- Practice · 4+4 marks
Properties of gasoline and diesel fuels
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).
(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.
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.
✓ Answer checked by Claude Opus 5.5 (high effort)
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