Piston Engine

Most aircraft piston engines are four-stroke (induction, compression, power, exhaust) horizontally opposed, air-cooled, direct-drive units. Compression ratio = total cylinder volume ÷ clearance volume. Components: crankcase, crankshaft, connecting rods, pistons and rings, cylinders with cooling fins, valves operated by cam and pushrods. Two-stroke and rotary (Wankel) engines appear in light aircraft.

Float-type carburettors meter fuel by venturi depression and are prone to carburettor icing — expect it at high humidity and moderate power, cured by carb heat. Injection systems (e.g. Bendix RSA) deliver fuel to each cylinder and avoid induction icing. Mixture control compensates for falling air density with altitude; leaning is set by EGT peak or fuel flow.

Superchargers are engine-driven; turbochargers are exhaust-driven with a wastegate to regulate manifold pressure. Turbo-normalising restores sea-level manifold pressure at altitude. Exhaust systems also supply cabin/carb heat via a shroud — inspect carefully for cracks because of carbon monoxide risk.

Magnetos are self-contained and engine-driven — the engine will run with the master switch off, so treat every propeller as live. Dual magnetos give redundancy and better combustion, with two spark plugs per cylinder. An impulse coupling or booster retards and intensifies the spark for starting. A dead (P-lead disconnected) magneto is dangerous; magneto drop is checked at run-up (typical limit 125–175 rpm with limited differential).

Wet-sump or dry-sump systems circulate ashless dispersant oil; components include pump, filter, oil cooler, pressure relief and temperature control. Oil analysis and filter inspection reveal wear. Cooling is by airflow over cylinder fins directed by baffles and cowl flaps — missing baffling causes localised overheating. Shock cooling from rapid power reduction can crack cylinders.

Checks include compression testing (differential), spark-plug servicing and rotation, valve clearance, timing checks, and ground runs with reference to cylinder head temperature and oil temperature limits. Detonation (uncontrolled combustion) and pre-ignition (early ignition from a hot spot) both cause rapid damage — avoid with correct fuel grade, mixture and cooling.