7. The ideal Diesel cycle is shown in the figure, where a to b and c to d are adiabatic processes, b to c is constant pressure compression, and d to a is constant volume process. The temperature at a, b, c, d points are , and , respectively. The ratio of molar specific heat at constant pressure to that at constant volume is γ = Cp/Cv = 5/3. Find the entropy change (ΔS) of the engine after a complete cycle: a → b → c → d →a._____