Automotive engine systems: construction, components and working principles
Domain focus
This domain examines the construction, components and working principles of automotive engines, including four-stroke SI and CI operation, major castings and moving parts, valve train, cooling, lubrication and basic performance terms. The exam favours function and sequence over memorised labels.
Core rules
- In a four-stroke engine the sequence is intake, compression, power and exhaust; the crankshaft makes two revolutions per complete cycle.
- SI engines use a spark plug and a throttle-controlled air-fuel mixture; CI engines inject fuel into high-compression air and ignite by heat.
- Key components include cylinder block and head, piston, piston rings, gudgeon pin, connecting rod, crankshaft, flywheel, camshaft, valves and timing drive.
- Valve clearance is measured with a feeler gauge and set to the manufacturer’s cold or hot specification; timing marks must align.
- Lubrication reduces friction and carries heat; cooling maintains a stable temperature; both depend on correct grade and level.
Worked example
An engine has a swept volume of 400 cm3 and a clearance volume of 40 cm3. Compression ratio = (swept + clearance) ÷ clearance = (400 + 40) ÷ 40 = 11:1. This is a plausible SI ratio; a much higher value would change fuel and knock requirements. The reasoning matters because candidates often divide swept volume by clearance volume and wrongly get 10:1.
Traps
Saying a diesel engine needs a spark plug; confusing power stroke with compression stroke; measuring valve clearance at the wrong temperature; ignoring timing marks; treating compression ratio as swept volume alone.