JP4631759B2 - Failure diagnosis device for exhaust gas purification device of internal combustion engine - Google Patents

Description

  The present invention relates to an exhaust gas purification device in which exhaust gas is guided by a flow path switching valve to a bypass passage side provided with a catalytic converter relatively upstream of an exhaust system immediately after a cold start, and in particular, the flow path switching valve. The present invention relates to a failure diagnosis device for diagnosing leakage of a liquid.

  As conventionally known, in a configuration in which the main catalytic converter is disposed relatively downstream of the exhaust system such as under the floor of a vehicle, the temperature of the catalytic converter rises and is activated after a cold start of the internal combustion engine. In the meantime, a sufficient exhaust purification action cannot be expected. On the other hand, the closer the catalytic converter is to the upstream side of the exhaust system, that is, the internal combustion engine side, the lower the durability due to thermal degradation of the catalyst.

Therefore, as disclosed in Patent Document 1 and Patent Document 2, a bypass channel is provided in parallel with the upstream portion of the main channel including the main catalytic converter, and another bypass catalyst is provided in the bypass channel. 2. Description of the Related Art Conventionally, there has been proposed an exhaust device that guides exhaust gas to the bypass flow path side immediately after a cold start by a switching valve that interposes a converter and switches both of them. In this configuration, the bypass catalytic converter is positioned relatively upstream of the main catalytic converter in the exhaust system and is activated relatively early, so that exhaust purification can be started from an earlier stage. it can.
JP-A-5-321644 JP 2005-188374 A

  In the above configuration, when the flow path switching by the flow path switching valve is insufficient, for example, when the exhaust gas leaks even though the flow path switching valve that opens and closes the main passage is in the closed position. When the main catalytic converter is inactive, unpurified exhaust gas flows out to the outside as it is, which is not preferable. Accordingly, there is a need for a diagnostic device that diagnoses leakage of the flow path switching valve.

  In addition, although the said patent document 2 has proposed the method of detecting the flow volume fall by the adhesion of the deposit of a flow-path switching valve, etc., the leakage of unpurified exhaust gas cannot be diagnosed.

  According to the present invention, a bypass passage is provided in parallel with the upstream portion of the main passage provided with the main catalytic converter on the downstream side, the bypass passage is provided with the bypass catalytic converter, and the upstream portion of the main passage is provided with the bypass passage. In an exhaust gas purification apparatus for an internal combustion engine comprising a flow path switching valve for closing a main passage, first air-fuel ratio detection means for detecting an exhaust air-fuel ratio upstream of the bypass catalytic converter in the bypass passage, and a main passage in the main passage A second air-fuel ratio detecting means for detecting the exhaust air-fuel ratio upstream of the catalytic converter, and the air-fuel ratio of the internal combustion engine from lean or rich to the stoichiometric air-fuel ratio in a state where the flow path switching valve is controlled to the closed position; Air-fuel ratio control means that changes stepwise to rich or lean on the opposite side, and each air-fuel ratio after the engine air-fuel ratio changes stepwise Based on the detected air-fuel ratio of the ratio detecting means, characterized in that it and a diagnostic means for diagnosing leakage of the flow path switching valve.

  Desirably, the engine air-fuel ratio is kept lean for a period sufficient to saturate the oxygen storage capacity of the bypass catalytic converter, and then the step-by-step change is made rich.

  That is, in the exhaust emission control device having the above configuration, when the flow path switching valve that opens and closes the main passage is in the closed position, the entire amount of exhaust discharged from the internal combustion engine flows to the bypass passage side and passes through the bypass catalytic converter. On the other hand, when the flow path switching valve is in the open position, most of the exhaust discharged from the internal combustion engine flows through the main passage due to the difference in ventilation resistance.

  Diagnosis of leakage of the flow path switching valve is performed by changing the engine air-fuel ratio, for example, from lean to rich with the flow path switching valve controlled to the closed position. In this case, while the engine air-fuel ratio is lean, excess oxygen is stored by the oxygen storage capability of the catalyst of the bypass catalytic converter, and this oxygen is released when the engine air-fuel ratio becomes rich. Therefore, even if the engine air-fuel ratio changes richly, if there is no leakage of exhaust gas through the flow path switching valve, the detected air-fuel ratio of the second air-fuel ratio detecting means is immediately affected by the released oxygen. Not rich. That is, immediately after the change of the engine air-fuel ratio, the exhaust air-fuel ratio detected by the second air-fuel ratio detection means becomes relatively leaner than the engine air-fuel ratio. On the other hand, if there is a leak of exhaust gas through the flow path switching valve, the leaked exhaust gas is a rich component, so that the exhaust air-fuel ratio detected by the second air-fuel ratio detection means is richer. Become. Accordingly, it is possible to diagnose whether or not the flow path switching valve is leaking or the degree of the leak. Even when the engine air-fuel ratio is changed from rich to lean, the influence of leakage similarly occurs due to the oxygen storage capacity.

  In one aspect of the present invention, a difference between the average air-fuel ratio of the first air-fuel ratio detection means and the average air-fuel ratio of the second air-fuel ratio detection means in a predetermined period after the engine air-fuel ratio changes stepwise is obtained, Based on the difference, the presence or absence of leakage is determined.

  Since the above-described oxygen storage capacity is affected by catalyst deterioration, it is desirable to have means for diagnosing the degree of catalyst deterioration of the bypass catalytic converter, and to correct the leakage diagnosis according to the degree of catalyst deterioration.

  For example, when the above-described difference in average air-fuel ratio is compared with a determination reference value to determine the presence or absence of leakage, if the determination reference value is corrected in accordance with the degree of catalyst deterioration, the diagnostic accuracy is further improved. To do.

  According to the failure diagnosis apparatus for an exhaust gas purification apparatus for an internal combustion engine according to the present invention, it is possible to reliably diagnose the leakage of exhaust gas through the flow path switching valve, and to prevent outflow of unpurified exhaust gas to the outside. can do.

  In addition, diagnosis considering the degree of catalyst deterioration can be easily realized, and the diagnosis accuracy can be increased.

  Hereinafter, an embodiment in which the present invention is applied to an exhaust purification apparatus for an in-line four-cylinder internal combustion engine will be described in detail with reference to the drawings.

  FIG. 1 is an explanatory view schematically showing the piping layout and control system of the exhaust device of the internal combustion engine. First, the configuration of the exhaust device will be described based on FIG.

  In the cylinder head 1a of the internal combustion engine 1, exhaust ports 2 of cylinders # 1 to # 4 arranged in series are formed so as to open toward the side surfaces, respectively. In addition, the main passage 3 is connected. The four main passages 3 of the # 1 cylinder to the # 4 cylinder merge into one flow path, and the main catalytic converter 4 is disposed on the downstream side thereof. The main catalytic converter 4 has a large capacity arranged under the floor of the vehicle, and includes, for example, a three-way catalyst and an HC trap catalyst as the catalyst. The main passage 3 and the main catalytic converter 4 constitute a main passage through which exhaust flows during normal operation. In addition, a flow path switching valve 5 that opens and closes the main passages 3 at the same time is provided as a flow path switching means at the junction of the four main paths 3 from each cylinder. The flow path switching valve 5 is driven to open and close by an appropriate actuator 5a.

  On the other hand, bypass passages 7 each having a smaller passage sectional area than the main passage 3 are branched from the main passages 3 of the respective cylinders as bypass passages. The branch point 6 that is the upstream end of each bypass passage 7 is set to a position on the upstream side of the main passage 3 as much as possible. The four bypass passages 7 merge into one flow path on the downstream side, and a bypass catalytic converter 8 using a three-way catalyst is interposed immediately after the junction. The bypass catalytic converter 8 has a small capacity as compared with the main catalytic converter 4 and desirably uses a catalyst excellent in low temperature activity. The downstream end of the bypass passage 7 extending from the outlet side of the bypass catalytic converter 8 is connected to the main passage 3 at a junction 9 upstream of the main catalytic converter 4 in the main passage 3 and downstream of the flow path switching valve 5. ing.

  Here, a main upstream air-fuel ratio sensor 10 and a main downstream air-fuel ratio sensor 11 are arranged at the inlet and outlet of the main catalytic converter 4, respectively, and at the inlet and outlet of the bypass catalytic converter 8. A bypass upstream air-fuel ratio sensor 12 and a bypass downstream air-fuel ratio sensor 13 are disposed, respectively. The main upstream air-fuel ratio sensor 10 and the main downstream air-fuel ratio sensor 11 are for performing known air-fuel ratio feedback control after the activation of the main catalytic converter 4. The fuel ratio (fuel injection amount) is controlled, and the output signal of the downstream air-fuel ratio sensor 11 is used for correcting variations in the control characteristics. Similarly, the bypass upstream air-fuel ratio sensor 12 and the bypass downstream air-fuel ratio sensor 13 are for performing known air-fuel ratio feedback control when the bypass catalytic converter 8 is used. 12, the engine air-fuel ratio (fuel injection amount) is controlled, and the output signal of the downstream-side air-fuel ratio sensor 13 is used for correcting variations in the control characteristics. As these air-fuel ratio sensors 10 to 13, either a so-called wide-area type air-fuel ratio sensor having a substantially linear output characteristic corresponding to the exhaust air-fuel ratio or an oxygen sensor having a binary output characteristic of rich or lean is used. However, the upstream air-fuel ratio sensors 10 and 12 are desirably wide-area air-fuel ratio sensors, and the downstream air-fuel ratio sensors 11 and 13 are preferable in terms of controllability in the air-fuel ratio control described above. It is possible to use an oxygen sensor in terms of component costs. The bypass upstream air-fuel ratio sensor 12 corresponds to “first air-fuel ratio detection means”, and the main upstream air-fuel ratio sensor 10 corresponds to “second air-fuel ratio detection means”.

  The internal combustion engine 1 includes a spark plug 21, and a fuel injection valve 23 is disposed in the intake passage 22. Furthermore, a so-called electronically controlled throttle valve 24 that is opened and closed by an actuator such as a motor is disposed upstream of the intake passage 22, and an air flow meter 25 that detects the amount of intake air is provided downstream of the air cleaner 26. Yes.

  Various control parameters of the internal combustion engine 1, for example, the fuel injection amount by the fuel injection valve 23, the ignition timing by the spark plug 21, the opening degree of the throttle valve 24, the open / close state of the flow path switching valve 5, etc. 27. In addition to the sensors described above, the engine control unit 27 includes various sensors such as a coolant temperature sensor 28 and an accelerator opening sensor 29 that detects the opening (depression amount) of an accelerator pedal operated by a driver. Detection signal is input. Then, the engine control unit 27 appropriately executes a diagnosis of leakage of the flow path switching valve 5.

  In such a configuration, when the engine temperature or the exhaust temperature after the cold start is low, the flow path switching valve 5 is closed via the actuator 5a, and the main passage 3 is blocked. Therefore, the entire amount of exhaust discharged from each cylinder flows from the branch point 6 to the bypass catalytic converter 8 through the bypass passage 7. The bypass catalytic converter 8 is located upstream of the exhaust system, that is, at a position close to the exhaust port 2 and is small in size, so that it is activated quickly and exhaust purification is started at an early stage.

  On the other hand, when the engine warm-up proceeds and the engine temperature or the exhaust temperature becomes sufficiently high, it is considered that the catalyst of the main catalytic converter 4 has been activated, and the flow path switching valve 5 is opened. Thus, the exhaust discharged from each cylinder mainly passes through the main catalytic converter 4 from the main passage 3. At this time, the bypass passage 7 side is not particularly cut off, but the bypass passage 7 side has a smaller passage cross-sectional area than the main passage 3 side and the bypass catalytic converter 8 is interposed. Due to the difference, most of the exhaust flow passes through the main passage 3 side and hardly flows into the bypass passage 7 side. Therefore, the thermal deterioration of the bypass catalytic converter 8 is sufficiently suppressed.

  Next, the diagnosis of leakage of the flow path switching valve 5 will be described. In the following example, the two upstream air-fuel ratio sensors 10 and 12 are wide-area air-fuel ratio sensors, and the two downstream air-fuel ratio sensors 11 and 13 are oxygen sensors. In the present invention, at the time of diagnosis, the engine air-fuel ratio is changed stepwise from lean to rich, or from rich to lean.In the following, the case of changing from lean to rich is taken as an example. explain.

  FIG. 2 is a flowchart showing the flow of diagnostic processing. First, in step 1, the engine speed NE, load (fuel injection amount TP) and throttle valve opening TVO at that time are read. In step 2, these values are read. It is determined whether or not a leakage diagnosis is possible from the operating conditions. This diagnosis is basically executed under steady operation where the bypass catalytic converter 8 is active, the main catalytic converter 4 is not warmed up, and the flow path switching valve 5 is in the closed position. If the conditions are diagnosable, in step 3, the target air-fuel ratio of the internal combustion engine is made lean, and the process waits until the output voltage of the bypass downstream air-fuel ratio sensor 13 becomes lean (step 4). That is, the engine air-fuel ratio is made lean until the oxygen storage capacity of the bypass catalytic converter 8 is saturated, and then the target air-fuel ratio is changed stepwise to a rich (step 5). In this state, changes in the output signals of the bypass upstream air-fuel ratio sensor 12 and the main upstream air-fuel ratio sensor 10 are monitored, and diagnostic parameters described later are calculated (step 6). After the target air-fuel ratio is made rich, leakage diagnosis for a predetermined period ends (step 7), the target air-fuel ratio is returned to the theoretical air-fuel ratio (step 8), and the diagnosis parameter is compared with the determination reference value L (step 9). Note that the end of the diagnosis may be determined based on, for example, the elapsed time since the rich, or the diagnosis may be ended when the output voltage of the bypass downstream air-fuel ratio sensor 13 is reversed to the rich side. If the diagnosis parameter is larger than the determination reference value L in step 9, it is determined that there is a leak, and for example, a warning light (not shown) is turned on (step 10).

  FIG. 3 is a time chart for the above leakage diagnosis. The target air-fuel ratio of the internal combustion engine, the output voltage of the bypass downstream air-fuel ratio sensor 13, the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 and the bypass upstream side. Each change of the air-fuel ratio AFB detected by the air-fuel ratio sensor 12 is shown. As described above, the target air-fuel ratio is forcibly lean for diagnosis from the state of the stoichiometric air-fuel ratio, and thereafter changes to a rich stepwise manner. Since the flow path switching valve 5 is in the closed position, the exhaust gas of the internal combustion engine basically flows on the bypass passage 7 side, and oxygen is stored in the catalyst of the bypass catalytic converter 8 while the engine air-fuel ratio is lean. . Therefore, the output signal of the bypass downstream air-fuel ratio sensor 13 downstream of the bypass catalytic converter 8 is on the lean side with some delay. Further, both the bypass upstream air-fuel ratio sensor 12 and the main upstream air-fuel ratio sensor 10 show the lean exhaust air-fuel ratio. In this embodiment, the target air-fuel ratio is changed richly by giving an appropriate delay period after the signal of the bypass downstream air-fuel ratio sensor 13 becomes lean.

  When the engine air-fuel ratio changes from lean to rich, the detected air-fuel ratio AFB of the bypass upstream air-fuel ratio sensor 12 located upstream of the bypass catalytic converter 8 immediately becomes rich. On the other hand, since the oxygen stored in the catalyst of the bypass catalytic converter 8 is released downstream of the bypass catalytic converter 8, the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 is immediately rich. However, it changes to rich with a slight delay as shown by the solid line. This solid line is a characteristic when there is no leakage. That is, if there is no leakage, the detected air-fuel ratio AFB of the bypass upstream air-fuel ratio sensor 12 and the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 are greatly different.

  On the other hand, when there is an exhaust leakage through the flow path switching valve 5 due to a sealing failure of the valve body of the flow path switching valve 5 or the like, a part of the exhaust gas is not emptied through the main upstream side without passing through the bypass catalytic converter 8. Since the air-fuel ratio sensor 10 is reached, the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 shows a richer value, as indicated by a virtual line. In other words, if there is a leak, the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 approaches the detected air-fuel ratio AFB of the bypass upstream air-fuel ratio sensor 12 according to the degree, and the difference between the two becomes small. Since the output voltage of the bypass downstream air-fuel ratio sensor 13 reaches a level corresponding to a predetermined lean at the stage where all the oxygen stored in the catalyst is released, the target air-fuel ratio is theoretically determined at this point in this embodiment. Return to air-fuel ratio.

  As described above, the presence / absence of leakage can be diagnosed based on the output signals of the main upstream air-fuel ratio sensor 10 and the bypass upstream air-fuel ratio sensor 12, and more specifically, the timing of the change of the detected air-fuel ratio or the following As will be described in detail, a diagnosis can be made using the difference in the detected air-fuel ratio.

  That is, in order to quantify the degree of leakage, in the present embodiment, the average value AVAFM of the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 and the bypass upstream air-space for a predetermined period T after switching of the target air-fuel ratio. The average value AFAFB of the detected air-fuel ratio AFB of the fuel ratio sensor 12 is obtained, and the difference (AVAFM−AVAFB) is used as the above-described diagnostic parameter. This diagnostic parameter is compared with a predetermined determination reference value L as described above. The period T is set so as to include the entire period in which the target air-fuel ratio is rich, for example, but may be a part thereof.

  Diagnosis using such a diagnosis parameter based on the detected air-fuel ratio also has the following effects. In other words, the wide-range air-fuel ratio sensor responds sensitively to slight changes in the air-fuel ratio, so even if there is no leakage, the output value changes slightly but immediately if the exhaust air-fuel ratio changes (even downstream of the catalyst). (See, for example, “(2) No AFM leakage” in FIG. 3). Therefore, it is difficult to make a diagnosis at the change timing (time difference) of the output value. On the other hand, as described above, the change timing (time difference) is used by using, as a diagnostic parameter, the difference in detected air-fuel ratio that appears as a more obvious difference depending on the presence or absence of leakage, particularly the average value within a predetermined time. Compared with the case, the diagnostic accuracy is further improved.

  As described above, in the above embodiment, it is possible to easily diagnose whether or not the flow path switching valve 5 has leaked by using the existing air-fuel ratio sensors 10 to 13 used for air-fuel ratio feedback control. As a result, it is possible to avoid deterioration of exhaust emission due to poor sealing of the flow path switching valve 5 or the like.

  Furthermore, when the stoichiometric air-fuel ratio is changed to the opposite air-fuel ratio across the theoretical air-fuel ratio, the difference in the detected air-fuel ratio due to the presence or absence of leakage is greater when the amount of oxygen stored in the catalyst is made as saturated or empty as possible. This improves the diagnostic accuracy. In the above embodiment, the engine air-fuel ratio is made lean so as to increase the amount of oxygen stored in the catalyst of the bypass catalytic converter 8 at first, so that it becomes sufficiently saturated, and the oxygen amount is saturated in this way. In the meantime, unpurified HC is not discharged.

  Contrary to the above embodiment, when the engine air-fuel ratio is first made rich, the amount of oxygen stored in the catalyst is reduced, and for a while after the engine air-fuel ratio changes to lean, it flows into the catalyst. Oxygen is stored in the catalyst. Therefore, if there is no leakage, the main upstream air-fuel ratio sensor 10 changes to lean with a delay, and if it changes quickly to lean, it is known that the leakage is occurring.

  By the way, since the diagnosis of the leakage depends on the oxygen storage capacity of the catalyst of the bypass catalytic converter 8, it is affected when the catalyst deteriorates and the oxygen storage capacity decreases. Therefore, the determination reference value L may be a fixed value, but if the value is corrected according to the degree of catalyst deterioration, the leakage diagnosis becomes more accurate.

  FIG. 4 is a time chart for explaining an example of a method for diagnosing the degree of catalyst deterioration of the bypass catalytic converter 8, which is executed when the flow path switching valve 5 is in the open position. In addition, you may make it open compulsorily for a catalyst deterioration diagnosis. Then, with the flow path switching valve 5 open, the target air-fuel ratio of the internal combustion engine is changed stepwise from lean to rich, and the change timing of the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 and the downstream of the bypass A time difference ΔT with respect to the change timing of the output signal AFB of the side air-fuel ratio sensor 13 is measured. That is, since the flow path switching valve 5 is open, when the engine air-fuel ratio becomes rich, the detected air-fuel ratio AFM of the main upstream air-fuel ratio sensor 10 immediately becomes rich, whereas the bypass downstream air-fuel ratio The exhaust air-fuel ratio AFB detected by the sensor 13 changes richly with a delay due to the oxygen storage capacity of the catalyst. Therefore, in the case of a new bypass catalytic converter 8 with no deterioration, the time difference ΔT is large, and the time difference ΔT is small according to the degree of deterioration. Therefore, from this time difference ΔT, the degree of deterioration of the catalyst can be obtained as shown in FIG. The measurement of the degree of catalyst deterioration can be performed by utilizing the change in the air-fuel ratio at the time of so-called fuel cut recovery in which the fuel supply is resumed after the fuel cut at the time of deceleration, for example.

  A determination reference value L is obtained from the relationship shown in FIG. 6, for example, with respect to the degree of catalyst deterioration obtained as described above. As a result, the determination reference value L becomes a value considering the degree of catalyst deterioration, and the above-described diagnosis of the presence or absence of leakage becomes more accurate. In particular, since the degree of catalyst deterioration is diagnosed with the flow path switching valve 5 open, if there is a leakage at the closed position of the flow path switching valve 5, the measurement of the catalyst deterioration level is not affected by the leakage. Since the diagnosis of leakage is performed on the premise of the degree of catalyst deterioration, it is possible to accurately determine the presence or absence of leakage. 5 and 6 schematically illustrate linear characteristics, but it goes without saying that the present invention is not limited to this. The diagnosis of the degree of catalyst deterioration is not limited to the above method, and various known methods can be applied.

BRIEF DESCRIPTION OF THE DRAWINGS The structure explanatory drawing which shows the structure of the internal combustion engine which concerns on this invention. The flowchart which shows the flow of a process of a leakage diagnosis. The time chart which shows an example of a leakage diagnosis. The time chart which shows an example of the diagnostic method of a catalyst deterioration degree. The characteristic view which shows the relationship between time difference (DELTA) T and catalyst degradation degree. The characteristic view which shows the relationship between a catalyst deterioration degree and the criterion value L.

Explanation of symbols

DESCRIPTION OF SYMBOLS 3 ... Main passage 4 ... Main catalytic converter 5 ... Flow path switching valve 6 ... Branch point 7 ... Bypass passage 8 ... Bypass catalytic converter 9 ... Junction point 10 ... Main upstream air-fuel ratio sensor 11 ... Main downstream air-fuel ratio sensor 12 ... Bypass upstream air-fuel ratio sensor 13 ... Bypass downstream air-fuel ratio sensor 27 ... Engine control unit

Claims (8)

  A bypass passage is provided in parallel with the upstream portion of the main passage provided with the main catalytic converter on the downstream side, the bypass passage is provided with the bypass passage, and the main passage is blocked at the upstream portion of the main passage. In the exhaust gas purification apparatus for an internal combustion engine, the first air-fuel ratio detecting means for detecting the exhaust air-fuel ratio upstream of the bypass catalytic converter in the bypass passage, and the main catalytic converter upstream of the main passage The second air-fuel ratio detecting means for detecting the exhaust air-fuel ratio of the engine and the air-fuel ratio of the internal combustion engine from lean or rich to the rich on the opposite side of the stoichiometric air-fuel ratio while the flow path switching valve is controlled to the closed position Alternatively, the air-fuel ratio control means that changes stepwise to lean, and each air-fuel ratio detection means after the engine air-fuel ratio changes stepwise Based on the detected air-fuel ratio, the trouble diagnosis device for the exhaust gas purification system of an internal combustion engine, characterized in that it comprises a diagnostic means for diagnosing leakage of the flow path switching valve.   The presence or absence of leakage is determined based on the difference between the average air-fuel ratio of the first air-fuel ratio detection means and the average air-fuel ratio of the second air-fuel ratio detection means in a predetermined period after the engine air-fuel ratio changes stepwise. The failure diagnosis device for an exhaust gas purification device for an internal combustion engine according to claim 1.   3. The internal combustion engine according to claim 1, wherein the second air-fuel ratio detecting unit detects an air-fuel ratio downstream of a joining point where a downstream end of the bypass passage joins the main passage. Failure diagnosis device for exhaust purification system.   The failure diagnosis apparatus for an exhaust gas purification apparatus for an internal combustion engine according to any one of claims 1 to 3, wherein the air-fuel ratio control means changes the air-fuel ratio stepwise from lean to rich.   5. The exhaust gas purification apparatus for an internal combustion engine according to claim 4, wherein the engine air-fuel ratio is kept lean for a period sufficient to saturate the oxygen storage capacity of the bypass catalytic converter and then is changed in a rich stepwise manner. Fault diagnosis device.   The exhaust of the internal combustion engine according to any one of claims 1 to 5, further comprising means for diagnosing the degree of catalyst deterioration of the bypass catalytic converter, and correcting the diagnosis of leakage according to the degree of catalyst deterioration. Failure diagnosis device for purification device.   An air-fuel ratio detecting means for detecting an exhaust air-fuel ratio downstream of the bypass catalytic converter is further provided, and the bypass catalytic converter upstream when the engine air-fuel ratio changes in a state where the flow path switching valve is controlled to the open position. 7. The failure diagnosis device for an exhaust gas purification apparatus for an internal combustion engine according to claim 6, wherein the degree of catalyst deterioration is obtained based on changes in the exhaust air-fuel ratio on the side and downstream side.   The difference between the average air-fuel ratio of the first air-fuel ratio detection means and the average air-fuel ratio of the second air-fuel ratio detection means in a predetermined period after the engine air-fuel ratio changes stepwise is compared with a determination reference value to determine whether there is leakage. 8. The failure diagnosis device for an exhaust gas purification apparatus for an internal combustion engine according to claim 6, wherein the determination reference value is corrected in accordance with the degree of catalyst deterioration.

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