Apparatus for detecting purification factor of catalyst in catalytic converter of internal combustion engine

Claim 2

A catalyst purification factor detection apparatus for an internal combustion engine arrangement having a catalytic converter containing a catalyst disposed in an exhaust system thereof, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean airfuel ratio conditions, said catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means for judging whether said engine is operating in a condition which satisfies predetermined catalyst deterioration detection conditions, and functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, and for holding said feedback period fixed at said different value during said operation for judging the purification factor, so long as it is judged that said predetermined catalyst deterioration detection conditions continue to be satisfied; and detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor, while said feedback period is fixed at said different value.

Claim 3

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition; detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor; and means for deriving from said output signal from said first oxygen sensor a corresponding first air-fuel ratio signal which goes to first and second fixed levels in accordance with a rich and lean air-fuel ratio respectively, and means for deriving from said first air-fuel ratio signal a second air-fuel ratio signal wherein each transition between said first and second levels is delayed by a specific amount with respect to a corresponding transition of said first air-fuel ratio signal, with a period of alternation of said second air-fuel ratio signal defining said feedback period.

Claim 4

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition; detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor; and means for deriving from said output signal from said first oxygen sensor a corresponding first air-fuel ratio signal which goes to first and second fixed levels in accordance with a rich and lean air-fuel ratio respectively, and means for deriving from said first air-fuel ratio signal a second air-fuel ratio signal wherein each transition between said first and second levels is delayed by a specific amount with respect to a corresponding transition of said first air-fuel ratio signal, with a period of alternation of said second air-fuel ratio signal defining said feedback period.

Claim 5

A purification factor detection apparatus according to claim 2, comprising means for changing said amount of delay by a delay operational quantity (TD), for thereby implementing said changing of said feedback period to a different value during an operation of purification factor judgement.

Claim 6

A purification factor detection apparatus according to claim 3, wherein said feedback period is changed to a predetermined target value, and wherein said feedback correction coefficient control means comprises means for deriving a requisite value for said delay operational quantity, including means for measuring a current value of said feedback period, means for calculating a frequency difference between said current value and said target value of feedback period, and means for deriving said value of additional amount of delay based on said frequency difference and a predetermined relationship between values of said feedback period and amounts of delay applied to said first air-fuel ratio signal for obtaining said second air-fuel ratio signal.

Claim 7

A purification factor detection apparatus according to claim 4, wherein said target value of feedback period is selected to be longer than a value of feedback period which occurs during a normal emission control operating condition of said engine, and to be within a range of values of feedback period which do not result in a lowering of emission control quality.

Claim 8

A purification factor detection apparatus according to claim 4, wherein said target value is determined based on a current rate of flow of intake air to said engine.

Claim 9

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: Feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period to a predetermined target value during said operation for purification factor judgement, and wherein said detection means functions to detect whether an amplitude variation of said output signal from said second oxygen sensor reaches a predetermined threshold value, after said predetermined target value of feedback period has been reached; and detection means for judging said purification factor.

Claim 10

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: Feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period to a predetermined target value during said operation for purification factor judgement, and wherein said detection means functions to detect whether an amplitude variation of said output signal from said second oxygen sensor reaches a predetermined threshold value, after said predetermined target value of feedback period has been reached; and detection means for judging said purification factor.

Claim 11

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period by successive specific small amounts during said operation for purification factor judgement, and wherein said detection means functions to detect a value of said feedback period, reached at a point during said increasing of the feedback period by successive small amounts, at which a variation amplitude of said output signal from said second oxygen sensor reaches a predetermined threshold value; and detection means for judging said purification factor.

Claim 12

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period by successive specific small amounts during said operation for purification factor judgement, and wherein said detection means functions to detect a value of said feedback period, reached at a point during said increasing of the feedback period by successive small amounts, at which a variation amplitude of said output signal from said second oxygen sensor reaches a predetermined threshold value; and detection means for judging said purification factor.

Claim 13

A catalyst purification factor detecting apparatus for an internal combustion engine arrangement having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, said catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period to a predetermined target value of feedback period during said operation for purification factor judgement; and detection means for judging said purification factor, wherein said detection means functions to measure a value of phase difference between said output signals from the first and second oxygen sensors, and to obtain a value of purification factor of said catalyst from said phase difference value, based upon a predetermined relationship between respective values of said phase difference and purification factor at said target value of feedback period.

Claim 14

A catalyst purification factor detecting apparatus for an internal combustion engine arrangement having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, said catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction coefficient control means functions to increase said feedback period to a predetermined target value of feedback period during said operation for purification factor judgement; and detection means for judging said purification factor, wherein said detection means functions to measure a value of phase difference between said output signals from the first and second oxygen sensors, and to obtain a value of purification factor of said catalyst from said phase difference value, based upon a predetermined relationship between respective values of said phase difference and purification factor at said target value of feedback period.

Claim 15

A purification factor detection apparatus according to claim 9, wherein said phase difference value is measured by said detection means as a time difference between a transition between conditions of said output signal from the first oxygen sensor indicating a rich air-fuel ratio and a lean air-fuel ratio respectively, and a corresponding transition of said output signal from the second oxygen sensor.

Claim 16

A purification factor detection apparatus according to claim 10, wherein said phase difference value is measured as an average value of time interval (T RL +T LR )/2, where T RL is a time difference between a point at which said feedback correction coefficient changes from a condition for producing an increased value of said fuel injection quantity to a condition for producing a decreased value of said fuel injection quantity to a point at which said output signal from the second oxygen sensor changes from a condition indicating a rich air-fuel ratio to a condition indicating a lean air-fuel ratio, and T LR is a time difference between a point at which said feedback correction coefficient changes from a condition for producing a decreased value of said fuel injection quantity to a condition for producing an increased value of said fuel injection quantity and a point at which said output signal from the second oxygen sensor changes from a condition indicating a lean air-fuel ratio to a condition indicating a rich air-fuel ratio.

Claim 17

A catalyst purification factor detection apparatus for an internal combustion engine, comprising: a catalytic converter (15) containing a catalyst, disposed in an exhaust system of said internal combustion engine, said first and second oxygen sensors (16, 18) respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions; feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, transitions of said values occurring periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity, said correction means including means for delaying said transitions by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between said rich and lean indication conditions; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period from a value of feedback period which occurs during a normal emission control condition to a target feedback period (T T ), by changing said delay amount by a delay operational quantity (TD); and detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor.

Claim 18

A catalyst purification factor detection apparatus for an internal combustion engine, comprising: a catalytic converter (15) containing a catalyst, disposed in an exhaust system of said internal combustion engine, said first and second oxygen sensors (16, 18) respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions; feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, transitions of said values occurring periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity, said correction means including means for delaying said transitions by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between said rich and lean indication conditions; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period from a value of feedback period which occurs during a normal emission control condition to a target feedback period (T T ), by changing said delay amount by a delay operational quantity (TD); and detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor.

Claim 19

A catalyst purification factor detection apparatus according to claim 12, wherein said correction means includes means for measuring a current value of said feedback period, means for computing an amount of difference between said target feedback period and said current feedback period, and means for obtaining an appropriate value for said delay operational quantity (TD) based on said difference and upon a known relationship between values of delay applied to said transitions of the output signal from the first oxygen sensor and corresponding values of feedback period.

Claim 20

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction means includes means for executing each of said periodic changes of said feedback correction coefficient as a stepwise change in accordance with a predetermined fixed skip quantity; and detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor.

Claim 21

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, and first and second oxygen sensors respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions, a catalyst purification factor detection apparatus comprising: feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, said successive values changing periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for changing said feedback period to a different value from a feedback period which occurs during a normal air-fuel ratio control condition, wherein said feedback correction means includes means for executing each of said periodic changes of said feedback correction coefficient as a stepwise change in accordance with a predetermined fixed skip quantity; and detection means for judging said purification factor, based at least on said output signal from said second oxygen sensor.

Claim 22

A catalyst purification factor detection apparatus for an internal combustion engine, comprising: a catalytic converter (15) containing a catalyst, disposed in an exhaust system of said internal combustion engine, and first and second oxygen sensors (16, 18) respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions; feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, transitions of said values occurring periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity, said correction means including means for delaying said transitions by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between said rich and lean indication conditions; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for increasing said feedback period from a value of feedback period which occurs during a normal emission control condition to a target feedback period (T T ), by changing said delay amount by a predetermined delay operational quantity (TD) that is determined based on a difference between said target feedback period and a current value of feedback period; and detection means for measuring a value of phase difference between said output signal from the first and second oxygen sensors, for obtaining a value of purification factor of said catalyst from said phase difference value, based upon a predetermined relationship between respective values of said phase difference and purification factor at said target value of feedback period, and for comparing said obtained value of purification factor with a reference value of purification factor.

Claim 23

A catalyst purification factor detection apparatus for an internal combustion engine, comprising: a catalytic converter (15) containing a catalyst, disposed in an exhaust system of said internal combustion engine, and first and second oxygen sensors (16, 18) respectively disposed upstream and downstream from said catalyst within said exhaust system, said first and second oxygen sensors producing respective output signals, each signal selectively indicating rich and lean air-fuel ratio conditions; feedback correction means responsive to at least said output signal from said first oxygen sensor for obtaining successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said internal combustion engine, transitions of said values occurring periodically, with a feedback period, between a condition for producing an increase in said fuel injection quantity and a condition for producing a reduction of said fuel injection quantity, said correction means including means for delaying said transitions by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between said rich and lean indication conditions; feedback correction coefficient control means functioning during an operation for judging a purification factor of said catalyst, for increasing said feedback period from a value of feedback period which occurs during a normal emission control condition to a target feedback period (T T ), by changing said delay amount by a predetermined delay operational quantity (TD) that is determined based on a difference between said target feedback period and a current value of feedback period; and detection means for measuring a value of phase difference between said output signal from the first and second oxygen sensors, for obtaining a value of purification factor of said catalyst from said phase difference value, based upon a predetermined relationship between respective values of said phase difference and purification factor at said target value of feedback period, and for comparing said obtained value of purification factor with a reference value of purification factor.

Claim 24

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, a first oxygen sensor and a second oxygen sensor disposed in said exhaust system respectively upstream and downstream from said catalyst, and means for executing negative feedback control of an air-fuel ratio of said engine based upon at least an output signal produced from said first oxygen sensor by deriving successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said engine, said values varying periodically with a feedback period between a condition for increasing said fuel injection quantity and a condition for decreasing said fuel injection quantity, a method of detecting a purification factor of said catalyst, comprising steps of: judging whether said engine is operating in a condition which satisfies predetermined catalyst deterioration detection conditions; changing said feedback period to a value that is different from a feedback period which occurs during a normal air-fuel ratio control mode of operation, and holding said feedback period fixed at said different value during said operation for judging the purification factor, so long as said predetermined catalyst deterioration conditions continue to be satisfied; and detecting said purification factor, on the basis of a condition of at least an output signal produced from said second oxygen sensor, while said feedback period is fixed at said different value.

Claim 25

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, a first oxygen sensor and a second oxygen sensor disposed in said exhaust system respectively upstream and downstream from said catalyst, and means for executing negative feedback control of an air-fuel ratio of said engine based upon at least an output signal produced from said first oxygen sensor by deriving successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said engine, said values varying periodically with a feedback period between a condition for increasing said fuel injection quantity and a condition for decreasing said fuel injection quantity, a method of detecting a purification factor of said catalyst, comprising steps of: judging whether said engine is operating in a condition which satisfies predetermined catalyst deterioration detection conditions; changing said feedback period to a value that is different from a feedback period which occurs during a normal air-fuel ratio control mode of operation, and holding said feedback period fixed at said different value during said operation for judging the purification factor, so long as said predetermined catalyst deterioration conditions continue to be satisfied; and detecting said purification factor, on the basis of a condition of at least an output signal produced from said second oxygen sensor, while said feedback period is fixed at said different value.

Claim 26

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, a first oxygen sensor and a second oxygen sensor disposed in said exhaust system respectively upstream and downstream from said catalyst, and means for executing negative feedback control of an air-fuel ratio of said engine based upon at least an output signal produced from said first oxygen sensor by deriving successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said engine, said values varying periodically with a feedback period between a condition for increasing said fuel injection quantity and a condition for decreasing said fuel injection quantity, a method of detecting a purification factor of said catalyst, comprising steps of: changing said feedback period to a value that is different from a feedback period which occurs during a normal air-fuel ration control mode of operation wherein said values of the feedback correction coefficient are determined at time points which are delayed by a specific amount with respect to corresponding transitions of the feedback correction coefficient are determined at time points which are delayed by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between a first condition that indicates a rich air-fuel ration and a second conditions that indicates a lean air-fuel ratio, and in which said step of changing the feedback period is executed by changing said delay amount by a specific delay operational quantity.

Claim 27

In an internal combustion engine having a catalytic converter containing a catalyst disposed in an exhaust system of the engine, a first oxygen sensor and a second oxygen sensor disposed in said exhaust system respectively upstream and downstream from said catalyst, and means for executing negative feedback control of an air-fuel ratio of said engine based upon at least an output signal produced from said first oxygen sensor by deriving successive values of a feedback correction coefficient which is applied to control a fuel injection quantity of said engine, said values varying periodically with a feedback period between a condition for increasing said fuel injection quantity and a condition for decreasing said fuel injection quantity, a method of detecting a purification factor of said catalyst, comprising steps of: changing said feedback period to a value that is different from a feedback period which occurs during a normal air-fuel ration control mode of operation wherein said values of the feedback correction coefficient are determined at time points which are delayed by a specific amount with respect to corresponding transitions of the feedback correction coefficient are determined at time points which are delayed by a specific amount with respect to corresponding transitions of said output signal from the first oxygen sensor between a first condition that indicates a rich air-fuel ration and a second conditions that indicates a lean air-fuel ratio, and in which said step of changing the feedback period is executed by changing said delay amount by a specific delay operational quantity.

Claim 28

A purification factor detection method according to claim 17, in which said change in the delay amount consists of a single step change, for setting the feedback period to a target value.

Claim 29

A purification factor detection method according to claim 18, in which said step of detecting the purification factor is implemented by comparing a value of output signal amplitude from said second oxygen sensor with a predetermined threshold value, after said feedback period has been changed to said target value.

Claim 30

A purification factor detection method according to claim 18, in which said step of detecting the purification factor is implemented, after said feedback period has been changed to said target value, by: measuring an amount of phase difference between said output signals from the first and second oxygen sensors; obtaining a value of purification factor of said catalyst from said phase difference value, based upon said phase difference and a predetermined relationship between respective values of said phase difference and purification factor at said target value of feedback period; and comparing said obtained value of purification factor with a reference value of purification factor.

Claim 31

A purification factor detection method according to claim 20, in which said step of measuring an amount of phase difference is implemented by measuring a time difference between a transition of said output signal from the first oxygen sensor, between conditions indicating a rich air-fuel ratio and a lean air-fuel ratio respectively, and a corresponding transition of said output signal from the second oxygen sensor.

Claim 32

A purification factor detection method according to claim 20, in which said step of measuring an amount of phase difference is implemented by measuring an average value of time interval (T RL +T LR )/2, where T RL is a time difference between a point at which said feedback correction coefficient changes from a condition for producing an increased value of said fuel injection quantity to a condition for producing a decreased value of said fuel injection quantity to a point at which said output signal from the second oxygen sensor changes from a condition indicating a rich air-fuel ratio to a condition indicating a lean air-fuel ratio, and T LR is a time difference between a point at which said feedback correction coefficient changes from a condition for producing a decreased value of said fuel injection quantity to a condition for producing an increased value of said fuel injection quantity and a point at which said output signal from the second oxygen sensor changes from a condition indicating a lean air-fuel ratio to a condition indicating a rich air-fuel ratio.

Claim 33

A purification factor detection method according to claim 18, in which the value of said delay operational quantity (TD), for achieving said target value, is established by steps of: during normal air-fuel ratio control operation, measuring a current value of said feedback period; calculating an amount of difference between said target value and said current value of feedback period; obtaining an appropriate value for said delay operational quantity (TD) based on said difference and upon a known relationship between values of delay applied to said transitions of the output signal from the first oxygen sensor and corresponding values of feedback period.

Claim 34

A purification factor detection method according to claim 18, in which said target value is selected to be within a range of values of feedback period for which no substantial deterioration of a quality of emission control by said catalyst will occur.

Claim 35

A purification factor detection method according to claim 18, in which said target value is determined in accordance with an intake air flow rate of said engine.

Claim 36

A purification factor detection method according to claim 17, in which said change in the delay amount consists of successive small step increments, which are executed until said output signal from the second oxygen sensor reaches a predetermined threshold value of amplitude.