--- title: "Properties of gasoline and diesel fuels" subject: "Energy Resources" chapter: 1 key: "semester_6/energy_resources:gasoline-diesel-properties-comparison" source: https://www.bctengineer.com/q/semester_6/energy_resources/gasoline-diesel-properties-comparison site: BCT Engineering --- # Properties of gasoline and diesel fuels Energy Resources · Chapter 1: Fossil fuels and their characteristics - Asked in: year not given, 4+4 marks - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/energy_resources/gasoline-diesel-properties-comparison - Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-1 ## Question (a) Define octane number and cetane number. How is each measured, and why does a good petrol have a high octane number but a good diesel a high cetane number? (b) Differentiate between gasoline and diesel on the basis of volatility, self-ignition temperature, calorific value, density, flash point and sulphur content. ## Short answer Octane number measures knock resistance of spark-ignition fuel (high value resists knock). Cetane number measures ignition quality of diesel (high value ensures short ignition delay). Petrol needs high octane to avoid knock during spark ignition; diesel needs high cetane for rapid self-ignition. Gasoline has higher volatility (40-200°C), lower density (720-775 kg/m3), lower flash point (<-20°C), and higher self-ignition temperature (~250-280°C) than diesel, which has higher density (820-860 kg/m3), higher flash point (>52°C), and lower self-ignition temperature (~210-250°C). ## Answer #### Octane number It is a measure of the **knock resistance** of a spark-ignition fuel. It is the percentage by volume of iso-octane (octane number 100) in a mixture with n-heptane (octane number 0) that matches the knocking of the fuel in a standard CFR engine. A higher octane number allows a higher compression ratio to be used without knock. #### Cetane number It is a measure of the **ignition quality** of a diesel fuel. It is the percentage by volume of cetane (n-hexadecane, 100) in a mixture with alpha-methylnaphthalene (0) that has the same ignition delay as the fuel in the standard CFR engine. Diesel fuels need cetane numbers of 40–55. #### Why they differ In a petrol engine the charge is ignited by a spark, so the fuel must resist spontaneous ignition (knock) and have a high octane number. In a diesel engine the fuel is injected into hot compressed air and must ignite quickly by itself, so a short ignition delay (high cetane number) avoids diesel knock. The two scales run in opposite directions: straight-chain paraffins have high cetane but low octane numbers, while aromatics and branched chains have high octane but low cetane numbers. #### Gasoline versus diesel | Property | Gasoline | Diesel | |---|---|---| | Volatility | High, boils 40–200 °C | Low, boils 250–350 °C | | Self-ignition temperature | Higher (about 250–280 °C) | Lower (about 210–250 °C) | | Calorific value | About 44 MJ/kg | About 42–43 MJ/kg | | Density at 15 °C | 720–775 kg/m³ | 820–860 kg/m³ | | Flash point | Below −20 °C | Above 52 °C | | Sulphur | Low (limit around 10–50 ppm) | Higher in older grades; ultra-low limit now | | Quality index | Octane number 87–98 | Cetane number 40–55 | | Use | Spark-ignition engines | Compression-ignition engines | Diesel has more energy per litre because of its higher density. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. The octane number of a fuel is the percentage by volume of what in a mixture with n-heptane that matches its knocking in a CFR engine? A. Cetane B. Alpha-methylnaphthalene C. Benzene D. Iso-octane Answer: D. Iso-octane is 100 and n-heptane 0 on the octane scale. (Key point: Octane number definition) 2. Why should a diesel fuel have a high cetane number? A. It must resist self-ignition to avoid knock B. It must have a high compression ratio limit C. It must ignite quickly by itself in hot compressed air, giving a short ignition delay D. It must be ignited by a spark plug Answer: C. Diesel is compression ignited, so short ignition delay avoids diesel knock; petrol is the opposite and needs knock resistance. (Key point: Why diesel needs high cetane) 3. Which statement about self-ignition temperature is correct? A. Gasoline about 210-260 °C, diesel about 400-500 °C B. Gasoline about 400-500 °C, diesel about 210-260 °C C. Both are about 400-500 °C D. Both are below 100 °C Answer: B. Diesel ignites at the lower temperature, which suits compression ignition; petrol needs a spark. (Key point: Self-ignition temperature) 4. Diesel has a slightly lower calorific value per kg than gasoline, yet more energy per litre. Why? A. Its density is higher B. Its volatility is higher C. Its sulphur content is lower D. Its flash point is lower Answer: A. Density 820-860 vs 720-775 kg/m³ more than makes up for the small per-kg difference. 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