Engine Construction, Operation and Systems
This domain tests engine hardware and the working of the four strokes. Expect labelled sketches, stroke-by-stroke descriptions of SI and CI engines, valve-timing terms, firing order, and calculations of swept volume, compression ratio and engine capacity.
Construction
Cylinder block, liner, head, gasket, piston with compression and oil rings, gudgeon pin, connecting rod, crankshaft with counterweights, flywheel, camshaft, valves, springs, rockers, pushrods, tappets, timing belt or chain, and sump. The piston pin may be full-floating or press-fit. The flywheel smooths torque pulses and carries the starter ring gear.
Strokes and timing
- SI engine: suction, compression, power (spark-ignited), exhaust.
- CI engine: suction of air, compression to about 16-22:1, injection and self-ignition, exhaust.
- Valve overlap: both valves are slightly open around TDC at the end of exhaust; the exhaust valve closes after TDC and the inlet opens before TDC.
Worked example
A 4-cylinder engine has bore 80 mm and stroke 90 mm. Swept volume per cylinder = (pi/4) x d squared x L = 0.7854 x 8 squared x 9 = 452.4 cm cubed. Total capacity = 452.4 x 4 = 1809.6 cm cubed, about 1810 cc or 1.8 litre. If clearance volume is 45.2 cm cubed per cylinder, CR = (452.4 + 45.2) / 45.2 = 11.0:1. Reasoning: compression ratio is a volume ratio (swept plus clearance, divided by clearance), not a pressure ratio.
Traps that cost marks
Adding clearance volume into the capacity figure. Writing firing order 1-2-3-4 for a straight-four (correct: 1-3-4-2 or 1-2-4-3). Confusing stroke with clearance volume. Claiming a diesel engine has a spark plug, or that valve overlap improves slow-speed idling.