JP2998539B2 - Catalyst deterioration determination device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art As a system for purifying exhaust gas of an internal combustion engine, an air-fuel ratio is feedback-controlled to a stoichiometric air-fuel ratio based on an output of an oxygen sensor, and HC and C are supplied to an exhaust passage.
It is widely practical to install a three-way catalyst that simultaneously oxidizes O and reduces NO.

[0003] When the three-way catalyst changes over time and its performance deteriorates, the conversion efficiency gradually decreases, which hinders exhaust gas purification. It is preferable to take measures such as replacing the catalyst whose performance has deteriorated. Therefore, in order to determine such a state of deterioration of the catalyst, an apparatus as disclosed in Japanese Patent Application Laid-Open No. 4-187848 has been proposed. ing.

In this method, oxygen sensors are respectively installed upstream and downstream of a three-way catalyst, and when the inversion frequency of the output of the downstream oxygen sensor is higher than a predetermined value, it is determined that the catalyst has deteriorated. .

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 is slightly rich,
By virtue of lean swing and such slight rich change, the oxidation and reduction functions of the three-way catalyst are maintained to the maximum. On the other hand, the air-fuel ratio of the exhaust gas that has passed through the three-way catalyst has 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 appears directly downstream. If the three-way catalyst does not function at all, the upstream and downstream air-fuel ratios 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 swings rich or lean. On the other hand, as the deterioration progresses, the inversion frequency at which the oxygen sensor changes rich and lean gradually increases.

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

[0009]

However, in this case, the output characteristics of the upstream and downstream oxygen sensors are shifted to the rich or lean side, which may affect the determination and cause an erroneous diagnosis.

Since the comparison value (threshold) for counting the reversal of the output of the downstream oxygen sensor is fixed to a constant value, if the characteristics of the upstream oxygen sensor deviate from rich or lean. , The inverted output frequency cannot accurately reflect the change in the air-fuel ratio.

FIG. 6A shows the case where there is no difference between the characteristics of the oxygen sensor on the upstream side and the oxygen sensor on the downstream side, and the output of the oxygen sensor on the downstream side is inverted in a rich lean manner with equal amplitude based on the comparison value.

However, as shown in FIG. 6B, when the upstream oxygen sensor is shifted, for example, by a lean shift, the air-fuel ratio controlled based on this shift becomes relatively lean, and the output of the downstream oxygen sensor is reduced. Is also shifted to the lean side with respect to the comparison threshold value, and the number of times exceeding the comparison threshold value does not correspond to the number of inversions. For this reason, even if the inversion frequency is actually large,
In some cases, the number of times exceeding the comparison threshold value is reduced and the catalyst is erroneously diagnosed as normal despite the deterioration of the catalyst.

The present invention solves such a problem, that is,
An object of the present invention is to make it possible to correctly determine deterioration of a catalyst regardless of a deviation in characteristics of an oxygen sensor.

[0014]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a method as shown in FIG.
As shown in the figure, a catalyst (12) interposed in an exhaust passage and a first oxygen sensor (14) disposed upstream of the catalyst (12)
A means 1 for feedback-controlling the air-fuel ratio to a stoichiometric air-fuel ratio based on the output of the oxygen sensor (14); and a second oxygen sensor (15) arranged downstream of the catalyst (12).
When the output of the oxygen sensor (15) is a comparison threshold
The number of times the battery is turned off is taken as the number of reversals of the second oxygen sensor (15).
Counted and the catalyst deterioration determination device for an internal combustion engine provided with means for determining 2 deterioration of the catalyst (12) based on the number of reversals counted, the said air-fuel ratio is feedback controlled second oxygen sensor (15 and means 3 for correcting, based on an output of), and an adjustment unit 4 to increase or decrease the comparison threshold based on the correction value.

[0015]

The feedback control of the air-fuel ratio is performed based on the output of the first oxygen sensor. If the output characteristic of the first oxygen sensor is shifted to the rich side, the feedback control of the second oxygen sensor is performed. The output also shifts to the rich side.

However, due to the output of the second oxygen sensor,
When the air-fuel ratio is corrected and controlled, the correction value is a value in a direction for returning the air-fuel ratio to the lean side. Using this correction value, the threshold value (slice level) for comparing the output of the second oxygen sensor is adjusted, that is, the comparison threshold value is displaced to the rich side, so that the displacement is shifted to the rich side. It moves toward the center of the inversion amplitude of the sensor output, and the number of inversions can be accurately counted. Based on the number of inversions, it is possible to correctly determine the deterioration of the catalyst.

[0017]

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

Reference numeral 16 denotes a control unit which performs feedback control on the amount of fuel supplied to the intake system of the engine 1 through the fuel injection valve 17 so that the fuel supply basically reaches the stoichiometric air-fuel ratio. Therefore, the control unit 16 receives a signal representative of an operating state such as an intake air amount and an engine speed, and also receives signals from the oxygen sensors 14 and 15 so that the control unit 16 has a constant value with respect to the intake air amount. The fuel supply amount set to be the ratio is supplied to the first oxygen sensor 14.
The feedback control is performed based on the output of the second oxygen sensor 15, and the feedback control is further performed based on the output of the second oxygen sensor 15 so that the stoichiometric air-fuel ratio is correctly obtained.

The control unit 16 has a second
From the number of reversals of the output of the oxygen sensor 15 from the three-way catalyst 12
Is operating normally, that is, the number of reversals per unit time is equal to or more than a predetermined value, or the second oxygen sensor 1
When the ratio of the number of reversals of the output of No. 5 exceeds a predetermined value, it is determined that the catalyst has deteriorated. At this time, a comparison threshold (slice level) for counting the number of reversals of the second oxygen sensor 15 Is corrected based on the correction value of the air-fuel ratio so that the number of reversals can be correctly counted.

The operation of counting the number of inversions will be described with reference to the flowchart of FIG.

First, in step 1, the first oxygen sensor 14
The feedback control value of the air-fuel ratio is further adjusted by the second oxygen sensor 15 to determine whether the correction control is being performed, and to determine in step 2 operating conditions for diagnosing catalyst deterioration (for example, during a constant steady operation). It is determined whether or not there is, and if the diagnosis condition is satisfied, the correction value PH of the air-fuel ratio control based on the output of the second oxygen sensor 15 is determined in step 3.
Read the OS. If the diagnosis condition is not satisfied, the process returns to the beginning.

For example, if the output characteristic of the first oxygen sensor 14 is leaner than that of the second oxygen sensor 15,
As shown in FIG. 4, the air-fuel ratio that is feedback-controlled based on this is basically relatively deviated to the lean side, so that the output of the second oxygen sensor 15 is also deviated to lean. Therefore, the correction value PHOS calculated based on the output of the second oxygen sensor 15 increases so as to richly correct the air-fuel ratio.

Conversely, if the output of the first oxygen sensor 14 has richer characteristics than the output of the second oxygen sensor 15, the correction value PHOS of the air-fuel ratio decreases to correct lean.

Therefore, the correction value PHOS is a parameter indicating the magnitude of the deviation from the output center point of the first and second oxygen sensors 14 and 15.

In step 4, this correction value PHOS is compared with the previous correction value PHOSP, and if PHOS> PHO
If it is SP, that is, if the current correction value is deviated to the rich side, the actual air-fuel ratio is lean, and the correction value is rich to correct this. The slice level SLR, which is a comparison threshold value for calculating the number of inversions of the second oxygen sensor 15, is represented by SL
As R = SLR-DSLR1, the temperature is lowered to be at the center of the amplitude of the lean air-fuel ratio.

On the other hand, in step 4, when the correction value is smaller than the previous value, that is, when the correction value is shifted to the lean side, in step 6, the slice level SLR = SLR + D
As SLR2, the slice level is raised in accordance with the rich air-fuel ratio.

As a result, the slice level SLR of the second oxygen sensor 15 relatively moves in accordance with the air-fuel ratio, and approaches the center of the amplitude of the air-fuel ratio.

In step 7, the output VRO2 of the second oxygen sensor 15 is compared with the corrected slice level SLR. If VRO2> SLR, in step 8, the previous output VRO2P of the second oxygen sensor 15 is output. And SLR
If VRO2P ≦ SLR, it is determined that the output of the oxygen sensor 15 has been inverted, and the process proceeds to step 10 to change the counter value CT of the inversion counter to CT = CT +
Let it be 1.

Similarly, in step 7, VRO2 <SL
If R, in step 9 the previous sensor output VRO2P
Is compared with the SLR. If VRO2P> SLR, it is determined that the sensor output has been inverted, and the counter value CT is incremented in step 10. When the output of the second oxygen sensor 15 does not reverse, each returns to the beginning,
Repeat the same operation.

In step 11, the correction value P
HOSP is PHOS, and the sensor output value VRO2
Return to the beginning with P as VRO2.

The operation of the control unit 16 will now be described. During normal operation, the control unit 16 performs feedback of the air-fuel ratio based on the output of the first oxygen sensor 14 upstream of the three-way catalyst 12. The control value is corrected based on the output of the second oxygen sensor 15 on the downstream side, and the amount of fuel supplied from the fuel injection valve 17 becomes
Control is performed to achieve the stoichiometric air-fuel ratio.

When the specific operating condition of the engine becomes the catalyst diagnosis condition, the slice level SLR is adjusted based on the air-fuel ratio correction value based on the output of the second oxygen sensor 15.

As shown in FIG. 5A, when the characteristic of the first oxygen sensor (O ₂ / S) shifts to a richer side than the second oxygen sensor 15, the output of the second oxygen sensor 15 Is easily biased toward the rich side, and as shown in FIG. 5B, the characteristic of the first oxygen sensor 14 is
When the position of the second oxygen sensor 15 is shifted to the lean side from the position 5, the output of the second oxygen sensor 15 is biased to the lean side.

For this reason, as shown in FIG.
When the first oxygen sensor 14 shifts lean and the output of the second oxygen sensor 15 also becomes lean, the correction value of the air-fuel ratio based on this shifts to the rich side. When the slice level SLR is corrected using this correction value, the slice level SLR is lowered toward the lean side and moves toward the center of the inverted amplitude of the sensor output. In this way, the comparison threshold value for determining the number of reversals of the output of the second oxygen sensor 15 decreases in accordance with the air-fuel ratio lean,
The reversal output of the sensor always crosses the comparison threshold, the number of reversals can be counted correctly, and the deterioration of the three-way catalyst 12 can be accurately determined.

In order to correct and control the air-fuel ratio based on the output of the second oxygen sensor 15, the first oxygen sensor 15
The air-fuel ratio can be accurately converged to the stoichiometric air-fuel ratio even if the output characteristic of the air-fuel ratio is biased to rich or lean.

[0036]

As described above, according to the present invention, the catalyst disposed in the exhaust passage, the first oxygen sensor disposed upstream of the catalyst, and the air-fuel ratio converted to the stoichiometric air-fuel ratio based on the output of the oxygen sensor. means for feedback control, and the second oxygen sensor disposed downstream of the catalyst, the output of the oxygen sensor
The number of times exceeding the comparison threshold is determined by the number of times the second oxygen sensor is inverted.
And a means for determining the deterioration of the catalyst based on the counted number of inversions, wherein the feedback-controlled air-fuel ratio is determined based on the output of the second oxygen sensor. Means for correcting
Due to an adjusting means for increasing or decreasing the comparison threshold based on the correction value, comparison threshold for counting the number of inversions second oxygen sensor, in accordance with the deviation of the sensor output characteristic, always The correction is made toward the center of the reversal amplitude. As a result, the number of reversals can be accurately counted without being affected by the output characteristics of the first oxygen sensor, and the deterioration of the catalyst can be correctly determined.

[Brief description of the 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 a control operation.

FIG. 4 is an explanatory diagram showing a relationship between a correction value of an air-fuel ratio and a change in a slice level.

5A and 5B are explanatory diagrams showing the relationship between the characteristics of an upstream oxygen sensor and the output characteristics of a downstream oxygen sensor. FIG. 5A shows a case where the upstream side is richer than the downstream side, and FIG. Shows the case of lean.

FIG. 6 is an explanatory diagram of output characteristics of a downstream oxygen sensor;
(A) is when there is no deviation in the characteristics of the upstream oxygen sensor,
(B) shows a case where the upstream oxygen sensor has undergone a lean shift.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 feedback control means 2 catalyst deterioration determination means 3 air-fuel ratio correction means 4 comparison threshold value adjustment means 12 three-way catalyst 14 first oxygen sensor 15 second oxygen sensor

Claims (1)

(57) [Claims] 1. A catalyst interposed in an exhaust passage, a first oxygen sensor disposed upstream of the catalyst, means for feedback-controlling an air-fuel ratio to a stoichiometric air-fuel ratio based on an output of the oxygen sensor, and the catalyst And a number of times that the output of the oxygen sensor crosses the comparison threshold value.
Means for counting the number of inversions of the oxygen sensor of No. 2 and means for determining deterioration of the catalyst based on the counted number of inversions. An apparatus for determining deterioration of a catalyst in an internal combustion engine, comprising: means for correcting based on an output of an oxygen sensor; and adjusting means for increasing or decreasing the comparison threshold value based on the correction value.