Global air travel has become so commonplace that we seldom stop to think about the amazing thing that it is – how it’s possible that enormous metal objects carrying hundreds of people at a time routinely travel thousands of miles without incident.

Right at the heart of what makes this all possible is the modern jet engine (technically, the turbofan engine) – one of the most extreme machines ever built. A single GE90 turbofan, for example, can produce over 400,000 Newtons (N) of thrust, survive temperatures hotter than molten lava, and operate flawlessly for hundreds of thousands of hours, ensuring that people get where they’re going safely and reliably.

Early jet engines pushed a small amount of air very fast. Modern turbofan engines do it differently – they trade speed for mass, which turns out to be far more efficient.

A turbofan engine has two main airflow paths:

• The combustion flow — this is the air that goes through the engine’s core, including the compressors, combustor, and turbines
• The bypass flow — this is the air that goes around the core through the bypass duct

Here’s the surprising part. It might seem silly to waste all that bypass air. However, most of the thrust in a modern turbofan (80% - 90%) comes from the bypass flow, not the exhaust jet from the core. That big fan at the front is what creates the bypass flow and hence most of the engine’s thrust.

The ratio of bypass to combustion air is called the bypass ratio (BPR). Some modern turbojets, like the GE9X, achieve bypass ratios of 10:1 (10 kg of air goes around the core for every 1 kg that goes through it). Generally speaking, the higher the bypass ratio, the greater the engine’s efficiency and the quieter it is.

There are 4 key parts of a turbofan engine:

1. The Fan: The large fan at the front pulls huge amounts of air into the engine. As noted, some of this goes into the core but most is bypass air. This fan is driven by the low pressure turbine at the back of the engine.
2. The Compressor: This is the first part of the core. A series of spinning rotors squeezes (or compresses) the air entering the core to much higher pressures, each rotor adding a little more pressure. Compressing air makes it hotter and denser, something you may have noticed when using a bicycle pump. Compressed air produces far more efficient combustion.
3. The Combustor: Here jet fuel is sprayed into the compressed air, and the mixture ignites and burns continuously. This produces extremely hot, high-energy gas with temperatures exceeding the melting point of metal.
4. The Turbine: The hot, rapidly expanding gas from the combustor blows through turbine blades, spinning them. The high pressure turbine drives the compressor while the low pressure turbine behind it drives the large fan. The energy still left in this air exits the engine at high speed as exhaust, contributing to the engine’s overall thrust.

This continuous, 4-stage cycle is often summarized as "suck, squeeze, bang, blow".