---
title: "Spark ignition vs compression ignition engine"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 6
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:spark-vs-compression-ignition"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/spark-vs-compression-ignition
site: BCT Engineering
---

# Spark ignition vs compression ignition engine

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

- Asked in: 2071 Magh, 6 marks
- Video: [Difference between Spark & Compression Ignition Engines | Internal Combustion Engine](https://www.youtube.com/watch?v=qMG31yfx-DQ) — Magic Marks
- 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/spark-vs-compression-ignition
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-6

## Question

Compare spark ignition and compression ignition engine. Sketch P-V and T-S diagrams for them.

## Short answer

Spark ignition (SI) engines use the Otto cycle with petrol, low compression ratio (6–10), spark plug, constant-volume heat addition. Compression ignition (CI) engines use the Diesel cycle with diesel, high compression ratio (14–22), self-ignition, constant-pressure heat addition. P-V and T-S diagrams for each show isentropic compression/expansion; heat addition differs.

## Answer

A spark ignition (SI) engine follows the Otto cycle and a compression ignition (CI) engine follows the Diesel cycle.

| Point | SI engine (Otto) | CI engine (Diesel) |
|---|---|---|
| Fuel | Petrol, light and volatile | Diesel oil, heavier |
| Air-fuel intake | Air + fuel mixed (carburettor or injection) during suction | Only air drawn in; fuel injected at end of compression |
| Ignition | By spark plug | By heat of compressed air (self-ignition) |
| Compression ratio | Low, 6-10 (knock limit) | High, 14-22 |
| Heat addition | At constant volume | At constant pressure |
| Efficiency | Lower in practice, ~25-30 % | Higher in practice, ~35-40 % |
| Weight and cost | Light, cheaper | Heavier, costlier |
| Speed | High-speed | Lower speed |
| Governing | Quantity governing (throttle) | Quality governing (fuel quantity) |
| Pollution | More CO and HC | More NOx and smoke |

#### Diagrams
**Otto cycle (SI engine)**
```text
 P-V diagram                      T-s diagram
 P                                T
 |   3                            |             3
 |   |\                           |            /|
 |   | \                          |           / |
 |   |  '.                        |          /  |
 |   |    '.                      |        .'   |
 |   2      '-.                   |    2 .'     |
 |    \        '-.                |    |        4
 |     '.         '-.             |    |       /
 |       '-.         '-. 4        |    |     .'
 |          '-._         |        |    |   .'
 |              '-.._    |        |    1.-'
 |                   '-- 1        |
 +-----------------------> V      +--------------------> s
    V2                   V1
```

**Diesel cycle (CI engine)**
```text
 P-V diagram                      T-s diagram
 P                                T
 |   2 _______ 3                  |               3
 |   |          \                 |             .'|
 |   |           '.               |           .'  |
 |   |             '.             |         .'    |
 |    \              '-.          |    2 _.'      |
 |     \                '-. 4     |    |          4
 |      '.                  |     |    |        .'
 |        '-.               |     |    |      .'
 |           '-._           |     |    |    .'
 |               '--.._     |     |    1.-'
 |                     '--- 1     |
 +-------------------------> V    +--------------------> s
    V2                      V1
```

Otto: 1-2 isentropic compression, 2-3 constant-volume heat addition, 3-4 isentropic expansion, 4-1 constant-volume heat rejection. η = 1 − 1/r^(γ−1).

Diesel: the same, except that 2-3 is at constant pressure. η = 1 − (1/r^(γ−1)) (r_c^γ − 1)/(γ(r_c − 1)).

## Check yourself

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

1. How does ignition differ?
   A. SI by spark, CI by heat of compressed air
   B. Both by spark
   C. SI by compression, CI by spark
   D. Both by compression
   Answer: A. CI engines self-ignite fuel injected into hot compressed air. (Key point: Ignition)

2. Typical compression ratios:
   A. Both 14-22
   B. SI 14-22, CI 6-10
   C. Both 6-10
   D. SI 6-10, CI 14-22
   Answer: D. SI is limited by knock. (Key point: Compression ratio)

3. Heat addition in SI and CI cycles:
   A. Both constant volume
   B. SI constant volume, CI constant pressure
   C. SI constant pressure, CI constant volume
   D. Both constant pressure
   Answer: B. Otto adds heat at constant volume, Diesel at constant pressure. (Key point: Heat addition)

4. Load control:
   A. SI quantity (throttle), CI quality (fuel amount)
   B. Both quality
   C. SI quality, CI quantity
   D. Both quantity
   Answer: A. SI throttles the mixture; CI varies the fuel injected. (Key point: Governing)

## Related in Fundamental of Thermodynamics & Heat Transfer

- [Power vs refrigeration cycle; Rankine components and diagrams](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Apower-vs-refrigeration-cycle-rankine-components) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Vapor compression refrigeration cycle p-h and T-s](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Avapor-compression-refrigeration-ph-ts) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [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
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- [Air standard assumptions; diesel cycle; Otto vs diesel](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aair-standard-assumptions-diesel-vs-otto) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Gas vs vapor cycles; Otto cycle efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-vs-vapor-cycles-otto-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6

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