.This paper presents an original approach to modelling and simulating complex systems. The overall application used to illustrate this approach consists in modelling and simulating the transient behaviour of a diesel engine. Conventional modelling and simulation stages for small projects are presented. As a larger project needs additional tools, we have developed an original approach using two other models. We first introduced a new compound model which is a modelling approach where many white and/or black boxes are linked together. In the second stages of a project - implementation of the simulator - we introduced another Object-Oriented model. Variable granularity modelling and simulation are also presented in this project. Such a concept is a suitable for a project starting from scratch. At first the subsystems are not well understood; but as we start to understand them better, different kinds of models can be introduced. This paper concludes with some information on their implementation and future lines of investigation.