Answer Geek: How Jet Engines Work

-- Q U E S T I O N: I see you have analyzed the Otto, the Wankel, the diesel and even the ion engines but it seems that you have missed one. The jet engine. How, in fact, does a jet engine create compression and ignition through rotating and static fan blades? I think you have to complete your series on ALL types of engines. Thank you for your time. I think many people will be very interested. — Matthew

A N S W E R: I suppose you are right, Matthew. It does seem like a bit of an omission on my part to have delved into the inner workings of so many types of engines without taking on the gas turbine model that makes modern air travel possible. But keep in mind that the way this generally works is I have to get an actual questions from a real reader before I can write about a topic. That’s why they call me the Answer Geek.

One more thing, Matthew. What’s with all the jet engine jargon in your question — ”compression,” “ignition,” “rotating and static fan blades”? I have this sneaking suspicion that you already know how jet engines work. Is this a test of some sort to see if I’m up to the challenge? Are you doing a sort of public service by making sure the Answer Geek record is complete on engines of all sorts? Or are you one of those people who asks questions that they already know the answer to? When I was in college, there was someone like that in every class. And believe me, I was very annoying. Er, umm, I mean — they were really annoying.

No matter. The question is a good one. So let’s start with the physics that make the jet engine work. Anyone ever heard of Isaac Newton? (Matthew, you can put your hand down, I know you know the answer.) Newton’s third law of motion — to every action there is an equal and opposite reaction — provides the fundamental framework for understanding how a jet engine can get a plane weighing as much as 800,000 pounds moving fast enough to become airborne and stay that way for hours at a time.

The Thrust of the Issue

The deal here is that hot, rapidly expanding gases are funneled out of a nozzle at the back of a jet engine. As this stream of gas molecules is expelled rearward, it exerts the proverbial equal and opposite reaction on the inner walls of the engine itself. The force of this reaction, also called thrust, propels the engine and everything attached to it, including the rest of the plane, forward. Remember: it is the pressure exerted by thrust on the engine itself, and not the pressure of gas pushing against outside air, that propels the plane. A lot of people get this wrong. (Not Matthew, though.)

The big question then is what kind of engine can expel a large enough volume of gas at sufficiently high speeds to ensure that a fully loaded jumbo jet gets its wheels off the ground before the end of the runway? The answer is the gas turbine engine. An internal combustion engine like the Otto engine in your car, gas turbines power not only jet planes, but everything from large ships to small power plants, high-speed pumps, and the U.S. military’s M-1 tank.

There are some similarities between Otto engines and gas turbines. In both, a mixture of compressed air and vaporized fuel is ignited, causing the air and fuel mixture to expand — a process that results in power. In your car, however, the ignition happens intermittently, and the expanding gas causes a piston to move, which moves a crank, which spins a shaft, which turns the wheels of your car. In a gas turbine engine, ignition is continuous and the movement of the gas itself is the action that propels the airplane.

Deconstructing the Engine

A gas turbine engine consists of three main parts. At its front is acompressor, which is a series of fans that spin at very high speeds (around 16,000 revolutions per minute). These blades pull air in and send it through a channel that gets progressively narrow. As the air moves through this channel, it is compressed until it is anywhere from 10 to 30 times more dense than it was to start with. Alternating with these spinning blades are the stationary fan blades that Matthew mentioned in his question. Also called stators, they are there to make the flow of compressed air move in a straight line rather than in swirling currents that would make the air burn less efficiently when it reaches the combustion chamber.

That combustion chamber is the second main section of a gas turbine engine. There, fuel — a refined form of kerosene, in the case of jet airplanes — is injected into the pressurized gas. When the engine is first started, igniter plugs are used to get the mixture burning. Once initial ignition is achieved, combustion is continuous, with the gas that is already burning igniting the fuel-and-air mixture that flows into chamber. Temperatures of around 2,500 degrees Fahrenheit are typical inside a gas turbine combustion chamber.

The third section of a jet engine is the turbine. As the hot gasses are expelled — at speeds well above 1,000 miles per hour — they provide power in the form of thrust. They also turn a series of fans that are much like the blades found in the compressor portion of the engine. These fans in turn provide the power to turn those compressor blades. Depending on the type of engine, they may make other things spin too — in industrial gas turbine engines, a drive shaft attached to the spinning blades will turn a drill bit, say, or power a pump.

If the airplane engine in question is a turboprop, the drive shaft will turn a propeller. Because they are very efficient at relatively low speeds, turboprops are great for smaller planes that fly shorter routes.

These days, the most common jet engine is a type known as a turbofan. Turbofans take advantage of the fact that the amount of thrust that an engine produces equals the mass of the gas expelled from the engine multiplied by its acceleration. (That's Newton's second law of motion in action — force equals mass times acceleration, or f=ma.) What that means is you get equivalent amounts of thrust from small amounts of hot gas moving at really high speed or larger amounts of cooler gas moving at slower speed.

Over the years, aeronautic engineers have learned jet engines that move lots of air at lower temperatures are quieter and more efficient than those that burn gases at really high temperatures to achieve greater acceleration. In turbofan engines, a massive fan sits at the front of the compressor. Blades in the turbine portion of the engine turn this fan, which greatly increases the amount of air sucked in. Much of this air isn’t channeled into the compressor at all, but is propelled directly out of the back of the engine. In fact as much as 75 percent of the total thrust generated by a turbofan engine comes by the fan alone.

So, Matthew, there you have it. The Otto. The diesel. The Wankel. The ion engine. And now, the gas turbine. If there are other engines I’ve missed that you think will be of interest to the general public, just send me a question. That’s what I’m here for.

Todd Campbell is a writer and Internet consultant living in Seattle. The Answer Geek appears weekly, usually on Thursdays.