The calculation of the performance of gas turbines, turbochargers, compressors and turbines requires the thermodynamic properties of the gases. Tables of properties exist which are effectively exact, but using these tables is tedious and far from practical in computer-based calculations. Representing tabulated results with polynomial approximations is inconvenient and prone to error in implementation. For teaching and simple calculations simple approximations, such as γ = 1.4 for unburned air and γ = 1.3 for combustion products, are sometimes used, but this is far from wholly satisfactory. This paper describes and discusses a simple empirical approach which will give adequate accuracy for many purposes but is simple enough to be used as part of an educational course.

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