Day 7 Night – Thermal Engg → IC Engines (Otto Cycle)

Why this topic is important for ECET:
The Otto cycle forms the foundation of petrol engine operation, a crucial portion of Thermal Engineering in ECET Mechanical. Understanding it helps in solving cycle efficiency, work output, and P-V diagram problems—frequent numerical and conceptual questions in ECET exams. Mastery of this topic improves both speed and accuracy in Thermodynamics sections.

---

📘 Concept Notes

1. Introduction to Otto Cycle:

• The Otto cycle is an idealized cycle representing the functioning of a spark-ignition (SI) engine.
• It consists of 4 reversible processes:
  1. Adiabatic compression (1 → 2) – piston compresses air-fuel mixture, temperature rises.
  2. Constant volume heat addition (2 → 3) – fuel ignites at top dead center, pressure rises.
  3. Adiabatic expansion (3 → 4) – piston moves down, performing work on the crankshaft.
  4. Constant volume heat rejection (4 → 1) – exhaust gases release heat at bottom dead center.

2. P-V and T-S Diagrams:

• P-V diagram shows pressure variation with volume.
• T-S diagram (Temperature-Entropy) highlights heat addition and rejection at constant volume.
• Efficiency depends on compression ratio (r = V1/V2) and specific heat ratio (k = cp/cv).

3. Key Terms:

• Compression ratio (r):
• Cut-off ratio (for Diesel, not Otto): N/A for Otto cycle
• Mean effective pressure (MEP): Average pressure that produces net work during cycle.

4. Performance Parameters:

• Thermal efficiency of Otto cycle:

⚙️ Formulas

Where:

• = thermal efficiency
• = compression ratio
• = ratio of specific heats

---

🔟 10 MCQs (Easy → GATE-level)

1. The Otto cycle is used in which type of engines?
   a) Diesel engines
   b) Petrol engines
   c) Steam engines
   d) Gas turbines
2. In the ideal Otto cycle, heat addition occurs at:
   a) Constant pressure
   b) Constant volume
   c) Adiabatic process
   d) Isothermal process
3. Thermal efficiency of an Otto cycle depends primarily on:
   a) Cut-off ratio
   b) Compression ratio
   c) Stroke length
   d) Engine displacement
4. For an Otto cycle with compression ratio r = 8 and γ = 1.4, the thermal efficiency is approximately:
   a) 0.56
   b) 0.68
   c) 0.48
   d) 0.72
5. Which process in the Otto cycle is adiabatic expansion?
   a) 1 → 2
   b) 2 → 3
   c) 3 → 4
   d) 4 → 1
6. Work done in an ideal Otto cycle is:
   a) Area under T-S curve
   b) Area under P-V curve
   c) Difference of volumes
   d) Pressure difference
7. For an Otto cycle, increasing the compression ratio will:
   a) Decrease efficiency
   b) Increase efficiency
   c) No effect on efficiency
   d) Increase work but decrease efficiency
8. The ratio of specific heats γ for air is:
   a) 1.2
   b) 1.3
   c) 1.4
   d) 1.5
9. A petrol engine operates on the Otto cycle with . If γ = 1.4, find thermal efficiency.
   a) 0.60
   b) 0.61
   c) 0.64
   d) 0.55
10. If the pressure at the end of compression (state 2) is 10 bar and volume at start of compression (state 1) is 0.5 L, volume at top dead center is 0.0625 L. Find pressure at state 2 assuming γ = 1.4.
    a) 35 bar
    b) 38 bar
    c) 40 bar
    d) 42 bar

---

✅ Answer Key

Q.No	Answer
1	b
2	b
3	b
4	a
5	c
6	b
7	b
8	c
9	c
10	b

---

🧠 Explanations

1. b) Otto cycle is used in petrol engines (SI engines). Diesel uses Diesel cycle.
2. b) Heat addition occurs at constant volume (isochoric) in Otto cycle.
3. b) Efficiency formula depends on compression ratio:
4. a)
5. c) Expansion occurs after combustion, 3 → 4.
6. b) Work done equals area enclosed in P-V diagram.
7. b) Higher compression ratio increases thermal efficiency.
8. c) For air, γ ≈ 1.4.
9. c)
10. b)

---

🎯 Motivation / Why Practice Matters
Mastering Otto cycle questions is critical for ECET 2026 Mechanical. These numericals appear repeatedly, and fast calculation of efficiency and work can save valuable time. Conceptual clarity reduces mistakes and improves competitive advantage in Thermodynamics sections.

---

📲 CTA
Join our WHATSAPP group for ECET 2026 updates and discussions → https://chat.whatsapp.com/GniYuv3CYVDKjPWEN086X9