Stirling cycle

Stirling cycle . It is a process that allows the conversion between mechanical and heat energy .


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  • 1 The cycle
  • 2 Description of the cycle
  • 3 Performance
  • 4 Heat exchanger
  • 5 Bibliographic references
  • 6 See also
  • 7 Sources

The cycle

It is a highly ideal cycle, the practical realization of which, even roughly, involves serious difficulties. However, in recent years it has become relevant with the development of Stirling engines, which operate approximately according to this cycle.

Cycle Description

Stirling 1 motors scheme

Stirling 2 engine diagram

An ideal Stirling cycle is made up of four reversible processes:

  • Isothermal compression A → B

The gas is compressed from an initial volume VA to a final one VB, lower, maintaining its constant temperature at a value T1 (based on cooling the gas continuously).

  • Constant volume heating B → C

The gas is heated from temperature T1 to temperature T2 keeping its volume fixed.

  • C → D isotherm expansion

The gas expands while heat is supplied so that its temperature remains at its T2 value.

  • Isochoric cooling D → A

The gas temperature is reduced back to its T1 value in a constant volume process.


In this process, heat is absorbed in isochoric heating and isothermal expansion, and it is transferred in the other two processes. The net value of the heat absorbed is

and the loan


so that performance is


being the compression ratio.

We can verify that this performance is always less than that of a reversible machine that operates between these two temperatures

being the difference



Heat exchanger

What makes the Stirling cycle special (and the Ericsson cycle, which is analogous but with isobaric rather than isochoric processes) is the presence of a heat exchanger . In gas cooling , temperature T2 is passed to T1 releasing heat. In heating, it goes from T1 to T2, absorbing heat. Since it passes through the same temperatures, it is (theoretically) possible to take advantage of the heat released when cooling without violating the second principle of thermodynamics: the heat that is gradually released at one point of cooling is transferred to the point at the same temperature at heating. Since both points are at the same temperature, the process is reversible.

Bibliographic references

  • Giacosa, Dante. Internal combustion engines. Editorial Hoepli
  • Automobile technique manual


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