Abstract: In a closed loop air-fuel mixture control system for carburetor-equipped internal combustion engines, an exhaust gas sensor provides a feedback signal to a control unit where the signal is modified to meet the control characteristics of the closed loop. The modified feedback signal is converted into digital pulses whose width varies with the amplitude of the feedback signal. Additional air is supplied to the engine through an air bleed in accordance with the magnitude of the analog feedback signal to provide a coarse control of air-fuel ratio and in response to the digital pulses to provide a fine control of the ratio. The mixture is controlled in a wide range of ratios to eliminate the need for calibration which would be required for fitting the closed loop system to the time-varying characteristics of individual carburetors.

Abstract: An engine operational condition detector detects a mode among several modes, such as an idling state, a steady state and an accelerating state, where a signal is utilized for selectively actuating one of several integrators, each of which has a predetermined time-constant. One of the integrators, when actuated, integrates an output signal of a P-I controller of a closed loop fuel control system, the integrated signal being stored in a capacitor connected across the integrator. Electromagnetic valves disposed in the openings of main air-bleeds of main and slow circuits of a carburetor are energized in accordance with the output signal of each integrator while other electromagnetic valves disposed in the openings of auxiliary air-bleeds of the same are energized in dependence on the output signal of the P-I controller.

Abstract: An exhaust gas recirculation control system for an internal combustion engine, in which a feedback signal representing the amount of actually recirculated exhaust gas is produced by detecting the concentration of either CO.sub.2 or H.sub.2 O, which is contained in the exhaust gas in a nearly constant concentration, in combustible gas mixture diluted with recirculated exhaust gas.

Abstract: A closed-loop emission control apparatus for internal combustion engines includes exhaust composition sensor for feedback control of the air-fuel mixture. When the temperature in the exhaust passage is lower than the operating temperature of the sensor, control is switched to an open-loop mode. The feedback control signal is fluctuated above and below a predetermined DC bias at periodic intervals during the open-loop mode to switch the control to the closed-loop mode in a minimum transition time as soon as the sensor's temperature condition warrants feedback control.