JP2010163932A - Catalyst degradation diagnostic device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst degradation diagnostic device for an internal combustion engine, which considers degradation levels of both upstream catalyst and downstream catalyst. <P>SOLUTION: An ECU 4 calculates an accumulated degradation level of the upstream catalyst 38 disposed on the upstream side of an exhaust passage 36, calculates an accumulated degradation level of the downstream catalyst 40 disposed on the downstream side from the upstream catalyst 38, calculates an oxygen storage capacity of the upstream catalyst 38, calculates an oxygen storage capacity of the downstream catalyst 40 based on the accumulated degradation levels of the upstream catalyst 38 and the downstream catalyst 40 and the oxygen storage capacity of the upstream catalyst 38, and diagnoses degradation of the upstream catalyst 38 and the downstream catalyst 40 based on the oxygen storage capacities of the upstream catalyst 38 and the downstream catalyst 40. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a catalyst deterioration diagnosis apparatus for an internal combustion engine.

  Patent Document 1 discloses a technique for determining deterioration of an upstream catalyst by disposing oxygen sensors provided between an upstream catalyst and a downstream catalyst in an exhaust passage and downstream of the downstream catalyst.

JP 2003-97334 A

  The technique disclosed in Patent Document 1 determines the deterioration of the upstream catalyst without considering the deterioration of the downstream catalyst. For this reason, even if the downstream catalyst is deteriorated, the upstream catalyst may be determined to be normal.

  An object of the present invention is to provide a catalyst deterioration diagnosis device for an internal combustion engine that takes into account the deterioration degree of both an upstream catalyst and a downstream catalyst.

  The object is to provide a first cumulative deterioration degree calculating means for calculating a cumulative cumulative deterioration degree of an upstream catalyst disposed upstream of an exhaust passage of an internal combustion engine, and a downstream catalyst disposed downstream of the upstream catalyst. Second cumulative deterioration degree calculating means for calculating the accumulated cumulative deterioration degree of the first catalyst, first storage capacity calculating means for calculating the oxygen storage capacity of the upstream catalyst, the cumulative deterioration degree of the upstream catalyst and the downstream catalyst, and the upstream Second storage capacity calculation means for calculating the oxygen storage capacity of the downstream catalyst based on the oxygen storage capacity of the catalyst, and diagnosis of deterioration of the upstream catalyst and downstream catalyst based on the oxygen storage capacity of the upstream catalyst and downstream catalyst This can be achieved by a catalyst deterioration diagnosis device for an internal combustion engine.

  The oxygen storage capacity of the downstream catalyst is calculated in consideration of the cumulative deterioration degree of the downstream catalyst, and the deterioration of the upstream catalyst and the downstream catalyst is diagnosed based on the oxygen storage capacity of the upstream catalyst and the downstream catalyst. Thus, since the deterioration of the upstream catalyst and the downstream catalyst is diagnosed based on the oxygen storage capacity of the upstream catalyst and the downstream catalyst, the diagnostic accuracy of the catalyst deterioration is improved.

  In the above-described configuration, the second cumulative deterioration degree calculating means is an active air-fuel ratio that controls the target air-fuel ratio of the air-fuel mixture based on whether misfire has occurred, whether fuel cut is being performed, and the air-fuel ratio of the exhaust gas. A configuration in which the cumulative deterioration degree of the downstream catalyst is calculated in consideration of at least one of whether control is being executed or not can be adopted.

  Thereby, the cumulative deterioration degree of the downstream catalyst can be calculated in consideration of the state of the internal combustion engine. Accordingly, the calculation accuracy of the cumulative deterioration degree is improved.

  The said structure WHEREIN: The said 2nd cumulative deterioration degree calculation means can employ | adopt the structure which calculates the deterioration degree of the said downstream catalyst based on the temperature of the said downstream catalyst.

  The catalyst is more susceptible to deterioration at higher temperatures. Therefore, by calculating the degree of deterioration of the upstream catalyst based on the temperature of the downstream catalyst, the calculation accuracy of the degree of deterioration of the downstream catalyst is improved.

  In the above-described configuration, the second cumulative deterioration degree calculating means takes into account the output value of an oxygen sensor disposed between the upstream catalyst and the downstream catalyst during fuel cut, and the cumulative deterioration degree of the downstream catalyst. It is possible to adopt a configuration that calculates

  The period from when the fuel cut is performed to when the fuel cut is performed under the rich atmosphere to when the fuel cut is performed until the downstream catalyst becomes a lean atmosphere is different. Due to the difference in the period, the degree of deterioration of the downstream catalyst is also different. By considering the output value of the oxygen sensor, the difference in this period can be taken into account. Thereby, the calculation accuracy of the cumulative deterioration degree of the downstream catalyst is improved.

  The said structure can employ | adopt the structure which the said diagnostic means diagnoses based on the sum total of the oxygen storage capacity of the said upstream catalyst and a downstream catalyst.

  Thereby, the deterioration of the catalyst can be diagnosed in consideration of the deterioration degree of both the upstream catalyst and the downstream catalyst. Therefore, it is possible to prevent erroneous diagnosis that the upstream catalyst is not deteriorated even though the downstream catalyst is deteriorated.

  In the above configuration, the second storage capacity calculation means is configured to determine the oxygen content of the downstream catalyst based on the ratio of the cumulative deterioration degree of the upstream catalyst and the cumulative deterioration degree of the downstream catalyst and the oxygen storage capacity of the upstream catalyst. A configuration for calculating the storage capacity can be adopted.

  Thus, the oxygen storage capacity of the downstream catalyst can be calculated by a simple method.

  ADVANTAGE OF THE INVENTION According to this invention, the catalyst deterioration diagnostic apparatus of an internal combustion engine which considers the deterioration degree of both an upstream catalyst and a downstream catalyst can be provided.

FIG. 1 is a schematic diagram illustrating a configuration of an engine system according to the present embodiment. FIG. 2 is a flowchart showing an example of a catalyst deterioration diagnosis process executed by the ECU. FIG. 3 is an exemplary diagram of a diagnostic map. FIG. 4 is a flowchart showing an example of the calculation process of the cumulative deterioration degree of the upstream catalyst executed by the ECU. FIG. 5 is an exemplary diagram of a deterioration degree calculation map. FIG. 6 is a flowchart showing an example of a calculation process of the cumulative deterioration degree of the downstream catalyst executed by the ECU. FIG. 7 is an explanatory diagram of the behavior of the output value of the oxygen sensor before and after the fuel cut is performed. 8A and 8B are graphs showing the relationship between the degree of deterioration of the upstream catalyst and the purification rate of exhaust gas that has passed through the downstream catalyst. 9A and 9B are views showing the fuel cut period and the air-fuel ratio in the upstream catalyst bed and in the downstream catalyst bed. FIG. 10 is a graph showing the relationship between the deterioration degree of the catalyst and the travel distance.

  Embodiments according to the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic diagram showing a configuration of an engine system according to the present embodiment. A multi-cylinder in-cylinder injecting gasoline engine (hereinafter abbreviated as “engine”) 2 mounted on an automobile and its electronic control unit ( Hereinafter, the schematic configuration of the ECU 4 is referred to as “ECU”. In FIG. 1, the configuration of one cylinder is mainly shown. The engine 2 is provided with a fuel injection valve 12 that directly injects fuel into the combustion chamber 10 and an ignition plug 14 that ignites the injected fuel.

  The intake port 16 connected to the combustion chamber 10 is opened and closed by driving an intake valve (not shown). A surge tank 22 is provided in the middle of the intake passage 20 connected to the intake port 16, and a throttle valve 26 whose opening degree is adjusted by a throttle motor 24 is provided upstream of the surge tank 22.

  The intake air amount is adjusted by the opening degree of the throttle valve 26. The throttle opening is detected by a throttle opening sensor 28, and the intake pressure PM in the surge tank 22 is detected by an intake pressure sensor 30 provided in the surge tank 22, and is read into the ECU 4. An air flow meter 21 is disposed in the intake passage 20 to output the intake air amount to the ECU 4.

  The exhaust port 32 connected to the combustion chamber 10 is opened and closed by driving an exhaust valve (not shown). An exhaust passage 36 connected to the exhaust port 32 is provided with an upstream catalyst 38 and a downstream catalyst 40 that purify exhaust gas and store oxygen. In the exhaust passage 36, an air-fuel ratio sensor 64 is disposed upstream of the upstream catalyst 38, and an oxygen sensor 66 is disposed between the upstream catalyst 38 and the downstream catalyst 40.

  The ECU 4 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like, and controls the operation of the entire engine. The ROM stores a program for executing a catalyst deterioration diagnosis process described later. The ECU 4 corresponds to first cumulative deterioration degree calculating means, second cumulative deterioration degree calculating means, first storage amount calculating means, second storage amount calculating means, and diagnostic means.

  In addition to the throttle opening sensor 28 and the intake pressure sensor 30, the ECU 4 inputs a signal from an accelerator opening sensor 56 that detects the depression amount of the accelerator pedal 44. Further, the ECU 4 receives signals from an engine speed sensor 58, an air-fuel ratio sensor 64, and an oxygen sensor 66 that detect the engine speed NE from the rotation of the crankshaft 54, respectively. Further, a water temperature sensor 41 for detecting the engine coolant temperature is provided, and the detected engine coolant temperature is output to the ECU 4. The ECU 4 appropriately controls the fuel injection timing, the fuel injection amount, and the throttle opening of the engine 2 based on the detection contents from the various sensors described above.

  The ECU 4 actively controls the fuel injection amount so that the air-fuel ratio of the exhaust gas flowing into the upstream catalyst 38 becomes the stoichiometric air-fuel ratio. In the active control, the fuel injection amount is controlled based on the output of the air-fuel ratio sensor 64 or the output thereof and the output of the oxygen sensor 66.

In the active control, the fuel injection amount is decreased when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, and the fuel injection amount is increased when it is lean. As a result, the air-fuel ratio is maintained within a predetermined range near the stoichiometric air-fuel ratio. The above-described upstream catalyst 38 exhibits high purification performance when the air-fuel ratio is near the stoichiometric air-fuel ratio.

  Next, the catalyst deterioration diagnosis process executed by the ECU 4 will be described. FIG. 2 is a flowchart showing an example of the catalyst deterioration diagnosis process executed by the ECU 4.

  The ECU 4 determines whether or not a diagnosis condition for catalyst deterioration is satisfied (step S1). For example, if the engine 2 is in a steady operation state and the catalyst temperature is in the active temperature range, such as the intake air amount and the engine rotational speed are substantially constant, it is determined that the condition is satisfied. When the condition is not satisfied, the catalyst deterioration diagnosis process is not executed, and when the condition is satisfied, the process of step S2 is executed.

  The ECU 4 reads the cumulative deterioration levels of the upstream catalyst 38 and the downstream catalyst 40 (steps S2 and S3). Calculation of the cumulative deterioration degree of the upstream catalyst 38 and the downstream catalyst 40 will be described later. Next, the ECU 4 calculates the oxygen storage capacity of the upstream catalyst 38 (step S4). The calculation of the oxygen storage capacity of the upstream catalyst 38 is performed by the following method.

When the pre-catalyst air-fuel ratio is lean, the oxygen excess amount ΔOSA in the exhaust gas flowing into the upstream catalyst 38 can be obtained by the following equation.
ΔOSA = (A / F−A / Fstoichi) × fuel injection amount × α (1)
Where A / Fstoichi is the stoichiometric air-fuel ratio, and α is the ratio of oxygen in the air.
On the other hand, when the pre-catalyst air-fuel ratio is rich, the oxygen deficiency ΔOSA in the exhaust gas flowing into the upstream catalyst 38 can be obtained by the following equation.
ΔOSA = (A / Fstoichi−A / F) × fuel injection amount × α (2)

Therefore, if | A / F−A / Fstoichi | = ΔA / F, the exhaust gas flowing into the upstream catalyst 38 can be distinguished without distinguishing when the pre-catalyst air-fuel ratio is rich or lean. The oxygen excess / deficiency ΔOSA can be expressed by the following equation.
ΔOSA = ΔA / F × fuel injection amount × α (3)

The ECU 4 clears the oxygen storage amount OSA every time the output of the oxygen sensor 66 is inverted, and thereafter calculates the integrated value of the oxygen excess / deficiency ΔOSA as the oxygen storage amount OSA as shown in the following equation.
OSA = ΣΔOSA
= Σ (ΔA / F × fuel injection amount × α) (4)

  The output of the oxygen sensor 66 is inverted either when all the oxygen in the upstream catalyst 38 is released or when the oxygen is fully stored in the upstream catalyst 38. In the former case, thereafter, the oxygen storage capacity of the upstream catalyst 38 can be obtained by accumulating the excess oxygen amount ΔOSA until oxygen is fully stored in the upstream catalyst 38 and the output of the oxygen sensor 66 is reversed again. . Also in the latter case, thereafter, the oxygen storage capacity of the upstream catalyst 38 can be obtained by integrating the oxygen deficiency ΔOSA until all the oxygen in the upstream catalyst 38 is released and the output of the oxygen sensor 66 is reversed again. it can. That is, in any case, when the output of the oxygen sensor 66 is reversed, the oxygen storage amount OSA calculated at that time can be recognized as the oxygen storage capacity of the upstream catalyst 38. By such a method, the ECU 4 calculates the oxygen storage amount of the upstream catalyst 38. The ECU 4 may calculate the oxygen storage amount of the upstream catalyst 38 by other known methods.

Next, the ECU 4 calculates the oxygen storage capacity of the downstream catalyst 40 (step S5). Specifically, the ratio of the cumulative deterioration degree Fr_S of the upstream catalyst 38 and the cumulative deterioration degree Rr_S of the downstream catalyst 40 and the ratio of the oxygen storage capacity Fr_Cmax of the upstream catalyst 38 and the oxygen storage capacity Rr_Cmax of the downstream catalyst 40 are equal. Then, the oxygen storage capacity Rr_Cmax of the downstream catalyst 40 is calculated by the following equation.
Rr_Cmax = Fr_Cmax * Rr_S / Fr_S (5)

  In this way, the ECU 4 calculates the oxygen storage capacity of the downstream catalyst 40 by a simple method. It is also conceivable to calculate the oxygen storage capacity of the downstream catalyst 40 in the same manner as the calculation of the oxygen storage capacity of the upstream catalyst 38 described above. However, if the oxygen storage capacity is calculated by the same method as described above up to the downstream catalyst 40, the exhaust gas flowing into the downstream catalyst 40 until the air-fuel ratio of the exhaust gas downstream from the downstream catalyst 40 is rich and reverses between leans. It is necessary to control the air-fuel ratio of the engine to either the lean side or the rich side. For this reason, the emission may be deteriorated.

  Next, the ECU 4 adds the respective oxygen storage capacities of the upstream catalyst 38 and the downstream catalyst 40 to calculate the oxygen storage capacity T_Cmax of the entire upstream catalyst 38 and downstream catalyst 40 (step S6). Next, the ECU 4 determines whether or not the overall oxygen storage capacity T_Cmax is less than the deterioration determination value Kobd (step S7). Specifically, the ECU 4 diagnoses the deterioration state of the catalyst based on a diagnostic map stored in advance in the ROM. FIG. 3 is an exemplary diagram of a diagnostic map. It is determined whether the value of T_Cmax is located in the normal diagnosis region or the abnormal diagnosis region divided by the diagnosis curve K on the diagnosis map.

  When T_Cmax is less than the deterioration determination value Kobd, the ECU 4 is diagnosed as normal (step S8), and when T_Cmax exceeds the deterioration determination value Kobd, the ECU 4 is diagnosed as abnormal ( Step S9).

  As described above, the ECU 4 performs catalyst abnormality diagnosis in consideration of the deterioration degree of both the upstream catalyst 38 and the downstream catalyst 40. Thereby, the diagnostic accuracy of catalyst deterioration improves.

  Next, calculation of the cumulative deterioration degree of the upstream catalyst 38 executed by the ECU 4 will be described. FIG. 4 is a flowchart showing an example of a process for calculating the cumulative deterioration degree of the upstream catalyst 38 executed by the ECU 4. Note that the ECU 4 executes a process for calculating the cumulative deterioration degree of the upstream catalyst 38 every 200 msec, for example.

  As shown in FIG. 4, the ECU 4 calculates the temperature of the upstream catalyst 38 (step S10). The temperature is calculated based on at least one of the engine speed calculated based on the output of the crank angle sensor 58 detected by the air flow meter 21 and the engine load calculated based on the detected value of the throttle opening sensor 28. Based on this, the temperature of the upstream catalyst 38 is estimated using a map or function set in advance through experiments or the like. The temperature of the upstream catalyst 38 may be calculated by other known methods.

  Next, the ECU 4 determines whether the engine 2 has misfired (step S11). The determination of the presence or absence of misfire is made based on the change in the pressure in the combustion chamber 10 before and after the spark is blown by the spark plug 14. The determination of the presence or absence of misfire may be performed by other known methods.

  If misfire has occurred, the ECU 4 calculates the degree of deterioration of the upstream catalyst 38 according to the misfire (step S12). Specifically, the calculation is performed based on a map showing the degree of deterioration of the upstream catalyst 38 associated with the catalyst temperature. FIG. 5 is an exemplary diagram of a deterioration degree calculation map for calculating the deterioration degree. As shown in FIG. 5, the higher the catalyst temperature, the higher the degree of deterioration of the upstream catalyst 38 is set. In the deterioration degree calculation map, the degree of deterioration is defined for each of the presence / absence of fuel cut, presence / absence of active control, and presence / absence of rich atmosphere, which will be described later. Note that, when calculating the degree of deterioration of the downstream catalyst 40, which will be described later, a map similar to the map shown in FIG. 5 is referred to. However, if the types of the upstream catalyst 38 and the downstream catalyst 40 are different, Each deterioration degree is calculated based on the map corresponding to each.

  Next, the ECU 4 calculates the cumulative deterioration degree of the upstream catalyst 38 (step S18). Specifically, it is calculated by adding the currently calculated deterioration frequency to the cumulative value of the deterioration frequency of the upstream catalyst 38 calculated up to the previous time.

  If no misfire has occurred, the ECU 4 determines whether or not there is a fuel cut (step S13). The ECU 4 determines whether or not there is a fuel cut based on the fuel cut flag. During the fuel cut, the upstream catalyst 38 and its surroundings are in a lean atmosphere, so the deterioration of the upstream catalyst 38 proceeds.

  When the fuel is being cut, the ECU 4 calculates the deterioration frequency (step S14). Specifically, it is calculated based on the above-described deterioration degree calculation map. Next, the ECU 4 executes the process of step S18.

  If the fuel is not being cut, the ECU 4 determines whether or not active air-fuel ratio control is being executed (step S15). Specifically, the ECU 4 makes a determination based on an active air-fuel ratio control flag. When active control is being executed, the ECU 4 calculates the degree of deterioration of the upstream catalyst 38 according to the active control based on the deterioration degree calculation map (step S16).

  When the active control is not executed, the ECU 4 calculates the deterioration degree of the upstream catalyst 38 based on the deterioration degree calculation map, assuming that the exhaust gas flowing into the upstream catalyst 38 is in a rich atmosphere (step S17). .

  Note that the deterioration of the upstream catalyst 38 proceeds at the time of misfire or fuel cut. This is because at the time of misfire, the amount of intake air is large and oxygen is not consumed by combustion, so that a large amount of oxygen is supplied to the upstream catalyst 38. This is because when the fuel is cut, the degree of opening of the throttle valve 26 is controlled to some extent, but combustion is not performed, so that the vicinity of the upstream catalyst 38 is in a lean atmosphere.

  Thus, the ECU 4 determines whether misfire has occurred, whether fuel cut is being performed, and whether active air-fuel ratio control for controlling the target air-fuel ratio of the mixture based on the air-fuel ratio of the exhaust gas is being performed, Accordingly, the degree of deterioration of the upstream catalyst 38 is calculated. By calculating the deterioration degree according to the state of the engine 2 in this way, the calculation accuracy of the cumulative deterioration degree of the upstream catalyst 38 is improved.

  Next, calculation of the cumulative deterioration degree of the downstream catalyst 40 executed by the ECU 4 will be described. FIG. 6 is a flowchart showing an example of the calculation process of the cumulative deterioration degree of the downstream catalyst 40 executed by the ECU 4. The calculation process for the cumulative deterioration degree of the downstream catalyst 40 is in common with the cumulative deterioration calculation process for the upstream catalyst 38. For this reason, the description of the common part processing is omitted.

  As shown in FIG. 6, in the case of an affirmative determination in step S23, that is, when the fuel cut is being executed, the ECU 4 reads the output value of the oxygen sensor 66 (step S131), and the output value is less than 0.05V. It is determined whether or not (step S132). When the output voltage value of the oxygen sensor 66 is less than 0.05V, the ECU 4 executes the process of step S24. If it exceeds 0.05 V, the ECU 4 executes the process of step S25. The reason for this will be described.

  FIG. 7 is an explanatory diagram of the behavior of the output value of the oxygen sensor 66 before and after the fuel cut is executed. The oxygen sensor 66 outputs a voltage value within a range of 1 to 0 V, and outputs a lower voltage value as the amount of oxygen increases. When air that does not contain fuel is supplied to the oxygen sensor 66, the output value of the oxygen sensor 66 is less than 0.05V.

  As shown in FIG. 7, when a fuel cut is performed in a rich atmosphere, the output value of the oxygen sensor 66 indicates a lean state after a certain period of time has elapsed since the fuel flag was turned on. This is because the air in the lean atmosphere flows into the upstream catalyst 38 for a predetermined period after the fuel cut is executed, and the upstream catalyst 38 occludes oxygen. For this reason, the downstream side of the upstream catalyst 38 has a rich atmosphere for a predetermined period. When the amount of oxygen stored in the upstream catalyst 38 reaches the upper limit of the capacity, the downstream side of the upstream catalyst 38 becomes a lean atmosphere.

  Further, as shown in FIG. 7, when the fuel cut is executed in a lean atmosphere, the output value of the oxygen sensor 66 further decreases relatively quickly after the fuel cut is executed. This is based on the following reason. In the lean atmosphere, the amount of oxygen stored in the upstream catalyst 38 is already near the upper limit of the capacity, so that when the fuel cut is executed, the amount of oxygen stored in the upstream catalyst 38 immediately reaches the upper limit of the capacity. For this reason, air that is not mixed with fuel is supplied downstream from the upstream catalyst 38.

  Therefore, even when the fuel cut is executed, there are cases where the vicinity of the downstream catalyst 40 does not immediately become a lean atmosphere. In consideration of such a case, the ECU 4 accumulates the degree of deterioration of the downstream catalyst 40 based on the output value of the oxygen sensor 66. Thereby, the calculation accuracy of the cumulative deterioration degree of the downstream catalyst 40 is improved.

  As described above, the ECU 4 diagnoses the deterioration of the upstream catalyst 38 and the downstream catalyst 40 based on the oxygen storage capacity of the upstream catalyst 38 and the downstream catalyst 40. This improves the accuracy of diagnosis of catalyst deterioration.

  Here, the importance of performing the catalyst deterioration diagnosis by calculating the deterioration degrees of both the upstream catalyst 38 and the downstream catalyst 40 will be briefly described. 8A and 8B are graphs showing the relationship between the degree of deterioration of the upstream catalyst 38 and the purification rate of the exhaust gas that has passed through the downstream catalyst 40.

  The graph in FIG. 8A shows the relationship between the degree of deterioration of the upstream catalyst 38 and the total amount of HC components in the exhaust gas. The HC purification rate depends on the performance of the upstream catalyst 38 and is not greatly affected by the degree of deterioration of the downstream catalyst 40. The graph in FIG. 8B shows the relationship between the degree of deterioration of the upstream catalyst 38 and the amount of NOx component in the exhaust gas. The NOx purification rate greatly depends on the performance of the downstream catalyst 40. Therefore, when diagnosing the deterioration of only the upstream catalyst 38 as in the conventional method for diagnosing catalyst deterioration, in order to avoid the diagnosis, the deterioration of the upstream catalyst 38 is diagnosed on the assumption that the degree of deterioration of the downstream catalyst 40 is large. Was. For this reason, there is a possibility that it is diagnosed that the upstream catalyst 38 is deteriorated although it is normal. However, as in this embodiment, the diagnosis is performed in consideration of the degree of deterioration of both the upstream catalyst 38 and the downstream catalyst 40, so that the diagnostic accuracy is improved.

  Next, it will be described that the deterioration state of the upstream catalyst 38 and the downstream catalyst 40 changes depending on the fuel cut period. 9A and 9B are views showing the fuel cut period and the air-fuel ratio in the upstream catalyst 38 bed and the downstream catalyst 40 bed. FIG. 9A shows a case where the fuel cut period is relatively short, and FIG. 9B shows a case where the fuel cut period is relatively long. The fuel cut period is changed according to the operating state of the engine.

  FIG. 9A shows a case where the period during which the fuel cut flag is turned ON once is 2.0 sec and is executed five times in total. Therefore, in the example shown in FIG. 9A, the total period of the fuel cut period is 10.0 sec. The atmosphere around the upstream catalyst 38 shifts from rich to lean after a time lag of about 0.5 sec after the fuel flag is turned on. The air-fuel ratio in the downstream catalyst 40 bed shifts from rich to lean after a time lag of about 1.5 sec after the fuel flag is turned on. Therefore, the period in which the downstream catalyst 40 bed is in the oxygen-excess state is 0.5 sec for one fuel cut. Therefore, the total period in which the downstream catalyst 40 bed is in an oxygen-excess state is 2.5 sec.

  FIG. 9B shows a case where the period during which the fuel cut flag is turned ON once is 5.0 sec and is executed twice in total. Therefore, in the example shown in FIG. 9A, the total period of the fuel cut period is 10.0 sec as in the case shown in FIG. 9A. The period during which the downstream catalyst 40 bed is in the oxygen-excess state is 3.5 sec for one fuel cut. Therefore, the total period during which the downstream catalyst 40 bed is in the oxygen-excess state is 7.0 sec.

  Thus, even if the total period of fuel cut is the same, the period during which the downstream catalyst 40 bed is in an oxygen-excess state may differ. This indicates that there may be a difference in the degree of deterioration of the downstream catalyst 40 even if the total period of the fuel cut is the same.

  FIG. 10 is a graph showing the relationship between the deterioration degree of the catalyst and the travel distance. FIG. 10 shows the degree of deterioration of the upstream catalyst 38 and the downstream catalyst 40 when the fuel cut period is relatively long and short as described above.

  As shown in FIG. 10, when the engine is continuously operated with one fuel cut period set to be relatively long, both the upstream catalyst 38 and the downstream catalyst 40 are likely to deteriorate at an early stage. On the other hand, when the engine is continuously operated with a single fuel cut period set to be relatively short, both the upstream catalyst 38 and the downstream catalyst 40 are unlikely to deteriorate, but the deterioration of the upstream catalyst 38 and the downstream catalyst 40 There is a big difference between the degradation of Therefore, if the deterioration of the catalyst is diagnosed based on the deterioration state of the upstream catalyst 38 as in the prior art, an erroneous determination may occur.

  However, since the deterioration diagnosis of the catalyst is performed in consideration of both the deterioration of the upstream catalyst 38 and the downstream catalyst 40 as in this embodiment, the accuracy of the diagnosis can be maintained even in the above case.

  Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the gist of the present invention described in the claims. It can be changed.

2 Engine 4 ECU
36 Exhaust passage 38 Upstream catalyst 40 Downstream catalyst 64 Air-fuel ratio sensor 66 Oxygen sensor

Claims (6)

First cumulative deterioration degree calculating means for calculating a cumulative cumulative deterioration degree of an upstream catalyst disposed upstream of the exhaust passage of the internal combustion engine;
Second cumulative deterioration degree calculating means for calculating a cumulative cumulative deterioration degree of a downstream catalyst disposed downstream of the upstream catalyst;
First storage capacity calculation means for calculating the oxygen storage capacity of the upstream catalyst;
Second storage capacity calculation means for calculating the oxygen storage capacity of the downstream catalyst based on the cumulative deterioration degree of the upstream catalyst and the downstream catalyst and the oxygen storage capacity of the upstream catalyst;
And a diagnostic means for diagnosing degradation of the upstream catalyst and downstream catalyst based on oxygen storage capacities of the upstream catalyst and downstream catalyst.   The second cumulative deterioration degree calculating means is executing active air-fuel ratio control that controls the target air-fuel ratio of the air-fuel mixture based on whether misfire has occurred, whether fuel cut is being performed, and the air-fuel ratio of the exhaust gas. 2. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 1, further comprising: calculating a cumulative deterioration degree of the downstream catalyst in consideration of at least one of the presence or absence of the catalyst.   3. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 1, wherein the second cumulative deterioration degree calculation unit calculates a deterioration degree of the downstream catalyst based on a temperature of the downstream catalyst.   The second cumulative deterioration degree calculating means calculates a cumulative stress of the downstream catalyst in consideration of an output value of an oxygen sensor arranged between the upstream catalyst and the downstream catalyst during fuel cut. The catalyst deterioration diagnosis device for an internal combustion engine according to any one of claims 1 to 3.   5. The catalyst deterioration diagnosis apparatus for an internal combustion engine according to claim 1, wherein the diagnosis unit makes a diagnosis based on a total of oxygen storage capacities of the upstream catalyst and the downstream catalyst.   The second storage capacity calculation means calculates the oxygen storage capacity of the downstream catalyst based on the ratio of the cumulative deterioration degree of the upstream catalyst and the cumulative deterioration degree of the downstream catalyst and the oxygen storage capacity of the upstream catalyst. An apparatus for diagnosing catalyst deterioration in an internal combustion engine according to any one of claims 1 to 5, wherein:

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