The jet engine cycle
A simplified two-spool, high-bypass turbofan. This educational model shows the main assemblies; it is not a manufacturing drawing or a specific production engine.
The Brayton cycle
The ideal Brayton cycle describes compression, constant-pressure heat addition, expansion, and heat rejection. An aircraft engine is an open-flow system: fresh air enters and exhaust leaves. The real engine has pressure losses and imperfect compressors and turbines.
Intake: A large fan. Two air streams.
The inlet guides air into the fan. Most of the flow travels through the bypass duct; the rest enters the core. The fan accelerates bypass air to generate thrust.
Compression: Build pressure, stage by stage.
The booster and high-pressure compressor squeeze core air through rows of rotating blades and stationary vanes. Pressure and temperature rise before fuel is added.
Combustion: A steady flame supplies energy.
Fuel injectors spray fuel into the annular combustor. A stabilized flame heats the moving air continuously. Real combustors lose some pressure; ideal Brayton-cycle heat addition is at constant pressure.
Expansion: Drive the shafts. Generate thrust.
Hot gas expands through the turbines, powering both spools. The nozzle accelerates the remaining core flow rearward. Core and bypass streams transfer momentum to the air, producing thrust.
Two spools, continuous operation
The high-pressure turbine powers the high-pressure compressor. A separate inner shaft connects the low-pressure turbine to the fan and booster. The two spools can rotate at different speeds. Unlike a piston engine, a turbofan performs its processes continuously in different regions.
Reference: NASA Glenn — Turbofan engine
Connect the steps
- Compress
- Add heat
- Expand through turbines
- Discharge exhaust
A spatial sequence, not four piston strokes
The compressor, combustor and turbine operate continuously in different parts of the engine. Following air downstream reveals successive processes, even while all those regions work at once. The ideal Brayton cycle is a useful thermodynamic abstraction; real compressors and turbines have losses, and an aircraft engine exchanges air with its surroundings.
Try it in 3D
Pause the animation and locate each process. Then resume it and notice that the assemblies operate together.
Open the related explorer →Is the combustor a series of piston-like explosions?
No. Normal operation sustains combustion in a flowing air stream. The animation separates stages for learning; the engine does not pause between them.