What Makes It Go

Jet engines of one sort or another have been around since about 1942. The design has evolved considerably since the Luftwaffe was whizzing around amazing everybody but the basics are the same. Actually, only the very basics are the same. I’ll try to explain that part.

Only the core of a  modern jet engine has anything to do with fuel or combustion and all the machinery that makes so much noise and smoke. Most of the power comes from the fan part of the engine that never even gets the air hot.

The fan you see through the front of the engine is a giant propeller, it just has a lot of blades. This is where the name “Fan-Jet” or “Turbo-Fan” comes from. Older jet engines didn’t have the fan part and were just called “Turbo-jets”. On a windy day while the airplane is parked you can see the fan turning in the wind, even backwards if there is a tailwind. It makes a clacking sound, this is from the fan blades flopping around in their mounts and dragging against the outer edges. They are supposed to do that.

The fan is really efficient at moving a huge volume of air at relatively slow speed. This is important at low altitude where there is a lot of dense air to move. The turbo-jet core of a fan jet engine moves a small volume of air at super sonic speed. This is efficient only at high altitude. For this reason modern fan engines are much quieter and fuel efficient than the turbo-jets of 1960, especially at low altitude.

To understand a jet engine visualize a magic “stuff”.  Let’s call it air. Air is is really compressible. Squeezy, like a foam rubber sponge. Since we can’t see it in the first place this isn’t readily noticeable, and a little difficult to imagine.

The front part of a jet engine has a series of rotating discs of blades. Between each set of rotating blades is another set of blades that do not turn.  The rotating blades are mounted on a hollow drive-shaft.

Air coming in the front is compressed into a fluid with a very high oxygen content. Think of the air leaving the back of the compressor section as a volatile, unstable liquid. It would look like water if you could see it.

After the compressor is the burner section. Here fuel is metered through little injectors and mixed with the volatile compressed air liquid stuff. The resulting chemical reaction  is kind of amazing. Rapid expansion of the fuel/air mixture causes exhaust to jet out the back of the burner section like a rocket. Actually, it is a rocket. And a jet too. 

Here is the  pay off part. On the way out of the engine the exhaust gas turns another set of blades called “turbines” which also have stationary blades between them. The turbine blades are attached to … the same dang drive shaft that the compressor blades are driven by. Ain’t that just cooler than sheep dip! One section of the thing drives the other!  Wow.

Remember how I said the drive-shaft is hollow? The big fan on the front is attached to a separate drive-shaft rotating  inside the main one and is driven by a second set of turbines after the first set.

There is no ignition system used once the engine is started. The reaction of the fuel and compressed air is enough to keep the thing going. As a precaution during icing conditions or heavy rain, igniters are turned on that should help relight the engine if it should “un-start”.

The speed of the fan and core are allowed to be different and variable through this arrangement. This is why they often make that rung rung rung rung aaarung aarung  sound during cruise flight. It seems to work out just fine as far as the pilots can tell.

We control the power of the engine by limiting the amount of fuel available to it. On takeoff the CFM-56 engine of about 28,000 pounds of thrust uses more than 400 pounds of fuel per minute. Cruise power requires only about 50 pounds of fuel per minute per engine.

At average weights a flight will burn about 1,000 gallons the first hour, 800 gallons the second hour, and only about 150 gallons from cruise altitude to landing (with no delays).  We normally use pounds for fuel calculations since it is the mass of the fuel that the engines care about. A lighter fuel like ethanol would require many more gallons per hour but about the same number of pounds.

Since the compressor section produces an abundance of compressed air some of can be bled off and used to provide air for the cabin pressurization and wing or engine de-icing. It seems that if you have an abundance of compressed air it is fairly easy to heat or cool about anything.  The bleed air from the compressor is hot just because it has been compressed. 

The method of using hot air ( about 200 degrees F) to create cold air for cabin cooling is worthy of a article of its own. Most pilots don’t really understand it but they really don’t need to know why a system works if they know how to work the system. 

When the engines are used to help slow the plane on the ground the reverse thrust is created by ducting some of the thrust toward the front. This is done with sleeves that slide back and partially close off the exit of air from the engine and direct it forward. The engine doesn’t really start turning backwards, the air just gets blown forward.

Each engine drives fairly massive DC generator through an automatic transmission that runs at  constant speed. These provide enough electrical power for a few houses.

The bigger Boeings have engines as big around as the fuselage (body) of a 737. These are some really big air movers but the design is essentially the same.

The air doesn’t seem to mind too much what compresses it and moves along.

Happy landings


2 Responses to “What Makes It Go”

  1. The only part of a jet engine that doesn’t make sense is how that little explosion can tell front from back. You would think it would explode in both directions, turning the front fan backwards, and the back fan forwards – and then neither would turn.

  2. luckyjet1 Says:

    The condition you describe is called a compressor stall and is quite envigorating to witness. It happens sometimes at high power settings when the engine eats a bird, has a fuel controller problem, or an internal mechanical problem. A really low airspeed and high power setting in reverse thrust can also cause a compressor stall. There is an amazingly loud thump followed by flames shooting about 75 feet in front of the engine. This is sometimes followed by severe engine damage and failure but usually not.

    For understanding the concept of pressure balance inside the engine don’t think of it as an explosion but rathor a controlled rapid continuous expansion. If there were no air coming in the front and the fuel was not metered toward cut-off the bad things you describe would happen.

    Before the fancy new computer controlled fuel systems problems like this used to be much more common.

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