Air-fuel ratio control system for engine

Claim 1

An air-fuel ratio control system comprising: an internal combustion engine; a sensor for detecting a parameter from an exhaust gas discharged from the engine; a fuel injector for injecting fuel from a fuel tank into the engine; vaporized fuel supplying means for supplying vaporized fuel from the fuel tank to the engine; and control means for (1) feedback controlling an air-fuel ratio of a mixture to be supplied into the engine, based on the parameter detected from the exhaust gas discharged from the engine, so as to set said air-fuel ratio to a target air-fuel ratio, (2) gradually increasing an amount of vaporized fuel supplied to a necessary level during an initial interval of supplying said vaporized fuel, (3) learning and updating a control value used in feedback controlling said air-fuel ratio of the mixture based on a deviation of said air-fuel ratio from said target air-fuel ratio while supplying said vaporized fuel, (4) gradually changing said control value, upon a resumption of supplying said vaporized fuel, from zero (0) to a learned control value obtained during a preceding period of supplying vaporized fuel according to a gradually increasing amount of vaporized fuel supplied over an interval in which an amount of vaporized fuel supplied is gradually increased, and (5) operating said fuel injector to supply a specific amount of injected fuel from the fuel tank to the engine based on said control value.