The above states that there is no change of the internal energy ( U ) of the system over the cycle. E i n represents the total work and heat input during the cycle and E o u t would be the total work and heat output during the cycle. The repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics. Thermodynamic cycles are often represented mathematically as quasistatic processes in the modeling of the workings of an actual device.