Aircraft refrigeration using air as the working fluid Which thermodynamic cycle best represents air-cycle refrigeration used on aircraft?

Introduction / Context:
Environmental control systems in many aircraft rely on air-cycle refrigeration. Knowing the representative cycle clarifies why turbines and heat exchangers, rather than evaporators and expansion valves, dominate these systems.

Given Data / Assumptions:

• Working fluid: air (treated as an ideal gas).
• Key components: compressor, heat exchangers, turbine.
• Primary processes: isentropic compression/expansion and constant-pressure heat exchange.

Concept / Approach:
The reversed Brayton cycle (also called the reversed Joule cycle) consists of two isentropic and two constant-pressure processes. It is the standard model for air-cycle machines such as bootstrap or regenerative systems on aircraft.

Step-by-Step Solution:
Identify air as refrigerant → no phase change equipment.Map processes: compress, reject heat at pressure, expand in turbine, absorb heat at pressure.This sequence is the reversed Brayton cycle.

Verification / Alternative check:
System schematics for aircraft ECS show exactly these components and processes.

Why Other Options Are Wrong:
(A) Carnot is an ideal limit, not the actual cycle; (D) Otto is for spark-ignition engines; (E) absorption is a different technology using phase-change pairs. Option B is synonymous with C, but the standard name in refrigeration texts is “reversed Brayton.”

Common Pitfalls:
Thinking only vapour-compression cycles exist; air-cycle systems are common where bleed air and turbines are available.

Final Answer:
Reversed Brayton cycle