16.1–16.2 Piston engine fundamentals and performance (with worked calculations)
1,187 words · checked 2026-10-03 · Free
Scope and exam
Module 16 is taken by A2, A4, B1.2, B1.4 and B3 (EASA: 52 questions for A, 76 for B1 and B3). Both sub-modules: A level 1, B1/B3 level 2.
- 16.1: mechanical, thermal and volumetric efficiencies; 2-stroke, 4-stroke, Otto, Diesel and rotary (Wankel) principles; piston displacement and compression ratio; engine configuration and firing order.
- 16.2: power calculation and measurement; factors affecting power; mixtures/leaning; pre-ignition.
Four-stroke Otto cycle
| Stroke | Piston | Valves | Event |
|---|---|---|---|
| Induction (intake) | Down | Inlet open | Mixture drawn in |
| Compression | Up | Both closed | Mixture compressed; spark before TDC |
| Power | Down | Both closed | Combustion at nearly constant volume, gas pressure drives piston |
| Exhaust | Up | Exhaust open | Burnt gas expelled |
- One power stroke per cylinder every two crankshaft revolutions (720°); the camshaft turns at half crankshaft speed.
- Valve lead (inlet opens before TDC on the exhaust stroke), valve lag (exhaust closes after TDC; inlet closes after BDC) and valve overlap (both valves briefly open around TDC) use gas momentum to improve scavenging and volumetric efficiency.
- Ignition timing is advanced (e.g. 20–30° BTDC, type-dependent) so peak pressure occurs just after TDC.
Other cycles
| Engine | Principle |
|---|---|
| Two-stroke | Power stroke every revolution; ports in the cylinder wall replace valves; crankcase compression of the charge; simpler, lighter but less efficient and oil mixed with fuel (some microlight engines) |
| Diesel (compression ignition) | Air only is compressed to a high ratio (around 14–22:1); fuel injected ignites from the heat of compression; burns Jet A/Jet A-1 in aviation diesels; better fuel efficiency, no ignition system, usually FADEC-controlled |
| Rotary (Wankel) | Triangular rotor in an epitrochoidal housing; three combustion chambers; output shaft turns 3 times per rotor revolution; smooth, compact; apex seal wear |
Efficiencies
| Efficiency | Definition |
|---|---|
| Thermal | Power output ÷ heat energy in the fuel (typically only around 25–35 % in aviation piston engines). Brake thermal efficiency uses BHP |
| Mechanical | BHP ÷ IHP — losses due to friction and pumping (friction horsepower FHP = IHP − BHP) |
| Volumetric | Actual mass of charge drawn in ÷ mass that would fill the swept volume at ambient (standard) conditions. Reduced by restrictive intakes, high rpm, hot air, carb heat; can exceed 100 % with supercharging |
Displacement and compression ratio
Swept volume (one cylinder) = piston area × stroke = (π/4) × d² × L Total displacement = swept volume × number of cylinders
Worked example 1 (FAA): Bore 5.5 in, stroke 5.5 in, 14 cylinders: area = 0.7854 × 5.5² = 23.758 in²; swept volume = 23.758 × 5.5 = 130.67 in³; total = 130.67 × 14 ≈ 1,829 in³.
Worked example 2 (SI): Bore 100 mm (10 cm), stroke 90 mm (9 cm), 4 cylinders: area = 0.7854 × 10² = 78.54 cm²; swept volume = 78.54 × 9 = 706.9 cm³; total ≈ 2,827 cm³ (2.83 L).
Compression ratio = total volume (BDC) ÷ clearance volume (TDC) = (swept + clearance) ÷ clearance
Worked example 3 (FAA): 140 in³ at BDC, 20 in³ at TDC → CR = 140 ÷ 20 = 7:1. Worked example 4: Swept volume 700 cm³, clearance volume 100 cm³ → CR = (700 + 100) ÷ 100 = 8:1.
Higher CR → higher thermal efficiency but greater risk of detonation; it is limited by the fuel's anti-knock (octane) rating. The pressure just before ignition is roughly manifold pressure × CR (FAA: 30 inHg × 7 ≈ 210 inHg; at 60 inHg, ≈ 420 inHg) — which is why boosted engines need lower CR or higher-grade fuel.
Engine configurations and firing order
| Configuration | Notes | Example firing orders (FAA) |
|---|---|---|
| Horizontally opposed | Most common light-aircraft layout; air-cooled; flat and well balanced | 4-cyl: 1-3-2-4 or 1-4-2-3 (model dependent); 6-cyl: 1-4-5-2-3-6 |
| In-line | Narrow frontal area, often inverted | 6-cyl in-line: 1-5-3-6-2-4 |
| V-type | Two banks of in-line cylinders | — |
| Radial | Odd number of cylinders per row so alternate firing works | 9-cyl: 1-3-5-7-9-2-4-6-8 |
Firing order spreads power impulses evenly along the crankshaft and around the crank cycle for smoothness and balanced bearing loads. Cylinder numbering is defined by the manufacturer (it differs between makers).
Power calculation (16.2)
Indicated horsepower — the "PLANK" formula (FAA):
IHP = P × L × A × N × K ÷ 33,000
P = indicated mean effective pressure (psi); L = stroke (ft); A = piston area (in²); N = power strokes per minute per cylinder = rpm ÷ 2; K = number of cylinders.
Worked example 5: IMEP 165 psi, stroke 0.5 ft, piston area 20 in², 2,400 rpm, 4 cylinders: N = 2,400 ÷ 2 = 1,200; IHP = 165 × 0.5 × 20 × 1,200 × 4 ÷ 33,000 = 7,920,000 ÷ 33,000 = 240 IHP.
Brake horsepower — measured with a dynamometer (Prony brake, hydraulic or electric dynamometer) or a calibrated test club/propeller:
BHP = 2π × N × T ÷ 33,000 (T in lb·ft, N in rpm) — SI: P (kW) = 2π × N × T ÷ 60,000 (T in N·m)
Worked example 6: Torque 500 lb·ft at 2,400 rpm: BHP = 2π × 2,400 × 500 ÷ 33,000 ≈ 228.5 BHP. With IHP = 240 → mechanical efficiency = 228.5 ÷ 240 ≈ 95 %; friction HP ≈ 11.5.
Brake specific fuel consumption BSFC = fuel flow ÷ BHP (e.g. 0.45 lb/hp/h).
Factors that affect power
| Factor | Effect |
|---|---|
| Air density — altitude, temperature, pressure | Lower density → less mass of charge → less power (normally aspirated engines lose power steadily with altitude) |
| Humidity | Water vapour displaces air → less power (significant for piston engines) |
| Manifold pressure and rpm | Power ≈ proportional to both |
| Mixture | See below |
| Ignition timing, compression, valve condition | Wear and leaks reduce power |
| Intake restrictions, carb heat (hot, less dense air) | Reduce power |
Mixtures and leaning
- Combustible range of petrol–air mixtures: roughly 8:1 (rich) to 16:1 (lean) air–fuel by mass.
- Chemically correct (stoichiometric): about 15:1.
- Best power: slightly rich of stoichiometric (fuel–air ratio about 0.075–0.080, i.e. air–fuel ≈ 12.5–13:1) — maximum power for a given airflow.
- Best economy: leaner — maximum power per unit of fuel.
- Full rich at take-off/high power — extra fuel cools the cylinders and suppresses detonation.
- Leaning in cruise / at altitude: as density falls the mixture becomes over-rich; leaning by EGT (e.g. to peak EGT or a set amount rich of peak, per the POH) restores the correct ratio, saves fuel and prevents plug fouling.
- Too lean: high CHT, rough running, detonation, burnt valves; too rich: plug fouling, power loss, carbon build-up.
Detonation and pre-ignition
- Detonation: after the spark, the remaining unburnt charge explodes instead of burning progressively — sharp pressure spikes, high CHT, damage to pistons, rings and valves. Causes: low-octane fuel, too lean, high CR/manifold pressure, high temperatures, over-advanced timing.
- Pre-ignition: the charge is ignited before the spark by a hot spot (glowing carbon, overheated plug or valve edge). Causes rough running, power loss, high CHT; often leads to detonation.
Exam traps
- Four-stroke: one power stroke per two revolutions; camshaft at half crank speed.
- N = rpm ÷ 2 in PLANK for a four-stroke engine.
- Mechanical efficiency = BHP ÷ IHP.
- CR = (swept + clearance) ÷ clearance.
- Pre-ignition = before the spark; detonation = after the spark.