Section 16.3

Engine Construction

Cylinder arrangements, crankshaft and camshaft, cylinders, valves, pistons and valve timing.

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Cylinder arrangements: inline, V, horizontally-opposed (flat) and radial. The air-cooled horizontally-opposed layout dominates light aircraft — low frontal area, good cooling and balance; radials were common on older large aircraft.
The crankcase houses the crankshaft (turns the pistons' reciprocating motion into rotation via the connecting rods) and the camshaft (opens the valves through cam followers, pushrods and rocker arms), running in main bearings.
Each cylinder has a barrel and head with poppet valves (one or more inlet and exhaust), valve seats, guides and springs. Exhaust valves are often sodium-filled to carry heat from the hot valve head down the stem.
Pistons carry compression rings (seal the combustion gas) and an oil-control ring (meter oil on the bore), connected by the gudgeon (piston) pin to the con-rod.
Valve timing uses lead, lag and overlap (both valves briefly open around TDC) to improve cylinder filling and scavenging; higher-power engines drive the propeller through a reduction gearbox.
⚠ Exam trap: exhaust valves are commonly SODIUM-FILLED to conduct heat away from the valve head; valve OVERLAP (both valves open near TDC) improves breathing at speed but can cause rough running/blow-back at idle.

Solved examples

Why are the exhaust valves of many aircraft piston engines partly filled with sodium?
The exhaust valve runs extremely hot because burnt gas flows past it. Sodium melts at low temperature and sloshes inside the hollow valve as it operates, carrying heat from the hot valve head down to the cooler stem and guide, where it is conducted away. This keeps the valve head cool enough to avoid burning and warping.

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