A Rankine cycle engine is described that comprises close-coupled annular components (boiler, nozzle ring, and air condenser) corotating counter to an interior turbine wheel on a common axis. A stationary annular combustor surrounds the rotating boiler. Test runs up to 18 hp demonstrated several advantages for this kind of Rankine engine, which utilizes centrifugal force to achieve: boiler compactness, air condenser compactness (viscous drag air pumping), automatic condensate return (no separate pump), control simplicity, and few moving parts. The organic working fluid used does not support combustion and has low physiological reactivity in preliminary tests. The results appear significant for uses requiring low-polluting, quiet engines.