A jet engine is a highly complex piece of equipment with a straightforward job: to give an airplane the thrust it needs to fly. Anyone who has felt themselves pushed back slightly in their seat as an aircraft speeds down the runway and then takes to the sky can likely intuitively sense what’s happening. The turbofan engines beneath each wing are inhaling the air, and accelerating it out the back, producing thrust.
The details inside commercial engines from companies such as General Electric, Rolls-Royce, and Pratt & Whitney may vary, but the basics of what’s happening are the same.
“A modern turbofan jet engine works based on Newton’s Third Law,” says Emma Booth, a subsystem lead at Rolls-Royce. “Every action has an equal and opposite reaction.”
While the high-level description might sound simple, the process within the engine itself is both complex and fascinating. Here’s what to know about an engine’s inner workings, where air is compressed, fuel is ignited, and temperatures become extremely hot.
Fans at the front of the engine inhale the air
Take a look at an engine from the outside—you can see this from an airport gate—and you’ll notice the fan blades at the front, housed within the engine’s body. These can be absolutely enormous in diameter. For example, General Electric’s GE9X features a fan with 16 blades that spans over 11 feet in diameter. One of those engines can produce 105,000 pounds of thrust, although it’s cranked out even more than that, setting a record in 2017.
“There’s a big fan on the front—that actually provides about 90 percent of the thrust,” says Christopher Lorence, the chief engineer at GE Aerospace.
Consider a GE90 engine, which hangs below the wings of planes like the Boeing 777. The company says that one of those will suck in about 3,600 pounds of air every second when a plane is taking off.
The fan slurps in the air, and as the air travels through the engine, a proportionally smaller amount travels down one path through the center of the machine—its core. But most of the air bypasses the core, skipping it and going straight out the back. It’s the air that does not go through the core that does most of the work when it comes to propelling the aircraft.
The difference between the volume of air that bypasses the core versus the air that goes through the core is known as the engine’s bypass ratio. Engine makers want the ratio to be high for peak efficiency. “The most efficient way to do it is to take a lot of air and increase the pressure a little,” says Lorence. “The early engines had a very low bypass ratio—and so what they were doing is, most of the air was going through the core, a limited [amount of] air was going through the bypass, and it was going through it at pretty high velocity.” But today, turbofan engines have very high bypass ratios.
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An exception here are the jet engines on military aircraft, like fighter jets, which lack the large bypass ratios that engines on commercial planes have. These aircraft have other priorities besides pure fuel efficiency—like the ability to be highly maneuverable, hit supersonic speeds, and keep a low profile—and their engines, which are closely integrated with the body of the aircraft, can also make use of afterburners.
In the core, air is compressed, and fuel ignites
The fan blades in the front need power to spin, and that’s where the engine’s core comes into play. The small percentage of air that does go through the core (Booth, of Rolls-Royce, says it’s around 10 percent, while the other 90 percent bypasses the core) experiences a multistage process.
The first part of the core is the compressor stage, where the air is—you guessed it—compressed. The air becomes more dense, and it heats up. “There’s many stages of compressor blades, which are rotating, and compressor vanes, which are static, and the air is sort of progressively squeezed and squeezed and squeezed as those compressor blades get smaller and smaller and smaller,” says Booth.
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The air, of course, doesn’t want to be compressed; it takes work to make that happen. “It’s basically like you’re trying to brush water uphill,” Booth explains.
Then, after the compressor stage, comes the combustor. Jet fuel ignites and heats up the air even more. GE’s Lorence says that if the temperature of the air is around 1,200 to 1,300 degrees Fahrenheit at the tail end of the compressor, it could get as hot as 3,000 degrees Fahrenheit or so after going through the combustor. For comparison, lava from a volcano in Hawaii tends to be in the neighborhood of 2,140 degrees.
The scorching air that departs the combustor is, amazingly, “higher than the melting point of the turbine blades that follow it,” says Lorence. “We actually have to pump air through those blades to keep them from melting.” That relatively cooler air comes from the compressor stage. Rolls-Royce also does something similar to prevent the blades in its turbine from melting.
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And just like engine makers want a large bypass ratio, they also want the engine to be very hot inside. “The hotter you make that temperature, the more efficient that core operates,” Lorence says.
Turbines in the core harvest energy
After the air is superheated, it has an important job to do before it can clock out for the weekend and relax: spin some turbines. In a General Electric engine, there are two turbines—a high-pressure turbine and a low-pressure turbine. “You have a bunch of air that’s got a lot of energy in it,” says Lorence. “The reason you’ve done all that is so that the energy can be released through these turbine stages.”
Each of those two turbines has a specific task. First, the high-pressure turbine “takes that energy and spins the compressor, which basically runs the core,” says Lorence. “And then in the low-pressure turbine, it takes that energy and spins that shaft, which spins the fan [in the front of the engine].”
In Roll-Royce’s Trent engines, like those on Airbus A350s, there’s also an intermediate-pressure turbine, in between the high- and low-pressure turbines. In that case, those first two turbines make the compressor work, and the final one powers the large fan blades in the front.
In a nutshell: the air that enters the core is compressed and heated with burning fuel. It then drives turbines, and one of those turbines powers the fan blades at the front of the engine. And remember, it’s the air that bypasses the core that gives the engine most of its thrust, compared to the exhaust from the core.
The bypass air “is traveling at a lower speed than what’s gone through the core of the engine, but that air has such a lot of mass to it, that it still generates a lot of thrust,” says Booth. And because of that thrust, the aircraft can take to the sky.
