US5282383A
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US
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Method and apparatus for determining deterioration of three-way catalysts in double air-fuel ratio sensors system
Assignee: Toyota Motor Corp
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Filing: 1992-04-22
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Granted: 1994-02-01
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Expires: 2012-04-22
Abstract
Abstract In a double air-fuel ratio sensor system including two air-fuel ratio sensors disposed on the upstream and the downstream of the catalyst which is installed in the exhaust passage of the engine and has oxygen storage effect, an air-fuel ratio correction amount is calculated in accordance with the output of the upstream O 2 sensor, and the actual air-fuel ratio is adjusted in accordance with the calculated air-fuel ratio correction amount. Further, the determination of the deterioration of the catalyst is carried out in accordance with the output of the second O 2 sensor when the air-fuel ratio of the engine is adjusted.
Inventors
- 👤 Nobuaki Kayanuma
Bibliographic Details
Application Number
—
Jurisdiction
US
Data Source
google_patents
Last Synchronized
2026-11-06 04:07
CPC Classifications
F; F02; F02D; F02D41/00; F02D41/02; F02D41/14; F02D41/1438; F02D41/1439; F02D41/1441; F01; F01N; F01N11/00; F01N11/007; F01N2550/00; F01N2550/02; F01N2900/00; F01N2900/04; F01N2900/0421; F01N2900/0422; Y; Y02; Y02T; Y02T10/00; Y02T10/10; Y02T10/40
Claims Analysis
Total Claims: 39
Claim 1
Independent Claim
132 words
A method of detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is different from the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is controlled in accordance with the adjusting step.
Claim 2
Independent Claim
132 words
A method of detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is different from the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is controlled in accordance with the adjusting step.
Claim 3
Dependent Claim
45 words
A method as set forth in claim 1, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and monitoring whether the number of fluctuations is greater than a predetermined value.
Claim 4
Dependent Claim
99 words
A method as set forth in claim 1, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and calculating a difference between the number of upstream-side fluctuations and the number of downstream-side fluctuations when the number of upstream-side fluctuations is greater than a first predetermined value, and determining that said catalyst has deteriorated when the difference is greater than a second predetermined value.
Claim 5
Dependent Claim
85 words
A method as set forth in claim 1, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and determining that said catalyst has deteriorated when a ratio of the number of downstream-side fluctuations to the number of upstream-side fluctuations is greater than a predetermined value, which is about "1".
Claim 6
Independent Claim
165 words
A method of detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; shifting the air-fuel ratio that is controlled in accordance with said adjusting step to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio.
Claim 7
Independent Claim
165 words
A method of detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; shifting the air-fuel ratio that is controlled in accordance with said adjusting step to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio.
Claim 8
Dependent Claim
67 words
A method as set forth in claim 5, wherein said adjusting step comprises a step of: increasing the air-fuel ratio correction factor when the air-fuel ratio changes from the lean state to the rich state, and decreasing the air-fuel ratio correction factor when the air-fuel ratio changes from the rich state to the lean state; and said shifting step comprises a step of: varying said correction factor.
Claim 9
Dependent Claim
61 words
A method as set forth in claim 5, wherein said adjusting step comprises a step of: gradually increasing the air-fuel ratio correction factor while the air-fuel ratio remains in the rich state, and gradually decreasing the air-fuel ratio correction factor while the air-fuel ratio remains in the lean state; and said shifting step comprises a step of: varying said correction factor.
Claim 10
Dependent Claim
80 words
A method as set forth in claim 5, wherein said adjusting step comprises a step of: delaying a time at which the air-fuel ratio changes from the rich state to the lean state by a predetermined lean delay time, and a time at which the air-fuel ratio changes from the lean state to the rich state by a predetermined rich delay time; and said shifting step comprises a step of: varying said lean delay time, and said rich delay time.
Claim 11
Independent Claim
237 words
A method of detecting a deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensor disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice verse; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage, and determining that the degree of deterioration of said catalyst is such that a diagnostic procedure is required when the number of fluctuations during a predetermined interval is greater than a predetermined value; shifting the air-fuel ratio that is controlled in accordance with said adjusting step to an air-fuel ration that is not equal to the stoichiometric air-fuel ratio, when the degree of deterioration of said catalyst is such that it requires a diagnostic procedure; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio in accordance with said shifting step.
Claim 12
Independent Claim
237 words
A method of detecting a deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensor disposed upstream and downstream, respectively, of said catalyst converter, comprising the steps of: adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that an air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice verse; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage, and determining that the degree of deterioration of said catalyst is such that a diagnostic procedure is required when the number of fluctuations during a predetermined interval is greater than a predetermined value; shifting the air-fuel ratio that is controlled in accordance with said adjusting step to an air-fuel ration that is not equal to the stoichiometric air-fuel ratio, when the degree of deterioration of said catalyst is such that it requires a diagnostic procedure; and determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio in accordance with said shifting step.
Claim 13
Dependent Claim
67 words
A method as set forth in claim 9, wherein said adjusting step comprises a step of: increasing the air-fuel ratio correction factor when the air-fuel ratio changes from the lean state to the rich state, and decreasing the air-fuel ratio correction factor when the air-fuel ratio changes from the rich state to the lean state; and said shifting step comprises a step of: varying said correction factor.
Claim 14
Dependent Claim
61 words
A method as set forth in claim 9, wherein said adjusting step comprises a step of: gradually increasing the air-fuel ratio correction factor while the air-fuel ratio remains in the rich state, and gradually decreasing the air-fuel ratio correction factor while the air-fuel ratio remains in the lean state; and said shifting step comprises a step of: varying said correction factor.
Claim 15
Dependent Claim
80 words
A method as set forth in claim 9, wherein said adjusting step comprises a step of: delaying a time at which the air-fuel ratio changes from the rich state to the lean state by a predetermined lean delay time, and a time at which the air-fuel ratio changes from the lean state to the rich state by a predetermined rich delay time; and said shifting step comprises a step of: varying said lean delay time and said rich delay time.
Claim 16
Dependent Claim
45 words
A method as set forth in claim 9, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and monitoring whether the number of fluctuations is greater than the predetermined value.
Claim 17
Dependent Claim
98 words
A method as set forth in claim 9, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and calculating a difference between the number of upstream-side fluctuations and the number of downstream-side fluctuations when the number of upstream-side fluctuations is greater than the predetermined value, and determining that said catalyst has deteriorated when the difference is greater than a second predetermined value.
Claim 18
Dependent Claim
85 words
A method as set forth in claim 9, wherein said determining step comprises the steps of: monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and determining that said catalyst has deteriorated when a ratio of the number of downstream-side fluctuations to the number of upstream-side fluctuations is greater than the predetermined value, which is about "1".
Claim 19
Independent Claim
138 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is different from the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is controlled in accordance with said adjusting means.
Claim 20
Independent Claim
138 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is different from the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice versa; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is controlled in accordance with said adjusting means.
Claim 21
Dependent Claim
46 words
An apparatus as set forth in claim 16, wherein said determining means comprises: means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for monitoring whether the number of fluctuations is greater than a predetermined value.
Claim 22
Dependent Claim
102 words
An apparats as set forth in claim 16, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for calculating a difference between the number of upstream-side fluctuations and the number of downstream-side fluctuations when the number of upstream-side fluctuations is greater than a first predetermined value, and determining that said catalyst has deteriorated when the difference is greater than a second predetermined value.
Claim 23
Dependent Claim
88 words
An apparatus as set forth in claim 16, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for determining that said catalyst has deteriorated when a ratio of the number of downstream-side fluctuations to the number of upstream-side fluctuations is greater than a predetermined value, which is about "1".
Claim 24
Independent Claim
173 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice verse; means for shifting the air-fuel ratio that is controlled in accordance with said adjusting means to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio.
Claim 25
Independent Claim
173 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean state, or vice verse; means for shifting the air-fuel ratio that is controlled in accordance with said adjusting means to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio.
Claim 26
Dependent Claim
65 words
An apparatus as set forth in claim 20, wherein said adjusting means comprises: means for increasing the air-fuel ratio correction factor when the air-fuel ratio changes from the lean state to the rich state, and decreasing the air-fuel ratio correction factor when the air-fuel ratio changes from the rich state to the lean state; and said shifting means comprises: means for varying said correction factor.
Claim 27
Dependent Claim
59 words
An apparatus as set forth in claim 20, wherein said adjusting means comprises: means for gradually increasing the air-fuel ratio correction factor while the air-fuel ratio remains in the rich state, and gradually decreasing the air-fuel ratio correction factor while the air-fuel ratio remains in the lean state; and said shifting means comprises: means for varying said correction factor.
Claim 28
Dependent Claim
78 words
An apparatus as set forth in claim 22, wherein said adjusting means comprises: means for delaying a time at which the air-fuel ratio changes from the rich state to the lean state by a predetermined lean delay time, and a time at which the air-fuel ratio changes from the lean state to the rich state by a predetermined rich delay time; and said shifting means comprises: means for varying said lean delay time and said rich delay time.
Claim 29
Dependent Claim
46 words
An apparatus as set forth in claim 20, wherein said determining means comprises: means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for monitoring whether the number of fluctuations is greater than a predetermined value.
Claim 30
Dependent Claim
102 words
An apparats as set forth in claim 20, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for calculating a difference between the number of upstream-side fluctuations and the number of downstream-side fluctuations when the number of upstream-side fluctuations is greater than a first predetermined value, and determining that said catalyst has deteriorated when the difference is greater than a second predetermined value.
Claim 31
Dependent Claim
88 words
An apparatus as set forth in claim 20, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for determining that said catalyst has deteriorated when a ratio of the number of downstream-side fluctuations to the number of upstream-side fluctuations is greater than a predetermined value, which is about "1".
Claim 32
Independent Claim
246 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean sate, or vice versa; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage, and determining that the degree of deterioration of said catalyst is such that a diagnostic procedure is required when the number of fluctuations during a predetermined interval is greater than a predetermined value; means for shifting the air-fuel ratio that is controlled in accordance with said adjusting means to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio, when the degree of deterioration of said catalyst is such that a diagnostic procedure is required; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio in accordance with said shifting means.
Claim 33
Independent Claim
246 words
An apparatus for controlling an air-fuel ratio and detecting deterioration of a catalyst converter for removing pollutants in exhaust gas disposed within an exhaust gas passage of an internal combustion engine having upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, comprising: means for adjusting an air-fuel ratio correction factor in accordance with the output of said upstream-side air-fuel ratio sensor so that the air-fuel ratio is controlled at a predetermined target air-fuel ratio that is equal to the stoichiometric air-fuel ratio, the air-fuel ratio changing from the rich state to the lean sate, or vice versa; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage, and determining that the degree of deterioration of said catalyst is such that a diagnostic procedure is required when the number of fluctuations during a predetermined interval is greater than a predetermined value; means for shifting the air-fuel ratio that is controlled in accordance with said adjusting means to an air-fuel ratio that is not equal to the stoichiometric air-fuel ratio, when the degree of deterioration of said catalyst is such that a diagnostic procedure is required; and means for determining whether or not said catalyst converter has deteriorated in accordance with the output of said downstream-side air-fuel ratio sensor when the air-fuel ratio is shifted to the air-fuel ratio that is not equal to the stoichiometric air-fuel ratio in accordance with said shifting means.
Claim 34
Dependent Claim
65 words
An apparatus as set forth in claim 27, wherein said adjusting means comprises; means for increasing the air-fuel ratio correction factor when the air-fuel ratio changes from the lean state to the rich state, and decreasing the air-fuel ratio correction factor when the air-fuel ratio changes from the rich state to the lean state; and said shifting means comprises: means for varying said correction factor.
Claim 35
Dependent Claim
59 words
An apparatus as set forth in claim 27, wherein said adjusting means comprises: means for gradually increasing the air-fuel ratio correction factor while the air-fuel ratio remains in the rich state, and gradually decreasing the air-fuel ratio correction factor while the air-fuel ratio remains in the lean state; and said shifting means comprises: means for varying said correction factor.
Claim 36
Dependent Claim
78 words
An apparatus as set forth in claim 27, wherein said adjusting means comprises: means for delaying a time at which the air-fuel ratio changes from the rich state to the lean state by a predetermined lean delay time, and a time at which the air-fuel ratio changes from the lean state to the rich state by a predetermined rich delay time; and said shifting means comprises: means for varying said lean delay time and said rich delay time.
Claim 37
Dependent Claim
46 words
An apparatus as set forth in claim 27, wherein said determining means comprises: means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for monitoring whether the number of fluctuations is greater than the predetermined value.
Claim 38
Dependent Claim
101 words
An apparatus as set forth in claim 27, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for calculating a difference between the number of upstream-side fluctuations and the number of downstream-side fluctuations when the number of upstream-side fluctuations is greater than the predetermined value, and determining that said catalyst has deteriorated when the difference is greater than a second predetermined value.
Claim 39
Dependent Claim
88 words
An apparatus as set forth in claim 27, wherein said determining means comprises: means for monitoring fluctuations in the output of said upstream-side air-fuel ratio sensor around a reference voltage corresponding to the stoichiometric air-fuel ratio; means for monitoring fluctuations in the output of said downstream-side air-fuel ratio sensor around a predetermined reference voltage; and means for determining that said catalyst has deteriorated when a ratio of the number of downstream-side fluctuations to the number of upstream-side fluctuations is greater than the predetermined value, which is about "1".
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