A loaded passenger jet can weigh more than 70 tonnes at take-off, yet two engines push it into the sky within about a minute. How does a jet engine work? The principle is simple enough to explain in four words, which engineers have used for decades: suck, squeeze, bang, blow.
The principle: Newton's third law
A jet engine takes in a large mass of air, adds energy to it, and throws it out of the back much faster than it came in. Pushing air backwards creates an equal and opposite force forwards. That force is thrust.
The five stages
1. Intake (suck)
Air enters the front of the engine. In a modern turbofan, the large fan you see at the front of the engine pulls in an enormous volume of air.
2. Compression (squeeze)
Rows of compressor blades, spinning at very high speed, squeeze the air into a smaller space. Its pressure rises many times over, and it heats up as it is compressed.
3. Combustion (bang)
In the combustion chamber, fuel is sprayed into the compressed air and burns continuously. The temperature of the gas rises sharply and it expands with great energy. It is a steady flame, not a series of explosions.
4. Turbine
The hot gas rushes through turbine blades and spins them. The turbines are connected by shafts to the compressor and fan at the front, so the engine powers itself: the turbine drives the compressor, which feeds the combustion chamber, which drives the turbine.
5. Exhaust (blow)
The gas leaves through the exhaust nozzle at high speed, adding to the thrust.
Watch: a jet engine in 60 seconds
Turbojet vs turbofan
Early jets used turbojets, where all the air passes through the core. They are powerful but noisy and thirsty. Today's airliners use high-bypass turbofans: most of the air pushed by the fan flows around the core (the "bypass" air). Moving a large mass of air at a moderate speed is more efficient than moving a small mass very fast, so in a modern airliner engine most of the thrust comes from the fan, not the exhaust jet. That is why airliner engines have such large diameters, and why they are quieter and more fuel-efficient.
Related engine types
- Turboprop: the turbine drives a propeller through a gearbox. Common on regional aircraft like the ATR 72.
- Turboshaft: the turbine drives a shaft, as in helicopter rotors and aircraft APUs.
Why aviation students study engines
Pilots need engine knowledge for normal operation and for emergencies such as engine failure or fire. Airline operations staff need it to understand delays caused by technical defects, and airport and MRO managers deal with engine changes, spares and maintenance planning. Aircraft systems are part of our programs and of DGCA Technical General exams. Read next: how thrust reversers help aircraft stop.
Learn aviation the practical way
At the Department of Aviation, Dr. M.G.R. Educational and Research Institute, students learn concepts like this in the classroom and see them at work on airport visits and in industry sessions. Explore our programs, test yourself with the free aviation skill test, or call +91 96000 04836 to talk to the admissions team.