Question

In which of the following pairs of cycles, both cycles have at least one isothermal process?

• A. Diesel cycle and Otto cycle
• B. Carnot cycle and Stirling cycle
• C. Brayton cycle and Rankine cycle
• D. Bell-Coleman cycle and Vapour compression refrigeration cycle
  \textbf{Correct Answer:} (B) Carnot cycle and Stirling cycle

Hint:

Isothermal processes are characteristic of cycles like Carnot and Stirling, where the temperature remains constant during part of the process. These cycles are idealized models often used in thermodynamics to analyze heat engines.

Answer 1

The Correct Option is B

To solve this problem, we need to understand the concept of an isothermal process. An isothermal process is a thermodynamic process in which the temperature remains constant throughout. This is characteristic of certain cycles, where the heat added or removed is at a constant temperature. Let's analyze each of the options:
Statement (A): Diesel cycle and Otto cycle
- Diesel Cycle: This is a type of compression-ignition engine cycle. It involves adiabatic compression and expansion, meaning there are no isothermal processes involved in this cycle.
- Otto Cycle: Similar to the Diesel cycle, the Otto cycle involves adiabatic processes during compression and expansion, so there are no isothermal processes in the Otto cycle either.
Thus, Statement (A) is incorrect.
Statement (B): Carnot cycle and Stirling cycle
- Carnot Cycle: The Carnot cycle is a theoretical thermodynamic cycle that consists of two isothermal processes (one during expansion and one during compression). The isothermal processes in the Carnot cycle are idealized and are essential for its high efficiency.
- Stirling Cycle: The Stirling cycle involves two isothermal processes as well—one during the expansion of the gas and the other during its compression. This cycle operates on a constant-temperature basis during these steps, so it too has isothermal processes. Thus, Statement (B) is correct.
Statement (C): Brayton cycle and Rankine cycle
- Brayton Cycle: The Brayton cycle is an idealized thermodynamic cycle used in gas turbines. It involves adiabatic compression and expansion, so there are no isothermal processes in the Brayton cycle.
- Rankine Cycle: The Rankine cycle is used in steam engines, and although it involves heat transfer, it is typically not considered an isothermal process. Instead, it includes isentropic processes (constant entropy), not isothermal ones.
Thus, Statement (C) is incorrect.
Statement (D): Bell-Coleman cycle and Vapour compression refrigeration cycle - Bell-Coleman Cycle: This is a cycle used in air refrigeration systems and involves both isentropic and isochoric processes but does not necessarily include isothermal processes.
- Vapour Compression Refrigeration Cycle: This cycle is widely used in refrigeration systems, and it also does not include isothermal processes. It consists of compression, expansion, and heat exchange processes, but not isothermal compression or expansion.
Thus, Statement (D) is incorrect.
Conclusion: The correct answer is (B) because both the Carnot cycle and Stirling cycle have at least one isothermal process.