Exhaust gas cleaning device for an internal combustion engine

An exhaust gas cleaning device for controlling an air-to-fuel ratio of an air-fuel mixture supplied to an internal combustion engine including a catalytic converter disposed in an exhaust gas outlet passage of the engine for removing noxious components contained in an exhaust gas of the engine, said exhaust gas cleaning device comprising:

• air-to-fuel ratio sensor means, disposed in the exhaust gas outlet passage of the internal combustion engine, for detecting, from a concentration of a component of the exhaust gas, an air-to-fuel ratio parameter indicative of the air-to-fuel ratio of the air-fuel mixture supplied to the internal combustion engine;
• comparator means, coupled to said air-to-fuel sensor means, for comparing said air-to-fuel ratio parameter with a reference level to determine whether the air-to-fuel is in a rich or a lean state, said comparator means outputting a comparison judgement signal that take two levels indicative of the rich and the lean state of the air-to-fuel ratio, respectively;
• integrator means, coupled to said comparator means, for integrating said comparison judgement signal with predetermined integration characteristics, to output an integrated signal;
• operation condition detector means for detecting an operating condition of the internal combustion engine, to output a corresponding detection signal;
• oscillation signal generating means, coupled to said integrator means and said operating condition detector means, for generating an oscillation signal which oscillates around a central level of the integrated signal of the integrator means, wherein a frequency of the oscillation signal is varied in response to the detection signal of the operating condition detector means in accordance with the operating condition of the engine, a period of the oscillation signal being maintained shorter than an inversion half period of said comparison judgement signal;
• air-to-fuel ratio feedback control means for effecting a feedback control of the air-to-fuel ratio of the air-fuel mixture in accordance with said oscillation signal, and
• reference level modifying means, coupled to said operating condition detector means, for varying and modifying the reference level to which the air-to-fuel ratio parameter is compared by said comparator means, wherein said reference level modifying means modifies said reference level, in response to the detection signal of the operating condition detector means, for a predetermined length of time corresponding to the operating condition of the engine after each inversion of the level of the comparison judgement signal, by a predetermined amount in the reference level in such a direction as to make an inversion of the level of the comparison judgement signal more difficult to take place.