A micro power unit comprises a Rankine cycle circuit powered by combustion of hydrocarbon fuel from tank (2). Combustion takes place on a combustion catalyst (6) in a chamber (5). The fluid circuit includes a boiler (10), turbine (11), a condenser (12) and feed pump (13). The feed pump impeller (13A) is driven by magnetic coupling with the turbine rotor (11A) and further magnetic couplings may be used to drive e.g. a propeller (15) for a micro air vehicle and/or electrical generator (16).