Nineteen informants (n = 19) were asked to study and comment two computer animations of the Otto combustion engine. One animation was non-interactive and realistic in the sense of depicting a physical engine. The other animation was more idealised, interactive and synchronised with a dynamic PV-graph. The informants represented practical and theoretical traditions of knowledge: science students and teachers at upper secondary school level; vocational students and teachers in vehicle mechanics at upper secondary school level, and; MSc and PhD students in vehicle system engineering. The aim was to explore how they interpreted the animations against the background of their different traditions of knowledge and their experience of physical engines and models of engines. A key finding was that the PhD students saw the interactive animation as a familiar and useful model of engines, whereas the vehicle mechanics teachers saw it as a poor representation of reality. A general conclusion was that there is a variety of competent ways to interpret a model, depending on the tradition of knowledge.