The Brayton cycle idealizes gas-turbine power systems and many jet-engine core processes using steady-flow components.

The ideal cycle contains:

1. isentropic compression in a compressor;
2. constant-pressure heat addition;
3. isentropic expansion in a turbine;
4. constant-pressure heat rejection.

For an ideal gas with constant specific heats, the thermal efficiency can be written in terms of compressor pressure ratio

$$r_p=\frac{p_2}{p_1}$$

as

$$\eta_{Brayton}=1-\frac{1}{r_p^{(k-1)/k}}.$$

A complete component analysis uses enthalpy differences: compressor work is $h_2-h_1$, turbine work is $h_3-h_4$, and net work is their difference.

Real gas turbines have non-isentropic compressors and turbines, pressure losses and finite-temperature heat transfer. Isentropic component efficiencies therefore provide a direct bridge from the ideal Brayton cycle to realistic performance.