Synthetic atmosphere diesel (SAD) engines have been and are still being developed as air-independent power systems for use in naval and commercial underwater vehicles. Although the basic concept of such a system is relatively simple, its practical implementation is somewhat complicated and normally involves expensive and time consuming prototype development. If an analytical method existed which could be used to compare the overall performance of different configurations or highlight essential control aspects, system optimization could be attempted more readily and a close-to-optimum design produced prior to any subsequent practical development. Consequently, a thermodynamic simulation model has been formulated so that the performance and/or design of such systems can be investigated, and the effects of the various system variables can be identified. In this paper the development of the model and the associated experimental investigation is described.