Skip to content
Every Mechanism

Home How a Jet Engine Works

Suck, squeeze, bang, blow

How a jet engine works

A giant fan pulls in the air. The core squeezes some of it forty times, burns fuel in it and lets it rush out through turbines that turn the fan and the compressor. Everything left over pushes the plane forward. This airliner engine even has a gearbox, so its huge fan can turn slowly while the turbine behind it spins fast.

Spool it up yourself
1.76m
fan across
40×
squeezed
2,000°C
flame
10: 1
bypass air
3: 1
fan gearbox
An airliner engine under its wing at blue hour, cut in half along its axis: the dark fan at the front, the compressor's rows of blades, the glowing combustion chamber and the turbines, with blue air streaming round the core and orange hot gas through it.
Cut in half along its axis: blue air flows round the core, orange hot gas through it.

Video

How a Jet Engine Works - Inside a Geared Turbofan

Watch on YouTube

3D model · cut in half · 360°

Start it, then open the throttle

The engine is already cut in half along its axis. Press Start: air from the tail spins the core, the fuel lights, and the engine runs on its own. Then push the throttle to take-off. Drag to turn the view, scroll or pinch to zoom, and click a part to use it.

Try itThe engine is stopped. Press Start and watch the starter spin the core, the fuel light all the way round the combustion chamber, and the fan begin to turn.
Loading the 3D model…

Meet the parts

Fourteen parts, one engine

The engine cut in half along its axis, the air flowing from left to right. Tap any part of the picture or pick a name below. The fan and its gearbox are at the front, the core in the middle, and the gearbox that drives the engine’s helpers hangs underneath.

The jet engine cut in half: the fan and spinner at the front, the star gearbox behind the fan, the booster and the compressor’s rows of blades, the combustion chamber, the high and low pressure turbines, the shafts down the middle, the exhaust plug, the nacelle round it all, and the accessory gearbox and starter underneath.

Hover or tap the picture. Teal rings mark every part.

Parts diagram

From the intake to the jet

The engine cut in half, seen from the side. The numbers follow the air: 1 to 14 along its path from the spinner to the exhaust, 15 to 22 the shafts and gears that carry the turbines’ power, and 23 to 26 the fuel and the starting system.

The jet engine cut in half seen from the side, with twenty-six numbered parts from the spinner to the exhaust plug.
    How it works, step by step

    Twenty steps from the intake to the start

    The same steps as the video. Each picture is a moment from it; press Show me in 3D to set the model up for that step.

      An airliner at the gate at blue hour: the engine under its wing, from in front.
      Step 1 of 20

      What pushes an airliner?

        Demo · three speeds

        Spool it up

        Push the throttle and watch the three rotors speed up: the fan, the low pressure spool and the high pressure spool. Below them, follow the air through the engine and see its pressure and temperature climb, peak in the flame and fall again through the turbines.

        The throttle sets how much fuel burns. More fuel makes the gas hotter, so the turbines push harder and both spools speed up, and the faster the compressor spins, the harder it squeezes. The core speeds up least: it is already turning fast at idle. The fan is geared to the low pressure spool, so it always turns at exactly a third of its speed.

        • Blue: the low pressure spool, which drives the booster and, through the gearbox, the fan.
        • Amber: the high pressure spool: the compressor and its turbine.
        –fan, turns a minute
        –low pressure spool
        –high pressure spool
        –thrust

        Demo · the star gearbox

        Count the turns

        Drag the sun gear in the middle (it is the turbine’s shaft) or press Run, and count: every three turns of the shaft, the ring gear and the fan go round once, the other way. Then see why the engine needs the gearbox at all: what the fan’s tips would do without it.

        –turbine shaft turns
        –fan turns
        –fan tips at take-off
        –the fan’s turning force, times the shaft’s

        Demo · bypass ratio

        The big, slow push

        Give the same core a bigger and bigger fan. The core’s power then pushes a wider stream of air more gently, instead of a thin jet very hard. Watch the thrust, the jet’s speed, the noise and the fuel.

        Thrust comes from throwing air backward: the mass of air each second times how much faster it leaves than it came. Doubling the speed of a jet takes four times the energy, but doubling the amount of air takes only twice as much. So moving a lot of air a little is the cheaper way to push, and a slower jet is far quieter too. Only the drag and weight of an ever bigger fan and nacelle set a limit.

        • Orange: the hot, fast jet from the core.
        • Blue: the cool, slower bypass stream from the fan.
        –take-off thrust
        –jet speed
        –jet noise vs the turbojet
        –fuel for the same push in cruise

        Demo · the start

        Start it up

        Start the engine the way the engine’s computer does it, or do each step yourself: air from the APU, the starter, the igniters, the fuel. Then try what goes wrong if you do it in the wrong order.

        –core speed (N2)
        –exhaust gas temperature
        –fan speed
        –fuel flow

        Demo · the hot section

        Hotter than the metal

        The first turbine blades work in gas hotter than the melting point of the nickel alloy they are made of. Turn the cooling air and the ceramic coating off and on, and see what keeps them solid.

        Each blade is hollow. Air taken from the end of the compressor, at about 600 °C, runs up passages inside it and out of rows of tiny holes, where it spreads over the skin as a thin cool film. A ceramic coating a few tenths of a millimetre thick adds another layer of insulation. Every bit of cooling air costs fuel, though: it took work to squeeze, but it skips the flame and gives little of that work back in the turbine.

        • Blue: cooling air inside the blade and over its skin.
        • Orange: the hot gas from the combustion chamber.
        –gas around the blade
        –hottest metal
        –below the melting point
        –how long the blade lasts

        More to know

        Kinds, history and care

        Six kinds of jet engine

        TurbojetAll the air goes through the core and leaves as one fast, hot, very loud jet. It powered the first jet planes and Concorde, but it wastes a lot of fuel at airliner speeds.
        High-bypass turbofanA big fan pushes most of the air round the core. Nearly every airliner today flies on these: quieter and far more economical than a turbojet.
        Geared turbofanThe engine on this page. A gearbox lets the huge fan turn slowly while its turbine spins fast, so both can work at their best speed, with fewer turbine stages.
        Low-bypass turbofanA small fan and a fast jet for fighters. Many add an afterburner, which sprays extra fuel into the hot exhaust for a burst of thrust.
        TurbopropThe turbine turns a propeller through a reduction gearbox. Very economical for slower aircraft on short flights.
        TurboshaftAll the turbine’s power goes into a shaft instead of a jet: it drives helicopter rotors, ships, tanks and the APU in an airliner’s tail.

        From a patent to the geared fan

        1. Frank Whittle, a young Royal Air Force officer, files his patent for a jet engine on 16 January.
        2. Whittle’s first engine runs on a test bed on 12 April.
        3. The Heinkel He 178, with Hans von Ohain’s engine, makes the world’s first jet-powered flight on 27 August.
        4. The de Havilland Comet starts the first jet airliner service, London to Johannesburg, on 2 May.
        5. Lufthansa starts flying passengers in Boeing 707s with Rolls-Royce Conway engines in March: the first turbofans in airline service.
        6. The Boeing 747 enters service with Pan Am on 22 January, on four Pratt & Whitney JT9D high-bypass turbofans.
        7. The General Electric GE90 enters service on the Boeing 777 with British Airways in November.
        8. The first geared turbofan for airliners, the Pratt & Whitney PW1100G, is delivered to Lufthansa on an Airbus A320neo on 20 January.
        9. The General Electric GE9X, with a fan 3.4 m across, makes a record 134,300 pounds of thrust on test, on 10 November.

        Around a running engine

        • Keep clear of the intake. A running engine can suck loose objects, and even people, into it from several metres in front, so ground crews stay out of the marked danger zones.
        • Keep clear of the exhaust too. Behind the engine the jet blast can knock people over and roll vehicles, far beyond the end of the wing.
        • Keep the apron clean. A stone or a lost bolt sucked into the fan can nick its blades, so crews walk the ramp picking up debris.
        • Look inside without taking it apart. Engineers slide a borescope, a camera on a flexible probe, through small ports to check the blades for nicks, cracks and burns.
        • Wash the compressor. Dirt on the blades makes the engine work harder and run hotter; a wash with water brings the efficiency back.
        • Watch the start. The exhaust temperature must stay under its start limit, and the speed must keep rising. A temperature that climbs too fast is a hot start; a speed that stops rising is a hung start. Either way the start is stopped.

        Things worth knowing

        Hotter than its own metalThe gas reaching the first turbine blades of the newest engines can be 1,600 to 1,700 °C, while the nickel alloy they are cast from starts to melt at about 1,330 °C. Cooling air and a ceramic coating keep them solid.
        A racing car in every bladeRolls-Royce says that at take-off a single high pressure turbine blade in its big Trent engines produces as much power as a Formula 1 car.
        Tonnes of airAn engine the size of this one swallows around 400 kg of air every second at take-off. The giant GE90-115B of the Boeing 777 takes about 1,350 kg.
        Three speeds in one engineIn the geared PW1100G the fan turns at up to about 3,300 turns a minute, its low pressure turbine at about 10,000, and the core at up to about 22,000.
        Most of the push is coldIn a high-bypass engine the fan’s cool air makes most of the thrust. The hot core mainly provides the power to turn the fan.
        Started by air, or by electricityMost airliner engines start with compressed air from the APU, a ground cart or the other engine. The Boeing 787 starts its engines electrically instead, through their generators.
        Quick check

        Four questions