Air fuel ratio control system for an internal combustion engine with an improved open loop mode operation

In an air/fuel ratio control system for an internal combustion engine, the system including a plurality of sensors for sensing engine opertional parameters relating to an engine load, an exhaust gas sensor for sensing a concentration of an exhaust gas component of the internal combustion engine, and an air/fuel ratio controller for controlling an air/fuel ratio of a mixture supplied to the engine in response to an output value determined on the basis of signals from said sensors, the system comprising:

• means connected to said plurality of of sensors for setting a base value of air/fuel ratio control in response to said engine parameters at intervals of a predetermined time period, the base value being obtained from a plurality of predetermined values stored in a first memory means comprising a read only memory;
• means for adjusting said base value in response to a result of comparison between a target air/fuel ratio and a detected air/fuel ratio obtained by an output signal of said exhaust gas sensor to provide said output value;
• means for calculating correction values to compensate for possible errors of said base value each time said output value is produced;
• second memory means for storing each calculated value of said correction values in connection with each value of said plurality of engine operational parameters, said second memory means comprising a random access memory;
• means for detecting a predetermined operational condition of said internal combustion engine;
• and
• means for stopping said adjustment of said base value in response to the output signal of said exhaust gas sensor when said predetermined operational condition is detected, and correcting said base value by a value chosen from said correction values stored in said second memory means to provide said output value, said chosen value corresponding to present values of said plurality of engine operational parameters, wherein said means together form a control unit of the air/fuel ratio.