Air/fuel ratio control system for automotive vehicle using feedback control

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

• an upstream catalytic device installed in an exhaust pipe of an internal combustion engine;
• a downstream catalytic device installed in the exhaust pipe downstream of said upstream catalytic device;
• a first, a second, and a third air-fuel ratio sensor each working to measure an air-fuel ratio of an exhaust gas flowing through the exhaust pipe, said first air-fuel ratio sensor being disposed upstream of said upstream catalytic device, said second air-fuel ratio sensor being disposed between said upstream and downstream catalytic devices, said third air-fuel ratio sensor being disposed downstream of said downstream catalytic device;
• a first feedback controller working to perform feedback control to bring a value of the air-fuel ratio, as measured by said first air-fuel ratio sensor, into agreement with a target air-fuel ratio;
• and
• a second feedback controller working to sample values of the air-fuel ratios, as measured by said second and third air-fuel ratio sensors, to correct a predetermined controlled parameter in the feedback control of said first feedback controller.