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In order to elaborate accurate injection-control strategies aimed at limiting pollutant emissions resulting from transient S.I. engines operation, it is necessary to gain better understanding of the mixture formation. In this paper, we present the first stage of a software whose purpose is to model in a phenomenological way an S.I. engine cycle in order to study mixture formation during transient operation. With respect to crank angle, it computes pressures and flow-rates throughout the engine (from the throttle to the exhaust port), the description of liquid and vapor fuel in the intake ports and the instantaneous equivalence ratio of the mixture entering the cylinders. It is shown the non homogeneity of the intake port and in-cylinder mixture during the cycle, the importance of the liquid fuel film flow and the influence of the flow rates and pressures calculations on the fuel behavior.