JP2024055343A - Deterioration diagnosis method of exhaust emission control catalyst and deterioration diagnosis device of exhaust emission control catalyst - Google Patents

Abstract

To perform catalyst deterioration diagnosis accurately.SOLUTION: A prescribed value A is set, which is a threshold of the concentration combining an NOx component and an ammonia component in exhaust gas on the downstream side of a three-way catalyst 15 according to an exhaust air-fuel ratio. If an instantaneous value of the concentration combining an NOx component and an ammonia component in exhaust gas detected by an ammonia sensor 20 when an exhaust air-fuel ratio is richer than a theoretical air-fuel ratio is the prescribed value A or lower, and if the instantaneous value of the concentration combining an NOx component and an ammonia component in exhaust gas detected by the ammonia sensor 20 when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio is the prescribed value A or higher, the three-way catalyst 15 is diagnosed as being deteriorated.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for diagnosing the deterioration of an exhaust purification catalyst and a device for diagnosing the deterioration of an exhaust purification catalyst.

For example, Patent Document 1 discloses a technique for measuring the concentration of NOx in exhaust gas downstream of a three-way catalyst using a NOx detection means, and diagnosing the deterioration of the three-way catalyst using the measured NOx concentration.

In Patent Document 1, the degree of deterioration of the NOx reduction ability of the precious metal components Pd and Rh of the three-way catalyst is diagnosed based on the NOx concentration detected during the period from when the internal combustion engine shifts from a lean operating state to a rich operating state until it returns to a lean operating state.

JP 2020-45885 A

However, sensors that detect NOx concentrations are generally also sensitive to ammonia. Therefore, the NOx concentration detected by the NOx detection means in Patent Document 1 is considered to be a concentration that includes ammonia.

In other words, in Patent Document 1, the combined concentration of NOx and ammonia components in the exhaust gas is used as the NOx concentration in the exhaust gas to diagnose deterioration of the three-way catalyst. Therefore, the catalyst deterioration diagnosis in Patent Document 1 may result in poor accuracy of the value of the NOx concentration in the exhaust gas used for diagnosis, and there is a risk that the catalyst deterioration diagnosis cannot be performed accurately.

The catalyst deterioration diagnosis of the exhaust purification catalyst of the present invention is performed by providing an ammonia sensor that is sensitive to the NOx concentration and ammonia concentration in the exhaust gas downstream of the exhaust purification catalyst in the exhaust passage of the internal combustion engine, and a threshold value is set for the combined concentration of the NOx component and the ammonia component in the exhaust gas downstream of the exhaust purification catalyst according to the exhaust air-fuel ratio downstream of the exhaust purification catalyst or in the exhaust purification catalyst. When the combined concentration of the NOx component and the ammonia component in the exhaust gas detected by the ammonia sensor is below the threshold value when the exhaust air-fuel ratio is richer than the theoretical air-fuel ratio, and when the combined concentration of the NOx component and the ammonia component in the exhaust gas detected by the ammonia sensor is above the threshold value when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio, the exhaust purification catalyst is diagnosed as deteriorated.

When the inside of the exhaust purification catalyst is in an environment richer than the theoretical air-fuel ratio, ammonia is produced by the NOx and hydrogen in the exhaust gas through a catalytic reaction, and some of the produced ammonia is converted to nitrogen and the like through an oxidation reaction, with the remaining ammonia flowing out to the downstream side of the exhaust purification catalyst. Therefore, if the exhaust purification catalyst is deteriorated, even if the inside of the exhaust purification catalyst is in an environment richer than the theoretical air-fuel ratio, a sufficient catalytic reaction does not occur, and the amount of ammonia flowing out to the downstream side of the exhaust purification catalyst also decreases.

In addition, in an environment where the inside of the exhaust purification catalyst is richer than the theoretical air-fuel ratio, if the oxygen storage capacity of the exhaust purification catalyst has not deteriorated, the NOx in the exhaust gas is reduced by the exhaust purification catalyst, making it difficult for the NOx in the exhaust gas to flow downstream of the exhaust purification catalyst.

On the other hand, when the inside of the exhaust purification catalyst is in an environment leaner than the theoretical air-fuel ratio, the NOx in the exhaust gas is not reduced by the exhaust purification catalyst and tends to flow out downstream of the exhaust purification catalyst.

According to the present invention, by utilizing the characteristics of the exhaust purification catalyst related to NOx and ammonia according to the exhaust air-fuel ratio and the characteristics of the ammonia sensor that is sensitive to the NOx concentration and the ammonia concentration, it is possible to perform an accurate catalyst deterioration diagnosis of the exhaust purification catalyst.

1 is an explanatory diagram that illustrates a schematic configuration of an internal combustion engine to which the present invention is applied; FIG. 2 is an explanatory diagram showing a schematic overview of the catalyst deterioration diagnosis of the present invention; 4 is a flowchart showing a flow of a catalyst deterioration diagnosis according to the present invention.

Below, an embodiment of the present invention will be described in detail with reference to the drawings. Figure 1 is an explanatory diagram showing the general configuration of an internal combustion engine 1 according to an embodiment of the present invention.

The internal combustion engine 1 is a four-stroke cycle spark ignition internal combustion engine (a so-called gasoline engine), and each cylinder is equipped with an intake valve 2, an exhaust valve 3, and an ignition plug 4. The illustrated example is configured as an in-cylinder direct injection engine, and a fuel injection valve 5 that injects fuel into the cylinder is disposed, for example, on the intake valve 2 side. Note that the internal combustion engine 1 may also be configured as a port injection type that injects fuel into an intake port 6.

An electronically controlled throttle valve 10, whose opening is controlled by a control signal from an engine controller 9, is provided upstream of a collector section 8 in an intake passage 7 connected to the intake port 6 of each cylinder. An air flow meter 11 that detects the amount of intake air is provided upstream of the throttle valve 10 in the intake passage 7. An air cleaner 12 is provided upstream of the air flow meter 11 in the intake passage 7.

The exhaust ports 13 of each cylinder are gathered into a single exhaust passage 14, and a three-way catalyst 15 is provided in this exhaust passage 14 as an exhaust purification catalyst for purifying the exhaust gas. The three-way catalyst 15 is, for example, a monolith ceramic catalyst in which a catalyst layer containing a catalytic metal (precious metal) is coated on the surface of a monolith ceramic body in which fine passages are formed. The three-way catalyst 15 may also be configured to further include a downstream catalyst (a so-called underfloor catalyst) arranged in series.

An upstream air-fuel ratio sensor 19 is disposed on the inlet side of the three-way catalyst 15 in the exhaust passage 14, i.e., upstream of the three-way catalyst 15, to detect the air-fuel ratio of the exhaust gas discharged by the internal combustion engine 1 (in other words, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15). This upstream air-fuel ratio sensor 19 is a so-called wide-range air-fuel ratio sensor that provides an output according to the exhaust air-fuel ratio.

In addition, an ammonia sensor 20 having sensitivity to the ammonia concentration in the exhaust gas flowing out from the three-way catalyst 15 is disposed on the outlet side or downstream side of the three-way catalyst 15 in the exhaust passage 14. The ammonia sensor 20 in one embodiment is a type of sensor called a NOx sensor capable of detecting NOx in the exhaust gas. That is, it is sensitive to both the ammonia concentration and the NOx concentration, and both are output as one detection signal. In addition, in a preferred embodiment, the ammonia sensor 20 is configured to output an output signal corresponding to the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 separately from the detection signal of ammonia and NOx. In other words, the ammonia sensor 20 functions as a downstream air-fuel ratio sensor that detects the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15. The exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 is considered to correspond to the air-fuel ratio environment inside the three-way catalyst 15. If the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 is lean, the ammonia/NOx detection signal output by the ammonia sensor 20 is considered to indicate the NOx concentration, and if the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15 is rich, the ammonia/NOx detection signal output by the ammonia sensor 20 is considered to indicate the ammonia concentration.

The exhaust passage 14 may be provided with a downstream air-fuel ratio sensor that is separate from the ammonia sensor 20. Alternatively, the exhaust air-fuel ratio downstream of the three-way catalyst 15 may not be detected, and the air-fuel ratio environment inside the three-way catalyst 15 may be estimated based on the exhaust air-fuel ratio of the exhaust gas flowing into the three-way catalyst 15 (which is detected by the upstream air-fuel ratio sensor 19).

The three-way catalyst 15 further includes a catalyst temperature sensor 25 for detecting the temperature of the three-way catalyst 15. In one embodiment, the catalyst temperature sensor 25 detects the temperature of the carrier (monolith ceramic body) of the three-way catalyst 15. Note that instead of the catalyst temperature sensor 25 that directly detects the carrier temperature, an exhaust temperature sensor that detects the exhaust gas temperature may be provided on at least one of the upstream and downstream sides of the three-way catalyst 15, and the temperature of the three-way catalyst 15 may be estimated based on the exhaust gas temperature. Alternatively, the temperature of the three-way catalyst 15 may be estimated based on the amount of heat input provided to the three-way catalyst 15 from the internal combustion engine 1.

The detection signals of the upstream air-fuel ratio sensor 19, the ammonia sensor 20, the catalyst temperature sensor 25, and the air flow meter 11 are input to the engine controller 9. The engine controller 9 also receives detection signals from a number of sensors, such as a crank angle sensor 21 for detecting the engine speed, a water temperature sensor 22 for detecting the cooling water temperature, and an accelerator opening sensor 23 for detecting the amount of depression of the accelerator pedal operated by the driver. Based on these input signals, the engine controller 9 optimally controls the amount and timing of fuel injection by the fuel injection valve 5, the ignition timing by the spark plug 4, the opening of the throttle valve 10, etc.

As one of various controls of the internal combustion engine 1, the engine controller 9 performs air-fuel ratio control to maintain the oxygen storage amount of the three-way catalyst 15 at a target oxygen storage amount (for example, set to about 40 to 60%) in order to optimize the exhaust purification performance of the three-way catalyst 15. In the air-fuel ratio control, the fuel injection amount is feedback controlled (for example, PID control, etc.) so that the exhaust air-fuel ratio detected by the upstream air-fuel ratio sensor 19 (hereinafter, this is called the upstream exhaust air-fuel ratio) follows the target air-fuel ratio. Here, the target air-fuel ratio is calculated so that the oxygen storage amount of the three-way catalyst 15 estimated from the upstream exhaust air-fuel ratio matches the target oxygen storage amount. Therefore, basically, the oxygen storage amount of the three-way catalyst 15 is maintained near the target oxygen storage amount. When the oxygen storage amount is near the target oxygen storage amount, the air-fuel ratio environment inside the three-way catalyst 15 is equivalent to the theoretical air-fuel ratio. This effectively oxidizes CO and HC in the exhaust gas and reduces NOx.

The amount of oxygen storage can be estimated, for example, based on the output signal of the upstream air-fuel ratio sensor 19 located upstream of the three-way catalyst 15 and the exhaust flow rate. The exhaust flow rate can be estimated, for example, from the output signal of the air flow meter 11.

Here, when the inside of the three-way catalyst 15 is in an environment richer than the theoretical air-fuel ratio, ammonia is generated by the NOx and hydrogen in the exhaust gas through a catalytic reaction, and some of the generated ammonia is converted to nitrogen and the like through an oxidation reaction, and the remaining ammonia flows out to the downstream side of the three-way catalyst 15. Therefore, when the three-way catalyst 15 is deteriorated, even if the inside of the three-way catalyst 15 is in an environment richer than the theoretical air-fuel ratio, a sufficient catalytic reaction does not occur, and the amount of ammonia flowing out to the downstream side of the three-way catalyst 15 also decreases.

In addition, in an environment where the inside of the three-way catalyst 15 is richer than the theoretical air-fuel ratio, if the oxygen storage capacity of the three-way catalyst 15 has not deteriorated, the NOx in the exhaust gas is reduced within the three-way catalyst 15, making it difficult for the NOx in the exhaust gas to flow downstream of the three-way catalyst 15.

On the other hand, when the inside of the three-way catalyst 15 is in an environment leaner than the theoretical air-fuel ratio, the NOx in the exhaust gas is less likely to be reduced by the three-way catalyst 15 and is more likely to flow downstream of the three-way catalyst 15. In addition, the oxygen storage capacity of the three-way catalyst 15 (oxygen storage amount, oxygen adsorption/desorption speed) changes depending on the deterioration state of the three-way catalyst 15.

Therefore, when the inside of the three-way catalyst 15 is in an environment leaner than the theoretical air-fuel ratio, if the three-way catalyst 15 is deteriorated, the amount of NOx that can be processed by the three-way catalyst 15 decreases, and the amount of NOx that flows out downstream of the three-way catalyst 15 increases.

Therefore, a catalyst deterioration diagnosis of the three-way catalyst 15 is performed by utilizing the NOx processing characteristics and ammonia generation characteristics of the three-way catalyst 15 according to such an exhaust air-fuel ratio (the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15).

In other words, as shown in FIG. 2, a specified value A is set as a threshold value for the combined concentration of NOx and ammonia components in the exhaust gas downstream of the three-way catalyst 15 according to the exhaust air-fuel ratio, and the three-way catalyst 15 is diagnosed as degraded when the instantaneous value of the combined concentration of NOx and ammonia components in the exhaust gas detected by the ammonia sensor 20 when the exhaust air-fuel ratio is richer than the theoretical air-fuel ratio becomes equal to or less than the specified value A, or when the instantaneous value of the combined concentration of NOx and ammonia components in the exhaust gas detected by the ammonia sensor 20 when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio becomes equal to or more than the specified value A.

The specified value A is set according to the exhaust air-fuel ratio detected by the ammonia sensor 20. For example, the specified value A is determined by using a map that is created in advance and that assigns the specified value A using the exhaust air-fuel ratio as a parameter.

The specified value A is set so that it increases as the exhaust air-fuel ratio decreases when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, and increases as the exhaust air-fuel ratio increases when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio.

The deterioration diagnosis of the three-way catalyst 15 is performed within the engine controller 9. In other words, the engine controller 9 corresponds to a threshold setting unit and a diagnosis unit.

Figure 2 is an explanatory diagram that shows a schematic overview of the deterioration diagnosis of the three-way catalyst 15. As shown in Figure 2, a state in which the exhaust air-fuel ratio (the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15) is richer than the theoretical air-fuel ratio is, for example, a state in which the exhaust air-fuel ratio is smaller (richer) than a predetermined first air-fuel ratio (air-fuel ratio 14.6) that is smaller (richer) than the theoretical air-fuel ratio. Note that the value of the first air-fuel ratio is not limited to 14.6, and may be set to another value near the theoretical air-fuel ratio that is smaller (richer) than the theoretical air-fuel ratio of 14.7.

When the exhaust air-fuel ratio (the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15) is in a state richer than the theoretical air-fuel ratio, if the instantaneous value of the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor 20 falls below a specified value A, the three-way catalyst 15 is diagnosed as degraded.

When the exhaust air-fuel ratio is smaller than the first air-fuel ratio, the deterioration of the catalytic metal (precious metal) of the three-way catalyst 15 is diagnosed as a deterioration diagnosis of the three-way catalyst 15.

The state in which the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio is, for example, a state in which the exhaust air-fuel ratio is greater (leaner) than a predetermined second air-fuel ratio (air-fuel ratio of 14.8) that is greater (leaner) than the theoretical air-fuel ratio. Note that the value of the second air-fuel ratio is not limited to 14.8, and may be set to another value near the theoretical air-fuel ratio that is greater (leaner) than the theoretical air-fuel ratio of 14.7.

When the exhaust air-fuel ratio (the exhaust air-fuel ratio of the exhaust gas flowing out from the three-way catalyst 15) is leaner than the theoretical air-fuel ratio, if the instantaneous value of the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor 20 is greater than a specified value A, the three-way catalyst 15 is diagnosed as degraded.

When the exhaust air-fuel ratio is greater than the second air-fuel ratio, the deterioration of the catalytic metal (precious metal) of the three-way catalyst 15 is diagnosed as a deterioration diagnosis of the three-way catalyst 15. Note that when the air-fuel ratio is greater than the second air-fuel ratio, the deterioration of the oxygen storage capacity of the three-way catalyst 15 due to the deterioration of the catalytic metal (precious metal) of the three-way catalyst 15 is also diagnosed.

When the exhaust air-fuel ratio is in a stoichiometric region, which is equal to or greater than the first air-fuel ratio and equal to or less than the second air-fuel ratio, the exhaust air-fuel ratio is determined to be in a stoichiometric state, and if the instantaneous value of the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor 20 is greater than a specified value A, the three-way catalyst 15 is diagnosed as degraded.

In other words, if the instantaneous value of the combined concentration of NOx and ammonia components in the exhaust gas detected by the ammonia sensor 20 when the exhaust air-fuel ratio is at the theoretical air-fuel ratio becomes equal to or exceeds a specified value A, the performance of the three-way catalyst 15 is diagnosed as degraded.

When the exhaust air-fuel ratio is equal to or greater than the first air-fuel ratio and equal to or less than the second air-fuel ratio, the deterioration of the catalytic metal (precious metal) of the three-way catalyst 15 and the deterioration of the oxygen adsorption/desorption performance (adsorption/desorption speed) of the three-way catalyst 15 are diagnosed as deterioration diagnosis of the three-way catalyst 15. In other words, when the exhaust air-fuel ratio is equal to or greater than the first air-fuel ratio and equal to or less than the second air-fuel ratio, the performance diagnosis of the three-way catalyst 15 is performed, including the deterioration of the catalytic metal (precious metal) of the three-way catalyst 15 and the deterioration of the oxygen adsorption/desorption performance (adsorption/desorption speed) of the three-way catalyst 15.

Figure 3 is a flowchart showing the process for diagnosing catalyst deterioration in the above-described embodiment.

In step S1, the ammonia sensor 20 is used to detect the exhaust air-fuel ratio downstream of the three-way catalyst 15.

In step S2, the specified value A is calculated using the exhaust air-fuel ratio detected in step S2.

In step S3, the ammonia sensor 20 is used to detect the combined concentration of NOx and ammonia components in the exhaust gas downstream of the three-way catalyst 15.

In step S4, it is determined whether the value of the exhaust air-fuel ratio detected in step S1 is smaller than the first air-fuel ratio (14.6). If the value of the exhaust air-fuel ratio is smaller than the first air-fuel ratio (14.6) in step S4, the process proceeds to step S5. If the value of the exhaust air-fuel ratio is equal to or greater than the first air-fuel ratio (14.6) in step S4, the process proceeds to step S6.

In step S5, it is determined whether the combined concentration of NOx and ammonia components in the exhaust gas is less than a specified value A. If in step S5 the combined concentration of NOx and ammonia components in the exhaust gas is less than the specified value A, the process proceeds to step S7. If in step S5 the combined concentration of NOx and ammonia components in the exhaust gas is equal to or greater than the specified value A, the process proceeds to step S1.

In step S6, it is determined whether the combined concentration of NOx and ammonia components in the exhaust gas is greater than a specified value A. If the combined concentration of NOx and ammonia components in the exhaust gas is greater than the specified value A in step S6, the process proceeds to step S7. If the combined concentration of NOx and ammonia components in the exhaust gas is equal to or less than the specified value A in step S6, the process proceeds to step S1.

In step S7, the three-way catalyst 15 is diagnosed as degraded.

As explained above, in the above-mentioned embodiment, the ammonia sensor 20 can be used to accurately diagnose the deterioration of the three-way catalyst 15.

In addition, in the above-described embodiment, deterioration diagnosis of the three-way catalyst 15 is possible even when the internal combustion engine 1 is not being supplied with fuel while the engine speed is equal to or higher than a predetermined speed, i.e., when fuel supply to the internal combustion engine 1 is stopped. Therefore, even when the internal combustion engine 1 is mounted on a hybrid vehicle in which such fuel cut-off situations occur infrequently, it is possible to ensure sufficient opportunities to perform deterioration diagnosis of the three-way catalyst 15 and to improve the accuracy of deterioration diagnosis of the three-way catalyst 15.

Although specific examples of the present invention have been described above, the present invention is not limited to the above examples, and various modifications are possible without departing from the spirit of the present invention.

For example, when diagnosing deterioration of the three-way catalyst 15, a moving average value of the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor 20 may be used.

When diagnosing the deterioration of the three-way catalyst 15, the instantaneous value and the moving average value of the combined concentration of the NOx component and the ammonia component in the exhaust gas detected by the ammonia sensor 20 may be used depending on the operating state. Specifically, when the fluctuation in the intake air volume of the internal combustion engine 1 is small, the instantaneous value may be used, and when the fluctuation in the intake air volume of the internal combustion engine 1 is large (when acceleration and deceleration are severe), the moving average value may be used.

The specified value A may be corrected taking into account factors that affect the combined concentration of NOx and ammonia components in the exhaust gas at the outlet of the three-way catalyst 15, such as the EGR rate and catalyst temperature. For example, the specified value A may be corrected to be smaller as the EGR rate in the internal combustion engine 1 increases. Here, the EGR rate is the ratio of EGR gas to the intake side flow rate when a portion of the exhaust gas is introduced as EGR gas into the intake passage 7. In addition, the specified value A may be corrected to be larger as the catalyst temperature decreases, for example, if the three-way catalyst 15 is in an activated state. These corrections can further improve the accuracy of the deterioration diagnosis of the three-way catalyst 15.

The above-described embodiment relates to a method for diagnosing the deterioration of an exhaust gas purification catalyst and a device for diagnosing the deterioration of an exhaust gas purification catalyst.

Reference Signs List 1 internal combustion engine 9 engine controller 10 throttle valve 11 air flow meter 14 exhaust passage 15 three-way catalyst 19 upstream air-fuel ratio sensor 20 ammonia sensor 25 catalyst temperature sensor

Claims (7)

A method for diagnosing deterioration of an exhaust purification catalyst, comprising: providing an ammonia sensor, which is sensitive to a NOx concentration and an ammonia concentration in exhaust gas, downstream of an exhaust purification catalyst in an exhaust passage of an internal combustion engine; and diagnosing deterioration of the exhaust purification catalyst using a detection signal of the ammonia sensor,
setting a threshold value for a combined concentration of NOx components and ammonia components in the exhaust gas downstream of the exhaust purification catalyst according to an exhaust air-fuel ratio downstream of the exhaust purification catalyst or within the exhaust purification catalyst;
The deterioration diagnosis method for an exhaust purification catalyst diagnoses that the exhaust purification catalyst is deteriorated when the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or lower than the threshold value when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, and when the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or higher than the threshold value when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1, in which the exhaust purification catalyst is diagnosed as being deteriorated if the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or greater than the threshold value when the exhaust air-fuel ratio is in a theoretical air-fuel ratio state. The state in which the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio is a state in which the exhaust air-fuel ratio is smaller than a predetermined first air-fuel ratio that is smaller than the stoichiometric air-fuel ratio,
The state in which the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio is a state in which the exhaust air-fuel ratio is greater than a predetermined second air-fuel ratio that is greater than the stoichiometric air-fuel ratio,
3. The method for diagnosing deterioration of an exhaust gas purification catalyst according to claim 2, further comprising the step of diagnosing that the exhaust gas purification catalyst is deteriorated when a combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or higher than the threshold value while the exhaust gas air-fuel ratio is equal to or higher than the first air-fuel ratio and equal to or lower than the second air-fuel ratio. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1, wherein the threshold value increases as the exhaust air-fuel ratio decreases when the exhaust air-fuel ratio is richer than the theoretical air-fuel ratio, and increases as the exhaust air-fuel ratio increases when the exhaust air-fuel ratio is leaner than the theoretical air-fuel ratio. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1, wherein the combined concentration of NOx and ammonia components in the exhaust gas detected by the ammonia sensor is an instantaneous value or a moving average of the instantaneous values. The method for diagnosing deterioration of an exhaust purification catalyst according to claim 1, in which the threshold value is corrected taking into account factors that affect the combined concentration of NOx components and ammonia components in the exhaust gas at the outlet of the exhaust purification catalyst. an exhaust purification catalyst provided in an exhaust passage of the internal combustion engine;
an ammonia sensor provided downstream of the exhaust purification catalyst and sensitive to NOx concentration and ammonia concentration in the exhaust gas;
a threshold setting unit that sets a threshold value of a combined concentration of NOx components and ammonia components in exhaust gas downstream of the exhaust purification catalyst in accordance with an exhaust air-fuel ratio downstream of the exhaust purification catalyst or within the exhaust purification catalyst;
and a diagnosis unit which diagnoses that the exhaust purification catalyst is deteriorated when the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or lower than the threshold value when the exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, and when the combined concentration of NOx components and ammonia components in the exhaust gas detected by the ammonia sensor becomes equal to or higher than the threshold value when the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio.

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