The work done during the Carnot cycle is the shaded area on the PV graph, but this is difficult to calculate because of its curved shape. Instead, Carnot used the graph of temperature versus entropy (TS) to analyze this cycle. On the TS graph, area is equal to heat. An isothermal process is represented by a horizontal line (a → b). Because no heat flows (Q = 0), an adiabatic process is a vertical line (b → c) representing no change in entropy. The entire cycle is a rectangle on the TS graph defined by the two isothermal processes (a → b and c → d) and the two adiabatic processes (b → c and d → a).
