Abstract: An air-fuel ratio control apparatus for an internal combustion engine which is equipped with an air-fuel ratio sensor for sensing an actual air-fuel ratio of a mixture to be introduced into the engine and a target air-fuel ratio setting section for setting a target air-fuel ratio of the engine. Also included is a controlled-amount calculating section for setting an optimal feedback gain on the basis of a predetermined dynamic model of the engine to calculate a controlled amount in accordance with the predetermined optimal feedback gain so that the actual air-fuel ratio becomes equal to the target air-fuel ratio. A fuel supply amount to the engine is determined on the basis of the calculated controlled amount, and the control responsiveness of the controlled-amount calculating section is suppressed when the engine is in a speed-decreasing state. This arrangement can adequately control the air-fuel ratio of the engine irrespective of the engine speed-decreasing state.

Abstract: Outer air serving as a reference gas for determining oxygen concentration is guided to a hollow portion of a core. The core is formed in an insulating cylinder whose distal end is closed and whose proximal end is open to receive the reference gas. An opening is formed on the outer surface of the core, to communicate with the hollow portion. An oxygen concentration sensor element made of a solid-state electrolyte layer having first and second electrode layers one on each surface thereof is mounted such that it closes the opening. A sheet member made of an insulating sheet is wound around the sensor element mounted on the sensor element, and is fixed thereon. A window is formed in the sheet so as to expose the surface of the sensor element. A heater is formed around the window. Wiring layers respectively connected to both terminals of the heater are formed on the sheet. Wiring layers adapted to contact the first and second electrode layers of the sensor element are also formed on the sheet.