- Practice · 5 marks
Requirements of SI and CI engine fuels
Short answer
For SI engines, fuel needs high octane number to resist knock, suitable volatility, high calorific value, low sulphur. For CI engines, high cetane number for short ignition delay, proper viscosity, low sulphur and carbon residue. Octane number measures knock resistance (iso-octane/n-heptane scale); cetane number measures ignition quality (cetane/alpha-methyl naphthalene scale).
List and explain the basic requirements of a good fuel for (i) a spark ignition engine and (ii) a compression ignition engine. Explain octane number and cetane number.
Requirements of a good engine fuel
A good fuel should burn smoothly, give high energy per kg, be easy to store and supply, and cause little pollution.
Spark ignition (petrol) fuel
- High octane number (anti-knock quality), so that a high compression ratio can be used.
- Suitable volatility: high enough for easy cold starting, but not so high that it causes vapour lock.
- High calorific value (about 44 MJ/kg) and low sulphur and gum content.
- Non-corrosive, stable in storage and low in emissions.
Compression ignition (diesel) fuel
- High cetane number for a short ignition delay (smooth, quiet running).
- Proper viscosity, so that the injector atomises it well and it lubricates the pump.
- Low sulphur, ash and carbon residue (fewer deposits and less smoke).
- Suitable pour point and cloud point for cold weather.
Octane and cetane number
- Octane number: the percentage by volume of iso-octane in a blend of iso-octane (rating 100) and n-heptane (rating 0) that matches the knock behaviour of the fuel in a standard CFR engine. A higher value means better knock resistance. Regular petrol is typically about 91 RON, and premium grades 95 RON or more.
- Cetane number: the percentage by volume of cetane (n-hexadecane, rating 100) in a blend with alpha-methyl naphthalene (rating 0) that has the same ignition delay as the diesel fuel. Automotive diesel is typically 45 to 55.
| Property | Petrol | Diesel |
|---|---|---|
| Wanted quality | High octane (resists self-ignition) | High cetane (ignites readily) |
| Rating scale | Octane number | Cetane number |
✓ Answer checked by Claude Opus 5.5 (high effort)
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