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I was doing research about jet engines, and they seem really difficult to fully understand. So, can anyone explain it in a simple way?

Jet Engine Image
An afterburner glows on an F-15 Eagle engine, source: Wikimedia.

How do jet engines work?

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  • $\begingroup$ I gotta say, that engine doesn't seem to be terribly well tethered. I guess those two longer guy wires and the short turnbuckle (per side) are sturdier than they look. $\endgroup$ – FreeMan Jan 9 '15 at 16:19
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    $\begingroup$ Compare with physics.stackexchange.com/questions/114280/… $\endgroup$ – Peter Kämpf Jan 8 '16 at 9:09
  • $\begingroup$ Could anybody (OP ?) source the image ? $\endgroup$ – kebs Jan 9 '16 at 18:37
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In the simplest of terms:

  • Suck - Air is sucked into the turbine. For efficiency reasons, most aircraft let some of that just pass through the outer part of the fan, rather than through the whole engine.
  • Squeeze - The compressor squeezes that air together to a high pressure. This helps with ignition.
  • Boom - Fuel is injected and ignited. As the air gets hot, it expands.
  • Blow - The hot air drives the low-pressure turbine (driving the whole shaft holding the engine together), sucking new air in, and is itself blown out the back.

jet

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    $\begingroup$ There's a great interactive animated GIF at NASA's K-12 page on jet engines. $\endgroup$ – egid Mar 26 '14 at 18:44
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    $\begingroup$ if some of the air bypasses the engine it is called a turbofan engine. but that is semantics $\endgroup$ – ratchet freak Mar 26 '14 at 19:05
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    $\begingroup$ the "suck" part is in effect only at startup/on ground. During cruise is the motion of the aircraft w.r.t. the air that makes the air enter the engine. The fan effectively pushes the air towards the back. $\endgroup$ – Federico Mar 27 '14 at 13:43
  • $\begingroup$ @ratchetfreak I'd always heard that a "low-bypass" jet engine was still a turbojet and not a turbofan, but other definitions refute that. Some specialized engines, like those for the SR-71, had variable bypass; they were technically turbojets up to Mach 2, then the bypasses were opened and air flowed directly to the afterburners. $\endgroup$ – KeithS May 27 '15 at 2:56
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    $\begingroup$ @KeithS Federico is right, engines only "suck" in air when they are at no or very low speed. I think Federico was not commenting on the fact that in order to suck in air the pressure in the inlet has to be lower than the ambient pressure. This means there is a pressure gradient from high ambient pressure to low static pressure in the intake. During cruise flight this is not the case anymore, then the ambient static pressure gradient reverses. $\endgroup$ – rul30 Jan 8 '16 at 8:31
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Users reaching this question: How does a jet engine work? (explanation for non-aviation related people) are now redirected here. This illustrative answer is in the spirit of the initial question, easy to read for a non-aviation person without sacrificing accuracy.

To create a reaction, throw something

The physical principle at work has been studied by Isaac Newton and is known as principle of reaction (or the more mouthful Newton's third law of motion).

The reaction movement is the response to another movement. To create a large reaction, we just need to create a large action. In many cases, including the jet engine, this is done by moving something heavy, at the highest possible velocity.

While reaction seems magical, it's something experienced non-stop in everyday life, without giving attention to it. We can easily imagine the result of such experience:

  • Get a small boat and a well-sized rock, say 20 kg.
  • The boat is initially stationary on the water.
  • Throw the rock violently behind the boat.

    enter image description here
    (Legal disclaimer: Don't try this at home, I conducted this experiment a long time ago with trained personnel)
  • As soon as the stone gets speed rearwards, the boat gets speed forwards.
  • Both objects stop accelerating when the rock has left the hands of the thrower.

A note here about the velocity:

  • In the boat experiment, if we want to throw the rock without moving the boat, instinctively we'll move it very very slowly until it is above water, then let it drop into the water without pushing, and it'll work.

  • We also know what happens when a big gun throw a 500 g shell at the speed of sound: There is a huge reaction of the gun, in spite the gun is heavier than our boat and friction on the ground should prevent it to move.

Reaction from throwing air

Throwing stones is obviously not practical for an engine. But we can throw air, like in a balloon, again something very familiar:

enter image description here

This principle of ejecting air to create a reaction has been known since Antiquity with the aeolipile. It was an early steam engine. Air was ejected through tangential openings at the end of pipes.

enter image description here
Source

A jet engine works the same way, throwing air overboard, in large quantity and at very high velocity. Compared to the boat experiment, we have replaced:

  • The stone by air. As air is much less dense than rock we need to concentrate a lot of air to get the same effects.

  • Hand muscles by fuel combustion. Combustion delivers high velocity gases by effect of gas expansion at hot temperature. The more fuel, the higher the gases velocity.

Burning a lot of fuel requires a lot of air, we have already a need of air in large quantity, so this is not a problem. However air won't come in large quantity into the engine, at some point air already present in the engine prevents more air to enter. In flight, ram air pressure can increase the quantity of air in the engine, but this isn't enough.

The solution is to use a compressor to fight the pressure of air already in the engine, and push, on large airplanes, about one ton of air per second in it. It's only 50 times more than our stone, but the exhaust velocity is definitely not the same: About 1,000 km/h.

How do we collect a lot of air, and eject it at high velocity?

We have all the ingredients to make a jet engine, which is composed of three sections (to simplify, let's look at an early turbojet engine, refer to this question for common turbofan):

  • Air enters in the engine from forward and is compressed by a pump (named the compressor) to increase the quantity available.

  • Compressed air is mixed with fuel and lit. Air is required for fuel to burn (oxygen). During the combustion, the mix achieves a very high temperature and expands under the effect of heat, exactly like water vapor expands in a pressure cooker or an aeolipile.

  • Before being ejected as small part of the hot gases energy is used to turn a turbine (like wind turns a wind turbine). This rotating motion is transferred to the compressor we saw at the beginning. To start the compressor at the beginning a starter is used, like on a car.

  • Hot gases are ejected rearwards this creates the forwards reaction of the engine.

The components of a jet engine are therefore:

  • A compressor to force air into the engine.
  • A combustion chamber to create rapidly expanding gases from air and fuel.
  • A turbine to turn the compressor.
  • An exhaust to release the gases in an optimized way to increase efficiency.

enter image description here
Source

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  • $\begingroup$ I can't believe I've never seen this answer before, but +1 for the man in the boat! $\endgroup$ – FreeMan Sep 7 '17 at 17:30
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There are three basic parts to a gas turbine, a compressor, a combustion chamber and a turbine. Air is compressed by the compressor, heated by the combustion chamber and the heated air powers the turbine. The turbine in turn powers the compressor.

The key is that the heating in the combustion chamber causes the air to expand. This means that the work that can be done by gasses (air plus combustion products) exiting the pressurised combustion chamber is greater than the work needed to force the incoming air (and fuel) into the combustion chamber.

The available energy in the exhaust can be used in various ways. The simplest is the turbojet, the stream of hot gasses coming out the back of the engine directly provides forward propulsion.

Alternatively we can capture more of the energy from the exhaust with a turbine and use it to drive a fan resulting in a turbofan.

Or we can design our turbine to capture most of the usable energy in the exhaust and direct it to a shaft which can be used to power all sorts of things.

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A jet engine is an overcomplicated ramjet with extra turbines to let it work at lower speeds.

A ram jet works by igniting compressed incoming by mixing it with fuel and providing a spark.

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    $\begingroup$ hmmm... a ramjet lacks (moving) compressor stages, which are an essential component of a jet engine, and there is no bypass component and most of the time no supersonic flow. So perhaps more distant relatives than siblings? $\endgroup$ – yankeekilo Mar 26 '14 at 19:32
  • $\begingroup$ @yankeekilo that's why I said overcomplicated :) $\endgroup$ – ratchet freak Mar 26 '14 at 19:38
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    $\begingroup$ LOL, but then a barrel of fuel and some matches may also claim commen ancestry :D $\endgroup$ – yankeekilo Mar 26 '14 at 19:44
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    $\begingroup$ A ramjet is almost totally different from a turbojet (and its derivatives). It lacks a fan/compressor and a turbine, so it would only confuse people to compare them. Once you have a turbojet down, you can look at pulsejets, ramjets, scramjets, and other more esoteric forms. $\endgroup$ – Phil Perry Mar 26 '14 at 22:49
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    $\begingroup$ I don't see why you say it's "overcomplicated" -- it seems that a jet engine is exactly as complicated as it needs to be for its intended operating environment. $\endgroup$ – Johnny Mar 26 '14 at 23:43

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