Thermodynamic cycle

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Thermodynamic cycles
Atkinson cycle Brayton/Joule cycle Carnot cycle Combined cycle Crower cycle Diesel cycle Ericsson cycle Hirn cycle Kalina cycle Lenoir cycle Linde-Hampson cycle Miller cycle Mixed/Dual Cycle Otto cycle Rankine cycle Scuderi cycle Stirling cycle Two-stroke cycle One-stroke cycle Bourke cycle Wankel cycle
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A thermodynamic cycle is a series of thermodynamic processes which returns a system to its initial state. Properties depend only on the thermodynamic state and thus do not change over a cycle. Variables such as heat and work are not zero over a cycle, but rather are process dependent. The first law of thermodynamics dictates that the net heat input is equal to the net work output over any cycle. The repeating nature of the process path allows for continuous operation, making the cycle an important concept in thermodynamics. Thermodynamic cycles often use quasistatic processes to model the workings of actual devices.

1 Classes
- 1.1 Power cycles
- 1.2 Refrigeration cycles
2 Carnot cycle
3 See also

[edit] Classes

Two primary classes of thermodynamic cycles are power cycles and refrigeration cycles. Power cycles are cycles which convert a heat input into a work output, while refrigeration cycles transfer heat from low to high temperatures using work input. Cycles composed entirely of quasistatic processes can operate as power or refrigeration cycles by controlling the process direction. On a pressure-volume or Temperature-entropy diagram, the clockwise and counterclockwise directions indicate power and refrigeration cycles, respectively.

[edit] Power cycles

Power cycles are the basis for the operation of heat engines, which supply most of the world's electric power and run almost all motor vehicles. Power cycles can be divided according to the type of heat engine they seek to model. The most common cycles that model internal combustion engines are the Otto cycle, which models gasoline engines and the Diesel cycle, which models diesel engines. Cycles that model external combustion engines include the Brayton cycle, which models gas turbines, and the Rankine cycle, which models steam turbines.

[edit] Refrigeration cycles

Refrigeration cycles are the models for heat pumps and refrigerators. The difference between the two is that heat pumps are intended to keep a place warm and refrigerators designed to cool it. The most common refrigeration cycle is the vapor compression cycle, which models systems using refrigerants that change phase. The absorption refrigeration cycle is an alternative that absorbs the refrigerant in a liquid solution rather than evaporating it. Gas refrigeration cycles include the reversed Brayton cycle and the Linde-Hampson cycle. Regeneration in gas refrigeration allows for the liquefaction of gases.

[edit] Carnot cycle

Main article: Carnot 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 temperatures of the two reservoirs in which heat transfer takes place, and for a power cycle is:

$\eta=1-\frac{T_L}{T_H}$

where $T L$ is the lowest cycle temperature and $T H$ the highest. For Carnot refrigeration cycles the coefficient of performance for a heat pump is:

$\ COP = 1+\frac{T_L}{T_H - T_L}$

and for a refrigerator the coefficient of performance is:

$\ COP = \frac{T_L}{T_H - T_L}$

The second law of thermodynamics limits the efficiency and COP for all cyclic devices to levels at or below the Carnot efficiency. The Stirling cycle and Ericsson cycle are two other reversible cycles that use regeneration to obtain isothermal heat transfer.