How does a jet engine work?

Modern aviation owes its success to the jet engine. The technology was originally developed in the late 1930s and early 1940s for military use in World War Two, but it has since powered the passenger aircraft revolution. 

There are many different variations on the jet engine, but the one most commonly used in passenger planes is called a turbofan (because it contains a turbine and a fan). The description below is about turbofans in particular, but much of it applies more generally.

So how does a jet engine work?

At the simplest level the way a jet engine works can be reduced to just four words: suck, squeeze, bang, blow. Let’s break down what that means. 

Suck

When you look at a jet engine, the first thing you will generally notice is that the front is a giant many-bladed fan, inside what is known as the intake. The blades act in exactly the same way the blades on a propeller or desk fan work, sucking air in and shoving it out the other side at high speed. The fan in a jet engine does have a lot more blades than a desk fan, though: often more than 20. Think of the fan as a propeller on steroids. 

In most modern jet engines, the fan alone can generate up to 90% of the thrust, or ‘pushing power’ of the engine. To find out where the other 10% comes from, we must continue to follow the air on its journey.

Squeeze

We are now leaving pre-jet engine technology behind. Once the fan sucks in the air, some of it is not just forced around the engine, but is funnelled to what is known as the compressor. Inside, air is pushed along by many spinning disks loaded with small blades along a tube that gets smaller and smaller. This quickly squeezes the air, making it much denser, hotter and more explosive when fuel is added.

Bang

For the pyromaniacs out there, there is where the fun begins. Fuel is added to the compressed air, creating a highly volatile mix requiring a simple spark to burn. This is what happens in the combustion chamber, where the fuel/air mix is sprayed and ignited, rapidly expanding the air and generating the rest of the thrust of the engine. 

Blow

The rapid expansion of the air during combustion generates a massive amount of pressure that needs to find a way out.  The way out of a jet engine is at the end of another tube full of spinning disks bristling with blades that are spun by the force of the expanding gas. This part is known as the turbine. Once at the end of the turbine, the gases leave the engine at high speed, exerting a force on the engine in the opposite direction. (In accord with Newton’s third law: for every action, there is an equal and opposite reaction.) 

The ingenious part of the modern jet engine is that the intake fan, compressor, combustion chamber and turbine are linked by a single shaft running along the inside of the engine. So when the expanding gases spin the turbine at the back, it helps spin the fan at the front, which keeps the process going and generates more thrust.

Related reading: Australia researchers create 3D-printed jet engine