This paper describes the development of an interactive model to simulate a direct injection diesel engine under both steady and transient conditions, based on the application of concurrent process computing methods. Initially the engine is modelled operating under steady conditions and induction, injection, air entrainment, fuel air mixing, combustion, emission and the mechanical friction processes are considered. The fuel pump, governor, engine crankshaft and external load dynamics are incorporated in the model to study the transient behaviour of a 2.5 litre D. I. engine and its associated load. Employing a two zone combustion model enables detailed performance and exhaust emissions predictions to be produced with economic use of computing time. The model written in FORTRAN is implemented in parallel on a transputer based concurrent computer by using the transputer operating system language OCCAM as a harness. Model predictions compare favourably with experimental results.