System Engineering projects are characterized by knowledge intensive processes. Creating effective Knowledge Management Systems is one of the key success factors in engineering process improvement. In this paper, we focus on the knowledge modeling issue. Our main objective is to provide a semantic description for knowledge items created and/or used in system engineering processes in order to facilitate their reuse. The framework is based on a set of layered ontologies where entities such as domain concepts, engineering processes, artefacts models, are interlinked to capture explicit as well as implicit engineering project knowledge.