Piston engines

021 Aircraft General KnowledgeChapter 3

Four-stroke Otto cycle

Key fact

Induction: Inlet open, exhaust closed — mixture drawn in (valve overlap: both open briefly). Compression: Both close; ignition before TDC. Power: Both closed; combustion expands piston. Exhaust: Exhaust open, inlet closed (overlap again).

Camshaft rotates at ½ crankshaft speed. Crankshaft = 720° per complete cycle. Thermal efficiency ~30%. Combustion ideally at constant volume.

Formula — Compression ratio and power

CR = Total volume ÷ Clearance volume
BHP = Torque × RPM
Power = Torque × RPM (at gearbox for prop torque)

Key fact

Diesel: Air-compression engine — no throttle, no mixture control; power set by fuel flow only; higher compression ratio, higher thermal efficiency; produces soot; intake needs no icing protection.

Fuel and mixture

Key fact

AVGAS colours: 100LL = blue, 100 = green, 80 = red. MOGAS: Increased carburettor icing risk.

Chemically correct mixture: 14.7:1 (by weight air:fuel) — max EGT. Best power ~12:1. Best economy ~16:1. Mixture ratio = mass of fuel : mass of air.

Condition	Mixture effect
Altitude increases (not leaned)	Becomes richer — reduce fuel flow
Enriching	Lower CHT; fouls plugs if excessive
Leaning	Higher CHT; weak mixture causes overheating
Climb	Lean mixture
Descent	Richen mixture

Exam trap

Detonation (after spark): Low octane, high CHT, too lean, high MAP + low RPM, excessive boost, carb heat misuse. Action: retard throttle, reduce MAP, enrich mixture.

Pre-ignition (before spark): Hot spots in cylinder — more likely with RPM increase. Engine run-on with mag OFF = excessive carbon in cylinder.

Carburettor

Key fact

Venturi throat reduces pressure — draws fuel from float chamber. Carb icing: Most likely OAT −5°C to +18°C with visible moisture or RH above 60%. Can occur at OAT above 10°C. Types: impact, throttle, fuel (venturi) ice.

Carb heat applied: Air density reduces → mixture richer; FPP: RPM drops then rises; CSP: MAP drops, RPM constant. Not used on take-off/landing (power loss, detonation risk). MAP increase after carb heat = ice was forming.

Key fact

Primer pump: Extra fuel for start — excessive priming washes lubricant from cylinder walls (most correct exam answer), risks fire/flooding, fouls plugs. Accelerator pump: Enriches during rapid throttle advance. Diffuser/compensating jet: Constant mixture across power range.

Fuel injection

Key fact

Direct injection: Fuel into cylinder end of compression — air-compression engine; higher fuel pressure. Indirect: Ahead of inlet valve — mixture-compression; poppet valve opens on pressure differential.

Advantages over carb: No carb icing/throttle ice; no inverted-flight limitation; even cylinder distribution. Disadvantages: Hot start difficulty, vapour lock on ground, restart after starvation.

Electric fuel pump: Take-off, landing, emergency, vapour lock, engine pump failure.

Lubrication and cooling

Key fact

Wet sump: Oil in crankcase — simple, limited capacity, not for aerobatics. Dry sump: External tank; scavenge pump capacity ~3× pressure pump; oil cooler in return line after scavenge pump.

Oil functions: lubricate, seal, cool, clean, prevent corrosion; CSP pitch change (hydraulic). Oil pressure gauge: Outlet side of pressure pump. Higher than normal pressure on start-up is normal if it returns to normal.

Key fact

Air cooled: RAM air, cylinder fins, baffle plates (venturi effect), cowl flaps. Liquid cooled: ~70% water, 30% ethylene glycol. CHT: Hottest cylinder via thermocouple at spark plug gasket.

High CHT actions: reduce power, increase speed, mixture rich, open cowl flaps, carb heat OFF.

Ignition

Key fact

Magneto: Independent of aircraft electrical system once running. Dual ignition improves reliability, combustion, reduces detonation. Impulse coupling: Strong retarded spark at start; inhibited at high RPM by centrifugal flyweights.

Mag selector OFF: Grounds primary (more important) and secondary. One mag failure = slight RPM drop. Live mag with broken ground wire — shut off fuel if engine continues.

Propellers and power augmentation

Key fact

Constant speed prop (CSP): RPM + MAP; blade angle changed by engine oil; double-acting (oil both directions) or single-acting (oil fine, spring coarse). CTM tends to fine pitch; ATM tends to coarse.

Feathering: Pull lever aft; electric unfeathering pump; feathering stop prevents over-travel. P-factor: Upgoing blade higher AoA at high aircraft AoA.

Exam trap

Engine overload avoidance: Increase RPM before MAP (MRT: Mixture–RPM–Throttle). Decrease MAP before RPM (TRM: Throttle–RPM–Mixture). High MAP + low RPM = detonation risk.

Turbocharger waste gate seized: MAP may over-exceed on descent, decrease on climb. Intercooler fitted between turbo and manifold to reduce detonation risk.

Key fact

Normally aspirated engine: MAP always lower than ambient when running. MAP decreases ~1 in.Hg per 1000 ft altitude gain. Supercharger: Crankshaft-driven, compresses mixture downstream of carb. Turbocharger: Exhaust-driven, compresses air upstream of carb — uses otherwise wasted exhaust energy.