Section 16.1

Fundamentals

The Otto cycle, the four-stroke and two-stroke sequences, compression ratio and engine efficiencies.

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Main ideas

The aircraft piston engine works on the Otto cycle — heat added at essentially constant VOLUME, unlike the gas turbine's constant-pressure Brayton cycle. The four-stroke sequence is induction, compression, power (combustion), exhaust.
A four-stroke engine gives ONE power stroke every TWO crankshaft revolutions (720°): only one of the four strokes produces power, the others are driven by the flywheel/other cylinders.
A two-stroke completes the cycle in one revolution (a power stroke every rev) using ports instead of valves — lighter and simpler but less efficient and 'dirtier', rare in certified aircraft.
Key geometry: top and bottom dead centre (TDC/BDC), bore and stroke, swept volume and clearance volume, and the compression ratio (total ÷ clearance volume) — higher compression gives more power but needs higher-grade fuel to resist detonation.
Efficiencies: thermal (heat → work), mechanical (brake ÷ indicated power) and volumetric (how well the cylinder fills with charge). The firing order spaces the power impulses evenly for smooth running.
⚠ Exam trap: a four-stroke produces one power stroke per TWO revolutions (720°), not one per revolution; the piston engine uses the constant-volume Otto cycle, the gas turbine the constant-pressure Brayton cycle.

Key formulas

Solved examples

A four-stroke engine runs at 2400 rpm. How many power strokes does one cylinder make per minute?
One cylinder fires once every two revolutions, so power strokes = rpm ÷ 2 = 2400 ÷ 2 = 1200 power strokes per minute for that cylinder. This '1 in 2 revolutions' relationship is the defining feature of the four-stroke cycle.

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Four-Stroke Engine — the Otto CycleFull screen ↗

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