A gas turbine engine includes: a compressor, a combustor, and a turbine arranged in sequential flow relationship along a primary flowpath, the turbine being connected in mechanical driving relationship to the compressor, so as to define at least one engine rotor that is rotatable about a centerline axis of the engine; a secondary flowpath connected in flow communication with the primary flowpath; and an air cycle machine including an air cycle rotor carrying an at least one air cycle compressor and at least one air cycle expander, wherein: the air cycle rotor is coupled in mechanical driving relationship with the at least one engine rotor; the air cycle rotor is coupled in fluid flow communication with the secondary flowpath; and the air cycle rotor is coupled in fluid flow communication with at least one heat exchanger.