Abstract:
A control apparatus for an internal combustion engine can detect degradation of a three-way catalyst with high accuracy without causing deterioration in an exhaust. A pair of first and second air fuel ratio detectors are disposed in an exhaust system at locations upstream and downstream of the three-way catalyst for detecting a first and a second air fuel ratio of an exhaust gas. A target oxygen change amount calculator calculates a target oxygen change amount of the three-way catalyst, and an oxygen change amount calculator calculates an oxygen change amount of the three-way catalyst from an amount of exhaust gas passing through the three-way catalyst and the first air fuel ratio. An air fuel ratio operator inversely controls the air fuel ratio to a rich side and a lean side with a prescribed air fuel ratio width each time the oxygen change amount reaches a target oxygen change amount.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a control apparatus for an internal combustion engine with a three-way catalyst for purification of exhaust gas installed on an exhaust system, and more particularly, it relates to a new technique to detect the degradation of the three-way catalyst in a reliable manner.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, in internal combustion engines, a three-way catalyst is used to purify harmful components of an exhaust gas. The three-way catalyst has an oxygen occlusion capability to keep the atmosphere inside the three-way catalyst at a stoichiometric air fuel ratio by occluding oxygen in the exhaust gas when the air fuel ratio of the exhaust gas is leaner than the stoichiometric air fuel ratio, while releasing the oxygen occluded therein when the air fuel ratio of the exhaust gas is richer than the stoichiometric air fuel ratio.  
         [0005]     In addition, the three-way catalyst also has a capability to oxidize HC and CO among three harmful components contained in the exhaust gas and to reduce NOx, thereby purifying these respective components into harmless gases. Further, since the purification ability of the three-way catalyst becomes maximum in the vicinity of the stoichiometric air fuel ratio, the exhaust gas is excellently purified by combining the oxygen occlusion ability and the purification ability of the three-way catalyst with each other.  
         [0006]     However, when the exhaust gas becomes leaner than the stoichiometric air fuel ratio to cause the amount of oxygen occluded in the three-way catalyst to exceed the oxygen occlusion capacity thereof, the atmosphere in the three-way catalyst becomes no longer kept at the stoichiometric air fuel ratio, so the NOx purification rate of the catalyst is deteriorated to a remarkable extent.  
         [0007]     In addition, when the exhaust gas becomes richer than the stoichiometric air fuel ratio so the amount of oxygen occluded in the three-way catalyst becomes lacking or insufficient, the atmosphere in the three-way catalyst can not be kept at the stoichiometric air fuel ratio, thus deteriorating the purification rate of HC and CO. Here, it is known that as the three-way catalyst is deteriorated, the oxygen occlusion capacity thereof decreases, thus worsening the purification performance thereof.  
         [0008]     Accordingly, there has been proposed a control apparatus for an internal combustion engine in which a pair of air fuel ratio sensors are arranged at an upstream side and at a downstream side, respectively, of a three-way catalyst so as to directly measure an oxygen occlusion capacity thereof to detect the degradation of the three-way catalyst (see, for instance, a first patent document: Japanese patent No. 2812023). In this case, in changes of the air fuel ratios at the upstream and downstream sides of the three-way catalyst and in a change of the concentration of harmful components in the exhaust gas at the downstream side of the three-way catalyst, the air fuel ratio at the upstream side of the three-way catalyst is switched from a predetermined air fuel ratio, which is preset to a lean side, into a first prescribed air fuel ratio, which is preset to a rich side, across the stoichiometric air fuel ratio.  
         [0009]     At this time, even if the air fuel ratio at the upstream side of the three-way catalyst changes into the rich side, the oxygen adsorbed and held in the three-way catalyst is released. As a result, the air fuel ratio at the downstream side of the three-way catalyst is first maintained at the stoichiometric air fuel ratio only for a first predetermined period of time, and thereafter reaches a first air fuel ratio which is at the rich side. Subsequently, the air fuel ratio at the upstream side of the three-way catalyst is switched from the predetermined air fuel ratio, which is preset at the rich side, into a second prescribed air fuel ratio, which is preset at the lean side, across the stoichiometric air fuel ratio.  
         [0010]     At this time, oxygen is adsorbed and held in the three-way catalyst, contrary to the above case. As a result, the air fuel ratio at the downstream side of the three-way catalyst is first maintained at the stoichiometric air fuel ratio only for a second predetermined period of time, and thereafter reaches the second air fuel ratio at the lean side. Hereinafter, an absolute amount of the oxygen adsorbed and held by the three-way catalyst is calculated from a difference between the switched air fuel ratio and the stoichiometric air fuel ratio, and from the amount of the exhaust gas that has passed through the three-way catalyst for the first or second predetermined period of time, so that the degradation level of the three-way catalyst is detected from the absolute amount of adsorbed oxygen thus calculated (see FIG. 6 of the above-mentioned first patent document).  
         [0011]     In the known control apparatus for an internal combustion engine as referred to above, after the amount of oxygen occluded in the three-way catalyst has been decreased to zero (or saturated) without fail, the air fuel ratio at the upstream side of the three-way catalyst is switched, and then the absolute amount of the oxygen adsorbed and held by the three-way catalyst is calculated. Accordingly, the purification rate of the three-way catalyst in a period of time in which the amount of the oxygen occluded in the three-way catalyst is being decreased to zero or saturated is reduced, thus posing a problem that the exhaust gas is deteriorated.  
       SUMMARY OF THE INVENTION  
       [0012]     Accordingly, the present invention is intended to solve the problem as referred to above, and has for its object to obtain a control apparatus for an internal combustion engine which is capable of detecting the degradation of a three-way catalyst with high accuracy without causing deterioration in an exhaust gas by controlling the amount of change (i.e., the amount of occlusion or release) of the oxygen in the three-way catalyst to such an amount slightly more than the oxygen occlusion capacity of a degraded three-way catalyst specified by the relevant laws.  
         [0013]     A control apparatus for an internal combustion engine according to the present invention includes: a three-way catalyst disposed in an exhaust system of the internal combustion engine; a first air fuel ratio detection part disposed in the exhaust system at a location upstream of the three-way catalyst for detecting a first air fuel ratio of an exhaust gas; a second air fuel ratio detection part disposed in the exhaust system at a location downstream of the three-way catalyst for detecting a second air fuel ratio of the exhaust gas; a target oxygen change amount calculation part that calculates a target oxygen change amount of the three-way catalyst; an oxygen change amount calculation part that calculates an oxygen change amount of the three-way catalyst from an amount of exhaust gas passing through the three-way catalyst and the first air fuel ratio; and an air fuel ratio operation part that inversely operates the first air fuel ratio in accordance with the oxygen change amount. The air fuel ratio operation part controls, in an inverting manner, the first air fuel ratio to a rich side and a lean side across a stoichiometric air fuel ratio with a prescribed air fuel ratio width each time the oxygen change amount in the three-way catalyst reaches the target oxygen change amount.  
         [0014]     According to the present invention, by controlling the amount of change of the oxygen in the three-way catalyst to an amount slightly more than the oxygen occlusion capacity of the deteriorated three-way catalyst, the variation of the oxygen in the three-way catalyst, if in its normal state, does not exceed the oxygen occlusion capacity of the three-way catalyst. As a result, the oxygen concentration at the downstream side of the three-way catalyst does not vary, thus making it possible to avoid the deterioration of the exhaust gas at the downstream side of the three-way catalyst. On the other hand, if the three-way catalyst is in a deteriorated state, the variation of the oxygen in the three-way catalyst exceeds the oxygen occlusion capacity of the three-way catalyst. Consequently, the oxygen concentration at the downstream side of the three-way catalyst is inversely varied to a rich side and a lean side across the stoichiometric air fuel ratio, thus making it possible to detect the degradation of the three-way catalyst with high accuracy.  
         [0015]     The above and other objects, features and advantages of the present invention will become more readily apparent to those skilled in the art from the following detailed description of a preferred embodiment of the present invention taken in conjunction with the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a configuration view showing a control apparatus for an internal combustion engine according to one embodiment of the present invention together with its peripheral equipment.  
         [0017]      FIG. 2  is a flow chart illustrating an oxygen change amount control routine for a three-way catalyst for purification of an exhaust gas according to this embodiment of the present invention.  
         [0018]      FIG. 3  is a flow chart illustrating a degradation detection routine for the three-way catalyst according to this embodiment of the present invention.  
         [0019]      FIG. 4  is a timing chart illustrating the operation of a three-way catalyst degradation detection device when the three-way catalyst is in a normal state, according to this embodiment of the present invention.  
         [0020]      FIG. 5  is a timing chart illustrating the operation of the three-way catalyst degradation detection device when the three-way catalyst is in a degraded state, according to this embodiment of the present invention.  
         [0021]      FIG. 6  is a view illustrating a relation between the oxygen occlusion capacity of the three-way catalyst and the amount of exhaust gas emission during travelling in a US FTP mode when the three-way catalyst is degraded. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     Now, a preferred embodiment of the present invention will be described below in detail while referring to the accompanying drawings.  
       Embodiment 1  
       [0023]      FIG. 1  is a block diagram that shows an entire arrangement of a control apparatus for an internal combustion engine according to one embodiment of the present invention together with its peripheral equipment.  
         [0024]     In  FIG. 1 , an internal combustion engine  101  includes, as an air intake system, an intake pipe  105  having an air cleaner  102 , a throttle valve  103  and a surge tank  104  install thereon.  
         [0025]     The intake pipe  105  further includes an air flow sensor  106  for detecting the amount of intake air Qa, an injector  107  for injecting fuel into the intake pipe  105 , a throttle sensor  117  for detecting the throttle opening θ of a throttle valve  103 , and an idle switch  118  for detecting when the internal convention engine  101  is idling. The idle switch  118  generates an idle signal DL that becomes on at an idling opening (i.e., the throttle opening θ is in a fully closed state).  
         [0026]     In addition, the internal combustion engine  101  further includes, as an exhaust system, an exhaust pipe  108 . A three-way catalyst  109  is arranged in the exhaust pipe  108  for purifying harmful components in the exhaust gas, and a linear air fuel (A/F) ratio sensor  110  and a λ oxygen sensor  111  are arranged at an upstream side and at a downstream side, respectively, of the three-way catalyst  109 .  
         [0027]     An internal combustion engine control unit  112  (hereinafter referred to as an “ECU ”), being constituted by a microcomputer, includes a central processing unit  113  (hereinafter referred to as a “CPU ”), a read-only memory  114  (hereinafter referred to as a “ROM ”), a random-access memory  115  (hereinafter referred to as a “RAM ”), an input and output interface  116  (hereinafter referred to as an “I/O interface ”), and a drive circuit  122 .  
         [0028]     The internal combustion engine  101  further includes a water temperature sensor  119  for detecting the temperature of cooling water WT, a crank angle sensor  120  four generating a crank angle signal CA corresponding to a crank angle position (i.e., the rotational angle or position of a crankshaft), and a cam angle sensor  121  for generating a cam angle signal corresponding to a cam angle position (i.e., the rotational angle or position of a camshaft).  
         [0029]     The water temperature sensor  119 , the crank angle sensor  120 , and the cam angle sensor  121  together constitute various kinds of sensors for detecting the operating conditions of the internal combustion engine  101 , together with other sensors (e.g., the air flow sensor  103 , the linear air-fuel ratio sensor  110 , the λ oxygen sensor  111 , the throttle sensor  117 , the idle switch  118 , and so on). The respective detection signals from these sensors are input to the ECU  112  as engine operating condition information.  
         [0030]     In the internal combustion engine  101  as shown in  FIG. 1 , the intake air cleaned by the air cleaner  102  is sucked into the respective engine cylinders through the surge tank  104  and the intake pipe  105  while being controlled by the throttle valve  103  into an amount corresponding to a load on the internal combustion engine  101 . At this time, the amount of intake air Qa sucked into the internal combustion engine  101  is detected by the air flow sensor  106 , and fuel supplied to the respective cylinders of the internal combustion engine  101  is injected into the intake pipe  105  through the injector  107 .  
         [0031]     A mixture (i.e., air and fuel) sucked into the respective cylinders of the internal combustion engine  101  is burned therein in combustion stroke and turned into an exhaust gas, which is then exhausted into the ambient atmosphere through the three-way catalyst  109  arranged on the exhaust pipe  108  while the harmful components in the exhaust gas are purified by the three-way catalyst  109 .  
         [0032]     At this time, the linear A/F sensor  110  arranged at the upstream side of the three-way catalyst  109  detects the air fuel ratio A/F of the mixture by linearly detecting an oxygen concentration in the exhaust gas at the upstream side of the three-way catalyst  109 . The λ oxygen sensor  111  arranged at the downstream side of the three-way catalyst  109  detects an oxygen concentration λ O2 in the exhaust gas at the downstream side of the three-way catalyst  109 . The detection signals of the respective sensors  110 ,  111  contribute to the detection processing of the states of the exhaust gas upstream and downstream of the three-way catalyst  109  according to the ECU  112 .  
         [0033]     In the ECU  112 , various pieces of operating condition information (e.g., the amount of intake air Qa, the throttle opening θ, the idle signal DL, the cooling water temperature WT, the air fuel ratio A/F, the oxygen concentration λ O2, the crank angle signal CA, the cam angle signal from the cam angle sensor  121 , etc.) are taken into the CPU  113  through the I/O interface  116 .  
         [0034]     The ECU  112  constitutes an air fuel ratio feedback control system which generates a drive signal for the injector  107  based on the air fuel ratio A/F and the oxygen concentration λ O2 from the respective sensors  110 ,  111  arranged before and after (upstream and downstream of) the three-way catalyst  109 , so that a required amount of fuel can be injected by the injector  107 .  
         [0035]     In the air fuel ratio feedback control system in the ECU  112 , the CPU  113  drives the injector  107  through the drive circuit  122  in such a manner that the internal combustion engine  101  can be operated in a predetermined air fuel ratio on the basis of a control program and various maps stored in the ROM  114 . The actual air fuel ratio (A/F) is controlled to a target air fuel ratio A/Fo according to this air fuel ratio feedback control.  
         [0036]     Here, note that the ECU  112  also functions as a degradation detection device for the three-way catalyst  109  so as to control the internal combustion engine  101  at an optimal manner, as will be described later. In addition, the drive circuit  122  in the ECU  112  drives not only the injector  107  but also various kinds of actuators such as, for instance, an ISC valve (not shown), associated with the internal combustion engine  101 .  
         [0037]     That is, the ECU  112  performs, in addition to the air fuel ratio control, various kinds of control such as ignition timing control, idling speed control, etc., and at the same time detects, as a self-diagnosis function, failure of various kinds of components from which deterioration in the exhaust gas results.  
         [0038]     The CPU  113  and the drive circuit  122 , which serve to drive the injector  107  in a controlled manner, together constitute an air fuel ratio operation part which controls, in an inverting manner, the air fuel ratio A/F to a rich side and a lean side across the stoichiometric air fuel ratio with a prescribed width of the air fuel ratio each time the amount of change of the oxygen in the three-way catalyst  109  reaches a target oxygen change amount.  
         [0039]     Moreover, the ROM  114  stores therein not only a routine for controlling the amount of change of the oxygen in the three-way catalyst  109 , but also control programs such as a degradation detection routine for the three-way catalyst  109 , etc., along with necessary maps for these control processes.  
         [0040]     Hereinafter, the control apparatus for an internal combustion engine according to the this embodiment of the present invention shown in  FIG. 1  will be described in detail while referring to flow charts of  FIGS. 2, 3 , timing charts of  FIGS. 4, 5  and an explanatory view of  FIG. 6 . Here, the description will be made along the contents of processing according to a control routine for controlling the amount of change of the oxygen in the three-way catalyst  109  as well as a degradation detection routine for detecting the degradation of the three-way catalyst  109 , these routines featuring the present invention.  
         [0041]      FIG. 2  is a flow chart that illustrates an air fuel ratio control routine, which is the control routine for controlling the amount of change of the oxygen in the three-way catalyst  109  for purification of an exhaust gas, according to the this embodiment of the present invention.  
         [0042]     In  FIG. 2 , first of all, it is determined whether a degradation determination execution condition for the three-way catalyst  109  holds (step  201 ). When it is determined that the degradation determination execution condition does not hold (that is, NO), the processing routine of  FIG. 2  is immediately terminated and a return is performed.  
         [0043]     On the other hand, when it is determined in step  201  that the degradation determination execution condition holds (that is, YES), it is subsequently determined whether the value of a degradation determination execution counter CN 1  is greater than “0” (step  202 ). When it is determined as CN 1 =0 (that is, NO), a predetermined number of degradation determination executions have been completed, so the processing routine of  FIG. 2  is terminated at once and a return is carried out.  
         [0044]     At this time, it is assumed that only when the degradation determination execution condition holds for the first time (step  201 ), the degradation determination execution counter CN 1  and an oxygen occlusion amount initialization counter CN 2  are set to initial values, respectively. In addition, it can be determined that the degradation determination execution condition holds, for example, when the following conditions are satisfied: that is, the internal combustion engine  101  is after warmed up; the amount of intake air Qa is in a predetermined range; and the number of revolutions per minute and the load of the engine are in predetermined ranges, respectively.  
         [0045]     The degradation determination execution counter CN 1  is a subtraction counter which is decremented by “1” each time a lean flag FL (to be described later) is switched. Accordingly, if the initial value of the degradation determination execution counter CN 1  is set to “5”, for example, the target air fuel ratio A/Fo, after first enriched, is controlled to be made lean and rich in an alternate manner. Such enriching and leaning are carried out a total of five times.  
         [0046]     On the other hand, when it is determined as CN 1 &gt;0 in step  202  (that is, YES), it is subsequently determined whether the value of the oxygen occlusion amount initialization counter CN 2  is greater than “0” (step  203 ).  
         [0047]     The oxygen occlusion amount initialization counter CN 2  is a subtraction counter which is decremented by “1” each time the processing of enriching the target air fuel ratio A/Fo is carried out during the initialization of the amount of oxygen occlusion (to be described later). In this case, by making the target air fuel ratio A/Fo richer than the stoichiometric air fuel ratio over a predetermined period of time, it is possible to suppress deterioration in the amount of NOx emission resulting from the oxygen occlusion capacity of the three-way catalyst  109  being saturated by the amount of oxygen in the three-way catalyst  109  in the lean processing of the target air fuel ratio A/Fo executed when the value of the oxygen occlusion amount initialization counter CN 2  is “0”.  
         [0048]     When it is determined as CN 2 &gt;0 in step  203  (that is, YES), the amount of change of the oxygen QOX in the three-way catalyst  109  is set to “0” (step  204 ), and the target air fuel ratio A/Fo is enriched by a predetermined amount from the stoichiometric air fuel ratio based on a map that is set in accordance with the number of revolutions per minute and the load of the internal combustion engine  101  (step  205 ).  
         [0049]     Also, the oxygen occlusion amount initialization counter CN 2  is decremented by “1” (step  206 ), and the lean flag FL is set to “1 (hold) ” in preparation for the post termination of the initialization of the amount of change of the oxygen QOX in the three-way catalyst  109 , after which the processing routine of  FIG. 2  is ended. If the lean flag FL is set to “1(hold) ” after the termination of the initialization of the oxygen occlusion amount, the target air fuel ratio A/Fo is made lean, whereas if the lean flag FL is cleared to “0 (not hold) ”, it functions as a determination flag to enrich the target air fuel ratio A/Fo.  
         [0050]     On the other hand, if it is determined as CN 2 =0 (that is, NO) in step  203 , the target oxygen change amount QOXo is obtained by arithmetic calculations (step  208 ). Here, note that the target oxygen change amount QOXo is set to a value that is equal to the oxygen occlusion capacity of the three-way catalyst  109 , at which the catalyst  109  should be detected as degraded according to the relevant laws, added by a predetermined amount of margin (e.g., about 20%).  
         [0051]      FIG. 6  is an explanatory view that illustrates a relation between the oxygen occlusion capacity [g] at the time when the three-way catalyst  109  is degraded and the amount of exhaust gas emission at the time of travelling in a US FTP (Federal Test Procedure: Exhaust Gas Measuring Procedure) mode, wherein an alternate long and short dash line indicates NMHC (Non-Methane Hydro Carbon: HCs other than methane), and a broken line indicates an amount of NOx emission. In  FIG. 6 , a regulation value (with respect to a self-diagnosis function for emission failure) according to the United States OBD (On Board Diagnosis)-2 is defined in such a manner that a failure should be detected when the amount of exhaust gas emission at the time of travelling in the US FTP mode exceeds a predetermined times of an emission regulation value.  
         [0052]     Here, the oxygen occlusion capacity of the three-way catalyst  109 , at which the degradation of the catalyst  109  should be detected, is set so as to satisfy the United States OBD-2 regulation value. Here, it is assumed that characteristic data of  FIG. 6  is stored in the ROM  114  in the ECU  112 . Thus, when the target oxygen change amount QOXo is calculated in step  208 , the amount of change of the oxygen QOX in the three-way catalyst  109  is subsequently calculated by the following expression (1) (step  209 ). 
 
 QOX=QOX (last value)+| A/F−A/Fb|÷A/Fb×Qa×ΔT×α   (1) 
 
 where ΔT is a calculation period or cycle of the oxygen change amount QOX; α is an oxygen amount conversion factor; and A/Fb is a basic target air fuel ratio. The basic target air fuel ratio A/Fb is a target air fuel ratio which is set when the air fuel ratio of the mixture is not made rich or lean, and which is a stoichiometric air fuel ratio corresponding to a driving operation point of the internal combustion engine  101 . Further, it is assumed that the amount of intake air Qa is substantially equal to the amount of gas that has passed the three-way catalyst  109 . 
 
         [0053]     After the oxygen change amount QOX is calculated according to expression (1) above in step  209 , it is subsequently determined whether the oxygen change amount QOX is smaller than the target oxygen change amount QOXo (step  210 ). When it is determined as QOX≧QOXo in step  210  (that is, NO), the oxygen change amount QOX reaches the target oxygen change amount QOXo. Accordingly, the oxygen change amount QOX is set to “0” (step  212 ), the lean flag FL is inverted (e.g., from “1 (hold) ” to “0 (not hold)”) (step  213 ), and the control flow proceeds to step  211 .  
         [0054]     On the other hand, when it is determined as QOX&lt;QOXo in step  210  (that is, YES), it is subsequently determined whether the lean flag FL is set as “1 (hold) ” (step  211 ). When it is determined as FL=1 in step  211  (that is, YES), the target air fuel ratio A/Fo is increased or made leaner by a predetermined amount (e.g., 0.4) than the basic target air fuel ratio A/Fb (step  214 ), and the processing routine of  FIG. 2  is terminated.  
         [0055]     On the other hand, when it is determined as FL=0 in step  211  (that is, NO), the target air fuel ratio A/Fo is decreased or made richer by a predetermined amount (e.g., 0.4) than the basic target air fuel ratio A/Fb (step  215 ), and the processing routine of  FIG. 2  is terminated.  
         [0056]      FIG. 3  is a flow chart that illustrates a routine for detecting the degradation of the three-way catalyst  109  according to the this embodiment of the present invention. In  FIG. 3 , first of all, it is determined whether the degradation determination execution condition holds (step  301 ), and when it is determined that the degradation determination execution condition does not hold (that is, NO), the processing routine of  FIG. 3  is immediately terminated and a return is performed.  
         [0057]     On the other hand, when it is determined in step  301  that the degradation determination execution condition holds (that is, YES), it is subsequently determined whether the value of the degradation determination execution counter CN 1  is greater than “0” (step  302 ), and when determined as CN 1 =0 (that is, NO), the control flow advances to step  312  (to be described later).  
         [0058]     When it is determined as CN 1 &gt;0 in step  302  (that is, YES), it is subsequently determined whether the value of the oxygen occlusion amount initialization counter CN 2  is greater than “0” (step  303 ), and when determined as CN 2 &gt;0 (that is, NO), the processing routine of  FIG. 3  is terminated at once, whereas when determined as CN 2 =0 in step  303  (that is, YES), it is subsequently determined whether the lean flag FL is the same as the last value (step  304 ), and when determined that the current FL is not equal to the last FL (that is, NO), the control flow advances to step  310  (to be described later).  
         [0059]     On the other hand, when it is determined in step  304  that the current FL is equal to the last FL (that is, YES), it is subsequently determined whether a λ O2 inversion flag Fλ downstream of the three-way catalyst  109  is equal to “0 (not hold)” (step  305 ), and when determined that the degradation determination execution condition does not hold (that is, NO), the processing routine of  FIG. 3  is immediately terminated.  
         [0060]     When it is determined as Fλ=0 in step  305  (that is, YES), inversion determination processing for the oxygen concentration λ O2 detected by the λ oxygen sensor  111  is carried out (step  306 ). Specifically, it is determined whether the output value of the λ oxygen sensor  111  downstream of the three-way catalyst  109  is lower than a lean inversion threshold value during the lean processing of the target air fuel ratio A/Fo, or it is determined whether the output value of the λ oxygen sensor  111  downstream of the three-way catalyst  109  exceeds a rich inversion threshold value during the rich processing of the target air fuel ratio A/Fo.  
         [0061]     Here, note that when the value of the oxygen occlusion amount initialization counter CN 2  is greater than zero (i.e., CN 2 &gt;0), the λ O2 inversion flag F λ downstream of the three-way catalyst  109  is set to “0 (not hold)”, and in case of CN 2 =0, the λ O2 inversion flag F λ downstream of the three-way catalyst  109  is set to “1 (hold)” if it becomes less than the lean inversion threshold value when inverted (i.e., at the time of the lean processing of the target air fuel ratio A/Fo), or if it becomes greater than the rich inversion threshold value at the time of the rich processing of the target air fuel ratio A/Fo. In addition, the lean inversion threshold value is set to 0.3 [V] for instance, and the rich inversion threshold value is set to 0.7 [V ], for instance.  
         [0062]     Then, by detecting the timing at which the inversion determination holds in step  306 , it is determined whether the inversion determination of the oxygen concentration λ O2 holds (step  307 ), and when determined that the inversion determination of the oxygen concentration λ O2 does not hold (that is, NO), the processing routine of  FIG. 3  is immediately terminated.  
         [0063]     On the other hand, when it is determined in step  307  that the inversion determination of the oxygen concentration λ O2 holds (that is, YES), it is further determined whether the λ O2 inversion flag Fλ downstream of the three-way catalyst  109  is set to “1 (hold)” (step  308 ), and a degradation determination hold counter CN 3  is incremented (i.e., added by “1”)(step  309 ), after which the processing routine of  FIG. 3  is terminated.  
         [0064]     Here, note that each time the output value of the λ oxygen sensor  111  downstream of the three-way catalyst  109  is inverted, the degradation determination hold counter CN 3  is incremented by “1”, and when the degradation determination execution counter CN 1  (e.g., initial value=5) becomes “0”, a final degradation determination (to be described later) holds if the value of the degradation determination hold counter CN 3  becomes equal to or greater than “4”.  
         [0065]     On the other hand, when in step  304  the lean flag FL is inverted and hence it is determined that the current FL is not equal to the last FL (that is, NO), the λ O2 inversion flag Fλ downstream of the three-way catalyst  109  is set to “0 (not hold)” (step  310 ), and the degradation determination execution counter CN 1  is subtracted or decremented (step  311 ), after which the processing routine of  FIG. 3  is terminated.  
         [0066]     Further, when it is determined in the above step  302  that the degradation determination execution counter CN 1  is equal to zero (CN 1 =0) (that is, NO), final degradation determination processing is carried out in which it is determined whether the degradation determination hold counter CN 3  exceeds a final degradation determination threshold value (preset in the ROM  114 ) (step  312 ).  
         [0067]     Subsequently, in step  313 , it is determined whether the degradation determination execution condition holds in step  312 , and when the degradation determination hold counter CN 3  is less than 4 (i.e., CN 3 &lt;4) and hence it is determined that the degradation determination execution condition does not hold (that is, NO), the processing routine of  FIG. 3  is immediately terminated. On the other hand, when in step  313  the degradation determination hold counter CN 3  is equal to or greater than  4  (i.e., CN 3 ≧4) and hence it is determined that the final degradation determination holds (that is, YES), a MIL (Malfunction Indicator Light) lamp is lit so as to inform the driver of the fact that the three-way catalyst  109  is in its deteriorated state (step  314 ), and the processing routine of  FIG. 3  is terminated.  
         [0068]      FIG. 4  and  FIG. 5  are timing charts that illustrate the operation of the degradation detection device for detecting the degradation of the three-way catalyst  109  according to the this embodiment of the present invention, wherein  FIG. 4  shows an operation when it should be detected that the three-way catalyst  109  has been deteriorated, and  FIG. 5  shows an operation when the three-way catalyst  109  is normal.  
         [0069]     In  FIG. 4  and  FIG. 5 , there are illustrated the time-related changes in the respective values of the target air fuel ratio A/Fo (≠A/F at the upstream side of the three-way catalyst  109 ), the oxygen change amount QOX, the oxygen concentration λ O2, the downstream λ O2 inversion flag F λ, the degradation determination execution counter CN 1 , the oxygen occlusion amount initialization counter CN 2 , the degradation determination hold counter CN 3 , and the lean flag FL.  
         [0070]     Hereinafter, reference will be made to the specific contents of processing of the oxygen change amount control routine and the degradation detection routine for the three-way catalyst  109  as stated above along respective steps  401  through  439  while referring to  FIG. 4 .  
         [0071]     In  FIG. 4 , when the degradation determination execution condition holds by a first time point a, initial values are set in the degradation determination execution counter CN 1  and the oxygen occlusion amount initialization counter CN 2 , respectively (steps  401 ,  402 ), and in step  404 , the target air fuel ratio A/Fo is enriched until the value of the oxygen occlusion amount initialization counter CN 2  becomes “0” (steps  403  through  408 ).  
         [0072]     In the enriching period by the first time point a, the oxygen change amount QOX is set to “0” (step  405 ), and the lean flag FL is set to “1” (step  406 ), and the λ O2 inversion flag Fλ downstream of the three-way catalyst  109  is fixed to “0” (step  407 ).  
         [0073]     Subsequently, when the oxygen occlusion amount initialization counter CN 2  becomes “0” within a period from time point a to time point b (step  408 ), the target oxygen change amount QOXo is calculated (step  409 ), and the oxygen change amount QOX is also calculated (step  410 ). Hereinafter, the target air fuel ratio A/Fo is made leaner until the oxygen change amount QOX reaches the target oxygen change amount QOXo (step  411 ).  
         [0074]     When the output value λ O2 (oxygen concentration) of the λ oxygen sensor  111  downstream of the three-way catalyst  109  falls below the lean inversion threshold value within the above-mentioned period from time point a to time point b (step  412 ), the λ O2 inversion flag Fλ downstream of the three-way catalyst  109  is set to “1” (step  413 ), and the degradation determination hold counter CN 3  is incremented (step  414 ).  
         [0075]     Hereinafter, when the oxygen change amount QOX reaches the target oxygen change amount QOXo at time point b as shown in  FIG. 5 , the oxygen change amount QOX is set to “0” (step  415 ), and the lean flag FL is set “0” (step  416 ), and the target air fuel ratio A/Fo is made richer. At the same time, the λ O2 inversion flag F λ downstream of the three-way catalyst  109  is set to “0” (step  418 ), and the degradation determination execution counter CN 1  is decremented (step  419 ).  
         [0076]     Subsequently, the target oxygen change amount QOXo and the oxygen change amount QOX are calculated within a period from time point b to time point c, similar to the period from time point a to time point b (steps  420 ,  421 ). Thereafter, the target air fuel ratio A/Fo is made richer until the oxygen change amount QOX reaches the target oxygen change amount QOXo (step  417 ).  
         [0077]     When the output value λ O2 (oxygen concentration) of the λ oxygen sensor  111  downstream of the three-way catalyst  109  exceeds the rich inversion threshold value within the period from time point b to time point c (step  422 ), the λ O2 inversion flag F λ is set to “1” (step  423 ), and the degradation determination hold counter CN 3  is incremented (step  424 ).  
         [0078]     Hereinafter, when the oxygen change amount QOX reaches the target oxygen change amount QOXo at time point c as shown in  FIG. 5 , the oxygen change amount QOX is set to “0” (step  425 ), and the lean flag FL is set to “1” (step  426 ), and the target air fuel ratio A/Fo is made leaner (step  427 ). At the same time, the λ O2 inversion flag F λ downstream of the three-way catalyst  109  is set to “0” (step  428 ), and the degradation determination execution counter CN 1  is decremented (step  429 ).  
         [0079]     Subsequently, the target air fuel ratio A/Fo is repeatedly made leaner and richer in an alternate manner (steps  427 ,  431 ,  432 ,  433 ) until the value of the degradation determination execution counter CN 1  becomes “0” (step  430 ) within a period from time point c to time point d, similar to the period from time point a to time point c, and the degradation determination hold counter CN 3  is incremented each time the λ O2 inversion flag F λ is set to “1” (steps  434 ,  435 ,  436 ).  
         [0080]     Hereinafter, when the degradation determination execution counter CN 1  reaches “0” at time point d (step  430 ), as shown in  FIG. 5 , the final degradation determination is executed (step  437 ). At this time, when the final degradation determination holds, the MIL lamp is lit (step  438 ), and the degradation detection routine for the three-way catalyst  109  is completed.  
         [0081]     In addition, the target air fuel ratio A/Fo is set to the basic target air fuel ratio A/Fb, and the oxygen change amount control routine for the three-way catalyst  109  is completed (step  439 ). In  FIG. 4 , the operation at the time of the degradation detection of the three-way catalyst  109  is illustrated, and the oxygen concentration λ O2 downstream of the three-way catalyst  109  alternately indicates an oxygen occlusion excess state and an oxygen release shortage state for the oxygen change amount QOX.  
         [0082]     On the other hand, the respective operation sequences or steps  401  through  439  in  FIG. 5  (at the time of normal operation) are similar to those in the case of  FIG. 4  (at the time of degradation detection), but since the three-way catalyst  109  is normal, the output value λ O2 (oxygen concentration) of the λ oxygen sensor  111  downstream of the three-way catalyst  109  is not inverted within the period until time point d, so the degradation determination hold counter CN 3  is not incremented. Also, the oxygen concentration λ O2 downstream of the three-way catalyst  109  continuously indicates a normal value, so at time point d, the final degradation determination becomes “not hold ”, and the MIL lamp is accordingly not lit.  
         [0083]     As described above, the control apparatus for an internal combustion engine according to the this embodiment of the present invention comprises, as shown in  FIG. 1 , the three-way catalyst  109  that is arranged in the exhaust system of the internal combustion engine  101 , air fuel ratio detection parts  110 ,  111  that are arranged at the upstream side and at the downstream side, respectively, of the three-way catalyst  109  for detecting an air fuel ratio in an exhaust gas, and the ECU  112  that includes a target oxygen change amount calculation part, an oxygen change amount calculation part and an air fuel ratio operation part.  
         [0084]     In the ECU  112 , the target oxygen change amount calculation part calculates a target oxygen change amount, and the oxygen change amount calculation part calculates the amount of change of the oxygen in the three-way catalyst  109  based on the amount of gas having passed through the three-way catalyst  109  (the amount of intake air Qa) and the air fuel ratio A/F upstream of the three-way catalyst  109 .  
         [0085]     In addition, the air fuel ratio operation part controls, in an inverting manner, the air fuel ratio A/F at the upstream side of the three-way catalyst  109  to a rich side and a lean side across the stoichiometric air fuel ratio with a prescribed width of the air fuel ratio each time the amount of change of the oxygen in the three-way catalyst  109  reaches the target oxygen change amount. Moreover, the air fuel ratio operation part controls the air fuel ratio at the upstream side of the three-way catalyst  109  to a prescribed air fuel ratio, which is at a side richer than the stoichiometric air fuel ratio, over a predetermined period of time prior to the start of the operation of the air fuel ratio upstream of the three-way catalyst  109  based on the target oxygen change amount.  
         [0086]     By controlling the amount of change of the oxygen in the three-way catalyst  109  to an amount slightly more than the oxygen occlusion capacity of the deteriorated three-way catalyst  109 , the variation of the oxygen in the three-way catalyst  109 , if in its normal state, does not exceed the oxygen occlusion capacity of the three-way catalyst  109 . As a result, the oxygen concentration λ O2 at the downstream side of the three-way catalyst  109  does not vary, thus making it possible to avoid deterioration of the exhaust gas at the downstream side of the three-way catalyst  109 . On the other hand, if the three-way catalyst  109  is in a deteriorated state, the variation of the oxygen in the three-way catalyst exceeds the oxygen occlusion capacity of the three-way catalyst  109 . Consequently, the oxygen concentration λ O2 at the downstream side of the three-way catalyst  109  is inversely varied to a rich side and a lean side across the stoichiometric air fuel ratio, thus making it possible to detect the degradation of the three-way catalyst  109  with high accuracy.  
         [0087]     Further, by making the air fuel ratio at the upstream side of the three-way catalyst  109  richer than the stoichiometric air fuel ratio over a predetermined period of time prior to the start of the operation of the air fuel ratio upstream of the three-way catalyst  109  based on the target oxygen change amount, it is possible to control the amount of change of the oxygen in the three-way catalyst  109  within a range in which the amount of oxygen occluded in the three-way catalyst  109  does not saturate the oxygen occlusion capacity thereof, thus making it possible to prevent deterioration in the amount of NOx emission.  
         [0088]     Although in the this embodiment of the present invention, the relatively inexpensive λ oxygen sensor  111  is arranged at the downstream side of the three-way catalyst  109 , the present invention is not limited to this, but for example, a linear A/F sensor may be arranged in place of the λ oxygen sensor  111  so as to improve the control accuracy for the air fuel ratio. In addition, although inversion of the output value λ O2 (oxygen concentration) of the λ oxygen sensor  111  at the downstream side of the three-way catalyst  109  is used as a degradation determination reference for the three-way catalyst  109 , the present invention is not limited to this, but for example, an inversion frequency ratio between the respective output values of the sensors arranged at the upstream side and at the downstream side, respectively, of the three-way catalyst  109  may instead be used.  
         [0089]     While the invention has been described in terms of a preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modifications within the spirit and scope of the appended claims.