AIR/FUEL RATIO CONTROL SYSTEM FOR AN INTERNAL COMBUSTION ENGINE

A system for controlling an air/fuel ratio of an internal combustion engine, including a feedback control loop having an adaptive controller (STR controller) that receives as inputs a desired value r and a controlled variable y output from the plant (engine) and an adaptation mechanism that estimates adaptive parameters. The STR controller calculates a feedback correction coefficient KSTR, as an output u based upon at least the adaptive parameters, for correcting a basic amount of fuel supply calculated by retrieving mapped data, prepared beforehand, such that the controlled variable y converges to the desired value r. The system is configured such that the controlled variable y is determined based upon the detected air/fuel ratio KACT and a desired air/fuel ratio KCMD so that the desired value r is a predetermined value. More specifically, the controlled variable is determined to be a ratio between the detected air/fuel ratio and the desired air/fuel ratio such that the desired value r is 1.0 or thereabout. With this arrangement, when the desired air/fuel ratio is changed or corrected frequently, the estimation of the adaptive parameters is not affected, thereby improving the stability of air/fuel ratio control.