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Ethanol is being increasingly used as an alternative fuel to petroleum-based gasoline and diesel derivatives. Currently available flexible fuel vehicles (FFVs) can operate on a blend of gasoline and ethanol in any concentration of up to 85% ethanol (93% in Brazil) with minimum hardware modifications. This flexibility is partly achieved through the closed-loop air-to-fuel ratio control which maintains automatically operation around the stoichiometric ratio. Precise transient air-to-fuel ratio (AFR) control depends however on a feedforward compensator that reduces the transient effects of fuel puddle dynamics. An accurate and tunable model of the fuel puddle dynamics for gasoline-ethanol blends is, thus, necessary for the purpose of AFR control. In this paper, we propose a physics-based fuel puddle model that may be used for control purposes in flex-fuel vehicles. In particular, the gasoline-ethanol blend is modeled using several chemical compounds and is parameterized by ethanol content. The model consists of a droplet evaporation model and a single-puddle vaporization model. The droplet evaporation model is simulated offline to generate port wall-impacting factors of injected fuel to be used in a single-puddle vaporization model. The single-puddle vaporization model is a cycle-based model that may be simulated online to characterize fuel puddle dynamics in port fuel injected engines. To verify the validity of the model, simulation results are compared with limited experimental data. A transient fuel compensator based on the proposed model is also formulated.