Which of the following best describes a gas cycle heat engine?

A gas cycle heat engine predominantly uses air as its main working fluid, which is in line with the principles of thermodynamics employed in such systems. In this context, air is compressed and heated, and its ensuing expansion drives a piston or turbine, producing mechanical work. The ideal gas law applies effectively to the workings of gas cycle engines, and combustion engines are prime examples of how gases function within this framework.

The emphasis on air as the working fluid is significant, as it highlights the function of the heat engine in utilizing a continuously circulating gas, which is often mixed with fuel for combustion in practical applications. This is distinctly different from other types of engines that utilize different fluids, such as steam engines, which rely on condensable vapors. The cycle relies on the specific properties of gases, including their behavior under pressure changes and temperature variations, to convert thermal energy into mechanical work efficiently.

Other options do not accurately describe the characteristics of a gas cycle heat engine, as they either refer to systems using different working fluids or imply conditions not typically relevant to gas cycle operations. Understanding that air, as a working fluid, distinguishes gas cycle heat engines is crucial for recognizing their operational mechanics in thermal systems.