In this paper, a CFD analysis was carried out to study the scavenging process of a two stroke engine. Particularly, an Otto, 125cc, single-cylinder, loop scavenging and direct injection engine was studied. Transient solutions of pressure, velocity and mass fraction fielld have been obtained numerically. In order to validate this model, the numerically obtained results were compared to experimental obtained ones, which means that this work can perfectly simulate what happens inside the cylinder. In future projects, the aim is to use this model to optimize the efficiency of the new engines, reducing the habitual problems of short circuiting and losses of fresh charge.