AIR-FUEL RATIO CONTROL DEVICE FOR INTERNAL COMBUSTION ENGINE

PROBLEM TO BE SOLVED: To realize accurate air-fuel ratio control while concentration of evaporated fuel (evaporation gas) is quickly further accurately estimated. SOLUTION: A canister 13 of absorbing evaporation gas is connected to a fuel tank 7, to arrange a purge valve 16 in a release passage 15 of communicating the canister 13 and an intake pipe 2 of an engine 1. An injector 4 is arranged in the intake pipe 2. In a CPU21, at evaporation purge time, in accordance with a degree of changing a feedback correction coefficient FAF to a fuel amount increasing side (lean time) or to a fuel amount decreasing side (rich time), evaporation concentration FLPRG is adjusted, thus to estimate an actual concentration value. In the CPU21, in accordance with an engine operating condition (engine speed, intake air temperature, etc.) detected by various sensor groups, a concentration updating width (increasing amount width or decreasing amount width) of evaporation concentration is changed. Further in the CPU21, evaporation concentration thus estimated is reflected to air-fuel ratio feedback control.