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Transient operation in port-injected engines often results in long lasting air-fuel ratio excursions. These excursions lead to high levels of pollutant emissions even in three-way catalyst equipped vehicles. The reduction of these excursions can be achieved by controlling fuel injection, but the exact amount of fuel to be injected is dependant on air flow and deposited fuel dynamics. This paper describes a complete model of the intake port, taking into account physical evolutions of three phases in a bidimensional geometry. Back-flow of hot burned gases, droplet trajectories, droplet evaporation, deposited fuel film flow, deposited fuel film evaporation, and air flow are taken into account. The model is used to predict air-fuel ratio excursions during engine transients. Results are compared to measurements made on a real four-cylinder engine with an oxygen sensor on the exhaust port of one cylinder, for several operating conditions.