Diesel Cycle MCQ – Objective Questions with Answers

Diesel Cycle MCQ – Objective Questions with Answers

Mechanical Engineering › Thermodynamics | Free practice MCQs with detailed explanations

Last Updated: June 2026

📌 About this MCQ Set

The Diesel cycle is the ideal cycle for compression-ignition (diesel) engines. Heat is added at constant pressure.

These MCQs cover its processes, efficiency, and the cut-off ratio.

8 questions • every answer comes with a worked explanation. Click Show Answer to check yourself.

📖 New to this topic? Read the full concept guide: Diesel Cycle →

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Diesel Cycle MCQs

Q1. The Diesel cycle is the ideal cycle for:

  1. Petrol engines
  2. Compression-ignition (diesel) engines
  3. Gas turbines
  4. Refrigerators
Show Answer

Answer: B. Compression-ignition (diesel) engines

It models compression-ignition (diesel) engines.

Q2. In the Diesel cycle, heat is added at constant:

  1. Volume
  2. Pressure
  3. Temperature
  4. Entropy
Show Answer

Answer: B. Pressure

Heat addition occurs at constant pressure; heat rejection at constant volume.

Q3. The Diesel cycle efficiency depends on compression ratio and:

  1. Cut-off ratio
  2. Pressure ratio
  3. Bore diameter
  4. Stroke length
Show Answer

Answer: A. Cut-off ratio

η depends on compression ratio r, cut-off ratio ρ and γ.

Q4. The cut-off ratio is the ratio of volumes:

  1. Before and after heat addition
  2. At top and bottom dead centre
  3. Of clearance to swept
  4. Of inlet to exhaust
Show Answer

Answer: A. Before and after heat addition

Cut-off ratio ρ = V3/V2, the volume ratio during constant-pressure heat addition.

Q5. For the same compression ratio, Diesel cycle efficiency is ____ Otto cycle efficiency.

  1. Higher than
  2. Lower than
  3. Equal to
  4. Double
Show Answer

Answer: B. Lower than

At equal compression ratio, the Diesel cycle is less efficient than the Otto cycle due to the cut-off term.

Q6. Diesel engines can use higher compression ratios than petrol engines because:

  1. They are bigger
  2. Only air is compressed, avoiding knocking
  3. They are cheaper
  4. They run slower
Show Answer

Answer: B. Only air is compressed, avoiding knocking

Compressing only air (fuel injected later) avoids pre-ignition, allowing much higher compression ratios.

Q7. As the cut-off ratio increases (at fixed compression ratio), Diesel efficiency:

  1. Increases
  2. Decreases
  3. Stays constant
  4. Doubles
Show Answer

Answer: B. Decreases

A larger cut-off ratio reduces the air-standard efficiency of the Diesel cycle.

Q8. The Diesel cycle consists of two adiabatics, one constant-pressure and one:

  1. Constant-volume process
  2. Isothermal process
  3. Constant-entropy process
  4. Polytropic process
Show Answer

Answer: A. Constant-volume process

Two reversible adiabatics, constant-pressure heat addition, and constant-volume heat rejection.

Frequently Asked Questions

What is the Diesel cycle?

The ideal air-standard cycle for compression-ignition engines, with heat added at constant pressure.

Why are diesel engines more efficient in practice?

They run at much higher compression ratios (only air is compressed, so no knocking), which raises thermal efficiency.

What is the cut-off ratio?

The ratio of cylinder volumes at the end and start of the constant-pressure heat-addition process (ρ = V3/V2).

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