Closed and recycle diesel engine systems have been developed for use on board conventional manned military and commercial submarines. This type of power system is now being considered for unmanned Autonomous Underwater Vehicle (AUV) applications. However, with a simple recycle system, the recirculated carbon dioxide rich combustion products can be expected to have an adverse effect on engine performance. Recent developments in this field have been concerned with producing synthetic atmospheres whose properties match those of free air so enabling the non air performance at least to match that of the normally aspirated unit. This philosophy has resulted in bulky and complicated exhaust gas management systems. The possibility of a diesel engine operating on a carbon dioxide/oxygen atmosphere whilst still retaining acceptable engine operating performance would therefore seem to require further examination. This paper describes experimental investigations currently being pursued at the Royal Naval Engineering College to determine the performance limits of carbon dioxide breathing diesel engines for AUV applications.