Abstract: An air-fuel ratio control system for an internal combustion engine includes an air-fuel ratio sensor arranged in an exhaust system thereof for generating an output substantially proportional in value to an air-fuel ratio of exhaust gases emitted from the engine. An amount of variation in combustion of the engine is detected. A desired air-fuel ratio of a mixture supplied to the engine is set to a value leaner than a stoichiometric air-fuel ratio based on the amount of variation in combustion of the engine, when the engine is in a predetermined operating condition. Feedback control of an air-fuel ratio of the mixture to the desired air-fuel ratio is carried out in response to the output from the air-fuel ratio sensor.

Abstract: An air-fuel ratio control system for an internal combustion engine includes an air-fuel ratio sensor arranged in the exhaust system of the engine, for generating an output indicative of the air-fuel ratio of exhaust gases emitted from the engine. An ECU carries out feedback control of controlling the air-fuel ratio of a mixture supplied to the engine to a predetermined desired air-fuel ratio in response to the output from the air-fuel ratio sensor, by using at least one feedback control gain. A purge control valve purges evaporative fuel generated in the fuel tank to the engine. The at least one feedback control gain is set to a smaller value when the degree of influence of evaporative fuel purged by the purge control valve upon the air-fuel ratio of the mixture is larger, and set to a larger value when the degree of influence of the purged evaporative fuel upon the air-fuel ratio of the mixture is smaller.

Abstract: An evaporative fuel-processing system for an internal combustion engine includes a canister, a charging passage extending between the canister and the fuel tank, a purging passage extending between the canister and the intake system of the engine, an open-to-atmosphere passage for relieving the interior of the canister to atmosphere, a charge control valve, a purge control valve, a vent shut valve, a pressure sensor for detecting pressure within the charging passage, and a float valve arranged at an end of the charging passage opening into the fuel tank. When the engine is in a predetermined operating condition, the interior of the canister is negatively pressurized into a predetermined negatively pressurized state by opening the purge control valve and the charge control valve and closing the vent shut valve. The interior of the changing passage is stabilized by closing the charge control valve and the purge control valve over a predetermined time period after the negative pressurization.

Abstract: There is provided an air-fuel ratio control system for an internal combustion engine. The air-fuel ratio control system has an air-fuel ratio sensor arranged in an exhaust passage of the engine, which generates an output proportional to concentration of oxygen in exhaust gases emitted from the engine. The air-fuel ratio of a mixture supplied to the engine is controlled to a desired air-fuel ratio in response to the output from the air-fuel ratio sensor by the use of a proportional term and an integral term. A repetition period of inversion of the output from the air-fuel ratio sensor with respect to a predetermined reference value is calculated. Deterioration of the air-fuel ratio sensor is detected based the repetition period of inversion of the output from the air-fuel ratio sensor.