One mole of ideal gas undergoes cycle: isothermal expansion at T₁, then cooling to T₂, then isothermal compression, then heating. This is a:

The Carnot cycle alternates two isothermal steps (heat absorbed at T_H, released at T_C) with two adiabatic steps (no heat exchange); the enclosed loop area equals the net work output, and η = 1−T_C/T_H is the maximum possible efficiency between those two temperatures.