Thermodynamic cycle

A thermodynamic cycle consists of linked sequences 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.

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.

During a closed cycle, the system returns to its original thermodynamic state of temperature and pressure.

represents the total work and heat input during the cycle and

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.

Because the net variation in state properties during a thermodynamic cycle is zero, it forms a closed loop on a P-V diagram.

will be negative, the cyclic machine will require work to absorb heat at a low temperature and reject it at a higher temperature and it represents a heat pump.

Thermodynamic power cycles are the basis for the operation of heat engines, which supply most of the world's electric power and run the vast majority of motor vehicles.

Cycles encountered in real world devices (real cycles) are difficult to analyze because of the presence of complicating effects (friction), and the absence of sufficient time for the establishment of equilibrium conditions.

For the purpose of analysis and design, idealized models (ideal cycles) are created; these ideal models allow engineers to study the effects of major parameters that dominate the cycle without having to spend significant time working out intricate details present in the real cycle model.

Power cycles can also be divided according to the type of heat engine they seek to model.

For example :--the pressure-volume mechanical work output from the ideal Stirling cycle (net work out), consisting of 4 thermodynamic processes, is[citation needed][dubious – discuss]: For the ideal Stirling cycle, no volume change happens in process 4-1 and 2-3, thus equation (3) simplifies to: Thermodynamic heat pump cycles are the models for household heat pumps and refrigerators.

Multiple compression and expansion cycles allow gas refrigeration systems to liquify gases.

Thermodynamic cycles may be used to model real devices and systems, typically by making a series of assumptions to reduce the problem to a more manageable form.

[2] For example, as shown in the figure, devices such a gas turbine or jet engine can be modeled as a Brayton cycle.

The actual device is made up of a series of stages, each of which is itself modeled as an idealized thermodynamic process.

If energy is added by means other than combustion, then a further assumption is that the exhaust gases would be passed from the exhaust to a heat exchanger that would sink the waste heat to the environment and the working gas would be reused at the inlet stage.

The difference between an idealized cycle and actual performance may be significant.

[2] For example, the following images illustrate the differences in work output predicted by an ideal Stirling cycle and the actual performance of a Stirling engine: As the net work output for a cycle is represented by the interior of the cycle, there is a significant difference between the predicted work output of the ideal cycle and the actual work output shown by a real engine.

It may also be observed that the real individual processes diverge from their idealized counterparts; e.g., isochoric expansion (process 1-2) occurs with some actual volume change.

Therefore, the internal energy changes of a perfect gas undergoing various processes connecting initial state

If the total heat flow per cycle is required, this is easily obtained.

Thus, the total heat flow per cycle is calculated without knowing the heat capacities and temperature changes for each step (although this information would be needed to assess the thermodynamic efficiency of the cycle).

The Carnot cycle is a cycle composed of the totally reversible processes of isentropic compression and expansion and isothermal heat addition and rejection.

The thermal efficiency of a Carnot cycle depends only on the absolute temperatures of the two reservoirs in which heat transfer takes place, and for a power cycle is: where

For Carnot power cycles the coefficient of performance for a heat pump is: and for a refrigerator the coefficient of performance is: The second law of thermodynamics limits the efficiency and COP for all cyclic devices to levels at or below the Carnot efficiency.

This makes sense since all the work done by the cycle is done by the pair of isothermal processes, which are described by Q=W.

This suggests that all the net heat comes in through the top isotherm.

If Z is a state function then the balance of Z remains unchanged during a cyclic process: Entropy is a state function and is defined in an absolute sense through the Third Law of Thermodynamics as where a reversible path is chosen from absolute zero to the final state, so that for an isothermal reversible process In general, for any cyclic process the state points can be connected by reversible paths, so that meaning that the net entropy change of the working fluid over a cycle is zero.