Apparatus and method for controlling air-fuel ratio of engine

An air-fuel ratio control apparatus for controlling the air-fuel ratio of air-fuel mixture drawn into a combustion chamber of an engine, wherein a canister is connected to an intake system of the engine through a purge line, wherein the canister includes an adsorbent, an air layer located between the adsorbent and the purge line, and an air hole for introducing air into the canister, wherein the adsorbent adsorbs fuel vapor generated in a fuel tank and permits adsorbed fuel vapor to be desorbed, wherein air introduced into the canister through the air hole flows to the purge line through the adsorbent, and wherein gas containing fuel vapor is purged to the intake system from the canister through the purge line, the apparatus comprising:

• a computer, which performs feedback correction of the amount of fuel supplied to the combustion chamber such that the air-fuel ratio of the air-fuel mixture seeks a target air-fuel ratio, wherein, by using a physical model related to the fuel vapor behaviors, the computer estimates a total vapor flow rate, which represents the flow rate of fuel vapor in gas purged to the intake system, according to a total purge flow rate representing the total flow rate of the purged gas, wherein the physical model is based on a physical status quantity representing the fuel vapor stored state of the air layer, a physical status quantity representing the fuel vapor stored state of the adsorbent, and a physical status quantity representing the vapor generating state in the fuel tank, and wherein, according to the estimated total vapor flow rate, the computer corrects the fuel supply amount, which is subjected to the feedback correction.