<div class="htmlview paragraph">An underwater powerplant is described based on a conventional diesel engine using normal hydrocarbon fuel but aspirating a mixture of stored oxygen diluted with recycled exhaust gas, the remaining exhaust gas being passed to the sea by a compressor. The theoretical aspects of the system are covered, followed by details of the mechanical and control system developments carried out in the construction of an operational prototype underwater powerplant. Comparisons are drawn between the recycle unit using various forms of oxygen storage and lead acid batteries, other alternative power systems also being discussed.</div>