---
title: "Cetane number and rating of CI fuels"
subject: "Internal Combustion Engines"
chapter: 3
key: "semester_6/internal_combustion_engines:cetane-number-diesel-fuel-rating"
source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/cetane-number-diesel-fuel-rating
site: BCT Engineering
---

# Cetane number and rating of CI fuels

Internal Combustion Engines · Chapter 3: Engine fuels

- Asked in: year not given, 6 marks
- Video: [Cetane Number of diesel Fuels in compression engine](https://www.youtube.com/watch?v=JyWaApDEbpY) — C for Chemistry
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/cetane-number-diesel-fuel-rating
- Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-3

## Question

Define the cetane number. Explain how CI engine fuels are rated. Differentiate between octane number and cetane number, and state the effect of cetane number on diesel knock.

## Short answer

Cetane number indicates ignition quality; it is the % n-cetane in a reference blend giving the same delay. CI fuels are rated in a CFR engine by matching ignition delay. High cetane (40–55) shortens delay, reducing diesel knock. Octane number measures knock resistance (SI), cetane number measures ignition ease (CI); they are inversely related.

## Answer

#### Cetane number
The cetane number (CN) of a diesel fuel is the percentage by volume of cetane (n-hexadecane, C$_{16}$H$_{34}$) in a mixture with alpha-methylnaphthalene that gives the same ignition delay as the fuel under test. Cetane (ignites very easily) has CN = 100 and alpha-methylnaphthalene (ignites poorly) has CN = 0. In current practice heptamethylnonane (HMN, CN = 15) is often used in place of alpha-methylnaphthalene, so that $\text{CN} = \%\ \text{cetane} + 0.15 \times \%\ \text{HMN}$.

#### Rating of CI fuels
- The fuel is tested in a CFR diesel engine at 900 rpm with fixed injection timing (13 degrees before TDC).
- The compression ratio is changed until ignition starts exactly at TDC, i.e. the ignition delay is 13 degrees.
- Reference blends are run to find the blend with the same delay at the same compression ratio.
- Another quick estimate is the **diesel index** from aniline point and API gravity: $\text{DI} = \text{aniline point}(^\circ\text{F}) \times \text{API}/100$.

Normal diesel has CN 40-55. A high CN means a short ignition delay.

#### Octane number versus cetane number
| Basis | Octane number | Cetane number |
|---|---|---|
| Measures | Resistance to auto-ignition | Readiness to auto-ignite |
| Used for | SI engine fuels | CI engine fuels |
| Reference fuels | Iso-octane (100), n-heptane (0) | n-cetane (100), alpha-methylnaphthalene (0) |
| Desirable value | High (85-98) | Moderate to high (40-55) |
| Fuel structure | Aromatics, branched chains good | Straight chains good |
| Relation | A good petrol is a poor diesel and vice versa | |

#### Effect on diesel knock
A low CN gives a long ignition delay. A large amount of fuel collects before burning and then burns suddenly, giving a very rapid pressure rise and the harsh knock. A high-CN fuel has a short delay, smooth combustion, easier starting and less noise.

## Check yourself

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

1. What is cetane number?
   A. Percent by volume of cetane in a blend with alpha-methylnaphthalene that gives the same ignition delay
   B. Percent iso-octane in n-heptane
   C. The octane number divided by 2
   D. Resistance to auto-ignition
   Answer: A. Cetane (CN 100) ignites readily; alpha-methylnaphthalene (CN 0) ignites poorly. (Key point: Cetane number definition)

2. Why does a low cetane number cause diesel knock?
   A. Short delay burns fuel too early
   B. Long ignition delay accumulates fuel which then burns suddenly, giving a rapid pressure rise
   C. Fuel does not vaporise
   D. Spark timing is wrong
   Answer: B. A large quantity of fuel burns at once after a long delay, causing the harsh knock. (Key point: Effect on diesel knock)

3. What do octane and cetane numbers measure respectively?
   A. Readiness to auto-ignite; resistance to auto-ignition
   B. Both measure resistance to auto-ignition
   C. Resistance to auto-ignition; readiness to auto-ignite
   D. Both measure calorific value
   Answer: C. They are opposite properties, so a good petrol is a poor diesel. (Key point: Octane vs cetane)

4. In the CFR diesel test, what is varied until ignition begins exactly at TDC?
   A. Injection pressure
   B. Bore
   C. Fuel temperature only
   D. Compression ratio
   Answer: D. With fixed injection timing (13 deg bTDC), compression ratio is changed to get a 13 degree delay. (Key point: Test method)

## Related in Internal Combustion Engines

- [Octane number and rating of SI fuels](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aoctane-number-rating-si-fuels) — Internal Combustion Engines, chapter 3
- [Stages of CI combustion and diesel knock](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-combustion-stages-knock) — Internal Combustion Engines, chapter 5

## Related in other subjects

- [Requirements of SI and CI engine fuels](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Asi-engine-fuel-requirements) — Automotive Technology, chapter 2
- [Properties of gasoline and diesel fuels](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Agasoline-diesel-properties-comparison) — Energy Resources, chapter 1
- [Phases of combustion in a CI engine](https://www.bctengineer.com/question.md?key=semester_5%2Fautomotive_technology%3Acombustion-ci-engine-phases) — Automotive Technology, chapter 3