Automotive Engine Systems and Components
What the exam tests
Engine questions cover the four-stroke cycle, major castings and moving parts, valve train, compression, firing order and the support systems that keep the engine running.
Core rules
- Four-stroke petrol cycle: intake, compression, power, exhaust. The crankshaft turns twice per complete cycle; the camshaft turns once.
- Main parts include cylinder block, cylinder head, piston, rings, connecting rod, crankshaft, camshaft, bearings, flywheel and valves.
- Compression ratio compares total cylinder volume with clearance volume: (swept volume + clearance volume) ÷ clearance volume.
- Valve clearance allows for thermal expansion; incorrect clearance causes noise, poor sealing or burned valves.
- Engine displacement for one cylinder is π/4 × bore² × stroke; multiply by the number of cylinders.
Worked example
A four-cylinder engine has a bore of 8.0 cm and a stroke of 9.0 cm. Area = π/4 × 8.0² = 50.27 cm². One-cylinder swept volume = 50.27 × 9.0 = 452.4 cm³. Four cylinders give about 1,810 cm³, or 1.81 litres. Reasoning: displacement is swept volume, not including combustion-chamber volume. Confusing the two gives the wrong answer.
Traps that cost marks
Saying the power stroke occurs every revolution, treating diesel as spark-ignition, ignoring valve timing, and mixing up compression ratio with engine capacity. Also remember that only the power stroke produces useful work; the other strokes absorb energy.