Air-fuel ratio control system for internal combustion engines

Claim 1

An air-fuel ratio control system for use in an internal combustion engine including an intake passage and exhaust passage, comprising: (a) a sensor provided in said exhaust passage of said engine for detecting the arrival of the air-fuel ratio of a mixture supplied to said engine from said intake passage at its stoichiometric level in response to the concentration of one ingredient of exhaust gases flowing thereover; (b) fuel supply means for supplying a controlled amount of fuel into said intake passage of said engine; (c) control means associated with said sensor for providing first and second control signals to said fuel supply means so as to control the air-fuel ratio of said mixture, said first control signal occurring for a predetermined period of time after the air-fuel ratio arrives at the stoichiometric level to cause the air-fuel ratio to vary away from the stoichiometric level to the rich side, and said second control signal occurring with the lapse of the predetermined period of time to cause the air-fuel ratio to vary toward the stoichiometric level, thereby controlling the average air-fuel ratio on the rich side of stoichiometric, said control means comprising: a Schmitt trigger circuit for providing a trigger pulse each time said sensor detects the arrival of the air-fuel ratio at the stoichiometric level; a timer circuit, responsive to said trigger pulse, for providing a high output held for a predetermined period of time; an integrating circuit; a sample-hold circuit for sampling the output of said integrating circuit when applied with a trigger pulse from said trigger circuit and holding the output of said integrating circuit when applied with no trigger pulse; a level shifter circuit for shifting the output of said sample-hold circuit up by a predetermined level; said integrating circuit, responsive to a high input from said timer circuit, for integrating the output of said sample-hold circuit relative to the output of said level shifter circuit to provide said second control signal and, responsive to a low input therefrom, for passing the output of said level shifter circuit to provide said first control signal.