Why This Topic is Important for ECET
The Diesel Cycle is the fundamental cycle for compression ignition (CI) engines, used in trucks, buses, ships, and power plants. In ECET exams, questions are often asked on its P-V/T-S diagrams, efficiency formulas, comparison with Otto cycle, and numerical problems. Understanding this cycle ensures you can solve both conceptual and numerical questions quickly, boosting your score in Thermal Engineering.
📘 Concept Notes
The Diesel Cycle (or Constant Pressure Cycle) is the ideal cycle for diesel engines where combustion occurs at constant pressure.
🔹 Four Processes of Diesel Cycle
- 1 → 2: Isentropic Compression
- Air is compressed adiabatically.
- Pressure and temperature rise.
- 2 → 3: Constant Pressure Heat Addition
- Fuel is injected and burns at constant pressure.
- Volume increases.
- 3 → 4: Isentropic Expansion (Power Stroke)
- High-pressure gases expand, doing work.
- 4 → 1: Constant Volume Heat Rejection
- Heat is rejected at constant volume.
🔹 P-V and T-S Diagrams
- P-V diagram: Compression, constant pressure addition, expansion, constant volume rejection.
- T-S diagram: Adiabatic processes are vertical, constant pressure = horizontal rising line, constant volume = horizontal falling line.
🔹 Efficiency of Diesel Cycle
- The efficiency depends on:
- Compression ratio (rrr)
- Cut-off ratio (rcr_crc)
- Ratio of specific heats (γ\gammaγ)
- Diesel Cycle is less efficient than Otto cycle for the same compression ratio because of higher cut-off losses.
- But practically, Diesel engines operate at higher rrr, so they are often more efficient.
🔹 Applications
- Heavy vehicles (trucks, buses).
- Marine engines.
- Railway locomotives.
- Power generation (diesel power plants).
⚙️ Formulas
- Compression Ratio
Cut-off Ratio
Air Standard Efficiency of Diesel Cycle
Mean Effective Pressure (MEP)
Work Output per kg of Air
🔟 10 MCQs
Q1. The Diesel cycle is also called:
a) Constant volume cycle
b) Constant pressure cycle
c) Dual cycle
d) Rankine cycle
Q2. In Diesel cycle, heat is added at:
a) Constant pressure
b) Constant volume
c) Constant temperature
d) Constant entropy
Q3. The compression ratio in diesel engines is usually:
a) 4–6
b) 8–10
c) 14–22
d) 25–35
Q4. For the same compression ratio, the Diesel cycle efficiency is:
a) Higher than Otto cycle
b) Lower than Otto cycle
c) Equal to Otto cycle
d) Can’t be compared
Q5. Cut-off ratio in Diesel cycle is defined as:
a) V1/V2V_1 / V_2V1/V2
b) V3/V2V_3 / V_2V3/V2
c) V2/V3V_2 / V_3V2/V3
d) V4/V1V_4 / V_1V4/V1
Q6. If r=16r = 16r=16, rc=2r_c = 2rc=2, and γ=1.4\gamma = 1.4γ=1.4, the air standard efficiency is approximately:
a) 48%
b) 55%
c) 60%
d) 70%
Q7. In the P-V diagram of Diesel cycle, the process 2 → 3 is:
a) Isothermal
b) Isentropic
c) Constant pressure
d) Constant volume
Q8. The efficiency of Diesel cycle increases when:
a) Cut-off ratio increases
b) Compression ratio increases
c) Heat rejected increases
d) Expansion ratio decreases
Q9. For the same compression ratio and heat input, which cycle has maximum efficiency?
a) Otto
b) Diesel
c) Dual
d) Carnot
Q10. Which engine actually works on the Diesel cycle?
a) Petrol engine
b) Diesel engine
c) Steam turbine
d) Jet engine
✅ Answer Key
| Q | Ans |
|---|---|
| 1 | b |
| 2 | a |
| 3 | c |
| 4 | b |
| 5 | b |
| 6 | b |
| 7 | c |
| 8 | b |
| 9 | d |
| 10 | b |
🧠 Explanations
- Q1: Diesel cycle = Constant pressure cycle → (b).
- Q2: Heat is supplied at constant pressure (fuel injection) → (a).
- Q3: Diesel engines use r=14–22r = 14–22r=14–22 → (c).
- Q4: For same rrr, Otto > Diesel because cut-off ratio reduces efficiency → (b).
- Q5: Cut-off ratio rc=V3/V2r_c = V_3/V_2rc=V3/V2 → (b).
- Q6: Substituting in efficiency formula gives ~55% → (b).
- Q7: Heat addition at constant pressure → (c).
- Q8: Higher compression ratio increases efficiency → (b).
- Q9: Carnot cycle is ideal maximum efficiency → (d).
- Q10: Diesel cycle is ideal for CI engines → (b).
🎯 Motivation / Why Practice Matters
Diesel cycle problems are high-scoring in ECET because they combine simple theory + direct formulas.
👉 With practice, you can solve these in less than 1 minute per question.
👉 Accuracy in Thermal Engineering strengthens your overall rank since these are compulsory questions in every ECET set.
Consistency in solving numerical problems is the shortcut to beating competition.
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