Air-fuel mixture control for automobile engine having fuel injection system

An air-fuel ratio control system for an internal combustion engine of a type having a fuel injection system, said control system comprising, in combination:

• a fuel intake passage having an injection nozzle for injecting a mist of fuel thereinto for the preparation of a combustible mixture in admixture with incoming air flowing therethrough and a throttle valve;
• an exhaust passage means for the discharge of exhaust gases from the engine towards the atmosphere;
• an EGR passage means for providing communication between the intake passage means and the exhaust passage means for recirculating a portion of the exhaust gases from the exhaust passage means into the intake passage means;
• an EGR control valve means disposed in said EGR passage means for controlling the rate of flow of that portion of the exhaust gases;
• a composition sensor disposed in the exhaust passage means for detecting the concentration of a component of the exhaust gases which is a function of the air-fuel mixing ratio of the combustible mixture burned in the engine;
• a fuel adjusting means for controlling the amount of fuel to be injected into the intake passage means in dependence on the rate of flow the incoming air through the intake passage means, said rate of flow of the incoming air being detected by a combination of at least two of parameters including the engine speed, the negative pressure developed inside the intake passage means at a position downstream of the throttle valve with respect to the direction of flow of the combustible mixture towards the engine and the opening of the throttle valve, said fuel adjusting means being so adjusted that the air-fuel mixing ratio of the combustible mixture to be supplied to the engine when the engine operates within a particular operating region is lower than that to be supplied to the engine when the latter operates within the other operating region than said particular operating region;
• a first EGR control means for controlling the EGR control valve means according to a control value stored in a memory means when the engine operates within the particular operating region;
• and
• a second EGR control means operable when the engine operates within said other operating region for controlling the EGR control valve means in dependence on the concentration of the exhaust gas component detected by the composition sensor.