The ideal gas equation is not only a good approximation to the behaviour of real gases at high temperatures and low pressures, but it also has irreplaceable role in thermodynamic education. Sloping straight line is seemingly simple process in pv diagram. The calculation of work delivered by the gas system during linear expansion is easy. The description of heat flow is more complicated task. Temperature entropy diagram is convenient in the case. The example of calculation of thermodynamic efficiency of reversible cycle was created in the paper. The cycle consists of a linear expansion and of an adiabatic compression. Example leads to better understanding of concept of entropy. The application of Ts diagram to calculation of thermodynamic cycles is shown.