Automotive Engines: Construction and Working
What the exam tests
Engine construction and working covers the cylinder block, head, piston, rings, connecting rod, crankshaft, flywheel, camshaft and valve train. You must explain the four strokes of SI and CI engines, valve timing, firing order and compression ratio.
Core rules and concepts
- In the SI engine, a spark ignites a petrol-air mixture. In the CI engine, high compression raises air temperature and diesel is injected.
- Four strokes: intake, compression, power and exhaust. Two crankshaft revolutions complete one cycle.
- Swept volume Vs = π/4 × bore² × stroke. Compression ratio CR = (Vs + Vc)/Vc, where Vc is clearance volume.
- Valve overlap improves scavenging; excessive overlap causes poor idling.
- Firing order depends on crankshaft and camshaft design; it affects balance and smoothness.
- Piston rings seal combustion gas, control oil and transfer heat.
Worked example
A single-cylinder engine has bore 80 mm, stroke 90 mm and clearance volume 50 cm³. Swept volume = π/4 × 8² × 9 = 452.4 cm³. CR = (452.4 + 50)/50 = 10.05:1. The engine is petrol, so this is a plausible ratio. If the clearance volume were 40 cm³, CR would rise to 12.31:1, increasing the risk of detonation unless fuel octane and ignition timing suit it.
Traps that cost marks
- Using bore instead of radius, or mixing mm and cm in volume.
- Confusing compression ratio with compression pressure.
- Treating valve overlap as a defect rather than a design feature.
- Mixing the order of strokes or the number of crankshaft turns.