CFD has been widely used to predict the flow behavior inside 2-stroke engines over the past twenty years. Usually a mass flow profile or a simple 0D model is used for the inlet boundary condition, which replaces the complete intake geometry, such as reed valve, throttle, and air box geometries. For a CFD simulation which takes into account the exact reed valve geometry, a simulation of all above mentioned domains is required, as these domains are coupled together and thus interact. As the high speed of the engine affects the opening dynamic and closure of the reed valve, the transient data from the crank case volume and the section upstream the reed valve have an important influence on the reed petal dynamic and therewith on the sucked fresh air mass of the engine.This paper covers a methodology for the transient CFD simulation of the reed petals of a 2-stroke engine by using a 2D model. It is a mathematical model, which describes the displacement and the dynamic behavior of a reed petal, applying a Fluid-Structure Interaction (FSI). This bending beam model of the petal is coupled with the commercial CFD Code Fluent via a user defined function. The used coupling and the mesh moving strategy is presented and discussed in detail. The 3D engine domain includes the complete engine geometry. The main focus lies on the requirements of a reed valve simulation for a 2-cylinder 2-stroke engine. Finally, test bench results of the fired engine were used for the validation of the gas dynamic behavior inside the engine.