Piston Engines
Domain 14.2 covers the reciprocating engine as fitted to light aircraft: the four-stroke cycle, layout and valve train, mixture formation and ignition, and the abnormal combustion modes that destroy engines. Expect compression-ratio and displacement arithmetic alongside detonation/pre-ignition discrimination.
Cycle and geometry
Four strokes - induction, compression, power, exhaust - make one Otto cycle per cylinder every two crankshaft revolutions. Swept volume per cylinder = (π/4) × bore² × stroke; compression ratio CR = (swept + clearance volume) ÷ clearance volume. Brake mean effective pressure (BMEP) relates torque to displacement and is the fairest way to compare engines of different size. Valve timing is quoted relative to TDC, with overlap around the exhaust/intake changeover to exploit gas inertia.
Detonation and pre-ignition
Detonation is spontaneous auto-ignition of the end gas after normal spark initiation; it gives the characteristic knock, pressure spikes and rising CHT, and is controlled by mixture, manifold pressure, compression ratio and fuel octane. Pre-ignition is ignition before the spark, typically from a glowing plug, carbon deposit or overheated valve; it can hole a piston within seconds and is often triggered by prolonged detonation. They are different faults with different causes.
Fuel and ignition
Carburettors meter by venturi depression and are prone to icing in roughly the −5 to +20 °C band at high humidity; fuel injection gives better distribution and avoids float-chamber icing. Magnetos are self-contained and independent of the battery, and use an impulse coupling to retard timing for starting. Correct firing order and plug leads matter: a crossed lead gives rough running and abnormal EGT.
Worked example
Bore 120 mm, stroke 100 mm, four cylinders, clearance volume 100 cm³ each. Swept per cylinder = 0.7854 × 12² × 10 = 1 131 cm³. CR = (1 131 + 100)/100 = 12.3:1. Total displacement = 4 × 1 131 = 4 524 cm³ (4.5 litres). Had the clearance figure been quoted for the whole engine, dividing it by four before applying the formula is the step candidates miss.
Traps
Do not compute CR from total displacement unless clearance is also total. Detonation is not pre-ignition. Valve clearances are set cold unless the manual says otherwise. A magneto drop that is within limits but unequal between magnetos points to timing or lead faults, not a healthy engine.