 Heat transfer from point 4 to 1 and point 2 to 3 are equal to zero.The behaviour of a Carnot engine or refrigerator is best understood by using a temperature–entropy diagram (T–S diagram), in which the thermodynamic state is specified by a point on a graph with entropy (S) as the horizontal axis and temperature (T) as the vertical axis ().For a simple closed system (control mass analysis), any point on the graph will represent a particular state of the system.