An electrical signal representative of engine speed is derived; a sensor senses the composition of the exhaust gases and provides an electrical signal to an integrating controller which is connected to adjust the air-fuel ratio applied to the internal combustion engine. In accordance with the invention, a switching circuit is connected to the integral controller which is controlled to switch in synchronism with rotation of the engine to command the integrator to integrate in steps dependent on engine speed so that, as the engine speed changes, the integration rate changes.