Inside a Stirling engine is transformed heat to work through a closed heat cycle. From a design of heat cycle in p-V diagram can be approximately calculated an internal work of the engine, torque and others engine parameters. An accuracy of these computations is directly proportional to a similarity of shape of the designed cycle to real Stirling engine cycle. The Stirling engine cycle, which is described here is based on several types of heat cycles used to calculation of the Stirling engine cycle, because these cycles have common features (so called assumptions of solving).
Pressure of a working gas inside working volume of the Stirling engine is changed under a change of temperature and a volume of the working volume. From description of the Stirling engine can be the working volume separated to three volumes, the death volume, the cylinder volume on the hot side engine and the cylinder volume on the cold side engine of the engine. The mean working gas temperature in all volumes varies during one cycle between their maximum value and their minimum value:
The calculation of the Stirling engine cycle, which is described here is can use under these simplifying assumptions: (1) Value of mean polytropic index of thermodynamics processes inside working volumes of engine is the same during one cycle. – (2) Temperature ratio at border of regenerator is constant, τ=TTR/TSR=const. – (3) There is no pressure loss, pressure of working gas is same in all working volumes. (4) Working gas is ideal gas. – (5) Stirling engine is perfect sealed. – (6) Stirling engine cycle is steady (the same cycle is repeated).
An equation for working gas pressure inside the engine as function its volume can be derived from the assumptions of solvings, which are presented in a previous paragraph:
Last equation shows that only temperatures on boundaries regenerator influences progress of pressure and not mean temperatures in the cylinders. For running engine is necessary difference of temperature between hot and cold side of the engine.
Physical interpretation of the integration constant can be obtainable through a derivation of pressure equation:
|3.438 Physical interpretation of Cint|
The integration constant can be computed from any point of cycle, in which be known pressure and a value of reduced volume.
The polytropic index inside the Stirling engine may be in the interval <1; κ>; (κ [-] heat capacity ratio – adiabatic index) for steady cycle. The polytropic index can not be less than 1. If n is equal κ, then an engine is perfectly thermally isolated and between the hot and cold side of the engine can not occur temperature difference (τ is equal 1 and internal work of the engine is zero). In case n=1 only isothermal processes are performed in the engine, therefore the isothermal processes can be regarded as comparative processes for real processes:
|4.446 Definition of mean polytropic index|
ν [-] isothermal ratio*.
The isothermal ratio about 0,5 is the usual value for engines with ideal heat transfer between the working gas and heat flow area (e.g. Strojírny Bohdalice Stirling engines, United Stirling V160). The isothermal ratio smaller than 0,5 is the usual value for engines with small heat flow area, higher speed, small death volumes. The isothermal ratio bigger than 0,5 is the usual value for engines with bigger heat flow area, small speed, bigger death volume or engines with control heat flow (difficult to achieve). Polytropic index is function engine speed, engine geometry and working gas.
Measured of the isothermal ratio can shows constructional deficiencies of the engine.
Equtions for calculation of mass and temperature of the working gas, the internal work of the engine, heat input, heat rejection and regenerated heat during one cycle are shown in article Energy balance of Stirling engine cycle.
The motion of the piston is often performed through a crankshaft, then the volumes of the engine are function of an angle of rotation φ (VTV(φ); VSV(φ)):
In this case for calculation of the pistons position can be used the equation for piston position conencted with crankshaft:
Combination of Equation 2 with Equation 6 be obtained the equation of pressure as function φ. From extremes of function p(φ) can be calculate minimum, maximum pressure and pressure ratio during one cycle φ<0; 2π):
The mean pressure of cycle be computed according to the mean value theorem which is applied on function p(φ):
|8.443 Mean pressure of cycle|
If the enter of calculation contains the mean pressure of cycle then Cint can be determined through iteration from result Equation 8.
Through same procedure can be derived of equations of piston motion for others configurations of the Stirling engine.
If n≠1 then temperature of the working gas is changed inside individual volumes according equations:
From these equations is evident, that temperature change follows pressure change and this change is the bigger the bigger is the mean temperature of the working gas in the individual volume. If mean temperature of the working gas is known only, then for a calculation of the temperature TT,max be must used iteration calculation. It means that during first step of the calculation be must estimate of TT,max and by the Equation 9 computes the temperature TT,st. This result must be equal with the required mean temperature on hot side the engine.
|Results of Problem 2.|
The Stirling cycle and a Schmidt cycle* are simplified Stirling engine cycles with assumtion n=1. This methods are very very popular. The Stirling cycle assumes linear motion of pistons and zero death volumes, the Schmidt cycle assumes sinusoidal move of pistons and non-zero death volumes. Details about these cycles are shown in , .
Theodor Finkelstein be published cycle where n=κ, this cycle be computed by Finite element methods , [2, p. 87]. The most widely is cycle with adiabatic processes in cylinders and isothermal processes in death volumes. Authors this cycle are Israel Urieli and David Berchowitz  (authors assembled set of differential equations, which are solved by Runge – Kuttak method).
Thermodynamic cycle described here is based on mean values of temperature ratio and polytropic index, but these quantities are variable during one cycle at real process. Mass of the working gas in the working volume is not constant also at real process (alternate ingress/leakage of the working gas through piston rings, see article Losses in Stirling engines). These factors (especially the last said) significantly influence calculated diagrams.
ŠKORPÍK, Jiří. Oběh Stirlingova motoru, Transformační technologie, 2009-07, [last updated 2012-01]. Brno: Jiří Škorpík, [on-line] pokračující zdroj, ISSN 1804-8293. Dostupné z https://www.transformacni-technologie.cz/seznam-clanku.html#34. English version: Stirling engine cycle. Web: https://www.transformacni-technologie.cz/en_seznam-clanku.html#34.