How a jet engine works
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.
Two streams, one purpose
The fan sends most incoming air around the engine core. The smaller core stream is compressed, heated by burning fuel, and expanded through turbines. The turbines power the fan and compressors; accelerated bypass air and core exhaust produce thrust.
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
- Intake and fan
- Core compression
- Combustion
- Turbines
- Exhaust acceleration
Two streams share one engine
Follow one parcel of air into the fan. In a high-bypass turbofan, much of the flow bypasses the core. Core air follows the compressor, combustor and turbines. The turbines take energy from hot gas to drive compressors and the fan; they are not an extra source of fuel energy. Both flow paths contribute to the overall thrust.
Try it in 3D
Compare the bypass route with the core route in the cutaway. Find the shaft connection that allows a core turbine to drive the fan.
Open the related explorer →Does all incoming air pass through combustion?
No. Bypass air goes around the core. Its acceleration by the fan is an important part of how the illustrated turbofan produces thrust.