Aircraft environmental control system

An environmental control system of an aircraft comprising:

• a plurality of inlets for receiving a plurality of mediums including a first medium, a second medium, and a third medium;
• an outlet for delivering a conditioned flow of the second medium to one or more loads of the aircraft;
• at least one compressing device in fluid communication with the inlet and the outlet, the at least one compressing device including a four-wheel compressing device having a compressor and at least one turbine operably coupled via a shaft, wherein in a first mode of operation, the compressor of the at least one compressing device is driven by the first medium, and in a second mode of operation, the compressor of the at least one compressing device is driven by the third medium.