This paper presents a methodology to determine and to formalize all the variables involved in a CFD model of a 2-stroke diesel engine. The formalization includes the establishing of the values domain and the research of relations between the variables. The formalization can be followed by a reduction of the variables and the ranges of values. The methodology concludes on the qualification of the model. The methodology has to be used before solving the model. It helps not to forget any variable and to well understand the characteristics of the final model. It also provides the validity domain of the model and the limits of its exploitation. The methodology is applies to the 2-stroke Diesel engines to optimize its design and characterize the aerodynamics inside the combustion chamber.