Complex sentence: Its use is sometimes extended to describe engines employing other permanent gasses as their working fluid, but specifically excludes any engine performing a thermodynamic cycle such as the Rankine cycle, in which the working fluid undergoes a phase change.
Split into simpler sentences: Its use is also often extended to describe engines employing other permanent gasses as their working fluid. Thought, as noted above, the term hot air engine may refer to engines not employing atmospheric air, it specifically excludes any engine performing a thermodynamic cycle such as the Rankine cycle, in which the working fluid undergoes a phase change.