SUCK, SQUEEZE, BANG, AND BLOW! Or, How a Jet Engine Works!

Here at Science… sort of, we know that our listeners are high-flying important-types, who spend more time in the air than they do on the ground.

So while you’re flyin’ in first class on a Boeing Triple-Seven, sippin on Courvoisier, watching Soul Plane while writing a business pitch on your MacBook Air, I’m sure you’ve occasionally stopped to wonder… “Why is this plane in the air, and not falling towards the ground?”

It’s because Snoop’s gotchya back

In a word, it’s Magic Physics. In three words, it’s 1) Thrust, 2) Lift, and 3) Control.  Today, I’m covering the most interesting of those three, THhhhhrust!!!  And what provides thrust on a commercial airliner? One of man-kind’s most awesome inventions: Jet Engines.

Bad….Ass….

So, lets get down and dirty.  How do these bad boys work?  Well, if you’re under the age of 13, you might want to navigate away from this page, because it’s about to get dirty…

Suck, Squeeze, Bang, and Blow

Suck

The first major component of the type of jet engines that are on your airliners is the one that you’re most likely to see: the Fan.

THE FAN SUCKS!!!!   That’s not me being mean, that’s just the truth man…deal with it.

Fans suck by design.  They are typically very large so that they can pull in large amounts of air, some of which bypasses the rest of the engine and exits directly through the rear (depending on the engine bypass ratio), and some is directed into the next major component, the Compressor.

Squeeze

The compressor could be considered to be the most important part of a jet engine.  It’s what separates a jet engine from a simple controlled explosion, and allows you to extract useful work from a heat source.  (If you’re particularly interested, try some learning on Thermodynamic Cycles, and particularly, the Brayton Cycle)

The compressor is named very accurately.  It compresses (get it?) the incoming air through a long series of successively smaller rows of alternating spinning-and-stationary airfoils.  The spinning blades are called Rotors, and the stationary blades are called Stators.  The number of successive rotor-stator combinations you have is the total number of Stages in your compressor.

Typically, if you have more stages, you’ll have a higher compression-ratio, and are able to extract more energy from the engine, which results in a more efficient engine.  For this reason, most of the past 20 years in jet engine design has focused on improvements the compressor stages.

By the time the air has gone through your compressor stages, it can be compressed as much as 40 times that of the outside pressure.  And once it’s nice and irritated about being squeezed so tight, you slap it in the face, tell it it’s your bitch, and light it on fire.

Bang

The combustor is obviously a very important component of a jet engine.  It injects fuel into the flow, and ignites it, which serves as the source of energy for your thermodynamic cycle.  However, for the purposes of understanding how a jet engine works, it’s simple enough to say that the combustor in a jet engine is not much different in theory than a butane lighter or a flame thrower.

In terms of our thermodynamic cycle, the combustion is what creates most of the energy that we will capture to turn into work.  And how do we turn it into work? Meet the close-cousin of the compressor, the Turbine.

2 HOT 4 U

Turbines take the hot, expanded air from the combustor, and captures it like a water-wheel capturing the flow of water from a stream.  The turbines are designed in stages, just like the compressor with rotors and stators. Unlike the compressor, though, these blades are interacting with HOT HOT HOT air! For that reason, they are made from some of the most advanced materials known to man (like single-crystal titanium-nickel alloys, called superalloys), and even use advanced cooling techniques to allow them to operate in temperatures higher than their own melting point.  Turbine’s are freakin H-O-T-T.  Like your mom.

But here’s the most important part! –> The rotating turbine stages are DIRECTLY CONNECTED to the compressor stages and the main Fan (via a long shaft…I told you Jet Engine’s are dirty). So the turbines capture the super-fast flow aft of the combustor, and then transfer the kinetic energy back to the compressor and fan to draw in more air, and keep the cycle going.

So wait a second, the compressor brings air to be combusted, and then the turbine slows down the air in order to create energy to bring in more air, and the air exits at the same speed it came in… How can we create any thrust if there’s no velocity differential?  If you’ve been observant, you would have  seen through my sexual innuendos to find the answer…

Blow

Remember how I said that the Fan only allowed SOME of the air to go into the compressor?  Well in the engines on your commercial airliners, the amount of air that is “bypassed” (meaning not allowed to go through the compressor) can be as much as 10 times the amount of air that goes through the main components of the engine.  This is called the “Bypass Ratio“, and it’s defined as the amount of air that bypasses, divided-by the amount of air that goes through the main components of the engine.

So part of the energy that is captured by the turbine stages is used to turn the main fan, and MOST of the air that is sucked in by the fan is blown right out the back, creating THhhhrust!!  And that’s how a Jet Engine works!

110,000 pounds of thrust, to be exact…

Remember how I said Fans suck earlier?  Well, they go both ways…they Suck and Blow.

Disclaimer

This has been a very simplified explanation of how typical high-bypass turbofan jet engines work, as they are the type that power most of our commercial airliners.  Military jet engines are quite different in some ways, and quite similar in others.  Also, there are many components of jet engines that I’ve left out, and the ones that I have included actually serve slightly different purposes than what I’ve described here.  But for the most part, this post has enough to impress your friends.  And if you’re interested in more details (like how to build a jet engine at home from an old car turbocharger and a leafblower!), just send me an email at jacob@sciencesortof.com and I’ll be glad to help you out.

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9 Responses to SUCK, SQUEEZE, BANG, AND BLOW! Or, How a Jet Engine Works!

  1. PauloNo Gravatar 22 June, 2010 at 11:23 am #

    “Suck, Squeeze, Bang, and Blow” sounds like a perfect date!

    • JacobNo Gravatar 22 June, 2010 at 1:19 pm #

      I think you missed the point of the blog post… it was about JET ENGINES. What were you thinking?

      ;-)

  2. KenMNo Gravatar 22 June, 2010 at 12:44 pm #

    Maybe next you could do one on F*$&ing magnets. How do they work?

    • JacobNo Gravatar 22 June, 2010 at 1:18 pm #

      Lol! Sorry, as the infallible Insane Clown Posse has asserted, it’s F*$&ing MAGIC

      =P

  3. Kyle U.No Gravatar 23 June, 2010 at 12:38 pm #

    Stealth technology, Rocket engines, jet engines… what’s next? Cold fusion?! Way to go Jacob!

  4. dkNo Gravatar 30 August, 2010 at 11:05 am #

    Wait, I’m confused. Is it only in military applications that the midi-chlorian count matters?!

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