General performance characteristics of real heat engines

Realistic upper bounds can be placed on the power and efficiency of real heat engines via a relatively simple analytic treatment of primary sources of irreversibility. Generalized curves for heat engine performance, their universal nature, and quantitative evaluation of upper bounds for power and efficiency are derived for several engine types, specifically: Brayton cycle (gas turbines), Rankine cycle (steam turbines), and cycles with sizable heat leaks, such as thermoelectric generators. The key irreversibility sources include fluid friction, the constraint of the equation of state of the engine's working fluid, and heat leak. It is demonstrated that maximum power and maximum efficiency operating points are usually relatively close, with the associated implications for the selection of optimal heat engine operating conditions. The limitations of past analyses of endoreversible cycles as models for real heat engines will be discussed and the fortuitous nature of agreement between their predictions and actual heat engine performance will be explained.