---
title: "Ideal diesel cycle working principle"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 6
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:diesel-cycle-working-principle"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/diesel-cycle-working-principle
site: BCT Engineering
---

# Ideal diesel cycle working principle

Fundamental of Thermodynamics & Heat Transfer · Chapter 6: Thermodynamic Cycles

- Asked in: 2073 Magh, 6 marks
- Video: [Diesel Cycle](https://www.youtube.com/watch?v=Op1b1j0ViJg) — TutorialsPoint
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/diesel-cycle-working-principle
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-6

## Question

Explain the working principle of an ideal diesel cycle with P-V and T-S diagrams.

## Short answer

The ideal diesel cycle models CI engines: isentropic compression, constant-pressure heat addition as fuel burns, isentropic expansion, and constant-volume heat rejection. Efficiency decreases with increasing cutoff ratio and increases with compression ratio.

## Answer

The ideal (air-standard) diesel cycle models the compression-ignition engine. Air is compressed so much that fuel injected at the end of compression ignites by itself. Heat is added at constant pressure while the piston moves outward, and rejected at constant volume.

#### Processes
1. **1-2: Isentropic compression.** Air is compressed from V₁ to V₂ (compression ratio r = V₁/V₂, typically 14-22). The temperature rises above the ignition temperature of the fuel.
2. **2-3: Constant-pressure heat addition.** Fuel is injected and burns while the piston starts moving down; the volume increases from V₂ to V₃. Cut-off ratio r_c = V₃/V₂.
3. **3-4: Isentropic expansion.** The gas expands to V₄ = V₁ and produces the power stroke.
4. **4-1: Constant-volume heat rejection.** Heat is rejected as the exhaust valve opens and the pressure falls to the initial value.

#### Diagrams
```text
 P-V diagram                      T-s diagram
 P                                T
 |   2 _______ 3                  |               3
 |   |          \                 |             .'|
 |   |           '.               |           .'  |
 |   |             '.             |         .'    |
 |    \              '-.          |    2 _.'      |
 |     \                '-. 4     |    |          4
 |      '.                  |     |    |        .'
 |        '-.               |     |    |      .'
 |           '-._           |     |    |    .'
 |               '--.._     |     |    1.-'
 |                     '--- 1     |
 +-------------------------> V    +--------------------> s
    V2                      V1
 1-2 isentropic compression       2-3 constant-P heat addition
 3-4 isentropic expansion         4-1 constant-V heat rejection
```

#### Efficiency
q_in = cp (T₃ − T₂), q_out = cv (T₄ − T₁)

**η = 1 − (1/r^(γ−1)) × (r_c^γ − 1)/(γ (r_c − 1))**

The efficiency rises with the compression ratio and falls as the cut-off ratio increases. As r_c → 1, the diesel efficiency approaches the Otto efficiency 1 − 1/r^(γ−1).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does fuel ignite in the diesel cycle?
   A. By spark plug
   B. By glow of exhaust
   C. Air is compressed so hot that injected fuel self-ignites
   D. By a pilot flame
   Answer: C. Compression raises temperature above the fuel ignition temperature. (Key point: Ignition)

2. Heat addition in the ideal diesel cycle is at:
   A. Constant temperature
   B. Constant volume
   C. Constant entropy
   D. Constant pressure
   Answer: D. Fuel burns as the piston moves down; heat is added at constant pressure, and rejected at constant volume. (Key point: Heat addition)

3. The cut-off ratio is:
   A. P3/P2
   B. V3/V2
   C. V4/V3
   D. V1/V2
   Answer: B. Cut-off ratio r_c = V3/V2. (Key point: Cut-off ratio)

4. As cut-off ratio r_c approaches 1, diesel efficiency:
   A. Approaches Otto efficiency
   B. Approaches zero
   C. Becomes Brayton
   D. Approaches 100%
   Answer: A. The bracket tends to 1, giving 1 - 1/r^(g-1). (Key point: Cut-off effect)

## Related in Fundamental of Thermodynamics & Heat Transfer

- [Ideal diesel cycle: diagrams and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cycle-efficiency-derivation) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Ideal Brayton cycle: diagrams and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Abrayton-cycle-efficiency-derivation) — Fundamental of Thermodynamics & Heat Transfer, chapter 6, asked 2×
- [Ideal Otto cycle diagrams and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aotto-cycle-efficiency-compression-ratio) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Brayton cycle working principle with P-v and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Abrayton-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Rankine cycle working principle](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Arankine-cycle-working-principle) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Ideal diesel engine CR 20: efficiency and MEP](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cr20-max-2200k) — Fundamental of Thermodynamics & Heat Transfer, chapter 6

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- [Ideal vapour compression cycle for 5 TR](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Avcr-ideal-cycle-numerical) — Applied Thermodynamics, chapter 3
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