Fundamentals & Engine Performance
The Brayton cycle, engine types, thrust production, thermal and propulsive efficiency, ram effect and flat rating.
Fundamentals & performance, summary notes
- A gas turbine works on the Brayton (constant-pressure) cycle in four continuous stages, suck, squeeze, bang, blow: the compressor raises pressure, the combustor adds heat at constant pressure, and the turbine and nozzle extract energy and accelerate the gas rearward.
- Thrust is Newton's third law in action: F = ṁ·(V_jet − V_inlet). The engine produces forward thrust by accelerating a mass of air rearward; a high-bypass turbofan moves a large mass slowly (efficient, quiet), a turbojet a small mass fast.
- Engine types trade the same core differently: turbojet (all thrust from the jet), turbofan (most thrust from the bypass fan), turboprop (most power to a propeller), turboshaft (all power to a shaft, e.g. helicopters/APUs).
- Efficiency has two parts: thermal efficiency (how well heat becomes mechanical energy, favoured by high pressure ratio and turbine entry temperature) and propulsive efficiency (how well that energy becomes thrust, favoured by a jet velocity close to the aircraft speed, i.e. high bypass).
- Thrust falls with altitude (lower air density/mass flow) and with rising ambient temperature, and rises with forward speed through ram effect. 'Flat rating' holds a guaranteed thrust up to a corner-point temperature by setting the engine to a limit.
- ⚠ Exam trap: specific fuel consumption (SFC) is fuel burned per unit thrust per hour, LOWER is better; and a high-bypass fan gives high propulsive efficiency because it accelerates a large mass of air only slightly, not because the jet is fast.
- Thrust
- F = ṁ·(V_jet − V_inlet)
- Brayton cycle
- compress → heat (const P) → expand
Why does a modern high-bypass turbofan burn less fuel and make less noise than an early turbojet of the same thrust?
It generates thrust by accelerating a very large mass of bypass air by a small amount, rather than a small mass to a very high jet velocity. A jet velocity closer to the aircraft's speed means higher propulsive efficiency (less energy wasted as leftover kinetic energy in the jet) and a lower, quieter exhaust velocity.
Gas turbine, the running turbojet
Fundamentals concept map
Fundamentals & Performance
Fundamentals & performance quiz
Fundamentals & Engine Performance, quiz
1. An engine delivers 120 kN of thrust and burns 4200 kg of fuel per hour. Its specific fuel consumption is:
0.035 kg/h per newton28.6 N per kg/h504 kg/h per kN2. A two-spool engine has a fan passing 300 kg/s and a core passing 60 kg/s. Its bypass ratio is:
5:16:10.2:13. On the engine shown, station 3 lies immediately before the burner. The pressure and temperature there are:
The highest pressure in the engine, at compressor deliveryThe lowest pressure in the engineThe highest temperature in the engine4. The cycle diagram shown is for a gas turbine. Heat is added along the upper line at:
Constant pressureConstant volumeConstant temperature5. A turbojet passes 60 kg/s and accelerates it from 250 m/s to 550 m/s. The thrust produced is:
18 kN33 kN15 kN6. An engine passes 400 kg/s through the bypass duct and 100 kg/s through the core. Its bypass ratio is:
4:11:45:17. An engine of mass flow 50 kg/s accelerates air from 200 m/s to 400 m/s. The thrust produced is:
10 kN20 kN30 kN8. A gas turbine operates on which thermodynamic cycle?
The Brayton (constant-pressure) cycleThe Otto cycleThe Carnot cycle at constant volume