Exhaust purification apparatus for internal combustion engine

1. An exhaust purification apparatus for an internal combustion engine equipped with fuel injection means for injecting fuel in a quantity changed in accordance with operating condition of the engine, comprising:

a catalyst unit disposed in an exhaust passage of the engine;

an upstream air-fuel-ratio sensor disposed in the exhaust passage to be located upstream of the catalyst unit;

a downstream air-fuel-ratio sensor disposed in the exhaust passage to be located downstream of the catalyst unit;

main-feedback control means for calculating a main-feedback control quantity on the basis of an output value of the upstream air-fuel-ratio sensor and a predetennined upstream-side target value, and correcting, on the basis of the main-feedback control quantity, the quantity of fuel injected by the fuel injection means, to thereby feedback-control the air-fuel ratio of the engine; and

sub-feedback control means for calculating a sub-feedback control quantity on the basis of an output value of the downstream air-fuel-ratio sensor and a predetermined downstream-side target value, and correcting, on the basis of the sub-feedback control quantity, the quantity of fuel injected by the fuel injection means, independently of the correction of the fuel injection quantity by the main-feedback control means, to thereby feedback-control the air-fuel ratio of the engine.

2. An exhaust purification apparatus for an internal combustion engine according to claim 1 , wherein the main-feedback control means is configured to calculate the main-feedback control quantity on the basis of a value obtained through performance of high-pass filter processing on a value determined on the basis of the deviation of the output value of the upstream air-fuel-ratio sensor from the predetermined upstream-side target value.

3. An exhaust purification apparatus for an internal combustion engine equipped with fuel injection means for injecting fuel in a quantity changed in accordance with operating condition of the engine, comprising:

a catalyst unit disposed in an exhaust passage of the engine;

an upstream air-fuel-ratio sensor disposed in the exhaust passage to be located upstream of the catalyst unit;

a downstream air-fuel-ratio sensor disposed in the exhaust passage to be located downstream of the catalyst unit;

main-feedback control means for calculating a main-feedback control quantity on the basis of a value obtained through performance of high-pass-filter processing on an output value of the upstream air-fuel-ratio sensor, and correcting, on the basis of the main-feedback control quantity, the quantity of fuel injected by the fuel injection means, to thereby feedback-control the air-fuel ratio of the engine; and

sub-feedback control means for calculating a sub-feedback control quantity on the basis of an output value of the downstream air-fuel-ratio sensor and a predetermined downstream-side target value, and correcting, on the basis of the sub-feedback control quantity, the quantity of fuel injected by the fuel injection means, independently of the correction of the fuel injection quantity by the main-feedback control means, to thereby feedback-control the air-fuel ratio of the engine.

4. An exhaust purification apparatus for an internal combustion engine according to claim 2 , wherein the sub-feedback control means is configured to calculate the sub-feedback control quantity on the basis of a value obtained through performance of low-pass filter processing on a value determined on the basis of the deviation of the output value of the downstream air-fuel-ratio sensor from the predetermined downstream-side target value, or on the basis of a value determined on the basis of the deviation, from the predetermined downstream-side target value, of a value obtained through performance of low-pass filter processing on the output value of the downstream air-fuel-ratio sensor.

5. An exhaust purification apparatus for an internal combustion engine according to claim 3 , wherein the sub-feedback control means is configured to calculate the sub-feedback control quantity on the basis of a value obtained through performance of low-pass filter processing on a value determined on the basis of the deviation of the output value of the downstream air-fuel-ratio sensor from the predetermined downstream-side target value, or on the basis of a value determined on the basis of the deviation, from the predetermined downstream-side target value, of a value obtained through performance of low-pass filter processing on the output value of the downstream air-fuel-ratio sensor.

6. An exhaust purification apparatus for an internal combustion engine according to claim 2 , further comprising factor acquisition means for acquiring a value of a factor which affects the flow rate of exhaust gas flowing through the exhaust passage, wherein

the main-feedback control means is configured to change the cutoff frequency of the high-pass filter processing in accordance with the acquired value of the factor.

7. An exhaust purification apparatus for an internal combustion engine according to claim 3 , further comprising factor acquisition means for acquiring a value of a factor which affects the flow rate of exhaust gas flowing through the exhaust passage, wherein

the main-feedback control means is configured to change the cutoff frequency of the high-pass filter processing in accordance with the acquired value of the factor.

8. An exhaust purification apparatus for an internal combustion engine according to claim 2 , further comprising deterioration-index-value acquisition means for acquiring a deterioration index value which represents the degree of deterioration of the catalyst unit,

wherein the main-feedback control means is configured to change the cutoff frequency of the high-pass filter processing in accordance with the acquired deterioration index value.

9. An exhaust purification apparatus for an internal combustion engine according to claim 3 , further comprising deterioration-index-value acquisition means for acquiring a deterioration index value which represents the degree of deterioration of the catalyst unit,

wherein the main-feedback control means is configured to change the cutoff frequency of the high-pass filter processing in accordance with the acquired deterioration index value.

10. An exhaust purification apparatus for an internal combustion engine according to claim 4 , further comprising factor acquisition means for acquiring a value of a factor which affects the flow rate of exhaust gas flowing through the exhaust passage,

wherein the main-feedback control means and the sub-feedback control means are configured to change the cutoff frequency of the high-pass filter processing and the cutoff frequency of the low-pass filter processing, respectively, in accordance with the acquired value of the factor.

11. An exhaust purification apparatus for an internal combustion engine according to claim 5 , further comprising factor acquisition means for acquiring a value of a factor which affects the flow rate of exhaust gas flowing through the exhaust passage,

wherein the main-feedback control means and the sub-feedback control means are configured to change the cutoff frequency of the high-pass filter processing and the cutoff frequency of the low-pass filter processing, respectively, in accordance with the acquired value of the factor.

12. An exhaust purification apparatus for an internal combustion engine according to claim 4 , further comprising deterioration-index-value acquisition means for acquiring a deterioration index value which represents the degree of deterioration of the catalyst unit,

wherein the main-feedback control means and the sub-feedback control means are configured to change the cutoff frequency of the high-pass filter processing and the cutoff frequency of the low-pass filter processing, respectively, in accordance with the acquired deterioration index value.

13. An exhaust purification apparatus for an internal combustion engine according to claim 5 , further comprising deterioration-index-value acquisition means for acquiring a deterioration index value which represents the degree of deterioration of the catalyst unit,

wherein the main-feedback control means and the sub-feedback control means are configured to change the cutoff frequency of the high-pass filter processing and the cutoff frequency of the low-pass filter processing, respectively, in accordance with the acquired deterioration index value.

14. An exhaust purification apparatus for an internal combustion engine according to claim 2 , wherein

the sub-feedback control means is configured to calculate the sub-feedback control quantity on the basis of at least a time-integration value of a value determined on the basis of the deviation of the output value of the downstream air-fuel-ratio sensor from the predetermined downstream-side target value; and

the main-feedback control means includes dead-band setting means for setting its output value to zero when an input value falls within a predetermined range including zero, and for setting the output value to a value corresponding to the input value when the input value falls outside the predetermined range, wherein the main-feedback control means calculates an integral-processed main-feedback control quantity on the basis of a time-integration value of the output value of the dead-band setting means at the time when a value determined on the basis of the deviation of the output value of the upstream air-fuel-ratio sensor from the predetermined upstream-side target value is applied as the input value of the dead-band setting means, and the main-feedback control means corrects the quantity of fuel injected by the fuel injection means, on the basis of a regular main-feedback control quantity obtained by adding the integral-processed main-feedback control quantity to the main-feedback control quantity, to thereby feedback-control the air-fuel ratio of the engine.

15. An exhaust purification apparatus for an internal combustion engine according to claim 3 , wherein

the sub-feedback control means is configured to calculate the sub-feedback control quantity on the basis of at least a time-integration value of a value determined on the basis of the deviation of the output value of the downstream air-fuel-ratio sensor from the predetermined downstream-side target value; and

the main-feedback control means includes dead-band setting means for setting its output value to zero when an input value falls within a predetermined range including zero, and for setting the output value to a value corresponding to the input value when the input value falls outside the predetermined range, wherein the main-feedback control means calculates an integral-processed main-feedback control quantity on the basis of a time-integration value of the output value of the dead-band setting means at the time when a value determined on the basis of the deviation of the output value of the upstream air-fuel-ratio sensor from the predetermined upstream-side target value is applied as the input value of the dead-band setting means, and the main-feedback control means corrects the quantity of fuel injected by the fuel injection means, on the basis of a regular main-feedback control quantity obtained by adding the integral-processed main-feedback control quantity to the main-feedback control quantity, to thereby feedback-control the air-fuel ratio of the engine.

16. An exhaust purification apparatus for an internal combustion engine according to claim 1 , wherein the main-feedback control means is configured to change the predetermined upstream-side target value in accordance with the sub-feedback control quantity calculated by the sub-feedback control means.