JPH07189780A - Catalyst deterioration diagnosis device for internal combustion engine - Google Patents

Abstract

PURPOSE:To secure diagnosis opportunity of catalyst deterioration by comparing output fluctuation frequencies of first and second sensors detected by an output fluctuation frequency detection means, and judging deterioration of exhaust purifying catalyst, even when control points are different at upstream and downstream sides of the catalyst. CONSTITUTION:An output fluctuation range detection means 12 detects an output fluctuation range of a second oxygen sensor 17 on a downstream side. The air-fuel ratio control property correction means 12 corrects air-fuel ratio feedback control by the air-fuel ratio feedback control means 12 for making the output fluctuation range of the second oxygen sensor 17 on the downstream side approach a specified output range including an output level corresponding to a target air-fuel ratio. A fluctuation frequency is detected by the output fluctuation frequency detection means 12, which frequency has a boundary composed of target air-fuel ratio corresponding values of outputs of upstream and downstream oxygen sensors 16, 17. The output fluctuation frequencies of the first and second oxygen sensors 16, 17 are compared to each other, and deterioration of an exhaust emission catalyst 10 is judged by the catalyst deterioration diagnosis means 12.

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 specifically, it detects oxygen concentrations on the upstream side and the downstream side of an exhaust purification catalyst, and based on these detected values, air-fuel ratio feedback control. The present invention relates to an apparatus for diagnosing catalyst deterioration by comparing output fluctuation frequencies of an oxygen sensor on the upstream side of a catalyst and an oxygen sensor on the downstream side of the catalyst in an internal combustion engine configured to execute.

[0002]

2. Description of the Related Art Conventionally, oxygen sensors are provided on the upstream side and the downstream side of a three-way catalyst provided in an exhaust system for exhaust gas purification, and the air-fuel ratio is fed back using the detection values of these two oxygen sensors. Various controls have been proposed (see Japanese Patent Application Laid-Open No. 4-72438, etc.).

Further, when the air-fuel ratio feedback control is performed by using the oxygen sensors respectively provided on the upstream side and the downstream side of the catalyst as described above, that is, the rich state and the lean state with respect to the target air-fuel ratio are set. In the repeated state, due to the oxygen storage effect of the catalyst, the output fluctuation frequency of the oxygen sensor on the downstream side tends to become smaller than the output fluctuation frequency of the oxygen sensor on the upstream side. When it decreases, the output fluctuation frequency of the oxygen sensor on the downstream side comes closer to the value on the upstream side. Therefore, the deterioration of the catalyst is diagnosed by comparing the output fluctuation frequency of the oxygen sensor between upstream and downstream during the air-fuel ratio feedback control.

[0004]

By the way, the oxygen sensors provided on the upstream side and the downstream side of the catalyst are deteriorated with different characteristics, and particularly, the output characteristics of the second oxygen sensor on the downstream side are lean or When the deterioration deviates to the rich side, the output of the oxygen sensor on the downstream side may stick to the lean output side or the rich output side during the air-fuel ratio feedback control due to the correction limit based on the output of the second oxygen sensor. There is.

In such a case, the above-mentioned catalyst deterioration diagnosis is carried out based on whether or not the output fluctuation frequency of the upstream oxygen sensor is close to the output fluctuation frequency of the downstream oxygen sensor. because there is,
If sticking of the downstream oxygen sensor occurs and the frequency cannot be measured, there is a problem that even if catalyst deterioration occurs, such deterioration cannot be diagnosed.

The present invention has been made in view of the above problems, and in an engine in which oxygen sensors are provided on the upstream side and the downstream side of the catalyst, and the air-fuel ratio feedback control is performed based on the outputs of these oxygen sensors, An object of the present invention is to ensure a chance of diagnosing catalyst deterioration even if the control points of air-fuel ratio feedback control differ between upstream and downstream of the catalyst due to deterioration of the oxygen sensor.

[0007]

Therefore, a catalyst deterioration diagnosing device for an internal combustion engine according to the present invention is constructed as shown in FIG. In FIG. 1, the first and second oxygen sensors are
The sensors are provided on the upstream side and the downstream side of the exhaust purification catalyst provided in the exhaust passage of the engine, and the output value changes in response to the oxygen concentration in the exhaust.

The air-fuel ratio feedback control means feedback-controls the amount of fuel supplied to the engine in a direction to bring the air-fuel ratio of the engine intake air-fuel mixture closer to the target air-fuel ratio based on the output values of the first and second oxygen sensors. To do. On the other hand, the output fluctuation range detection means detects the output fluctuation range of the second oxygen sensor on the downstream side, and the air-fuel ratio control characteristic correction means detects the second fluctuation on the downstream side detected by the output fluctuation range detection means.
The air-fuel ratio feedback control by the air-fuel ratio feedback control means is corrected so that the output variation range of the oxygen sensor approaches a predetermined output range including the output level corresponding to the target air-fuel ratio.

Here, the output fluctuating frequency detecting means detects the number of fluctuating frequencies of the outputs of the first oxygen sensor on the upstream side and the second oxygen sensor on the downstream side with the target air-fuel ratio equivalent value as a boundary. . Then, the catalyst deterioration diagnosing means compares the output fluctuation frequencies of the first and second oxygen sensors detected by the output fluctuation frequency detecting means to diagnose the deterioration of the exhaust purification catalyst.

[0010]

According to this structure, the air-fuel ratio feedback control is performed by the oxygen sensors provided on the upstream side and the downstream side of the catalyst, respectively, and the fluctuation frequency of the oxygen sensor output at this time is compared between the upstream and downstream sides. , Deterioration of the catalyst is diagnosed. On the other hand, in performing the catalyst deterioration diagnosis, the output fluctuation range of the second oxygen sensor on the downstream side is detected, and the air-fuel ratio feedback control is corrected so that the output fluctuation range approaches the predetermined output range.

That is, when the second oxygen sensor on the downstream side shows a desired fluctuation due to the air-fuel ratio feedback control, the air-fuel ratio feedback control is not corrected,
Although the catalyst deterioration diagnosis is performed based on the comparison of the output fluctuation frequency as it is, if the output of the second oxygen sensor on the downstream side does not show the fluctuation in the desired range, can the diagnosis based on the output fluctuation frequency be performed? Or, the diagnostic accuracy will deteriorate.

Therefore, by correcting the air-fuel ratio feedback control, the output fluctuation of the second oxygen sensor is forcibly caused, and the condition for catalyst deterioration diagnosis based on the comparison of the output fluctuation frequencies is created.

[0013]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 showing an embodiment, the internal combustion engine 1 includes an air cleaner 2
Air is sucked through the intake duct 3, the throttle valve 4 and the intake manifold 5. At the branch portion of the intake manifold 5, a fuel injection valve 6 is provided for each cylinder. The fuel injection valve 6 is an electromagnetic fuel injection valve that is energized by a solenoid to open the valve, is deenergized and is closed, and is energized by an injection pulse signal from a control unit 12 described later to open the valve. Fuel that is pressure-fed from a fuel pump (not shown) and adjusted to a predetermined pressure by a pressure regulator is injected and supplied into the intake manifold 5.

Spark plugs 7 are provided in the combustion chambers of the engine 1 to ignite sparks to ignite and burn the air-fuel mixture. Exhaust gas is discharged from the engine 1 through the exhaust manifold 8, the exhaust duct 9, the exhaust purification three-way catalyst 10 (exhaust purification catalyst), and the muffler 11. The three-way catalyst 10 has an oxygen storage effect, is a catalyst that oxidizes CO and HC in exhaust components and reduces NOx to convert them into other harmless substances. Both conversion efficiency becomes the best when the air-fuel mixture is burned at the stoichiometric air-fuel ratio.

The control unit 12 includes a CPU, RO
A microcomputer including an M, a RAM, an A / D converter, and an input / output interface is provided, detection signals from various sensors are input, and arithmetic processing is performed as described later to make the fuel injection valve 6 operate. Control operation. As the various sensors, a hot wire type or flap type air flow meter 13 is provided in the intake duct 3 and outputs a voltage signal according to the intake air amount Q of the engine 1.

A crank angle sensor 14 is provided to output a reference angle signal REF for each predetermined piston position and a unit angle signal POS for each unit angle. Here, the engine rotation speed Ne can be calculated by measuring the generation cycle of the reference angle signal REF or the number of generations of the unit angle signal POS within a predetermined time. Further, a water temperature sensor 15 for detecting the cooling water temperature Tw of the water jacket of the engine 1 is provided.

Further, a first oxygen sensor 16 is provided at a collecting portion of the exhaust manifold 8 which is an upstream side of the three-way catalyst 10, and a downstream side of the three-way catalyst 10 is upstream of a muffler 11. Is provided with a second oxygen sensor 17. The first oxygen sensor 16 and the second oxygen sensor 17 are known sensors (for example, a zirconia tube type oxygen sensor) whose output value changes in response to the oxygen concentration in the exhaust gas. This is a rich lean sensor that can detect the rich lean of the exhaust air-fuel ratio with respect to the stoichiometric air-fuel ratio by utilizing the sudden change in the oxygen concentration of.

In the present embodiment, the first and second oxygen sensors 16 and 17 output a high voltage (rich output) near 1 V when the air-fuel ratio is richer than the stoichiometric air-fuel ratio, When is leaner than the stoichiometric air-fuel ratio, a low voltage near 0 V (lean output) is output. Here, the CPU of the microcomputer incorporated in the control unit 12 calculates the basic fuel injection amount Tp based on the intake air flow rate Q and the engine rotation speed Ne detected by each sensor, while the cooling water temperature Tw is calculated. Based on the above, various correction coefficients COEF for correcting the basic fuel injection amount Tp are calculated and set.

Further, the control unit 12 having the function as the air-fuel ratio feedback control means, when the predetermined feedback control condition is satisfied, sets the air-fuel ratio feedback correction coefficient LMD for correcting the basic injection amount Tp, The calculation is performed as follows based on the outputs of the first oxygen sensor 16 and the second oxygen sensor 17. That is, as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-72438, the air-fuel ratio is controlled by proportional / integral control according to rich / lean with respect to the target air-fuel ratio determined based on the output of the first oxygen sensor 16 on the upstream side. Feedback correction coefficient LMD
On the other hand, the proportional operation amount (proportional amount P) in the proportional / integral control is corrected based on the rich / lean with respect to the target air-fuel ratio detected by the second oxygen sensor 17 on the downstream side.

However, the air-fuel ratio feedback control using the first oxygen sensor 16 and the second oxygen sensor 17 is not limited to the correction of the proportional operation amount described above, and the timing of performing the proportional control is determined by the second oxygen sensor. It may be configured such that it is corrected based on the detection result of 17 or the reference level used for rich / lean determination based on the output of the first oxygen sensor 16 is corrected based on the output of the second oxygen sensor 17. good.

Then, the basic fuel injection amount Tp is set to the various correction coefficients COEF and the air-fuel ratio feedback correction coefficient L.
The final fuel injection amount Ti is obtained by correcting the MD, and further by the correction amount Ts by the battery voltage, and an injection pulse signal having a pulse width corresponding to the fuel injection amount Ti is output to the fuel injection valve 6 at a predetermined timing. To do. On the other hand, the control unit 12 has a self-diagnosis function for diagnosing the deterioration of the three-way catalyst 10, as shown in the flow charts of FIGS. 3 and 4.

In this embodiment, the functions of the output fluctuation range detecting means, the air-fuel ratio control characteristic correcting means, the output fluctuation frequency detecting means, and the catalyst deterioration diagnosing means are as shown in the flow charts of FIGS. 3 and 4. The control unit 12 is equipped with software. In the flowcharts of FIGS. 3 and 4, in step 1 (denoted as S1 in the drawings; the same applies hereinafter) to step 3, the vehicle speed, the engine rotation speed Ne, and the basic fuel injection amount Tp (engine load) are each subject to catalyst deterioration. It is determined whether or not the conditions for diagnosing are met.

As a condition for diagnosing the catalyst deterioration, it is preferable that the exhaust temperature is relatively high and the three-way catalyst 10 is surely activated. Further, the fact that the air-fuel ratio feedback control using the first and second oxygen sensors 16 and 17 is being performed is a prerequisite for the catalyst deterioration diagnosis. Vehicle speed and engine speed N in steps 1 to 3
If it is determined that all three conditions of e and the basic fuel injection amount Tp meet the predetermined diagnostic conditions, the process proceeds to step 4.

In step 4, the second oxygen sensor on the downstream side is
Monitor the output voltage of 17. Then, in the next step 5, based on the monitoring result in step 4, the second
Rich output RichEs and lean output LeanEs of oxygen sensor 17
Is calculated, for example, as the average of the electromotive force peak values during 5 cycles. In step 6, the deviation V _PP between the rich output RichEs and the lean output LeanEs obtained in step 5, that is, the output fluctuation range of the second oxygen sensor 17 is calculated.

In the next step 7, it is judged whether or not the deviation V _PP is equal to or more than a predetermined value. If the deviation V _PP is greater than or equal to the predetermined value, it is determined that the output of the second oxygen sensor 17 is fluctuating within the desired fluctuation range including the target air-fuel ratio equivalent level, and the routine proceeds to step 8. The deterioration of the three-way catalyst 10 is diagnosed based on the comparison between the output fluctuation frequency of the first oxygen sensor 16 and the output fluctuation frequency of the second oxygen sensor 17.

Incidentally, the deterioration diagnosis based on the comparison of the output fluctuation frequencies is specifically carried out by measuring the cycle at which the oxygen sensor output crosses the theoretical air-fuel ratio equivalent value and comparing the cycles, or This is performed by comparing the number of times the oxygen sensor output crosses the fuel ratio in a predetermined time frame. That is, when the deviation V _PP is greater than or equal to a predetermined value, the exhaust air-fuel ratio on the downstream side of the three-way catalyst 10 repeatedly becomes rich and lean with respect to the stoichiometric air-fuel ratio, and the air that the second oxygen sensor 17 applies It is judged that the output shows an output substantially corresponding to the change in the fuel ratio.

At this time, if the three-way catalyst 10 has not deteriorated, the three-way catalyst 10 is changed due to its oxygen storage capacity.
The exhaust air-fuel ratio fluctuation frequency on the downstream side tends to be smaller than the fluctuation frequency of the exhaust air-fuel ratio (oxygen concentration) on the upstream side, and when the oxygen storage capacity decreases due to deterioration of the catalyst, the exhaust gas on the downstream side of the catalyst Since the fluctuation frequency of the air-fuel ratio comes close to the fluctuation frequency of the upstream side,
The second oxygen sensor for the output fluctuation frequency of the oxygen sensor 16
When the output fluctuation frequency of 17 shows a value closer to that in the normal state, the deterioration of the three-way catalyst 10 is determined.

On the other hand, if it is determined in step 7 that the deviation V _PP is not greater than the predetermined value, the second oxygen sensor 17
Output does not show the normal output fluctuation expected during air-fuel ratio feedback control. In this case, there is a risk of erroneous diagnosis if the normal catalyst deterioration diagnosis in step 8 is performed as it is, so the process proceeds to step 9. In step 9, the rich output RichEs of the second oxygen sensor 17 is
It is within a predetermined range (for example, 200 to 800 mV) including the stoichiometric air-fuel ratio equivalent value (for example, 500 mV), a value exceeding the predetermined range (800 mV or more), or a value below the predetermined range ( It is 200 mV or less).

Here, the rich output Ri of the second oxygen sensor 17
When it is determined that chEs is 800 mV or more, it is already detected in step 7 that the output fluctuation range is narrow, and therefore the output of the second oxygen sensor 17 shifts to the rich side as shown in FIG. 5, for example. It can be regarded as being in a sticky state. Then, in the situation as shown in FIG. 5, even if the output fluctuation frequencies of the first oxygen sensor 16 and the second oxygen sensor 17 are compared, it is not possible to diagnose a decrease in the oxygen storage capacity of the catalyst.

Therefore, when it is determined in step 9 that the rich output RichEs of the second oxygen sensor 17 is 800 mV or more, the first oxygen sensor 17 is adjusted so that the output of the second oxygen sensor 17 also changes to the lean side. In the proportional / integral control of the air-fuel ratio feedback correction coefficient LMD using 16 outputs,
When the correction coefficient LMD is corrected in the decreasing direction (air-fuel ratio leaning direction), the proportional amount P used for correction is increased and the control point of the air-fuel ratio feedback control using the first oxygen sensor 16 is forcibly shifted to the lean side. Perform processing.

That is, in a situation in which rich lean is repeated around the target air-fuel ratio by the air-fuel ratio feedback control using the first oxygen sensor 16 on the upstream side, the second oxygen sensor 17 on the downstream side extends to the rich side. If it is attached, it is estimated that a difference in output characteristics (difference in control points) between the first oxygen sensor 16 and the second oxygen sensor 17 as shown in FIG. 6, for example, occurs. In such a case, the control point of the air-fuel ratio feedback control is forcibly corrected to the lean side so that the output of the second oxygen sensor 17 repeats rich / lean as shown in FIG. Can be expected to become.

When the process for correcting the control point of the air-fuel ratio feedback control to the lean side is performed in step 10, in the next step 11, the output fluctuation range V _PP of the second oxygen sensor 17 after the lean correction is obtained. Then, in the next step 12, by comparing the output fluctuation range V _PP obtained in step 10 with a predetermined value, the output of the second oxygen sensor 17 is diagnosed for deterioration by the lean correction in step 10. It is determined whether or not a possible output fluctuation is exhibited.

When the output of the second oxygen sensor 17 fluctuates within a predetermined range including the stoichiometric air-fuel ratio equivalent value by the lean correction in step 10, the routine proceeds to step 8, where the first oxygen sensor 16 and the second oxygen sensor 16 Deterioration diagnosis of the three-way catalyst 10 is executed based on comparison of the output fluctuation frequency with the sensor 17. On the other hand, even though the control point of the air-fuel ratio feedback control is lean-corrected in step 10, the second oxygen sensor
When the output of 17 is still stuck to the rich side and the output fluctuation width V _PP is less than the predetermined value, the second oxygen sensor is actually used even though the oxygen concentration in the exhaust gas is increasing. Since 17 does not show any reaction, the process proceeds to step 13 to determine whether the second oxygen sensor 17 is defective.

When it is determined in step 9 that the rich output RichEs of the second oxygen sensor 17 is near the stoichiometric air-fuel ratio equivalent value, the routine proceeds to step 14. In step 14,
The proportional amount P in the leaning direction (decrease correction direction of the correction coefficient LMD) and the proportional amount P in the rich direction (increasing correction direction of the correction coefficient LMD) used in the proportional / integral control of the correction coefficient LMD are both increased and corrected. As a result, the amplitude of the air-fuel ratio fluctuation associated with the air-fuel ratio feedback control is expanded, and the output fluctuation range of the second oxygen sensor 17 is expanded.

That is, the rich output Rich of the second oxygen sensor 17
When Es is near the stoichiometric air-fuel ratio, it becomes difficult to detect the output fluctuation frequency with the stoichiometric air-fuel ratio equivalent value as a boundary with high accuracy, so that the output fluctuation can be grasped more clearly. The amplitude is increased, thereby expanding the output fluctuation range of the second oxygen sensor 17. In step 14, the proportional amount is corrected to increase the amplitude of the air-fuel ratio, and in the next step 15, the second oxygen sensor 17 after the correction is corrected.
The output fluctuation range V _{PP of} is calculated.

Then, in the next step 16, it is determined whether or not the rich RichEs becomes high as a result of the correction, and thus the output fluctuation width V _PP becomes equal to or larger than a predetermined value. Here, if the output fluctuation width V _PP of the second oxygen sensor 17 does not reach the predetermined value or more even if the proportional correction is performed in step 14, the second oxygen sensor 17 is defective and the proportional amount is proportional. It is considered that the expansion of the air-fuel ratio amplitude due to the correction did not affect the sensor output, and the procedure proceeds to step 13, where the second
A defect of the oxygen sensor 17 is determined.

On the other hand, when it is determined in step 16 that the output fluctuation width V _PP is equal to or greater than the predetermined value, the process proceeds to step 8 and the output fluctuation frequency of the first and second oxygen sensors 16 and 17 is set. Deterioration diagnosis of the catalyst 10 is executed based on this. Also,
When it is determined in step 9 that the rich output RichEs of the second oxygen sensor 17 is stuck to the lean side, the routine proceeds to step 17, where the air-fuel ratio feedback control of the air-fuel ratio feedback control is performed in order to change the output of the second oxygen sensor 17 to the rich side. The proportional amount in the enrichment direction (increasing correction direction of the correction coefficient LMD) is increased and corrected.

Then, in step 18, the output fluctuation width V _PP of the second oxygen sensor 17 after the correction of the proportional portion is obtained, and in the next step 19, the output fluctuation width V _PP is equal to or more than a predetermined value. Determine whether. If it is determined in step 19 that the output fluctuation range V _PP is not equal to or greater than the predetermined value, it means that the change in the air-fuel ratio due to the proportional correction has not been detected by the second oxygen sensor 17, and thus the same as in step 13 above.
Then, the process goes to step 2 to judge whether the second oxygen sensor 17 is defective.

On the other hand, when the output fluctuation width V _PP of the second oxygen sensor 17 becomes equal to or greater than the predetermined value due to the proportional correction in step 17, the process proceeds to step 8 and the three-way catalyst based on the comparison of the output fluctuation frequency is used. Execute 10 deterioration diagnosis.
As described above, according to the present embodiment, when the air-fuel ratio feedback control is performed using the first oxygen sensor 16 provided on the upstream side of the three-way catalyst 10 and the second oxygen sensor 17 provided on the downstream side. If the downstream second oxygen sensor 17 does not show the expected output fluctuation, the second oxygen sensor 17 is forcibly forced.
The proportional portion of the air-fuel ratio feedback control is corrected so that the output fluctuation range of the
After making the output of the oxygen sensor 17 show the desired fluctuation centered on the theoretical air-fuel ratio equivalent value, the catalyst deterioration diagnosis is performed.

Therefore, due to the difference in deterioration characteristics between the first oxygen sensor 16 and the second oxygen sensor 17, the output characteristics of the respective sensors become different, and the output of the second oxygen sensor 17 extends to the rich side or the lean side. Even if it becomes attached or changes only in the vicinity of the stoichiometric air-fuel ratio equivalent value, the desired output fluctuation of the second oxygen sensor 17 is forcibly generated and the catalyst 10
The deterioration of can be diagnosed.

In the above embodiment, the output fluctuation range of the second oxygen sensor 17 is corrected by correcting the proportional portion of the control operation amount in the proportional / integral control of the air-fuel ratio feedback correction coefficient LMD. However, instead of the proportional component, or the integral component may be corrected together with the proportional component. Further, instead of correcting the control operation amount in the air-fuel ratio feedback control, the characteristics of the air-fuel ratio feedback control are corrected by correcting the execution timing of the proportional control and the reference level used for rich / lean determination based on the oxygen sensor output. It may be configured to correct.

Further, the air-fuel ratio feedback correction coefficient LM
The D control is not limited to the proportional / integral control.

[0043]

As described above, according to the present invention, in the engine in which the oxygen sensors are provided on the upstream side and the downstream side of the exhaust purification catalyst, and the air-fuel ratio feedback control is performed based on the outputs of these oxygen sensors, When diagnosing the deterioration of the catalyst by comparing the output fluctuation frequencies of the upstream and downstream oxygen sensors, the air-fuel ratio feedback control is corrected to bring the output fluctuation of the downstream side into a desired state where deterioration diagnosis can be performed. Therefore, even if the output of the downstream side oxygen sensor becomes stuck to the rich or lean due to the deterioration of the oxygen sensor, etc., it becomes possible to perform the catalyst deterioration diagnosis, and secure the opportunity of the catalyst deterioration diagnosis to perform the diagnosis. There is an effect that reliability can be improved.

[Brief description of drawings]

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

FIG. 2 is a system schematic diagram showing an embodiment of the present invention.

FIG. 3 is a flowchart showing catalyst deterioration diagnosis control of the embodiment.

FIG. 4 is a flowchart showing catalyst deterioration diagnosis control of the embodiment.

FIG. 5 is a time chart showing control characteristics in the example.

FIG. 6 is a diagram showing an example of a deteriorated state of an oxygen sensor.

[Explanation of symbols]

 1 engine 6 fuel injection valve 10 three-way catalyst (exhaust gas purification catalyst) 12 control unit 13 air flow meter 14 crank angle sensor 16 first oxygen sensor 17 second oxygen sensor

Claims (1)

[Claims] 1. An exhaust purification catalyst provided in an exhaust passage of an engine, and an exhaust purification catalyst which is provided upstream and downstream of the exhaust purification catalyst and whose output value changes in response to oxygen concentration in exhaust gas.
And a second oxygen sensor, and an air feed-back control for controlling the amount of fuel supplied to the engine in a direction to bring the air-fuel ratio of the engine intake air-fuel mixture closer to the target air-fuel ratio based on the output values of the first and second oxygen sensors. In an internal combustion engine comprising: a fuel ratio feedback control means, an output fluctuation range detecting means for detecting an output fluctuation range of the second oxygen sensor on the downstream side, and a downstream side of the downstream side detected by the output fluctuation range detecting means. Second
An output fluctuation range of the oxygen sensor, in a direction approaching a predetermined output range including an output level corresponding to the target air-fuel ratio, an air-fuel ratio control characteristic correction means for correcting the air-fuel ratio feedback control by the air-fuel ratio feedback control means, Output fluctuation frequency detecting means for detecting a fluctuation frequency of the outputs of the first oxygen sensor on the upstream side and the second oxygen sensor on the downstream side with the target air-fuel ratio equivalent value as a boundary; and the output fluctuation frequency detecting means. A catalyst deterioration of an internal combustion engine, comprising: catalyst deterioration diagnosis means for diagnosing deterioration of the exhaust gas purification catalyst by comparing detected output fluctuation frequencies of the first and second oxygen sensors. Diagnostic device.