Explained
How a jet engine works.
Suck, squeeze, bang, blow
A jet engine makes thrust by speeding up a stream of air and throwing it out of the back. Engineers sum up the process as suck, squeeze, bang, blow. Air is drawn in at the front, compressed, heated by burning fuel, and then expanded through a turbine and out of a nozzle. Unlike a car engine, which repeats the same steps in a cylinder, a jet engine does all four at once in different places, so the power flows continuously.
The engine in this model is a turbofan, the type used on almost every modern airliner. Its defining feature is the large fan at the front. Only a small share of the air the fan moves goes into the core of the engine; the rest is pushed around the outside through the bypass duct. Moving a large mass of air fairly slowly is more efficient and quieter than moving a small mass very fast, which is why the bypass air produces most of the thrust on today's airliners.
Inside the core
The core starts with the compressor, a stack of rotating blade rows separated by fixed vanes. Each stage raises the pressure a little, and together they squeeze the air to many times atmospheric pressure. The compressed air then enters the combustor, an annular chamber where fuel is sprayed in and burned continuously. The flame never goes out in normal running, and the gas leaving the combustor is extremely hot.
That hot, high-pressure gas rushes through the turbine. Turbine blades work like windmill sails, taking energy from the flow to turn the shafts that drive the compressor and the fan. The blades survive temperatures above their alloy's melting point only because cool air is fed through tiny passages inside them and because they are protected by ceramic coatings. Whatever energy the turbine doesn't take is turned into a fast jet by the exhaust nozzle.
Shafts and speeds
The fan, the compressor and the turbine don't all spin at the same speed. Large turbofans have two or three concentric shafts, each joining one part of the turbine to the part of the compressor or fan it drives. The fan is big, so its tips would go supersonic if it spun as fast as the small high-pressure stages, and separate shafts let each part run at its best speed. In the model, the inner high-pressure stages spin faster than the fan.
Parts glossary
- Fan
- The big fan at the front pulls in a huge mass of air. In a high-bypass turbofan most of that air flows around the core rather than through it. Fun fact: In modern airliner turbofans, the bypass air from the fan produces most of the engine's thrust.
- Compressor
- Rows of small spinning blades, each followed by fixed vanes, squeeze the core air to many times atmospheric pressure before it reaches the combustor.
- Combustor
- Fuel is sprayed into the compressed air and burns continuously in a ring-shaped chamber. The gas leaves far hotter, and with much more energy, than it came in.
- Turbine
- The hot, fast gas spins the turbine blades, which drive the compressor and fan through the shafts. Only part of the gas's energy is taken out here; the rest leaves as thrust. Fun fact: Turbine blades run in gas hotter than the melting point of their own nickel alloy, protected by internal cooling air and ceramic coatings.
- Shafts
- Concentric shafts link each turbine to the compressor or fan it drives. Most large turbofans have two or three shafts that spin at different speeds.
- Exhaust nozzle
- The nozzle and tail cone shape the exhaust so it leaves as a fast jet. Pushing gas backwards pushes the engine forwards, as Newton's third law describes.
- Nacelle
- The streamlined cowling around the engine guides air into the fan, keeps the outside airflow smooth and forms the bypass duct.
