# Thermodynamic cycle

A process in which a fluid (water, air, ammonia, etc) successively changes state (from a liquid to a gas and back to a liquid) for the purpose of producing or transferring energy.[1]

### Wikipedia Definition

A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer of heat and work into and out of the system, while varying pressure, temperature, and other state variables within the system, and that eventually returns the system to its initial state. In the process of passing through a cycle, the working fluid (system) may convert heat from a warm source into useful work, and dispose of the remaining heat to a cold sink, thereby acting as a heat engine. Conversely, the cycle may be reversed and use work to move heat from a cold source and transfer it to a warm sink thereby acting as a heat pump. At every point in the cycle, the system is in thermodynamic equilibrium, so the cycle is reversible (its entropy change is zero, as entropy is a state function).During a closed cycle, the system returns to its original thermodynamic state of temperature and pressure. Process quantities (or path quantities), such as heat and work are process dependent. For a cycle for which the system returns to its initial state the first law of thermodynamics applies: Δ E = E o u t − E i n = 0 {\displaystyle \Delta E=E_{out}-E_{in}=0} The above states that there is no change of the energy of the system over the cycle. Ein might be the work and heat input during the cycle and Eout would be the work and heat output during the cycle. The first law of thermodynamics also dictates that the net heat input is equal to the net work output over a cycle (we account for heat, Qin, as positive and Qout as negative). 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.

Related Terms
Rankine cycleBrayton cycleThermoelectric power generationheat