Closed-loop mixture control system for an internal combustion engine with improved response characteristic to idling condition

Claim 1

A mixture control system for internal combustion engines of the type including air-fuel supply means for supplying air and fuel for combustion, and exhaust means for carrying away exhaust gases after combustion, comprising: an exhaust gas sensor for detecting the deviation from a reference value of the concentration of a composition in said exhaust gases determined by said air-fuel supply means; means for generating a control signal varying proportionally to the magnitude of the deviation of said concentration from said reference value in a direction opposite to the direction of said deviation and for controlling said air-fuel supply means so that the ratio of air to fuel is controlled at a predetermined value; means for detecting an operating parameter of the engine indicative of an idling condition; and a switched resistor network providing a variable resistance value which increases in response to the detection of said operating parameter of the engine, said network being connected between the output of said comparator and the input to said control signal generating means to thereby reduce the amplitude of said control signal in response to the detection of an idling condition, whereby the time taken for air and fuel supplied to said engine to approach said predetermined value of air to fuel ratio is reduced.