Patent Publication Number: US-2006010854-A1

Title: Exhaust gas clarifying device for internal combustion engine

Description:
TECHNICAL FIELD  
      The present invention relates to an exhaust purification device of an internal combustion engine.  
     BACKGROUND ART  
      As a catalyst for purifying NOR contained in exhaust gas when fuel is burned under a lean air-fuel ratio, there is known a catalyst comprised of a carrier made of alumina on the surface of which a layer of a NO x  absorbent comprised of an alkali metal, or alkali earth is formed and on that surface of which a precious metal catalyst such as platinum is carried (for example, see Japanese Patent No. 2600492). In this catalyst, when the air-fuel ratio of the exhaust gas is lean, the NO x  contained in the exhaust gas is oxidized by the platinum and absorbed in the NO x  absorbent in the form of a nitrate. Next, when the air-fuel ratio of the exhaust gas is made rich in a short time, the NOR which had been absorbed in the NO x  absorbent during that time is released and reduced. Next, when the air-fuel ratio of the exhaust gas returns to lean, the action of absorption of the NO x  in the NOR absorbent is started.  
      On the other hand, exhaust gas also contains SO x . The NO x  absorbent absorbs the SO x  in addition to the NO x . In this case, the SO x  is absorbed in the form of a sulfate. However, this sulfate is harder to break down compared with a nitrate and will not break down if simply making the air-fuel ratio of the exhaust gas rich. Therefore, the NO x  absorbent gradually increases in the amount of absorption of the SO x . Along with this, it can no longer absorb NO x . Therefore, when using such a NO x  absorbent, it is necessary to make it release the SO x . In this regard, a sulfate becomes easy to breakdown when the temperature of the catalyst becomes 600° C. or more. If making the air-fuel ratio of the exhaust gas rich at this time, the SO x  is released from the NO x  absorbent. Accordingly, if using such a NO x  absorbent, when making the NO x  absorbent release the SO x , the temperature of the catalyst is maintained at 600° C. or more and the air-fuel ratio of the exhaust gas is maintained rich.  
      Further, if providing a layer of such a NO x  absorbent, SO x  is inevitably also absorbed in addition to the NO x , so to prevent the SO x  from being absorbed, it would be sufficient not to provide such a layer of a NO x  absorbent. Therefore, a catalyst comprised of a carrier made of alumina on which only platinum is carried has been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 11-285624). This publication describes that NO x  can be purified even when a carrier made of alumina carries only platinum if trapping NO x  in the catalyst when the air-fuel ratio is lean and switching the air-fuel ratio alternately between lean and rich.  
      Further, as a catalyst able to purify the NO x  generated when fuel is burned under a lean air-fuel ratio, a lean NO x  catalyst comprised of zeolite carrying a transition metal or precious metal is known. This lean NO x  catalyst has the function of absorbing the HC and NO x  in the exhaust gas and reducing the NO x , but if oxygen is adsorbed, the NO x  purification performance remarkably drops. Therefore, an internal combustion engine designed to cause the adsorbed oxygen to disassociate by periodically making the air-fuel ratio of the exhaust gas flowing into a lean NO x  catalyst rich is known (see Japanese Patent No. 3154110). This lean NO x  catalyst has the feature of being able to reduce NO x  even when fuel is burned under a lean air-fuel ratio, but has the defects that the exhaust gas has to be supplied with HC for reducing the NO x , the heat resistance is low, and a purification rate of only 50 percent or less can be obtained.  
      The inventors researched catalysts comprised of a carrier formed with a layer of a NO x  absorbent, but also researched catalysts comprised of a carrier not having a layer of a NO x  absorbent. As a result, they learned that with a catalyst comprised of carrier not having a layer of a NO x  absorbent, for example, a catalyst comprised of a carrier made of alumina on which only platinum is carried, if temporarily making the air-fuel ratio rich when burning fuel under a lean air-fuel ratio, a NO x  purification rate of 90 percent or more can be obtained when the catalyst temperature is a low temperature of 256° C. or less.  
      The inventors engaged in repeated studies on the reasons for this from various angles and as a result reached the following conclusion. That is, generally speaking, platinum inherently has activity at a low temperature. The NO x  contained in exhaust gas is directly broken down or selectively reduced on the surface of the platinum. Further, a carrier made of alumina has base points on its surface. NO x  oxidized on the surface of the platinum is adsorbed on the surface of the carrier in the form of NO 2  or is held on the base points on the surface of the carrier in the form of nitrate ions NO 3   − . When purifying the NO x , these various actions are performed simultaneously. As a result, a high purification rate of 90 percent or more is obtained.  
      However, if exposing a catalyst comprised of a carrier made of alumina on which only platinum is carried to exhaust gas of a lean air-fuel ratio, the NO x  purification rate gradually falls. This is because the surface of the platinum is covered by oxygen atoms, that is, the surface of the platinum suffers from oxygen poisoning, whereby the direct breakdown of NO x  or selective reduction of NO x  on the platinum surface becomes difficult. In practice, if making the air-fuel ratio temporarily rich at this time, the oxygen atoms covering the platinum surface will be consumed for oxidation of the HC or CO, that is, the oxygen poisoning of the platinum surface will be eliminated. When the air-fuel ratio returns to lean next, the direct breakdown of NO x  or selective reduction of NO x  will again be performed well.  
      On the other hands if the surface of the platinum is covered by oxygen atoms, the NO x  will become more easily oxidized on the surface of the platinum and therefore the amount of the NO x  adsorbed or held on the carrier will increase. Regardless of this, the fall in the NO x  purification rate means that the direct breakdown of NO x  or selective reduction of NO x  governs the purification action of NO x . Therefore, when carrying only platinum on a carrier made of alumina, preventing the entire surface of the platinum from becoming poisoned by oxygen is the most important issue. Therefore, it becomes necessary to temporarily switch the air-fuel ratio of the exhaust gas from lean to rich before the entire surface of the platinum suffers from oxygen poisoning.  
      Note that if temporarily switching the air-fuel ratio of the exhaust gas from lean to rich, the NO x  adsorbed on the carrier or the nitrate ions NO 3   −  held on the carrier is reduced by the HC and CO. That is, if temporarily switching the air-fuel ratio of the exhaust gas from lean to rich to eliminate the oxygen poisoning of the surface of the platinum, the NO x  adsorbed or hold on the carrier is removed. Therefore, when the air-fuel ratio is returned from rich to lean, the action of adsorption of NO x  or the action of holding the nitrate ions NO 3   −  is started.  
      As explained above, when carrying only platinum on a carrier made of alumina, to secure a high purification rate of NO x , it is necessary to prevent the entire surface of the platinum from becoming poisoned by oxygen. However, neither Japanese Unexamined Patent Publication (Kokai) No. 11-285624 nor Japanese Patent No. 31541.10 suggests anything regarding this. That is, Japanese Unexamined Patent Publication (Kokai) No. 11-285624 shows the results of studies all predicated on NO x  being purified based on&#39;the action of adsorption of NO x . It does not notice that oxygen poisoning of platinum governs the purification rate of NO x . Accordingly, only naturally, Japanese Unexamined Patent Publication (Kokai) No. 11-285624 does not suggest anything regarding obtaining a high purification rate even with a low temperature of 250° C. or less.  
      Further, Japanese Patent No. 3154110 covers a lean NO x  catalyst comprised of zeolite and discloses that the adsorption of oxygen at this lean NOR catalyst has an effect on the NO x  purification rate, but does not suggest anything regarding the fact that oxygen poisoning of the surface of platinum governs the NO purification rate. This zeolite has no base points, so not only does the method of purification of NO x  differ from when using alumina, but also obtaining a NO x  purification rate of 50 percent or more is difficult. Therefore, Japanese Patent No. 3154110 cannot serve as a document suggesting obtaining a high purification rate of 90 percent or more at 250° C. or less.  
     DISCLOSURE OF THE INVENTION  
      The present invention finds that oxygen poisoning of the surface of platinum, that is, the surface of a precious metal, governs the purification rate of NO x  and provides an exhaust purification device of an internal combustion engine designed to secure a high NO x  purification rate based on this.  
      According to the present invention, there is provided an exhaust purification device of an internal combustion engine designed to purify NO x  generated when burning fuel under a lean air-fuel ratio by an exhaust purification catalyst arranged in an exhaust passage, which device uses as a catalyst carrier of the exhaust purification catalyst a carrier having base points on the carrier surface, carries a precious metal catalyst dispersed on the carrier surface without forming a layer of a NO x  absorbent able to absorb NO x , and temporarily switches air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst from lean to rich before the entire surface of the precious metal catalyst suffers from oxygen poisoning. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an overview of a compression ignition type internal combustion engine;  
       FIG. 2  is a view schematically showing a cross-section of the carrier surface part of an exhaust purification catalyst;  
       FIG. 3  is a view showing the change in the air-fuel ratio of exhaust gas due to feed of a reducing agent;  
       FIG. 4  is a view of the NO x  purification rate,  
       FIG. 5A  to  FIG. 5S  are views of the amount of oxygen poisoning per unit time;  
       FIG. 6  is a view of a time chart of control for eliminating oxygen poisoning and control for releasing SO x ;  
       FIG. 7  is a view of various fuel injection patterns;  
       FIG. 8  is a flow chart of control of different flags;  
       FIG. 9  and  FIG. 10  are flow charts of control for feeding a reducing agent;  
       FIG. 11A  and  FIG. 11B  are views for explaining control of the air-fuel ratio of exhaust gas;  
       FIG. 12  is a flow chart of control for feeding a reducing agent;  
       FIG. 13  is a view of changes in the air-fuel ratio of exhaust gas;  
       FIG. 14  is a flow chart of control for feeding a reducing agent;  
       FIG. 15A  and  FIG. 15B  are views of a particulate filter;  
       FIG. 16  is an overview of another embodiment of a compression ignition internal combustion engine;  
       FIG. 17  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 18  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 19  is a view of the NO x  purification rate;  
       FIG. 20  is a flow chart of control for feeding a urea aqueous solution;  
       FIG. 21  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 22  is a view schematically illustrating a cross-section of the part of the carrier surface of a NO x  storing catalyst;  
       FIG. 23  is a view of changes, in the air-fuel ratio of exhaust gas due to feeding a reducing agent;  
       FIG. 24  is a view of the exhaust gas purification rate;  
       FIG. 25  is a view of a time chart of control for eliminating oxygen poisoning and control for releasing SO x ;  
       FIG. 26A  and  FIG. 26B  are views for explaining the amount of NO x  absorption per unit time;  
       FIG. 27  is a view of a times chart of control for releasing NO x  and SO x ;  
       FIG. 28  is a flow chart of control for feeding a reducing agent;  
       FIG. 29  is a flow chart of processing for eliminating poisoning;  
       FIG. 30  is a flow chart of processing I for releasing SO x ;  
       FIG. 31  is a flow chart of processing for releasing NO x ;  
       FIG. 32  is a flow chart of processing II for releasing SO x ;  
       FIG. 33  is a flow chart of control for feeding a reducing agent;  
       FIG. 34  is a flow chart for processing for eliminating poisoning;  
       FIG. 35  is a flow chart of processing for releasing NO x ;  
       FIG. 36  is a view showing the exhaust gas temperature and catalyst basicity degree when releasing NO x ;  
       FIG. 37  is a view of the relationship between the SO x  release temperature and catalyst basicity degree;  
       FIG. 38  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 39  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 40  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 41  is an overview of still another embodiment of a compression ignition internal combustion engine;  
       FIG. 42  is a view of the amount of generation of smoke;  
       FIG. 43A  and  FIG. 43B  are views of the gas temperature etc. in a combustion chamber;  
       FIG. 44  is a view of operating regions I and II;  
       FIG. 45  is a view of the air-fuel ratio A/F; and  FIG. 46  is a view of the changes in the throttle valve opening degree etc. 
    
    
     BEST MODE FOR WORKING THE INVENTION  
       FIG. 1  shows the case of application of the present invention to a compression ignition type internal combustion engine. Note that the present invention may also be applied to a spark ignition type internal combustion engine.  
      Referring to  FIG. 1, 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 . The inlet of the compressor  7   a  is connected to an air cleaner  8 . Inside the intake duct  6  is arranged a throttle valve  9  driven by a step motor. Further, around the intake duct  6  is arranged a cooling device  10  for cooling the intake air flowing through the inside of the intake duct  6 . In the embodiment shown in  FIG. 1 , the engine cooling water is guided into the cooling device  10 . The engine cooling water cools 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 , while the outlet of the exhaust turbine  7   b  is connected to a casing  12  housing a exhaust purification catalyst  11 . The outlet of the collecting portion of the exhaust manifold  5  is provided with a reducing agent feed valve  13  for feeding a reducing agent comprised of for example a hydrocarbon into the exhaust gas flowing through the inside of the exhaust manifold  5 .  
      The exhaust manifold  5  and the intake manifold  4  are connected through an exhaust gas recirculation (hereinafter referred to as an “EGR”) passage  14 . The EGR passage.  14  is provided with an electronically controlled EGR control valve  15 . Further, around the EGR passage  14  is arranged a cooling device  16  for cooling the EGR gas flowing through the inside of the EGR passage  14 . In the embodiment shown in  FIG. 1 , the engine cooling water is guided into the cooling device  16 . The engine cooling water cools the EGR gas. On the other hand, each fuel injector  3  is connected through a fuel feed tube  17  to a fuel reservoir, that is, a so-called “common rails”  18 . This common rail  18  is supplied with fuel from an electronically controlled variable discharge fuel pump  19 . The fuel supplied into the common rail  18  is supplied through each fuel feed tube  17  to the fuel injector  3 .  
      An electronic control unit  30  is comprised of a digital computer provided with 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  all connected to each other by a bidirectional bus  31 . The exhaust purification catalyst  11  is provided with a temperature sensor  20  for detecting the temperature of the exhaust purification catalyst  11 . The output signal of the temperature sensor  20  is input to the input port  35  through a corresponding AD converter  37 . Further, the exhaust pipe  21  connected to the outlet of the casing  0 . 12  if necessary has various types of sensors  22  arranged in it. An accelerator pedal  40  has a load sensor  41  generating an output voltage proportional to the amount of depression L connected to it. The output voltage of the load sensor  41  is input to the input port  35  through a corresponding AD converter  37 . Further, the input port  35  has a crank angle sensor  42  generating an output pulse each time the crankshaft turns for example by 15 degrees connected to it. On the other hand, the output port  36  is connected through corresponding drive circuits  38  to the fuel injectors  3 , throttle valve driving step motor  96  EGR control valve  15 , and fuel pump  19 .  
      The exhaust purification catalyst  11  shown in  FIG. 1  is comprised of a monolithic catalyst. A base of the exhaust purification catalyst  11  carries a catalyst carrier.  FIG. 2  schematically shows the cross-section of the surface part of this catalyst carrier  50 . As shown in  FIG. 2 , the catalyst carrier  50  carries a precious metal catalyst  51  dispersed on its surface. In the present invention, as the catalyst carrier  50 , a carrier  50  on the surface of which there are base points exhibiting basicity is used. Further, in this embodiment of the present invention, platinum is used as the precious metal catalyst  51 .  
      In this way, in this embodiment, the surface of the catalyst carrier  50  made of alumina carries only platinum  51  and is not formed with a layer of an NO x  absorbent comprised of an alkali metal or alkali earth able to absorb NO x . The inventors studied an exhaust purification catalyst  11  comprised of a carrier  50  made of alumina carrying only platinum  51  on its surface and as a result learned that with such an exhaust purification catalyst  11 , if temporarily making the air-fuel ratio rich when burning fuel under a lean air-fuel ratio, a NO, purification rate of 90 percent or more can be obtained when the temperature of the exhaust purification catalyst  11  is a low temperature of 250° C. or less.  
      The inventors engaged in studies on the reasons for this from various angles and as a result reached the conclusion that when purifying NO x , an action of direct breakdown of NO x  at the surface of the platinum  51  or action of selective reduction of NO x  or an action of adsorption of NO x  on the catalyst carrier  50  or action of holding the NO x  on the catalyst carrier  50  occur simultaneously in parallel and that due to these actions occurring simultaneously in parallel, a high NO x  purification rate of 90 percent or more is obtained.  
      That is, platinum  51  inherently has activity at a low temperature. The first action which occurs when NO x  is being purified is the action, when the air-fuel ratio of the exhaust gas is lean, of the NO x  in the exhaust gas being absorbed on the surface of the platinum  51  in the state separated into N and O and the separated N forming N 2  and being diassociated from the platinum  51 , that is, action of direct breakdown of NO x . This direct breakdown action forms part of the NO x  purification action.  
      The second action which occurs when NO x  is being purified is the action, when the air-fuel ratio of the exhaust gas is lean, of the NO x  adsorbed on the surface of the platinum  51  being selectively reduced by the HC adsorbed on the catalyst carrier  50 . This NO x  selective reduction action forms part of the NO x  purification action.  
      On the other hand, the NO x  in the exhaust gas, that is, the NO x  is oxidized on the surface of the platinum  51  to become NO 2  and is further oxidized to become nitrate ions NO 3   − . The third action which occurs when, NO x  is being purified is the action of the NO 2  being adsorbed on the catalyst carrier  50 . This adsorption action forms part of the NO x  purification action. Further, the catalyst carrier  50  made of alumina has base points on its surface. The fourth action which occurs when NO x  is being purified is the action of the nitrate ions NO 3   −  being held at the base points on the surface of the catalyst carrier  10 . This holding action forms part of the NO x  purification action.  
      In this way, when NO x  is being purified, these various actions occur simultaneously. As a result, a high purification rate of 90 percent or more is obtained.  
      However, if exposing an exhaust purification catalyst  11  comprised of a catalyst carrier  50  made of alumina on which only platinum  51  is carried to exhaust gas of a lean air-fuel ratio, the NO x  purification rate gradually falls. This is because the surface of the platinum  51  becomes covered by oxygen atoms, that is, the surface of the platinum  51  suffers from oxygen poisoning, whereby the direct breakdown of NO x  or selective reduction of NO x  on the surface of the platinum  51  becomes difficult. That is, if the surface of the platinum  51  is covered by oxygen atoms, the NO in the exhaust gas will no longer be able to be adsorbed on the surface of the platinum  51 , so direct breakdown of the NO x  will become difficult. If the surface of the platinum  51  is covered by oxygen atoms, the NO will no longer be able to be absorbed on the surface of the platinum  51 , so the selective reduction of the NO x  will become difficult.  
      However, if temporarily making the air-fuel ratio rich at this time, the oxygen atoms covering the surface of the platinum  51  will be consumed for oxidation of the HC or CO, that is, the oxygen poisoning of the surface of the platinum  51  will be eliminated, therefore when the air-fuel ratio is returned to lean, direct breakdown of NO x  or selective reduction of NO x  again becomes performed well.  
      However, if the surface of the platinum  51  is covered by oxygen atoms, the NO x  will easily be oxidized on the surface of the platinum  51  and therefore the amount of the NO x  adsorbed or held on the catalyst carrier  50  will increase. Despite this, the fact that the NO x  purification rate drops means that the direct breakdown of the NO x  or the selective reduction of the NO x  governs the NO x  purification action. Therefore, when carrying only platinum  51  on a catalyst carrier  50  made of alumina, preventing the surface of the platinum  51  as a whole from becoming poisoned by oxygen is the most important issue. Therefore, it becomes necessary to temporarily switch the air-fuel ratio of the exhaust gas from lean to rich before the entire surface of the platinum  51  suffers from oxygen poisoning.  
      Next, this will be explained with reference to experimental results.  
       FIG. 3  shows the case of injecting a reducing agent from a reducing agent feed valve  13  for exactly the time t1 at time intervals of the time t2 and thereby maintaining the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  (ratio of the amount of air supplied to the intake passage, the combustion chambers  2 , and the exhaust passage upstream of the exhaust purification catalyst and the amount of fuel and reducing agent) lean for exactly the time t2, then making it rich for exactly the time t1.  
       FIG. 4  shows the relationship between the temperature TC (° C.) of the exhaust purification catalyst  11  and the NO x  purification rate (%) when temporarily switching the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  from lean to rich for exactly the time t1 shown in  FIG. 3  before the entire, surface of the platinum  51  suffers from oxygen poisoning in an exhaust purification catalyst  11  comprised of a catalyst carrier  50  made of alumina on which only platinum  51  is carried. Note that  FIG. 4  shows the case where the amount of coating of the catalyst carrier  50  made of the alumina is 150 (g) and the amount of platinum  51  carried is 3 (g).  
      From  FIG. 4 , it will be understood that a NO x  purification rate of 90 percent or more, that is, close to 100 percent, is obtained when the temperature TC of the exhaust purification catalyst  11  is a low temperature of 250° C. or less. Note that it is learned that when the temperature TC of the exhaust purification catalyst  11  becomes 200° C. or less, the NO x  purification rate falls somewhat, but even if the temperature TC of the exhaust purification catalyst  11  falls to 150° C., the NO x  purification rate is 80 percent or more and is still high. Further, if the temperature TC of the exhaust purification catalyst  11  becomes higher than 250° C., the NO x  purification rate will gradually fall. That is, if the temperature TC of the exhaust purification catalyst  11  becomes higher, the NO will have difficultly being adsorbed on the surface of the platinum  51  and as a result not only will the direct breakdown action of the NO x  become difficult, but also the selective reduction action of the NO x  will becomes difficult, so the NO x  purification rate will gradually fall.  
      Note that even if increasing the amount of the platinum  51  carried over 3 (g), the NO x  purification rate will not increase much at all, but if reducing the amount of platinum  51  carried to less than 3 (g), the NO x  purification rate will fall.  
      Further,  FIG. 4  shows the case of making the lean time t2 when the air-fuel ratio of the exhaust gas is lean in  FIG. 3  60 seconds and making the rich time  51  where the air-fuel ratio of the exhaust gas is made rich 3 seconds. In this case, a rich time t1 of 3 seconds is sufficient to completely eliminate the oxygen poisoning of the platinum  51 , so if seen from the viewpoint of eliminating the oxygen poisoning, there is no sense even if making the rich time t1 more than 3 seconds. As opposed to this, if making the rich time  51  shorter than 3 seconds, the NO x  purification rate will gradually fall.  
      Further, it is also possible to use rhodium as the precious metal catalyst  51  in addition to platinum. In this case, in  FIG. 4 , the region of the temperature TC (° C.) where the NO x  purification rate becomes 90 percent or more spreads to the high temperature side and the NO x  purification rate at the high temperature side becomes higher.  
      If temporarily switching the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  from lean to rich before the precious metal catalyst  51  suffers from oxygen poisoning at its entire surface, it is possible to obtain a NO x  purification rate of 90 percent or more. Note that if temporarily switching the air-fuel ratio of the exhaust gas from lean to rich, the NO 2  adsorbed on the catalyst carrier  50  or the nitrate ions NO 3  held on the catalyst carrier  50  will be reduced by the HC and CO. That is, if temporarily switching the air-fuel ratio of the exhaust gas from lean to rich so as to eliminate the oxygen poisoning of the surface of the precious metal catalyst  51 , the NO x  adsorbed or held on the catalyst carrier  50  will be removed and therefore when the air-fuel ratio is returned from rich to lean, the NO x  adsorption action or the nitrate ion NO 3   −  holding action will be started again.  
      As explained above, when carrying only platinum  51  on a catalyst carrier  50  made of alumina, the direct breakdown of the NO x  and selective reduction of the NO, will govern the NO x  purification ratio. However, the action of adsorption of NO 2  to the catalyst carrier  50  and the action of holding nitrate ions NO 3   −  on the catalyst carrier  50  also contribute to purification of NO x . However, it has been known in the past that it there is NO 2  in the exhaust gas, some NO 2  will be adsorbed on the catalyst no matter what the catalyst. In this embodiment of the present invention, the NO in the exhaust gas is oxidized at the platinum  51 , whereby NO 2  is produced and therefore NO 2  is adsorbed on the exhaust purification catalyst  11 .  
      As opposed to this, the nitrate ions NO 3   −  are not held at all catalysts. In order to get nitrate ions NO 3  held on a catalyst, the surface of the catalyst must exhibit basicity. In the embodiment according to the present invention, as explained above, since the catalyst carrier  50  is comprised of alumina, the catalyst carrier  50  has base points having basicity on its surface and therefore the nitrate ions NO 3   −  are held at the base points present on the surface of the catalyst carrier  50 .  
      However, the basicity of the base points present on the surface of a catalyst carrier  50  comprised of alumina is not that strong. Therefore, the holding force on the nitrate ions NO 3 — is also not that strong. Accordingly, if the temperature TC of the exhaust purification catalyst  11  rises, the N x  held on the exhaust purification catalyst  11  is disassociated from the exhaust purification catalyst  11 . As shown in  FIG. 4 , the NO x  purification rate gradually falls along with the rise of the temperature TC of the exhaust purification catalyst  11  because of the presence of this NO x  disassociation action.  
      On the other hand, the higher the basicity of the base points on the surface of the catalyst carrier  50 , the greater the amount of NO x  held in the form of nitrate ions NO 3   − . Therefore, to get the amount of NO x  held on the exhaust purification catalyst  11  to increase, it is sufficient to increase the number of base points or raise the basicity of the base points. In this case, as shown by reference numeral  52  in  FIG. 2 , if adding to the inside of the catalyst carrier  50  made of alumina at least one element selected from potassium K, sodium Na, lithium Li, cesium Cs, rubidium Rb, or another alkali metal, barium Ba, calcium Ca, strontium Sr, or another alkali earth, lanthanum La, yttrium Y, or another rare earth, it is possible to increase the number of base points or raise the basicity of the base points. In this case, these lanthanum La, barium Ba, or other additive  52  can be added to the inside of the catalyst carrier  50  so as to form part of the crystal structure of the alumina for stabilization of the structure or can be added to the inside of the catalyst carrier  50  so as to form a salt between the alumina and additive  52 . Note that only naturally if increasing the amount of the lanthanum La, barium Ba, or other additive  52 , the amount of NO x  held at the exhaust purification catalyst  11  increases when the air-fuel ratio of the exhaust gas is lean.  
      On the other hand, if raising the basicity of the base points in this way, the holding force on the nitrate ions NO 3   −  becomes stronger. Therefore, the nitrate ions NO 3   −  become harder to disassociate even if the temperature TC of the exhaust purification catalyst  11  rises. Therefore, if raising the basicity of the base points, the NO x  purification rate at the high temperature side becomes higher in  FIG. 4 .  
      However, exhaust gas also includes SO 2 . This SO 2  is oxidized on the platinum  51  and becomes SO 3 . Next, this SO 3  is further oxidized on the platinum  51  and becomes sulfate ions SO 4   2− . If the catalyst has basicity, the sulfate ions SO 4   2−  are held on the catalyst. Further, the sulfate ions SO 4   2−  are held on the catalyst more easily than the nitrate ions NO 3   − . Therefore, if nitrate ions NO 3   −  are held on the catalyst, sulfate ions SO 4   2−  will also necessarily be held on the catalyst. In this embodiment according to the present invention, nitrate ions NO 3   −  are held on the catalyst carrier  50 . Therefore, in this embodiment according td the present invention, the sulfate ions SO 4   2−  are also held on the catalyst carrier  50 .  
      On the other hand, as explained at the start, if forming a layer of a NCH absorbent comprised of an alkali metal or alkali earth on the catalyst carrier, SO x  forms a sulfate in the layer of the NO x  absorbent. However, this sulfate is hard to break down. Unless raising the temperature of the catalyst to 600° C. or more and making the air-fuel ratio of the exhaust gas rich in that state, it is not possible to get the SO x  released from the catalyst.  
      However, in this embodiment, the basicity of the base points present on the surface of the catalyst carrier  50  comprised of the alumina is extremely low compared with the basicity of the NO x  absorbent. Therefore, the SO x  is held at the base points on the surface of the catalyst carrier  50  not in the form of a sulfate, but in the form of sulfate ions SO 4   2− . Further, in this case, the holding force on the sulfate ions SO 4   2−  is considerably small.  
      If the holding force on the sulfate ions SO 4   2−  is small in this way, the sulfate ions SO 4   2−  will break down and disassociate at a low temperature. In fact, in this embodiment, if raising the temperature TC of the exhaust purification catalyst  11  to about 500° C. and making the air-fuel ratio of the exhaust gas rich, it is possible to get the SO x  held at the exhaust purification catalyst  11  released from the exhaust purification catalyst  11 .  
      However, as explained above, by adding lanthanum La, barium Ba, or another additive  52  to the catalyst carrier  50  so as to raise the basicity of the base points on the surface of the catalyst carrier  50 , it is possible to increase the amount of NO x  held on the catalyst carrier  50  when the air-fuel ratio of the exhaust gas is lean and therefore in particular possible to raise the NO x  purification rate at the high temperature side. However, if raising the basicity of the base points of the surface of the catalyst carrier  50 , the amount of SO x  held on the catalyst carrier  50  will increase and further the holding force on the SO x  will increase. As a result, the SO x  release temperature of the exhaust purification catalyst  11  required for releasing the SO x  will rise.  
      Note that as the catalyst carrier  50 , not only alumina, but also various other carriers known from the past can be used so long as they are carriers having base points on the catalyst carrier surface.  
      Next, the processing of the NO x  and SO x  will be explained based on specific embodiments.  
      First, a first embodiment of calculating the amount of oxygen poisoning of the precious metal catalyst; for example, the platinum  51 , switching the air-fuel ratio of the exhaust gas from lean to rich when the calculated amount of oxygen poisoning exceeds a predetermined allowable value, and thereby eliminating oxygen poisoning of the platinum  51  will be explained.  
      As shown in  FIG. 5A , the amount W of oxygen poisoning of platinum  51  per unit time is proportional to the oxygen concentration in the exhaust gas. Further, as shown in  FIG. 5B , the amount W of oxygen poisoning of platinum  51  per unit time increases the higher the temperature of the exhaust purification catalyst  11 . Here, the oxygen concentration in the exhaust gas and the temperature of the exhaust purification catalyst  11  are determined from the operating state of the engine. That is, these are functions of the fuel injection amount Q and engine speed N. Therefore, the amount W of the oxygen poisoning of the platinum  51  per unit time becomes a function of the fuel injection amount Q and engine speed N. In the first embodiment, the amount W of oxygen poisoning of the platinum  51  per unit time is found in advance by experiments in accordance with the fuel injection amount Q and engine speed N. This amount W of oxygen poisoning is stored as a function of the fuel injection amount Q and engine speed N in advance in the form of a map in the ROM  32  as shown in  FIG. 5C .  
       FIG. 6  shows a time chart of the control for eliminating oxygen poisoning and the control for releasing SO x . As shown in  FIG. 6 , each time the cumulative value ΣW of the amount W of oxygen poisoning exceeds the allowable value WX, a reducing agent is supplied from the reducing agent feed valve  13  and the air-fuel ratio A/F of the exhaust gas flowing into the exhaust purification catalyst  11  is temporarily switched from lean to rich. At this time, the oxygen poisoning of the platinum  51  is eliminated and the NO x  adsorbed or held on the catalyst carrier  50  is released from the catalyst carrier  50  and reduced.  
      On the other hand, the cumulative value ΣSOX of the amount of SO x  held on the exhaust purification catalyst  11  is also calculated and, when the cumulative value ΣSOX of the amount of SO x  exceeds an allowable value SX, the action of releasing SO x  from the exhaust purification catalyst  11  is performed. That is, first, the temperature TC of the exhaust purification catalyst  11  is raised to the SO x  release temperature TX. The SO x  release temperature TX is about 500° C. when no additive  52  is added to the catalyst carrier  41 . When additive  52  is added to the catalyst carrier  51 , it is a temperature between about 500° C. to 550° C. depending on the amount of addition of the additive  52 .  
      When the temperature TC of the exhaust purification catalyst  11  reaches the SO x  release temperature TX, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  is switched from lean to rich and the SO x  starts to be released from the exhaust purification catalyst  11 . While the SO x  is being released, the temperature TC of the exhaust purification catalyst  11  is held at the SO x  release temperature TX or more and the air-fuel ratio of the exhaust gas is held rich. Next, when the SO x  release action ends, the action of raising the temperature of the exhaust purification catalyst  11  is stopped and the air-fuel ratio of the exhaust gas is returned to lean.  
      As explained above, when SO x  should be released from the exhaust purification catalyst  11 , the temperature of the exhaust purification catalyst  11  is raised until reaching the NO x  release temperature TX. Next, the method of raising the temperature TC of the exhaust purification catalyst  11  will be explained with reference to  FIG. 7 .  
      One of the methods effective for raising the temperature TC of the exhaust purification catalyst  11  is the method of retarding the fuel injection timing to compression top dead center or on. That is, normally the main fuel Q m , in  FIG. 7 , is injected near compression top dead center as shown by (I). In this case, as shown by (II) of  FIG. 7 , if the injection timing of the main fuel Qm is retarded, the after burn time becomes longer and therefore the exhaust gas temperature rises, If the exhaust gas temperature rises, the temperature TC of the exhaust purification catalyst  11  will rise along with it.  
      Further, to raise the temperature TC of the exhaust purification catalyst  11 , as shown by (III) of  FIG. 7 , it is possible to inject auxiliary fuel Qv near suction top dead center in addition to the main fuel Qm. If additionally injecting auxiliary fuel Qv in this way, the fuel which can be burned is increased by exactly the auxiliary fuel Qv, so the exhaust gas temperature rises and therefore the temperature TC of the exhaust gasc  11  rises.  
      On the other hand, if injecting auxiliary fuel Qv near suction top dead center in this way, during the compression stroke, the heat of compression causes aldehydes, ketones, peroxides, carbon monoxide, and other intermediate products to be produced from this auxiliary fuel Qv. These intermediate products cause the reaction of the main fuel Qm to be accelerated. Therefore, in this case, as shown by (III) of  FIG. 7 , even if greatly retarding the injection timing of the main fuel Qm, good combustion is obtained without causing misfires. That is, since it is possible to greatly retard the injection timing of the main fuel Qm in this way, the temperature of the exhaust gas becomes considerably higher and therefore the temperature TC of the exhaust purification, catalyst  11  can be made to quickly rise.  
      Further, the temperature TC of the exhaust purification catalyst  11  may be made to rise as shown in (IV) of  FIG. 7  by injecting auxiliary fuel Op in addition to the main fuel Qm during the expansion stroke or the exhaust stroke. That is, in this case, the majority of the auxiliary fuel Qp is exhausted into the exhaust passage in the form of unburned HC without being burned. This unburned RC is oxidized by the excess oxygen in the exhaust purification catalyst  11 . The heat of oxidation reaction occurring at that time causes the temperature TC of the exhaust purification catalyst  11  to rise.  
       FIG. 8  shows a routine for control of an oxygen poisoning elimination flag showing that oxygen poisoning of the platinum  51  should be eliminated and a SO x  release flag showing that SO x  should be released. This routine is executed by interruption every predetermined time interval.  
      Referring to  FIG. 8 , first, at step  100 , the amount W of oxygen poisoning per unit time is calculated from the map shown in  FIG. 5C . Next, at step  101 , the amount W of oxygen poisoning is added to ΣW to calculate the cumulative value ΣW of the amount of oxygen poisoning. Next, at step  102 , it is judged if the cumulative value ΣW of the amount of oxygen poisoning has exceeded an allowable value WX, that is, if the situation is a little before the entire surface of the platinum  51  suffers from oxygen poisoning. When ΣW≦WX, the routine jumps to step  104 . As opposed to this, when ΣW&gt;WX, the routine proceeds to step  103 , where the poisoning elimination flag is set, then the routine proceeds to step  104 .  
      At step  104 , the value k·Q of a constant k multiplied with the fuel injection amount Q is added to ΣSOX. The fuel contains a certain amount of sulfur. Therefore, the amount of SO x  held in the exhaust purification catalyst  11  per unit time can be expressed by k·Q. Therefore, the ΣSOX obtained by adding ΣSOX to k·Q expresses the cumulative value of the amount of SO x  held on the exhaust purification catalyst  11 . Next, at step  105 , it is judged if the cumulative amount ΣSOX of the amount of SO x  exceeds an allowable value SX. When ΣSOX≦SX, the processing cycle is ended. When ΣSOX&gt;SX, the routine proceeds to step  106 , where the SO x  release flag is set.  
      Next, a routine for control for feeding a reducing agent will be explained while referring to  FIG. 9 .  
      Referring to  FIG. 9 , first, at step  200 , it is judged if the poisoning elimination flag is set. When the poisoning elimination flag is not set, the routine jumps to step  208 . As opposed to this, when the poisoning elimination flag is set, the routine proceeds to step  201 , where it is judged if the temperature of the exhaust purification catalyst  11  is lower than an allowable temperature TL. This allowable temperature TL is for example the temperature TC of the exhaust purification catalyst  11  when the NO x  purification rate becomes 30 percent. When carrying only platinum  51  on a catalyst carrier  50  made of alumina, this allowable temperature TL is about 400° C. When TC≧TL, that is, when a high NO x  purification rate cannot be obtained even if periodically making the air-fuel ratio of the exhaust gas rich, the routine jumps to step  208 . That is, when the temperature TC of the exhaust purification catalyst  11  exceeds about 400° C., the action of switching the air-fuel ratio from lean to rich is prohibited. As opposed to this, when TC&lt;TL, that is, when a high NO x  purification rate can be obtained by periodically making the air-fuel ratio of the exhaust gas rich, the routine jumps to step  202 .  
      At step  202 , the feed amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of for example about 13 is calculated. Next, at step  203 , the feed time of the reducing agent is calculated. This reducing agent feed time is normally 10 seconds or less. Next, at step  204 , the feed of the reducing agent from the reducing agent is feed valve  13  is started. Next, at step  205 , it is judged if the feed time of the reducing agent calculated at step  203  has elapsed. When the feed time of the reducing agent has not elapsed, the routine jumps to step  208 , where the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 13. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the oxygen poisoning of the platinum  51  has been eliminated, the routine proceeds to step  206 , where the feed of the reducing agent is stopped, then the routine proceeds to step  207 , where the ΣW and the oxygen poisoning elimination flag are cleared. Next, the routine proceeds to step  208 .  
      At step  208 , it is judged if the SO x  release flag has been set. When the SO x  release flag has not been set, the processing cycle is ended. As opposed to this, when the SO x  release flag has been set, the routine proceeds to step  209 , where the control is performed for raising the temperature of the exhaust purification catalyst  11 . That is, the fuel injection pattern from the fuel injector  3  is changed to an injection pattern of any of (II) to (IV) of  FIG. 7 . If the fuel injection pattern is changed to any injection pattern of (II) to (IV) of  FIG. 7 , the exhaust gas temperature rises and therefore the temperature of the exhaust purification catalyst  11  rises. Next, the routine proceeds to step  210 .  
      At step  210 , it is judged if the temperature TC of the exhaust purification catalyst  11  detected by the temperature sensor  20  has reached the SO x  release temperature TX or more. When TC&lt;TX, the processing cycle is ended. As opposed to this, when TC≧TX, the routine proceeds to step  211 , where the feed amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 14 is calculated. Next, at step  212 , the feed time of the reducing agent is calculated. The feed time of the reducing agent is several minutes. Next, at step  213 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  214 , it is judged if the feed time of the reducing agent calculated at step  212  has elapsed. When the feed time of the reducing agent has not elapsed, the processing cycle is ended. At this time, the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 14. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the release of the SO x  held in the exhaust purification catalyst  11  has been completed, the routine proceeds to step  215 , where the feed of the reducing agent is stopped. Next, at step  216 , the action of raising the temperature of the exhaust purification catalyst  11  is stopped, then the routine proceeds to step  217 , where the ΣSOX, ΣW, and SO x  release flag are cleared.  
       FIG. 11A ,  FIG. 11B , and  FIG. 12  show another embodiment. In this embodiment, as the sensor  22  arranged in the exhaust pipe  21 , a NO x  concentration sensor able to detect the concentration of NO x  in the exhaust gas is used. This NO x  concentration sensor  22 , as shown in FIG.  11 B, generates an output voltage V proportional to the NO x  concentration.  
      If the oxygen poisoning of the platinum  51  progresses, the NO x  purification rate gradually falls. As a result, the NO x  concentration in the exhaust gas gradually increases. Therefore, the amount of oxygen poisoning of the precious metal catalyst, for example, the platinum  51 , can be estimated from the NO x  concentration in the exhaust gas. In this embodiment, when the amount of oxygen poisoning estimated from the NO x  concentration in the exhaust gas exceeds a predetermined allowable value, that is, as shown in  FIG. 11A , when the output voltage V of the NO x  concentration sensor  22  exceeds a set value VX, the air-fuel ratio of the exhaust gas is switched from lean to rich.  
       FIG. 12  shows the routine for control for feeding a reducing agent in this embodiment.  
      Referring to  FIG. 12 , first, at step  300 , it is judged if the output voltage V of the NO x  concentration sensor  22  has exceeded a set value VX. When V≦VX, the routine jumps to step  208  of  FIG. 10 . As opposed to this, when V&gt;VX, the routine proceeds to step  301 , where the feed amount of the reducing agent required for making the air-fuel ratio of the exhaust gas the rich air-fuel ratio of about 13 is calculated. Next, at step  302 , the feed time of the reducing agent is calculated. This feed time of the reducing agent is normally 10 seconds or less. Next, at step  303 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  304 , it is judged if the feed time of the reducing agent calculated at step  302  has elapsed. When the feed time of the reducing agent has not elapsed, the routine jumps to step  208  of  FIG. 10 , where the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at a rich air-fuel ratio of about 13. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the oxygen poisoning of the platinum  51  has been eliminated, the routine proceeds to stop  365 , where the feed of the reducing agent is stopped, then the routine proceeds to step  208  of  FIG. 10 .  
      Note that in this embodiment as well, the routine for control of the flags shown in  FIG. 8  is used, but in this embodiment, it is not necessary to calculate the amount W of oxygen poisoning, so in the routine for control of the flags shown in  FIG. 8 , only step  104  to step  106  are executed. Further, in this embodiment, as explained above, after the routine shown in  FIG. 12 , the routine shown in  FIG. 10  is executed, but at step  217  in the routine shown in  FIG. 10 , only the ΣSOX and SO x  release flag are cleared.  
       FIG. 13  and  FIG. 14  show still another embodiment.  
      In this embodiment, to eliminate the oxygen poisoning of the precious metal catalyst, for example, the platinum  51 , it is judged if the oxygen poisoning of the platinum  51  has been eliminated when the air-fuel ratio of the exhaust gas is made rich. When it is judged that the oxygen poisoning of the platinum  51  has been eliminated, the air-fuel ratio of the exhaust gas is switched from rich to lean.  
      More specifically speaking, in this embodiment, as the sensor  22  arranged in the exhaust pipe  21 , an air-fuel ratio for detecting the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst  11  is used. As shown in  FIG. 13 , when the air-fuel ratio (A/F) of the exhaust gas flowing into the exhaust purification catalyst  11  is switched from lean to rich, that is, when a reducing agent is supplied from the reducing agent feed valve  13 , the reducing agent, that is, the hydrocarbon, is oxidized by the oxygen on the platinum  51 . While there is oxygen on the platinum  51 , the air-fuel ratio(A/F)out of the exhaust gas flowing out from the exhaust purification catalyst  11  is maintained at about the stoichiometric air-fuel ratio. Next, when there is no longer any oxygen on the platinum  51 , the hydrocarbon passes through the exhaust purification catalyst  11 , so the air-fuel ratio(A/F)out of the exhaust gas flowing out from the exhaust purification catalyst  11  becomes rich. Therefore, when the air-fuel ratio(A/F)in of the exhaust gas flowing into the exhaust purification catalyst  11  is switched from lean to rich, it is possible to judge that the oxygen poisoning of the platinum  51  has been eliminated when the air-fuel ratio (A/F) out of the exhaust gas flowing out from the exhaust purification catalyst  11  becomes rich.  
       FIG. 14  shows a routine for control for feeding a reducing agent in this embodiment.  
      Referring to  FIG. 14 , first, at step  400  it is judged if the poisoning elimination flag has been set. When the poisoning elimination flag has not been set, the routine-jumps to step  208  of  FIG. 10 . As opposed to this, when the poisoning elimination flag is set, the routine proceeds to step  401 , where the feed amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 13 or so is calculated. Next, the routine proceeds to step  402 , where the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  403 , it is judged if the air-fuel ratio(A/F)out of the exhaust gas detected by the air-fuel ratio sensor  22  has become rich. When the air-fuel ratio(A/F)out is not rich, the routine jumps to step  208  of  FIG. 10 . As opposed to this, when the air-fuel ratio(A/F)out becomes rich, that is, when the oxygen poisoning of the platinum  51  is eliminated, the routine proceeds to step  404 , where the feed of the reducing agent is stopped, then the routine proceeds to step  405 , where the ΣW and poisoning elimination flag are cleared. Next, the routine proceeds to step  208  of  FIG. 10 ,  
      Next, an embodiment using a particulate filter instead of the exhaust purification catalyst  11  will be explained.  
       FIG. 15A  and  FIG. 15B  show the structure of this particulate filter  11 . Note that  FIG. 15A  is a front view of the particulate filter  11 , while  FIG. 15B  is a side sectional view of the particulate filter  11 . As shown in  FIGS. 15A and 15B , the particulate filter  11  forms a honeycomb structure and is provided with a plurality of exhaust passage pipes  60  and  61  extending in parallel with each other. These exhaust passage pipes are comprised by exhaust gas inflow passages  69  with downstream ends sealed by plugs  62  and exhaust gas outflow passages  61  with upstream ends sealed by plugs  63 . Note that the hatched portions in  FIG. 15A  show plugs  63 . Therefore, the exhaust gas inflow passages  60  and the exhaust gas outflow passages  61  are arranged alternately through thin wall partitions  64 . In other words, the exhaust gas inflow passages  60  and the exhaust gas outflow passages  61  are arranged so that each exhaust gas inflow passage  60  is surrounded by four exhaust gas outflow passages  61 , and each exhaust gas outflow passage  61  is surrounded by four exhaust gas inflow passages  60 .  
      The particulate filter  11  is formed from a porous material such as for example cordierite. Therefore, the exhaust gas flowing into the exhaust gas inflow passages  60  flows out into the adjoining exhaust gas outflow passages  61  through the surrounding partitions  64  as shown by the arrows in  FIG. 15B .  
      In this embodiment, the peripheral walls of the exhaust gas inflow passages  60  and exhaust gas outflow passages  61 , that is, the surfaces of the two sides of the partitions  64  and inside walls of the fine holes of the partitions  64  are formed on them with a layer of a catalyst carrier comprised of alumina. The catalyst carrier carries a precious metal catalyst on it. Note that in this embodiment, platinum Pt is used as the precious metal catalyst.  
      In this embodiment as well, platinum is carried on the catalyst carrier made of alumina. Therefore, in this embodiment as well, the NO x  purification rate shown in  FIG. 4  is obtained.  
      Further, in this embodiment, the particulate contained in the exhaust gas is trapped in the particulate filter  11  and the trapped particulate is successively made to burn by the heat of the exhaust gas. If a large amount of particulate deposits on the particulate filter  11 , the injection pattern is switched to any one of the injection patterns (II) to (XV) of  FIG. 7  and the exhaust gas temperature is made to rise. Due to this, the deposited particulate is ignited and burned.  
       FIG. 16  and  FIG. 17  show other embodiments of a compression ignition internal combustion engine.  
      In the embodiment shown in  FIG. 16 , the exhaust passage upstream of the exhaust purification catalyst  11  has arranged in it an exhaust purification catalyst the same as the exhaust purification catalyst  11  or a particulate filter or NO x  selective reducing catalyst  23  having the function of selectively reducing NO x  but not having the function of absorbing NO x . In the embodiment shown in  FIG. 17 , the exhaust passage downstream of the exhaust purification catalyst  11  has arranged in it a particulate filter or a NON selective reducing catalyst  23  having the function of selectively reducing NO x , but not having the function of absorbing NO x .  
      If arranging in the exhaust passage upstream of the exhaust purification catalyst  11  an exhaust purification catalyst  23  the same as the exhaust purification catalyst  11 , the downstream exhaust purification catalyst  11  will become lower in temperature than the upstream exhaust purification catalyst  23 , so when the temperature of the upstream exhaust purification catalyst  23  becomes high and the NO x  purification rate drops, a high NO x  purification rate can be obtained at the downstream exhaust purification catalyst it. Further, the particulate filter  23  may be one not having a precious metal catalyst and catalyst carrier or one having a precious metal catalyst and a catalyst carrier. Further, as the NO x  selective reducing catalyst  23 , a Cu-zeolite catalyst may be used. However, a Cu-zeolite catalyst  23  is low in heat resistance, so when using a Cu-zeolite catalyst  23 , as shown in  FIG. 17 , it is preferable to arrange the Cu-zeolite catalyst  23 . at  the downstream side of the exhaust purification catalyst  11 . Note that in the embodiments shown in  FIG. 16  and  FIG. 17  as well, the feed of the reducing agent is controlled by a method similar to the method shown in  FIG. 6 .  
       FIG. 18  shows still another embodiment of the compression ignition internal combustion engine.  
      In this embodiment, the exhaust passage downstream of the exhaust purification catalyst  11  has arranged in it a NO x  selective reducing catalyst  24  having the function of selectively reducing NO x , but not having the function of absorbing NO x . As this NO x  selective reducing catalyst  24 , use is made of a catalyst V 2 O 5 /TiO 2  having titania as a carrier and carrying vanadium oxide on this carrier (hereinafter referred to as a “vanadium-titania catalyst”) or a catalyst Cu/ZSM 5 having zeolite as a carrier and carrying copper on the carrier (hereinafter referred to as a “copper-zeolite carrier”).  
      Further, the exhaust passage between the NO x  selective reducing catalyst  24  and the exhaust purification catalyst  11  has arranged in it a urea feed valve  25  for feeding a urea aqueous solution. This urea feed valve  25  feeds a urea aqueous solution by a feed pump  26 . Further, the intake passage has an intake air detector  27  arranged inside it. A NO x  concentration sensor is used as the sensor  22  arranged in the exhaust pipe  21 .  
      If feeding urea aqueous solution from the urea feed valve  25  into the exhaust gas when the air-fuel ratio of the exhaust gas is lean, the NO contained in the exhaust gas is reduced by the ammonia NH 3  generated from the urea CO(NH 2 ) 2  on the NO x  selective reducing catalyst  24  (for example, 2NH 3 +2NH+1/2O 2 →2N 2 +3H 2 O). In this case, a certain amount of urea is required for reducing the NO x  contained in the exhaust gas and completely removing the NO x  in the exhaust gas. Below, the amount of urea required for reducing and completely removing the NO x  in the exhaust gas will be referred to as an amount of urea of an equivalence ratio of the urea/NO x  of 1. Note that an equivalence ratio of urea/NO x  of 1 will be referred to below simply as an equivalence ratio of 1.  
      The solid line of  FIG. 19  shows the relationship between the NO x  purification rate due to the exhaust purification catalyst  11  and the temperature TC of the exhaust purification catalyst  11  of the same value as shown in  FIG. 4 . The broken line of  FIG. 19  shows the relationship between the NO x  purification rate when feeding a urea aqueous solution to give an amount of urea of an equivalence ratio of 1 with respect to the amount of NO x  in the exhaust gas and the temperature TC of the NO x  selective reducing catalyst  24 . From  FIG. 19 , when a urea aqueous solution is fed so as to give an amount of urea of an equivalence ratio of 1 with respect to the amount of NO x  in the exhaust gas, if the temperature TC of the NO x  selective reducing catalyst  24  becomes about 300° C. or more, the NO x  purification rate becomes about 100 percent. As the temperature TC of the NO x  selective reducing catalyst  24  falls, it is learned that the NO x  purification rate falls.  
      In this embodiment, the feed of the reducing agent from the reducing agent feed valve  13  is controlled by the routine for control of the flags shown in  FIG. 8  and the routine for control for feeding the reducing agent shown in  FIG. 10  in the region I with a temperature TC of the exhaust purification catalyst  11  lower than the set temperature TL, for example, 300° C., in  FIG. 19 . Therefore, a high NO x  purification rate is obtained by the exhaust purification catalyst  11  in the region I. Note that in this case, as will be understood from  FIG. 19 , the TL at step  201  of  FIG. 9  is 300° C.  
      On the other hand, in the region where the temperature TC of the NO x  selective reducing catalyst  24  is higher than the set temperature TN (&lt;TL) in  FIG. 19 , a urea aqueous solution is fed by the urea aqueous solution feed control routine shown in  FIG. 20 , whereby the NO x  is purified by the NO x  selective reducing catalyst  24 .  
      That is, referring to  FIG. 20 , first, at step  500 , it is judged if the temperature TC of the NO x  selective reducing catalyst  24  is higher than a set temperature TN, for example, 250° C. When TC≦TN, the processing cycle is ended. As opposed to this, when TC&gt;TN, the routine proceeds to step  501 , where the amount of NO x  exhausted from a combustion chamber  2  per unit time is found from the NO x  concentration detected by the NO x  concentration sensor  22  and the amount of intake air detected by the intake air detector  27 . Based on this amount of NO x , the amount of urea per unit time giving an equivalence ratio of 1 with respect to the amount of NO x  is calculated. Next, at step  502 , the feed amount of the urea aqueous solution is calculated from the calculated amount of urea. Next, at step  503 , the amount of the urea aqueous solution calculated at step  502  is fed from the urea feed valve  13 . Therefore, in the region II, a high NO x  purification rate is obtained by the NO x  selective reducing catalyst  24 .  
      As will be understood from  FIG. 19 , in the region where the region I and region II overlap, the action of purification of NO x  by the exhaust purification catalyst  11  and the action of purification of NO x  by the NO x  selective reducing catalyst  24  are performed. Therefore, the NO x  purification rate in this region becomes substantially 100 percent. Therefore, a high NO x  purification rate can be obtained over a wide temperature region.  
      Next, another embodiment enabling a high NO x  purification rate to be obtained over a wide temperature region will be explained.  
      In this embodiment, as shown in  FIG. 21 , a NO x  storing catalyst  29  housed in a casing  28  is arranged upstream of the exhaust purification catalyst  11 . That is, in this embodiment, the casing  28  housing the NO x  storing catalyst  29  is connected to the outlet of the exhaust turbine  7   b  of the exhaust turbocharger  7 , while the outlet of the casing  28  is connected to a casing  12  housing the exhaust purification catalyst  11  through the exhaust pipe  43 .  
      Further, in this embodiment, in addition to the temperature sensor  20  for detecting the temperature of the exhaust purification catalyst  11 , a temperature sensor  48  for detecting the temperature of the NO x  storing catalyst  29  is attached to the NO x  storing catalyst  11 . The exhaust pipe  43  connecting the outlet of the NO x  storing catalyst  29  and the inlet of the exhaust purification catalyst  11  has arranged in it a temperature sensor  49  for detecting the temperature of the temperature of the exhaust gas flowing through these catalysts  29  and  11 . Note that in practice at least one of these temperature sensors  20 ,  48 , and  49  is provided.  
      The NO x  storing catalyst  29  shown in  FIG. 21  is comprised of a monolithic catalyst. A base of the NO x  storing catalyst  29  carries a catalyst carrier made of for example alumina.  FIG. 22  schematically shows the cross-section of the surface part of this catalyst carrier  45 . As shown in  FIG. 22 , the catalyst carrier  45  carries a precious metal catalyst  46  dispersed on its surface. Further, the catalyst carrier  45  is formed on its surface with a layer of a NO x  absorbent  47 .  
      In this embodiment, platinum Pt is used as the precious metal catalyst  46 . As the ingredient forming the NO x  absorbent  47 , for example, at least one element selected from potassium K, sodium Na, cesium Cs, or another alkali metal, barium Ba, calcium Ca, or another alkali earth, lanthanum La, yttrium Y, or another rare earth may be used.  
      If the ratio of the air and fuel (hydrocarbons) supplied to the engine intake passage, combustion chambers  2 , and exhaust passage upstream of the NO x  storing catalyst  29  is referred to as the “air-fuel ratio of the exhaust gas”, the NO x  absorbent  47  performs an NO x  absorption and release action of absorbing, the NO x  when the air-fuel ratio of the exhaust gas is lean and releasing the absorbed NO x  when the oxygen concentration in the exhaust gas falls. Note that when the inside of the exhaust passage upstream of the NO x  storing catalyst  29  is not supplied with fuel (hydrocarbons) or air, the air-fuel ratio of the exhaust gas matches with the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber  2 . Therefore, in this case, the NO x  absorbent  47  absorbs the NO x  when the air-fuel ratio of the air-fuel mixture supplied into the combustion chamber  2  is lean, while releases the absorbed NO x  when the oxygen concentration in the air-fuel mixture supplied to the combustion chamber  2  falls.  
      That is, if explaining this taking as an example the case of using barium Ba as the ingredient forming the NO x  absorbent  47 , when the air-fuel ratio of the exhaust gas is lean, that is, when the oxygen concentration in the exhaust gas is high, the NO contained in the exhaust gas is oxidized on the platinum Pt  46  such as shown in  FIG. 22  to become NO 2 , then is absorbed in the NO x  absorbent  47  and diffuses in the NO x  absorbent  47  in the form of nitrate ions NO 3   −  while bonding with the barium oxide BaO. In this way, the NO x  is absorbed in the NO x  absorbent  47 . So long as the oxygen concentration in the exhaust gas is high, NO 2  is produced on the surface of the platinum Pt  46 . So long as the NO x  absorbing capability of the NO x  absorbent  47  is not saturated, the NO 2  is absorbed in the NO x  absorbent  47  and nitrate ions NO 3   −  are produced.  
      As opposed to this, by making the air-fuel ratio in the combustion chamber  2  rich or the stoichiometric air-fuel ratio or feeding a reducing agent from the reducing agent feed valve  13  to make the air-fuel ratio of the exhaust gas rich or the stoichiometric air-fuel ratio, since the oxygen concentration in the exhaust gas falls, the reaction proceeds in the reverse direction (NO 3   − →NO 2 ) and therefore the nitrate ions NO 3   −  in the NO x  absorbent  47  are released from the NO x  absorbent  47  in the form of NO 2 . Next, the released NO x  is reduced by the unburned HC or CO included in the exhaust gas.  
      In this way, when the air-fuel ratio of the exhaust gas is lean, that is, when burning fuel under a lean air-fuel ratio, the NO x  in the exhaust gas is absorbed in the NO x  absorbent  47 . However, if continuing to burn fuel under a lean air-fuel ratio, during that time the NO x  absorbing capability of the NO x  absorbent  47  will end up becoming saturated and therefore NO x  will end up no longer being able to be absorbed by the NO x  absorbent  47 . Therefore, in this embodiment, as shown in  FIG. 23 , before the absorbing capability of the NO x  absorbent  47  becomes saturated, a reducing agent is supplied from the reducing agent feed valve  14  so as to temporarily make the air-fuel ratio of the exhaust gas rich and thereby release the NO x  from the NO x  absorbent  47 .  
      However, platinum Pt  46  inherently has activity at a low temperature. However, the basicity of the NO x  absorbent  47  is considerably strong. Therefore, the activity of the platinum Pt  46  at a low temperature, that is, the oxidation ability, ends up being weakened. As a result, if the temperature TC of the NO x  storing catalyst  11  falls, the NO oxidation action weakens and the NO x  purification rate falls. The solid line of  FIG. 24  shows the relationship between the NO x  purification rate due to the exhaust purification catalyst  11 , which is the same as the value shown in  FIG. 4 , and the temperature TC of the exhaust purification catalyst  11 , while the broken line of  FIG. 24  shows the relationship between the NO x  purification rate by the NO x  storing catalyst  29  and the temperature TC of the NO x  storing catalyst  29 . In this embodiment, as will be understood from  FIG. 24 , when the temperature TC of the NO x  storing catalyst  29  becomes lower than about 250° C., the NO x  purification rate rapidly falls.  
      On the other hand, exhaust gas contains SO 2 . This SO 2  is oxidized at the platinum Pt  46  and becomes SO 3 . Next, this SO 2  is absorbed in the NO x  absorbent  47  and bonds with the barium oxide BaO while diffusing in the NO x  absorbent  47  in the form of sulfate ions SO 4   2−  to produce the stable sulfate BaSO 4 . However, the NO x  absorbent  47  has a strong basicity, so this sulfate BaSO 4  is stable and hard to break down. If just making the air-fuel ratio of the exhaust gas rich, the sulfate BaSO 4  will remain without being broken down. Therefore, in the NOR absorbent  47 , the sulfate BaSO 4  will increase along with the elapse of time and therefore the amount of NO x  which the NO x  absorbent  47  can absorb will fall along with the elapse of time.  
      However, if raising the temperature of the NO x  storing catalyst  29  to 600° C. or more and in that state making the air-fuel ratio of the exhaust gas rich, SO x  will be released from the NO x  absorbent  47 . Therefore, in this embodiment, when the amount of SO x  absorbed in the NO x  absorbent  47  increases, the temperature of the NO x  storing catalyst  29  is raised up to 600° C. or more and the air-fuel ratio of the exhaust gas is made rich.  
      As will be understood from the above explanation, in this embodiment, an exhaust purification catalyst  11  using a carrier  50  having base points on its surface and having a precious metal catalyst  51  carried dispersed on the surface of the carrier  50  without forming a layer of a NO x  absorbent able to absorb NO x  under a lean air-fuel ratio and a NO x  storing catalyst  29  forming on the surface of a carrier  45  a layer of a NO x  absorbent  47  able to absorb NO x  under a lean air-fuel ratio and having a precious metal catalyst  46  carried dispersed on the surface are arranged in series in the engine exhaust passage. When the NO x  in the exhaust gas is mainly being purified by the NO x  purification catalyst  11 , the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  is temporarily switched from lean to rich before the entire surface of the precious metal catalyst  51  carried on the surface of the carrier  50  of the NO x  purification catalyst  11  suffers from oxygen poisoning. When the NO x  in the exhaust gas is mainly being purified by the NO x  storing catalyst  29 , the air-fuel ratio of the exhaust gas flowing into the NO x  storing catalyst  29  is temporarily switched from lean to rich before the NO x  storing capacity of the NO x  storing catalyst  29  becomes saturated.  
      Note that in this case, as will be understood from  FIG. 24 , when the temperature of the NO x  purification catalyst  11  is in a first temperature region lower than a set temperature Ts, the NO x  in the exhaust gas is mainly purified by the exhaust purification catalyst  11 , while when the temperature of the NO x  storing catalyst  29  is in the second temperature region at the higher temperature side from the first temperature region, that is, higher than the set temperature Ts, the NO x  in the exhaust gas is mainly purified by the NO x  storing catalyst  29 . In the example shown in  FIG. 24 , the set temperature Ts is about 250° C.  
      Further, as the temperature TC of the catalyst at  FIG. 24 , a representative temperature representing the temperature of the exhaust purification catalyst  11  and the temperature of the NO x  storing catalyst  29  is used. As this representative temperature TC, the temperature of the NO x  storing catalyst  29  or the temperature of the exhaust purification catalyst  11  detected by the temperature sensor  20  or the temperature of the exhaust gas detected by the temperature sensor  49  is used. In this case, when the representative temperature TC is lower than the predetermined set temperature Ts, for example, 250° C., it is judged that the temperature of the exhaust purification catalyst  11  is in the first temperature region, while when the representative temperature TC is higher than the predetermined set temperature TS, for example 250° C., it is judged that the temperature of the NO x  storing catalyst  29  is in the second region.  
      Next, the processing of the NO x  and SO x  will be explained.  
      In this embodiment as well, when NO x  is mainly purified in the NO x  purification catalyse  11 , the amount of oxygen poisoning of the precious metal catalyst of the exhaust purification catalyst  11 , for example, platinum Pt  51 , is calculated using the map shown in  FIG. 5C . When the calculated amount of oxygen poisoning exceeds a predetermined allowable value, the air-fuel ratio of the exhaust gas is switched from lean to rich and thereby the oxygen poisoning of the platinum Pt  51  is eliminated.  
       FIG. 25  shows the time chart of the control for elimination of oxygen poisoning and the control for release of SO x . The control shown in  FIG. 25  is substantially the same as the control shown in  FIG. 6 . That is, as shown in  FIG. 25 , every time the cumulative value ΣW of the amount W of oxygen poisoning exceeds an allowable value WX, the reducing agent is supplied from the reducing agent feed valve  13  and the air-fuel ratio A/F of the exhaust gas flowing into the NO x  purification catalyst  11  is temporarily switched from lean to rich. At this time, the oxygen poisoning of the platinum  51  is eliminated and the NO x  adsorbed or held on the catalyst carrier  50  is released from the catalyst carrier  50  and reduced.  
      On the other hand, the cumulative value ΣSOX1 of the amount of SO x  held at the exhaust purification catalyst  11  is also calculated. When the cumulative value ΣSOX1 of this amount of SO x  exceeds an allowable value SX1, the action of release of the SO x  from the NO x  purification catalyst  11  is performed. That is, first, the methods shown in (II) to (IV) of  FIG. 7  are used to raise the temperature TC of the NO x  purification catalyst  11  until the SO x  release temperature TX1 ins reached. This SO x  release temperature TX1, as explained above, is about 500° C. when no additive  52  is added to the catalyst carrier  51 . When an additive  52  is added to the catalyst carrier  51 , it is a temperature between about 500° C. to 550° C. corresponding to the amount of addition of the additive  52 .  
      If the temperature TC of the exhaust purification catalyst  11  reaches the SO x  release temperature TX1, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  11  is switched from lean to rich and the release of SO x  from the exhaust purification catalyst  11  is started. While the SO x  is being released, the temperature TC of the exhaust purification catalyst  11  is held at the SO x  release temperature TX1 or more and the air-fuel ratio of the exhaust gas is held rich. Next, when the SO x  release action ends, the action of raising the temperature of the exhaust purification catalyst  11  is stopped and the air-fuel ratio of the exhaust gas is returned to lean.  
      Further, in this embodiment, when the NO x  is mainly purified by the NO x  storing catalyst  29 , the amount of NO x  absorbed in the NO x  absorbent  47  of the NO x  storing catalyst  29  is calculated. When the calculated amount of NO x  absorbed exceeds a predetermined allowable value, the air-fuel ratio of the exhaust gas is switched from lean to rich, whereby the NO x  is made to be released from the NO x  absorbent  47 .  
      The amount of NO x  exhausted from the engine per unit time is a function of the fuel injection amount Q and the engine speed N. Therefore, the amount NOXA of NO x  absorbed in the NO x  absorbent  47  per unit time becomes a function of the fuel injection amount Q and the engine speed N. In this embodiment, the amount NOXA of NO x  absorbed per unit time corresponding to the fuel injection amount Q and the engine speed N is found in advance by experiments. This amount NOXA of NO x  absorbed is stored in advance in the ROM  32  in the form of a map as shown by  FIG. 26A  as a function of the fuel injection amount Q and engine speed N.  
      On the other hand,  FIG. 26B  shows the relationship between the NO x  absorption rate KN to the NO x  absorbent  47  and the temperature TC of the NO x  storing, catalyst  29 . This NO x  absorption rate KN has the same tendency as the NO x  absorption rate shown by the broken line of  FIG. 24  with respect to the temperature TC of the NO x  storing catalyst  29 . The actual amount of NO x  absorbed in the NO x  absorbent  45  is shown by the product of NOXA and KN.  
       FIG. 27  shows a time chart of the control for release of NO x  and SO x . As shown in  FIG. 27 , every time the cumulative value ΣNOX of the amount NOXA·KN of NO x  absorbed exceeds an allowable value NX, a reducing agent is supplied from the reducing agent feed valve  13  and the air-fuel ratio A/F of the exhaust gas flowing into the NO x  storing catalyst  29  is temporarily switched from lean to rich. At this time, NO x  is released from the NO x  absorbent  47  and reduced.  
      On the other hand, the ΣSOX2 of the amount of SO x  absorbed in the NO x  absorbent  47  is also calculated. When the cumulative value ΣSOX2 of this amount of SO x  exceeds an allowable value SX2, the action of release of the SO x  from the NO x  absorbent  47  is performed. That is, first the methods shown in (II) to (IV) of  FIG. 7  are used to raise the temperature TC of the NO x  storing catalyst  29  until the SO x  release temperature TX2 is reached. This SO x  release temperature TX2 is 600° C. or more.  
      If the temperature TC of the NO x  storing catalyst  29  reaches the SO x  release temperature TX2, the air-fuel ratio of the exhaust gas flowing into the NO x  storing, catalyst  29  is switched from lean to rich and the release of SO x  from the NO x  absorbent  47  is started. While the SO x  is being released, the temperature TC of the NO x  storing catalyst  29  is held at the SO x  release temperature TX2 or more and the air-fuel ratio of the exhaust gas is held rich. Next, when the SO x  release action ends, the action of raising the temperature of the NO x  storing catalyst is stopped and the air-fuel ratio of the exhaust gas is returned to lean.  
      Note that the t 0  shown in  FIG. 27  and the to shown in  FIG. 25  express the same time. Therefore, the rich interval when getting the NO x  released from the NO x  absorbent  47  and the rich time when getting the SO x  released from the NO x  absorbent  45  are longer than the rich interval for eliminating oxygen poisoning at the exhaust purification catalyst  11  and the rich time for releasing the SO x .  
       FIG. 28  shows the routine for control for feeding a reducing agent from the reducing agent feed valve  13 . This routine is executed by interruption every predetermined time interval.  
      Referring to  FIG. 28 , first, at step  600 , it is judged if a representative temperature TC representing the temperature of the NO x  storing catalyst  29  and the exhaust purification catalyst is lower than a set temperature Ts, for example, 250° C. When TC&lt;Ts, the routine proceeds to step  601 , where the amount W of oxygen poisoning per unit time is calculated from the map shown in  FIG. 5 (C). Next, at step  602 , the amount W of oxygen poisoning W is added to ΣW so as to calculate the cumulative value ΣW of the amount of oxygen poisoning. Next, at step  603 , it is judged if the cumulative value ΣW of the amount of oxygen poisoning has exceeded an allowable value WX, that is, if the state is a little before the entire surface of the platinum  51  suffers from oxygen poisoning. When ΣW≦WX, the routine jumps to step  605 . As opposed to this, when ΣW&gt;WX, the routine proceeds to step  604 , where the poisoning elimination processing is performed, then the routine proceeds to step  605 .  
      At step  605 , the value k1·Q of a constant k1 multiplied with the fuel injection amount Q is added to ΣSOX1. This ΣSOX1 expresses the cumulative value of the amount of SO x  held on the exhaust purification catalyst  11 . Next, at step  606 , it is judged if the cumulative value ΣSOX1 of the amount of SO x  has exceeded an allowable value SX1. When ΣSOX1≦SX1, the processing cycle is ended, while when ΣSOX1&gt;SX1, the routine proceeds to step  607 , where the SO x  release processing I is performed.  
      On the other hand, when it is judged at step  600 , that TC≧Ts, the routine proceeds to step  608 , where the amount NOXA of NO x  absorbed per unit time is calculated from the map shown in  FIG. 26A , and the NO x  absorption rate KN shown in  FIG. 26B  is calculated. Next, at step  609 , by adding the actual amount KN·NOXA of NO x  absorbed to ΣNOX, the cumulative value ΣNOX of the amount of NO x  absorbed is calculated. Next, at step  610 , it is judged if the cumulative value ΣNOX of the amount of NO x  absorbed has exceeded an allowable value NX. When ΣNOX≦NX, the routine jumps to step  612 . As opposed to this, when ΣNOX&gt;NX, the routine proceeds to step  611 , where the NO x  release processing is performed, then the routine proceeds to step  612 .  
      At step  612 , the value k2·Q of the constant k2 multiplied with the fuel injection amount Q is added to ΣSOX2. This ΣSOX2 shows the cumulative value of the amount of SO x  absorbed in the NO x  absorbent  47 . Next, at step  613 , it is judged if the cumulative value ΣSOX2 of the amount of SO x  has exceeded an allowable value SX2. When ΣSOX2≦SX2, the processing cycle is ended. When ΣSOX2&gt;SX2, the routine proceeds to step  614 , where the SO x  release processing II is performed.  
       FIG. 29  shows the routine for processing for elimination of poisoning executed at step  604  of  FIG. 28 .  
      Referring to  FIG. 29 , first, at step  620 , the amount of feed of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of for example about 13 is calculated. Next, at step  621 , the feed time of the reducing agent is calculated. This reducing agent feed time is normally 10 seconds or less. Next, at step  622 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  623 , it is judged if the feed time of the reducing agent calculated at step  621  has elapsed. When the feed time of the reducing agent has not elapsed, the routine returns to step  623  again. At this time, the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 13. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the oxygen poisoning of the platinum  51  has been eliminated, the routine proceeds to step  624 , where the feed of the reducing agent is stopped, then the routine proceeds to step  625 , where the ΣW is cleared. Next, the routine proceeds to step  605  of  FIG. 28 .  
       FIG. 30  shows the routine for processing of the SO x  release processing I executed at step  607  of  FIG. 28 .  
      Referring to  FIG. 30 , first, at step  630 , control is performed for raising the temperature of the exhaust purification catalyst  11 . That is, the fuel injection pattern from the fuel injector  3  is changed to an injection pattern of any of (II) to (IV) of  FIG. 7 . If the fuel injection pattern is changed to any injection pattern of (II) to (IV) of  FIG. 7 , the exhaust gas temperature rises and therefore the temperature of the exhaust purification catalyst  11  rises. Next, the routine proceeds to step  631 , where it is judged if the representative temperature TC representing the temperature of the exhaust purification catalyst  11  has reached the SO x  release temperature TX1 or more. When TC&lt;TX1, the routine returns to step  631  again. As opposed to this, when TC≧TX1, the routine proceeds to step  632 , where the amount of feed of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 14 is calculated. Next, at step  633 , the feed time of the reducing agent is calculated. The feed time of the reducing agent is several minutes. Next, at step  634 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  635 , it is judged if the feed time of the reducing agent calculated at step  633  has elapsed. When the feed time of the reducing agent has not elapsed, the routine returns to step  635 . At this time, the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 14. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the release of the SO x  held in the exhaust purification catalyst  11  has been completed, the routine proceeds to step  636 , where the feed of the reducing agent is stopped. Next, at step  637 , the action of raising the temperature of the exhaust purification catalyst  11  is stopped, then the routine proceeds to step  638 , where the ΣSOX1 and ΣW are cleared.  
       FIG. 31  shows the routine for processing for release of NO x  executed at step  611  of  FIG. 28 .  
      Referring to  FIG. 31 , first, at step  640 , the amount of feed of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of for example about 13 is Calculated. Next, at step  641 , the feed time of the reducing agent is calculated. This reducing agent feed time is normally 10 seconds or less. Next, at step  642 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  643 , it is judged if the feed time of the reducing agent calculated at step  641  has elapsed. When the feed time of the reducing agent has not elapsed, the routine returns to step  643 . At this time, the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 13. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the action of release of NO x  from the NO x  absorbent  47  has been completed, the routine proceeds to step  644 , where the feed of the reducing agent is stopped, then the routine proceeds to step  645 , where the ΣNOX is cleared. Next, the routine proceeds to step  612  of  FIG. 28 .  
       FIG. 32  shows the routine for processing of the SO x  release processing II executed at step  614  of  FIG. 28 .  
      Referring to  FIG. 32 , first, at step  650 , control is performed for raising the temperature of the NO x  storing catalyst  29 . That is, the fuel injection pattern from the fuel injector  3  is changed to an injection pattern of any of (II) to (IV) of  FIG. 7 . If the fuel injection pattern is changed to any injection pattern of (II) to (IV) of  FIG. 7 , the exhaust gas temperature rises and therefore the temperature of the NO x  storing catalyst  29  rises, Next, the routine proceeds to step  651 , where it is judged if a representative temperature TC representing the temperature of the NO x  storing catalyst  29  has reached the SO x  release temperature TX2 or more. When TC&lt;TX2, the routine returns to step  651 . As opposed to this, when TC≧TX2, the routine proceeds to step  652 , where the feed amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of about 14 is calculated. Next, at step  653 , the feed time of the reducing agent is calculated. The feed time of the reducing agent is around 10 minutes. Next, at step  654 , the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  655 , it is judged if the feed time of the reducing agent calculated at step  653  has elapsed. When the feed time of the reducing agent has not elapsed, the routine returns to step  655 . At this time, the feed of the reducing agent is continued and the air-fuel ratio of the exhaust gas is maintained at the rich air-fuel ratio of about 14. As opposed to this, when the feed time of the reducing agent has elapsed, that is, when the release of the SO x  held in the NO x  absorbent  47  has been completed, the routine proceeds to step  656 , where the feed of the reducing agent is stopped. Next, at step  657 , the action of raising the temperature of the NO x  storing catalyst  29  is stopped, then the routine proceeds to step  658 , where the ΣSOX2 and ΣFOX are cleared.  
       FIG. 33  shows still another embodiment. In this embodiment, as the sensor  22  arranged in the exhaust pipe  21 , a NO x  concentration sensor which can detect the concentration of NO x  in the exhaust gas is used. This NO x  concentration sensor  22  generates an output voltage V proportional to the NO x  concentration. In the exhaust purification catalyst  11 , if the oxygen poisoning of the platinum Pt  51  proceeds, the NON purification rate gradually falls and as a result the concentration of NO x  in the exhaust gas gradually increases. Therefore, in this embodiment, when the amount of oxygen poisoning estimated from the concentration of NO x  in the exhaust gas has exceeded a predetermined allowable value, that is, when the output voltage V of the NO x  concentration sensor has exceeded the set value VX1, the air-fuel ratio of the exhaust gas is switched from lean to rich.  
      Further, with the NO x  storing catalyst  29 , as the amount of NO x  absorbed of the NO x  absorbent  47  approaches saturation, the NO x  purification rate gradually falls and as a result the concentration of NO x  in the exhaust gas gradually increases. Therefore, the amount of NO x  absorbed in the NO x  absorbent  47  can be estimated from the concentration of NO x  in the exhaust gas. In this embodiment, when the amount of NO x  absorbed estimated from the concentration of NO x  in the exhaust gas exceeds a predetermined allowable value, that is, when the output voltage V of the NO x  concentration sensor  22  has exceeded a set value VX2, the air-fuel ratio of the exhaust gas is switched from lean to rich.  
       FIG. 33  shows the routine for control for feeding a reducing agent from the reducing agent feed valve  13  in this embodiment. This routine is executed every predetermined time interval.  
      Referring to  FIG. 33 , first, at step  700 , it is judged if a representative temperature TC representing the temperature of the NO x  storing catalyst  29  and the exhaust purification catalyst  11  is lower than a set temperature Ts, for example, 250° C. When TC&lt;Ts, the routine returns to step  701  where it is judged if the output voltage V of the NO x  concentration sensor  22  has exceeded a set value VX1. When V≦VX1, the routine jumps to step  703 . As opposed to this, when V&gt;VX1, the routine proceeds to step  702 , where the routine for processing for elimination of poisoning is executed, then the routine proceeds to step  703 .  
      At step  703 , the value k1·Q of the constant k1 multiplied with the fuel injection amount Q is added to ΣSOX1. This ΣSOX1 expresses the cumulative value of the amount of SO x  held on the exhaust purification catalyst  11 . Next, at step  704 , it is judged if the cumulative value ΣSOX1 of the amount of SO x  has exceeded an allowable value SX1. When ΣSOX1≦SX1, the processing cycle is ended, while when ΣSOX1&gt;SX1, the routine proceeds to step  705 , where the SO x  release processing I shown in  FIG. 30  is performed.  
      On the other hand, when it is judged at step  700  that TC≧Ts, the routine proceeds to step  706 , where it is judged if the output voltage V of the NO x  concentration sensor  22  has exceeded a set value VX2. When V≦VX2, the routine jumps to step  708 . As opposed to this, when V&gt;VX2, the routine proceeds to step  707 , where the NO x  release processing shown in  FIG. 31  is executed. Next, the routine proceeds td step  708 .  
      At step  708 , the value k2. ·Q of the constant k2 multiplied with the fuel injection amount Q is added to ΣSOX2. This ΣSOX2 expresses the cumulative value of the amount of SO x  held in the NO x  absorbent  47 . Next, at step  709 , it is judged if the cumulative value ΣSOX2 of the amount of SO x  has exceeded an allowable value SX2. When ΣSOX2≦SX2, the processing cycle is ended, while when ΣSOX2&gt;SX2, the routine proceeds to step  710 , where the SO x  release processing II shown in  FIG. 32  is performed.  
       FIG. 34 , and  FIG. 35  show still another embodiment. In this embodiment, as the sensor  22  arranged in the exhaust pipe  21 , the air-fuel ratio sensor for detecting the air-fuel ratio of the exhaust gas is used. As shown in  FIG. 13 , when the air-fuel ratio(A/F)out of the exhaust gas flowing out from the exhaust purification catalyst  11  becomes rich after the air-fuel ratio(A/F)in of the exhaust gas flowing into the exhaust purification catalyst  11  is switched from lean to rich, it is judged that the oxygen poisoning of platinum Pt  51  has been eliminated. At this time, the air-fuel ratio of the exhaust gas, is switched from rich to lean.  
      Further, in this embodiment, when the air-fuel ratio of the exhaust gas for releasing the NO x  from the NO x  absorbent  47  of the NO x  storing catalyst  29  is made rich, it is judged if the action of release of NO x  from the NO x  absorbent  47  has been completed from the change of the output of the air-fuel ratio sensor  22 . When it is judged that the action of release of NO x  from the NO x  absorbent  47  has been completed, the air-fuel ratio of the exhaust gas is switched from rich to lean.  
      Specifically speaking, in this case as well, as shown in  FIG. 13 , when the air-fuel ratio(A/F)in of the exhaust gas flowing into the NO x  storing catalyst  29  is switched from lean to rich, that is, when the reducing agent is supplied from the reducing agent feed valve  13 , the reducing agent, that is, the hydrocarbon, is used for reducing the NO x  released from the NO x  absorbent  47 . While the NO x  is continuing to be released from the NO x  absorbent  47 , the air-fuel ratio(A/F)out of the exhaust gas flowing out from the NO x  storing catalyst  29  is maintained at substantially the stoichiometric air-fuel ratio or somewhat lean. Next, when NO x  is no longer released from the NO x  absorbent  47 , the hydrocarbons pass through the NO x  storing catalyst  29 , so the air-fuel ratio (A/F) out of the exhaust gas flowing out from the NO x  storing catalyst  29  becomes rich. Therefore, when the air-fuel ratio(A/F)out of the exhaust gas flowing out from the NO x  storing catalyst  29  becomes rich after the air-fuel ratio(A/F)in of the exhaust gas flowing into the NO x  storing catalyst  29  is switched from lean to rich, it can be judged that the action of release of NO x  from the NO x  absorbent  47  has been completed.  
      The control for feeding the reducing agent in this embodiment is performed using the routine shown in  FIG. 28 . However, the processing for elimination of poisoning at step  604  in  FIG. 28  uses the routine shown in  FIG. 34 , while the processing for release of NO x  at step  611  of  FIG. 28  uses the routine shown in  FIG. 35 .  
      Referring to the routine for processing for elimination of poisoning shown in  FIG. 34 , first, at step  800 , the amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of for example about 13 is calculated. Next, the routine proceeds to step  801 , where the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  802 , it is judged if the air-fuel ratio(A/F)out of the exhaust gas detected by the air-fuel ratio sensor  22  has become rich. When the air-fuel ratio(A/F)out is not rich, the routine returns to step  802 . As opposed to this, when the air-fuel ratio(A/F)out is rich, that is, when the oxygen poisoning of the platinum  41  has been eliminated, the routine proceeds to step  803 , where the feed of the reducing agent is stopped, then the routine proceeds to step  804 , where the ΣW is cleared. Next, the routine proceeds to step  605  of  FIG. 28 .  
      On the other hand, referring to the NO x  release processing routine shown in  FIG. 35 , first, at step  810 , the amount of the reducing agent required for making the air-fuel ratio of the exhaust gas a rich air-fuel ratio of for example about 13 is calculated. Next, the routine proceeds to step  811 , where the feed of the reducing agent from the reducing agent feed valve  13  is started. Next, at step  812 , it is judged if the air-fuel ratio(A/F)out of the exhaust gas detected by the air-fuel ratio sensor  22  has become rich. When the air-fuel ratio(A/F)out is not rich, the routine returns to step  812 . As opposed to this, when the air-fuel ratio(A/F)out is rich, that is, when the action of release of NO x  from the NO x  absorbent  47  has been completed, the routine proceeds to step  813 , where the feed of the reducing agent is stopped, then the routine proceeds to step  814 , where the ΣNOX is cleared. Next, the routine proceeds to step  612  of  FIG. 28 .  
       FIG. 36  and  FIG. 37  show a further embodiment of the present invention.  
      As shown in  FIG. 36 , in this embodiment as well, like in the embodiment shown in  FIG. 21 , a NO x  storing catalyst  29  is arranged at the upstream side of the engine exhaust passage, while an exhaust purification catalyst  11  is arranged at the downstream side of the engine exhaust passage. However, in this embodiment, an acidic catalyst  70  such as an oxidation catalyst is arranged at the upstream side of the NO x  storing catalyst  29 . Further,  FIG. 36  shows the change of the temperature of the exhaust gas when performing control for raising the temperature to get the SO x  released from the NO x  storing catalyst  29  or the exhaust purification catalyst  11  and the strengths of the basicities of the catalysts  70 ,  29 , and  11 , that is, the basicity degrees.  
      As explained above, the basicity of the NO x  absorbent  47  of the NO x  storing catalyst.  29  is considerably strong, while the basicity of the exhaust purification catalyst  11  is weak. In other words, the basicity degree of the NO x  storing catalyst  29  is considerably higher than the basicity degree of the exhaust purification catalyst  11 . In this case, as explained above, if the basicity degree of the catalyst becomes higher, the holding force of the SO x  becomes stronger along with this. If the holding force of the SO x  becomes stronger, the SO x  will no longer be easily released even if raising the temperature of the catalyst. That is, as shown in  FIG. 37 , the SO x  release temperature becomes higher as the basicity degree of the catalyst becomes higher.  
      On the other hand, the temperature of the exhaust gas at the time of control for raising the temperature for releasing the SO x  becomes higher at a catalyst positioned at the upstream side than a catalyst positioned at the downstream side. Therefore, if seen from the viewpoint of releasing the SO x , it is preferable to arrange the catalyst with a high NO x  release temperature, that is, the catalyst with a high basicity degree, at the upstream side. That is, seen from the viewpoint of the release of SO x , it can be said to be preferable to raise the basicity degree the higher the catalyst bed temperature at the time of control for raising the temperature. In the embodiment shown in  FIG. 21  and  FIG. 36 , if seen from this viewpoint, the order of arrangement of the exhaust purification catalyst  11  and the NO x  storing catalyst  29  is determined by the strengths of the basicities of the catalysts. The catalyst with a strong basicity, that is, the NO x  storing catalyst  29 , is arranged upstream of the catalyst with a weak basicity, that is, the NO x  purification catalyst  11 .  
      Note that the temperature raising action of the exhaust gas due to the heat of the oxidation reaction of the unburned HC in the exhaust gas is the most powerful. Therefore, in the embodiment shown in  FIG. 36 , the acidic catalyst  70  is arranged at the upstream side of the NO x  storing catalyst  29 .  
      Note that each of the NO x  storing catalysts  29  shown in  FIG. 21  and  FIG. 36  may also be comprised of a particulate filter shown in  FIG. 15A  and  FIG. 15B .  
      In this way, when configuring the NO x  storing catalyst  29  by a particulate filter, the peripheral walls, of the exhaust gas inflow passages  60  and exhaust gas outflow passages  61 , that is, the surfacer of the two sides of the partitions  64  and inside walls of the fine holes of the partitions  64  are formed on them with a layer of a catalyst carrier comprised of alumina. As shown in  FIG. 22 , this catalyst carrier  45  carries a precious metal catalyst  46  and NO x  absorbent  47  on it. Note that in this case as well, platinum Pt is used as the precious metal catalyst. In this way, even when configuring the NO x  storing catalyst  29  by a particulate filter, when the air-fuel ratio of the exhaust gas is lean, the NO x  absorbent  47  absorbs the NO x  and the SO x . Therefore, in this case as well, the control for release of the NO x  and SO x  similar to the control for release of the NO x  and SO x  for the NO x  storing catalyst  29  is performed.  
      Further, when configuring the NO x  storing catalyst  29  by a particulate filter, the particulate contained in the exhaust gas is trapped in the particulate filter and the trapped particulate is successively made to burn by the heat of the exhaust gas. If a large amount of particulate deposits on the particulate filter, the injection pattern is switched to any one of the injection patterns (II) to (IV) of  FIG. 7  or a reducing agent is supplied from the reducing agent feed valve  13  and thereby the exhaust gas temperature is made to rise. Due to this, the deposited particulate is ignited and burned.  
       FIG. 38  to  FIG. 41  show various examples of arrangement of the NO x  storing catalyst  29  and the exhaust purification catalyst  11 .  
      In the example shown in  FIG. 38 , the exhaust purification catalyst  11  is arranged at the upstream side of the NO x  storing catalyst  29 . In this case, even when the temperature of the exhaust gas is low, the exhaust purification catalyst  11  can purify the NO x . Further, when the exhaust gas is lean, the exhaust purification catalyst  11  converts part of the NO contained in the exhaust gas to NO 2 . This NO 2  is easily stored in the NO x  storing catalyst  29 . On the other hand, when feeding a reducing agent from the reducing agent feed valve  13  to make the air-fuel ratio of the exhaust gas rich, this reducing agent is changed to a low molecular weight hydrocarbon in the exhaust purification catalyst  11 . Therefore, it is possible to reduce the NO x  released from the NO x  absorbent  47  of the NO x  storing catalyst  29  well.  
      On the other hand, in the example shown in  FIG. 38 , the NO x  storing catalyst  29  may also be comprised of a particulate filter. In this case, the NO 2  produced in the exhaust purification catalyst  11  promotes the oxidation of the particulate deposited on the particulate filter (NO 2 +C→CO 2 +N 2 ).  
      In the example shown in  FIG. 39 , exhaust purification catalysts  11  are arranged upstream and downstream of the NO x  storing catalyst  29 . In this case, the NO x  storing catalyst  29  may be formed from a particulate filter.  
      In the example shown in  FIG. 40 , an exhaust purification catalyst  11  is arranged downstream of the NO x  storing catalyst  29  and a monolithic catalyst  71  is arranged upstream of the NO x  storing catalyst  29 . The upstream half of the monolithic catalyst  71  is comprised of the exhaust purification catalyst  11 , while the downstream half is comprised of the NO x  storing catalyst  29 . In this example as well, the NO x  storing catalyst  29  may be formed from a particulate filter.  
      In the example shown in  FIG. 41 , a monolithic catalyst  72  is arranged in the engine exhaust passage. The center part of this monolithic catalyst  72  is comprised of the NO x  storing catalyst  29 , while the upstream part and downstream part are comprised of exhaust purification catalysts  11 . In this example as well, the NO x  storing catalyst  29  may be formed from a particulate filter.  
      Next, a low temperature combustion method suitable for raising the temperature of the exhaust purification catalyst  11  etc. and making the air-fuel ratio of the exhaust gas rich will be explained.  
      In the compression ignition type internal combustion engine shown in  FIG. 1  etc., if increasing the EGR rate (amount of EGR gas/(amount of EGR gas+amount of intake air)), the amount of production of smoke will gradually increase and then peak. If further increasing the EGR rate, the amount of production of smoke will then rapidly drop. This will be explained while referring to  FIG. 42  showing the relationship between the EGR rate and the smoke when changing the degree of cooling of the EGR rate. Note that in  FIG. 42 , the curve A shows the case of powerfully cooling the EGR gas to maintain the temperature of the EGR gas at about 90° C., the curve B shows the case of using a small cooling device to cool the EGR gas, and the curve C shows the case of forcibly cooling the EGR gas.  
      As shown by the curve A of  FIG. 42 , when powerfully cooling the EGR gas, the amount of production of smoke peaks when the EGR rate becomes a little lower than 50 percent. In this case, if making the EGR rate about 55 percent or more, almost no smoke will be produced any longer. On the other hand, as shown by the curve B of  FIG. 42 , when slightly cooling the EGR gas, the amount of production of smoke will peak at an EGR rate slightly higher than 50 percent. In this case, if making the EGR rate about 65 percent or more, almost no smoke will be produced any longer. Further, as shown by the curve C of  FIG. 42 , when not forcibly cooling the EGR gas, the amount of production of smoke peaks at an EGR rate of near 55 percent. In this case, if making the EGR rate about 70 percent or more, almost no smoke will be produced any more.  
      Smoke is no longer produced if making the EGR gas rate 55 percent or more in this way because the temperature of the fuel and its surrounding gas at the time of combustion does not become that high due to the heat absorbing action of the EGR gas, that is, low temperature combustion is performed, and as a result the hydrocarbons will not grow to soot.  
      This low temperature combustion has the feature of enabling the amount of production of NO x  to be reduced while suppressing the production of smoke regardless of the air-fuel ratio. That is, if the air-fuel ratio is made rich, the fuel becomes in excess, but the combustion temperature is suppressed at a low temperature, so the excess fuel will not grow into soot and therefore no smoke will be generated. Further, at this time, only a very small amount of NO x  will be generated. On the other hand, even when the average air-fuel ratio is lean or when the air-fuel ratio is the stoichiometric air-fuel ratio, if the combustion temperature becomes high, a small amount of soot will be produced, but in low temperature combustion, the combustion temperature is suppressed to a low temperature, so no smoke at all will be produced and only a very small amount of NO x  will be produced either.  
      On the other hand, during this low temperature combustion, the temperature of the fuel and its surrounding gas becomes lower, but the temperature of the exhaust gas rises. This will be explained with reference to  FIG. 43A  and  FIG. 43B .  
      The solid line of  FIG. 43A  shows the relationship between the average gas temperature Tg in the combustion chamber  5  and the crank angle at the time of low temperature combustion, while the broken line of  FIG. 43A  shows the relationship between the average gas temperature Tg in the combustion chamber S and the crank angle at the time of normal combustion. Further, the solid line of  FIG. 43B  shows the relationship between the temperature Tf of the fuel and its surrounding gas and the crank angle at the time of low temperature combustion, while the broken line of  FIG. 43B  shows the relationship between the temperature Tf of the fuel and its surrounding gas and the crank angle at the time of normal combustion.  
      During low temperature combustion, the amount of EGR is greater than during normal combustion. Therefore, as shown in  FIG. 43A , before compression top dead center, that is, during the compression stroke, the average gas temperature Tg at the time of low temperature combustion shown by the solid line becomes higher than the average gas temperature Tg at the time of normal combustion shown by the broken line. Note that at this time, as shown  FIG. 43B , the temperature Tf of the fuel and its surrounding gas becomes substantially the same temperature as the average gas temperature Tg.  
      Next, the fuel starts to be burned near compression top dead center, but in this case, when low temperature combustion is performed, as shown by the solid line of  FIG. 43B , the temperature Tf of the fuel and its surrounding gas does not become that much higher due to the heat absorbing action of the EGR gas. As opposed to this, during normal combustion, there is a large amount of oxygen around the fuel, so as shown by the broken line in  FIG. 43B , the temperature Tf of the fuel and its surrounding gas becomes extremely high. In this way, during normal combustion, the temperature Tf of the fuel and its surrounding gas becomes considerably higher than the case of low temperature combustion, but temperature of the other gas, which accounts for the majority of the gas, becomes lower at the time of normal combustion compared with low temperature combustion. Therefore, as shown in  FIG. 43A , the average gas temperature Tg in a combustion chamber  2  near compression top dead center becomes higher during low temperature combustion than normal combustion. As a result, as shown in  FIG. 43A , the temperature of the burnt gas in the combustion chamber  2  after the combustion has been completed becomes higher during low temperature combustion than normal combustion. Consequently, if performing low temperature combustion, the temperature of the exhaust gas becomes higher.  
      However, if the required torque TQ of the engine becomes high, that is, if the fuel injection amount becomes greater, the temperature of the fuel and the surrounding gas at the time of the combustion becomes higher, so low temperature combustion becomes difficult. That is, low temperature combustion is only possible at the time of engine medium and low load operation where the amount of heat generated due to combustion is relatively small. In  FIG. 44 , the region I shows the operating region where first combustion where the amount of inert gas of the combustion chamber  5  is larger than the amount of inert gas where the amount of generation of soot peaks, that is, low temperature combustion, can be performed, while the region II shows the operating region where only second combustion where the amount of inert gas in the combustion chamber is smaller than the amount of inert gas where the amount of generation of soot peaks, that is, normal combustion is possible.  
       FIG. 45  shows the target air-fuel ratio A/F in the case of low temperature combustion in the operating region I, while  FIG. 46  shows the opening degree of the throttle valve  9 , the opening degree of the EGR control valve  15 , the EGR rate, the air-fuel ratio, the injection start timing θS, the injection end timing θE, and the injection amount in accordance with the required torque TQ in the case of low temperature combustion in the operating region I. Note that  FIG. 46  also shows the opening degree of the throttle valve  9  at the time of normal combustion performed in the operating region II.  
      From  FIG. 45  and  FIG. 46 , it is learned that at the time of low temperature combustion in the operating region I, the EGR rate is made 55 percent or more and the air-fuel ratio A/F is made a lean air-fuel ratio of 15.5 to 18 or so. Note that as explained above, at the time of low temperature combustion in the operating region I, even if the air-fuel ratio is made rich, almost no smoke is generated.  
      In this way, at the time of low temperature combustion, it is possible to make the air-fuel ratio rich without causing almost any generation of smoke. Therefore, when the air-fuel ratio of the exhaust gas should be made rich to eliminate oxygen poisoning or release SO x , it is possible to perform low temperature combustion and make the air-fuel ratio rich under low temperature combustion.  
      Further, as explained above, if performing low temperature combustion, the exhaust gas temperature rises. Therefore, to release SO x  or make the deposited particulate ignite and burn, it is also possible to perform the low temperature combustion when the exhaust gas temperature should be raised.  
      As explained above, according to the present invention, a high NO x  purification rate can be obtained.