Fundamentals
The Otto cycle, the four-stroke and two-stroke sequences, compression ratio and engine efficiencies.
Fundamentals, summary notes
- 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.
- Compression ratio
- CR = (swept + clearance volume) / clearance volume
- Mechanical efficiency
- η_mech = BHP / IHP
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.
Four-stroke engine, the Otto cycle
Fundamentals concept map
Fundamentals
Fundamentals quiz
Fundamentals, quiz
1. A six-cylinder engine has a bore of 130 mm and a stroke of 100 mm. Its total swept volume is approximately:
7963 cm³1327 cm³10 140 cm³2. An engine has a swept volume of 1350 cm³ and a clearance volume of 150 cm³. Its compression ratio is:
10:19:111:13. A cylinder has a swept volume of 800 cm³ and a clearance volume of 100 cm³. Its compression ratio is:
9:18:110:14. A four-cylinder engine has a bore of 100 mm and a stroke of 80 mm. Its total swept volume is approximately:
2513 cm³628 cm³3200 cm³5. The aircraft piston engine works on which cycle?
The Otto (constant-volume) cycleThe Brayton (constant-pressure) cycleThe Carnot cycle6. The four strokes in order are:
Induction, compression, power, exhaustCompression, induction, exhaust, powerPower, exhaust, induction, compression7. A four-stroke engine produces one power stroke every:
Two crankshaft revolutions (720°)One revolution (360°)Four revolutions8. A two-stroke engine produces a power stroke every:
One revolutionTwo revolutionsFour revolutions