Engine Construction
Cylinder arrangements, crankshaft and camshaft, cylinders, valves, pistons and valve timing.
Engine construction, summary notes
- 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.
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.
Construction concept map
Engine Construction
Engine construction quiz
Engine Construction, quiz
1. The purpose of the crankshaft counterweights is to:
Balance the rotating and reciprocating masses and reduce vibrationIncrease the compression ratio, by reducing the clearance volume left at top dead centreDrive the propeller directly, the counterweights forming the hub the blades are mounted in2. The main purpose of piston rings is to:
Seal the combustion gases and control oil on the cylinder wallSupport the crankshaft in its bearings and carry the side thrust from the connecting rodTime the opening of the valves by uncovering ports as the piston passes up the bore3. The most common cylinder arrangement in light aircraft is:
Horizontally-opposed (flat), air-cooledRadialV-12 liquid-cooled4. The crankshaft's job is to:
Convert reciprocating motion to rotaryOpen the valvesStore fuel5. The camshaft operates the valves via:
Cam followers, pushrods and rocker armsThe crankshaft directlyThe propeller6. Exhaust valves are often sodium-filled to:
Conduct heat away from the valve headAdd weightImprove sealing only7. A piston's compression rings:
Seal the combustion gas above the pistonMeter oil onlyCool the valve8. The oil-control ring:
Meters/scrapes oil on the cylinder wallSeals combustion gasLocates the gudgeon pin