Air-fuel ratio control apparatus for internal combustion engine

Abstract

The output from an air-fuel ratio sensor is corrected using an integral term that is calculated by integrating values of deviation of the output from an oxygen sensor with respect to a reference output that would be obtained when the combustion air-fuel ratio is stoichiometric, when an engine operates with a target combustion air-fuel ratio set to the stoichiometric air-fuel ratio. A fuel-cutoff prohibition period in which fuel supply cutoff is prohibited is set so that the engine continues to operate with the target combustion air-fuel ratio set to the stoichiometric air-fuel ratio, until the integral term is updated at least once. The integral term may be updated when a predetermined updating condition is satisfied.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of the invention will become more apparent from the following description of example embodiments with reference to the accompanying drawings, in which the same or corresponding portions are denoted by the same reference numerals and wherein:

FIG. 1 is a schematic diagram showing the exhaust system for an internal combustion engine controlled by an air-fuel ratio control apparatus according to the invention;

FIG. 2 is a time-chart showing a target combustion air-fuel ratio for the air-fuel ratio control apparatus according to the invention;

FIG. 3 is a first flowchart showing a routine executed by the air-fuel ratio control apparatus according to the invention; and

FIG. 4 is a second flowchart showing another routine executed by the air-fuel ratio control apparatus according to the invention.

Claims

1. An air-fuel ratio control apparatus for an internal combustion engine, which includes an air-fuel ratio sensor that is disposed upstream of a three-way catalyst device provided in an engine exhaust system, and an output from which changes according to an air-fuel ratio of exhaust gas; and an oxygen sensor that is disposed downstream of the three-way catalyst device, and an output from which sharply changes when the air-fuel ratio of the exhaust gas approaches a stoichiometric air-fuel ratio, wherein the air-fuel ratio control apparatus controls a combustion air-fuel ratio to a desired air-fuel ratio, based on the output from the air-fuel ratio sensor, wherein: the output from the air-fuel ratio sensor is corrected using an integral term, which is calculated by integrating values of deviation of the output from the oxygen sensor with respect to a reference output that would be obtained when the combustion air-fuel ratio is stoichiometric, when the engine operates with a target combustion air-fuel ratio set to the stoichiometric air-fuel ratio;the integral term is updated when a predetermined updating condition is satisfied; anda fuel-cutoff prohibition period in which fuel supply cutoff is prohibited is set so that the engine continues to operate with the target combustion air-fuel ratio set to the stoichiometric air-fuel ratio, until the integral term is updated at least once.

2. The air-fuel ratio control apparatus according to claim 1, wherein the fuel-cutoff prohibition period continues until the integral term substantially converges to a value appropriate for correcting the deviation of the output from the air-fuel ratio sensor as a result of updating the integral term a plurality of times.

3. The air-fuel ratio control apparatus according to claim 1, wherein the fuel-cutoff prohibition period continues until an absolute value of a difference between a newly calculated integral term and a current integral term is smaller than a predetermined value.

4. The air-fuel ratio control apparatus according to claim 1, wherein, after the fuel-cutoff prohibition period ends, updating of the integral term is prohibited.

5. The air-fuel ratio control apparatus according to claim 1, wherein, after the fuel-cutoff prohibition period ends, the integral term is updated when an amount by which the integral term has changed exceeds a prescribed value.

6. The air-fuel ratio control apparatus according to claim 1, wherein, after the fuel-cutoff prohibition period ends, a current integral term is updated to a newly calculated integral term only when an absolute value of a difference between the newly calculated integral term and the current integral term exceeds the prescribed value.

7. An air-fuel ratio control method for an internal combustion engine, comprising: calculating an integral term by integrating values of deviation of an output from an oxygen sensor, disposed downstream of a three-way catalyst device provided in an engine exhaust system, with respect to a reference output that would be obtained when a combustion air-fuel ratio is stoichiometric;correcting an output from an air-fuel ratio sensor, disposed upstream of the three-way catalyst device, using the integral term when an engine operates with a target combustion air-fuel ratio set to the stoichiometric air-fuel ratio;controlling the combustion air-fuel ratio to a desired air-fuel ratio based on the corrected output from the air-fuel ratio sensor;updating the integral term when a predetermined updating condition is satisfied; andsetting a fuel-cutoff prohibition period in which fuel supply cutoff is prohibited so that the engine continues to operate with the target combustion air-fuel ratio set to the stoichiometric air-fuel ratio until the integral term is updated at least once,wherein, the output from the air-fuel ratio sensor changes according to an air-fuel ratio of exhaust gas, and the output from the oxygen sensor sharply changes when the air-fuel ratio of the exhaust gas approaches a stoichiometric air-fuel ratio.

8. The air-fuel ratio control method according to claim 7, wherein the fuel-cutoff prohibition period continues until the integral term substantially converges to a value appropriate for correcting the deviation of the output from the air-fuel ratio sensor as a result of updating the integral term a plurality of times.

9. The air-fuel ratio control method according to claim 8, wherein the fuel-cutoff prohibition period continues until an absolute value of a difference between a newly calculated integral term and a current integral term is smaller than a predetermined value.

10. The air-fuel ratio control method according to claim 7, wherein, after the fuel-cutoff prohibition period ends, updating of the integral term is prohibited.

11. The air-fuel ratio control method according to claim 7, wherein, after the fuel-cutoff prohibition period ends, the integral term is updated when an amount by which the integral term has changed exceeds a prescribed value.

12. The air-fuel ratio control method according to claim 11, wherein, after the fuel-cutoff prohibition period ends, a current integral term is updated to a newly calculated integral term only when an absolute value of a difference between the newly calculated integral term and the current integral term exceeds the prescribed value.