Question

Which one of the following makes an ideal air-standard Stirling cycle?

• A. Two reversible isobars, and two reversible adiabatics
• B. Two reversible isotherms, and two reversible isobars
• C. Two reversible isotherms, and two reversible isochores
• D. Isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection

Hint:

The Stirling cycle is the constant temperature counterpart of the Otto cycle. It swaps the two isentropic legs of the Otto cycle for two isothermal legs, while keeping the two constant volume legs.

Answer 1

The Correct Option is C

The air-standard Stirling cycle is an idealized thermodynamic cycle originally associated with the Stirling engine, and it is defined by a specific pair of process types repeated twice around the cycle. This question checks whether that exact pair, isotherms plus isochores, can be picked out from other similar sounding cycles.

1. Two reversible isobars, and two reversible adiabatics: This combination, constant pressure plus isentropic legs, describes the ideal Brayton, or Joule, cycle used for gas turbines, not the Stirling cycle.
2. Two reversible isotherms, and two reversible isobars: Mixing constant temperature legs with constant pressure legs does not correspond to any of the standard named air-standard power cycles taught alongside Stirling, Otto, Diesel, and Brayton. This option does not describe the Stirling cycle.
3. Two reversible isotherms, and two reversible isochores: This is exactly the definition of the ideal Stirling cycle. Heat is added at constant temperature during isothermal expansion, then heat is absorbed internally at constant volume, then heat is rejected at constant temperature during isothermal compression, then heat is rejected at constant volume to complete the loop, aided in a real machine by a regenerator that stores and returns the heat exchanged during the two constant volume legs.
4. Isentropic compression, constant volume heat addition, isentropic expansion, and constant volume heat rejection: This four process description, isentropic, isochoric, isentropic, isochoric, is the ideal air-standard Otto cycle, the idealization of a spark ignition petrol engine, not the Stirling cycle.

The correct option is two reversible isotherms and two reversible isochores, since that is the precise textbook definition of the ideal air-standard Stirling cycle.

Let's sum up:

• Stirling cycle: two isotherms plus two isochores.
• Otto cycle: two isentropes plus two isochores.
• Diesel cycle: two isentropes, one isobar, one isochore.
• Brayton cycle: two isentropes plus two isobars.

So the ideal air-standard Stirling cycle is made of two reversible isotherms and two reversible isochores.