Engine configuration and construction (cylinder arrangements, crankcase, crankshaft, connecting rods, bearings)
Engine configuration and construction
This section tests how cylinder layout, crankcase, crankshaft, connecting rods and bearings carry gas, inertia and propeller loads without distortion or fatigue.
Core rules and concepts
- Light aircraft commonly use horizontally opposed, air-cooled engines; inline, V and radial layouts also appear. Opposed cylinders are staggered so both banks share a crankpin.
- The crankcase carries the crankshaft main bearings and cylinder mountings, resists gas and inertia loads, and contains oil passages and sump space.
- The crankshaft turns reciprocating force into torque. Throw spacing sets firing interval; counterweights reduce bending; a torsional vibration damper protects the crank from resonant loads.
- Connecting rods have an I-beam shank and plain shell bearings at the small and big ends. In a radial engine one master rod carries the crankpin and the remaining rods articulate on it.
- Main bearings support the crankshaft and take thrust; rod bearings take firing loads. Correct crush, oil clearance and lubrication are essential.
Worked example
A nine-cylinder four-stroke radial completes its cycle in 720°. Even firing spacing is 720 ÷ 9 = 80°. The firing order must place the next power pulse 80° later; an answer of 40° wrongly treats the engine as a two-stroke.
Traps
Only the master rod attaches directly to the crankpin in a radial-do not say all rods do. Do not confuse bearing crush with running clearance, ignore thrust loads, or assume opposed cylinders fire together.