Catalyst deterioration diagnosis device and catalyst deterioration diagnosis method

Abstract

This invention provides a catalyst degradation diagnosis device and method that eliminates the need for floating-point calculations, enabling catalyst degradation diagnosis through simple calculations. The catalyst degradation diagnosis device includes: a catalyst disposed in the exhaust passage of an internal combustion engine; an upstream oxygen concentration detection unit disposed upstream of the catalyst; a downstream oxygen concentration detection unit disposed downstream of the catalyst; an upstream determination counting unit that counts the number of upstream determinations; and a downstream determination counting unit that counts the number of downstream determinations. When the number of upstream determinations in the upstream determination counting unit reaches a predetermined number, a catalyst degradation index value is calculated using the upstream and downstream determination counts. The counts in the upstream and downstream determination counting units are then reset. Catalyst degradation is diagnosed by comparing the catalyst degradation index value with a catalyst degradation index threshold.

Description

Technical Field

[0001] This invention relates to a catalyst degradation diagnostic device and a catalyst degradation diagnostic method, and more particularly to a method for diagnosing the degradation of a three-way catalytic converter used for exhaust gas purification in internal combustion engines such as automobiles and motorcycles. Background Technology

[0002] Previously, catalysts such as three-way catalytic converters in the exhaust pipes of internal combustion engines, such as cars and motorcycles, which are used to purify exhaust gases, were diagnosed for deterioration while the vehicle was in motion.

[0003] As a method for diagnosing catalyst deterioration, the following method is known: if the catalyst deteriorates, the purification performance decreases and the oxygen storage capacity (OSC) also decreases, and the catalyst layer cannot store enough oxygen. With an eye toward easily releasing oxygen molecules downstream of the catalyst, for example, Patent Document 1 uses an oxygen concentration detection unit (air-fuel ratio sensor) disposed on the upstream side of the catalyst and an oxygen concentration detection unit (oxygen sensor) disposed on the downstream side of the catalyst.

[0004] The catalyst degradation diagnosis method in Patent Document 1 is as follows: by using the oxygen concentration detection units, the change in oxygen concentration upstream and downstream of the catalyst is calculated as the standard deviation (σ), and the catalyst degradation index value (IDX) is calculated according to the following formula.

[0005] Formula 1

[0006] Where, σ F σ represents the standard deviation of the oxygen concentration change upstream of the catalyst. R This represents the standard deviation of oxygen concentration changes downstream of the catalyst. The larger the value, the greater the standard dispersion of the oxygen concentration change, and the greater the oxygen concentration change.

[0007] The catalyst without degradation shows virtually no change on the downstream side of the catalyst, therefore σ R A value ≥0 indicates that the catalyst degradation index (IDX) = 1. If degradation progresses, the oxygen storage capacity (OSC) decreases, therefore the change in oxygen concentration before and after catalyst degradation becomes the same (σ... F -σ R Since )≒0, it means that the catalyst degradation index value (IDX)≒0.

[0008] That is, the closer the catalyst degradation index (IDX) value is to 1, the closer it is to no degradation (new product state), and the closer the catalyst degradation index (IDX) value is to 0, the more advanced the degradation state.

[0009] Thus, in the catalyst degradation diagnosis method of Patent Document 1, it is necessary to calculate the standard deviation based on the oxygen concentration signal output by each oxygen concentration detection unit. Therefore, a high-performance computing device capable of floating-point calculation is required.

[0010] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2013-083195 Summary of the Invention The problem the invention aims to solve Therefore, components such as motorcycle engine systems are subject to cost limitations, and they are difficult to apply to ECUs (Electronic Control Units) that do not have floating-point arithmetic capabilities.

[0011] Therefore, the problem to be solved by the present invention is that catalyst degradation diagnosis can be performed through simple calculations without the need for floating-point calculations.

[0012] means for solving problems The catalyst degradation diagnostic apparatus and method of the present invention, made to solve the above-mentioned problems, comprises: a catalyst disposed in the exhaust passage of an internal combustion engine to purify exhaust gas; an upstream oxygen concentration detection unit disposed upstream of the catalyst to detect the oxygen concentration in the exhaust gas and output a signal; a downstream oxygen concentration detection unit disposed downstream of the catalyst to detect the oxygen concentration in the exhaust gas and output a signal; an upstream determination counting unit that counts the number of times the signal output from the upstream oxygen concentration detection unit crosses a voltage threshold, as the upstream determination count; and a downstream determination counting unit that counts the number of times the signal output from the downstream oxygen concentration detection unit crosses a voltage threshold, as the downstream determination count. When the upstream determination count in the upstream determination counting unit reaches a predetermined number, a catalyst degradation index value (IDX) is calculated using the upstream determination count and the downstream determination count, and the counts of the upstream determination counting unit and the downstream determination counting unit are reset. By comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold, the degradation of the catalyst is diagnosed.

[0013] Invention Effects According to the present invention, a catalyst degradation diagnosis device and a catalyst degradation diagnosis method can be provided, which do not require floating-point calculations and can perform catalyst degradation diagnosis through simple calculations. Attached Figure Description

[0014] Figure 1 A schematic diagram illustrating an application example of the catalyst degradation diagnostic device according to an embodiment of the present invention.

[0015] Figure 2 This is a block diagram illustrating an example of a catalyst degradation diagnostic device according to an embodiment of the present invention.

[0016] Figure 3 To demonstrate execution Figure 2 A flowchart illustrating the conditions of an example degradation diagnosis method.

[0017] Figure 4 To show Figure 2 A block diagram of an example degradation diagnosis method.

[0018] Figure 5 To show Figure 2 A graph showing the output voltage of the upstream oxygen concentration detection unit when the catalyst is functioning normally.

[0019] Figure 6 To show Figure 2 A graph showing the number of determinations made by the upstream oxygen concentration detection unit when the catalyst is in normal operation, as an example.

[0020] Figure 7 To show Figure 2 A graph showing the output voltage of the downstream oxygen concentration detection unit when the catalyst is functioning normally.

[0021] Figure 8 To show Figure 2 A graph showing the number of determinations made by the downstream oxygen concentration detection unit when the catalyst is functioning normally.

[0022] Figure 9A , Figure 9B To show Figure 2 A schematic diagram of an example of a degradation diagnosis method. Figure 9A A graph showing the relationship between the exhaust gas and oxygen concentration detection unit before and after catalyst deterioration (new product) is presented. Figure 9B A graph showing the relationship between the exhaust gas and oxygen concentration detection unit before and after catalyst degradation.

[0023] Figure 10 To show Figure 2 A graph showing the number of counts for the upstream and downstream determination counters in one example.

[0024] Figure 11 To show Figure 2 A graph showing the measured values ​​of catalyst degradation index for an example.

[0025] Figure 12 To show Figure 2 A graph showing the catalyst degradation index values ​​after filtering, as an example.

[0026] Figure 13 To show Figure 2 A graph showing the number of times the catalyst degradation index value was calculated, for example.

[0027] Figure 14 To show Figure 2 A graph illustrating an example of how to set the threshold for catalyst degradation indicators.

[0028] Explanation of reference numerals in the attached figures 1. Catalyst deterioration diagnostic device 10. Internal combustion engine 11. Exhaust passage 12. Intake passage 13. Intake pressure sensor 14. Water temperature sensor 15. Speed ​​sensor 20 Catalysts 30. Upstream oxygen concentration detection unit 40 Downstream oxygen concentration detection unit 50 Upstream side determination and counting unit, 60 Downstream side determination and counting unit, 70 Catalyst Deterioration Index Value Calculation and Counting Department 80 ECU. Detailed Implementation

[0029] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.

[0030] Figures 1 to 14 The diagram illustrates an embodiment of the catalyst degradation diagnostic apparatus of the present invention. In this embodiment, degradation diagnosis is performed on a three-way catalytic converter used for purifying exhaust gases from internal combustion engines such as automobiles and motorcycles.

[0031] The catalyst degradation diagnostic device 1 includes: a catalyst 20 disposed in the exhaust passage 11 of an internal combustion engine 10; an upstream oxygen concentration detection unit 30 disposed upstream of the catalyst 20; a downstream oxygen concentration detection unit 40 disposed downstream of the catalyst 20; an upstream determination counting unit 50 that counts the number of determinations made by the upstream oxygen concentration detection unit 30; a downstream determination counting unit 60 that counts the number of determinations made by the downstream oxygen concentration detection unit 40; and a catalyst degradation index value calculation counting unit 70 that counts the number of calculations made for the catalyst degradation index value (IDX).

[0032] In addition, as a component for determining the operating conditions of the internal combustion engine 10, it includes: an intake pressure sensor 13, which is disposed upstream of the throttle valve of the intake passage 12 to detect the pressure of the intake passage 12; a coolant temperature sensor 14, which is disposed on the coolant path of the internal combustion engine 10 to detect the coolant temperature; and a speed sensor 15, which is disposed close to the flywheel or camshaft of the internal combustion engine 10 to detect the speed.

[0033] The upstream oxygen concentration detection unit 30, the downstream oxygen concentration detection unit 40, the intake pressure sensor 13, the water temperature sensor 14, and the speed sensor 15 are connected to the ECU 80.

[0034] ECU80 is a computer that controls the operation of automobiles and motorcycles. In this embodiment, a microcomputer without floating-point arithmetic function, such as that used in motorcycle engine systems, is used.

[0035] The upstream determination counting unit 50, the downstream determination counting unit 60, and the catalyst deterioration index value calculation counting unit 70 are installed in the ECU80 either internally or externally.

[0036] Catalyst 20 is a three-way catalyst. Three-way catalysts, through oxidation or reduction, remove harmful substances in the exhaust gas, namely hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). X It is converted into water (H2O), carbon dioxide (CO2) and nitrogen (N2) and discharged.

[0037] The upstream oxygen concentration detection unit 30 and the downstream oxygen concentration detection unit 40 are devices that output signals based on the oxygen concentration at the measurement location. In this embodiment, both use oxygen sensors (O2 sensors).

[0038] An oxygen sensor is a sensor that outputs a voltage signal in the range of 0V to 1V based on the oxygen concentration at the measurement location. When the air-fuel ratio at the measurement location is richer than the stoichiometric air-fuel ratio, it outputs a voltage close to 1V, and when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, it outputs a voltage close to 0V.

[0039] Alternatively, an air-fuel ratio sensor (A / F sensor) can be used as an alternative to an oxygen sensor, for example, to output a signal based on the level of current flowing through the oxygen concentration at the measurement location.

[0040] The degradation diagnosis of catalyst 20 is performed when the internal combustion engine 10 is under specified operating conditions.

[0041] The reason for this is that, when evaluating catalyst performance, it is important to determine whether the catalyst, as a three-way catalytic converter, is in an active state. Typically, the activity of a three-way catalytic converter requires a catalyst temperature above 400°C. Therefore, it is necessary to determine whether the catalyst temperature is within this temperature range using direct methods such as inserting a temperature sensor into the catalyst layer, or indirect methods such as driving history records.

[0042] As a specific example of the prescribed operating conditions, such as Figure 3 The flowchart shown indicates that when all of the following conditions A to H are met, the system is deemed to be in an operating condition where catalyst degradation diagnosis can be performed, thus enabling the catalyst diagnosis permission flag to be set and executing the catalyst degradation diagnosis.

[0043] A. Activation of a ternary catalyst.

[0044] B. Activation of the upstream oxygen concentration detection unit and the downstream oxygen concentration detection unit.

[0045] C. The engine temperature is above the specified value (e.g., 20°C).

[0046] D. The engine speed is within the specified range.

[0047] E. The intake manifold pressure sensor is functioning correctly.

[0048] F. The intake manifold pressure is within the specified range.

[0049] G. Not under fuel cut-off control.

[0050] H. Fuel injection is under closed-loop control.

[0051] Next, the catalyst degradation diagnosis method of this embodiment will be described.

[0052] The diagnostic method of this embodiment is characterized by diagnosing catalyst degradation based on the catalyst degradation index value (IDX) obtained from the number of judgments made by the upstream oxygen concentration detection unit and the downstream oxygen concentration detection unit.

[0053] Figure 4 This is a block diagram illustrating the function of the catalyst degradation diagnostic device. The number of times the signal output from the upstream oxygen concentration detection unit crosses the voltage threshold (hereinafter referred to as the "upstream determination count") is counted by the upstream determination counter (see reference). Figure 5 , Figure 6 ).

[0054] Furthermore, the number of times the signal output from the downstream oxygen concentration detection unit passes through the voltage threshold (hereinafter referred to as the "downstream determination count") is counted by the downstream determination counter (see reference). Figure 7 , Figure 8 ).

[0055] In this embodiment, the voltage threshold is set to 0.45V, but it can also be set to 0.5V, and can be set arbitrarily.

[0056] Whenever the number of upstream side judgments reaches the specified number (hereinafter referred to as the "upstream side specified number of judgments"), the catalyst degradation index value (IDX) is calculated using the upstream side judgment number and the downstream side judgment number and the following formula (1).

[0057] Formula 2

[0058] In this embodiment, the number of upstream determinations is set to 30. This is determined by referring to vehicle data to ensure the diagnostic execution rate is achieved; however, it can also be set arbitrarily.

[0059] Figure 9A , Figure 9B To illustrate a schematic diagram of a catalyst degradation diagnosis method, Figure 9A A graph showing the relationship between the exhaust gas and the oxygen concentration detection unit (oxygen sensor) before and after catalyst deterioration (new product) is presented. Figure 9B A graph showing the relationship between exhaust gas and oxygen concentration detection unit (oxygen sensor) before and after catalyst degradation.

[0060] exist Figure 9A as well as Figure 9B In the middle, the oxygen concentration detection unit, which is an oxygen sensor, as shown by the solid line, outputs a voltage signal based on the oxygen concentration at the measurement location, which serves as the voltage signal of the upstream oxygen concentration detection unit or the voltage signal of the downstream oxygen concentration detection unit.

[0061] Furthermore, when the voltage signal of the upstream oxygen concentration detection unit or the voltage signal of the downstream oxygen concentration detection unit passes the voltage threshold shown by the dashed line, it is counted as the number of upstream determinations or the number of downstream determinations.

[0062] like Figure 9A As shown, under the new state of the catalyst, through oxidation or reduction, the harmful substances contained in the exhaust gas, namely hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), are removed. X It is converted into reaction products, namely water (H2O), carbon dioxide (CO2), and nitrogen (N2), and discharged.

[0063] As a result, the oxygen concentration downstream of the catalyst decreases, so the voltage signal of the downstream oxygen concentration detection unit does not exceed the voltage threshold, and the number of downstream determinations decreases.

[0064] Therefore, the number of downstream determinations is less than the number of upstream determinations, and thus the catalyst degradation index (IDX) calculated by the formula (1) is close to 1.

[0065] On the other hand, such as Figure 9B As shown, when the catalyst is in a deteriorated state, the oxidation or reduction processes cannot be fully utilized, and unreacted products pass through the catalyst.

[0066] In this way, the oxygen concentration on the downstream side of the catalyst is the same as that on the upstream side of the catalyst. When the voltage signal of the oxygen concentration detection unit on the downstream side exceeds the voltage threshold, the number of determinations on the downstream side increases.

[0067] Therefore, the number of upstream and downstream determinations is close, and thus, the catalyst degradation index (IDX) calculated by the formula (1) is close to 0.

[0068] If the number of determinations on the upstream side and the number of determinations on the downstream side are equal, the catalyst deterioration index value (IDX) calculated by the formula (1) becomes 0, indicating that the catalyst is completely deteriorated.

[0069] Using the coefficient K1 pre-calculated by the following formula (2) which includes the number of determinations specified on the upstream side, the catalyst degradation index (IDX) value is calculated by the following formula (3). Thus, the catalyst degradation index (IDX) value can be calculated without division, thereby further reducing the computational burden.

[0070] Formula 3

[0071] Formula 4

[0072] This explains the case where the catalyst degradation index (IDX) value is calculated once in the above-described catalyst degradation diagnosis method. In practice, the calculation of the catalyst degradation index (IDX) value is performed continuously. In addition, in order to smooth out and reduce the fluctuation of the measured value caused by sensor noise and external interference, noise countermeasures (filtering) are implemented during the calculation of the catalyst degradation index (IDX) value.

[0073] like Figure 10 As shown, when the number of judgments by the upstream oxygen concentration detection unit reaches the specified number of judgments by the upstream side, the catalyst degradation index value (idx0) before filtering is calculated by the following formula (4), and the upstream judgment counting unit and the downstream judgment counting unit are reset.

[0074] Formula 5

[0075] After resetting, the counting of upstream and downstream determinations begins again. Whenever the number of determinations by the upstream oxygen concentration detection unit reaches the specified number of determinations by the upstream side, the calculation of the catalyst degradation index value (idx0) before filtering and the resetting of the upstream and downstream determination counting units are repeated.

[0076] like Figure 11 As shown, the catalyst degradation index value (idx0) before filtering includes fluctuations in the measured value caused by factors such as sensor noise or external interference, and is not suitable for direct diagnosis.

[0077] Therefore, as Figure 12 As shown, noise countermeasures are applied to the obtained catalyst degradation index value (idx0) before filtering to calculate the catalyst degradation index value (IDX). A low-pass filter composed of digital circuitry is used as the noise countermeasure. During the calculation of the catalyst degradation index value (IDX), a counter is used to count the number of times the catalyst degradation index value is calculated. Thus, by performing noise countermeasures through the low-pass filter, the obtained catalyst degradation index value (IDX) becomes smoother after removing noise.

[0078] like Figure 13 As shown, when the number of times the catalyst degradation index value (IDX) is calculated in the catalyst degradation index value calculation count unit reaches a predetermined number, catalyst degradation is diagnosed by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold. In this embodiment, the predetermined number of calculations for the catalyst degradation index value is set to 35. This is determined based on vehicle data to ensure a high diagnostic execution rate; however, it can also be set arbitrarily.

[0079] The catalyst degradation index count unit does not reset during a driving cycle, but resets at the end of the driving cycle (when the key is turned off). Then, at the beginning of the next driving cycle, the catalyst degradation index count unit starts counting from 0.

[0080] The catalyst degradation index threshold is a pre-set value in the range of 0 to 1, which serves as the basis for diagnosing the fault. When the catalyst degradation index value (IDX) is lower than the catalyst degradation index threshold, it is used as the fault diagnosis result and the catalyst is judged to be faulty.

[0081] In this embodiment, similar to the embodiment described in Patent Document 1, which is a prior example, the catalyst degradation index (IDX) value is taken in the range of 0 to 1. The closer the catalyst degradation index (IDX) value is to 1, the closer it is to no degradation (new product state), and the closer the catalyst degradation index (IDX) value is to 0, the more degraded the state. In this way, by integrating the numerical range and meaning of the catalyst degradation index (IDX) value with the prior example, high portability and versatility can be achieved.

[0082] Regarding the setting of catalyst degradation thresholds, exhaust gas fault determination thresholds can be used. The exhaust gas fault determination threshold (OTL: OBD Threshold Level) is a value that uses the degree of exhaust gas deterioration as a detection criterion, for example, based on the emission amounts of carbon monoxide, non-methane hydrocarbons, and nitrogen oxides in the exhaust gas. When catalyst degradation occurs, fault determination must be performed before exceeding this exhaust gas fault determination threshold.

[0083] Therefore, for example Figure 14 As shown, two degraded catalysts with different degrees of degradation are prepared, and catalyst degradation thresholds can be set based on the catalyst degradation index (IDX) values ​​of these degraded catalysts A and B. The degradation degree of degraded catalyst A (OK boundary) is defined as the exhaust gas value not exceeding the exhaust gas fault determination threshold, but with a large margin from the threshold. The degradation degree of degraded catalyst B (NG boundary) is defined as the exhaust gas value not exceeding the exhaust gas fault determination threshold, but near the threshold. To improve the accuracy of fault determination, degraded catalyst B is preferably selected to have an exhaust gas value as close as possible to the exhaust gas fault determination threshold.

[0084] Furthermore, the value between the catalyst degradation index (IDX) of degraded catalyst A and the catalyst degradation index (IDX) of degraded catalyst B is used as the catalyst degradation index threshold. The catalyst degradation index threshold is set such that it can be reliably determined to be normal when using degraded catalyst A and to be reliably determined to be faulty when using degraded catalyst B. Specifically, it is preferably a value that is smaller than the lower limit of the catalyst degradation index (IDX) of degraded catalyst A and larger than the upper limit of the catalyst degradation index (IDX) of degraded catalyst B.

[0085] Furthermore, in the embodiment described in Patent Document 1, which serves as a prior example, an air-fuel ratio sensor (A / F sensor), which is more expensive than an oxygen sensor (O2 sensor), is used on the upstream side of the oxygen concentration detection unit (refer to Patent Document 1). Figure 2 (A) This also contributes to the increase in component and management costs.

[0086] In contrast, according to this embodiment, by using an oxygen sensor (O2 sensor) as both an upstream oxygen concentration detection device and a downstream oxygen concentration detection device, cost reduction can be achieved in terms of sensor types.

[0087] As described above, according to the present invention, catalyst degradation diagnosis can be achieved by simply calculating the number of determinations made by each oxygen concentration detection unit upstream and downstream of the catalyst.

[0088] Therefore, it is possible to use low-cost computing devices without floating-point arithmetic functions, such as microcomputers, which are also applicable to ECUs that have cost limitations, such as those used in motorcycle engine systems.

[0089] In addition, by using an oxygen sensor (O2 sensor) as both an upstream and downstream oxygen concentration detection device, and eliminating the need for an air-fuel ratio sensor (A / F sensor), costs can be reduced in terms of sensor types.

[0090] Furthermore, by integrating the numerical range and meaning of the Catalyst Deterioration Index (IDX) with previous examples, high portability and versatility can be achieved.

Claims

1. A catalyst degradation diagnostic device, characterized in that, have: Catalysts, placed in the exhaust passage of internal combustion engines, purify exhaust gases. An upstream oxygen concentration detection unit, located upstream of the catalyst, detects the oxygen concentration in the exhaust gas and outputs a signal. A downstream oxygen concentration detection unit, located downstream of the catalyst, detects the oxygen concentration in the exhaust gas and outputs a signal. The upstream-side determination and counting unit counts the number of times the signal output from the upstream-side oxygen concentration detection unit crosses a voltage threshold, and uses this count as the upstream-side determination count. The downstream determination counting unit counts the number of times the signal output from the downstream oxygen concentration detection unit passes through a voltage threshold, and uses this count as the downstream determination count. When the number of upstream determinations in the upstream determination counting unit reaches a predetermined number, the catalyst degradation index (IDX) value is calculated using the upstream determination count and the downstream determination count, and the counts of the upstream determination counting unit and the downstream determination counting unit are reset. The degradation of the catalyst is diagnosed by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold.

2. The catalyst deterioration diagnostic device according to claim 1, characterized in that, The catalyst deterioration diagnostic device includes a catalyst deterioration index value calculation and counting unit, which counts the number of times the catalyst deterioration index value (IDX) is calculated, and uses this count as the catalyst deterioration index value calculation count. When the number of times the catalyst deterioration index value is calculated in the catalyst deterioration index value calculation and counting unit reaches a predetermined number, Compare the catalyst degradation index value (IDX) with the catalyst degradation index threshold.

3. The catalyst degradation diagnostic device according to claim 1 or 2, characterized in that, Using the number of upstream and downstream determinations, the catalyst degradation index (IDX) is calculated using the following formula (1). 。 4. The catalyst degradation diagnostic device according to claim 1 or 2, characterized in that, Using the upstream side determination number, the downstream side determination number, and the predetermined coefficient K1 pre-calculated by the following formula (2), the catalyst degradation index value (idx0) before filtering is calculated by the following formula (3). Then, the catalyst degradation index value (idx0) before filtering is subjected to low-pass filtering to obtain the catalyst degradation index value (IDX). 。 5. A method for diagnosing catalyst deterioration, characterized in that, have: Catalysts, placed in the exhaust passage of internal combustion engines, purify exhaust gases. An upstream oxygen concentration detection unit, located upstream of the catalyst, detects the oxygen concentration in the exhaust gas and outputs a signal. A downstream oxygen concentration detection unit, located downstream of the catalyst, detects the oxygen concentration in the exhaust gas and outputs a signal. The upstream-side determination and counting unit counts the number of times the signal output from the upstream-side oxygen concentration detection unit crosses a voltage threshold, and uses this count as the upstream-side determination count. The downstream determination counting unit counts the number of times the signal output from the downstream oxygen concentration detection unit passes through a voltage threshold, and uses this count as the downstream determination count. When the number of upstream determinations in the upstream determination counting unit reaches a predetermined number, the catalyst degradation index (IDX) value is calculated using the upstream determination count and the downstream determination count, and the counts of the upstream determination counting unit and the downstream determination counting unit are reset. The degradation of the catalyst is diagnosed by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold.

6. The catalyst degradation diagnosis method according to claim 5, characterized in that, It also includes a catalyst degradation index value calculation and counting unit, which counts the number of times the catalyst degradation index value (IDX) is calculated, and uses this count as the catalyst degradation index value calculation count. When the number of times the catalyst deterioration index value is calculated in the catalyst deterioration index value calculation and counting unit reaches a predetermined number, Compare the catalyst degradation index value (IDX) with the catalyst degradation index threshold.

7. The catalyst degradation diagnosis method according to claim 5 or 6, characterized in that, Using the number of upstream and downstream determinations, the catalyst degradation index (IDX) is calculated using the following formula (1). 。 8. The catalyst degradation diagnosis method according to claim 5 or 6, characterized in that, Using the upstream side determination number, the downstream side determination number, and the predetermined coefficient K1 pre-calculated by the following formula (2), the catalyst degradation index value (idx0) before filtering is calculated by the following formula (3). Then, the catalyst degradation index value (idx0) before filtering is subjected to low-pass filtering to obtain the catalyst degradation index value (IDX). 。

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