Engine Systems and Components
Domain 2 tests the four-stroke cycle, the function of engine parts, valve timing and simple compression-ratio arithmetic.
The cycle
A petrol engine draws a fuel-air mixture and ignites it with a spark; a diesel draws air only, compresses it to roughly 30–45 bar and injects fuel that self-ignites. Suction, compression, power and exhaust take two crankshaft revolutions, so the camshaft turns at half crankshaft speed - which is why timing marks must align exactly.
Components and their jobs
Block, cylinder head and gasket; piston with two or three compression rings and one oil ring; gudgeon pin; connecting rod; crankshaft with counterweights; flywheel; camshaft, tappets, pushrods, rockers and valves; timing belt or chain. Rings seal combustion gas, carry heat to the bore and control oil; the flywheel smooths the power strokes. A typical four-cylinder in-line firing order is 1-3-4-2.
Worked example
Bore 80 mm, stroke 90 mm, clearance volume 60 cm³. Swept volume = (π/4) × 8² × 9 = 0.7854 × 64 × 9 = 452.4 cm³. Compression ratio = (452.4 + 60) ÷ 60 = 8.54, i.e. about 8.5 : 1 - normal for petrol. A diesel at 17 : 1 needs far less clearance volume for the same swept volume, which is why its combustion chamber shape differs.
Traps that cost marks
Using total cylinder volume instead of swept volume; adding the clearance volume twice; forgetting the camshaft runs at half engine speed; setting valve clearance hot when the specification is cold; assuming only one firing order is possible.