Abstract:
An ambient NO x  adsorption catalyst that can adsorb NO x  contained in an exhaust gas in the presence of CO under standard conditions is placed in an engine exhaust gas passage, in an internal combustion engine. Until an engine post-initiation catalyst is activated, the amounts of a high-boiling-point hydrocarbon and an unsaturated hydrocarbon that are contained in the exhaust gas flowing into the catalyst are reduced so that the NO x -adsorbing activity cannot be deteriorated by the adhesion activity of the hydrocarbons while maintaining the CO concentration in the exhaust gas flowing into the catalyst at a level higher than the concentration required for the adsorption of NO x .

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an exhaust purification device of an internal combustion engine. 
       BACKGROUND ART 
       [0002]    It is already known that if using an NO x  adsorbent in which a metal is carried on a carrier which is made of an oxide of at least one metal element selected from Co, Fe, Cu, Ce, and Mn, where the metal carried on the carrier is comprised of a metal which is selected from Cu, Co, Ag, and Pd and which is different from the metal contained in the carrier, and if running through this NO x  adsorbent a gas which contains NO and CO, even at an ordinary temperature, the NO x  adsorbent will adsorb the NO x  (see Patent Literature 1). 
       CITATIONS LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Publication No. 2007-160167 A1  
     SUMMARY OF INVENTION 
     Technical Problem 
       [0003]    In this regard, the inventors have long been researching exhaust purification systems which use a catalyst which contains a carrier and carried metal which are similar to the carrier of this NO x  adsorbent and the metal carried on this carrier and, when studying the exhaust purification action of the catalyst in the process of this research, it is confirmed that even with this catalyst, if the exhaust gas contains a certain extent or more of carbon monoxide CO, the NO x  contained in the exhaust gas is adsorbed well at the catalyst from right after engine startup, that is, at ordinary temperature. 
         [0004]    However, it has been proven that if using this catalyst to try to perform an NO x  adsorption action, certain specific hydrocarbons which are contained in the exhaust gas inhibit the action of adsorption of NO x  at the catalyst. These certain specific hydrocarbons are unsaturated hydrocarbons and high boiling point hydrocarbons which are liquid in a reference state (temperature 25° C., pressure 100 kPa). If the specific hydrocarbons are present in the exhaust gas in large amounts, the action of adsorption of NO x  at the catalyst is inhibited. Therefore, to secure a good action of adsorption of NO x  at the catalyst, the amount of the specific hydrocarbons in the exhaust gas has to be lowered. However, the above-mentioned Patent Literature 1 does not allude to this at all. 
         [0005]    An object of the present invention is to provide an exhaust purification device of an internal combustion engine which is designed to be able to adsorb the NO x  which is contained in exhaust gas well at a catalyst after engine startup and until the catalyst is activated. 
       Solution to Problem 
       [0006]    According to the present invention, there is provided an exhaust purification system of internal combustion engine comprising an ordinary temperature NO x  adsorption catalyst arranged in an engine exhaust passage, the ordinary temperature NO x  adsorption catalyst being comprised of a carrier and a metal carried on the carrier and being able to adsorb NO x  in a reference state, 
         [0007]    the carrier of the catalyst being comprised of an oxide of at least one metal element which is selected from Co, Fe, Cu, Ce, and Mn or a complex oxide which contains the metal element, 
         [0008]    the metal carried on the carrier being comprised of a metal which is selected from Cu, Co, Ag, Fe, Pt, Rh, and Pd and which is different from the metal contained in said carrier, 
         [0009]    the ordinary temperature NO x  adsorption catalyst having a property of adsorbing NO x  in the presence of carbon monoxide in the reference state and having a property in which an action of adsorption of NO x  is inhibited by an action of deposition or adsorption of hydrocarbons at the catalyst when the hydrocarbons in the exhaust gas flowing into the catalyst are unsaturated hydrocarbons or high boiling point hydrocarbons which become liquid in the reference state, 
         [0010]    wherein the amount of high boiling point hydrocarbons or the amount of unsaturated hydrocarbons which are contained in the exhaust gas flowing into the catalyst is made to lower while maintaining a concentration of carbon monoxide in the exhaust gas flowing into the catalyst at a concentration of more than the concentration which is required for adsorption of NO x  until the ordinary temperature NO x  adsorption catalyst is activated after engine startup. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0011]    After engine startup and until the ordinary temperature NO x  adsorption catalyst is activated, it is possible to secure a good NO x  adsorption action at the ordinary temperature NO x  adsorption catalyst. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  is an overall view of an internal combustion engine. 
           [0013]      FIG. 2  is a view which shows the relationship between a copresent CO gas concentration and amount of adsorption of NO x . 
           [0014]      FIG. 3  is a view which schematically shows the surface of the carrier of an ordinary temperature NO x  adsorption catalyst. 
           [0015]      FIG. 4  is a view which schematically shows the surface of the carrier of an ordinary temperature NO x  adsorption catalyst. 
           [0016]      FIG. 5  is a view which shows the relationship between the number of carbon atoms C of hydrocarbons and the HC deposition rate. 
           [0017]      FIG. 6  is a view which shows the relationship between the amount of deposition or adsorption of HC and the amount of adsorption of NO x . 
           [0018]      FIG. 7  is a view which shows a comparison of the ingredients of compressed natural gas (CNG) and gasoline. 
           [0019]      FIG. 8  is a flow chart for performing injection control of fuel. 
           [0020]      FIG. 9  is a flow chart for performing injection control of fuel. 
           [0021]      FIG. 10  is an overall view which shows another embodiment of an internal combustion engine. 
           [0022]      FIG. 11  is an overall view which shows still another embodiment of an internal combustion engine. 
           [0023]      FIG. 12  is an overall view which shows still another embodiment of an internal combustion engine. 
           [0024]      FIG. 13  is a flow chart for performing injection control of fuel. 
           [0025]      FIG. 14  is a flow chart for performing injection control of fuel. 
           [0026]      FIG. 15  is a flow chart for performing injection control of fuel. 
           [0027]      FIG. 16  is a flow chart for performing injection control of fuel. 
           [0028]      FIG. 17  is an overall view which shows still another embodiment of an internal combustion engine. 
           [0029]      FIG. 18  is a flow chart for performing injection control of fuel. 
           [0030]      FIG. 19  is a flow chart for performing injection control of fuel. 
           [0031]      FIG. 20  is an overall view which shows still another embodiment of an internal combustion engine. 
           [0032]      FIG. 21  is a flow chart for performing injection control of fuel. 
           [0033]      FIG. 22  is a flow chart for performing injection control of fuel. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]      FIG. 1  shows an overall view of an internal combustion engine. 
         [0035]    Referring to  FIG. 1 ,  1  indicates an internal combustion engine body,  2  a cylinder block,  3  a cylinder head,  4  a piston,  5  a combustion chamber,  6  a spark plug which is arranged at the center of the top of the combustion chamber  5 ,  7  an intake valve,  8  an intake port,  9  an exhaust valve, and  10  an exhaust port. Each intake port  8  is connected through an intake branch pipe  11  to a surge tank  12 , while the surge tank  12  is connected through an intake duct  13  to an air cleaner  14 . Inside the intake duct  13 , a throttle valve  15  driven by the actuator and an intake air amount detector  16  are arranged. 
         [0036]    In the embodiment shown in  FIG. 1 , as the fuel, two types of fuel of a first fuel and a second fuel are used. As shown in  FIG. 1 , in each intake port  8 , a first fuel injector  17  for injecting the first fuel is arranged, while in each intake branch pipe  11 , a second fuel injector  18  for injecting the second fuel is arranged. The first fuel which is stored in a fuel tank  19  is fed to the first fuel injector  17  by a feed pump  20 , and this first fuel is injected from the first fuel injector  17 . On the other hand, the second fuel which is stored in a fuel tank  21  is fed to the second fuel injector through a pressure regulator  22 , and this second fuel is injected from the second fuel injector  18 . 
         [0037]    In the embodiment which is shown in  FIG. 1 , the first fuel is comprised of a plant-derived alcohol fuel or a mixed fuel of a plant-derived alcohol fuel and gasoline, while the second fuel is comprised of compressed natural gas (CNG). Therefore, in the embodiment which is shown in  FIG. 1 , a plant-derived alcohol fuel or a mixed fuel of a plant-derived alcohol fuel and gasoline is injected from the first fuel injector  17 , while compressed natural gas is injected from the second fuel injector  18 . 
         [0038]    On the other hand, as shown in  FIG. 1 , each exhaust port  10  is connected through an exhaust manifold  23  to a catalyst  24 . In the embodiment shown in  FIG. 1 , this catalyst  24  performs the role of an NO x  adsorption catalyst which can absorb the NO x  contained in the exhaust gas before the catalyst  24  is activated and performs the role of a three-way catalyst which can simultaneously reduce the HC, CO, and NO x  in the exhaust gas under a stoichiometric air-fuel ratio when the catalyst  24  is activated. An air-fuel ratio sensor  25  for detecting the air-fuel ratio of the exhaust gas is attached to the exhaust manifold  23 , while a temperature sensor  26  for judging if the catalyst  24  is activated is attached to the catalyst  24 . 
         [0039]    The electronic control unit  30  is comprised of a digital computer which is provided with a ROM (read only memory)  32 , RAM (random access memory)  33 , CPU (microprocessor)  34 , input port  35 , and output port  36 , which are connected to each other by a bidirectional bus  31 . An output signal of the intake air amount detector  16 , an output signal of the air-fuel ratio sensor  23 , and an output signal of the temperature sensor  26  are input through respectively corresponding AD converters  37  to an input port  35 . Further, an accelerator pedal  40  has a load sensor  41  connected to it which generates an output voltage which is proportional to the amount of depression of the accelerator pedal  40 . The output voltage of the load sensor  41  is input through a corresponding AD converter  37  to the input port  35 . Further, the input port  35  has connected to it a crank angle sensor  42  which generates an output pulse each time the crankshaft for example rotates by 30°. On the other hand, the output port  36  is connected through corresponding drive circuits  38  to each spark plug  6 , actuator for driving the throttle valve  15 , first fuel injector  17 , second fuel injector  18 , and feed pump  20 . 
         [0040]    Now then, the catalyst  24  shown in  FIG. 1  is comprised of a carrier and a metal which is carried on the carrier. In this catalyst  24 , the carrier of the catalyst is comprised of an oxide of at least one metal element which is selected from Co, Fe, Cu, Ce, and Mn or a complex oxide which contains such a metal element, and the metal carried on the carrier is a metal which is selected from Cu, Co, Ag, Fe, Pt, Rh, and Pd and which is different from the metal contained in the carrier. 
         [0041]    As the catalyst  24 , for example, a catalyst which is comprised of a carrier made of ceria CeO 2  on which 5 wt % of palladium Pd is carried is used.  FIG. 2  shows the results of experiments on the amount of adsorption of NO x  when a model gas is made to run over this catalyst in a reference state (temperature 25° C., pressure 100 kPa). As this model gas, N 2  gas which contains NO and CO is used. Experiments were run for the cases of changing the CO gas concentration (ppm) in various ways with respect to 1000 ppm of NO. 
         [0042]    As will be understood from  FIG. 2 , when the copresent CO gas concentration is lower than the NO concentration, the NO x  adsorption amount increases as the CO gas concentration becomes higher. If the CO gas concentration becomes slightly higher than the NO concentration, even if the CO gas concentration becomes higher somewhat above that, the NO x  adsorption amount no longer increases. Therefore, this catalyst  24  has the property of adsorbing NO x  in the reference state in the presence of carbon monoxide CO. Therefore, hereinafter, this catalyst  24  will be called the “ordinary temperature NO x  adsorption catalyst” which can adsorb NO x  in the reference state. Such an action of adsorption of NO x  at the catalyst  24  similarly occurs even when using as the carrier of the catalyst an oxide of a metal element other than cesium Ce such as Co, Fe, Cu, or Mn or a complex oxide which contains these metal elements and using as the carried metal on the carrier a metal other than palladium Pd such as Cu, Co, Ag, Fe, Pt, or Rh. 
         [0043]      FIG. 3  schematically shows the surface of the carrier  45  which is made of ceria CeO 2 . Further,  FIG. 3  shows the NO x  adsorption mechanism which is believed to occur when the air-fuel ratio of the exhaust gas is made the stoichiometric air-fuel ratio, somewhat rich, or somewhat lean. According to this NO x  adsorption mechanism, the CO which is contained in the exhaust gas pulls out the O of the ceria CeO 2  and is adsorbed on the carrier  45 . On the other hand, the NO which is contained in the exhaust gas reacts with the O which is pulled out from the ceria CeO 2  on the carrier metal  46  made of palladium Pd to become NO 2 . This NO 2  is chemically adsorbed at the cerium Ce. In this way, the NO x  is adsorbed on the carrier  45 . 
         [0044]    CO is strong in ability to attract O, that is, in reducing ability. It is confirmed that CO pulls out the O of ceria CeO 2  and is adsorbed. Further, it is also confirmed that NO 2  is chemically adsorbed at the cerium Ce. As opposed to this, it can be considered that NO reacts with the O which is pulled out from the ceria CeO 2 , but this remains the realm of speculation. Whatever the case, it is certain that CO and NO x  are co-adsorbed and thereby NO x  is adsorbed. 
         [0045]    NO x  is not adsorbed in the form of NO. To make NO be adsorbed, it is necessary to make the NO oxidize to make it NO 2 . In this case, in the past, it had been thought that if the exhaust gas contained CO, this CO would cause the carried metal  46  to be poisoned and as a result the formation of NO 2  would be inhibited, so the NO x  adsorption action would be inhibited. However, with the method which is used in the present invention, the adsorption of NO x  requires the presence of CO. Therefore, the method of adsorption of NO x  which is used in the present invention can be said to be a method which overturns what was thought common knowledge in the past. 
         [0046]    Further, in the NO x  adsorption method which is used the present invention, as explained above, NO 2  is chemically adsorbed at the cerium Ce. If chemically adsorbed in this way, the holding force on the NO 2  becomes stronger and the adsorbed NO 2  is desorbed when the temperature of the catalyst  24  rises to 300° C. or more. If the temperature of the catalyst  24  rises to 300° C. or more, the catalyst  24  is activated. Therefore, in the NO x  adsorption method which is used the present invention, when the catalyst  24  is activated, NO 2  is desorbed. If NO 2  is desorbed when the catalyst  24  is activated, this desorbed NO 2  is removed at the catalyst  24 , therefore, NO x  is not exhausted into the atmosphere at all. 
         [0047]    Now then, the ordinary temperature NO x  adsorption catalyst  24  according to the present invention, as explained up to here, can adsorb the NO x  which is contained in the exhaust gas well in the reference state even in the presence of CO. However, when studying the exhaust purification action of this ordinary temperature NO x  adsorption catalyst  24 , it was learned that this ordinary temperature NO x  adsorption catalyst  24  is not poisoned by CO, but is poisoned by specific hydrocarbons HC. That is, as shown in  FIG. 4 , if the surface of the carried metal  46  is covered by hydrocarbons HC, the NO which is contained in the exhaust gas has trouble being converted to NO 2  and therefore the action of adsorption of NO x  is inhibited. 
         [0048]    In this regard, the surface of the carried metal  46  is gradually covered by deposition or adsorption of hydrocarbons HC, but in this case, the degree by which the surface of the carried metal  46  is covered by the hydrocarbons HC differs considerably depending on the type of the hydrocarbons HC. This will be explained while referring to  FIG. 5 . 
         [0049]    In  FIG. 5 , the HC deposition rate of the ordinate indicates the rate by which hydrocarbons HC deposit or are adsorbed on the surface of the carried metal  46  in the reference state, while the abscissa shows the number of carbon atoms C of the hydrocarbons HC. Now then, in  FIG. 5 , the solid line shows the changes in the HC deposition rate when using n-paraffin as fuel and increasing the number of carbon atoms C of the n-paraffin. As will be understood from the solid line, n-paraffin is a gas in the reference state up to four carbon atoms C and, in this case, the n-paraffin does not deposit on the surface of the carried metal  46 , so the HC deposition rate becomes 0%. As opposed to this, n-paraffin becomes a liquid in the reference state if the number of carbon atoms C becomes five or more. At this time, n-paraffin more easily deposits on the surface of the carried metal  46 , so the HC deposition rate increases. 
         [0050]    On the other hand, if the hydrocarbons HC have branched chains, even if the number of carbon atoms C is  4 , the hydrocarbons HC becomes liquid in the reference state. Accordingly, as shown in  FIG. 5  by the dot and dash line, when the hydrocarbons HC have branched chains, if the number of carbon atoms C is 4, the HC deposition rate becomes higher. In this way, hydrocarbons which become liquid in the reference state are referred to in the Description of the present application as “high boiling point hydrocarbons”. 
         [0051]    On the other hand, even if an olefin were a gas in the reference state, the active chemical adsorption action of the double bond parts make deposition on the surface of the carried metal  46  easier. Therefore, as shown in  FIG. 5  by the broken line, when the hydrocarbons are an olefin, that is, unsaturated hydrocarbons, the HC deposition rate becomes higher. Note that, if the number of carbon atoms C of the olefin becomes the number of carbon atoms resulting in a liquid state in the reference state, as shown by the solid line or dot and dash line, the HC deposition rate becomes further higher. 
         [0052]    In this way, the ordinary temperature NO x  adsorption catalyst  24  according to the present invention has the property of depositing or adsorbing hydrocarbons on the catalyst  24  and has the property in which, when the hydrocarbons in the exhaust gas which flows to the catalyst  24  are unsaturated hydrocarbons or high boiling point hydrocarbons which become liquid in the reference state, these hydrocarbons deposit or are adsorbed on the catalyst  24 , and an action of adsorption of NO x  is inhibited by an action of deposition or adsorption of hydrocarbons at the catalyst  24 . In this case, the amount of adsorption of NO x , as shown in  FIG. 6 , is decreased the more the amount of HC deposition or adsorption at the ordinary temperature NO x  adsorption catalyst  24  is increased. 
         [0053]    If the ordinary temperature NO x  adsorption catalyst  24  is activated, the NO 2  which is deposited or adsorbed at the catalyst  24  is made to desorb, and this desorbed NO 2  and NO x  contained in the exhaust gas are made to be reduced at the catalyst  24 . Therefore, after the catalyst  24  is activated, NO x  will never be exhausted into the atmosphere. As opposed to this, before the catalyst  24  is activated, NO x  cannot be reduced. Therefore, at this time, to keep the NO x  from being exhausted into the atmosphere, it is necessary to make the NO x  which is contained in the exhaust gas be deposited or adsorbed at the catalyst  24  as much as possible. 
         [0054]    Therefore, in the present invention, to make the NO x  deposit or be adsorbed at the catalyst  24  as much as possible until the catalyst  24  is activated, the amount of high boiling point hydrocarbons or the amount of unsaturated hydrocarbons, which are contained in the exhaust gas flowing into the catalyst  24 , is made to lower while maintaining a concentration of carbon monoxide, that is, CO concentration, in the exhaust gas flowing into the catalyst  24  at a concentration of more than the concentration which is required for adsorption of NO x  until the ordinary temperature NO x  adsorption catalyst is activated after engine startup. 
         [0055]    Here, the CO concentration which is required for adsorption of NO x , as explained above, is substantially equal to the NO x  concentration which is contained in the exhaust gas. Therefore, in the present invention, after engine startup and until the ordinary temperature NO adsorption catalyst  24  is activated, the CO concentration in the exhaust gas which flows into the catalyst  24  is made higher than the NO x  concentration in the exhaust gas. Usually, in an internal combustion engine, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the CO concentration in the exhaust gas becomes higher than the NO x  concentration. However, depending on the internal combustion engine, sometimes, after engine startup and until the ordinary temperature NO adsorption catalyst  24  is activated, the CO concentration in the exhaust gas becomes lower than the NO concentration. In this case, by making the air-fuel ratio smaller or by retarding the ignition timing, the CO concentration in the exhaust gas is made higher than the NO x  concentration. 
         [0056]    On the other hand, in the present invention, as explained above, after engine startup and until the ordinary temperature NO adsorption catalyst  24  is activated, the amount of high boiling point hydrocarbons or the amount of unsaturated hydrocarbons, which are contained in the exhaust gas which flows into the catalyst  24 , is lowered. In this case, in the first embodiment according to the present invention, before the catalyst  24  is activated, compared with after the catalyst  24  is activated, fuel with less high boiling point hydrocarbons and unsaturated hydrocarbons is used. Due to this, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the amounts of high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas which flows into the catalyst  24  is lowered. 
         [0057]    Explained in another way, in the first embodiment according to the present invention, as the fuel, at least two types of fuel, that is, the catalyst activation use fuel which is used after the ordinary temperature NOX adsorption catalyst  24  is activated and the catalyst nonactivation use fuel which is used before the ordinary temperature NO x  adsorption catalyst  24  is activated and which has fewer high boiling point hydrocarbons and unsaturated hydrocarbons compared with the catalyst activation use fuel, are used. After engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the catalyst nonactivation use fuel is used as the fuel. 
         [0058]    In this regard, when the catalyst  24  is in an unactivated state, to make the NO x  be adsorbed at the catalyst  24  as much as possible, at the time of nonactivation of the catalyst, it is preferable to use as the fuel a fuel which as little an amount of high boiling point hydrocarbons and unsaturated hydrocarbons as possible. Therefore, in the first embodiment according to the present invention, as the catalyst nonactivation use fuel, compressed natural gas which has methane as its main ingredient is used. 
         [0059]      FIG. 7  shows the ingredients of this compressed natural gas (CNG) in a form compared with the ingredients of gasoline. Note that, in  FIG. 7 , C 1 , C 2 , . . . respectively indicate a hydrocarbon with one carbon atom C, a hydrocarbon with two carbon atoms C . . . As will be understood from  FIG. 7 , compressed natural gas is comprised close to 90% by a hydrocarbon with one carbon atom C, that is, methane. In addition to this methane, if including hydrocarbons with two and three carbon atoms C, that is, ethane and propane, the figure becomes 90% or more. On the other hand, from  FIG. 5 , it will be understood that if using a hydrocarbon with one, two, or three carbon atoms C, that is, methane, ethane, or propane, as the fuel, the HC deposition rate becomes  0 . That is, if using compressed natural gas as the fuel, the HC deposition rate becomes extremely low and therefore almost all of the NO x  which is contained in the exhaust gas becomes adsorbed at the catalyst  24 . 
         [0060]    In fact, it is confirmed by experiments that after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, if using compressed natural gas as fuel and maintaining the air-fuel ratio at the stoichiometric air-fuel ratio or somewhat rich or somewhat lean, the exhaust gas which flows out from the ordinary temperature NO x  adsorption catalyst  24  does not contain almost any NO x . 
         [0061]    Needless to say, this compressed natural gas can be used as fuel even after the ordinary temperature NO x  adsorption catalyst  24  is activated. However, this compressed natural gas requires a large volume for storage of the same amount of fuel compared with liquid fuel. Therefore, in the first embodiment according to the present invention, until the catalyst  24  is activated, as the fuel, compressed natural gas is used, while after the catalyst  24  is activated, as the fuel, liquid fuel is used. Due to this, the storage volume for the fuel is made smaller. 
         [0062]    Incidentally, in the embodiment which is shown in  FIG. 1 , after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the compressed natural gas stored in the fuel tank  21  is injected from the second fuel injector  18 , while after the catalyst  24  is activated, the plant-derived alcohol fuel or other liquid fuel which is stored in the fuel tank  19  is injected from the first fuel injector  17 . Note that, in this embodiment, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the ordinary temperature NOX adsorption catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
         [0063]      FIG. 8  shows the injection control routine which is executed in the embodiment which is shown in  FIG. 1 . 
         [0064]    Referring to  FIG. 8 , first, at step  50 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  51  where the amount of injection of the catalyst nonactivation use fuel, that is, compressed natural gas, is calculated. Next, at step  52 , the compressed natural gas is injected from the second fuel injector  18 . At this time, the air-fuel ratio is controlled based on the output signal of the air-fuel ratio sensor  25  to the stoichiometric air-fuel ratio or somewhat rich. 
         [0065]    As opposed to this, when it is judged at step  50  that TC&gt;TC 0 , that is, when it is judged that the catalyst  24  is activated, the routine proceeds to step  53  where the amount of injection of the catalyst activation use fuel, that is, plant-derived alcohol fuel etc., is calculated. Next, at step  54 , the plant-derived alcohol fuel etc. is injected from the first fuel injector  17 . At this time, the air-fuel ratio is controlled based on the output signal of the air-fuel ratio sensor  25  to the stoichiometric air-fuel ratio. 
         [0066]      FIG. 9  shows the injection control routine in the case where the compressed natural gas is used as the fuel irrespective of whether before activation of the catalyst  24  or after activation. In this case, in  FIG. 1 , the first fuel injector  17  does not have to be provided. 
         [0067]    Referring to  FIG. 9 , first, at step  60 , the amount of injection of compressed natural gas is calculated. Next, at step  61 , the compressed natural gas is injected from the second fuel injector  18 . Note that, in this case as well, before the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
         [0068]      FIG. 10  shows another embodiment. In this embodiment, as shown in  FIG. 10 , an HC adsorbent  70  which adsorbs high boiling point hydrocarbons and unsaturated hydrocarbons is arranged in the engine exhaust passage upstream of the ordinary temperature NO x  adsorption catalyst  24 . As this HC adsorbent  70 , an HC adsorbent which has the property of adsorbing high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in exhaust gas after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated and releasing the adsorbed high boiling point hydrocarbons and unsaturated hydrocarbons after the ordinary temperature NOX adsorption catalyst  24  is activated is used. 
         [0069]    Therefore, in this embodiment, even if the exhaust gas contains high boiling point hydrocarbons and unsaturated hydrocarbons, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, these high boiling point hydrocarbons and unsaturated hydrocarbons are adsorbed at the HC adsorbent  70 . Therefore, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas are kept from flowing into the ordinary temperature NO x  adsorption catalyst  24  and therefore a good action of adsorption of NO x  at the catalyst  24  is secured. 
         [0070]    Further, instead of an HC adsorbent  70 , it is possible to use an HC adsorption catalyst which has the property of adsorbing high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in exhaust gas after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated and releasing the adsorbed high boiling point hydrocarbons and unsaturated hydrocarbons after the ordinary temperature NO x  adsorption catalyst  24  is activated. 
         [0071]    Therefore, in this case as well, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas are adsorbed at the HC adsorption catalyst  70  and therefore a good action of adsorption of NO x  at the ordinary temperature NO x  adsorption catalyst  24  is secured. 
         [0072]    As this HC adsorption catalyst  70 , it is possible to use the same catalyst as the ordinary temperature NO x  adsorption catalyst  24 . In this case, the additional adsorption catalyst  70  is arranged in the engine exhaust passage in addition to the adsorption catalyst  24 . Further, when using as the HC adsorption catalyst  70  a catalyst the same as the ordinary temperature NO x  adsorption catalyst  24 , as shown in  FIG. 11 , the additional adsorption catalyst  70  and the ordinary temperature NO x  adsorption catalyst  24  can be formed integrally. 
         [0073]    In the embodiment which is shown in  FIG. 10  and  FIG. 11 , even if the high boiling point hydrocarbons and unsaturated hydrocarbons are contained in the exhaust gas, these high boiling point hydrocarbons and unsaturated hydrocarbons are adsorbed at the HC adsorbent or the additional adsorption catalyst  70 . Therefore, in the embodiment shown in  FIG. 10  and  FIG. 11 , after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, as the fuel, in addition to compressed natural gas which does not contain almost any high boiling point hydrocarbons and unsaturated hydrocarbons, a fuel which contains large amounts of high boiling point hydrocarbons and unsaturated hydrocarbons, for example, a plant-derived alcohol fuel, gasoline, or a mixed fuel of a plant-derived alcohol fuel and gasoline can be used. 
         [0074]    That is, in the embodiment shown in  FIG. 10  and  FIG. 11 , as the fuel, it is possible to use at least two types of fuel of the catalyst activation use fuel which is used after the ordinary temperature NOX adsorption catalyst  24  is activated and the catalyst nonactivation use fuel which is used until the ordinary temperature NO x  adsorption catalyst  24  is activated after engine startup and which has less high boiling point hydrocarbons and unsaturated hydrocarbons compared with the catalyst activation use fuel, it is possible to use, as the fuel, only compressed natural gas which has methane as its main ingredient, or it is possible to use, as the fuel, only a fuel which has more high boiling point hydrocarbons and unsaturated hydrocarbons compared with compressed natural gas. 
         [0075]      FIG. 12  to  FIG. 22  show various embodiments which are able to use various fuels in this way. 
         [0076]    Referring to  FIG. 12 , in this embodiment, a bypass passage  81  is provided with respect to a main exhaust passage  80  which connects an outlet of the exhaust manifold  23  and an inlet of the ordinary temperature NO x  adsorption catalyst  24 , and flow switching valves  82  and  83  are arranged respectively at the inlet part and outlet part of the bypass passage  81 . That is, in this embodiment, the bypass passage  81  is juxtaposed to the engine exhaust passage upstream of the ordinary temperature NO x  adsorption catalyst  24 . Further, in this embodiment, an adsorbent which adsorbs the high boiling point hydrocarbons and unsaturated hydrocarbons or the additional adsorbent  70  is arranged in bypass passage  81 . 
         [0077]    In this embodiment, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, as shown in  FIG. 12  by the solid lines, the flow control valve  82  opens the inlet part of the bypass passage  81  while the flow control valve  83  opens the outlet part of the bypass passage  81 . Due to this, the exhaust gas exhausted from the engine passes through the bypass passage  81  and is fed to the ordinary temperature NO x  adsorption catalyst  24 . At this time, the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas are adsorbed by the adsorbent or adsorption catalyst  70 . Next, when the ordinary temperature NO x  adsorption catalyst  24  is activated, the adsorbed high boiling point hydrocarbons and unsaturated hydrocarbons are made to desorb from the adsorbent or adsorption catalyst  70 . 
         [0078]    On the other hand, if the ordinary temperature NO x  adsorption catalyst is activated, after that, as shown in  FIG. 12  by the broken lines, the flow switching valve  82  closes the inlet part of the bypass passage  81  and the flow switching valve  83  closes the outlet part of the bypass passage  81 . As a result, the exhaust gas which is exhausted from the engine is fed to the ordinary temperature NO x  adsorption catalyst  24  without routing the bypass passage  81 . 
         [0079]      FIG. 13  shows an injection control routine for the embodiment which is shown in  FIG. 12 . 
         [0080]    Referring to  FIG. 13 , first, at step  90 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 +constant temperature α. 
         [0081]    When TC≦TC 0 +α, the routine proceeds to step  91  where the amount of injection of the catalyst nonactivation use fuel, that is, for example, compressed natural gas, is calculated. Next, at step  92 , for example, compressed natural gas is injected from the second fuel injector  18 . 
         [0082]    At this time, the air-fuel ratio is controlled based on the output signal of the air-fuel ratio sensor  25  to the stoichiometric air-fuel ratio or somewhat rich. Next, at step  93 , the flow switching valves  82  and  83  are held at the positions which are shown by the solid lines in  FIG. 12 . 
         [0083]    As opposed to this, when it is judged at step  90  that TC&gt;TC 0 +α, the routine proceeds to step  94  where the amount of injection of the catalyst activation use fuel, for example, the plant-derived alcohol fuel, is calculated. Next, at step  95 , for example, the plant-derived alcohol fuel is injected from the first fuel injector  17 . At this time, the air-fuel ratio is controlled based on the output signal of the air-fuel ratio sensor  25  to the stoichiometric air-fuel ratio. Next, at step  96 , the flow switching valves  82  and  83  are held at the positions which are shown by the broken lines in  FIG. 12   
         [0084]      FIG. 14  shows the injection control routine in the case where only compressed natural gas, only a plant-derived alcohol fuel, only gasoline, or only a mixed fuel of a plant-derived alcohol and gasoline is used as the fuel irrespective of whether before activation of the catalyst  24  or after activation thereof. 
         [0085]    Referring to  FIG. 14 , first, at step  100 , the amount of injection of fuel is calculated. Next, at step  101 , fuel injection processing is performed. Next, at step  102 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 +constant temperature α. When TC≦TC 0 +α, the routine proceeds to step  103  where the flow switching valves  82  and  83  are held at the positions which are shown in  FIG. 12  by the solid lines. 
         [0086]    As opposed to this, when it is judged at step  102  that TC&gt;TC 0 +α, the routine proceeds to step  104  where the flow switching valves  82  and  83  are held at the positions which are shown in  FIG. 12  by the broken lines. Note that, in this case as well, before the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
         [0087]      FIG. 15  and  FIG. 16  show an embodiment where after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the ignition timing is retarded so as to make the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas burn before flowing into the ordinary temperature NO x  adsorption catalyst  24  and thereby keep the high boiling point hydrocarbons and unsaturated hydrocarbons from flowing into the ordinary temperature NO x  adsorption catalyst  24 . 
         [0088]    That is, if retarding the ignition timing, the afterburn period becomes longer, so the high boiling point hydrocarbons and unsaturated hydrocarbons which are made to burn in the combustion chamber  5  and the exhaust passage leading to the catalyst  24  are increased and therefore the amount of high boiling point hydrocarbons and unsaturated hydrocarbons which flows into the ordinary temperature NO x  adsorption catalyst  24  can be reduced. 
         [0089]    On the other hand, in the internal combustion engine which is shown in  FIG. 1 , a reference ignition timing corresponding to the operating state of the engine after completion of engine warmup is preset. Therefore, in this embodiment, to make the high boiling point hydrocarbons and unsaturated hydrocarbons burn in the combustion chamber  5  or in the exhaust passage leading to the catalyst  24 , after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the ignition timing is retarded from the reference ignition timing. 
         [0090]      FIG. 15  shows an injection control routine for working this embodiment. 
         [0091]    Referring to  FIG. 15 , first, at step  110 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  111  where the amount of injection of the catalyst nonactivation use fuel, for example, compressed natural gas, is calculated. Next, at step  112 , for example, compressed natural gas is injected from the second fuel injector  18 . Next, at step  113 , the ignition timing is retarded from the reference ignition timing. 
         [0092]    As opposed to this, when it is judged at step  110  that TC&gt;TC 0 , that is, when it is judged that the catalyst  24  is activated, the routine proceeds to step  114  where the amount of injection of the catalyst activation use fuel, for example, the plant-derived alcohol fuel, is calculated. Next, at step  115 , for example, plant-derived alcohol fuel is injected from the first fuel injector  17 . 
         [0093]      FIG. 16  shows the injection control routine in the case where only compressed natural gas, only a plant-derived alcohol fuel, only gasoline, or only a mixed fuel of a plant-derived alcohol and gasoline is used as the fuel irrespective of whether before activation of the catalyst  24  or after activation thereof. 
         [0094]    Referring to  FIG. 16 , first, at step  120 , the amount of injection of fuel is calculated. Next, at step  121 , fuel injection processing is performed. Next, at step  122 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  123  where the ignition timing is retarded from the reference ignition timing. Note that, in the case shown in each of  FIG. 15  and  FIG. 16  as well, before the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
         [0095]      FIG. 17  to  FIG. 19  show an embodiment wherein after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the opening timing of the exhaust valve  9  is retarded to thereby make the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas burn before flowing into the ordinary temperature NO x  adsorption catalyst  24  and thereby keep the high boiling point hydrocarbons and unsaturated hydrocarbons from flowing into the ordinary temperature NO x  adsorption catalyst  24 . 
         [0096]    That is, if retarding the opening timing of the exhaust valve  9 , the time period during which the high boiling point hydrocarbons and unsaturated hydrocarbons are made to burn in the combustion chamber  5  becomes longer. As a result, the high boiling point hydrocarbons and unsaturated hydrocarbons which are made to burn in the combustion chamber  5  are increased and therefore the amount of high boiling point hydrocarbons and unsaturated hydrocarbons which flow into the ordinary temperature NO x  adsorption catalyst  24  can be reduced. 
         [0097]    Therefore, in this embodiment, to make the high boiling point hydrocarbons and unsaturated hydrocarbons burn in the combustion chamber  5 , after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the opening timing of the exhaust valve  9  is retarded from a preset opening timing. Note that, in this embodiment, to control the opening timing of the exhaust valve  9 , as shown in  FIG. 17 , a variable valve timing mechanism  130  is provided for the exhaust valve  9 . 
         [0098]      FIG. 18  shows an injection control routine for working this embodiment. 
         [0099]    Referring to  FIG. 18 , first, at step  140 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  141  where the amount of injection of the catalyst nonactivation use fuel, for example, compressed natural gas, is calculated. Next, at step  142 , for example, compressed natural gas is injected from the second fuel injector  18 . Next, at step  143 , the opening timing of the exhaust valve  9  is retarded from the reference opening timing. 
         [0100]    As opposed to this, when it was judged at step  140  that TC&gt;TC 0 , that is, when it is judged that the catalyst  24  is activated, the routine proceeds to step  144  where the amount of injection of the catalyst activation use fuel, for example, plant-derived alcohol fuel, is calculated. Next, at step  145 , for example, plant-derived alcohol fuel is injected from the first fuel injector  17 . 
         [0101]      FIG. 19  shows the injection control routine in the case where only compressed natural gas, only a plant-derived alcohol fuel, only gasoline, or only a mixed fuel of a plant-derived alcohol and gasoline is used as the fuel irrespective of whether before activation of the catalyst  24  or after activation thereof. 
         [0102]    Referring to  FIG. 19 , first, at step  150 , the amount of injection of fuel is calculated. Next, at step  151 , fuel injection processing is performed. Next, at step  152 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  153  where the opening timing of the exhaust valve  9  is retarded from the reference opening timing. Note that, in the case shown in each of  FIG. 18  and  FIG. 19  as well, before the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
         [0103]      FIG. 20  shows still another embodiment. Referring to  FIG. 20 , in this embodiment, the surge tank  12  and the exhaust manifold  23  are connected through an exhaust gas recirculation (EGR) passage  160 , and an EGR control valve  161  is arranged in the EGR passage  160 . In this embodiment, after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the EGR rate is made to increase so that the high boiling point hydrocarbons and unsaturated hydrocarbons which are contained in the exhaust gas are made to burn before flowing into the ordinary temperature NO x  adsorption catalyst  24  and thereby the high boiling point hydrocarbons and unsaturated hydrocarbons are kept from flowing into the ordinary temperature NO x  adsorption catalyst  24 . 
         [0104]    That is, if recirculating the exhaust gas, the high boiling point hydrocarbons and unsaturated hydrocarbons which are exhausted into the exhaust manifold  23  are recirculated into the combustion chamber  5 . These high boiling point hydrocarbons and unsaturated hydrocarbons are made to burn in the combustion chamber  5 . Therefore, if increasing the EGR rate, the high boiling point hydrocarbons and unsaturated hydrocarbons which are made to burn in the combustion chamber  5  increase and therefore the amount of high boiling point hydrocarbons and unsaturated hydrocarbons which flow into the ordinary temperature NO x  adsorption catalyst  24  can be reduced. 
         [0105]    On the other hand, in the internal combustion engine which is shown in  FIG. 20 , a reference EGR rate corresponding to the operating state of the engine after completion of engine warmup is preset. Therefore, in this embodiment, to make the high boiling point hydrocarbons and unsaturated hydrocarbons burn in the combustion chamber  5 , after engine startup and until the ordinary temperature NO x  adsorption catalyst  24  is activated, the EGR rate is made to increase over the reference EGR rate. 
         [0106]      FIG. 21  shows an injection control routine for working this embodiment. 
         [0107]    Referring to  FIG. 21 , first, at step  170 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  171  where the amount of injection of the catalyst nonactivation use fuel, for example, compressed natural gas, is calculated. Next, at step  172 , for example, compressed natural gas is injected from the second fuel injector  18 . Next, at step  173 , the EGR rate is made to increase over the reference EGR rate. 
         [0108]    As opposed to this, when it is judged at step  170  that TC&gt;TC 0 , that is, when it is judged that the catalyst  24  is activated, the routine proceeds to step  174  where the amount of injection of the catalyst activation use fuel, for example, the plant-derived alcohol fuel, is calculated. Next, at step  175 , for example, a plant-derived alcohol fuel is injected from the first fuel injector  17 . 
         [0109]      FIG. 22  shows an injection control routine in the case where only compressed natural gas, only a plant-derived alcohol fuel, only gasoline, or only a mixed fuel of a plant-derived alcohol and gasoline is used as the fuel irrespective of whether before activation of the catalyst  24  or after activation thereof. 
         [0110]    Referring to  FIG. 22 , first, at step  180 , the amount of injection of fuel is calculated. Next, at step  181 , the fuel injection processing is performed. Next, at step  182 , it is judged based on the output signal of the temperature sensor  26  whether or not the temperature TC of the ordinary temperature NO x  adsorption catalyst  24  exceeds the activation temperature TC 0 . When TC≦TC 0 , that is, when the catalyst  24  is not activated, the routine proceeds to step  183  where the EGR rate is made to increase over the reference EGR rate. Note that, in the case shown in either of  FIG. 21  and  FIG. 22  as well, before the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio or somewhat rich, while after the catalyst  24  is activated, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           5  . . . combustion chamber 
           6  . . . spark plug 
           7  . . . intake valve 
           9  . . . exhaust valve 
           17  . . . first fuel injector 
           18  . . . second fuel injector 
           24  . . . ordinary temperature NO x  adsorption catalyst