Abstract: In a double air-fuel sensor system including two air-fuel ratio sensors upstream and downstream of a catalyst converter provided in an exhaust gas passage, an actual air-fuel ratio is adjusted in accordance with the outputs of the upstream-side and downstream-side air-fuel ratio sensors including an air-fuel ratio correction amount. When a change of an air-fuel ratio correction amount or a change of an air-fuel ratio feedback control parameter calculated in accordance with the output of the downstream-side air-fuel ratio sensor is small, a learning correction amount is calculated so that a mean value of the air-fuel ratio correction amount is brought close to a reference value. The actual air-fuel ratio is further adjusted in accordance with the learning correction amount.

Abstract: In a double air-fuel sensor system including two air-fuel ratio sensors upstream and downstream of a catalyst converter provided in an exhaust gas passage, an actual air-fuel ratio is adjusted in accordance with the outputs of the upstream-side air-fuel ratio sensor and the downstream-side air-fuel ratio sensor. The adjustment of the actual air-fuel ratio by the downstream-side air-fuel ratio sensor is prohibited in accordance with a coolant temperature of the engine.

Abstract: In a double air-fuel sensor system including two air-fuel ratio sensors upstream and downstream of a catalyst converter provided in an exhaust gas passage, an air-fuel ratio correction amount is remarkably changed when the output of the upstream-side air-fuel ratio sensor is switched from the lean side to the rich side or vice versa, and the actual air-fuel ratio is adjusted in accordance with the air-fuel ratio correction amount. The remarkable-change speed of the air-fuel ratio correction amount is changed in accordance with the output of the downstream-side air-fuel ratio sensor.

Abstract: An engine with three intake ports and three intake valves for each cylinder. The first intake port is a helical port. The remaining two intake ports have intake control valves, which open when the engine speed and the engine load are large and close when the engine speed and the engine are small. Air thus is introduced alternatively from the first port only or from all the ports. Fuel is injected in the third port between the corresponding intake valve and intake control valve.