JPH0754641A - Catalyst deterioration diagnotic device for internal combustion engine - Google Patents

Abstract

PURPOSE:To diagnose deterioration in NOx conversion performance in a exhaust gas purifying catalyst capable of purifying NOx in an oxygen excessive state. CONSTITUTION:NOx sensors 16a, 16b, 18a, 18b are respectively provided on the upstream side and the downstream side of catalysts 9a, 9b. And the outputs of respective NOx sensors are measured, and the ratio R of NOx concentration on the upstream side to that on the downstream side of the catalyst, namely, NOx conversion ratio, is calculated. Next, the reference value R0 of the NOx concentration ratio R is set on the basis of the engine speed Ne and the basic fuel injection amount Tp. And NOx concentration ratio R and the reference value R0 are compared with each other, when decrease of the NOx conversion ratio is judged on the basis of the compared result, an alarm lamp 19 is turned on.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a catalyst deterioration diagnosing device for an internal combustion engine, and more particularly to a device for diagnosing a decrease in NOx conversion performance of an exhaust purification catalyst that converts NOx.

[0002]

2. Description of the Related Art Conventionally, an apparatus for diagnosing deterioration (decrease in conversion performance) of an exhaust purification catalyst provided in an exhaust passage of an internal combustion engine is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-91440. There is. The internal combustion engine disclosed in Japanese Unexamined Patent Publication No. 2-91440 is provided with oxygen sensors on the upstream side and the downstream side of the three-way catalyst interposed in the exhaust passage,
The air-fuel ratio feedback control is performed based on the outputs of these oxygen sensors.

When the inversion cycle of the oxygen sensor on the upstream side is fu and the inversion cycle of the oxygen sensor on the downstream side is fd during the air-fuel ratio feedback control, fd /
When fu ≧ a predetermined value, the conversion performance of the three-way catalyst is normal, and when fd / fu <a predetermined value, the three-way catalyst is diagnosed as being deteriorated. That is, the above-mentioned diagnosis method is to judge the degree of oxygen storage effect in the three-way catalyst by the ratio fd / fu of the above-mentioned cycle, and thereby perform the deterioration diagnosis of the catalyst.

On the other hand, in recent years, a lean-burn engine has been developed which burns in an air-fuel ratio range (for example, an air-fuel ratio of about 20 to 22) that is significantly leaner than the stoichiometric air-fuel ratio. N in the air-fuel ratio range (in excess air atmosphere)
A so-called lean NOx catalyst that can purify Ox is used. The lean NOx catalyst is mainly composed of zeolite and temporarily adsorbs HC in the exhaust gas to form H
It is estimated that C reduces NOx.

[0005]

By the way, in the conventional diagnosis method, when the air-fuel ratio feedback control is performed in which the target air-fuel ratio is the theoretical air-fuel ratio which is the boundary air-fuel ratio at which the oxygen concentration in the exhaust gas suddenly changes, By utilizing the fact that the oxygen storage effect of the three-way catalyst causes a response delay in the oxygen concentration change on the downstream side of the catalyst, the degree of the oxygen storage effect of the catalyst,
Finally, the deterioration (deterioration) of the conversion performance of the three-way catalyst is diagnosed.

For this reason, the lean NOx for purifying NOx in the lean air-fuel ratio range (state where the oxygen concentration in the exhaust gas is excessive) is obtained.
It cannot be applied to the diagnosis of the catalyst, and it has been desired to provide a diagnosis device that can perform the deterioration diagnosis of the lean NOx catalyst. The present invention has been made in view of the above problems, and it is an object of the present invention to provide a catalyst deterioration diagnosis device capable of diagnosing deterioration (deterioration) of NOx conversion performance of a lean NOx catalyst that purifies NOx in a combustion state with a lean air-fuel ratio. To aim.

[0007]

Therefore, a catalyst deterioration diagnosing device for an internal combustion engine according to the present invention is provided with at least N in exhaust gas.
An exhaust gas purification catalyst for converting Ox is a catalyst deterioration diagnosis device for an internal combustion engine, which is provided in an exhaust passage, and is configured as shown in FIG. In FIG. 1, the NOx sensor indicates that the NO in the exhaust gas correlates with the NOx conversion rate in the exhaust purification catalyst.
It is a sensor that detects x concentration.

The reference value setting means sets a reference value for discriminating the detection result of the NOx sensor based on the engine operating condition detected by the operating condition detecting means.
Then, the diagnosing means judges the detection result by the NOx sensor based on the reference value set by the reference value setting means, and diagnoses the deterioration of the NOx conversion performance of the exhaust purification catalyst.

Here, the exhaust purification catalyst is a catalyst having a lean NOx catalytic action for converting NOx in an oxygen excess atmosphere and a three-way catalytic action, and the internal combustion engine has a lean air-fuel ratio in a predetermined operating region. In the case where the engine is operated with the target air-fuel ratio as the target air-fuel ratio, the diagnosis means determines the lean air-fuel ratio as the target air-fuel ratio when the deterioration of NOx conversion performance is diagnosed when the engine is operated with the lean air-fuel ratio. It is advisable to provide an air-fuel ratio switching control means for forcibly switching the target air-fuel ratio in the predetermined operating region where the fuel ratio is set to the stoichiometric air-fuel ratio.

[0010]

According to the catalyst deterioration diagnosing device having such a configuration, the NO in the exhaust gas that correlates with the NOx conversion rate in the exhaust purification catalyst.
The x concentration is detected by the NOx sensor, and the detection result by the NOx sensor is determined according to the engine operating condition,
The deterioration of the NOx conversion performance of the catalyst is diagnosed.

That is, by providing a NOx sensor, NO
By directly detecting the conversion rate of x and comparing the detection result with a value expected according to the engine operating state, the expected NO
It was made to diagnose whether or not the x conversion performance was exhibited. Further, the exhaust purification catalyst has a function of converting NOx in an oxygen excess atmosphere, and accordingly, when the engine is operated at a lean air-fuel ratio, if the NOx conversion performance in the oxygen excess atmosphere deteriorates, the lean air-fuel ratio is reduced. NOx in the combustion state at the fuel ratio
Emissions will increase. Therefore, when the deterioration of the NOx conversion performance is diagnosed as described above during the operation at the lean air-fuel ratio, the NOx conversion performance is deteriorated in the oxygen excess atmosphere, so the target air-fuel ratio is set to the original lean air-fuel ratio. The fuel ratio is switched to the stoichiometric air-fuel ratio so that the exhaust purification catalyst can purify the exhaust gas including NOx by the three-way catalytic action.

[0012]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 showing the system configuration of the embodiment, air is sucked into each cylinder of the V-type internal combustion engine 1 through an air cleaner 2, a throttle valve 3 and an intake manifold 4. An electromagnetic fuel injection valve 5 is provided at each branch of the intake manifold 4.

Exhaust gas from the engine 1 is exhausted from an exhaust manifold 6
After being collected for each bank by a and 6b, they are guided to the muffler 8 by exhaust pipes 7a and 7b, respectively. Catalytic converters (exhaust gas purification catalysts) 9a and 9b are provided in the exhaust pipes 7a and 7b, respectively. The catalytic converters 9a, 9b are configured to reduce NOx, HC, and
It has both a three-way catalytic action for simultaneously purifying CO at a high conversion rate and a lean NOx catalytic action for reducing (converting) NOx in an oxygen excess state (lean air-fuel ratio combustion state).

The control unit 10 has a built-in microcomputer, calculates the fuel injection amount Ti (injection pulse width) by the fuel injection valve 5 based on the detection signals from various sensors, and calculates the fuel injection amount Ti. The fuel supply to the engine 1 is electronically controlled by controlling the open drive of the fuel injection valve 5 based on The engine of the present embodiment is a so-called lean combustion engine that burns at an air-fuel ratio (for example, 22) that is significantly leaner than the stoichiometric air-fuel ratio in a predetermined operating region. Combustion is performed at an air-fuel ratio (output air-fuel ratio) near the fuel ratio to ensure acceleration performance and the like.

The various sensors include a throttle valve 3
Upstream, the air flow meter 11 for detecting the intake air amount Qa of the engine 1, the crank angle sensor 12 for extracting the engine rotation signal from the cam shaft, the water temperature sensor 13 for detecting the cooling water temperature Tw of the engine 1, and the exhaust manifolds 6a, 6b. The oxygen sensors 14a, 14b for detecting the oxygen concentration in the exhaust gas and the potentiometer-type throttle sensor 15 for detecting the opening of the throttle valve 3 are provided for each bank.

Reference numeral 17 is a control valve for adjusting the intake air amount at the time of idling, and the engine 1 is provided through a bypass passage 18 which bypasses the throttle valve 3.
Adjust the amount of auxiliary air supplied to the. Further, as will be described later, the control unit 10 has a catalyst deterioration diagnosis function for diagnosing a decrease in NOx conversion performance of the catalytic converters 9a, 9b. NO to each upstream side
x sensors 16a and 16b are provided, and NO is also provided on the downstream side.
x sensors 18a and 18b are provided.

The NOx sensors 16a, 16b, 18a, 18b
Is a semiconductor type thin film sensor of Ag _0.04 V ₂ O ₅ . This NOx sensor is a known sensor whose resistance value changes as shown in FIG. 3 due to the adsorption of NOx on the sensor surface. Then, the control unit 10 is
Based on the outputs of the Ox sensors 16a, 16b, 18a, 18b, the deterioration of NOx conversion performance in the catalytic converters 9a, 9b is diagnosed, and as a result of the diagnosis, the catalytic converter 9 is detected.
When it is determined that the NOx conversion performances of a and 9b are deteriorated, the warning light 19 (alarm means) provided in the driver's seat of the vehicle in which the engine 1 is mounted is turned on, and the diagnosis result is output to the driver. To warn you.

That is, the NOx conversion rate in the catalytic converters 9a and 9b is defined by the ratio of the NOx concentration on the upstream side of the catalyst to the NOx concentration on the downstream side of the catalyst.
In the present embodiment, the NOx sensors 16a, 16b, 18a, 18b provided on the upstream side and the downstream side respectively detect the NOx concentration in the exhaust gas that correlates with the NOx conversion rate in the catalytic converters 9a, 9b. Equivalent to.

Here, the structure of the catalyst deterioration diagnosing device of this embodiment is shown in a simplified block diagram of FIG. As shown in FIG. 4, the control unit 10 functions as a reference value setting means A that sets a reference value based on the detection signals of the crank angle sensor 12 and the air flow meter 11, the upstream NOx sensors 16a, 16b, and the downstream NOx sensors 16a, 16b. Side NOx sensor 18
Catalytic converters 9a, 9 based on the detection signals of a, 18b
b, the function as the NOx concentration ratio calculating means B for calculating the NOx concentration ratio between the upstream side and the downstream side, and the reference value and NO
Based on comparison with x concentration ratio, catalytic converters 9a, 9b
It has a function as a diagnostic means C for diagnosing the deterioration of the NOx conversion performance. Further, in the present embodiment, the alarm means D including the warning light 19 which is selectively operated in response to the diagnosis result (diagnosis signal) of the diagnosis means C is provided.

Here, the reference value setting means A sets the basic fuel injection amount Tp calculated based on the detection signals of the crank angle sensor 12 and the air flow meter 11 as an engine load equivalent value, and the basic fuel injection amount Tp The reference value R _O is set based on the engine speed Ne. On the other hand, the NOx concentration ratio calculation means B determines the conversion efficiency of the catalyst based on the output of the NOx sensor.
Calculate the ratio of Ox concentration. Then, the diagnosis means C is
By comparing the reference value R _O and the NOx concentration ratio, N
When it is determined that the Ox conversion efficiency has decreased more than a predetermined value and the NOx conversion efficiency has decreased, the warning means D is activated to warn the decrease of the NOx conversion performance.

Next, the state of the catalyst deterioration diagnosis will be described in detail with reference to the flowchart of FIG. In the flowchart of FIG. 5, at P1, the engine speed Ne is detected based on the detection signal of the crank angle sensor 12. Then
At P2, the intake air amount Qa of the engine 1 is detected based on the detection signal of the air flow meter 11.

At P3, the basic injection amount (basic injection pulse width) Tp = K × Qa / Ne in the fuel injection valve 5 is calculated based on the engine speed Ne and the intake air amount Qa.
(K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5.) is calculated. The basic injection amount Tp is used to set a reference value, which will be described later, as a value representing the engine load. Further, at P4, NOx sensors 16a and 16b (first NOx sensor) provided on the upstream side of the catalytic converters 9a and 9b.
Output V _RU , V _LU of

Further, in P5, the catalytic converters 9a, 9a
NOx sensors 18a, 18b (second
The outputs V _RD and V _LD of the NOx sensor) are measured. And
Next, at P6, the NOx concentration ratio R between the upstream and downstream of the catalyst, which indicates the NOx conversion rate in the catalytic converters 9a and 9b, is calculated based on the measured output according to the following equation.

[0024]

[Equation 1]

The above-mentioned arithmetic expression is an arithmetic expression for averaging the NOx conversion rates in the two catalytic converters 9a and 9b provided for each bank, and the NOx concentration for each catalytic converter 9a, 9b. You may make it calculate a ratio. On the other hand, at P7, from the reference value R ₀ of the NOx concentration ratio R stored in advance in the map (see FIG. 6) using the engine speed Ne and the basic fuel injection amount Tp (engine load) as parameters. , The data corresponding to the current engine speed Ne and the basic fuel injection amount Tp (engine load) are retrieved and read.

As described above, in this embodiment, the reference value R ₀ is variably set based on the engine speed Ne and the basic fuel injection amount Tp as the engine operating conditions. Therefore, the crank angle sensor 12 and the air flow are set. The meter 11 corresponds to the operating condition detecting means. The reference value R ₀ map reference value stored in R ₀ is expected the safety factor of the NOx concentration detected, corresponding to a state where the NOx emission amount is increased to about 150% of the initial state (state without deterioration) Has been set.

In the map shown in FIG. 6, the low load / low speed operation region surrounded by a dotted line is a region (lean combustion region) in which combustion is performed at an air-fuel ratio significantly leaner than the theoretical air-fuel ratio. , The lean NOx catalytic action in the catalytic converters 9a and 9b is diagnosed. Further, the operating region other than the lean burn region surrounded by the dotted line in the map shown in FIG. 6 is a region in which combustion is performed at an air-fuel ratio near the stoichiometric air-fuel ratio (output air-fuel ratio region), and in this region, the catalytic converter 9a , 3b NOx including three-way catalysis
The conversion performance will be diagnosed.

Next, at P8, the N calculated at P6 is calculated.
Compared with Ox concentration ratio R, and the reference value R ₀ which has been determined by the P7, when the reference value the actual NOx concentration ratio R than R ₀ is large, NOx conversion performance catalytic converters 9a, at 9b is normal Assuming that there is, the warning light 19 is controlled to be turned off at P10 and indirectly the catalytic converters 9a and 9b are normally operated.
Indicates that Ox purification is being performed.

On the other hand, at P8, the actual N is larger than the reference value R _0.
When it is determined that the Ox concentration ratio R is small, it is determined that the NOx conversion performance is deteriorated due to deterioration of the catalytic converters 9a and 9b, and the warning light 19 is turned on at P9,
The driver or the like is warned that the catalytic converters 9a and 9b have deteriorated NOx conversion performance. As described above, according to the above embodiment, the lean N that purifies NOx in the lean air-fuel ratio combustion state is used.
It becomes possible to diagnose the Ox catalytic action, and when the NOx conversion performance deteriorates, the warning lamp 19 is turned on to notify the operator, so that the engine is prevented from operating in the lean combustion state where NOx cannot be purified. it can.

By the way, by the catalyst deterioration diagnosis as described above, the lean NOx in the catalytic converters 9a and 9b is obtained.
If the lean combustion is executed when it is diagnosed that the catalytic action is lowered, NOx is exhausted without being purified. Therefore, when a decrease in the lean NOx catalytic action is diagnosed based on the determination of the NOx concentration ratio during lean combustion, the lean combustion is canceled and the target air-fuel ratio in the region initially set as the lean burn region is forced. It is preferable that the theoretical air-fuel ratio is set to NOx and NOx is surely purified by the three-way catalytic action in the catalytic converters 9a and 9b.

A second embodiment for forcibly switching the target air-fuel ratio in the lean burn region to the stoichiometric air-fuel ratio based on the deterioration diagnosis of the lean NOx catalytic action will be described below. FIG. 7 is a block diagram showing a simplified configuration of the catalyst deterioration diagnosis device and the fuel injection control device of the second embodiment.
In the block diagram of FIG. 7, in addition to the configuration shown in FIG. 4, basic fuel injection amount calculation means E for calculating the basic fuel injection amount Tp based on the outputs of the crank angle sensor 12 and the air flow meter 11, and the basic fuel An injection amount calculation unit F (air-fuel ratio switching control unit) that calculates a final fuel injection amount Ti based on the basic fuel injection amount Tp calculated by the injection amount calculation unit E and the diagnosis result of the diagnosis unit C. Quantity calculation means F
An injection valve driving means G for driving and controlling the fuel injection valve 5 on the basis of the fuel injection amount Ti calculated in (3) is provided as a function of the control unit 10.

Here, when the diagnosis means C determines that the lean NOx catalytic action in the catalytic converters 9a, 9b is decreased, the injection amount calculation means F sets the injection amount corresponding to the lean air-fuel ratio. Is canceled, and instead, the injection amount corresponding to the theoretical air-fuel ratio is calculated. Then, receiving the calculation result by the injection amount calculation means F,
The injection valve driving means G outputs an injection pulse signal having a pulse width corresponding to the injection amount Ti to the fuel injection valve 5 at a predetermined timing synchronized with the engine rotation.

Next, according to the flow chart of FIG. 8, diagnosis of the lean NOx catalytic action outlined above, and
The state of the air-fuel ratio control that has received the diagnosis result will be described in detail. In the flowchart of FIG. 8, first, in P21,
Based on the detection signal of the crank angle sensor 12, the engine speed N
e is detected. Next, at P22, the air flow meter 11
The intake air amount Qa of the engine 1 is detected based on the detection signal of.

At P23, the basic injection amount (basic injection pulse width) Tp = K × Qa / Ne in the fuel injection valve 5 is calculated based on the engine speed Ne and the intake air amount Qa.
(K is a proportional constant corresponding to the flow rate characteristic of the injection valve 5.) is calculated. The basic injection amount Tp is calculated as a theoretical air-fuel ratio equivalent value. At the next P24, it is judged from the engine speed Ne and the basic injection amount Tp whether or not it is a preset lean combustion region. If it is at the lean combustion region, the routine proceeds to P25.

At P25, based on the basic injection amount Tp,
The injection amount Ti is calculated according to the following formula. Ti = (14.6 / 22) × Tp + Ts In the above calculation formula, the basic injection amount Tp calculated as the theoretical air-fuel ratio (air-fuel ratio = 14.6) equivalent value corresponds to the target lean air-fuel ratio (= 22) in the lean combustion region. And Ts is a value for correcting the invalid injection time of the injection valve 5 and is determined by the battery voltage which is the power source of the injection valve 5.

Then, in the state where the injection of the injection valve 5 is controlled in accordance with the fuel injection amount Ti calculated with the above lean air-fuel ratio (= 22) as the target air-fuel ratio, NOx after P26.
By diagnosing the conversion performance, the catalytic converter 9
The deterioration of the lean NOx catalytic action in a and 9b is diagnosed. At P26, the outputs V _RU and V _LU of the NOx sensors 16a and 16b (first NOx sensor) provided on the upstream side of the catalytic converters 9a and 9b are measured.

Further, at P27, the catalytic converters 9a, 9
NOx sensors 18a, 18b (second
The outputs V _RD and V _LD of the NOx sensor) are measured. And
Next, at P28, the NOx concentration ratio R between the upstream and downstream of the catalyst, which indicates the NOx conversion rate in the catalytic converters 9a and 9b, is calculated based on the measured output according to the above mathematical expression 1.

On the other hand, at P29, the NOx is calculated based on the engine speed Ne and the basic fuel injection amount Tp (engine load).
The reference value R ₀ used to determine the density ratio R is variably set (see FIG. 6). Next, at P30, the NO calculated at P28
and x concentration ratio R, compared with a reference value R ₀ which has been determined by the P29, when the reference value R ₀ actual NOx concentration ratio R than is large, the catalytic converter 9a, lean in 9b NO
It is determined that the x-catalytic action (the action of reducing NOx in the lean burn state) is functioning normally. Then, in this case, the routine proceeds to P31, the warning light 19 is turned off, and the fuel control that normally makes the lean air-fuel ratio in the lean burn region the target air-fuel ratio is continued.

On the other hand, at P30, the actual NO is lower than the reference value R _0.
If it is determined that the x concentration ratio R is small, it is determined that the lean NOx catalytic action in the catalytic converters 9a and 9b is reduced, and the warning light 19 is turned on at P32 to turn NO.
In addition to notifying the decrease in the x conversion performance, in P33 as the air-fuel ratio switching control means, the theoretical air-fuel ratio is replaced with the lean air-fuel ratio (= 22) calculated in P25 instead of the fuel injection amount Ti which is the target air-fuel ratio. Fuel injection amount Ti = T for target air-fuel ratio
Calculate p + Ts. Then, by controlling the drive of the injection valve 5 based on the injection amount Ti corresponding to the theoretical air-fuel ratio,
The target air-fuel ratio in the lean burn region is switched from the original lean air-fuel ratio to the stoichiometric air-fuel ratio so that fuel injection by the injection valve 5 is controlled.

That is, when a decrease in lean NOx catalytic action in the catalytic converters 9a, 9b is diagnosed, if lean combustion is continued as it is, NOx will be discharged without being purified, so lean combustion will be stopped. Then, the combustion at the stoichiometric air-fuel ratio is performed even in the original lean combustion region, and the exhaust (NOx) can be purified by the three-way catalytic action in the catalytic converters 9a and 9b.

Therefore, even if the lean NOx catalytic action in the catalytic converters 9a and 9b is lowered, lean combustion cannot be performed if the three-way catalytic action is normal, but at least a large amount of NOx is discharged. You can avoid it. In each of the above embodiments, the catalytic converter 9
NOx sensors 16a, 16b, 18a provided on the upstream side and the downstream side of each of the catalytic converters 9a, 9b as NOx sensors for detecting the NOx concentration in the exhaust gas that correlates with the NOx conversion rate of a, 9b (exhaust gas purification catalyst). , 18b are shown, but for the sake of simplicity, the NOx sensor is provided only on the downstream side of each of the catalytic converters 9a, 9b (NOx of the above-described embodiment.
The sensors 18a, 18b) and the NOx conversion performance based on the comparison between the NOx concentration in the exhaust gas after purification by the catalyst detected by the NOx sensor and the reference concentration value set from the engine operating conditions (load, rotation). May be diagnosed.

[0042]

As described above, according to the present invention,
In particular, it becomes possible to diagnose a so-called lean NOx catalytic action that can purify NOx in a combustion state (in an excess air atmosphere) at a lean air-fuel ratio, and a state in which the lean NOx catalytic action is deteriorated due to catalyst deterioration. There is an effect that it is possible to prevent a large amount of NOx from being discharged by continuing the lean combustion as it is.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention.

FIG. 2 is a schematic diagram showing a system configuration of an embodiment.

FIG. 3 is a diagram showing characteristics of a NOx sensor.

FIG. 4 is a block diagram showing the configuration of the first embodiment.

FIG. 5 is a flowchart showing diagnostic control of the first embodiment.

FIG. 6 is a diagram showing a map storing a reference value of a NOx concentration ratio.

FIG. 7 is a block diagram showing the configuration of a second embodiment.

FIG. 8 is a flowchart showing diagnostic control and air-fuel ratio control of the second embodiment.

[Explanation of symbols]

 1 Internal Combustion Engine 5 Fuel Injection Valve 9a, 9b Catalytic Converter (Exhaust Gas Purification Catalyst) 10 Control Unit 11 Air Flow Meter 12 Crank Angle Sensor 16a, 16b, 18a, 18b NOx Sensor

Claims (2)

[Claims] 1. A catalyst deterioration diagnosing device for an internal combustion engine in which an exhaust purification catalyst having a function of converting at least NOx in exhaust gas is interposed in an exhaust passage, and is related to a NOx conversion rate in the exhaust purification catalyst. A NOx sensor for detecting the NOx concentration in exhaust gas, an operating condition detecting means for detecting engine operating conditions, and an operating condition detecting means for determining the detection result by the NOx sensor based on the engine operating conditions detected by the operating condition detecting means. Reference value setting means for setting a reference value and diagnostic means for diagnosing a decrease in NOx conversion performance of the exhaust purification catalyst by discriminating the detection result by the NOx sensor based on the reference value set by the reference value setting means. And a catalyst deterioration diagnosing device for an internal combustion engine, comprising: 2. The exhaust purification catalyst is N 2 in an oxygen excess atmosphere.
A catalyst having a lean NOx catalytic action for converting Ox and a three-way catalytic action, and the internal combustion engine is operated at a lean air-fuel ratio as a target air-fuel ratio in a predetermined operating region. When diagnosing a decrease in NOx conversion performance while the engine is operating at the lean air-fuel ratio, the target air-fuel ratio in a predetermined operating region where the lean air-fuel ratio is the target air-fuel ratio is forcibly set to the theoretical air-fuel ratio. 2. A catalyst deterioration diagnosing device for an internal combustion engine according to claim 1, further comprising an air-fuel ratio switching control means for switching.