Air-standard assumptions are a simplified means of modeling processes, evaluating properties and calculating performance of gas-based cycles.

The approximations, commonly known as the air-standard assumptions, are the following:

1. Air, behaving as an ideal gas, circulates continuously through the cycle.
2. All the processes within the cycle are internally reversible.
3. Combustion processes are viewed as heat addition steps.
4. Exhaust processes are viewed as heat rejection steps.

When the working fluid in a cycle remains as a vapor or gas, changes of enthalpy or internal energy can be approximated through the use of constant pressure (C_p) or constant volume (C_v) specific heats. This approach works for cycles including the Otto, Diesel and Brayton cycles.

The air-standard assumption can be further restricted and simplified by what is known as the cold-air standard assumption. The cold-air standard assumption assumes that the air has constant specific heats evaluated at room temperature (25^oC or 77^oF).

All of the air cycle problems illustrated herein make use of SI units.

Equations for the four basic properties of refrigerants (liquid density, vapor pressure, P-v-T relationships of the vapor, and heat capacity of the vapor) were taken from Refrigerant Equations published by R.C. Downing, Freon Products Division, The Dupont Company, Wilmington DE. Numerical methods were adapted as needed to exploit the published equations.

Equations for the steam properties calculated in the Rankine Cycles were taken from Steam and Gas Tables with Computer Equations, Thomas F. Irvine and Peter E. Liley, Academic Press, Inc., 1984.