Reciprocating Engines
What the questions test
The reciprocating engine domain is mostly about the four-stroke cycle and the hardware that carries it out. Be able to name the events in order - intake, compression, power, exhaust - and relate each to piston position, valve position, and crankshaft rotation. Two revolutions of the crankshaft (720 degrees) complete one cycle; the camshaft turns at half crankshaft speed.
Know the geometry: bore, stroke, displacement, compression ratio, and firing order. Displacement is the swept volume of all cylinders. Compression ratio is (swept volume of one cylinder plus clearance volume) divided by clearance volume. A higher compression ratio raises thermal efficiency but increases detonation tendency and demands higher octane.
Question writers love valve timing and overlap. The intake valve opens before TDC and closes after BDC; the exhaust valve opens before BDC and closes after TDC. Overlap is the period when both are open near TDC on the exhaust stroke. Ignition timing is a separate matter - the spark occurs before TDC (typically 20-25 degrees BTDC) and is not the same as valve overlap.
Also expect crankshaft counterweights, dynamic balance, bearing clearance checks, hydraulic valve lifters (no periodic lash adjustment), and the difference between direct-drive and geared engines.
Worked example
Displacement = (pi/4) x bore squared x stroke x number of cylinders. For a 5.125 in bore, 4.375 in stroke, six-cylinder engine: 0.7854 x 26.27 = 20.63 square inches; times 4.375 = 90.25 cubic inches; times 6 = 541.5 cubic inches - the familiar 540 series. Note displacement scales with the square of bore, so a small bore change moves the number quickly.