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. Of the dynamic heat engines available, one strong contender for underwater vehicle operation is the diesel engine. In order function underwater the engine must utilize a non-air or synthetic atmosphere for the combustion process. However, there is a lack of published data on how the engine actually performs with gaseous composition variations in the intake atmosphere.This paper reviews the development and technology requirements for AUVs and details the experimental evaluation of a diesel engine whilst operating with non-conventional atmospheres.