Propulsion (Gas Turbine and Piston Engines)
Gas turbines
The Brayton cycle: intake, compression, combustion at near-constant pressure, exhaust. Compressors (axial or centrifugal) raise pressure; the turbine extracts energy to drive the compressor, and the remaining energy either accelerates bypass air (turbofan) or drives a propeller (turboprop). Thrust equals mass flow × velocity change. Know the parameters: N1 (fan/LP spool), N2 (HP spool), EGT, EPR and fuel flow. FADEC schedules fuel and enforces limits. Start-sequence questions (normal, hot, hung and wet start) are routine.
Piston engines
The four-stroke Otto cycle: induction, compression, power, exhaust. Understand valve and ignition timing, mixture control, carburettor icing, and why magnetos are duplicated. Detonation is uncontrolled burning after normal spark ignition, driven by high pressure and temperature; pre-ignition is ignition before the spark, often from a hot spot.
The traps
Mixing N1 and N2; confusing detonation with pre-ignition; forgetting that bypass ratio is bypass mass flow divided by core mass flow; assuming a turbofan accelerates all its air through the combustion section (it does not).
Worked example
A turbofan passes 400 kg/s through the bypass duct and 100 kg/s through the core. Bypass ratio = 400 ÷ 100 = 4:1. A high bypass ratio gives better propulsive efficiency and lower noise. Rule of thumb: the larger the bypass ratio, the more thrust comes from accelerated cold air rather than the hot core.