JPH08218853A - Deterioration diagnosing device for exhaust gas purifying catalyst - Google Patents

Abstract

PURPOSE: To perform high-precise determination of a three-dimensional catalyst even when an output signal from a downstream side O2 sensor is shifted to the rich side or the lean side. CONSTITUTION: After an ECU 40 calculates an average value VORave based on the output voltage VOR of a downstream side O2 sensor 15, a hysteresis constant α is added thereto and subtracted therefrom to calculate upper and lower inversion reference values THH and THL. Thereafter, after a downstream inversion frequency fR is determined from the number of times which an output voltage VOR crosses the inversion reference values THH and THL, the ECU 40 calculates an inversion frequency ratio fR/fF from the output voltage VOF of an upper stream side O2 sensor 14 determined by the number of times which the output voltage VOF of an upper stream side O2 sensor 14 crosses a threshold value VTH. Thereafter, the ECU 40 diagnoses deterioration of a three- dimensional catalyst 13 when a value of the inversion frequency ratio fR/fF exceeds a given value E1, diagnoses that a purification efficiency ECAT is decreased to a value lower than a lower limit value E1 and lights ON an alarm lamp 41, and stores a trouble cord at an RAM.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a device for diagnosing deterioration of an exhaust purification catalyst, and more particularly to a device provided with air-fuel ratio sensors on the upstream side and the downstream side of the exhaust purification catalyst.

[0002]

2. Description of the Related Art In recent years, an exhaust system of an automobile gasoline engine is provided with an oxidation-reduction type exhaust purification catalyst (hereinafter, three-way catalyst) in order to reduce harmful exhaust gas components. A three-way catalyst is a device that purifies exhaust gas by oxidizing hydrocarbons (HC) and carbon monoxide (CO) while reducing nitrogen oxides (NO _x ). Since the redox reaction of the three-way catalyst is performed only in a narrow region (window) near the theoretical air-fuel ratio, an air-fuel ratio sensor (O ₂ sensor) is installed in the exhaust manifold and the air-fuel ratio is fed back based on the output signal. Have control.
On the other hand, the three-way catalyst deteriorates as the usage period increases and the purification efficiency decreases, but this deterioration can be confirmed only by a method using an exhaust gas tester at the time of periodic inspections, etc. However, there was a risk that it would continue to be used.

Therefore, in Japanese Patent Laid-Open No. 61-286550, O ₂ sensors are provided on the upstream side and the downstream side of the three-way catalyst, and the three-way catalyst is deteriorated based on the output signals of these O ₂ sensors. There is disclosed a deterioration diagnosis device for determining. In this deterioration diagnosing device, attention is paid to the fact that the air-fuel ratio fluctuates in a short cycle with the target air-fuel ratio (for example, the theoretical air-fuel ratio) as a boundary by the feedback control, and inversion of the upstream O ₂ sensor correspondingly changes. By comparing the frequency with that on the downstream side, the deterioration of the three-way catalyst is determined.

That is, since the three-way catalyst has the ability to store the residual oxygen in the exhaust gas, the exhaust gas passing through the three-way catalyst contains only a small amount of oxygen. Therefore, when the three-way catalyst maintains normal purification efficiency, the output signal of the downstream O ₂ sensor is less likely to change,
As the amplitude becomes smaller, the inversion frequency also becomes very low.
Therefore, when calculating the inversion frequency ratio of the upstream O ₂ sensor output signal with the downstream O ₂ sensor and the air-fuel ratio is varied (inversion frequency of the reversal frequency / upstream O ₂ sensor downstream O ₂ sensor), Its value is very small.

However, when the three-way catalyst deteriorates and the purification efficiency decreases, the harmful exhaust gas components are not purified and the ability to store oxygen also decreases, and the residual oxygen in the exhaust gas remains as it is. Start passing through. As a result, the output signal of the downstream O ₂ sensor also changes in the same manner as the output signal of the upstream O ₂ sensor, and the above-mentioned inversion frequency ratio gradually approaches 1. Therefore, in the deterioration diagnosis device described above, the deterioration determination of the three-way catalyst is performed based on the relationship between the purification efficiency of the three-way catalyst and the inversion frequency ratio.

[0006]

By the way, when the deterioration of the three-way catalyst has not progressed so much, the output signal of the downstream O ₂ sensor is not only small in amplitude but also in a rich side or a lean side in a relatively large cycle. Shift to the side. Therefore, as shown in FIG. 8, when the upper and lower reversal reference values THH and THL that form hysteresis are set near the stoichiometric air-fuel ratio and the reversal frequency is detected, when the shift to the rich side or the lean side is performed, The output signal is these inversion reference values THH,
The reversal frequency is counted lower than it really is, without crossing THL. For example, in FIG. 8, the point indicated by-is a reversal detection point, and although the signal is actually reversed 16 times, the signal is output only four times. Therefore, even if the deterioration progresses to some extent, the value of the above-mentioned inversion frequency ratio does not easily increase, and the deterioration diagnosis device determines that the three-way catalyst is normal.

Further, the above-described shift to the rich side or the lean side has a problem that the value of the reversal frequency ratio greatly varies depending on the individual engine because the cycle and the amplitude are different depending on the state of the engine main body, auxiliary machinery and the like. There was also. Therefore, conventionally, it is not possible to set the reversal frequency ratio, which is a criterion for deterioration determination, to a too high value, and even if the three-way catalyst has a purification efficiency within an allowable limit, it may be determined to be deteriorated and replaced. was there. As is well known, a three-way catalyst is a very expensive part that uses a precious metal such as platinum, and early replacement is
Not only the number of maintenance man-hours increased, but also resource saving and running cost were problems.

The present invention has been made in view of the above situation,
It is an object of the present invention to provide an exhaust purification catalyst deterioration diagnosing device capable of highly accurately determining deterioration of a three-way catalyst even if the output signal of the downstream O ₂ sensor is shifted to the rich side or the lean side.

[0009]

Therefore, in claim 1 of the present invention, a downstream side air-fuel ratio sensor provided downstream of the exhaust purification catalyst in the exhaust passage of the internal combustion engine, and the output of this downstream side air-fuel ratio sensor. Average value calculating means for calculating the average value of the signal, reference value setting means for setting an inversion reference value using the average value calculated by this average value calculating means, and inversion reference set by this reference value setting means A deterioration diagnosis device for an exhaust purification catalyst, comprising: deterioration determination means for determining deterioration of the exhaust purification catalyst based on the number of times the output signal of the downstream side air-fuel ratio sensor crosses a value.

According to a second aspect of the present invention, in the deterioration diagnosing device according to the first aspect, the deterioration determining means uses the inversion reference value set by the reference value setting means as an output signal of the downstream side air-fuel ratio sensor. Is provided with first inversion frequency calculation means for calculating the inversion frequency of the output signal of the downstream side air-fuel ratio sensor, and the deterioration determination means is provided with the first inversion frequency calculation means.
It is proposed that the deterioration of the exhaust gas purification catalyst is determined based on the inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the inversion frequency calculation means.

According to a third aspect of the present invention, in the deterioration diagnosing device according to the second aspect, the upstream side air-fuel ratio sensor provided upstream of the exhaust purification catalyst in the exhaust passage and the upstream side air-fuel ratio sensor. A second inversion frequency calculating means for calculating an inversion frequency of the output signal, wherein the deterioration determining means has an inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculating means; It is proposed that the deterioration of the exhaust gas purification catalyst is determined by comparing with the inversion frequency of the output signal of the upstream side air-fuel ratio sensor calculated by the second inversion frequency calculation means.

According to a fourth aspect of the present invention, in the deterioration diagnosing device according to the first aspect, the average value calculating means sets the average value O ₂ ave of the output signals of the downstream side air-fuel ratio sensor to the previous average value. O ₂ ave (n-1), the value of the output signal of the downstream side air-fuel ratio sensor this time is O ₂ real, and the filter constant is a, the following arithmetic expression O ₂ ave = a × O ₂ ave (n- 1) We propose the one characterized by being calculated by + (1-a) × O ₂ real.

[0013]

In the deterioration diagnosing device of claim 1, when the output signal of the downstream side air-fuel ratio sensor shifts to the rich side or the lean side,
The average value and the inversion reference value also follow and shift. Therefore, the output signal crosses the inversion reference value even if it is inverted with a relatively small amplitude, and the number of inversions is accurately measured.

Further, in the deterioration diagnosing device of the second aspect, for example, when the inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculation means becomes larger than a predetermined value, the deterioration judgment is made. The means determines that the exhaust purification catalyst has deteriorated. Further, in the deterioration diagnosis device according to claim 3, for example, the inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculation means and the upstream side air-fuel ratio sensor calculated by the second inversion frequency calculation means. When the ratio of the output signal to the reversal frequency approaches 1, the exhaust purification efficiency of the exhaust purification catalyst has fallen below the limit, and therefore the deterioration determination means has deteriorated the exhaust purification catalyst. To determine.

Further, in the deterioration diagnosing device of claim 4, for example, by appropriately setting the filter constant a, the average value and the inversion reference value follow when the output signal is shifted to the rich side or the lean side. The accuracy and response when shifting are adjusted. Become.

[0016]

An embodiment of the present invention will be described below with reference to the drawings.
Reveal FIG. 1 is a view showing a deterioration diagnosis device according to the present invention.
It is a schematic structure figure showing a fuel engine. In the figure,
The fuel injection valve 3 is installed in the intake port 2 of the engine 1 for each cylinder.
Air intake via the attached intake manifold 4
NA5, Karman vortex type air flow sensor 6, throttle
Valve 7, ISC (idle speed controller) 8
An intake pipe 9 equipped with etc. is connected. Also the exhaust port
10 has an exhaust pipe 12 through an exhaust manifold 11.
The exhaust pipe 12 is connected to the three-way catalyst 13 and
A muffler (not shown) is attached. Of the exhaust pipe 12
In the pipeline, upstream and downstream of the three-way catalyst 13, respectively
Upstream O₂Sensor 14 and downstream O ₂With sensor 15
It is worn. These O₂The sensors 14 and 15 are three-dimensional touch
Reacts with oxygen in the exhaust gas before and after passing through the medium 13,
The voltage is generated according to the concentration of.

The engine 1 is provided with a crank angle sensor 20 for detecting the engine rotation speed Ne and the cooling water temperature TW.
A water temperature sensor 21 for detecting the temperature is attached, and the intake system has a throttle sensor 22 for detecting the opening θTH of the throttle valve 7 and an atmospheric pressure sensor 2 for detecting the atmospheric pressure Ta.
3, various sensors such as the intake air temperature sensor 24 for detecting the intake air temperature Ta are connected. In the figure, 30 is a spark plug disposed above the combustion chamber 31, and 32 is a spark plug 30.
It is an ignition coil that outputs a high voltage to.

On the other hand, in the passenger compartment, an input / output device (not shown) and a storage device (RO
M, RAM, nonvolatile RAM, etc., central processing unit (CP)
U), an ECU (engine control unit) 40 including a timer counter and the like are installed. ECU40
On the input side of, various sensors are connected in addition to the O ₂ sensors 14 and 15 and the detection information from these is input. On the output side, the fuel injection valve 3, ISC8,
The ignition coil 32 and the like are connected, and the optimum value calculated based on the input information from various sensors is output to them. The ECU 40 controls fuel injection, ignition timing, ISC, etc., and also controls both O ₂ sensors 14,
The catalyst deterioration determination is also executed based on the output signal of 15. In the figure, reference numeral 41 denotes a warning light installed in the vehicle compartment, which is turned on when the three-way catalyst 13 is deteriorated to call the driver's attention.

First, the fuel injection control in this embodiment will be briefly described. At the same time when the driver starts the engine 1, the fuel injection control by the ECU 40 is executed. When this control is started, the ECU 40 obtains the intake air amount information A / N per intake stroke based on the output signals of the air flow sensor 6 and the crank angle sensor 20, and the value and the target air-fuel ratio (usually, the theoretical air-fuel ratio). ) And the basic fuel injection time TBASE. Next, the ECU 40 corrects the basic fuel injection time TBASE based on the output signals of the atmospheric pressure sensor 23 and the intake air temperature sensor 24, and further warms it up based on the output signals of the water temperature sensor 21 and the throttle sensor 22. The fuel injection time TINJ is calculated by performing the machine increase correction and the acceleration increase correction. Then, the ECU 40 adds the invalid injection time TD that complements the valve opening delay of the fuel injection valve 3 to the fuel injection time TINJ obtained in this way, and then adds the fuel injection valve TD via a fuel injection valve driver (not shown). Drive 3

By the way, the activation of the upstream O ₂ sensor 14 is completed, the engine 1 is not in a high load / high speed operation state, etc.
When the predetermined operating condition is satisfied, the ECU 40 starts the air-fuel ratio feedback control. When this control starts, EC
U40 compares the output voltage VOF of the upstream O ₂ sensor 14 with a predetermined threshold value VTH (for example, 0.5 V) to perform feedback correction of the air-fuel ratio. That is, upstream side O
_{In the 2} sensor 14, the air-fuel ratio of the air-fuel mixture is the theoretical air-fuel ratio (14.
Before and after 7), the output voltage VOF is the lowest voltage (for example, 0
Since the output voltage VOF falls below the threshold value VTH (for example, 0.5V), the fuel injection time TINJ is gradually lengthened to shift to the rich side when the output voltage VOF falls below the threshold value VTH (for example, 0.5V). On the contrary, when the output voltage VOF exceeds the threshold value VTH, the fuel injection time TINJ is gradually shortened to shift to the lean side. As a result, the air-fuel ratio of the air-fuel mixture is always maintained near the stoichiometric air-fuel ratio, and the exhaust gas is purified by the three-way catalyst 13 with high efficiency.

In the air-fuel ratio feedback control in this embodiment, the learning correction is performed so that the central value of the feedback correction coefficient becomes 1.0, and the learning correction amount is stored in the nonvolatile RAM.
To store. Then, by using the learning correction amount,
The accuracy of the open loop control described above is improved, and the deviation amount at the time of rising of the feedback control is reduced.

The procedure of catalyst deterioration diagnosis in this embodiment will be described below with reference to the flowcharts of FIGS. 2 to 5 and the graphs of FIGS. When the driver turns on the ignition switch to start the engine 1, the catalyst deterioration diagnosis subroutine of FIGS. 2 and 3 is executed. When this subroutine is started, the ECU 40 first determines in step S2 of FIG. 2 whether the normal flag FOK indicating that the three-way catalyst 13 is functioning normally is 1. Normal flag F
OK is 0 every time the ignition switch is turned off.
Is set to 1, and is set to 1 when the three-way catalyst 13 is diagnosed to be normal. Therefore, the normal flag FOK is always 0 immediately after the engine 1 is started, and the ECU 4
For 0, since the determination in step S2 is No (negative),
Go to step S4.

The ECU 40 determines in step S4 whether or not the conditions for carrying out the catalyst deterioration diagnosis are satisfied. Here, it is sequentially confirmed that the air-fuel ratio feedback control is carried out, that the engine rotation speed Ne and the volumetric efficiency ηv are within a predetermined range, that both O ₂ sensors 14 and 15 are operating normally. When all the conditions are satisfied, the determination result is Yes (affirmative). The reason for confirming that the engine rotation speed Ne and the volumetric efficiency ηv are within the predetermined ranges is that the O ₂ concentration of the exhaust gas is not stable when these are not stable and the normal feedback control is performed. This is because the engine rotation speed Ne and the volumetric efficiency ηv are within the ranges of the following expressions (1) and (2). In the formula below, Ne1, Ne2, ηv1, ηv2
Indicate the determination thresholds, and specific values thereof are, for example, when the engine 1 is connected to an automatic transmission, Ne1 is 1400 rpm, Ne2 is 3000 rpm,
ηv1 is 25% and ηv2 is 60%.

Ne1 <Ne <Ne2 (1) ηv1 <ηv <ηv2 (2) When the determination result of step S4 is Yes, the ECU 40
Next, in step S6, the upstream reversal frequency fF is calculated based on the output signal from the upstream O ₂ sensor 14.
As the calculation method, for example, a predetermined time (for example, 1
The number of times that the output voltage VOF of the upstream O ₂ sensor 14 crosses the threshold value VTH (for example, 0.5 V) within 0 second) is obtained, and this number is defined as the upstream inversion frequency fF.

Next, the ECU 40 determines in step S8 whether the upstream reversal frequency fF is smaller than a predetermined value THp. This determination is to stop the catalyst deterioration diagnosis when the upstream O ₂ sensor 14 deteriorates and does not function normally. That is, since the upstream O ₂ sensor 14 is constantly exposed to high-temperature exhaust gas, the downstream O ₂ sensor 1
5 is more likely to be thermally deteriorated, the responsiveness is lowered, and a normal electromotive force is not output in many cases. Therefore, the output voltage from the upstream O ₂ sensor 14 may not be changed clearly, and in this case, the upstream reversal frequency fF becomes lower than that in the normal state. Thus, the upstream reversal frequency f
When F decreases, the reversal frequency ratio fR / fF with the downstream reversal frequency fR increases, and there is a risk of erroneously determining that the three-way catalyst 13 has deteriorated. The predetermined value THp used here is a determination threshold value THc for the inversion frequency ratio fR / fF which will be described later.
Is a value (for example, 0.1 Hz) set in advance based on experimental data.

If the result of the determination in step S8 is also No, the ECU 40 proceeds to step S10, where the downstream O ₂
Based on the output voltage VOR of the sensor 15, its average value VOR
ave is calculated by the following formula. In the following equation, VORave (n-1) is the previous average value, VOR is the current output voltage of the downstream O ₂ sensor 15, and a is the filter constant. VORave = a × VORave (n-1) + (1-a) × VOR By this calculation, the output voltage VOR of the downstream O ₂ sensor 15
Average value VORave even if shifts to the rich side or lean side
Will also follow and shift. The filter constant a
By appropriately setting, the accuracy and response when the average value VORave shifts following the output voltage VOR are determined.

When the calculation of the average value VORave is completed in step S10, the ECU 40 calculates the upper and lower reversal reference values THH and THL by the following equation in step S12. here,
α is a hysteresis constant, which is set to about 0.05 V in this embodiment. THH = VORave + α THL = VORave−α Next, in step S14, the ECU 40 determines the number of times the output voltage VOR of the downstream O ₂ sensor 15 crosses the upper and lower inversion reference values THH and THL within the above-described predetermined time. Then, this number is set as the downstream side inversion frequency fR. FIG. 6 is a graph showing the relationship between the inversion reference values THH and THL and the output voltage VOR. In this figure, the number of inversion detection points indicated by-is equal to the actual number of inversions of the output voltage VOR. There is. That is, even if the output voltage VOR of the downstream O ₂ sensor 15 shifts to the rich side or the lean side, the inversion reference value THH
, THL are similarly shifted, so that the downstream side reversal frequency fR is accurately measured. Further, the reason why the upper and lower inversion reference values THH and THL are used to measure the downstream side inversion frequency fR is that the output voltage VOR of the single inversion reference value is smaller than the output voltage VOR because the output voltage VOR of the downstream side O ₂ sensor 15 has a small amplitude. This is because there is a risk of excessive measurement due to minute shake.

Next, the ECU 40 executes step S1 of FIG.
In step 6, the inversion frequency ratio fR / fF is calculated from the upstream inversion frequency fF and the downstream inversion frequency fR, and the calculated value is the predetermined value THc.
(For example, 0.8) is determined. If the result of this determination is No, then the three-way catalyst 13 has not deteriorated and, as shown in FIG. 7, functions normally above the lower limit value E1 (for example, about 85%) of the purification efficiency ECAT. Is diagnosed, and the normal process subroutine of FIG. 4 is executed in step S18.

In the normal processing subroutine, first, in step S32, the warning lamp 41 is turned off, and the three-way catalyst 1
Indicate to the driver that 3 is functioning normally. Next, in step S34, the ECU 40 performs an operation of erasing the failure code so that the failure code corresponding to the deterioration of the three-way catalyst 13 does not remain in the RAM. Then, in step S36, the normal flag FOK is set to 1, and the three-way catalyst 13
Remember that is working properly.

In this way, once the normal flag FOK is set to 1, the determination at step S2 becomes Yes at the next execution of the catalyst deterioration diagnosis subroutine. Therefore, in this case, the subroutine is terminated without performing the catalyst deterioration determination again until the ignition switch is turned off. On the other hand, the inversion frequency ratio f
If R / fF exceeds a predetermined value THc (0.8), step S16
If the result of the determination is Yes, it is diagnosed that the three-way catalyst 13 has deteriorated and the purification efficiency ECAT has become equal to or lower than the lower limit value E1 (about 85%), and in step S20, the deterioration time processing subroutine of FIG. To execute.

In the processing routine for deterioration, first, in step S42, the warning light 41 is turned on to notify the driver of the deterioration of the three-way catalyst 13 and prompt the repair. Then, in step S44, the ECU 40 stores the failure code corresponding to the deterioration of the three-way catalyst 13 in the RAM. As a result, when repairing, the failure content can be easily known by reading out the failure code, and the three-way catalyst 13 can be replaced promptly.

Although the description of the specific embodiment has been completed, the embodiment of the present invention is not limited to this embodiment. That is, in the above embodiment, the catalyst deterioration determination is stopped only when the upstream O ₂ sensor, which is likely to be thermally deteriorated, deteriorates, but the same measure may be taken when the downstream O ₂ sensor deteriorates. Further, in the above embodiment, the air-fuel ratio sensors on the upstream side and the downstream side are both voltage output type O ₂ sensors,
A current output type sensor such as a linear air-fuel ratio sensor may be used. Furthermore, start the specific procedure of control,
Specific values such as the threshold values can be changed without departing from the gist of the present invention.

[0033]

According to the deterioration diagnosing device of the first aspect of the present invention, the downstream side air-fuel ratio sensor provided downstream of the exhaust purification catalyst in the exhaust passage of the internal combustion engine and the output of the downstream side air-fuel ratio sensor. Average value calculating means for calculating the average value of the signal, reference value setting means for setting an inversion reference value using the average value calculated by this average value calculating means, and inversion reference set by this reference value setting means Since the output signal of the downstream side air-fuel ratio sensor is provided with the deterioration determining means for determining the deterioration of the exhaust purification catalyst based on the number of times the output signal of the downstream side air-fuel ratio sensor crosses, the output signal of the downstream side air-fuel ratio sensor is set to the rich side or the lean side. Even if it shifts, the average value and the inversion reference value also follow and shift, and the number of inversions of the output signal is accurately measured. As a result, the deterioration determination accuracy of the exhaust purification catalyst is improved, so the reference value for the deterioration determination can be set high,
Exhaust gas purification catalyst whose purification efficiency is within the allowable limit is less likely to be replaced.

According to the deterioration diagnosis device of claim 2,
The deterioration diagnosis device according to claim 1, wherein the deterioration determination means determines the output of the downstream side air-fuel ratio sensor from the number of times the output signal of the downstream side air-fuel ratio sensor crosses the inversion reference value set by the reference value setting means. A first reversal frequency calculation means for calculating a reversal frequency of the signal is provided, and the deterioration determination means is based on the reversal frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first reversal frequency calculation means. Since the deterioration is determined, the deterioration can be quantitatively determined. As a result, the accuracy of determining the deterioration of the exhaust purification catalyst is further improved, so that the reference value for the deterioration determination can be set high, and replacement of the exhaust purification catalyst whose purification efficiency is within the allowable limit is reduced.

According to the deterioration diagnosis device of claim 3,
The deterioration diagnosis device according to claim 2, wherein an upstream side air-fuel ratio sensor provided upstream of the exhaust purification catalyst in the exhaust passage and a second inversion frequency for calculating an inversion frequency of an output signal of the upstream side air-fuel ratio sensor. Further comprising a computing means,
The deterioration determining unit includes an inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculating unit and an output signal of the upstream side air-fuel ratio sensor calculated by the second inversion frequency calculating unit. Since the deterioration of the exhaust purification catalyst is determined by comparing with the inversion frequency, the purification efficiency of the exhaust purification catalyst can be directly and quantitatively determined, and the exhaust purification catalyst whose purification efficiency is within the allowable limit is replaced. Less to do.

According to the deterioration diagnosis device of claim 4,
The deterioration diagnosis device according to claim 1, wherein the average value calculating means is an average value O ₂ ave of an output signal of the downstream side air-fuel ratio sensor.
And the last average value _{O 2 ave (n-1)} , when the value of the O ₂ real of this output signal of the downstream air-fuel ratio sensor, the filter constant was a, the following calculation formula O ₂ ave = a Since it is calculated by × O ₂ ave (n-1) + (1-a) × O ₂ real, the average value and the inversion reference value are shifted according to the output signal by appropriately setting the filter constant a. It is possible to adjust the accuracy and response when performing, and it is possible to perform deterioration determination according to the layout of the exhaust purification catalyst, the operating condition, and the like.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an internal combustion engine equipped with a deterioration diagnosis device according to the present invention.

FIG. 2 is a flowchart showing a procedure of a catalyst deterioration diagnosis subroutine.

FIG. 3 is a flowchart showing a procedure of a catalyst deterioration diagnosis subroutine.

FIG. 4 is a flowchart showing a procedure of a normal processing subroutine.

FIG. 5 is a flowchart showing a procedure of a deterioration time processing subroutine.

FIG. 6 is a graph showing the relationship between the inversion reference values THH and THL and the output voltage VOR in the embodiment.

FIG. 7 is a graph showing the relationship between the inversion frequency ratio fR / fF and the purification efficiency ECAT.

FIG. 8 is a graph showing the relationship between the inversion reference values THH and THL and the output voltage VOR in the conventional device.

[Explanation of symbols]

1 Engine 12 Exhaust Pipe 13 Three-Way Catalyst 14 Upstream O ₂ Sensor 15 Downstream O ₂ Sensor 40 ECU 41 Warning Light

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Toshiro Nomura 5-3-8 Shiba, Minato-ku, Tokyo Mitsubishi Motors Corporation (72) Inventor Hidetsugu Kaneo 5-33-8 Shiba, Minato-ku, Tokyo Within Mitsubishi Motors Corporation

Claims (4)

[Claims] 1. A downstream side air-fuel ratio sensor provided downstream of an exhaust purification catalyst in an exhaust passage of an internal combustion engine, and an average value calculation means for calculating an average value of output signals of the downstream side air-fuel ratio sensor, Reference value setting means for setting an inversion reference value using the average value calculated by the average value calculation means, and the number of times the output signal of the downstream side air-fuel ratio sensor crosses the inversion reference value set by this reference value setting means. And a deterioration determining unit that determines deterioration of the exhaust purification catalyst based on the above. 2. The deterioration determining means determines the inversion frequency of the output signal of the downstream side air-fuel ratio sensor from the number of times the output signal of the downstream side air-fuel ratio sensor crosses the inversion reference value set by the reference value setting means. The deterioration determining means determines the deterioration of the exhaust purification catalyst based on the inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculating means. The deterioration diagnosis device for an exhaust purification catalyst according to claim 1, characterized in that. 3. An upstream air-fuel ratio sensor provided upstream of the exhaust purification catalyst in the exhaust passage, and second inversion frequency calculation means for calculating an inversion frequency of an output signal of the upstream air-fuel ratio sensor. The deterioration determining means includes an inversion frequency of the output signal of the downstream side air-fuel ratio sensor calculated by the first inversion frequency calculating means and an output of the upstream side air-fuel ratio sensor calculated by the second inversion frequency calculating means. The exhaust gas purification catalyst deterioration diagnosis device according to claim 2, wherein deterioration of the exhaust gas purification catalyst is determined by comparing with a signal inversion frequency. 4. The average value calculating means calculates the average value O ₂ ave of the output signals of the downstream side air-fuel ratio sensor, the previous average value O ₂ ave (n-1), and the current value of the downstream side air-fuel ratio sensor of this time. When the value of the output signal is O ₂ real and the filter constant is a, the calculation is performed by the following arithmetic expression O ₂ ave = a × O ₂ ave (n-1) + (1-a) × O ₂ real. The deterioration diagnosis device for an exhaust purification catalyst according to claim 1, which is characterized in that.