Internal Combustion Engines and Fuels
This section covers Internal Combustion Engines and Fuels. The exam tests four-stroke and two-stroke operation, SI and CI engine differences, compression ratio, and fuel ratings such as octane and cetane.
Core concepts
- A four-stroke SI engine has suction, compression, power and exhaust strokes. A spark plug ignites a petrol-air mixture. A CI engine draws air, compresses it to high temperature, then injects diesel; ignition is by compression heat.
- In a two-stroke engine, transfer and exhaust ports are uncovered by the piston; a power stroke occurs each revolution, but scavenging and lubrication need care.
- Compression ratio = (swept volume + clearance volume) ÷ clearance volume. Higher compression generally improves efficiency but can cause knock.
- Petrol is rated by octane number; higher octane resists knock in SI engines. Diesel is rated by cetane number; higher cetane aids ignition quality.
Worked example
A four-cylinder engine has bore 80 mm, stroke 90 mm and clearance volume 50 cm³ per cylinder. Swept volume = π/4 × 8² × 9 = 452.4 cm³. Compression ratio = (452.4 + 50) ÷ 50 = 10.05:1. Total displacement = 4 × 452.4 = 1,809.6 cm³. The reasoning converts dimensions to centimetres before using the cylinder formula.
Traps that cost marks
Do not put a spark plug in a CI engine or an injector in a petrol engine as the main ignition device. Do not say a two-stroke engine fires every second revolution. Do not confuse octane with cetane. Use clearance volume, not swept volume alone, in compression ratio.