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

Abstract

A fuel injection amount is controlled so that the air-fuel ratio of exhaust gas flowing into a catalyst ( 6 ) oscillates around a stoichiometric air-fuel ratio. When it is estimated that the level of oxygen storage capacity of the catalyst ( 6 ) is lower than a predetermined reference capacity, the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio is reduced by controlling the fuel injection amount. The level of the oxygen storage capacity of the catalyst ( 6 ) is estimated based on the amplitude of the output signal from an oxygen sensor ( 14 ) disposed downstream of the catalyst ( 6 ).

1 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when the amplitude of the signal output from the oxygen sensor is smaller than a predetermined value.   
   
   
       2 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when the amplitude of the signal output from the air-fuel ratio sensor remains smaller than a predetermined value more than a prescribed period.   
   
   
       3 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       4 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by limiting a correction amount based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       5 . An air-fuel ratio control apparatus for an internal combustion engine that includes: a catalyst, disposed in an exhaust passage, which has oxygen storage capacity; an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst; and an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the output signal from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       6 . An air-fuel ratio control apparatus for an internal combustion engine that includes: a catalyst, disposed in an exhaust passage, which has oxygen storage capacity: an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst; and an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the output signal from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio; and   amplitude reduction portion that reduces an amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by limiting a correction amount based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       7 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   amplitude reduction stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       8 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 7 , further comprising
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio remains reduced more than a prescribed period.   
   
   
       9 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 7 , further comprising
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when an accumulated amount of air taken into the internal combustion engine exceeds a prescribed amount after the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio is reduced.   
   
   
       10 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio, and   wherein the amplitude reduction portion reduces the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       11 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio, and   wherein the amplitude reduction portion reduces the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by limiting a correction amount based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       12 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the signal output from the oxygen sensor and a reference value corresponding to the stoichiometric air-fuel ratio, and   wherein the amplitude reduction portion reduces the amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       13 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the signal output from the oxygen sensor and a reference value corresponding to the stoichiometric air-fuel ratio, and   wherein the amplitude reduction portion reduces the amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by limiting a correction amount based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       14 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising,
 amplitude reduction stop portion that stops reducing the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       15 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising,
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio remains reduced more than a prescribed period.   
   
   
       16 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 1 , further comprising,
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when an accumulated amount of air taken into the internal combustion engine exceeds a prescribed amount after the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio is reduced.   
   
   
       17 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   oxygen storage capacity estimation portion that estimates that a level of the oxygen storage capacity of the catalyst is lower than the predetermined reference capacity when the amplitude of the signal output from the oxygen sensor is smaller than a predetermined value.   
   
   
       18 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   oxygen storage capacity estimation portion that estimates that a level of the oxygen storage capacity of the catalyst is lower than the predetermined reference capacity when the amplitude of the signal output from the air-fuel ratio sensor remains smaller than a predetermined value more than a predetermined period.   
   
   
       19 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when the amplitude of the signal output from the oxygen sensor is smaller than a predetermined value.   
   
   
       20 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, and an air-fuel ratio sensor that outputs a signal according to the air-fuel ratio of the exhaust gas flowing into the catalyst, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when the amplitude of the signal output from the air-fuel ratio sensor remains smaller than a predetermined value more than a prescribed period.   
   
   
       21 . An air-fuel ratio control apparatus for an internal combustion engine that includes a catalyst, disposed in an exhaust passage, which has oxygen storage capacity, comprising:
 fuel injection amount control portion that controls an amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio; and   oxygen storage capacity estimation portion that estimates that a level of the oxygen storage capacity of the catalyst is lower than the predetermined reference capacity when the amplitude of air-fuel ratio of exhaust gas flowing into the catalyst remains smaller than a predetermined value more than a predetermined period.   
   
   
       22 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 ,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio; and   wherein the amplitude reduction portion reduces the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       23 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 ,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio; and   wherein the amplitude reduction portion reduces the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio on the feedback control by limiting a correction amount based on the difference between the signal output from the air-fuel ratio sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       24 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 , further comprising an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the output signal from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio; and   wherein the amplitude reduction portion reduces the amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by reducing a gain used to correct the amount of fuel injected to the internal combustion engine based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       25 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 , further comprising an oxygen sensor that outputs a signal according to a concentration of oxygen in the exhaust gas that has passed through the catalyst,
 wherein the fuel injection amount control portion controls the amount of fuel injected to the internal combustion engine so that an air-fuel ratio of exhaust gas flowing into the catalyst oscillates around a stoichiometric air-fuel ratio by executing a feedback control on the air-fuel ratio based on a difference between the signal output from the air-fuel ratio sensor and a reference value corresponding to the stoichiometric air-fuel ratio while the fuel injection amount control portion corrects the amount of fuel injected to the internal combustion engine based on a difference between the output signal from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio; and   wherein the amplitude reduction portion reduces the amplitude of the oscillation of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio by limiting a correction amount based on the difference between the signal output from the oxygen sensor and the reference value corresponding to the stoichiometric air-fuel ratio.   
   
   
       26 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 , further comprising,
 amplitude reduction stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       27 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 3 , further comprising,
 amplitude reduction stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       28 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 4 , further comprising,
 amplitude recution stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       29 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 5 , further comprising,
 amplitude reduction stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       30 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 6 , further comprising,
 amplitude reduction stop portion that stops reducing an amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio when one of fuel supply cutoff control and fuel injection amount increase control is executed.   
   
   
       31 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 , further comprising,
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio remains reduced more than a prescribed period.   
   
   
       32 . The air-fuel ratio control apparatus for the internal combustion engine according to  claim 2 , further comprising,
 execution frequency increase portion that increases at least one of an execution frequency of the fuel supply cutoff control and an execution frequency of the fuel injection amount increase control when an accumulated amount of air taken into the internal combustion engine exceeds a prescribed amount after the amplitude of the air-fuel ratio of the exhaust gas which oscillates around the stoichiometric air-fuel ratio is reduced.