A jet engine is a remarkable piece of engineering that propels aircraft by expelling a fast-moving stream of gas. Instead of using a propeller, it draws in air, compresses it, mixes it with fuel, ignites the mixture, and then ejects the exhaust at high speed. The basic principle is Newton's third law: for every action, there is an equal and opposite reaction. By forcing hot gases backward, the engine experiences a forward thrust. This technology, first developed in the 1930s, revolutionised aviation, enabling faster and higher flights. Modern jet engines are complex machines, but their core operation can be understood through four main stages: intake, compression, combustion, and exhaust.

Each stage plays a crucial role in generating the immense power needed to lift an aeroplane into the sky. The first stage is the intake, where large quantities of air are drawn into the engine. At the front, a spinning fan pulls air in and directs it toward the compressor. The fan itself is the large visible blades you see on a modern turbofan engine. Some of this air bypasses the core to create additional thrust, but a portion enters the compressor section. The compressor consists of multiple rows of rotating blades called rotors, alternating with stationary blades called stators.

As air passes through each stage, it is progressively squeezed into a smaller volume, which raises its pressure dramatically. By the time the air reaches the combustion chamber, its pressure can be 30 to 40 times higher than atmospheric pressure. The third stage is combustion. Inside the combustion chamber, compressed air is mixed with a fine spray of jet fuel, typically kerosene. Igniters (similar to spark plugs) create a continuous flame that burns the fuel-air mixture. The combustion process releases a tremendous amount of heat, raising the temperature of the gases to over 1,000 degrees Celsius.

The compressor consists of multiple rows of rotating blades called rotors, alternating with stationary blades called stators.

This hot, high-pressure gas expands rapidly and is directed toward the turbine. Unlike a car engine, combustion in a jet engine is continuous and steady, not intermittent. The design ensures that the flame remains stable despite the high-speed airflow. All this energy comes from the chemical bonds in the fuel, which are broken apart by the heat of combustion. After combustion, the hot gases flow into the turbine section. The turbine looks similar to the compressor but works in reverse: the gases spin the turbine blades as they expand. The turbine is connected to the compressor by a central shaft, so as the turbine spins, it drives the compressor at the front.

This is a key feature: the engine is self-sustaining once started. Part of the energy from the combustion is used to keep the compressor running, and the remaining energy goes into accelerating the exhaust gases. The gases then pass through a nozzle at the rear of the engine. The nozzle narrows, forcing the gases to accelerate to supersonic speeds, creating the powerful thrust that propels the aircraft forward. There are several types of jet engines, each optimised for different purposes. The turbojet was the original design, where all the air passes through the core.

It is efficient at very high speeds but noisy and less fuel-efficient at lower speeds. The turbofan, used on most commercial airliners, has a large fan at the front that accelerates a huge volume of air around the core. This "bypass" air provides additional thrust and reduces noise. Turbofans are more efficient and quieter, making them ideal for passenger jets. Other variants include turboprops (where a turbine drives a propeller) and ramjets (which rely on forward motion to compress air). Each type applies the same basic principles in slightly different ways to achieve specific performance goals.

Jet engines have transformed travel and commerce, allowing people to cross oceans in hours. They are highly reliable, often running for tens of thousands of hours between overhauls. However, they consume large amounts of fossil fuels and release carbon dioxide and other emissions, contributing to climate change. Engineers are developing more efficient designs, such as geared turbofans, and exploring alternative fuels like sustainable aviation fuel (SAF) made from waste oils or plants. Hydrogen-powered jets are also on the horizon. For instance, new engine designs aim to reduce fuel burn and noise simultaneously. Understanding how jet engines work helps us appreciate the ingenuity behind modern flight and the challenges we face, such as reducing emissions and noise, in making air travel more sustainable for the future.