Exhaust gas recirculation control system for an internal combustion engine

An exhaust gas recirculation control system for an internal combustion engien, comprising a passage for conducting exhaust gases to be recirculated, a diaphragm type exhaust gas recirculation control valve having a diaphragm chamber and controlling said passage in accordance with the intake manifold vacuum supplied to said diaphragm chamber, a throttling means provided in said passage upstream of said exhaust gas recirculation control valve as seen in the direction of recirculating gas flow, a vacuum modulator for modulating said intake manifold vacuum supplied to said diaphragm chamber by selective atmospheric air bleeding, said vacuum modulator having a valve chamber through which said atmospheric air bleeding is effected and a diaphragm chamber connected to said passage at a portion located between said exhaust gas recirculation control valve and said throttling means and controlling said atmospheric air bleeding in accordance with the balance between the fluid pressures existing in said valve chamber and said diaphragm chamber connected to said intermediate passage portion, a bleed controlling valve having a diaphragm chamber and controlling atmospheric air bleeding to said valve chamber of said vacuum modulator in accordance with the fluid pressure supplied to said diaphragm chamber of said bleed controlling valve, a pressure control valve for controlling said fluid pressure, a pressure sensor for detecting said fluid pressure, a memory means for retaining target values for said fluid pressure which provide the optimum exhaust gas recirculation in various operating conditions of the engine, said operating conditions of the engine being expressed by the combination of two quantities selected from the rotational speed, amount of intake air and intake manifold vacuum of the engine, means for detecting current values of said two selected quantities, and a control means which reads out a particular target value for said fluid pressure from said memory means in accordance with the current values of said two selected quantities, compares the read-out target value with the value of said fluid pressure detected by said pressure sensor, and controls said pressure control valve so as to accord said fluid pressure to the read-out target value.