Posted by: Korry | February 14, 2012

Techie Tuesday: How Does a Jet Engine Work?

In last week’s Techie Tuesday feature, I wrote about a crazy three day trip I recently flew that included a problem getting one of our engines to start correctly. Since today is Valentine’s Day and there’s a chance you may be taking your significant other someplace to celebrate, I thought I’d try and help you understand just how those massive turbine (a.k.a. jet) engines actually work to propel your plane into the sky!

In it’s most basic form, a jet engine functions much the same way as a regular internal combustion engine like you have in your car. For an internal combustion engine, there are four main steps to generating power. You will often hear the four steps listed as Suck, Squeeze, Bang and Blow. That is, air is sucked into the engine, then it is squeezed together by the piston, then fuel and ignition cause a “bang” that powers the piston back up, and finally the piston goes down again, blowing the exhaust air out of the cylinder. The process repeats again and again and again.

Well, a turbine “jet” engine works the same way…just without pistons. Instead, the process is linear. It starts with air entering the front of the engine at the big fan…like this:

Front of Jet Engine

The air passes through the big fan and then through many smaller “compressor” fans. Each time the air passes another fan (or “stage”), the air is squeezed together more and more, just as though the piston in a car’s engine squeezes the air together.

Once it is compacted to the appropriate level, the air moves into the combustion chamber where fuel is injected and ignitors (think big spark plugs) light off the fuel/air mixture. The “bang” continues to accelerate the air out the back of the engine where it passes through several “turbine” fans. Generally, the final turbine fan is mechanically linked to the big fan at the front of the engine, so as the air spins the last turbine on its way out the back of the engine, it also spins the big fan in the front.

The amazing thing is that because the last turbine and the first compressor fan are connected, the process of making power can run continuously. Air comes in the front, is compressed by the compressor fans, enters the combustion chamber, is lit off, pushes past the turbine blades which turn the front fan and suck in more air. On and on, again and again, continuously powering the engine. In fact, because the engine never stops having a “bang” in the combustion section of the engine, once the ignitors do their job to initially light off the engine, we turn the ignitors off because they’re no longer needed. Pretty cool, right?

This continuous cycle makes turbine engines incredibly reliable once they’re started. They will just run and run barring some major mechanical malfunction, which thankfully is incredibly rare.

If that doesn’t clear things up, maybe this video will. It’s all about the GE CFM-56 engine, the same engines that power the Boeing 737s I fly.

But, Korry, how does a turbine engine actually move an airplane forward? 

That’s a great question, too. In your car, the pistons and rods of the internal combustion engine are connected to the crankshaft which mechanically links power from the engine to the wheels.

In an airplane, a jet engine uses physics, most notably Newton’s third law that states that for every action there is an equal and opposite reaction. The turbine engine pushes air backwards at a high speed (the action) and then the engine, thanks to strong connections to the body of the aircraft, gets pushed forward (the reaction). Of course, Newton’s second law plays a part, too. That law says that the amount of force is determined by an object’s mass times its acceleration. For the jet engine, the mass is the mass of the air and the acceleration is based on how fast the engine accelerates the air backwards. So the faster the turbine engine spins, the faster it accelerates the air rearward and the greater the force of that air is, which in turn means the greater the forward speed of the jet.

Does that make sense? If not, leave a comment with your question and I’ll try and get an answer for you.

Do you have a Techie Tuesday question? Email me at lifesflightplan@gmail.com.

Did you enjoy this post? If so, maybe your friends would, too! Try sharing it with them by using one of the buttons below. 

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Responses

  1. […] recent weeks, we’ve discussed how an airplane generates Lift and how a turbine engine works. With lift and power, an airplane can easily take flight. But that doesn’t clear up one of […]


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