Air-fuel ratio control device for an internal combustion engine

An air-fuel ratio control device for an internal combustion engine comprising:

• a catalytic converter disposed in an exhaust gas passage of an engine;
• an upstream air-fuel ratio sensor disposed in the exhaust gas passage upstream of the catalytic converter for detecting an air-fuel ratio of the exhaust gas upstream of the catalytic converter;
• a downstream air-fuel ratio sensor disposed in the exhaust passage downstream of the catalytic converter for detecting the air-fuel ratio of the exhaust gas downstream of the catalytic converter;
• first air-fuel ratio control means for setting the value of a first air-fuel ratio correction factor in accordance with the value of a second air-fuel ratio correction factor and the output of the upstream air-fuel ratio sensor;
• second air-fuel ratio control means for setting the value of the second air-fuel ratio correction factor in accordance with the output of the downstream air-fuel ratio sensor;
• learning correction means for performing a learning correction of the first air-fuel ratio correction factor by adjusting the value of a learning correction factor in such a manner that a center value of the fluctuation of the first air-fuel ratio correction factor agrees with a predetermined reference value;
• fuel supply control means for controlling the amount of fuel supplied to the engine in accordance with the values of said first air-fuel ratio correction factor and said learning correction factor;
• determining means for determining whether the learning correction by the learning correction means has been completed;
• and
• transient control means for controlling said second air-fuel ratio control means in such a manner that the rate of change in the value of the second air-fuel ratio correction factor becomes smaller when the learning correction has not completed than after the learning correction has completed.