I was reading about Carnot cycle in "Thermodynamics by Cengel & Boles", now it is stated that Carnot cycle is a totally reversible cycle, i.e it is an internally as well as externally reversible cycle (in all of the processes involved) and it is very well explained how it is like that(pages attached) but what I don't understand is why then we still get an entropy change in the T-S diagram of Carnot cycle, that is if it is completely reversible then shouldn't entropy change of the system must be zero?
The four processes explained as reversible.]1[It says in the first line in section 9-2 that Carnot is composed of four totally reversible processes ]2