Patent Publication Number: US-11028757-B2

Title: Exhaust purification system of internal combustion engine

Description:
FIELD 
     The present invention relates to an exhaust purification system of an internal combustion engine. 
     BACKGROUND 
     Known in the past has been an internal combustion engine able to perform fuel cut control stopping a feed of fuel to a combustion chamber in the state where the internal combustion engine is operating, for example, at the time of deceleration of a vehicle mounting the internal combustion engine. In addition, an exhaust purification system of an internal combustion engine provided with an exhaust purification catalyst supporting palladium or another precious metal in an exhaust passage of the internal combustion engine is known. It is known that in such an exhaust purification catalyst, if fuel cut control is performed in a state of a high temperature of the exhaust purification catalyst, the precious metal supported on the exhaust purification catalyst is liable to deteriorate (for example, PTL 1). 
     Therefore, in the exhaust purification system described in PTL 1, an exhaust shut valve is provided in an exhaust passage of the internal combustion engine and an EGR mechanism returning part of the exhaust gas flowing through the exhaust passage to the intake passage is provided. When the temperature of the exhaust purification catalyst is high, the throttle valve is closed, the shut valve is closed, and the EGR mechanism is used to make part of the exhaust gas flow into the intake passage. Due to this, even during a fuel cut operation, only EGR gas flows into the engine body, therefore the concentration of oxygen in the exhaust gas can be kept low and accordingly it is considered that the precious metal supported at the exhaust purification catalyst can be kept from deteriorating. In addition, it is considered that by closing the shut valve, the pumping loss becomes greater and accordingly the driver can obtain a feeling of deceleration. 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] Japanese Unexamined Patent Publication No. 2018-009535 
       
    
     SUMMARY 
     Technical Problem 
     In this regard, in the exhaust purification system described in PTL 1, high concentration EGR gas is fed into the combustion chamber during a fuel cut operation. Therefore, even when returning to normal operation after the end of a fuel cut operation, EGR gas fills the inside of the combustion chamber and thus combustion cannot be started immediately even if feeding fuel into the combustion chamber. Therefore, in the exhaust purification system described in PTL 1, it takes time to return to normal operation after a fuel cut operation. In addition, in the exhaust purification system described in PTL 1, it becomes necessary to set an exhaust shut valve inside the exhaust passage, therefore the manufacturing cost increases. Therefore, there is room for improvement in the exhaust purification catalyst described in PTL 1. 
     On the other hand, according to research of the inventors, it was learned that one reason for deterioration of the precious metal is the reaction on the precious metal between the HC adsorbed on the precious metal and the oxygen flowing into the exhaust purification catalyst during fuel cut control, and thus local generation of heat by the precious metal. 
     In consideration of the above technical problem, an object of the present disclosure is to provide an exhaust purification system able to keep the precious metal from locally generating heat and keep the precious metal from deteriorating. 
     Solution to Problem 
     The present invention was made so as to solve the above problem and has as its gist the following. 
     (1) An exhaust purification system of an internal combustion engine, comprising: an exhaust purification catalyst supporting a precious metal and able to store oxygen; and a control device controlling an amount of fuel fed to a combustion chamber, wherein
         when a predetermined condition for performing a fuel cut operation stands, the control device is configured to perform fuel feed control in which fuel is temporarily fed to the combustion chamber so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is a rich air-fuel ratio richer than the stoichiometric air-fuel ratio, then start fuel cut control stopping the feed of fuel to the combustion chamber in the state the internal combustion engine is operating.       

     (2) The exhaust purification system of the internal combustion engine according to above (1), wherein the control device is configured to control the amount of feed of fuel to the combustion chamber so that the total amount of feed of fuel during the fuel feed control is greater, when the oxygen storage amount of the exhaust purification catalyst when the condition for performing a fuel cut operation stands is relatively large, compared to when it is relatively small. 
     (3) The exhaust purification system of the internal combustion engine according to above (2), wherein the control device is configured to control the amount of feed of fuel to the combustion chamber so that the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst during the fuel feed control is greater, when the oxygen storage amount of the exhaust purification catalyst when the condition for performing a fuel cut operation stands is relatively large, compared to when it is relatively small. 
     (4) The exhaust purification system of the internal combustion engine according to any one of above (1) to (3), wherein the control device is configured to perform the fuel cut control without performing the fuel feed control even if the condition for performing the fuel cut operation stands, if the oxygen storage amount of the exhaust purification catalyst when the condition for performing the fuel cut operation stands is smaller than a predetermined reference oxygen storage amount the maximum storable oxygen amount of the exhaust purification catalyst and smaller than greater than zero. 
     (5) The exhaust purification system of the internal combustion engine according to any one of above (1) to (4), wherein the control device is configured to control the amount of feed of fuel to the combustion chamber so that the total amount of feed of fuel during the fuel feed control becomes smaller, when the degree of deterioration of the exhaust purification catalyst when the condition for performing a fuel cut operation stands is relatively large, compared to when it is relatively small. 
     (6) The exhaust purification system of the internal combustion engine according to any one of above (1) to (5), wherein the control device is configured to perform the fuel cut control without performing the fuel feed control if the amount of adsorption of hydrocarbons at the exhaust purification catalyst when the condition for performing the fuel cut operation stands is equal to or greater than a predetermined reference adsorption amount. 
     (7) The exhaust purification system of the internal combustion engine according to above (6), wherein the control device is configured to control the feed of fuel to the combustion chamber so that the greater the amount of adsorption of hydrocarbons at the exhaust purification catalyst, the smaller the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst during the fuel feed control becomes, if the amount of adsorption of hydrocarbons at the exhaust purification catalyst when the condition for performing the fuel cut operation stands is less than the reference adsorption amount. 
     Advantageous Effects of Invention 
     According to the present disclosure, an exhaust purification system able to keep precious metal from locally generating heat and keep the precious metal from deteriorating is provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view schematically showing an internal combustion engine in which an exhaust purification system according to one embodiment is used. 
         FIG. 2  is a schematic cross-sectional view schematically showing a surface of a support of the exhaust purification catalyst. 
         FIGS. 3A and 3B  are schematic cross-sectional views similar to  FIG. 2  schematically showing a surface of a support of the exhaust purification catalyst. 
         FIG. 4  is a time chart of an FC flag, an output of the internal combustion engine, an air-fuel ratio of exhaust gas, and an oxygen storage amount of the exhaust purification catalyst when fuel cut control is performed. 
         FIG. 5  is a flow chart showing a control routine of flag setting processing for setting the FC flag. 
         FIG. 6  is a flow chart showing a control routine of fuel cut processing for performing fuel cut control. 
         FIG. 7  is a view showing a relationship between the oxygen storage amount of the exhaust purification catalyst and a rich degree of the air-fuel ratio of the exhaust gas. 
         FIG. 8  is a view showing a relationship between the oxygen storage amount of the exhaust purification catalyst and time performing the fuel feed control. 
         FIG. 9  is a flow chart showing a control routine of fuel cut processing for performing fuel cut control. 
         FIG. 10  is a view showing a relationship between a degree of deterioration of the exhaust purification catalyst and a rich degree of the air-fuel ratio of the exhaust gas. 
         FIG. 11  is a view showing a relationship between an amount of adsorption of unburned HC at the exhaust purification catalyst and a rich degree of the air-fuel ratio of the exhaust gas. 
         FIG. 12  is a flow chart showing a control routine of fuel cut processing. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Below, referring to the drawings, embodiments of the present invention will be explained in detail. Note that, in the following explanation, similar component elements are assigned the same reference numerals. 
     First Embodiment 
     &lt;&lt;Explanation of Internal Combustion Engine as a Whole&gt;&gt; 
       FIG. 1  is a view which schematically shows an internal combustion engine in which an exhaust purification system according to a first embodiment of the present invention is used. As shown in  FIG. 1 , an internal combustion engine  1  includes an engine body  2 , a cylinder block  3 , a piston  4  which reciprocates inside the cylinder block  3 , a cylinder head  5  which is fastened to the cylinder block  3 , an intake valve  6 , an intake port  7 , an exhaust valve  8 , and an exhaust port  9 . A combustion chamber  10  is formed between the piston  4  and the cylinder head  5 . The intake valve  6  opens and closes the intake port  7 , while the exhaust valve  8  opens and closes the exhaust port  9 . Further, the engine body  2  is provided with a variable valve timing mechanism  28  which controls a valve timing of the intake valve  6 . Note that, the engine body  2  may be provided with a variable valve timing mechanism which controls a valve timing of the exhaust valve  8 . 
     As shown in  FIG. 1 , a spark plug  11  is arranged at a center part of an inside wall surface of the cylinder head  5 , while a fuel injector  12  is arranged at a side part of the inner wall surface of the cylinder head  5 . The spark plug  11  is configured to generate a spark in accordance with an ignition signal. Further, the fuel injector  12  injects a predetermined amount of fuel into the combustion chamber  10  in accordance with an injection signal. Note that, the fuel injector  12  may also be arranged so as to inject fuel into the intake port  7 , as long as able to supply fuel into the combustion chamber  10 . 
     The intake port  7  of each cylinder is connected to a surge tank  14  through a corresponding intake runner  13 , while the surge tank  14  is connected to an air cleaner  16  through an intake pipe  15 . The intake port  7 , intake runner  13 , surge tank  14 , and intake pipe  15  form an intake passage. Further, inside the intake pipe  15 , a throttle valve  18  which is driven by a throttle valve drive actuator  17  is arranged. 
     On the other hand, the exhaust port  9  of each cylinder is connected to an exhaust manifold  19 , which is connected to an upstream side casing  21  which houses an exhaust purification catalyst  20 . The upstream side casing  21  is connected to an exhaust pipe  22 . The exhaust port  9 , exhaust manifold  19 , upstream side casing  21  and exhaust pipe  22  form an exhaust passage. 
     In addition, the internal combustion engine  1  is provided with an electronic control unit (ECU)  31 . ECU  31  is comprised of a digital computer which is provided with components which are connected together through a bidirectional bus  32  such as a RAM (random access memory)  33 , ROM (read only memory)  34 , CPU (microprocessor)  35 , input port  36 , and output port  37 . 
     In the intake pipe  15 , an air flow meter  39  is arranged for detecting the flow rate of air which flows through the intake pipe  15 . At the throttle valve  18 , a throttle valve opening sensor  40  is arranged for detecting an opening degree of the throttle valve  18 . In addition, at the exhaust manifold  19  in the upstream side of the exhaust purification catalyst  20  in the flow direction of exhaust, an upstream side air-fuel ratio sensor  41  is provided, which detects the air-fuel ratio of the exhaust gas flowing through the exhaust manifold  19  (that is, the exhaust gas flowing into the exhaust purification catalyst  20 ). Further, in the exhaust pipe  22  in the downstream side of the exhaust purification catalyst  20  in the flow direction of exhaust, a downstream side air-fuel ratio sensor  42  is provided, which detects the air-fuel ratio of the exhaust gas flowing through the exhaust pipe  22  (that is, the exhaust gas flowing out from the exhaust purification catalyst  20  and flows into the downstream side exhaust purification catalyst  24 ). The outputs of the air flow meter  39 , throttle opening sensor  40 , upstream side air-fuel ratio sensor  40 , and downstream side air-fuel ratio sensor  41  are input through the corresponding AD converters  38  to the input port  36 . 
     Further, a load sensor  44  generating an output voltage proportional to the amount of depression of the accelerator pedal  43  is connected to the accelerator pedal  43 . The output voltage of the load sensor  44  is input through a corresponding AD converter  38  to the input port  36 . The crank angle sensor  45 , for example, generates an output pulse every time the crank shaft rotates by 15 degrees. This output pulse is input to the input port  36 . At the CPU  35 , the engine speed is calculated from the output pulse of this crank angle sensor  45 . 
     On the other hand, the output port  37  is connected through corresponding drive circuits  46  to the spark plugs  11 , fuel injectors  12 , throttle valve drive actuator  17 , and variable valve timing mechanism  28 . Therefore, ECU  31  functions as a control device for controlling an ignition timing of the ignition plug  11 , fuel injection timing or amount from the fuel injector  12 , opening degree of the throttle valve  18  and valve timing of the intake valve  6 . 
     In the present embodiment, the control device controls an air-fuel ratio of the exhaust gas flowing out from the engine body  2 , i.e., the exhaust gas flowing into the exhaust purification catalyst  20 , by adjusting the fuel injection amount from the fuel injector  12 . When changing the air-fuel ratio of the exhaust gas flowing out from the engine body  2  to the rich side, the fuel injection amount from the fuel injector  12  is increased, while when changing the air-fuel ratio of the exhaust gas flowing out from the engine body  2  to the lean side, the fuel injection amount from the fuel injector  12  is decreased. 
     &lt;&lt;Explanation of Exhaust Purification Catalyst&gt;&gt; 
     The exhaust purification catalyst  20  is a three-way catalyst which has an oxygen storage ability. Specifically, the exhaust purification catalyst is a three-way catalyst which comprises a carrier made of ceramic on which a precious metal (for example, platinum Pt) having a catalyst effect and a substance having an oxygen storage ability (for example, ceria CeO 2 ) are carried. A three-way catalyst has the function of simultaneously purifying unburned HC, CO and NO X  when the air-fuel ratio of the exhaust gas flowing into the three-way catalyst is maintained at the stoichiometric air-fuel ratio. In addition, when the exhaust purification catalyst  20  stores a certain extent of oxygen, the unburned HC and CO and NO X  are simultaneously purified even if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  somewhat deviates from the stoichiometric air-fuel ratio to the rich side or lean side. 
     Accordingly, if the exhaust purification catalyst  20  has an oxygen storage ability, that is, if the oxygen storage amount of the exhaust purification catalyst  20  is less than the maximum storage oxygen amount, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is somewhat leaner than the stoichiometric air-fuel ratio, the excess oxygen contained in the exhaust gas is stored in the exhaust purification catalyst  20 . Therefore, the surfaces of the exhaust purification catalyst  20  are maintained at the stoichiometric air-fuel ratio. As a result, on the surfaces of the exhaust purification catalyst  20 , the unburned HC, CO and NO X  are simultaneously purified. At this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst  20  is the stoichiometric air-fuel ratio. 
     On the other hand, if exhaust purification catalyst  20  can release oxygen, that is, the oxygen storage amount of the exhaust purification catalyst  20  is more than zero, when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is somewhat richer than the stoichiometric air-fuel ratio, the oxygen which is insufficient for reducing the unburned HC and CO contained in the exhaust gas, is released from the exhaust purification catalyst  20 . Therefore, the surfaces of the exhaust purification catalyst  20  are maintained at the stoichiometric air-fuel ratio. As a result, on the surfaces of the exhaust purification catalyst  20 , the unburned HC, CO and NO X  are simultaneously purified. At this time, the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst is the stoichiometric air-fuel ratio. 
     In this way, when the exhaust purification catalyst  20  stores a certain extent of oxygen, even if the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  deviates somewhat from the stoichiometric air-fuel ratio to the rich side or lean side, the unburned HC, CO and NO X  are simultaneously purified and the air-fuel ratio of the exhaust gas flowing out from the exhaust purification catalyst  20  is the stoichiometric air-fuel ratio. 
     &lt;&lt;Deterioration of Catalyst During Fuel Cut Control&gt;&gt; 
     In this regard, in the internal combustion engine  1  according to the present embodiment, when the vehicle mounting the internal combustion engine  1  is decelerating, fuel cut control, in which the injection of fuel from the fuel injector  12  is stopped in the state while the internal combustion engine  1  is operating, is performed. If such fuel cut control is performed, the air flowing into the combustion chamber  10  flows out as is from the combustion chamber  10 , therefore air flows into the exhaust purification catalyst  20 . 
     If air flows into the exhaust purification catalyst  20  in this way, the exhaust purification catalyst  20  deteriorates. One of the reasons why the exhaust purification catalyst  20  deteriorates has been elucidated, therefore below the reason for deterioration will be explained while referring to  FIG. 2 . 
       FIG. 2  is a schematic cross-sectional view schematically showing a surface of a support of the exhaust purification catalyst  20 . In the example shown in  FIG. 2 , a support including alumina (A 1   2 O 3 ) supports the precious metal palladium (Pd) and ceria (CeO 2 ) functioning as an oxygen storing agent. 
     As explained above, exhaust gas discharged from the engine body  2  and flowing into the exhaust purification catalyst  20  contains unburned HC, CO, and NOx. Among these constituents, unburned HC is adsorbed on the precious metal when the temperature of the exhaust purification catalyst  20  is low. 
     If in this way fuel cut control is performed and thus a large amount of oxygen flows into the exhaust purification catalyst  20  in the state where unburned HC is adsorbed on the precious metal, part of the inflowing oxygen reacts with the unburned HC adsorbed on the precious metal. Due to this reaction, carbon dioxide and water are generated. Such a reaction is an exothermic reaction, therefore the precious metal is locally heated. 
     Almost all of the heat of reaction at this time is used for heating the precious metal, therefore the temperature of the precious metal becomes extremely high. As a result, the precious metal is sintered. If the precious metal is sintered, the total surface area of the precious metal becomes smaller. As a result, the catalytic action due to the precious metal falls, that is, the exhaust purification catalyst  20  deteriorates. 
     &lt;&lt;Suppression of Deterioration of Catalyst&gt;&gt; 
     If considering this mechanism of deterioration of the exhaust purification catalyst  20 , to keep the exhaust purification catalyst  20  from deteriorating during fuel cut control, it may be considered to keep the unburned HC adsorbed at the precious metal and the oxygen from rapidly reacting during fuel cut control. Below, referring to  FIGS. 3A and 3B , the mechanism for keeping the exhaust purification catalyst  20  from deteriorating during fuel cut operation will be explained. 
       FIGS. 3A and 3B  are schematic cross-sectional views similar to  FIG. 2  schematically showing the surface of the support of the exhaust purification catalyst  20 .  FIG. 3A  shows the state of the support surface when exhaust gas of an air-fuel ratio richer than the stoichiometric air-fuel ratio (below, also referred to as a “rich air-fuel ratio”) flows into the exhaust purification catalyst  20 , while  FIG. 3B  shows the state of the support surface when air flows into the exhaust purification catalyst  20  due to fuel cut control. 
     As shown in  FIG. 3A , if making the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  a rich air-fuel ratio, the oxygen partial pressure in the exhaust gas is low, therefore the oxygen stored in the oxygen storing agent of the exhaust purification catalyst  20  is released into the exhaust gas. The oxygen released into the exhaust gas reacts with the unburned HC or CO in the exhaust gas and the oxygen partial pressure in the exhaust gas remains low. As a result, the oxygen storage amount of the oxygen storing agent decreases and the amount of oxygen which the oxygen storing agent can store increases. 
     In this way, if fuel cut control is started in the state where the amount of oxygen which the oxygen storing agent can store is increased, as shown in  FIG. 3B , part of the oxygen flowing into the exhaust purification catalyst  20  is stored in the oxygen storing agent. As a result, the amount of oxygen reacting with the unburned HC adsorbed at the precious metal becomes smaller and, accordingly, the precious metal is no longer excessively raised in temperature. Therefore, the precious metal is kept from sintering and the exhaust purification catalyst  20  is kept from deteriorating. 
     &lt;&lt;Control at Time of Fuel Cut Operation&gt;&gt; 
     Therefore, in the present embodiment, when a predetermined condition for performing fuel cut operation stands, the control device starts fuel cut control after performing fuel feed control temporarily feeding fuel to the combustion chamber  10  so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is a rich air-fuel ratio. Further, in the present embodiment, if the oxygen storage amount of the exhaust purification catalyst  20  when the condition for performing the fuel cut operation stands is smaller than a predetermined reference oxygen storage amount, which is smaller than the maximum storable oxygen amount (the maximum value of oxygen which the exhaust purification catalyst  20  can store) and greater than zero, the control device performs fuel cut control without performing the fuel feed control even if the condition for performing fuel cut operation stands. Below, this control will be specifically explained. 
       FIG. 4  is a time chart of an FC flag, an output of the internal combustion engine  1 , an air-fuel ratio of exhaust gas flowing into the exhaust purification catalyst  20 , and an oxygen storage amount of the exhaust purification catalyst  20 , at the time when fuel cut control is performed. The FC flag is a flag which is set ON if the condition for starting fuel cut control stands and is set OFF if the condition for ending fuel cut control stands. In the illustrated example, the stoichiometric air-fuel ratio of the exhaust gas is 14.6. 
     In the example shown in  FIG. 4 , before the timing t 2 , usual air-fuel ratio control is performed. In the air-fuel ratio control of the present embodiment, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is controlled so that the oxygen storage amount OSA of the exhaust purification catalyst  20  is maintained in the vicinity of a predetermined oxygen storage amount, which is smaller than the maximum storable oxygen amount and greater than zero. In the present embodiment, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is controlled to alternately change between an air-fuel ratio slightly richer than the stoichiometric air-fuel ratio (timings t 0  to t 1 ) and an air-fuel ratio slightly leaner than the stoichiometric air-fuel ratio (timings t 1  to t 2 ). 
     Note that, the usual air-fuel ratio control shown in  FIG. 4  is one example. As the usual air-fuel ratio control, another mode of air-fuel ratio control may be performed. Specifically, for example, in the usual air-fuel ratio control, the control device may control the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  to constantly be the stoichiometric air-fuel ratio. Alternatively, in usual air-fuel ratio control, the control device may control so as to switch the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  from a rich air-fuel ratio to a lean air-fuel ratio when the oxygen storage amount of the exhaust purification catalyst  20  becomes substantially zero and to switch it from a lean air-fuel ratio to a rich air-fuel ratio when the oxygen storage amount of the exhaust purification catalyst  20  becomes substantially the maximum possible storage amount. 
     In the illustrated figure, at the timing t 2 , the condition for performing fuel cut control stands. At this time, in the illustrated example, the oxygen storage amount of the exhaust purification catalyst  20  is smaller than the reference oxygen storage amount OSAuc. Therefore, even if fuel cut control is started in this state, part of the oxygen flowing into the exhaust purification catalyst  20  is stored at the oxygen storing agent of the exhaust purification catalyst  20 . As a result, the reaction rate of unburned HC adsorbed on the precious metal of the exhaust purification catalyst  20  and oxygen is slow and accordingly the possibility of the precious metal being excessively raised in temperature is low. 
     For this reason, if at the timing t 2  the condition for performing fuel cut control stands, fuel cut control is started immediately without performing fuel feed control. As a result, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  becomes extremely high, and the oxygen storage amount of the exhaust purification catalyst  20  rapidly increases and immediately reaches the maximum storable oxygen amount Cmax. If the oxygen storage amount of the exhaust purification catalyst  20  reaches the maximum storable oxygen amount Cmax, the exhaust purification catalyst  20  can no longer store any more oxygen. 
     After that, if at timing t 3  the condition for ending fuel cut control stands, the fuel cut control is ended. Therefore, after the timing t 3 , fuel injection from the fuel injector  12  is restarted and the engine output is increased from zero. 
     If fuel cut control is performed, the oxygen storage amount of the exhaust purification catalyst  20  reaches the maximum storable oxygen amount, therefore after fuel cut control ends, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is controlled to be a rich air-fuel ratio. As a result, in the illustrated example, after the timing t 3 , the oxygen storage amount of the exhaust purification catalyst  20  gradually decreases. 
     In the illustrated example, at the timing t 4 , the condition for performing fuel cut control again stands. At this time, the oxygen storage amount of the exhaust purification catalyst  20  is greater than the reference oxygen storage amount OSAuc. Therefore, if fuel cut control is started in this state, most of the oxygen flowing into the exhaust purification catalyst  20  reacts with the unburned HC adsorbed on the precious metal of the exhaust purification catalyst  20 . Therefore, the reaction rate of unburned HC and oxygen is fast and therefore the precious metal is excessively raised in temperature and there is a high possibility of sintering of the precious metal ending up being invited. 
     For this reason, if at the timing t 4  the condition for performing fuel cut control stands, fuel feed control is performed for temporarily feeding fuel to the combustion chamber  10  so that the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  becomes a rich air-fuel ratio before fuel cut control is started. In particular, in the present embodiment, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  while performing fuel feed control is set to a predetermined constant air-fuel ratio richer than the rich air-fuel ratio able to be taken when usual air-fuel ratio control is being performed. For this reason, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is controlled so that a rich degree (difference from stoichiometric air-fuel ratio in rich direction) becomes greater after the timing t 4  when fuel feed control is started compared with before the timing t 4  when usual air-fuel ratio control is performed. 
     If at the timing t 4  fuel feed control is started, the oxygen storage amount of the exhaust purification catalyst  20  decreases. In the present embodiment, at the timing t 5  when a predetermined time has elapsed from the start of fuel feed control (or the internal combustion engine has been driven for a predetermined number of cycles), the fuel feed control is ended. The time (or the crank angle) for performing fuel feed control is set to a certain time (or crank angle) predetermined so that the oxygen storage amount becomes at least less than the reference oxygen storage amount OSAuc regardless of the oxygen storage amount at the time of start of fuel feed control. 
     At the same time as the fuel feed control ended at the timing t 5 , fuel cut control is started. As a result, the oxygen storage amount of the exhaust purification catalyst  20  rapidly increases and immediately reaches the maximum storable oxygen amount Cmax. After that, if, at the timing t 6 , the condition for ending fuel cut control stands, the fuel cut control is ended. Therefore, after the timing t 6 , fuel injection from the fuel injector  12  is restarted and the engine output is increased from zero. 
     In the present embodiment, when the oxygen storage amount of the exhaust purification catalyst  20  is large, fuel cut control is started after performing fuel feed control to reduce the oxygen storage amount once. For this reason, even if fuel cut control is started, part of the oxygen flowing into the exhaust purification catalyst  20  is stored at the oxygen storing agent. As a result, it is possible to keep the amount of oxygen reacting with the unburned HC adsorbed on the precious metal small and accordingly possible to keep the exhaust purification catalyst  20  from deteriorating. 
     On the other hand, when the oxygen storage amount of the exhaust purification catalyst  20  is small, fuel cut control is started without performing fuel feed control. At this time, even if not performing fuel feed control, if fuel cut control is started, part of the oxygen flowing into the exhaust purification catalyst  20  is stored in the oxygen storing agent, therefore the exhaust purification catalyst  20  can be kept from deteriorating. In addition, by not performing fuel feed control, it is possible to start fuel cut control immediately if the condition for performing the fuel cut operation stands, therefore the response of the vehicle operation can be raised. 
     &lt;&lt;Specific Control&gt;&gt; 
       FIG. 5  is a flow chart showing a control routine of flag setting processing for setting the FC flag. The illustrated control routine is performed in the control device every certain time interval. 
     First, at step S 11 , it is judged if the FC flag is ON. If at step S 11  it is judged that the FC flag is not ON, the routine proceeds to step S 12 . 
     At step S 12 , it is judged if the condition for performing fuel cut control stands. Whether or not the condition for performing fuel cut control stands is, for example, judged based on the engine load or engine rotational speed. Specifically, for example, the condition stands if the amount of depression of the accelerator pedal  43  is zero and thus the engine load detected by the load sensor  44  is zero, the engine rotational speed calculated based on the output of the crank angle sensor  45  is equal to or greater than a predetermined first rotational speed, and the speed of the vehicle mounting the internal combustion engine  1  is equal to or greater than a predetermined speed. 
     If at step S 12  it is judged that the condition for performing fuel cut control does not stand, the routine proceeds to step S 14 . At step S 14 , the FC flag is set OFF and the control routine is ended. On the other hand, if at step S 12  it is judged that the condition for performing fuel cut control stands, the routine proceeds to step S 13 . At step S 13 , the FC flag is set ON and the control routine is made to end. 
     If the FC flag is set ON, at the next control routine, the routine proceeds from step S 11  to step S 15 . At step S 15 , it is judged if the condition for ending fuel cut control stands. Whether or not the condition for ending fuel cut control stands is, for example, judged based on the engine load or engine rotational speed. Specifically, the ending condition stands if the engine load detected by the load sensor  44  becomes a value larger than zero, or if the engine rotational speed calculated based on the output of the crank angle sensor  45  is equal to or less than a predetermined second rotational speed (speed lower than first rotational speed), etc. 
     If at step S 15  it is judged that the condition for ending fuel cut control does not stand, the routine proceeds to step S 16 . At step S 16 , the FC flag is maintained as set ON, then the control routine is ended. On the other hand, if at step S 15  it is judged that the condition for ending fuel cut control stands, the routine proceeds to step S 14  where the FC flag is set OFF. 
       FIG. 6  is a flow chart showing a control routine of fuel cut processing for performing fuel cut control. The illustrated control routine is performed in the control device every certain time interval. 
     First, at step S 21 , it is judged if fuel cut control is underway. When fuel cut control is not underway, the routine proceeds to step S 22 . At step S 22 , it is judged if the FC flag, set by the processing for setting the flag shown in  FIG. 5 , is ON. If at step S 22  it is judged that the FC flag is not ON, the control routine is ended. On the other hand, if at step S 22  it is judged that the FC flag is ON, the routine proceeds to step S 23 . At step S 23 , it is judged if fuel feed control is currently being performed. If it is judged that fuel feed control is not being performed, the routine proceeds to step S 24 . 
     At step S 24 , it is judged if the current oxygen storage amount OSA of the exhaust purification catalyst  20  is smaller than the reference oxygen storage amount OSAuc. The current oxygen storage amount OSA is, for example, calculated based on the flow rate of the exhaust gas flowing into the exhaust purification catalyst  20  calculated based on the output of the air flow meter  39  and the air-fuel ratio of the exhaust gas detected by the upstream side air-fuel ratio sensor  41  (below, also referred to as the “output air-fuel ratio”). If at step S 24  it is judged that the oxygen storage amount OSA is smaller than the reference oxygen storage amount OSAuc, the routine proceeds to step S 26  where fuel cut control is started. On the other hand, if at step S 24  it is judged that the oxygen storage amount OSA is equal to or greater than the reference oxygen storage amount OSAuc, the routine proceeds to step S 25  where fuel feed control is performed. 
     If fuel feed control is started, at the next control routine, the routine proceeds from step S 23  to step S 27 . At step S 27 , it is judged if the time ti from starting fuel feed control is equal to or greater than a predetermined reference time tref. If at step S 27  it is judged that the time ti is less than the reference time tref, the routine proceeds to step S 25  where fuel feed control is continued. On the other hand, if at step S 27  it is judged that the time ti is equal to or greater than the reference time tref, the routine proceeds to step S 28  where fuel cut control is started. 
     If fuel cut control is started at step S 26  or step S 28 , at the next control routine, the routine proceeds from step S 21  to step S 29 . At step S 29 , it is judged if the FC flag is ON. If at step S 29  it is judged that the FC flag is ON, fuel cut control is continued via proceeding to step S 30 . On the other hand, if at step S 29  it is judged that the FC flag is not ON, the routine proceeds to step S 31  where fuel cut control is ended. 
     &lt;&lt;Modifications&gt;&gt; 
     In the above embodiment, the fuel feed control is performed by continuing the state where the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is a predetermined constant rich air-fuel ratio for a predetermined constant time (constant crank angle). However, the rich degree of the air-fuel ratio of the exhaust gas in fuel feed control and the time for performing fuel feed control do not necessarily have to be constant. 
     Referring to  FIG. 7 , a first modification of the above embodiment will be explained. In the first modification, if the oxygen storage amount OSA of the exhaust purification catalyst  20  when the condition for performing the fuel cut operation stands is relatively large, compared to when it is relatively small, in fuel feed control, the amount of feed of fuel to the combustion chamber  10  is controlled so that the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is larger. 
       FIG. 7  is a view showing the relationship between the oxygen storage amount of the exhaust purification catalyst  20  and the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20 , in fuel feed control. Specifically, in the present modification, as shown in  FIG. 7 , if the oxygen storage amount OSA increases over the reference oxygen storage amount OSAuc, the amount of fuel injection is controlled so that the greater the oxygen storage amount OSA, the larger the rich degree in fuel feed control becomes. In the present modification, the time for performing fuel feed control is set a predetermined constant time, therefore it can be said that the greater the oxygen storage amount OSA, the greater the total amount of feed of fuel until starting fuel cut control in fuel feed control. In other words, in the present modification, it can be said as the oxygen storage amount OSA is greater, in fuel feed control, the value calculated by multiplying the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time with the rich degree of the air-fuel ratio of the exhaust gas at that time and cumulatively adding the multiplied values over the time of performing fuel feed control, becomes larger. 
     Referring to  FIG. 8 , a second modification of the above embodiment will be explained. In the second modification, if the oxygen storage amount OSA of the exhaust purification catalyst  20  when the condition for performing the fuel cut operation stands is relatively large, compared with when it is relatively small, the time of performing fuel feed control is set longer. 
       FIG. 8  is a view showing the relationship between the oxygen storage amount of the exhaust purification catalyst  20  and the time of performing fuel feed control. Specifically, in the present modification, as shown in  FIG. 8 , if the oxygen storage amount OSA increases over the reference oxygen storage amount OSAuc, the greater the oxygen storage amount OSA, the longer the time of performing fuel feed control (crank angle) becomes. In the present modification, since the rich degree of the air-fuel ratio of the exhaust gas in fuel feed control is set a predetermined constant value, the greater the oxygen storage amount OSA, the greater the amount of feed of fuel in fuel feed control until starting fuel cut control. In other words, in this modification, as the oxygen storage amount OSA is greater, in fuel feed control, the value calculated by multiplying the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time with the rich degree of the air-fuel ratio of the exhaust gas at that time and cumulatively adding the multiplied values over the time of performing fuel feed control becomes larger. 
     If summarizing the above-mentioned first modification and second modification, in these modifications, if the oxygen storage amount OSA of the exhaust purification catalyst  20  when the condition for performing the fuel cut operation stands is relatively large, compared to when it is relatively small, the amount of feed of fuel is controlled so that the total amount of feed of fuel in the fuel feed control until starting fuel cut control is larger. In other words, in these modifications, if the oxygen storage amount OSA of the exhaust purification catalyst  20  when the condition for performing the fuel cut operation stands is relatively large, compared to when it is relatively small, in the fuel feed control, the amount of feed of fuel is controlled so that the value calculated by multiplying the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time with the rich degree of the air-fuel ratio of the exhaust gas at that time and cumulatively adding the multiplied values over the time of performing fuel feed control becomes larger. 
     Referring to  FIG. 9 , a third modification of the above embodiment will be explained. In the third modification, during fuel feed control as well, the oxygen storage amount OSA of the exhaust purification catalyst  20  is estimated and fuel feed control is performed until the estimated oxygen storage amount OSA reaches a predetermined lower limit oxygen storage amount OSA 1   c  (see  FIG. 4 ). In this regard, the lower limit oxygen storage amount OSA 1   c  is set a value of equal to or greater than zero and smaller than the reference oxygen storage amount OSAuc. 
       FIG. 9  is a flow chart showing a control routine of processing for a fuel cut operation according to a third modification. The illustrated control routine is performed by the control device every constant time interval. Note that, steps S 41  to S 46  and S 48  to S 51  of  FIG. 9  are respectively similar to steps S 21  to S 26  and S 28  to S 31  of  FIG. 6 , therefore explanations will be omitted. 
     If at step S 43  it is judged that fuel feed control is currently underway, the routine proceeds to step S 47 . At step S 47 , it is judged if the current oxygen storage amount OSA is equal to or less than the lower limit oxygen storage amount OSA 1   c . The current oxygen storage amount OSA, like at step S 24  of  FIG. 6 , is, for example, calculated based on the flow rate of the exhaust gas flowing into the exhaust purification catalyst  20  and the air-fuel ratio of the exhaust gas. If it is judged that the current oxygen storage amount OSA is greater than the lower limit oxygen storage amount OSA 1   c , the routine proceeds to step S 45  where fuel feed control is continued. On the other hand, if at step S 47  it is judged that the current oxygen storage amount OSA is equal to or less than the lower limit oxygen storage amount OSA 1   c , the routine proceeds to step S 48  where fuel cut control is started. 
     Second Embodiment 
     Next, referring to  FIG. 10 , an exhaust purification system according to a second embodiment will be explained. The configuration and control of the exhaust purification system according to the second embodiment are basically similar to the configuration and control of the exhaust purification system according to the first embodiment. Below, exhaust purification system according to the second embodiment will be explained focusing on parts different from the first embodiment. 
     As explained above, if the exhaust purification catalyst  20  increasingly deteriorates, the total surface area of the precious metal becomes smaller due to sintering of the precious metal. If in this way the total surface area of the precious metal becomes smaller, the amount of unburned HC adsorbed at the surface of the precious metal is also reduced. Therefore, when the exhaust purification catalyst  20  increasingly deteriorates, compared to when the exhaust purification catalyst  20  does not deteriorate, even if reducing the total amount of feed of fuel in the fuel feed control, it is possible to sufficiently keep the exhaust purification catalyst  20  from further deteriorating. 
     Further, if the exhaust purification catalyst  20  increasingly deteriorates, the oxygen storing agent falls in oxygen storage ability. Therefore, if the exhaust purification catalyst  20  increasingly deteriorates, the exhaust purification catalyst  20  falls in the maximum storable oxygen amount. For this reason, even when the exhaust purification catalyst  20  increasingly deteriorates, if performing fuel feed control in the same way as when the exhaust purification catalyst  20  does not deteriorate, the total amount of feed of fuel may become too great, the oxygen storage amount of the exhaust purification catalyst  20  may reach zero, and part of the unburned HC fed to the exhaust purification catalyst  20  by fuel feed control may flow out from the exhaust purification catalyst  20 . 
     Therefore, in the present embodiment, when the degree of deterioration of the exhaust purification catalyst  20  when the condition for fuel cut operation stands is relatively high, compared to when it is relatively low, the total amount of feed of fuel during fuel feed control is made smaller. In other words, in the present embodiment, when the degree of deterioration of the exhaust purification catalyst  20  when the condition for fuel cut operation stands is relatively high, compared to when it is relatively low, the value calculated by multiplying the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time with the rich degree of the air-fuel ratio of the exhaust gas at that time and cumulatively adding the multiplied values over the time of performing fuel feed control is set smaller. 
       FIG. 10  is a view showing the relationship between a degree of deterioration of the exhaust purification catalyst  20  and a rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  in fuel feed control. As will be understood from  FIG. 10 , in the present embodiment, the amount of fuel injection is controlled so that the greater the degree of deterioration of the exhaust purification catalyst  20 , the smaller the rich degree in fuel feed control becomes. In the present embodiment, the time of performing fuel feed control is set a predetermined constant time, therefore the greater the degree of deterioration of the exhaust purification catalyst  20 , the smaller the total amount of feed of fuel until starting fuel cut control in fuel feed control. In other words, in the present embodiment, the greater the degree of deterioration of the exhaust purification catalyst  20 , the smaller the value calculated by multiplying the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time with the rich degree of the air-fuel ratio of the exhaust gas at that time and cumulatively adding the multiplied values over the time of performing fuel feed control becomes. 
     Further, the degree of deterioration of the exhaust purification catalyst  20  is detected by a known method. Specifically, for example, it is detected by the following method. First, in the state where the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  is a rich air-fuel ratio, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is changed to a lean air-fuel ratio and is maintained as is until the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  becomes a lean air-fuel ratio. Then, the degree of deterioration of the exhaust purification catalyst  20  is estimated based on the total amount of excess oxygen flowing into the exhaust purification catalyst  20  from when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is changed to a lean air-fuel ratio to when the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  becomes a lean air-fuel ratio (alternatively, the value cumulatively adding the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time multiplied with the lean degree of the air-fuel ratio of the exhaust gas at that time). The smaller the total amount of excess oxygen at this time, the higher the degree of deterioration of the exhaust purification catalyst  20  that is estimated. 
     Alternatively, the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is changed to a rich air-fuel ratio in the state where the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  is a lean air-fuel ratio and is maintained until the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  becomes a rich air-fuel ratio. Then, the degree of deterioration of the exhaust purification catalyst  20  is estimated based on the total amount of the excess unburned HC or CO flowing into the exhaust purification catalyst  20  from when the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is changed to a rich air-fuel ratio to when the output air-fuel ratio of the downstream side air-fuel ratio sensor  42  becomes a rich air-fuel ratio (alternatively, the value cumulatively adding the amount of exhaust gas flowing into the exhaust purification catalyst  20  per unit time multiplied with the rich degree of the air-fuel ratio of the exhaust gas at that time). The smaller the total amount of excess oxygen at this time, the higher the degree of deterioration of the exhaust purification catalyst  20  that is estimated. 
     According to the present embodiment, when the exhaust purification catalyst  20  increasingly deteriorates, compared with when the exhaust purification catalyst  20  does not deteriorate, the total amount of fuel feed during fuel feed control is set smaller, therefore the exhaust purification catalyst  20  can be kept from deteriorating while the amount of feed of fuel can be reduced. For this reason, the fuel efficiency can be kept from deteriorating. In addition, unburned HC can be kept from flowing out from the exhaust purification catalyst  20 . 
     Note that, in the present embodiment, the total amount of feed in fuel feed control is controlled based on only the degree of deterioration of the exhaust purification catalyst  20 . However, considering the modification of the first embodiment, it may also be changed based on the oxygen storage amount of the exhaust purification catalyst  20 , etc. In this case, for example, the amount of feed of fuel is controlled so that the greater the degree of deterioration of the exhaust purification catalyst  20  and the smaller the oxygen storage amount of the exhaust purification catalyst  20 , the smaller the total amount of feed of fuel during fuel feed control becomes. 
     Third Embodiment 
     Next, referring to  FIGS. 11 and 12 , an exhaust purification system according to a third embodiment will be explained. The configuration and control of the exhaust purification system according to the third embodiment are basically similar to the configuration and control of the exhaust purification system according to the first embodiment. Below, exhaust purification system according to the third embodiment will be explained focusing on parts different from the first embodiment. 
     In this regard, if the amount of adsorption of unburned HC per unit surface area of the precious metal of the exhaust purification catalyst  20  becomes greater, the catalytic action by the precious metal falls. If, in such a state, a large amount of unburned HC flows into the exhaust purification catalyst  20  due to fuel feed control, part of the inflowing unburned HC may flow out as is from the exhaust purification catalyst  20  without being removed at the exhaust purification catalyst  20 . 
     Therefore, in the present embodiment, if the amount of adsorption of hydrocarbons at the exhaust purification catalyst  20  when the condition for a fuel cut operation stands is equal to or greater than a predetermined reference adsorption amount, fuel cut control is performed without fuel feed control being performed. In addition, in the present embodiment, if the amount of adsorption of hydrocarbons at the exhaust purification catalyst  20  when the condition for a fuel cut operation stands is less than the reference adsorption amount, the amount of feed of fuel to the combustion chamber  10  is controlled so that the greater the amount of adsorption of hydrocarbons at the exhaust purification catalyst  20 , the smaller the amount of feed of fuel per unit time in the fuel feed control. 
       FIG. 11  is a view showing the relationship between the amount of unburned HC adsorbed at the exhaust purification catalyst  20  and the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  in fuel feed control. As will be understood from  FIG. 11 , in the present embodiment, if the amount of adsorption of the unburned HC is equal to or greater than the reference adsorption amount Qhcref, no fuel feed control is performed and accordingly the rich degree is also zero. 
     On the other hand, if the amount of unburned HC adsorbed at the exhaust purification catalyst  20  is smaller than the reference adsorption amount Qhcref, as shown in  FIG. 11 , the amount of fuel injection to the combustion chamber  10  is controlled so that the greater the amount of adsorption of unburned HC, the smaller the rich degree at fuel feed control. 
     According to the present embodiment, when the amount of unburned HC adsorbed at the exhaust purification catalyst  20  is large, that is, when the amount of adsorption of unburned HC per unit surface area of the precious metal is large, fuel feed control is not performed. For this reason, unburned HC is kept from flowing out from the exhaust purification catalyst  20 . Further, the greater the amount of unburned HC adsorbed at the exhaust purification catalyst  20 , the smaller the catalytic action of the precious metal. In that regard, in the present embodiment, the greater the amount of adsorption of the unburned HC, the smaller the rich degree is made, therefore it is possible to sufficiently remove the unburned HC even if the catalytic action is small. Due to this as well, unburned HC is kept from flowing out from the exhaust purification catalyst  20 . 
     Note that, in the present embodiment, the rich degree at the fuel feed control is controlled based on the amount of unburned HC adsorbed at the exhaust purification catalyst  20 . However, in addition to such control, it is also possible to control the total amount of feed of fuel to the exhaust purification catalyst  20  during fuel feed control based on the oxygen storage amount of the exhaust purification catalyst  20 , etc., in consideration of the first embodiment and second embodiment. 
     &lt;&lt;Specific Control&gt;&gt; 
       FIG. 12  is a flow chart showing a control routine of processing for a fuel cut operation according to the third embodiment. The illustrated control routine is performed by the control device every constant time interval. Note that, steps S 61  to S 64  and S 66  to S 72  of  FIG. 12  are respectively similar to steps S 21  to S 24  and S 25  to S 31  of  FIG. 6 , therefore explanations will be omitted. 
     If at step S 64  it is judged that the oxygen storage amount OSA is equal to or greater than the reference oxygen storage amount OSAuc, the routine proceeds to step S 65 . At step S 65 , it is judged if the amount of unburned HC adsorbed at the exhaust purification catalyst  20  is equal to or greater than a reference adsorption amount Qhcref. 
     The amount of unburned HC adsorbed at the exhaust purification catalyst  20  is, for example, estimated based on the flow rate of the unburned HC flowing into the exhaust purification catalyst  20  and the temperature of the exhaust purification catalyst  20 . The flow rate of the unburned HC flowing into the exhaust purification catalyst  20  is, for example, calculated based on the flow rate of the exhaust gas flowing into the exhaust purification catalyst  20  (for example, estimated based on the output of the air flow meter  39 ) and the output air-fuel ratio of the downstream side air-fuel ratio sensor  42 . The temperature of the exhaust purification catalyst  20 , for example, is detected by a temperature sensor (not shown) detecting the temperature of the exhaust purification catalyst  20 . 
     Specifically, the adsorption amount is calculated based on that the greater the flow rate of the unburned HC flowing into the exhaust purification catalyst  20 , the greater the amount of unburned HC adsorbed at the exhaust purification catalyst  20 . Further, the adsorption amount is calculated based on that the lower the temperature of the exhaust purification catalyst  20 , the greater the amount of unburned HC adsorbed at the exhaust purification catalyst  20 . 
     If at step S 65 , the amount of unburned HC adsorbed at the exhaust purification catalyst  20  is equal to or greater than the reference adsorption amount Qhcref, the routine proceeds to step S 67  where fuel cut control is started. On the other hand, if at step S 65  it is judged that the amount of unburned HC adsorbed at the exhaust purification catalyst  20  is smaller than the reference adsorption amount Qhcref, the routine proceeds to step S 66  where fuel feed control is performed. At this time, the rich degree of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst  20  is set based on the amount of adsorption of the unburned HC using a map such as shown in  FIG. 11 . 
     REFERENCE SIGNS LIST 
     
         
           1 . internal combustion engine 
           2 . engine body 
           10 . combustion chamber 
           12 . fuel injector 
           20 . exhaust purification catalyst 
           31 . electronic control unit (ECU) 
           41 . upstream side air-fuel ratio sensor 
           42 . downstream side air-fuel ratio sensor