Catalyst degradation diagnostic device and catalyst degradation diagnostic method
The catalyst degradation diagnostic device facilitates accurate diagnosis in cost-constrained systems by using simple calculations and oxygen sensors, addressing the limitations of existing methods that require high-performance computing devices.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NIKKI CO LTD
- Filing Date
- 2024-12-25
- Publication Date
- 2026-07-07
AI Technical Summary
Existing catalyst degradation diagnosis methods require high-performance computing devices with floating-point arithmetic capabilities, making them unsuitable for cost-constrained systems like motorcycle engines.
A catalyst degradation diagnostic device and method using upstream and downstream oxygen concentration detection means, determination counters, and a catalyst degradation index value calculation, which can be implemented in microcontrollers without floating-point capabilities, allowing for simple calculations.
Enables catalyst degradation diagnosis in cost-constrained systems by reducing computational load and sensor costs, achieving accurate diagnosis without the need for floating-point methods.
Smart Images
Figure 2026112665000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a catalyst deterioration diagnosis device and a catalyst deterioration diagnosis method, and particularly to a method for diagnosing the deterioration of a three-way catalyst for exhaust gas purification in an internal combustion engine such as an automobile or a motorcycle.
Background Art
[0002] Conventionally, for a catalyst such as a three-way catalyst provided for purifying exhaust gas in an exhaust pipe of an internal combustion engine such as an automobile or a motorcycle, diagnosis of whether it is deteriorated has been performed during driving.
[0003] As a method for diagnosing catalyst deterioration, when the catalyst deteriorates, the oxygen storage capacity (OSC) decreases along with the decrease in purification performance, sufficient oxygen cannot be stored in the catalyst layer, and oxygen molecules are likely to be released downstream of the catalyst. Focusing on this, for example, as in Patent Document 1, there is known a method that uses an oxygen concentration detection means (air-fuel ratio sensor) arranged upstream of the catalyst and an oxygen concentration detection means (oxygen sensor) arranged downstream of the catalyst.
[0004] The catalyst deterioration diagnosis method in Patent Document 1 calculates the oxygen concentration fluctuations upstream and downstream of the catalyst as standard deviations (σ) from the respective oxygen concentration detection means, and calculates a catalyst deterioration index value (IDX) according to the following mathematical formula.
Equation
[0005] Here, σ F represents the standard deviation of the oxygen concentration fluctuation upstream of the catalyst, and σ R represents the standard deviation of the oxygen concentration fluctuation downstream of the catalyst, respectively. The larger the value, the greater the standard degree of dispersion of the oxygen concentration fluctuation value, indicating a large oxygen concentration fluctuation.
[0006] For a non-deteriorated catalyst, there is almost no variation on the downstream side of the catalyst, so σ RIt becomes approximately 0, and the catalyst degradation index (IDX) value (σ) = 1. As degradation progresses, the oxygen storage capacity (OSC) decreases, so the fluctuations in oxygen concentration before and after the catalyst become similar, (σ F -σ R Since ) ≈ 0, the catalyst degradation index value (IDX) ≈ 0.
[0007] In other words, a catalyst degradation index (IDX) value closer to 1 indicates that the catalyst is in a near-new condition with no degradation, while a catalyst degradation index (IDX) value closer to 0 indicates that the catalyst has degraded more significantly.
[0008] Thus, the catalyst degradation diagnosis method described in Patent Document 1 requires the calculation of the standard deviation based on the oxygen concentration signals output from each of the oxygen concentration detection means, and therefore necessitates the use of a relatively high-performance computing device capable of performing calculations using a floating-point method. [Prior art documents] [Patent Documents]
[0009] [Patent Document 1] Japanese Patent Publication No. 2013-083195 [Overview of the project] [Problems that the invention aims to solve]
[0010] Therefore, there was a problem in that it was difficult to apply to ECUs that use microcontrollers without floating-point arithmetic capabilities, such as engine systems for motorcycles, where there are cost constraints on the installed components.
[0011] Therefore, the objective of the present invention is to enable catalyst degradation diagnosis by simple calculations, without requiring calculations using floating-point methods. [Means for solving the problem]
[0012] To solve the above problems, the catalyst degradation diagnostic device and catalyst degradation diagnostic method according to the present invention comprises: a catalyst arranged in the exhaust passage of an internal combustion engine to purify exhaust gas; an upstream oxygen concentration detection means arranged upstream of the catalyst to detect the oxygen concentration in the exhaust gas and output a signal; a downstream oxygen concentration detection means arranged downstream of the catalyst to detect the oxygen concentration in the exhaust gas and output a signal; an upstream determination counter unit that counts the number of times the signal output from the upstream oxygen concentration detection means passes a voltage threshold as the upstream determination count; and a downstream determination counter unit that counts the number of times the signal output from the downstream oxygen concentration detection means passes a voltage threshold as the downstream determination count. When the upstream determination count in the upstream determination counter unit reaches a predetermined number of times, a catalyst degradation index value (IDX) is calculated using the upstream determination count and the downstream determination count, the counts of the upstream determination counter unit and the downstream determination counter unit are reset, and the degradation of the catalyst is diagnosed by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold. [Effects of the Invention]
[0013] According to the present invention, a catalyst degradation diagnostic device and a catalyst degradation diagnostic method are provided that enable catalyst degradation diagnosis by simple calculations, without requiring calculations using a floating-point method. [Brief explanation of the drawing]
[0014] [Figure 1] A schematic diagram illustrating an example of the application of a catalyst degradation diagnostic device according to an embodiment of the present invention. [Figure 2] A block diagram showing an example of a catalyst degradation diagnostic device according to an embodiment of the present invention. [Figure 3] A flowchart showing the conditions for performing the deterioration diagnosis method in one example, as shown in Figure 2. [Figure 4] A block diagram showing a deterioration diagnosis method in one example shown in Figure 2. [Figure 5] A graph showing the output voltage of the upstream oxygen concentration detection means when the catalyst is functioning normally, as in one example shown in Figure 2. [Figure 6] Graph showing the number of determinations of the upstream oxygen concentration detection means when the catalyst is normal in an example of FIG. 2. [Figure 7] Graph showing the output voltage of the downstream oxygen concentration detection means when the catalyst is normal in an example of FIG. 2. [Figure 8] Graph showing the number of determinations of the downstream oxygen concentration detection means when the catalyst is normal in an example of FIG. 2. [Figure 9] Schematic diagram showing the deterioration diagnosis method in an example of FIG. 2, where (a) is a diagram showing the relationship between the exhaust gas before and after the catalyst and the oxygen concentration detection means in the state of no catalyst deterioration (new product), and (b) is a diagram showing the relationship between the exhaust gas before and after the catalyst and the oxygen concentration detection means in the state of catalyst deterioration. [Figure 10] Graph showing the number of counts of the upstream determination counter part and the downstream determination counter part in an example of FIG. 2. [Figure 11] Graph showing the actually measured catalyst deterioration index value in an example of FIG. 2. [Figure 12] Graph showing the catalyst deterioration index value after filter processing in an example of FIG. 2. [Figure 13] Graph showing the number of calculations of the catalyst deterioration index value in an example of FIG. 2. [Figure 14] Graph showing the method of setting the catalyst deterioration index threshold value in an example of FIG. 2.
Mode for Carrying Out the Invention
[0015] Hereinafter, embodiments of the present invention will be described based on the drawings.
[0016] FIGS. 1 to 14 are diagrams showing embodiments of a catalyst deterioration diagnosis apparatus according to the present invention. In this embodiment, deterioration diagnosis of a three-way catalyst used for purifying exhaust gas of an internal combustion engine such as an automobile or a motorcycle is performed.
[0017] The catalyst degradation diagnostic device 1 comprises a catalyst 20 positioned in the exhaust passage 11 of an internal combustion engine 10, an upstream oxygen concentration detection means 30 positioned upstream of the catalyst 20, a downstream oxygen concentration detection means 40 positioned downstream of the catalyst 20, an upstream determination counter unit 50 that counts the number of determinations made by the upstream oxygen concentration detection means 30, a downstream determination counter unit 60 that counts the number of determinations made by the downstream oxygen concentration detection means 40, and a catalyst degradation index value calculation counter unit 70 that counts the number of times the catalyst degradation index value (IDX) is calculated.
[0018] Furthermore, the system includes components for determining the operating conditions of the internal combustion engine 10, such as an intake pressure sensor 13 positioned upstream of the throttle in the intake passage 12 to detect the pressure in the intake passage 12, a water temperature sensor 14 positioned in the coolant path of the internal combustion engine 10 to detect the coolant temperature, and a rotational speed sensor 15 positioned in close proximity to the flywheel, camshaft, etc., of the internal combustion engine 10 to detect the rotational speed.
[0019] The upstream oxygen concentration detection means 30, the downstream oxygen concentration detection means 40, the intake pressure sensor 13, the water temperature sensor 14, and the rotation speed sensor 15 are connected to the ECU 80.
[0020] The ECU80 is a computer that controls the operation of automobiles, motorcycles, etc., and in this embodiment, a microcontroller without floating-point arithmetic capabilities is used, for example, in the engine system of a motorcycle.
[0021] The upstream determination counter unit 50, the downstream determination counter unit 60, and the catalyst degradation index value calculation counter unit 70 are implemented either internally or externally within the ECU 80.
[0022] Catalyst 20 is a three-way catalyst. The three-way catalyst removes harmful substances contained in exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO), through oxidation or reduction. X It releases ) as water (H2O), carbon dioxide (CO2), and nitrogen (N2).
[0023] The upstream oxygen concentration detection means 30 and the downstream oxygen concentration detection means 40 are devices that output a signal according to the oxygen concentration at the measurement location, and in this embodiment, both use oxygen sensors (O2 sensors).
[0024] An oxygen sensor is a sensor that outputs a signal in the range of 0V to 1V in terms of the oxygen concentration at the measurement point. When the air-fuel ratio at the measurement point is rich relative to the stoichiometric air-fuel ratio, it outputs a voltage close to 1V, and when the air-fuel ratio is lean, it outputs a voltage close to 0V.
[0025] Alternatively, as an alternative to the oxygen sensor, an air-fuel ratio sensor (A / F sensor), which outputs a signal based on the level of current according to the oxygen concentration at the measurement point, may be used.
[0026] The diagnosis of the deterioration of the catalyst 20 is performed when the internal combustion engine 10 is under predetermined operating conditions.
[0027] This is because, when evaluating catalytic performance, it is crucial to determine whether or not the three-way catalyst is in an active state. Typically, a catalyst temperature of 400°C or higher is required for a three-way catalyst to be active. Therefore, it is necessary to determine whether or not the catalyst temperature is within this range by direct means such as inserting a temperature sensor into the catalyst layer, or by indirect means such as driving history.
[0028] As a specific example of the specified operating conditions, as shown in the flowchart in Figure 3, when all of the following conditions A to H are met, it is determined that the operating conditions are suitable for catalyst degradation diagnosis, the catalyst diagnosis permission flag is set, and the catalyst degradation diagnosis is performed. A. The three-way catalyst is activated. B. The upstream and downstream oxygen concentration detection means are activated. C. The engine temperature is above the specified value (e.g., 20°C). D. The engine speed is within the specified range. E. The intake manifold pressure sensor is not faulty. F. Intake manifold pressure is within the specified range. G. Fuel cut control is not in operation. H. Fuel injection is under closed-loop control.
[0029] Next, the method for diagnosing catalyst degradation in this embodiment will be described.
[0030] The diagnostic method in this embodiment is characterized by diagnosing catalyst degradation using a catalyst degradation index value (IDX) obtained by the number of determinations made by the catalyst upstream oxygen concentration detection means and the catalyst downstream oxygen concentration detection means.
[0031] Figure 4 is a block diagram showing the functions of the catalyst degradation diagnostic device. The number of times the signal output from the upstream oxygen concentration detection means passes the voltage threshold (hereinafter referred to as the "upstream judgment count") is counted in the upstream judgment counter (see Figures 5 and 6).
[0032] Furthermore, the number of times the signal output from the downstream oxygen concentration detection means passes the voltage threshold (hereinafter referred to as the "downstream determination count") is counted in the downstream determination counter (see Figures 7 and 8).
[0033] In this embodiment, the voltage threshold is set to 0.45V, but it can also be set to 0.5V, for example, and can be set arbitrarily.
[0034] Each time the number of upstream judgments reaches a predetermined number (hereinafter referred to as the "predetermined number of upstream judgments"), the catalyst degradation index value (IDX) is calculated using the upstream judgment count and the downstream judgment count from the following formula (1).
[0035]
number
[0036] In this embodiment, the predetermined number of upstream judgments is set to 30. This is determined by referring to vehicle data to ensure a satisfactory diagnostic execution rate, but it can be set arbitrarily in other ways.
[0037] Figure 9 is a schematic diagram showing a method for diagnosing catalyst degradation. Figure 9(a) shows the relationship between exhaust gas before and after the catalyst and the oxygen concentration detection means (oxygen sensor) in a catalyst-free (new) state, and Figure 9(b) shows the relationship between exhaust gas before and after the catalyst and the oxygen concentration detection means (oxygen sensor) in a catalyst-degraded state.
[0038] In Figures 9(a) and 9(b), the oxygen sensor, which is the oxygen concentration detection means, outputs a signal whose voltage increases or decreases according to the oxygen concentration at the measurement location, as shown by the solid line, either as the upstream oxygen concentration detection means voltage signal or the downstream oxygen concentration detection means voltage signal.
[0039] Then, when the voltage signal from the upstream oxygen concentration detection means or the voltage signal from the downstream oxygen concentration detection means passes the voltage threshold indicated by the dotted line, it is counted as the upstream determination count or the downstream determination count.
[0040] As shown in Figure 9(a), when the catalyst is new, it removes harmful substances contained in exhaust gas, such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO), through oxidation or reduction. X The reaction products released are water (H2O), carbon dioxide (CO2), and nitrogen (N2).
[0041] As a result, the oxygen concentration downstream of the catalyst decreases, so the voltage signal from the downstream oxygen concentration detection means does not exceed the voltage threshold, and the number of downstream determinations is reduced.
[0042] Therefore, because the number of downstream checks is less than the number of upstream checks, the catalyst degradation index value (IDX) calculated using formula (1) above will be close to 1.
[0043] On the other hand, as shown in Figure 9(b), when the catalyst is degraded, the oxidation or reduction reaction does not function sufficiently, and the reaction products pass through the catalyst without being generated.
[0044] As a result, the oxygen concentration downstream of the catalyst becomes equivalent to that upstream of the catalyst, causing the voltage signal from the downstream oxygen concentration detection means to exceed the voltage threshold, and increasing the number of downstream determinations.
[0045] Therefore, as the number of upstream and downstream checks approaches, the catalyst degradation index value (IDX) calculated using formula (1) approaches 0.
[0046] When the number of upstream and downstream checks become equal, the catalyst degradation index value (IDX) calculated using formula (1) above becomes 0, indicating complete catalyst degradation.
[0047] By using the coefficient K1 calculated in advance with the following formula (2), which incorporates a predetermined number of upstream judgment cycles, and calculating the catalyst degradation index value (IDX) with the following formula (3), the catalyst degradation index value (IDX) can be calculated without division, thereby further reducing the computational load.
[0048]
number
[0049]
number
[0050] The catalyst degradation diagnosis method described above explains the case where the catalyst degradation index value (IDX) is calculated only once. However, in reality, the catalyst degradation index value (IDX) is calculated continuously. In addition, noise reduction processing (filtering) is performed when calculating the catalyst degradation index value (IDX) to smooth out and reduce fluctuations in measurement values caused by sensor noise and disturbances.
[0051] As shown in Figure 10, when the number of judgments made by the upstream oxygen concentration detection means reaches a predetermined number of judgments made upstream, the filter pre-catalyst degradation index value (idx0) is calculated using the following formula (4), and the upstream judgment counter unit and the downstream judgment counter unit are reset.
[0052]
number
[0053] After the reset, the counting of the upstream and downstream judgment counts is restarted, and each time the number of judgments of the upstream oxygen concentration detection means reaches a predetermined number of upstream judgment counts, the calculation of the pre-filter catalyst degradation index value (idx0) and the resetting of the upstream judgment counter and the downstream judgment counter are repeated.
[0054] As shown in Figure 11, the pre-filter catalyst degradation index value (idx0) includes fluctuations in measured values due to factors such as sensor noise and disturbances, and is therefore not suitable for diagnosis as is.
[0055] Therefore, as shown in Figure 12, noise reduction processing is performed on the obtained pre-filter catalyst degradation index value (idx0) to calculate the catalyst degradation index value (IDX). A low-pass filter configured with a digital circuit is applied as the noise reduction processing. When the catalyst degradation index value (IDX) is calculated, it is counted in the catalyst degradation index value calculation count counter. In this way, by performing noise reduction processing with a low-pass filter, the obtained catalyst degradation index value (IDX) is smoothed by removing noise.
[0056] As shown in Figure 13, when the number of times the catalyst degradation index value (IDX) is calculated in the catalyst degradation index value calculation counter reaches a predetermined number of times, the catalyst degradation is diagnosed by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold value. In this embodiment, the predetermined number of times the catalyst degradation index value is calculated is set to 35 times. This was determined by referring to vehicle data so that the diagnostic execution rate is maintained, but it can be set arbitrarily at other times.
[0057] The catalyst degradation index calculation counter is not reset during a single driving cycle, but is reset when driving is completed (when the key is turned off). Then, in the next driving cycle, the catalyst degradation index calculation counter starts counting from 0.
[0058] The catalyst degradation index threshold is a preset value between 0 and 1 that indicates a failure. If the catalyst degradation index value (IDX) falls below the catalyst degradation index threshold, the catalyst is determined to be faulty as a result of the failure diagnosis.
[0059] In this embodiment, similar to the embodiment described in Patent Document 1, which is a conventional example, the catalyst degradation index value (IDX) is set to take a range of 0 to 1. The closer the catalyst degradation index value (IDX) is to 1, the closer it is to no degradation (new condition), and the closer it is to 0, the more advanced the degradation. By aligning the numerical range and meaning of the catalyst degradation index value (IDX) with the conventional example in this way, high portability can be achieved and versatility can be improved.
[0060] For setting the catalyst degradation index threshold, the exhaust gas failure detection threshold can be used. The exhaust gas failure detection threshold (OTL: OBD Threshold Level) is a numerical value that uses the level of exhaust gas deterioration as a detection requirement, and is set, for example, based on the emissions of carbon monoxide, non-methane hydrocarbons, and nitrogen oxides contained in the exhaust gas. When the catalyst degrades, it is necessary to detect the failure before this exhaust gas failure detection threshold is exceeded.
[0061] Therefore, as shown in Figure 14, for example, two degraded catalysts with different degrees of degradation can be prepared, and a catalyst degradation index threshold can be set based on the catalyst degradation index values (IDX) of these degraded catalysts A and B. Degraded catalyst A (OK border) is one in which the exhaust gas value does not exceed the exhaust gas failure judgment threshold and has a relatively moderate degree of degradation with some margin before reaching the exhaust gas failure judgment threshold, while degraded catalyst B (NG border) is one in which the exhaust gas value does not exceed the exhaust gas failure judgment threshold but has a degree of degradation close to the exhaust gas failure judgment threshold. To improve the accuracy of failure judgment, it is preferable that degraded catalyst B has an exhaust gas value as close as possible to the exhaust gas failure judgment threshold.
[0062] The catalyst degradation index threshold is set to a value between the catalyst degradation index value (IDX) of degraded catalyst A and the catalyst degradation index value (IDX) of degraded catalyst B. The catalyst degradation index threshold is set so that normal detection is reliably performed when degraded catalyst A is used, and failure detection is reliably performed when degraded catalyst B is used. Specifically, it is preferable that the threshold is smaller than the lower limit of the catalyst degradation index value (IDX) of degraded catalyst A and larger than the upper limit of the catalyst degradation index value (IDX) of degraded catalyst B.
[0063] In the conventional embodiment described in Patent Document 1, an air-fuel ratio sensor (A / F sensor), which is relatively more expensive than an oxygen sensor (O2 sensor), is used on the upstream side of the oxygen concentration detection means (see Patent Document 1: Figure 2(a)), which also contributed to increased parts costs and management costs.
[0064] In contrast, this embodiment uses oxygen sensors (O2 sensors) for both the upstream and downstream oxygen concentration detection devices, enabling cost reduction in terms of sensor type as well.
[0065] As described above, the present invention makes it possible to diagnose catalyst degradation by a simple calculation using the number of determinations made by the oxygen concentration detection means upstream and downstream of the catalyst.
[0066] This makes it possible to use inexpensive arithmetic units without floating-point capabilities, such as microcontrollers, and can be applied to ECUs with cost constraints, such as those used in motorcycle engine systems.
[0067] Furthermore, since both the upstream and downstream oxygen concentration detection devices utilize oxygen sensors (O2 sensors) and do not use air-fuel ratio sensors (A / F sensors), cost reductions can be achieved in terms of sensor type.
[0068] Furthermore, by aligning the numerical range and meaning of the catalyst degradation index (IDX) with conventional examples, high portability and improved versatility can be achieved. [Explanation of Symbols]
[0069] 1. Catalyst degradation diagnostic device 10 Internal combustion engine 11 Exhaust passage 12 Intake passage 13. Intake pressure sensor 14. Water temperature sensor 15. Rotation speed sensor 20 Catalyst 30 Upstream oxygen concentration detection means 40 Downstream oxygen concentration detection means 50 Upstream determination counter unit 60 Downstream determination counter unit 70 Catalyst Degradation Index Value Calculation Counter Unit 80 ECU
Claims
1. A catalyst is placed in the exhaust passage of an internal combustion engine to purify exhaust gases, An upstream oxygen concentration detection means is positioned upstream of the catalyst and detects the oxygen concentration in the exhaust gas and outputs a signal; A downstream oxygen concentration detection means is positioned downstream of the catalyst and detects the oxygen concentration in the exhaust gas and outputs a signal. An upstream determination counter unit counts the number of times the signal output from the upstream oxygen concentration detection means passes a voltage threshold as the number of upstream determinations, The system includes a downstream determination counter unit that counts the number of times the signal output from the downstream oxygen concentration detection means passes a voltage threshold as the number of downstream determinations, When the number of upstream determinations in the upstream determination counter unit reaches a predetermined number, the catalyst degradation index value (IDX) is calculated using the number of upstream determinations and the number of downstream determinations, and the counts in the upstream determination counter unit and the downstream determination counter unit are reset. A catalyst degradation diagnostic device characterized by diagnosing the degradation of the catalyst by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold value.
2. The system includes a catalyst degradation index value calculation counter that counts the number of times the catalyst degradation index value (IDX) has been calculated as the number of catalyst degradation index value calculations, When the number of times the catalyst degradation index value is calculated in the catalyst degradation index value calculation counter unit reaches a predetermined number, The catalyst degradation diagnostic device according to claim 1, characterized by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold value.
3. The catalyst degradation diagnostic device according to claim 1 or 2, characterized in that the catalyst degradation index value (IDX) is calculated using the upstream number of judgments and the downstream number of judgments by the following formula (1). [Math 1]
4. The catalyst degradation index value (IDX) is determined by the number of upstream judgments, the number of downstream judgments, and a predetermined coefficient K calculated in advance using the following formula (2). １ A catalyst degradation diagnostic device according to claim 1 or 2, characterized in that, after calculating a pre-filter catalyst degradation index value (idx0) using the following formula (3), the pre-filter catalyst degradation index value (idx0) is subjected to low-pass filtering. [Math 2]
5. A catalyst is placed in the exhaust passage of an internal combustion engine to purify exhaust gases, An upstream oxygen concentration detection means is positioned upstream of the catalyst and detects the oxygen concentration in the exhaust gas and outputs a signal; A downstream oxygen concentration detection means is positioned downstream of the catalyst and detects the oxygen concentration in the exhaust gas and outputs a signal. An upstream determination counter unit counts the number of times the signal output from the upstream oxygen concentration detection means passes a voltage threshold as the number of upstream determinations, The system includes a downstream determination counter unit that counts the number of times the signal output from the downstream oxygen concentration detection means passes a voltage threshold as the number of downstream determinations, When the number of upstream determinations in the upstream determination counter unit reaches a predetermined number, the catalyst degradation index value (IDX) is calculated using the number of upstream determinations and the number of downstream determinations, and the counts in the upstream determination counter unit and the downstream determination counter unit are reset. A method for diagnosing catalyst degradation, characterized by diagnosing catalyst degradation by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold value.
6. The system includes a catalyst degradation index value calculation counter that counts the number of times the catalyst degradation index value (IDX) has been calculated as the number of catalyst degradation index value calculations, When the number of times the catalyst degradation index value is calculated in the catalyst degradation index value calculation counter unit reaches a predetermined number, The catalyst degradation diagnosis method according to claim 5, characterized by comparing the catalyst degradation index value (IDX) with the catalyst degradation index threshold value.
7. The catalyst degradation diagnosis method according to claim 5 or 6, characterized in that the catalyst degradation index value (IDX) is calculated using the upstream number of judgments and the downstream number of judgments by the following formula (1). [Math 3]
8. The catalyst degradation index value (IDX) is determined by the number of upstream judgments, the number of downstream judgments, and a predetermined coefficient K calculated in advance using the following formula (2). １ A method for diagnosing catalyst degradation according to claim 5 or 6, characterized in that, after calculating a pre-filter catalyst degradation index value (idx0) using the following formula (3), the pre-filter catalyst degradation index value (idx0) is subjected to low-pass filtering. [Math 4]