Air-fuel ratio feedback control system including at least downstreamside air-fuel ratio sensor

A method for controlling an air-fuel ratio in an internal combustion engine having a catalyst converter for removing pollutants in the exhaust gas thereof, and a downstream-side air-fuel ratio sensor disposed downstream of said catalyst converter, for detecting a concentration of a specific component in the exhaust gas, comprising the steps of:

• determining whether said engine is in an air-fuel ratio feedback control state for a stoichiometric air-fuel ratio by said downstream-side air-fuel ratio sensor or in an open loop control state for said downstream-side air-fuel ratio sensor;
• calculating an air-fuel ratio correction amount in accordance with the output of said air-fuel ratio sensor when said engine is in said air-fuel ratio feedback control state;
• determining whether or not a switching from the rich side to the lean side and a switching from the lean side occurs in the output of said downstream-side air-fuel ratio sensor;
• changing the air-fuel ratio correction amount in accordance with the output of said upstream-side and said downstream-side air-fuel ratio sensors;
• increasing the speed of change of said air-fuel ratio correction amount in accordance with the output of said downstream-side air-fuel ratio sensor for a predetermined time after a switching occurs in the output of said downstream-side air-fuel ratio sensor only the first time after said engine is switched from said open loop control state to said air-fuel ratio feedback control state;
• and
• adjusting an actual air-fuel ratio in accordance with said air-fuel ratio correction amount.

An apparatus for controlling an air-fuel ratio in an internal combustion engine having a catalyst converter for removing pollutants in the exhaust gas thereof, and upstream-side and downstream-side air-fuel ratio sensors disposed upstream and downstream, respectively, of said catalyst converter, for detecting a concentration of a specific component in the exhaust gas, comprising:

• means for determining whether said engine is in an air-fuel ratio feedback control state for a stoichiometric air-fuel ratio by said downstream-side air-fuel ratio sensor or in an open loop control state for said upstream-side and downstream-side air-fuel ratio sensors;
• means for calculating an air-fuel ratio correlation amount in accordance with the outputs of said upstream-side and downstream-side air-fuel ratio sensors when said engine is in said air-fuel ratio feedback control state;
• means for determining whether or not a switching from the rich side to the lean side and switching from the lean side occurs in the output of said downstream-side air-fuel ratio sensor ;
• means for changing the air-fuel ratio correction amount in accordance with the output of said upstream-side and said downstream-side air-fuel ratio sensors;
• means for increasing the speed of changing said air-fuel ratio correction amount in accordance with the output of said downstream-side air-fuel ratio sensor for a predetermined time after a switching occurs in the output of said downstream-side air-fuel ratio sensor only the first time after said engine is switched from said open loop control state to said air-fuel ratio feedback control state;
• and
• means for adjusting an actual air-fuel ratio in accordance with said air-fuel ratio correction amount.

An apparatus for controlling an air-fuel ratio in an internal combustion engine having a catalyst converter for removing pollutants in the exhaust gas thereof, and a downstream-side air-fuel ratio sensor disposed downstream of said catalyst converter, for detecting a concentration of a specific component in the exhaust gas, comprising:

• means for determining whether said engine is in an air-fuel ratio feedback control state for a stoichiometric air-fuel ratio by said downstream-side air-fuel ratio sensor or in an open loop control state for said downstream-side air-fuel ratio sensor;
• means for calculating an air-fuel ratio correction amount in accordance with the output of said air-fuel ratio sensor when said engine is in said air-fuel ratio feedback control state;
• means for determining whether or not a switching from the rich side to the lean side and a switching from the lean side occurs in the output of said downstream-side air-fuel ratio sensor;
• means for changing the air-fuel ratio correction amount in accordance with the output of said upstream-side and said downstream-side air-fuel ratio sensors;
• means for increasing the speed of change of said air-fuel ratio correction amount in accordance with the output of said downstream-side air-fuel ratio sensor for a predetermined time after a switching occurs in the output of said downstream-side air-fuel ratio sensor only the first time after said engine is switched from said open loop control state to said air-fuel ratio feedback control state;
• and
• means for adjusting an actual air-fuel ratio in accordance with said air-fuel ratio correction amount.