A jet engine turns chemical energy in fuel into fast-moving exhaust that produces thrust. The basic sequence is intake, compression, combustion, and exhaust. This sequence is modeled by the Brayton cycle, a thermodynamic cycle used for gas turbines.
Understanding the cycle helps explain why jet engines need compressors, burners, turbines, and nozzles working together.
Key Facts
- Thrust comes from accelerating air backward: F = mass flow rate x change in velocity.
- Ideal Brayton cycle stages are isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-pressure exhaust.
- Compressor work raises air pressure and temperature before combustion.
- Combustion adds heat at nearly constant pressure, greatly increasing gas temperature.
- The turbine extracts energy from hot gas to power the compressor through a shaft.
- Thermal efficiency of an ideal Brayton cycle increases with pressure ratio: efficiency = 1 - 1/(pressure ratio)^((gamma - 1)/gamma).
Vocabulary
- Brayton cycle
- A thermodynamic cycle that describes how gas turbines compress air, add heat, expand hot gas, and exhaust it to produce useful work or thrust.
- Compressor
- A set of rotating and stationary blades that raises the pressure of incoming air before it enters the combustor.
- Combustor
- The engine chamber where fuel mixes with compressed air and burns to add thermal energy to the gas.
- Turbine
- A bladed rotor driven by hot expanding gas that extracts energy to spin the compressor.
- Nozzle
- A shaped exit duct that converts thermal and pressure energy of the gas into high-speed exhaust.
Common Mistakes to Avoid
- Thinking the turbine directly creates all the thrust, which is wrong because the turbine mainly powers the compressor while the nozzle accelerates exhaust to produce thrust.
- Assuming pressure rises during combustion, which is wrong for the ideal Brayton cycle because heat is added at approximately constant pressure.
- Ignoring the compressor’s energy cost, which is wrong because some turbine work must be used to keep the compressor spinning.
- Confusing temperature with speed, which is wrong because hot gas does not produce thrust unless the engine expands and accelerates it through the turbine and nozzle.
Practice Questions
- 1 A turbojet takes in 80 kg/s of air and increases the exhaust speed by 350 m/s compared with the inlet air speed. Estimate the thrust using F = mass flow rate x change in velocity.
- 2 Air enters a compressor at 100 kPa and leaves at 900 kPa. What is the compressor pressure ratio?
- 3 Explain why a jet engine needs both a compressor and a turbine instead of simply burning fuel in an open tube.