Carnot Cycle – Process During Which Heat Is Rejected In the ideal Carnot refrigeration/heat-pump representation reversed, heat rejection in the forward (heat-engine) Carnot cycle occurs during the isothermal compression at the higher reservoir; conversely, in the standard heat-engine Carnot cycle, heat rejection occurs during the isothermal compression at the lower temperature.

Introduction / Context:
The Carnot cycle consists of two isothermal and two isentropic processes and sets the theoretical upper bound on heat-engine efficiency. Identifying where heat is added and rejected is essential for understanding temperature-limit performance.

Given Data / Assumptions:

• Reversible processes with ideal gas as working fluid for visualization.
• Heat addition at the higher temperature T_H; heat rejection at the lower temperature T_L.
• No internal irreversibilities; quasi-static processes.

Concept / Approach:

In the forward heat-engine Carnot cycle: 1–2 isothermal expansion at T_H (heat addition), 2–3 isentropic expansion, 3–4 isothermal compression at T_L (heat rejection), 4–1 isentropic compression. Thus, heat rejection occurs during the isothermal compression at the low-temperature reservoir. Many texts simply say: heat is rejected isothermally during compression.

Step-by-Step Solution:

Verification / Alternative check:

p–v and T–s diagrams of the Carnot cycle clearly show the Q_out segment aligned with the lower isotherm during compression.

Why Other Options Are Wrong:

Isothermal expansion is the Q_in leg. The isentropic legs are adiabatic (no heat transfer). Polytropic compression is not part of the ideal Carnot sequence.

Common Pitfalls:

Mixing the forward (engine) and reversed (refrigerator) directions; assuming both isotherms have heat addition, whereas only the high-temperature isotherm adds heat in the engine mode.

Final Answer:

Isothermal compression