System Engineering (SE) processes comprise knowledge-intensive tasks that involve extensive problem solving and decision making activities among interdisciplinary teams. Management of knowledge emerging in previous SE projects is vital for organizational process improvement. To fully exploit this intellectual capital, it must be made explicit and shared among system engineers. In this context, we propose a knowledge modeling framework for system engineering projects. 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, actors, decision processes, artefacts, are interlinked to capture explicit as well as implicit engineering project knowledge.