JPH07189781A - Catalyst deterioration judgment device for internal combustion engine - Google Patents

Abstract

PURPOSE:To properly judge deterioration of a catalyst irrespective of scattering of properties of oxygen sensors. CONSTITUTION:A catalyst 12 is arranged in an exhaust passage. A first oxygen sensor 14 is arranged on the upstream side of the catalyst 12. A means 1 which feedback controls an air-fuel ratio to be a theoretical one based on an output of the oxygen sensor 14. A second oxygen sensor 15 is arranged on the downstream side of the catalyst 12. A means 2 judges that an operation condition is a catalyst diagnosis condition. Under the catalyst diagnosis operation condition, feedback control is switched from one by means of the first oxygen sensor 14 to one by means of the second oxygen sensor 15. A means 3 counts inversion frequencies of the oxygen sensors 14, 15 in each feedback control mode. A means 4 judges a deteriorated condition of the catalyst 12 by comparing the inversion frequencies.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for determining a deterioration state of a catalyst for purifying exhaust gas of an internal combustion engine.

[0002]

As a system for cleaning the exhaust gas of an internal combustion engine, the air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio on the basis of the output of an oxygen sensor, and HC and C are provided in the exhaust passage.
It is widely practiced to install a three-way catalyst that simultaneously oxidizes O and reduces NO.

As the three-way catalyst deteriorates over time and its performance deteriorates, the conversion efficiency gradually decreases, which causes an obstacle to exhaust gas purification. It is preferable to take measures such as exchanging a catalyst whose performance has deteriorated. Therefore, in order to judge such a deteriorated state of the catalyst, an apparatus such as that disclosed in JP-A-4-18748 has been proposed. ing.

This is to determine that the catalyst has deteriorated when oxygen sensors are installed upstream and downstream of the three-way catalyst and the inversion frequency of the output of the downstream oxygen sensor is higher than a predetermined value. .

When feedback control is performed so that the air-fuel ratio becomes the stoichiometric air-fuel ratio based on the output of the upstream oxygen sensor,
The actual air-fuel ratio fluctuates slightly rich and lean with respect to the theoretical air-fuel ratio, and by changing slightly to rich lean in this way, the oxidation and reduction functions of the three-way catalyst are also maintained to the maximum. It On the other hand, the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst becomes a uniform stoichiometric air-fuel ratio because oxygen is stored by the action of the catalyst.

However, when the three-way catalyst deteriorates, the conversion efficiency of the catalyst decreases, and the change in the air-fuel ratio upstream of the three-way catalyst also appears in the downstream. If the three-way catalyst does not function at all, the upstream-downstream air-fuel ratio and the downstream-side air-fuel ratio change exactly the same.

When the oxygen concentration in the exhaust gas is measured by a downstream oxygen sensor, when the three-way catalyst is functioning normally,
The output of the oxygen sensor hardly fluctuates to rich or lean, while the deterioration frequency progresses, and the inversion frequency that changes to rich or lean gradually increases.

Therefore, the inversion frequency of the upstream oxygen sensor and the inversion frequency of the output of the downstream oxygen sensor are compared,
When the ratio exceeds a predetermined value, it is determined that the deterioration of the catalyst has progressed.

[0009]

In this case, however, the output characteristics of the upstream and downstream oxygen sensors may be rich or deviated to the lean side, which may affect the determination and cause an erroneous diagnosis.

Since the comparison value (threshold value) for counting the reversal of the output of the downstream oxygen sensor is fixed to a fixed value, it can be said that the characteristics of the upstream oxygen sensor are deviated to rich or lean. The frequency of the reverse output cannot accurately reflect the change in the air-fuel ratio.

That is, as shown in FIGS.
When the oxygen sensor on the upstream side shifts to rich or lean, the air-fuel ratio controlled based on this shifts to relatively rich or lean, and the output of the oxygen sensor on the downstream moves relatively above or below the comparison value. It changes, and in any case, the number of times crossing the comparison value changes. For this reason, even if the reversal frequency is actually high, the number of times that the comparison value is crossed is reduced, and it may be erroneously diagnosed as normal even though the catalyst is deteriorated.

The present invention solves such a problem, namely,
It is an object of the present invention to correctly determine the deterioration of the catalyst regardless of the deviation of the characteristics of the oxygen sensor.

[0013]

Therefore, according to the present invention, as shown in FIG.
As shown in, the catalyst (12) interposed in the exhaust passage and the first oxygen sensor (14) arranged upstream of the catalyst (12).
And a second oxygen sensor (15) installed downstream of the catalyst (12) in an internal combustion engine equipped with means (1) for feedback-controlling the air-fuel ratio to the stoichiometric air-fuel ratio based on the output of the oxygen sensor (14). ), Means 2 for judging that the operating condition is the catalyst diagnosis condition, and switching from the feedback control by the first oxygen sensor (14) to the feedback control by the second oxygen sensor (15) under the catalyst diagnosis operation condition, Oxygen sensor (14) during each control,
A means 3 for counting the inversion frequencies f1 and f2 of (15),
A means 4 for determining the deterioration state of the catalyst (12) by comparing these inversion frequencies.

[0014]

When the catalyst is operating normally, oxygen is reduced downstream of the catalyst by the amount of oxygen storage, and the rich lean inversion frequency f2 of the downstream oxygen sensor is lower than that of the upstream oxygen sensor. Get smaller. But,
As the catalyst deteriorates, the oxygen storage capacity decreases, and the downstream rich lean inversion frequency gradually increases. Therefore, the ratio f2 / f1 of these inversion frequencies f1 and f2 becomes large in accordance with the deterioration, and the deterioration state of the catalyst can be accurately judged by examining the change in this ratio.

In this case, during the feedback control by the oxygen sensor on the downstream side, the output of the oxygen sensor on the upstream side is irrelevant. Therefore, even if the characteristics of the oxygen sensor on the upstream side are deviated to any one, the downstream side The output of is not affected, and the rich lean inversion frequency correctly reflects the actual air-fuel ratio.

[0016]

FIG. 2 shows an embodiment of the present invention. 11 is an engine, 12 is an exhaust passage, 13 is a three-way catalyst interposed in the exhaust passage 12, and 14 is an upstream of the three-way catalyst 12. The first oxygen sensor 15 is a second oxygen sensor that is also installed downstream.

Reference numeral 16 is a control unit, which feedback-controls the fuel supply amount supplied to the intake system of the engine 1 via the fuel injection valve 17 so that it is basically at the stoichiometric air-fuel ratio. Therefore, the control unit 16 receives signals representative of operating states such as the intake air amount and the engine speed, and also receives signals from the oxygen sensors 14 and 15 to keep the intake air amount constant. The fuel supply amount set to be the ratio is corrected based on the output of the oxygen sensor 14 or 15 so that the ratio becomes the theoretical air-fuel ratio correctly.

The control unit 16 also determines from the outputs of the first and second oxygen sensors 14 and 15 whether the three-way catalyst 12 is operating normally. here,
This determination control operation will be described with reference to the flowcharts of FIGS.

First, in step 1, the diagnostic parameter average value FAV, which will be described later, and the i-th diagnostic parameter FCA.
T (i) is set to zero, and then, in step 2, it is judged whether or not the feedback control by the oxygen sensor 14 on the upstream side is being performed.

If the feedback control is being performed, the process proceeds to step 3 and it is determined whether or not a predetermined operating condition is satisfied. This is for performing a failure diagnosis of the three-way catalyst 12. For example, when a constant diagnostic operating condition such as a steady operating condition is established, the routine proceeds to the diagnostic routine of step 4 and below, and under other operating conditions, Normal upstream first oxygen sensor 14
The air-fuel ratio feedback control by is continued.

The output inversion frequency f1 of the first oxygen sensor 14 is maintained until the predetermined time elapses in steps 4 and 5.
To measure. The reversal frequency f1 is the number of times per unit time that the output of the oxygen sensor 14 changes to rich lean across the slice level (comparison value) corresponding to the theoretical air-fuel ratio.

Then, in step 6, this frequency f1
Is a preset lower limit frequency FL and upper limit frequency FU
If it is within the normal range,
Moving to step 7, the first feedback control of the air-fuel ratio
The control by the oxygen sensor 14 is switched to the feedback control by the output of the oxygen sensor 15 on the downstream side.

Then, in steps 8 and 9, the reversal frequency f2 of the output of the second oxygen sensor 15 is measured until a predetermined time elapses after this switching.

Since oxygen is normally stored by the action of the three-way catalyst 12, the reversal frequency f2 of the second oxygen sensor 15 is lower than the reversal frequency f1 of the first oxygen sensor 14.

When the sampling for a predetermined period is completed in this way, the feedback control of the air-fuel ratio is again made normal control in step 10, that is, the control based on the output of the first oxygen sensor 14 on the upstream side of the three-way catalyst 12. Switch to.

Then, in step 11, FCAT (N) = f2 / f1 is set as the Nth diagnostic parameter.
The ratio of the output reversal frequency at the time of air-fuel ratio control by the first oxygen sensor 14 and the second oxygen sensor 15 is calculated. In step 12, the average value FAV of the diagnostic parameters is
FAV = FAV + FCAT, including the previous diagnosis result
It is calculated as (N) / N.

Next, in step 13, the number of diagnoses N is set to N
= N + 1, it is determined in step 12 whether or not the number of diagnoses N has reached a preset total number of diagnoses NT, and the above-described diagnosis is repeated until the total number of diagnoses NT is reached. When the total number of diagnoses NT has been reached, in step 15, the diagnostic parameter average value FAV is compared with a preset catalyst diagnostic reference value FNG.

In the normal state, the reversal frequency of the second oxygen sensor 15 is smaller than that of the first oxygen sensor 14, and the three-way catalyst 1
Since the inversion frequency of the second oxygen sensor 15 increases as 2 deteriorates, the diagnostic parameter FA
V becomes a larger value as the deterioration of the three-way catalyst 12 progresses.

Therefore, when FAV is larger than FNG, it is judged that the three-way catalyst 12 is deteriorated from the normal state, and in steps 16 and 17, the failure code is recorded and the indicator lamp is turned on. And inform the driver.

In the normal operation, the control unit 16 is constructed as above, and the control unit 16 feeds back the air-fuel ratio based on the output of the first oxygen sensor 14 upstream of the three-way catalyst 12. Control is performed so that the amount of fuel supplied from the fuel injection valve 17 becomes the stoichiometric air-fuel ratio.

When the engine specific operating condition becomes the catalyst diagnostic condition, the output reversal frequency f1 within a predetermined time is maintained during the feedback control by the first oxygen sensor 14.
Is counted, and thereafter, the control is switched to the feedback control of the air-fuel ratio by the second oxygen sensor 15, and the output reversal frequency f2 within the predetermined time is also counted.

If the three-way catalyst 12 is operating normally, the amount of oxygen stored in the three-way catalyst 12 is low in oxygen downstream of the three-way catalyst 12, and the reversal frequency f2 of the downstream oxygen sensor 15 is the reversal frequency of the upstream oxygen sensor 14. It is smaller than f1. However, as the deterioration of the three-way catalyst 12 progresses, the oxygen storage capacity decreases, and the downstream rich lean inversion frequency gradually increases.

Therefore, these inversion frequencies f1 and f2
The ratio f2 / f1 of is increased according to deterioration,
Therefore, the deterioration state of the three-way catalyst 12 can be accurately determined by determining the average value of the diagnostic parameters for the predetermined time.

In this case, even if the characteristic of the first oxygen sensor 14 is biased toward the second oxygen sensor 15 to be rich or lean, the reverse output of the downstream oxygen sensor 15 is counted. During the feedback control by the downstream oxygen sensor 15, there is no influence of the upstream oxygen sensor 14, so that accurate determination can be performed.

There is a period during which the air-fuel ratio feedback control is performed by the downstream oxygen sensor 15 during this diagnostic control, but this period is short, and even if the accuracy of the feedback control during this period is slightly reduced, there is overall The adverse effect on exhaust emission etc. is negligible.

[0036]

As described above, according to the present invention, the catalyst disposed in the exhaust passage, the first oxygen sensor arranged upstream of the catalyst, and the air-fuel ratio based on the output of the oxygen sensor are set to the stoichiometric air-fuel ratio. In an internal combustion engine provided with a means for feedback control, a second oxygen sensor installed downstream of the catalyst, a means for judging that the operating condition is a catalyst diagnostic condition, and a first oxygen under the catalytic diagnostic operating condition. The feedback control by the sensor is switched to the feedback control by the second oxygen sensor, the means for counting the inversion frequency of the oxygen sensor at each control, and the means for judging the deterioration state of the catalyst by comparing these inversion frequencies are provided. Therefore, under the diagnostic operating condition, the output characteristic of the upstream oxygen sensor is affected during the feedback control by the downstream oxygen sensor. And no, it is possible to output the inverted frequency corresponding to the deterioration of the catalyst and thus it is possible deterioration of the catalyst is determined accurately.

[Brief description of drawings]

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

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

FIG. 3 is a flowchart of control operation.

FIG. 4 is a flowchart of the same.

FIG. 5 is an explanatory diagram showing the relationship between the characteristics of the upstream oxygen sensor and the output characteristics of the downstream oxygen sensor, where (A) is richer than the downstream side, and (B) is the downstream side. Shows the case of leaner than.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Feedback control means 2 Diagnostic condition determination means 3 Inversion frequency counting means 4 Catalyst deterioration determination means 12 Three-way catalyst 14 First oxygen sensor 15 Second oxygen sensor

Claims (1)

[Claims] 1. A catalyst provided in an exhaust passage, a first oxygen sensor arranged upstream of the catalyst, and means for feedback-controlling an air-fuel ratio to a stoichiometric air-fuel ratio based on the output of the oxygen sensor. In the internal combustion engine, a second oxygen sensor installed downstream of the catalyst, a means for judging that the operating condition is a catalyst diagnostic condition, and a second oxygen sensor based on the feedback control by the first oxygen sensor under the catalytic diagnostic operating condition. The internal combustion engine is characterized by including means for counting feedback frequencies of the oxygen sensor during each control by switching to feedback control by the oxygen sensor, and means for determining the deterioration state of the catalyst by comparing these inversion frequencies. An engine catalyst deterioration determination device.