How do jet engines work?

The principle

One idea, many engines. Every jet engine does the same four things to air, one after another and all the time: suck, squeeze, bang, blow. The different types just use the power in different ways.

Step 1: Suck and squeeze. Rows of spinning blades at the front pull air in and squeeze it into a smaller and smaller space, about 8 times tighter in a simple jet, and 10 to 40 times in big modern engines. It’s like pumping up a bike tyre: squeezed air gets warm.

Step 2: Bang, burn the fuel. Fuel is sprayed into the squeezed air and burns non-stop, not in separate bangs like a car engine. The air gets very hot, around 900 °C and often much more, and swells up. It takes roughly 50 parts air or more to burn 1 part fuel.

Step 3: Blow it out the back. On the way out, the hot air spins a second set of blades at the back. They sit on the same long rod as the front blades, so they keep the front blades turning. Then the air rushes out of the narrow back end at around 2,100 km/h, a typical speed for a simple jet. Throwing air backwards shoves the engine forwards, just like a balloon that shoots off when you let go of it. This simplest type is called a turbojet.

Step 4: Add a giant fan (turbofan). Airliners put a huge fan on the front, turned by the same engine. Most of its air goes around the hot middle instead of through it, about 10 parts around for every 1 part through the middle on the newest engines. Pushing lots of air gently uses less fuel and is much quieter than pushing a little air very fast, so the big fan gives most of the push. This is a turbofan.

Step 5: Turn a propeller or rotor. Take away the big casing and let the engine spin a propeller instead, and you get a turboprop: the propeller gives most of the push, which is great for slower planes. Let it turn a helicopter’s rotor and you get a turboshaft: almost all the power goes into the spinning rotor, and only a weak puff comes out the back. Same engine, different job.

Which engine for which job?

• Turbojet: the simplest kind and at its best at very high speed, like old fighter jets and Concorde. At normal airliner speeds it is thirsty and very loud, a bit like driving a race car to the shops.
• Turbofan: the big fan moves lots of air gently, so it uses much less fuel and is quieter, like rowing with long, calm strokes instead of fast splashy ones. Big turbofans also give the most push of all, which is why airliners use them. Fighter jets use slimmer turbofans with a smaller fan, often with an afterburner for extra bursts of speed.
• Turboprop: a propeller is the most fuel-saving choice at lower speeds, like short regional flights and cargo planes. The catch is speed: propellers lose their grip on the air as a plane goes faster, so a turboprop can’t fly as fast as a jet.
• Turboshaft: nearly all the power goes into a spinning shaft instead of a blast. That’s perfect for a helicopter’s rotor, and the same idea also drives some boats, ships, tanks and power-plant generators. In a helicopter, a separate “free” turbine behind the engine is spun only by the hot gas, like a pinwheel in the wind. Its own shaft runs forward to a small gear that turns a big gear, and angled gears turn the drive 90° up the rotor mast: like a bike’s low gear, slower but much stronger, with a one-way clutch so the rotor can keep spinning if the engine stops.