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An internal combustion engine includes an intake system, an exhaust system, and a mixture supply device which supplies primary fuel-air mixture into the intake system. An oxygen sensor assembly detects oxygen concentration at a certain point in the exhaust system and produces a first electrical sign

An internal combustion engine includes an intake system, an exhaust system, and a mixture supply device which supplies primary fuel-air mixture into the intake system. An oxygen sensor assembly detects oxygen concentration at a certain point in the exhaust system and produces a first electrical signal which represents this concentration. A secondary air flow control circuit receives this first electrical signal and produces a second electrical signal according thereto. A secondary air supply system feeds secondary air into the exhaust system upstream of the aforementioned certain point at a flow rate controlled by the second electrical signal. A primary air/fuel ratio correction circuit receives either the first or the second electrical signal and based on it produces a primary mixture control electrical signal. This primary mixture control electrical signal is received by a mixture control system, which alters the primary air/fuel ratio of the mixture provided by the mixture supply device in accordance therewith.

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1. For an internal combustion engine comprising: (a) an intake system; (b) an exhaust system; and (c) a mixture supply device, which includes an air bleed system incorporating a bleed air flow control device and supplies primary fuel-air mixture to said intake system, the air/fuel ratio of said

1. For an internal combustion engine comprising: (a) an intake system; (b) an exhaust system; and (c) a mixture supply device, which includes an air bleed system incorporating a bleed air flow control device and supplies primary fuel-air mixture to said intake system, the air/fuel ratio of said primary fuel-air mixture being increased as bleed air flow rate is increased; an air/fuel ratio control system, comprising: (d) a source of secondary air; (e) a secondary air supply system, which includes a secondary air flow rate control device and conducts a controlled flow of secondary air from said source of secondary air to an intermediate portion of said exhaust system; (f) an exhaust gas oxygen sensor, which detects the concentration of oxygen at a measurement point of said exhaust system downstream of said intermediate portion at which the air from said secondary air supply system is supplied into said exhaust system, and which produces a first secondary air flow control electrical signal representative of said concentration of oxygen; (g) a buffer amplifier, which receives said first secondary air flow control electrical signal and produces a second secondary air flow control electrical signal representative of the air/fuel ratio of exhaust gases at said measurement point in said exhaust system; (h) a reference value signal production circuit, which produces a reference value electrical signal representative of a predetermined air/fuel ratio of exhaust gases such as a stoichiometric air/fuel ratio; (i) a comparator, which receives said second secondary air flow control electrical signal and said reference value electrical signal, compares these two electrical signals, and produces a rectangular wave electrical signal which changes between first and second levels according to plus or minus of the balance between said two electrical signals; (j) a secondary air flow control circuit, which receives said rectangular wave electrical signal, and produces a third secondary air flow control electrical signal for controlling said secondary air flow rate control device; (k) a primary air/fuel ratio computing circuit, which receives said rectangular wave electrical signal, and integrates portions of said rectangular wave electrical signal of said first level with a first time constant which provides a predetermined positive tangent of angle x 1 while integrating portions of said rectangular wave electrical signal of said second level with a second time constant which provides a predetermined negative tangent of angle x 2, said first and second time constants being so determined that, when the ratio of duration of said first level portion to the sum of durations of said first and second level portions in one cycle of said rectangular wave electrical signal is a selected value, an amount of integration by said first time constant is exactly cancelled by an amount of integration by said second time constant in one cycle of said rectangular wave electrical signal; and (1) a primary fuel-air mixture control circuit, which controls said bleed air flow control device according to the result of integration by said primary air/fuel ratio computing circuit so as to increase bleed air flow rate from a standard value according to a positive value of said result of integration and to reduce bleed air flow rate according to a negative value of said result of integration. 2. An air/fuel ratio control system according to claim 1, wherein said primary fuel-air mixture control circuit includes a smoothing circuit which produces such an electrical signal that is constant during one cycle of said rectangular wave electrical signal when said result of integration in a cycle just preceding to said one cycle is zero, that gradually increases during one cycle of said rectangular wave electrical signal when said result of integration in a cycle just preceding to said one cycle is positive, and that gradually decreases during one cycle of said rectangular wave electrical signal when said result of integration in a cycle just preceding to said one cycle is negative.