Air-fuel ratio control system for internal combustion engine

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

• a catalytic converter;
• a first sensor, positioned upstream of said catalytic converter, for producing a first air-fuel indicative signal based on a preselected component which is contained in an exhaust gas upstream of said catalytic converter;
• a second sensor, positioned downstream of said catalytic converter, for producing a second air-fuel indicative signal based on a preselected component which is contained in an exhaust gas downstream of said catalytic converter;
• determining means for monitoring said first air-fuel indicative signal, for determining whether an input air-fuel ratio corresponding to a mixture of gas to be fed into an engine cylinder of said internal combustion engine is rich or lean relative to a target air-fuel ratio, and for generating an output signal based on said determination;
• control means for adjusting said input air-fuel ratio based on said output signal, said control means using a first feedback control constant to make said air-fuel ratio richer and said control means using a second feedback control constant to make said air-fuel ratio leaner;
• deriving means for determining whether said second air-fuel indicative signal represents a lean value or a rich value relative to a target air-fuel ratio indicative value and for deriving a correction amount to have a first sign when said second air-fuel indicative signal represents said lean value and a second sign, opposite to said first sign, when said second air-fuel indicative signal represents said rich value, wherein an absolute value of said correction amount increases as said second air-fuel indicative signal deviates more from said target air-fuel ratio indicative value within a given range across said target air-fuel ratio indicative value;
• and
• correction means for correcting said first feedback control constant using said correction amount when said second air-fuel indicative signal represents said lean value, and for correcting said second feedback control constant using said correction amount when said second air-fuel indicative signal represents said rich value.

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

• a catalytic converter;
• a first sensor, positioned upstream of said catalytic converter, for producing a first air-fuel indicative signal based on a preselected component which is contained in an exhaust gas upstream of said catalytic converter;
• a second sensor, positioned downstream of said catalytic converter, for producing a second air-fuel indicative signal based on a preselected component which is contained in an exhaust gas downstream of said catalytic converter;
• determining means for monitoring said first air-fuel indicative signal, for determining whether an input air-fuel ratio corresponding to a mixture of gas to be fed into an engine cylinder of said internal combustion engine is rich or lean relative to a target air-fuel ratio, and for generating an output signal based on said determination, wherein said determining means includes additional determining means for determining one of a first condition and a second condition based on a comparison between said first air-fuel indicative signal and a reference value which is indicative of said target air-fuel ratio, said first condition indicating a rich air-fuel ratio and said second condition indicating a lean air-fuel ratio, said determining means further including means for delaying an amount of time required to make a determination of said air-fuel ratio being rich or lean when said determination of said additional determining means changes between said first and second conditions;
• control means for adjusting said input air-fuel ratio based on said output signal, said control means using a first feedback control constant to make said air-fuel ratio richer and said control means using a second feedback control constant to make said air-fuel ratio leaner;
• deriving means for determining whether said second air-fuel indicative signal represents a lean value or a rich value relative to a target air-fuel ratio indicative value and for deriving a correction amount based on said determination;
• and
• correction means for correcting said first feedback control constant using said correction amount when said second air-fuel indicative signal represents said lean value, and for correcting said second feedback control constant using said correction amount when said second air-fuel indicative signal represents said rich value.