An aeronautical gas turbine engine includes a turbomachine including a compressor section, a combustion section, a turbine section, and an exhaust section in serial flow order. The aeronautical gas turbine engine additionally includes a closed cycle heat engine including a compressor configured to compress a working fluid; a primary heat exchanger in thermal communication with the turbomachine and the working fluid, the primary heat exchanger configured to transfer heat from the turbomachine to the working fluid; an expander coupled to the compressor for expanding the working fluid; and an output shaft driven by the expander.