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. These efforts have resulted in the evolution of increasingly complex systems and somewhat negated the attractiveness of the conceptual simplicity of the recycle diesel.However, initial engine test results conducted as part of a collaborative research project started in the late 1980s1, 2 and 3 indicated that the pernicious effects of carbon dioxide has perhaps been overemphasised in the extant technical literature. Thus, it was decided to ascertain the operating envelopes of both a DI and IDI non-air diesel engine within which acceptable performance could be obtained using purely carbon dioxide and oxygen mixtures. The rationale behind these investigations being that simpler and, therefore, more attractive underwater diesel engine systems may be possible.In this paper the main experimental results that have been obtained from tests on a multi-cylinder DI diesel engine configured to operate on oxygen and carbon dioxide atmospheres are presented. The results from the EDI tests can be found in references 2 and 3.