In the exhaust gas recirculation systems, it is of paramount importance to properly set the ratio of the flow rate of exhaust gas to be recirculated into the intake system to the amount of air taken into the engine.
It is known that changing the exhaust gas flow rate/air intake ratio (hereinafter referred to as EGR/AI ratio) according to the intake pipe negative pressure is a very effective method, and there are available some systems such as mentioned below for embodying such method: System A in which a variable constriction is provided between the exhaust pipe and pressure control valve and the pressure in a pressure chamber formed between said variable constriction and pressure control valve is maintained constant; System B in which a fixed constriction is provided between the exhaust pipe and pressure control valve and the pressure in a pressure chamber formed between said pressure control valve and fixed constriction is varied in correspondence to the intake pipe negative pressure; System C in which the pressure differential across the fixed constriction provided either upstream or downstream of the pressure control valve is proportioned to the pressure given by the product of the venturi pressure and the value determined according to the intake pipe negative pressure. In the said systems A and B, however, the unstable exhaust pressure is used for determining the EGR/AI ratio, so that in case the exhaust pressure in the low-load area is small and unstable, it is difficult to maintain the EGR/AI ratio controlling precision. On the other hand, system C is unstable in the engines other than those using a carburetor provided with a fixed venturi.