16.3 Piston engine construction

Scope

A level 1, B1/B3 level 2: crankcase, crankshaft, camshafts, sumps; accessory gearbox; cylinder and piston assemblies; connecting rods; inlet and exhaust manifolds; valve mechanisms; propeller reduction gearboxes.

Crankcase

• Foundation of the engine: supports the crankshaft and camshaft bearings, carries the cylinders, contains the oil and transmits propeller thrust and torque to the engine mounts.
• Opposed engines: two halves of cast aluminium alloy split vertically along the centreline, joined by through-bolts and studs; no gasket at the parting surface on many engines — a thin sealant and silk thread are used so bearing clearances are not disturbed.
• Radial engines: multi-section crankcase (nose section, power section, supercharger and accessory sections).
• Contains main bearing saddles, oil galleries, cam and accessory drives.

Crankshaft

• Converts reciprocating motion into rotation. Forged from high-strength alloy steel (chrome-nickel-molybdenum), with nitrided (surface-hardened) journals.
• Parts: main journals (run in main bearings), crankpins (connecting-rod journals, often hollow to save weight and act as oil passages/sludge traps), crank cheeks (webs), and counterweights.
• Types: single-throw (single-row radial), two-throw, four-throw, six-throw (opposed six).
• Dynamic dampers (pendulum counterweights) on many opposed engines absorb torsional vibration caused by power impulses, which could otherwise fatigue the crankshaft and propeller.
• Propeller flange (or spline/taper on larger engines) at the front, with a thrust bearing to take propeller thrust.