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Net entropy change means entropy change of the world (world means system plus environment). Carnot cycle is a reversible cycle. For a reversible cycle, world entropy change is zero. Because both of system and environment return to their initial states when cycle is completed. But, for an irreversible cycle, world entropy change (net entropy change) isn't zero. Although the system returns to its initial state, but the environment doesn't. So, net entropy change won't be zero and according to the principle of increase of entropy, it will be positive.