Carnot Cycle MCQ – Objective Questions with Answers
Mechanical Engineering › Thermodynamics | Free practice MCQs with detailed explanations
Last Updated: June 2026
📌 About this MCQ Set
The Carnot cycle is the most efficient reversible cycle operating between two temperature limits. It is the benchmark against which all real heat engines are measured.
These MCQs cover the four processes of the cycle, its efficiency, and Carnot’s theorem.
8 questions • every answer comes with a worked explanation. Click Show Answer to check yourself.
📖 New to this topic? Read the full concept guide: Carnot Cycle →
Carnot Cycle MCQs
Q1. The Carnot cycle consists of:
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Answer: A. Two isothermal and two adiabatic processes
The Carnot cycle has two reversible isothermal processes and two reversible adiabatic (isentropic) processes.
Q2. The efficiency of a Carnot engine is given by:
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Answer: A. 1 − T_L/T_H
η_Carnot = 1 − T_L/T_H, with temperatures in Kelvin.
Q3. Carnot efficiency depends on:
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Answer: B. Source and sink temperatures only
It depends only on the absolute temperatures of the hot and cold reservoirs.
Q4. A Carnot engine operates between 600 K and 300 K. Its efficiency is:
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Answer: B. 50%
η = 1 − 300/600 = 0.5 = 50%.
Q5. The Carnot cycle is significant because it:
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Answer: A. Has the highest efficiency between two temperatures
It sets the upper limit of efficiency for any heat engine working between the same two temperatures.
Q6. To increase Carnot efficiency, the most effective step is to:
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Answer: C. Raise the source temperature / lower the sink temperature
Efficiency rises as T_H increases or T_L decreases.
Q7. A reversed Carnot cycle is used as the ideal model of a:
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Answer: B. Refrigerator / heat pump
Running the Carnot cycle in reverse models an ideal refrigerator or heat pump.
Q8. 100% Carnot efficiency is theoretically possible only if the sink temperature is:
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Answer: A. 0 K (absolute zero)
η = 1 only if T_L = 0 K, which is unattainable (Third Law), so 100% efficiency is impossible.
Frequently Asked Questions
Why is the Carnot cycle the most efficient?
Because all its processes are reversible, it has no losses, giving the maximum possible efficiency between two temperature limits.
What are the four processes of the Carnot cycle?
Reversible isothermal expansion, reversible adiabatic expansion, reversible isothermal compression, and reversible adiabatic compression.
Why can’t real engines achieve Carnot efficiency?
Real processes involve friction, finite temperature differences and other irreversibilities that the ideal reversible Carnot cycle ignores.