I'm not entire sure, but I do know obviously the air is required to be compressed prior to it being introduced to the cabin. On an airplane I believe it might be possible to borrow some of the air from the engine compressors (which I would suspect would heat the air as the density increases) to achieve the goal of having air that can be safely introduced to the cabin. I'm not an airplane engineer, but if you could somehow get the compressed air before combustion that would seem like a requirement to me since I doubt you want to be consuming jet exhaust air. Not sure if that's possible. If it is indeed the case that the engine compressors are used, however, then I think the air coming into the system might ironically be warm, and it would be required to chill it again to get it down. Does anyone know?