Abstract: A closed-cycle heat engine system that operates within a sealed, gas tight enclosure uses CO2, water vapor and O2 in the intake and compression cycles, injecting fuel to promote combustion and using the high temperature and pressure product CO2 and water vapor as the post combustion working gas. By controlling the individual partial pressures of the O2 and CO2 in the enclosure and allowing H2O vapor to exist in saturation within the enclosure the gamma value of the ingested gas can be controlled to meet engine design and operating requirements. Excess CO2 produced by combustion is compressed, removed from the enclosure, liquefied and stored in a separate tank. Noncondensable gasses are returned to the engine enclosure where they are recycled to promote system efficiency. Excess water produced by combustion is also removed from the enclosure and stored in a separate tank.