With a view to controlling the formation of toxic nitrogen oxides (NO.sub.x) in the exhaust gases emitted from an internal combustion engine particularly for automotive use, an exhaust-gas recirculation system has been developed by means of which the exhaust gases delivered from the power cylinders of the engine are recirculated in regulated proportion to the intake system of the engine. The exhaust gases thus recirculated into the intake system of the engine give rise to a decrease in the maximum temperature of combustion in the power cylinders and contribute to reduction in the concentration of the toxic nitrogen oxides in the exhaust gases to be emitted from the power cylinders.
An exhaust-gas recirculation system of this nature is usually designed to remain inoperative when the engine is operating at low temperatures and to start the recirculation of the exhaust gases at a regulated rate after the engine is warmed up. This is because of the fact that the combustible charges supplied to the power cylinders are fired under unstable conditions when the engine is operating cold and accordingly that the recirculation of exhaust gases under such conditions would invite remarkable deterioration of the performance of the engine. The fact is, however, that toxic nitrogen oxides are contained in a not negligible concentration in the exhaust gases emitted from an engine under cold running conditions and, for this reason, it is desirable to effect the recirculation of the exhaust gases not only after the engine is warmed but when the engine is being warmed up.
It is, accordingly, an important object of the present invention to provide a method of controlling the exhaust-gas recirculation rate in an automotive internal combustion engine equipped with an exhaust-gas recirculation system.