amount WR is stored as a function of the amount of depression L of the accelerator pedal  40  and the engine speed N in the form of a map such as shown in  FIG. 15  in advance in the ROM  32 . Of course, in this case, it is also possible to make the amount of injection of hydrocarbons from the hydrocarbon feed valve  15  increase so as to make the air-fuel ratio (A/F) in of the exhaust gas rich. 
     Now then, in this embodiment according to the present invention, the NO x  removal action by the first NO x  removal method and the NO x  removal action by the second NO x  removal method are selectively performed. Which of the NO x  removal action by the first NO x  removal method and the NO x  removal action by the second NO x  removal method to perform is determined for example as follows. That is, the NO x  removal rate when the NO x  removal action by the first NO x  removal method is performed, as shown in  FIG. 5 , starts to rapidly fall when the temperature TC of the exhaust purification catalyst  13  becomes the limit temperature TX or less. As opposed to this, as shown in the figure, the NO x  removal rate when the NO x  removal action by the second NO x  removal method is performed falls relatively slowly when the temperature TC of the exhaust purification catalyst  13  falls. Therefore, in this embodiment according to the present invention, when the temperature TC of the exhaust purification catalyst  13  is higher than the limit temperature TX, the NO x  removal action by the first NO x  removal method is performed, while when the temperature TC of the exhaust purification catalyst  13  is lower than the limit temperature TX, the NO x  removal action by the second NO x  removal method is performed. 
     Now then, the exhaust gas which is exhausted from an engine contains various particulates, but usually these particulates slip through the exhaust purification catalyst  13  and therefore these particulates will not build up on the upstream side end face of the exhaust purification catalyst  13  or inside the exhaust purification catalyst  13 . However, if the above-mentioned new NO x  removal method, that is, if the NO x  removal by the first NO x  removal method, is performed, not only the particulates which are exhausted from the engine, but also the hydrocarbons which are injected from the hydrocarbon feed valve  15  will flow into the exhaust purification catalyst  13  with a high frequency, so the upstream side end face of the exhaust purification catalyst  13  gradually increases in buildup of particulates and hydrocarbons. Note that, in this case, if referring to the particulates which are exhausted from the engine and the hydrocarbons which are injected from the hydrocarbon feed valve  15  as the “particulates in the exhaust gas”, when the NO x  removal action by the first NO x  removal method is performed, the particulates in the exhaust gas will deposit on the upstream side end face of the exhaust purification catalyst  13 . 
     In this regard, generally speaking, exhaust gas will not flow uniformly to the upstream side end face of the exhaust purification catalyst  13  due to the effects of the structure of the engine exhaust system etc. Further, the particulates which are exhausted from the engine and the hydrocarbons which are injected from the hydrocarbon feed valve  15 , that is, the particulates in the exhaust gas, normally will not at all uniformly flow to the upstream side end face of the exhaust purification catalyst  13 . That is, the particulates in the exhaust gas normally flow lopsidedly to part of the region of the upstream side end face of the exhaust purification catalyst  13 . If the particulates in the exhaust gas continue to lopsidedly flow to part of the region of the upstream side end face of the exhaust purification catalyst  13  in this way, the catalyst will clog due to buildup of particulates in the exhaust gas. Next, this will be explained with reference to  FIGS. 16A and 16B . 
       FIG. 16A  is an enlarged view of the exhaust purification catalyst  13  of  FIG. 1 , while  FIG. 16B  is a perspective view of  FIG. 16A . In the embodiment according to the present invention, as shown in  FIGS. 16A and 16B , the exhaust purification catalyst  13  is housed inside a tubular casing  60 . At the back end of the inside of the casing  60 , a sensor arrangement space  61  which has a diameter the same as the downstream side end face of the exhaust purification catalyst  13  is formed. As will be understood from  FIGS. 16A and 16B , a temperature sensor  24  is arranged in this sensor arrangement space  61 . Further, as will be understood from  FIGS. 16A and 16B , in this embodiment of the present invention, the exhaust purification catalyst  13  is comprised of a straight flow type catalyst which has a plurality of exhaust flow passages which extend in an axial direction of the exhaust purification catalyst  13 . The exhaust gas which flows from the upstream side end face of the exhaust purification catalyst  13  to the exhaust purification catalyst  13  flows inside the exhaust flow passages inside of the exhaust purification catalyst  13  straight along the axis of the exhaust purification catalyst  13  and flows out from the downstream side end face of the exhaust purification catalyst  13 . 
     Now then, the particulates in the exhaust gas in many cases flow lopsidedly to a certain part of the peripheral region of the upstream side end face of the exhaust purification catalyst  13 .  FIGS. 16A and 16B  show the case where the particulates in the exhaust gas flow lopsidedly to a lower region CL of the peripheral area of the upstream side end face of the exhaust purification catalyst  13  and, as a result, the lower region CL of the peripheral area of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates in the exhaust gas. Normally, if the structure of the engine exhaust system or the mounting position of the hydrocarbon feed valve  15  is determined, a clogging region CL at the upstream side end face of the exhaust purification catalyst  13  is inevitably correspondingly determined.  FIGS. 17A and 17B  show a specific example where the exhaust pipe  12   a  is bent by 90 degrees or more in front of the upstream side end face of the exhaust purification catalyst  13  and where the hydrocarbon feed valve  15  is attached upstream from this bent part. In this specific example, it can be easily understood that a clogged region CL is formed at the peripheral part of the upstream side end face of the exhaust purification catalyst  13  in a direction opposite to the direction in which the exhaust pipe  12   a  extends. 
     In this way, it is possible to predict the limited part of the region at the peripheral area of the upstream side end face of the exhaust purification catalyst  13  where there is a possibility of clogging occurring due to buildup of particulates in the exhaust gas. Therefore, an embodiment according to the present invention predicts the limited part of the region CL of the peripheral area of the upstream side end face of the exhaust purification catalyst  13  where there is a possibility of clogging occurring due to buildup of particulates in the exhaust gas as the “particulate buildup region”. In this case, in actuality, this particulate buildup region CL is found by experiments. 
     Further, in an embodiment of the present invention, the exhaust purification catalyst  13  is comprised of a straight flow type of catalyst which has a plurality of exhaust flow passages which extend in the axial direction of the exhaust purification catalyst  13 . Therefore, in  FIGS. 16B and 17B , the exhaust gas which flows from the particulate buildup region CL to the exhaust flow passages of the exhaust purification catalyst  13  flows out from the corresponding region DL on the downstream side end face of the exhaust purification catalyst  13  which is positioned at the opposite side from the particulate buildup region CL on the longitudinal axis of the exhaust purification catalyst  13 . 
     Now then, when the NO x  removal action by the first NO x  removal method is being performed, the majority of the hydrocarbons which is injected from the hydrocarbon feed valve  15  is used for consumption of oxygen in the exhaust purification catalyst  13 . Only the remaining part of the hydrocarbons is used for producing the reducing intermediate. In this case, even if the amount of hydrocarbons which flow into the exhaust purification catalyst  13  decreases, the amount of hydrocarbons which is used for consumption of oxygen does not change. At this time, the amount of hydrocarbons which is used for producing the reducing intermediate decreases. Therefore, if the amount of hydrocarbons which flows into the exhaust purification catalyst  13  decreases, the amount of production of the reducing intermediate decreases and as a result the NO x  removal rate falls. 
     Now then, if part of the region of the upstream side end face of the exhaust purification catalyst  13 , that is, the particulate buildup region CL, clogs due to buildup of particulates, the amount of hydrocarbons which flow into the exhaust purification catalyst  13  decreases. As a result, as explained above, the amount of production of the reducing intermediate decreases and the NO x  removal rate falls. In this way, when the NO x  removal action by the first NO x  removal method is performed, if the amount of hydrocarbons which flows into the exhaust purification catalyst  13  slightly decreases, the NO x  removal rate greatly falls. Therefore, if just part of the region of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates, the NO x  removal rate will greatly fall. 
     That is, when the exhaust purification catalyst  13  clogs due to buildup of particulates, even if performing the NO x  removal action by the first NO x  removal method, the NO x  is liable to be unable to be reliably removed. Further, the hydrocarbons which are injected from the hydrocarbon feed valve  15  are liable to be unable to be effectively utilized for NO x  removal. Furthermore, sticking of the particulates in the exhaust gas to the particulates which form the clogging is promoted and the clogging which occurs at the exhaust purification catalyst  13  is liable to expand. 
     Therefore, in an embodiment of the present invention, it is judged if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas and, when it is judged that the exhaust purification catalyst  13  is clogged, the NO x  removal action by the second NO x  removal method is performed. 
     In this regard, when the catalyst temperature TC is considerably high, as will be understood from  FIG. 9 , it is difficult to obtain a high NO x  removal rate by the second NO x  removal method. Therefore, in an embodiment according to the present invention, when it is judged that the exhaust purification catalyst  13  is clogged, it is judged if the catalyst temperature TC is lower than a predetermined set temperature and, when it is judged that the catalyst temperature TC is lower than the set temperature, the NO x  removal action by the second NO x  removal method is performed. On the other hand, when it is judged that the catalyst temperature TC is higher than the set temperature, end face regeneration processing is performed to remove the particulates which have built up on the exhaust purification catalyst  13 . As a result, the clogging of the exhaust purification catalyst  13  is removed. Therefore, the NO x  removal action by the first NO x  removal method is allowed and the NO x  can be reliably removed. 
     In end face regeneration processing, temperature control is performed to make the temperature of the upstream side end face of the exhaust purification catalyst  13  rise, then maintain it at 500° C. or more, preferably 600° C. or more. In an embodiment according to the present invention, to make the air-fuel ratio (A/F) in of the exhaust gas temporarily rich, additional fuel is injected from the fuel injector  3  or hydrocarbons are injected from the hydrocarbon feed valve  15 . As a result, the exhaust gas is raised in temperature and the high temperature exhaust gas is used to raise the upstream side end face of the exhaust purification catalyst  13  in temperature. In other words, in this embodiment, temperature raising control is performed under a rich air-fuel ratio. Next, when the temperature raising control ends, the air-fuel ratio (A/F) in of the exhaust gas is returned to lean. As a result, a large amount of oxygen is supplied to the high temperature exhaust purification catalyst  13  and therefore the particulates which form the clogging are removed by oxidation. Note that, in the temperature raising control of this embodiment, the air-fuel ratio (A/F) in of the exhaust gas is made rich, so the NO x  which is stored in the exhaust purification catalyst  13  is released. 
     In another embodiment according to the present invention, temperature raising control is performed by injecting additional fuel from the fuel injector  3  or injecting hydrocarbons from the hydrocarbon feed valve  15  so that the air-fuel ratio (A/F) in of the exhaust gas is maintained lean. In other words, temperature raising control is performed under a lean air-fuel ratio. In this other embodiment as well, the particulates which form the clogging are removed by oxidation. However, if performing temperature raising processing under a rich air-fuel ratio, it is possible to raise the temperature of the exhaust purification catalyst more quickly than with temperature raising control under a lean air-fuel ratio. 
     In an embodiment according to the present invention, when the catalyst temperature TC is higher than the set temperature, end face regeneration processing is performed, so the upstream side end face of the exhaust purification catalyst  13  can be quickly raised in temperature. 
     On the other hand, in an embodiment according to the present invention, the pressure PCu upstream of the exhaust purification catalyst  13  inside the exhaust pipe  12   a  and the differential pressure APF across the particulate filter  14  are used as the basis to judge if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. This will be explained with reference to  FIG. 18 . 
       FIG. 18  shows the changes in the pressure PCu inside the exhaust pipe  12   a  and the differential pressure ΔPF across the particulate filter  14 . As shown in  FIG. 18  by the solid line, the pressure PCu inside of the exhaust pipe  12   a  increases a little at a time when the vehicle travel distance is low and the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas and increases rapidly when the vehicle travel distance is high and the exhaust purification catalyst  13  clogs. As opposed to this, even when the vehicle travel distance becomes high, the differential pressure ΔPF across the particulate filter  14  increases by a substantially constant ratio. Therefore, the difference “d” (=PCu−ΔPF) between the pressure PCu inside of the exhaust pipe  12  and the differential pressure ΔPF across the particulate filter  14  rapidly increases if the exhaust purification catalyst  13  becomes clogged. 
     Therefore, in an embodiment according to the present invention, when the difference “d” is larger than an allowable upper limit value, it is judged that the exhaust purification catalyst  13  is clogged due to buildup of particulates, while when the difference “d” is smaller than the allowable upper limit, it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates. Note that, in  FIG. 18 , the broken line shows the pressure PCu inside of the exhaust pipe  12   a  when the exhaust purification catalyst  13  is not clogged. 
     A particulate filter  14  is usually provided with a differential pressure sensor  26 . Therefore, by just providing a pressure sensor  25 , it becomes possible to judge if the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
       FIG. 19  snows the NO x  removal control routine for performing an NO x  removal control method of an embodiment according to the present invention. This routine is performed by interruption every predetermined time period. 
     Referring to  FIG. 19 , first, to start, at step  100 , it is determined which of the NO x  removal action by the first NO x  removal method and the NO x  removal action by the second NO x  removal method to perform. Next, at step  101 , it is judged if the NO x  removal action by the first NO x  removal method should be performed. When the NO x  removal action by the first NO x  removal method should be performed, the routine proceeds to step  102  where a routine is performed for judging if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. This routine is shown in  FIG. 20 . Next at step  103 , it is judged if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. When it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas, the routine proceeds to step  104  where the NO x  removal action by the first NO x  removal method is performed. That is, the hydrocarbon feed valve  15  injects the amount of injection W of hydrocarbons which is shown in  FIG. 11  in accordance with the operating state of the engine by a predetermined injection timing ΔT. 
     On the other hand, when it is judged at step  103  that the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas, the routine proceeds to step  105  where it is judged if the catalyst temperature TC is lower than the set temperature TC 1 . When TC≧TC 1 , the routine proceeds to step  106  where end face regeneration control is performed. As opposed to this, when TC&lt;TC 1 , the routine proceeds to step  107  where a routine for performing the NO x  removal action by the second NO x  removal method is performed. This routine is shown in  FIG. 21 . 
     When at step  101  the NO x  removal action by the second NO x  removal method should be performed, the routine proceeds to step  107 . 
       FIG. 20  shows a routine for judging clogging which is executed at step  102  of  FIG. 19 . 
     Referring to  FIG. 20 , first, to start, at step  120 , the difference “d” (=PCu−ΔPF) is calculated. Next, at step  121 , it is judged if the difference “d” is larger than an allowable upper limit value Ud. When d≦Ud, the routine proceeds to step  122  where it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas. As opposed to this, when d&gt;Ud, the routine proceeds to step  123  where it is judged that the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. 
       FIG. 21  shows the routine for performing the NO x  removal action by the second NO x  removal method which is performed at step  107  of  FIG. 19 . 
     Referring to  FIG. 21 , first, to start, at step  150 , the NO x  amount NOXA which is exhausted per unit time is calculated from the map which is shown in  FIG. 13 . Next at step  151 , the exhausted NO x  amount NOXA is added to ΣNOX to calculate the stored NO x  amount ΣNOX. Next, at step  152 , it is judged if the stored NO x  amount ΣNOX has exceeded the allowable value MAX. If ΣNOX&gt;MAX, the routine proceeds to step  153  where the amount of additional fuel WP is calculated from the map which is shown in  FIG. 15  and the action for injection of the additional fuel is performed. At this time, the exhaust gas which flows into the exhaust purification catalyst  13  is made rich in air-fuel ratio (A/F) in. Next, at step  154 , ΣNOX is cleared. 
     Next, another embodiment for judging if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas will be explained. 
     If the exhaust purification catalyst  13  becomes clogged due to buildup of particulates in the exhaust gas, the differential pressure across the exhaust purification catalyst  13  becomes larger. Therefore, it is judged if the differential pressure across the exhaust purification catalyst  13  has exceeded the allowable upper limit. When it is judged that the differential pressure across the exhaust purification catalyst  13  has exceeded the allowable upper limit, it is judged that the exhaust purification catalyst  13  is clogged due to buildup of particulates. As opposed to this, when it is judged that the differential pressure across the exhaust purification catalyst  13  is smaller than the allowable upper limit, it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas. 
     To detect the differential pressure across the exhaust purification catalyst  13 , in the embodiment which is shown in  FIG. 22A , a pressure sensor  25  which detects the pressure upstream of the exhaust purification catalyst  13  inside the exhaust pipe  12   a  and a pressure sensor  27  which detects the pressure downstream of the exhaust purification catalyst  13  inside the exhaust pipe  12   b  are respectively provided. In the embodiment which is shown in  FIG. 22B , a differential pressure sensor  28  is provided for detecting the differential pressure across the exhaust purification catalyst  13 . 
     Note that, in the embodiment which is shown in  FIG. 1 , when the difference “d” (=PCu−ΔPF) between the pressure PCu inside of the exhaust pipe  12   a  and the differential pressure ΔPF across the particulate filter  14  is larger than the allowable upper limit Ud, the differential pressure across the exhaust purification catalyst  13  can be considered to be larger than the allowable upper limit, while when the difference “d” is smaller than the allowable upper limit Ud, the differential pressure across the exhaust purification catalyst  13  can be considered to be smaller than the allowable upper limit. 
     Next, still another embodiment for judging if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas will be explained. 
     In still another embodiment of clogging judgment, the air-fuel ratio sensor  2  is arranged downstream of the exhaust purification catalyst  13 . That is, in the example which is shown in  FIGS. 23A and 23B  which correspond to  FIGS. 16A and 16B , an air-fuel ratio sensor  23  is arranged together with a temperature sensor  24  inside of the sensor arrangement space  61 . Further, in the example which is shown in  FIGS. 24A and 24B  corresponding to  FIGS. 17A and 17B  as well, an air-fuel ratio sensor  23  is arranged inside of the sensor arrangement space  61 . Furthermore, the air-fuel ratio sensor  23  is arranged immediately downstream of the corresponding region DL on the downstream side end face of the exhaust purification catalyst  13 . That is, in still another embodiment of clogging judgment, the air-fuel ratio sensor  23  is arranged downstream of the peripheral part of the downstream side end face of the exhaust purification catalyst  13  inside the exhaust gas flow region corresponding to the downstream side of the particulate buildup region CL when viewed along the longitudinal axis of the exhaust purification catalyst  13 . 
     Now then, in still another embodiment of clogging judgment, the blockage rate of the upstream side end face of the exhaust purification catalyst  13  is used as the basis to judge if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. Specifically, until the blockage rate of the upstream side end face of the exhaust purification catalyst  13  becomes a constant rate, it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas, while when the blockage rate of the upstream side end face of the exhaust purification catalyst  13  becomes the constant rate, it is judged that the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. This will be explained with reference to  FIG. 25 . 
       FIG. 25  shows the change in the blockage rate of the upstream side end face of the exhaust purification catalyst  13  with respect to the vehicle travel distance and the change in the outflow flow rate per unit cross-sectional area from the downstream side end face of the exhaust purification catalyst  13  downstream of particulate buildup region CL. As shown in  FIG. 25 , as the vehicle travel distance rises, the blockage rate of the upstream side end face of the exhaust purification catalyst  13  first increases a little at a time. After passing a certain point of time R, it begins to rapidly increase. In still another embodiment of clogging judgment, when the blockage rate of the upstream side end face of the exhaust purification catalyst  13  reaches this point R, it is judged that the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. 
     In this regard, even if part of the region of the upstream side end face of the exhaust purification catalyst  13 , that is, the particulate buildup region CL, clogs slightly due to buildup of particulates, the differential pressure across the exhaust purification catalyst  13  will not change that much. The differential pressure across the exhaust purification catalyst  13  becomes larger and clogging of the upstream side end face of the exhaust purification catalyst  13  can be detected when the amount of particulates which builds up at the upstream side end face of the exhaust purification catalyst  13  becomes considerably great. Note that,  FIG. 25  shows the detection limit at which the differential pressure across the exhaust purification catalyst  13  can be used to detect clogging of the upstream side end face of the exhaust purification catalyst  13 . From  FIG. 25 , the blockage rate of the upstream side end face of the exhaust purification catalyst  13  at the point R becomes considerably lower than the blockage rate which can be detected by the differential pressure across the exhaust purification catalyst  13 , therefore it is not possible to use the differential pressure across the exhaust purification catalyst  13  to judge if the blockage rate of the upstream side end face of the exhaust purification catalyst  13  has reached the point R. 
     On the other hand, in  FIG. 25 , GX shows the outflow flow rate per unit cross-sectional area from the downstream side end face of the exhaust purification catalyst  13  when the upstream side end face of the exhaust purification catalyst  13  has no particulates built up there at all, while GA and GB, as shown from  FIG. 23A  to  FIG. 24B , show the outflow flow rates per unit cross-sectional area from the downstream side end face of the exhaust purification catalyst  13  when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. Note that, GA shows the outflow flow rate from the corresponding region on the downstream side end face of the exhaust purification catalyst  13  which is positioned at the opposite side to the region of the upstream side end face where particulates have not built up on the longitudinal axis of the exhaust purification catalyst  13 , that is, the outflow flow rate at the point A in  FIG. 23A  to  FIG. 24B , while GB shows the outflow flow rate from the corresponding region DL on the downstream side end face of the exhaust purification catalyst  13  which is positioned at the opposite side to the particulate buildup region CL on the longitudinal axis of the exhaust purification catalyst  13 , that is, the outflow flow rate at the point B in  FIG. 23A  to  FIG. 24B . 
     As will be understood from  FIG. 25 , even if the vehicle travel distance rises and the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, the outflow flow rate GA at the point A of  FIG. 23A  to  FIG. 25B  only increases slightly from the outflow flow rate GX, while the outflow flow rate GB at the point B of  FIG. 23A  to  FIG. 25B  greatly decreases from the outflow flow rate GX. In this case, if making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio, a difference arises in the change of the air-fuel ratio of the exhaust gas which flows out from the exhaust purification catalyst  13  in accordance with the deviation in the outflow flow rate from: the outflow flow rate GX. That is, in the case where, like in point A of  FIG. 23A  to  FIG. 23B , the outflow flow rate GA is not deviated much at all from the outflow flow rate GX, when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio, the exhaust gas also instantaneously changes in air-fuel ratio at the point A of  FIG. 23A  to  FIG. 24B . As opposed to this, in the case where, like in point B of  FIG. 23A  to  FIG. 24 b   , the outflow flow rate GB greatly decreases from the outflow flow rate GX, even if the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change in air-fuel ratio, the exhaust gas will not instantaneously change in air-fuel ratio at the point B of  FIG. 23A  to  FIG. 24B . 
     That is, the exhaust gas instantaneously changes in air-fuel ratio at the point A from  FIG. 23A  to  FIG. 24B  when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio both when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates and when it is not. Therefore, it is not possible to judge if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates using the method of changing of the air-fuel ratio of the exhaust gas at the point A of  FIG. 23A  to  FIG. 24B  when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio. 
     On the other hand, the exhaust gas instantaneously changes in air-fuel ratio at the point B from  FIG. 23A  to  FIG. 24  when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is not clogged due to buildup of particulates. As opposed to this, the exhaust gas does not instantaneously change in air-fuel ratio at the point B from  FIG. 23A  to  FIG. 24B  when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     Therefore, it becomes possible to judge if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates using the method of changing of the air-fuel ratio of the exhaust gas at the point B of  FIG. 23A  to  FIG. 24B  when making the exhaust gas which flows into the exhaust purification catalyst  1  instantaneously change in air-fuel ratio. 
     Therefore, in still another embodiment of clogging judgment, to enable detection of change of the air-fuel ratio of the exhaust gas at the point B of  FIG. 23A  to  FIG. 24B , an air-fuel ratio sensor  23  is arranged downstream of the corresponding region DL on the downstream side end face of the exhaust purification catalyst  13  which is positioned at the opposite side to the particulate buildup region CL on the longitudinal axis of the exhaust purification catalyst  13  and the change in output value of this air-fuel ratio sensor  23  is used to judge if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. Next, the way of change of the output value of the air-fuel ratio sensor  23  when making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio will be explained, but before that, the characteristics of two types of air-fuel ratio sensors  23  which are used in still other embodiments of clogging judgment will be explained simply with reference to  FIGS. 26A and 26B . 
       FIG. 26A  shows the relationship between the output current I of a limit current type of air-fuel ratio sensor and an air-fuel ratio of the exhaust gas. As shown in  FIG. 26A , the output current I of this limit current type of air-fuel ratio sensor increases as the exhaust gas becomes larger in air-fuel ratio. Note that, in actuality, the change in this output current I is read into the electronic control unit  30  from the air-fuel ratio sensor  23  in the form of a change of voltage. On the other hand,  FIG. 26B  shows the relationship between the output voltage V of the air-fuel ratio sensor called the “oxygen concentration sensor” and the air-fuel ratio of the exhaust gas. As shown in  FIG. 26B , the output voltage V of this air-fuel ratio sensor becomes a lower voltage V 1  of an extent of 0.1(V) if the air-fuel ratio of the exhaust gas becomes larger than the stoichiometric air-fuel ratio and becomes a higher voltage V 1  of an extent of 0.9(V) if the air-fuel ratio of the exhaust gas becomes smaller than the stoichiometric air-fuel ratio. 
       FIG. 27  shows the change of the output voltage of the air-fuel ratio sensor  23  when using an air-fuel ratio sensor  23  constituted by a limit current type of air-fuel ratio sensor which has the output characteristic which is shown in  FIG. 26A  and making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio (A/F). Note that, in  FIG. 27 , VO shows the change in the output voltage of the air-fuel ratio sensor  23  when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is not clogged due to buildup of particulates, while VX shows the change in the output voltage of the air-fuel ratio sensor  23  when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     From  FIG. 27 , it is learned that when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is not clogged due to buildup of particulates, if the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change in air-fuel ratio, the output voltage Vo of the air-fuel ratio sensor  23  will also instantaneously change, while when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, if the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change in air-fuel ratio, the output voltage VX of the air-fuel ratio sensor  23  will change by a slower speed delayed from the instantaneous change of the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13 . The reason why the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  become slower in this way when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, is that the flow rate of the exhaust gas which flows out toward the air-fuel ratio sensor  23  from the exhaust purification catalyst  13  decreases as shown by GB in  FIG. 25 . 
     That is, if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  starts to clog due to buildup of particulate and the flow rate of the exhaust gas which flows through the inside of the exhaust purification catalyst  13  downstream of the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  decreases, time will be required for the exhaust gas which was changed in air-fuel ratio to flow out from the downstream side end face of the exhaust purification catalyst  13 . As a result, as shown in  FIG. 27 , the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  become slower. Further, if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is completely clogged due to buildup of particulates, the exhaust gas which changes in air-fuel ratio will circle around the air-fuel ratio sensor  23  for a while after flowing out from the downstream side end face of the exhaust purification catalyst  13 . Therefore, in this case as well, time will be required for the exhaust gas which was changed in air-fuel ratio to reach the air-fuel ratio sensor  23  and, as a result, as shown in  FIG. 27 , the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  will become slower. Whatever the case, if the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  will become slower. 
     Therefore, in still another embodiment of clogging judgment, when the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  fall, it is judged that the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. That is, in still another embodiment of clogging judgment, an exhaust purification catalyst  13  is arranged inside the engine exhaust passage and the hydrocarbon feed valve  15  is arranged upstream of the exhaust purification catalyst  13  inside the engine exhaust passage, the exhaust purification catalyst  13  is comprised of a straight flow type of catalyst which has a plurality of exhaust flow passages which extend in the longitudinal axial direction of the exhaust purification catalyst  13 , a limited part of the region at the peripheral area of the upstream side end face of the exhaust purification catalyst  13  where there is a possibility of clogging occurring due to buildup of particulates in the exhaust gas is predicted in advance, an air-fuel ratio sensor  23  is arranged downstream of the peripheral area of the downstream side end face of the exhaust purification catalyst  13  inside of the exhaust gas flow region corresponding to the downstream side of the particulate buildup region CL when viewed along the longitudinal axis of the exhaust purification catalyst  13 , when the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change in air-fuel ratio, if the particulate buildup region CL at the peripheral area of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates in the exhaust gas, compared to the case where the catalyst is not clogged due to buildup of particulates, the speeds of change of the output value of the air-fuel ratio sensor  23  will fall, when it is judged if the particulate buildup region CL at the peripheral area of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates in the exhaust gas, the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change in air-fuel ratio, and when the speed of change of the output value of the air-fuel ratio sensor  23  at this time falls, it is judged that the particulate buildup region CL at the peripheral area of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates in the exhaust gas. 
     In this case, the speed of change of the output voltage VX of the air-fuel ratio sensor  23  can be found by calculating the speed of change dV 1  of the output voltage VX of the air-fuel ratio sensor  23  when the output voltage VX of the air-fuel ratio sensor  23  changes from VX 1  to VX 2  in  FIG. 27 , while the speed of change of the output voltage VX of the air-fuel ratio sensor  23  can be found by calculating the speed of change dV 2  of the output voltage VX of the air-fuel ratio sensor  23  when the output voltage VX of the air-fuel ratio sensor  23  changes from VX 2  to VX 1  in  FIG. 27 . Further, the speed of change of the output voltage VX of the air-fuel ratio sensor  23  can be found by calculating the time t 1  until the output voltage VX of the air-fuel ratio sensor  23  changes from VX 1  to VX 2  in  FIG. 27 , while the speed of change of the output voltage VX of the air-fuel ratio sensor  23  can be found by calculating the time t 2  until the output voltage VX of the air-fuel ratio sensor  23  changes from VX 2  to VX 1  in  FIG. 27 . 
     That is, when the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  is made to instantaneously change, whether the speed of change of the output value of the air-fuel ratio sensor  23  falls can be judged based on any of the time t 1  which is required for the fall in the output voltage of the air-fuel ratio sensor  23  at this time, the speed of fall dV 1  of the output voltage of the air-fuel ratio sensor  23 , the time t 2  which is required for the rise in the output voltage of the air-fuel ratio sensor  23 , and the speed of rise dV 2  of the output voltage of the air-fuel ratio sensor  23 . The speed of change of the output voltage VX of the air-fuel ratio sensor  23  can be found by various methods, but below, the case of calculating the time t 1  for the output voltage VX of the air-fuel ratio sensor  23  to change from VX 1  to VX 2  in  FIG. 27  and thereby find the speed of change of the output voltage VX of the air-fuel ratio sensor  23  will be used as an example to explain still another embodiment of clogging judgment. 
       FIG. 28  shows a routine for executing still another embodiment of clogging judgment. This routine is executed at step  102  of  FIG. 19 . 
     Referring to  FIG. 28 , first, to start, at step  220 , air-fuel ratio change control is performed to make the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio. At this time, in still another embodiment of the clogging judgment, as shown in  FIG. 27 , additional fuel is fed to the inside of the combustion chamber  2  or the hydrocarbon feed valve  15  injects hydrocarbons to thereby make the exhaust gas which flows into the exhaust purification catalyst  13  temporarily change in air-fuel ratio (A/F) to the rich side. Next, at step  221 , the output voltage VX of the air-fuel ratio sensor  23  is read in. Next, at step  222 , the time t 1  which is required for the output voltage VX of the air-fuel ratio sensor  23  to change from VX 1  to VX 2  in  FIG. 27  is calculated. Next, at step  223 , it is judged if the time t 1  has exceeded the predetermined reference time Mt. 
     When it is judged at step  223  that the time t 1  has not exceeded the predetermined reference time Mt, the routine proceeds to step  224  where it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas. As opposed to this, when it is judged that the time t 1  has exceeded the predetermined reference time Mt, the routine proceeds to step  225  where it is judged that the catalyst is clogged due to buildup of particulates. 
       FIG. 29  and  FIG. 30  show another embodiment in the case of using an air-fuel ratio sensor  23  constituted by an air-fuel ratio sensor which has the output characteristic which is shown in  FIG. 26B .  FIG. 29  shows the change in the output voltage of the air-fuel ratio sensor  23  when the air-fuel ratio (A/F) of the exhaust gas which flows into the exhaust purification catalyst  13  in this case is made to temporarily change from lean to rich. Note that, in  FIG. 29 , VO shows the change in the output voltage of the air-fuel ratio sensor  23  when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is not clogged due to buildup of the particulates, while VX shows the change in the output voltage of the air-fuel ratio sensor  23  when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     From  FIG. 29 , when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is not clogged due to buildup of particulates, if making the exhaust gas which flows into the exhaust purification catalyst  13  instantaneously change in air-fuel ratio from lean to rich, the output voltage Vo of the air-fuel ratio sensor  23  will instantaneously rise from V 1  to VS, then the output voltage Vo of the air-fuel ratio sensor  23  will be maintained at VS. This VS, as shown in  FIG. 26B , shows the output voltage V of the air-fuel ratio sensor  23  when the air-fuel ratio of the exhaust gas is the stoichiometric air-fuel ratio. That is, when the exhaust purification catalyst  13  has an oxygen storage ability, if the exhaust gas which flows into the exhaust purification catalyst  13  is made to change in air-fuel ratio from lean to rich, the air-fuel ratio of the exhaust gas which flows out from the exhaust purification catalyst  13  will be maintained at the stoichiometric air-fuel ratio until the oxygen which is stored in the exhaust purification catalyst  13  is consumed. Therefore, as shown in  FIG. 29 , if the exhaust gas which flows into the exhaust purification catalyst  13  is made to change in air-fuel ratio from lean to rich, the output voltage Vo of the air-fuel ratio sensor  23  will be maintained at VS until the oxygen which is stored in the exhaust purification catalyst  13  is consumed, that is, during the time tS. Next, the output voltage Vo of the air-fuel ratio sensor  23  rises to V 2 . 
     On the other hand, it is learned that when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  clogs due to buildup of particulates, if the exhaust gas which flows into the exhaust purification catalyst  13  is made to change in air-fuel ratio from lean to rich, the output voltage VX of the air-fuel ratio sensor  23  will rise slowly by a speed dV 1  delayed from the instantaneous change of the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13 . It is learned that if the exhaust gas which flows into the exhaust purification catalyst  13  is made to change in air-fuel ratio from rich to lean, the output voltage VX of the air-fuel ratio sensor  23  will fall by a slow speed dV 2  delayed from the instantaneous change of the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13 . 
     Further, when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, when the exhaust gas which flows into the exhaust purification catalyst  13  is temporarily switched in air-fuel ratio from lean to rich, the time t 1  which is required for the output voltage VX of the air-fuel ratio sensor  23  to rise from V 1  to VS and the time t 2  which is required for the output voltage VX of the air-fuel ratio sensor  23  to fall from V 2  to V 1  increase. The fact that the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  become slow and the times t 1  and t 2  increase when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates as explained above, is caused by the flow rate of the exhaust gas which flows out toward the air-fuel ratio sensor  23  from the exhaust purification catalyst  13  decreases as shown by GB of  FIG. 25 . 
     Therefore, in a first example of the present invention, when the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  fall or the times t 1  and t 2  increase, it is judged that the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     Further, when the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates, as shown in  FIG. 29 , when the exhaust gas which flows into the exhaust purification catalyst  13  is made to change in air-fuel ratio from lean to rich, the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS increases. That is, at this time, the exhaust gas which flows out toward the air-fuel ratio sensor  23  from the exhaust purification catalyst  13  is decreased in flow rate, so more time is required for consuming the stored oxygen. As a result, the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS increases. Therefore, in this case, when the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS increases, it can be judged that the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     Note that, if the exhaust purification catalyst  13  deteriorates, the oxygen storage ability falls and, as a result, the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS decreases. That is, when the exhaust purification catalyst  13  deteriorates, the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS does not increase. The time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS increases when the particulate buildup region CL of the upstream side end face clogs due to buildup of particulates. Therefore, it becomes possible to use the change in the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS to reliably detect the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  clogging due to buildup of particulates. 
     Therefore, in a second example according to the present invention, when the speeds of change dV 1  and dV 2  of the output voltage VX of the air-fuel ratio sensor  23  fall or the times t 1  and t 2  increase and the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS increases, it is judged that the particulate buildup region CL of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates. 
     Next, taking as an example the case of using an air-fuel ratio sensor  23  constituted by the air-fuel ratio sensor which has the output characteristic which is shown in  FIG. 26B  so as to calculate the time t 1  until the output voltage VX of the air-fuel ratio sensor  23  changes from V 1  to VS in  FIG. 25  to thereby find the speed of change of the output voltage VX of the air-fuel ratio sensor  23 , another embodiment of clogging judgment will be explained. Note that, in this clogging judgment, when the exhaust gas which flows into the exhaust purification catalyst  13  is made rich in air-fuel ratio so as to make the exhaust purification catalyst  13  release NO x , it is judged if the particulate buildup region CL of the peripheral area of the upstream side end face of the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. 
       FIG. 30  shows a first example of a routine which executes still another embodiment of clogging judgment. This routine is executed at step  102  of  FIG. 19 . 
     If referring to  FIG. 30 , first, to start, at step  320 , rich control is performed for making the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  rich. That is, the additional fuel amount WR is calculated from the map which is shown in  FIG. 15  and an additional fuel injection action is performed. At this time, the exhaust gas which is exhausted from the combustion chamber  2  is made rich in air-fuel ratio and the exhaust gas which flows into the exhaust purification catalyst  13  is made rich in air-fuel ratio (A/F) in. Next, at step  321 , the output voltage V of the air-fuel ratio sensor  23  is read. Next, at step  322 , the time t 1  which is required for the output voltage VX of the air-fuel ratio sensor  23  to change from V 1  to VS in  FIG. 29  is calculated. Next, at step  323 , it is judged if the time t 1  has exceeded the predetermined reference time Mt. 
     When it is judged at step  323  that the time t 1  has not exceeded the predetermined reference time Mt, the routine proceeds to step  324  where it is judged if the exhaust purification catalyst  13  has become clogged due to buildup of particulates. Next, the routine jumps to step  326 . As opposed to this, when it was judged at step  323  that the time t 1  has exceeded the predetermined reference time Mt, the routine proceeds to step  325  where it is judged if the catalyst is clogged due to buildup of particulates. Next, the routine proceeds to step  326 . At step  326 , ΣNOX is cleared. 
       FIG. 31  shows a second example of a routine which executes still another embodiment of clogging judgment. This routine is executed at step  102  of  FIG. 19 . 
     Referring to  FIG. 31 , first, to start, at step  420 , rich control is performed for making the air-fuel ratio of the exhaust gas which flows into the exhaust purification catalyst  13  rich. That is, the additional fuel amount WR is calculated from the map which is shown in  FIG. 15  and an additional fuel injection action is performed. At this time, the exhaust gas which flows into the exhaust purification catalyst  13  is made rich in air-fuel ratio (A/F) in. Next, at step  421 , the output voltage V of the air-fuel ratio sensor  23  is read. Next at step  422 , the time t 1  which is required for the output voltage VX of the air-fuel ratio sensor  23  to change from V 1  to VS in  FIG. 29  is calculated. Next, at step  423 , the time tS during which the output voltage VX of the air-fuel ratio sensor  23  is maintained at VS is calculated. 
     Next, at step  424 , it is judged if the time t 1  has exceeded the predetermined reference time Mt. When it is judged at step  424  that the time t 1  has not exceeded the predetermined reference time Mt, the routine proceeds to step  426  where it is judged that the catalyst is not clogged due to buildup of particulates. Next, the routine jumps to step  428 . As opposed to this, when, at step  424 , it is judged that the time t 1  has exceeded the predetermined reference time Mt, the routine proceeds to step  425  where it is judged if the time tS has exceeded the predetermined reference time MS. When it is judged at step  425  that the time tS has not exceeded the predetermined reference time MS, the routine proceeds to step  426  where it is judged that the catalyst is not clogged due to buildup of particulates. Next, the routine jumps to step  428 . 
     As opposed to this, when it is judged at step  425  that the time tS has exceeded a predetermined reference time MS, the routine proceeds to step  427  where it is judged if the catalyst is clogged due to buildup of particulates. Next, the routine proceeds to step  428 . At step  428 , ΣNOX is cleared. 
     Next, referring to  FIG. 32 , another embodiment of NO x  removal control will be explained. 
     In the embodiment which is shown in  FIG. 19 , first, it is judged whether to perform the first NO x  removal method. When the NO x  removal action by the first NO x  removal method should be performed, it is judged if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. 
     As opposed to this, in the embodiment which is shown in  FIG. 32 , first, it is judged the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. When it is judged that it is not clogged, it is judged whether to perform the first NO x  removal method. 
       FIG. 32  shows an NO x  removal control routine for performing an NO x  removal control method of another embodiment according to the present invention. This routine is performed by interruption every certain time period. 
     Referring to  FIG. 32 , first, to start, at step  200 , a routine is performed to judge whether the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. This routine is for example shown in  FIG. 20 . Next, at step  201 , it is judged if the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas. When it is judged that the exhaust purification catalyst  13  is not clogged due to buildup of particulates in the exhaust gas, the routine proceeds to step  202  where it is decided which of the NO x  removal action by the first NO x  removal method and the NO x  removal action by the second NO x  removal method to perform. Next, at step  203 , it is judged whether the NO x  removal action by the first NO x  removal method should be performed. When the NO x  removal action by the first NO x  removal method should be performed, the routine proceeds to step  204  where the NO x  removal action by the first NO x  removal method is performed. That is, the hydrocarbon feed valve  15  injects the injection amount W of hydrocarbons which is shown in  FIG. 11  in accordance with the operating state of: the engine by a predetermined injection period ΔT. When at step  203  the NO x  removal action by the second NO x  removal method should be performed, the routine proceeds to step  205  where a routine is performed to perform the NO x  removal action by the second NO x  removal method. This routine is shown in  FIG. 21 . 
     When it is judged at step  201  that the exhaust purification catalyst  13  is clogged due to buildup of particulates in the exhaust gas, the routine proceeds to step  206  where it is judged if the catalyst temperature TC is lower than the set temperature TC 1 . When TC&lt;TC 1 , the routine proceeds to step  205  where a routine for execution of the NO x  removal action by the second NO x  removal method is performed. As opposed to this, when TC&gt;TC 1 , the routine proceeds to step  207  where end face regeneration control is performed. 
     Note that, as another embodiment, an oxidation catalyst for reforming the hydrocarbons may also be arranged upstream of the exhaust purification catalyst  13  inside the engine exhaust passage. 
     REFERENCE SIGNS LIST 
     
         
           4  intake manifold 
           5  exhaust manifold 
           12   a ,  12   b  exhaust pipe 
           13  exhaust purification catalyst 
           14  particulate filter 
           15  hydrocarbon feed valve 
           25  pressure sensor 
           26  differential pressure sensor