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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.

EGR diesel engines are used in an underwater environment, or in terrestrial applications that demand low exhaust emissions. In the underwater mode the intake mixture may contain up to 30% CO2 whereas with land-based EGR diesels the percentage will be much lower. In both applications noise is an important emission parameter for not only is it a pollutant but in the underwater environment a primary means of detection. Thus, in the research reported here the combustion noise levels and spectra have been measured for a diesel engine using a variety of precisely proscribed intake mixtures containing levels of CO2. It has been found that the presence of CO2 alters both the sound level and frequency spectra of the combustion noise and that in general, although not in all circumstances, the sound pressure levels are increased.

It has been known for nearly a century that by recycling the exhaust gas and adding renewal oxygen for combustion, it is possible to operate a standard diesel engine in air restricted conditions. However in order to operate under these conditions, such as found in underwater vessels, exhaust gas management systems are required to process the combustion products. The characteristics of recycled working fluids and the effective disposal of the exhaust gases leads to conflicting system operational requirements. In order to operate the whole system as a compact and efficient power unit, a compromise needs to be found between the performance of the engine with the recycled exhaust and the physical size and efficiency of the exhaust processing system. Previous research using non-conventional or contaminated atmospheres for underwater vehicles power systems, pollution control and mine engineering has mainly used three methods of supplying the intake atmosphere.

Even in today's age of underwater nuclear power the majority of the world's submarines still use diesel engines as their main source of mechanical power, as they have done since the turn of the century. The diesel-electric submarine propulsion system has changed little in concept since the start of the Great War of 1914-1919. Diesels are used to provide surface propulsion and underwater power is provided from battery driven motors. Diesels are also used to recharge the batteries when the vessel is on the surface or at snort depth. In the 1939-45 War efforts were made by the Germans to perfect the closed or recycled diesel so that the engine could operate underwater independent of a normal air supply. After sporadic revivals of the idea in recent years, the concept has been brought to technical maturity by British and German engineers. However, the history of the submarine diesel engine and its air-dependent versions are nearly as old as the engine itself.

Non-air-breathing diesel engine systems have, and continue to be developed for underwater applications. When the engine is operated in such an environment the intake oxidant mixture consists of a combination of oxygen and recycled exhaust gas. The latter will contain combustion gaseous products and may also include additional inert diluents. Since its initial conception in the late nineteenth century, a major problem encountered in the operation of the recycle diesel engine has been the detrimental effect of the recirculated exhaust carbon dioxide upon the engine's performance. To avoid this problem exhaust gas scrubbing systems have been developed to remove the carbon dioxide from the exhaust gases. In addition, inert gases such as argon and helium have been added to the non-air mixture to improve its thermodynamic and transport properties and hence engine performance.

This paper describes work aimed at developing an underwater power system and an environmental control EGR system based on the recycle and closed-cycle operations of conventional diesel engines. Particular emphasis is placed on one of the key problems associated with the recycling some of carbon dioxide in closed-cycle diesel engine (CCDE). A quasi-dimensional model has been developed to investigate the effects of different intake compositions on engine performances. The paper also introduces the development of instrumentaion for measurement and control combustion conditions in CCDE. With the objective of improving fuel ignitability and reducing the ignition delay, the paper experimentally investigates the effects of heated fuel on fuel injection characteristics, engine performance and exhaust emissions in DI and IDI diesel engines.

Long range, extended endurance, variable speed autonomous underwater vehicles (AUVs) appear to be an attractive solution to problems of environmental monitoring, geophysical exploration and military surveillance. To undertake their intended autonomous missions these vehicles require reliable and cost-effective power systems. Although there is presently an extensive interest in untethered AUVs, with far reaching efforts being made in a variety of activities, only limited headway has been made in the development of power systems which could be readily integrated into these vessels. The majority of current research is focusing on increasing the underwater endurance and hence cost effectiveness of the vehicle by developing compact, lightweight high energy density power systems for vessel propulsion. Subsequently, a number of different power systems have been investigated proposed, designed and developed.