Air-fuel ratio feedback control apparatus and method for internal combustion engine

1. An air-fuel ratio feedback control apparatus for an internal combustion engine, comprising:

an air-fuel ratio sensor disposed in an exhaust pipe of said engine, to output a detection signal of an air-fuel ratio;

a control section that receives the detection signal from said air-fuel ratio sensor, and that outputs an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

a diagnosing section that forcibly changes said target air-fuel ratio during an air-fuel ratio feedback control by said control section, that diagnoses a response characteristic of said air-fuel ratio sensor based on a change in detection signal from said air-fuel ratio sensor to the change in said target air-fuel ratio, and that outputs a signal indicating a diagnosis result;

a storing section that stores a value of the air-fuel ratio feedback control signal immediately before the starting of diagnosis by said diagnosing section; and

a reset section that returns said target air-fuel ratio to a normal value when the diagnosis by said diagnosing section is finished, and that simultaneously resets said air-fuel ratio feedback control signal to said value stored in said storing section immediately before the starting of the diagnosis.

2. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , further comprising:

a gain setting section that changes a value of a feedback gain in said control section to be larger than a normal value when the diagnosis is performed by said diagnosis section.

3. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , wherein said diagnosis section diagnoses the response characteristic of said air-fuel ratio sensor based on a time until the detection signal from said air-fuel ratio sensor indicates a predetermined change in air-fuel ratio after said target air-fuel ratio is forcibly changed.

4. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , wherein said diagnosis section diagnoses the response characteristic of said air-fuel ratio sensor based on a time of after said target air-fuel ratio is forcibly changed until the detection signal from said air-fuel ratio sensor converges in a value equivalent to the post-changed target air-fuel ratio.

5. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , wherein said diagnosis section diagnoses the response characteristic of said air-fuel ratio sensor based on a time until the detection signal from said air-fuel ratio sensor passes over a value equivalent to a predetermined air-fuel ratio after said target air-fuel ratio is forcibly changed.

6. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , wherein said diagnosis section diagnoses the response characteristic of said air-fuel ratio sensor based on a change speed in detection signal from said air-fuel ratio sensor after said target air-fuel ratio is forcibly changed.

7. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , further comprising:

a feedback control prohibition section that prohibits the air-fuel ratio feedback control by said control section, when a signal indicating response deterioration is output from said diagnosis section.

8. An air-fuel ratio feedback control apparatus for an internal combustion engine according to claim 1 , wherein said diagnosis section periodically changes said target air-fuel ratio stepwise.

9. An air-fuel ratio feedback control apparatus for an internal combustion engine, comprising:

an air-fuel ratio sensor disposed in an exhaust pipe of said engine, to output a detection signal of an air-fuel ratio;

a control section that receives the detection signal from said air-fuel ratio sensor, and that outputs an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

a diagnosing section that forcibly changes said target air-fuel ratio during an air-fuel ratio feedback control by said control section, that diagnoses a response characteristic of said air-fuel ratio sensor based on a change in detection signal from said air-fuel ratio sensor to the change in said target air-fuel ratio, and that outputs a signal indicating a diagnosis result;

a reset section that stores a value of air-fuel ratio feedback control signal immediately before the starting of diagnosis by said diagnosis section, and that resets said air-fuel ratio feedback control signal to said stored value when the diagnosis is finished; and

an exhaust purification catalyst disposed downstream of said air-fuel ratio sensor in said exhaust pipe;

wherein said diagnosis section diagnoses the response characteristic of said air-fuel ratio sensor, provided that a predetermined time or above has elapsed from the starting of engine operation, that said air-fuel ratio sensor is activated, and that said exhaust purification catalyst is activated.

10. An air-fuel ratio feedback control apparatus for an internal combustion engine, comprising:

an air-fuel ratio sensor disposed in an exhaust pipe of said engine, to output a detection signal of an air-fuel ratio, the air-fuel ratio sensor includes comprising:

a gas diffusion layer to which engine exhaust gas is introduced;

a rich/lean detecting section that detects whether an air-fuel ratio in said gas diffusion layer is richer or leaner than a stoichiometric air-fuel ratio; and

a pump section that is applied with a voltage based on the detection result of said rich/lean detecting section, to migrate oxygen ions in said gas diffusion layer;

a control section that receives the detection signal from said air-fuel ratio sensor, and that outputs an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

a diagnosing section that forcibly changes said target air-fuel ratio during an air-fuel ratio feedback control by said control section, that diagnoses a response characteristic of said air-fuel ratio sensor based on a change in detection signal from said air-fuel ratio sensor to the change in said target air-fuel ratio, and that outputs a signal indicating a diagnosis result; and

a reset section that stores a value of air-fuel ratio feedback control signal immediately before the starting of diagnosis by said diagnosis section, and that resets said air-fuel ratio feedback control signal to said stored value when the diagnosis is finished.

11. An air-fuel ratio feedback control apparatus for an internal combustion engine, comprising:

air-fuel ratio detecting means disposed in an exhaust pipe of said engine, for outputting a detection signal of an air-fuel ratio;

control means for receiving the detection signal from said air-fuel ratio detecting means, and for outputting an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

diagnosing means for forcibly changing said target air-fuel ratio during an air-fuel ratio feedback control by said control means, for diagnosing a response characteristic of said air-fuel ratio detecting means based on a change in detection signal from said air-fuel ratio detecting means to the change in said target air-fuel ratio, and for outputting a signal indicating a diagnosis result;

storing means for storing a value of the air-fuel ratio feedback control signal immediately before the starting of diagnosis by said diagnosing means; and

reset means for returning said target air-fuel ratio to a normal value when the diagnosis by said diagnosing means is finished, and for simultaneously resetting said air-fuel ratio feedback control signal to said value stored in said storing means immediately before the starting of the diagnosis.

12. An air-fuel ratio feedback control method for an internal combustion engine, comprising the steps of:

detecting an air-fuel ratio based on a detection signal from an air-fuel ratio sensor disposed in an exhaust pipe of said engine;

outputting an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

forcibly changing said target air-fuel ratio during an air-fuel ratio feedback control;

diagnosing a response characteristic of said air-fuel ratio sensor based on a change in detection signal to the change in said target air-fuel ratio;

storing a value of the air-fuel ratio feedback control signal in just immediately before the starting of diagnosis;

returning said target air-fuel ratio to a normal value when the step of diagnosis is finished; and

resetting said air-fuel ratio feedback control signal to said stored value when the step of diagnosing is finished.

13. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , further comprising the step of;

changing a value of a feedback gain to be larger than a normal value when the response characteristic of said air-fuel ratio sensor is diagnosed.

14. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

measuring a time until the detection signal from said air-fuel ratio sensor indicates a predetermined change in air-fuel ratio after said target air-fuel ratio is forcibly changed;

comparing said measured time with a predetermined time; and

outputting a signal indicating response deterioration when said measured time exceeds the predetermined time.

15. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

measuring a time of after said target air-fuel ratio is forcibly changed until the detection signal from said air-fuel ratio sensor converges in a value equivalent to the post-changed target air-fuel ratio;

comparing said measured time with a predetermined time; and

outputting a signal indicating response deterioration when said measured time exceeds the predetermined time.

16. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

measuring a time until the detection signal from said air-fuel ratio sensor passes over a value equivalent to a predetermined air-fuel ratio after said target air-fuel ratio is forcibly changed;

comparing said measured time with a predetermined time; and

outputting a signal indicating response deterioration when said measured time exceeds the predetermined time.

17. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

measuring a change speed in detection signal from said air-fuel ratio sensor after said target air-fuel ratio is forcibly changed;

comparing said measured change speed with a predetermined speed; and

outputting a signal indicating response deterioration when said measured change speed is below said predetermined speed.

18. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , further comprising the steps of:

prohibiting the air-fuel ratio feedback control, when response deterioration of said air-fuel ratio sensor is diagnosed.

19. An air-fuel ratio feedback control method for an internal combustion engine according to claim 12 , wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

periodically changing said target air-fuel ratio stepwise during the air-fuel ratio feedback control;

measuring a response time of said detection signal at a time when said target air-fuel ratio is changed stepwise to a richer side and at a time when said

target air-fuel ratio is changed stepwise to a leaner side;

comparing said response time with a predetermined time; and

outputting a signal indicating response deterioration when said response time exceeds the predetermined time.

20. An air-fuel ratio feedback control method for an internal combustion engine, comprising the steps of:

detecting an air-fuel ratio based on a detection signal from an air-fuel ratio sensor disposed in an exhaust pipe of said engine, wherein an exhaust purification catalyst is disposed downstream of said air-fuel ratio sensor in said exhaust pipe;

outputting an air-fuel ratio feedback control signal so that the air-fuel ratio detected based on the detection signal is brought into a target air-fuel ratio;

forcibly changing said target air-fuel ratio during an air-fuel ratio feedback control;

diagnosing a response characteristic of said air-fuel ratio sensor based on a change in detection signal to the change in said target air-fuel ratio, wherein said step of diagnosing the response characteristic of said air-fuel ratio sensor comprises the steps of:

judging whether or not a predetermined time or above has elapsed from the starting of engine operation;

judging whether or not said air-fuel ratio sensor is activated;

judging whether or not said exhaust purification catalyst is activated; and

outputting a diagnosis permission signal when the predetermined time or above has elapsed from the starting of engine operation, and said air-fuel ratio sensor is activated, and said exhaust purification catalyst is activated;

storing a value of the air-fuel ratio feedback control signal immediately before the starting of diagnosis; and

resetting said air-fuel ratio feedback control signal to said stored value when the diagnosis is finished.