An automatic mesh generation process for a body fitted 3D CFD code is presented in this paper along with the methodology to guarantee the mesh quality. This tool named OMEGA (Optimized MEsh Generation Automation) uses a direct coupling procedure between the IFP-C3D solver and a hybrid mesher Centaur. Thanks to this automatic procedure, the engineering time needed for body fitted 3D CFD simulation in internal combustion engines is drastically reduced from a few weeks to a few hours. Valve and piston motion laws are just given as input files and geometries and meshes are automatically moved and generated. Unlike other procedures, this automatic mesh generation does not use an intermediate geometry discretization (STL file, tetrahedral surface mesh) but directly the original CAD that has been modified thanks to the geometry motion functionalities integrated into the mesher. All the meshes generated by the tool discretize precisely the surface geometry (nodes are projected on the correct CAD surfaces) to guarantee a correct flow prediction around intake valves and piston. Using the automatic procedure and the IFP-C3D code, a 4-valves SI engine simulation is presented. In a first step, the meshes generated with the tool are analyzed in terms of quality and refinement. Then, 3D numerical results of the internal aerodynamics are presented along with a description of the engineering time spent from the CAD modification to the end of the numerical simulation while highlighting the parallel efficiency of the IFP-C3D code using an integrated load balancing.