Underwater diesel engines use a synthetic atmosphere made up of oxygen, recycled exhaust and other diluents such as argon which are added to enhance system performance. Although the basic concept of such systems is relatively simple, its practical implementation is quite complex and normally involves expensive and time consuming prototype development. However, published experimental data are scarce and complete system analytical models essentially non-existent. Without performance data on such systems it is impossible to optimise their design for underwater vehicular use. Thus, the aim of this investigation was to develop a thermodynamic model of such a system and compare the predicted performance data with that obtained from a comprehensively instrumented engine rig also to be developed as part of the investigation.

A complete system model has been developed whose main element was a thermodynamic model of a diesel engine working with non-air mixtures. It was also found necessary to develop a new heat release model and ignition delay equation to deal with the effects of non-air mixtures on engine performance. The engine model has been validated against experimental data. Subsequently the model of the complete underwater system has been used to carry out a preliminary investigation of the effects of sub-system operation on the whole system performance and operation.

As part of the modelling process a user-friendly system simulation has been developed which involves several thousand lines of Fortran coding. The simulation has an extensive graphical output capability and has been run on PCs, workstations and main frame hardware. Specialised kowledge of non-air diesel engine systems or oprations is not required by the inter-active user of the simulation.

Many interesting and sometimes unexpected results have been obtined during the investigation. It has been fund that non-air diesel engines can perfrm as well as their naturally aspirated equivalents under many operational conditions especially at high loads. Furthermore by careflly controlling the engine's intae conditions, high concentrations of carbon dioxide can be tolerate by the engine, often without signifcant effects on perfrmace. Simulation results have shown that if the fel injection systems on such engines were modified then even better perfrmance could result.

An underwater power system and an EGR diesel system used on land are very similar in many respects and thus cerin aspects of the operation of the latter were aso investigated in this research programme.