Preliminary design criteria of Stirling engines taking into account real gas effects

• The theoretical limit of the gas circuit performance considers real gas effects.
• The influences of physical gas properties are analysed.
• Simulations predict the feasibility of operating under real gas effects.
• Equations and correlations are proposed as preliminary design criteria.
• The procedure is supported by an experimental database.

This article deals with the main geometric parameters, operating variables and experimental results of engines with different size and characteristics, as well as simulation predictions at operating conditions for which the working fluid may evidence real gas effects. The concept of dimensionless quasi-static indicated work, introduced previously, is computed assuming both ideal and real gas models. Both values are compared with the Schmidt's model prediction, to evaluate separately how the mechanism simplification and the equation of state affect. The influence of physical gas properties are also analysed regarding the gas circuit performance and mechanical efficiency. Pending of experimental corroboration, some simulations show the possibility of obtaining interesting operating conditions under real gas effects.Semi-empirical equations and experimental correlations are proposed as preliminary design criteria. The ratio between the dimensionless values of the maximum indicated power (a sort of ‘indicated’ Beale number) and the quasi-static indicated work is analysed. The procedure to estimate the maximum indicated power is completed by analysing the characteristic Mach number (dimensionless rotation frequency). Analogous procedure is applied for the maximum brake power and corresponding engine speed. Use of dimensionless variables facilitates the generalization of analyses by means of dynamic similarity criteria.