"The Most Fascinating Machines Ever Made": How Jet Engines Work
When you board a plane, you might have noticed that little swirl, or white hash, in the very middle of the engine, slowly turning like an optical illusion. Behind that swirl is probably the most complex piece of engineering ever designed: One of the jet engines powering your aircraft.
"There's no metal-on-metal contact. They can go for thousands of hours — 60,000 hours — dependent only on air and fuel. The components are incredibly long-lasting," said Dr. Magdy Attia, Professor of Aerospace Engineering at Embry-Riddle Aeronautical University.
I spoke with Dr. Attia and James Speich, Marketing Director for Pratt & Whitney Commercial Engines, to understand how a jet engine works.
Attia is a longtime expert in aerospace engineering; he has several aerospace patents to his name along with an arm's length of peer-reviewed publications. He also runs a gas-turbine research facility at the university. Speich is a mechanical engineer who has been with Pratt for 45 years; he cut his teeth working on early computer models of jet engines and on the PW4000, the successor to the first engine Pratt designed for the Boeing 747. More about that later.
I think we're in good hands here.
First Things First: Lots of Air. Really Lots.
Jet engines work fundamentally by drawing in air, lots of air, mixing it with fuel and expelling the resulting gases out the back at great speed. That moves the engine forward by reaction, as well as the airplane attached to it.
But that's not quite how today's jet engines work. In fact, most of the thrust generated by a modern jet engine comes simply by moving an incredible volume of air, all at once, very quickly. A full 90% of the air that enters the engines passes right through without being mixed with fuel and ignited. The fan blades at the front are a slave to the core of the engine — and that core makes those fans do all the heavy lifting.
At the dawn of the jet engine, airplanes used a type of jet engine that's no longer made for commercial uses: a turbojet, in which all of the air sucked into the engine passes through its core. These days, jets instead use turbofans, which push almost all of the air they ingest around the engine core. They're quieter and far more efficient than turbojets.
The biggest jetliners in service today have extremely high-bypass engines, where there is a high ratio between the air accelerated through the engine — bypassing the core — and the air going into the core of the engine itself. The huge diameter of those engines, such as those on Boeing 777s, is due to the need to have a giant fan at the front.
Civilian turbojets stopped flying with Concorde, which even used something found only on supersonic fighters and bombers: afterburners — literally injecting fuel into the exhaust to create a huge thrust boost — to help accelerate on takeoff and, later in the flight, to break the sound barrier.
You're not going to see flames bursting out the back of civilian airplanes on takeoff these days.
Propulsive Energy is the Key
The theory put into practice with turbofans is something called propulsive efficiency. It is much more efficient to move a large volume of air at relatively slower speeds than to move a small volume of air at higher speeds. (Attia repeated this maxim to me by rote memory). "Generally, at takeoff, 70% to 80% of the thrust is provided by the bypass and around 20% is provided by the core itself. As the aircraft hits cruising altitude, this tends towards 95% to 100% of the thrust (being) provided by the bypass," Attia said. Turbojet engines, like those on the Concorde, did not have any bypass at all, which made them very expensive to operate. To make that jet roar, the engines had to burn a lot of fuel.
Suck, Squeeze, Bang and Blow
"Suck, Squeeze, Bang, Blow" is how pilots remember the various stages of an engine.
The fan at the front sucks in air. 10 percent of this air goes into the so-called "core" of the engine. 90 percent is sped up and pushed around the core.
The air that entered the core moves through a series of small, spinning blades attached to a shaft called the compressor. The act of spinning the air causes torque, which causes the air to speed up and increases its pressure.
Fuel is then injected into the compressed air and ignited in a combustor.
Next, the rapidly expanding, hot gas mixture passes through another set of fan blades called the turbine. These gases are caught by small blades on the turbine, causing the turbine to spin.
It's this turbine that's incredible.
The spinning turbine turns a shaft that makes the compressors spin and turns the fan at the very front. A key takeaway: the whole point of the engine core is to turn the fan at the front — not to provide most of the thrust itself.
"The turbine converts the thermal energy generated by combustion back into mechanical energy. It's the small turbine blades that spin, and they're connected to a shaft, which is connected to the compressor itself and the fan," Attia explained. That turbine shaft spins around 20,000 RPM — which is really, really fast.
So, how much air is needed to provide enough forward movement to get the wings working, and to generate lift?
53 UPS Trucks
A typical jet engine will ingest some 1,500 kilograms of air per second. Air density at sea level is about 1.2 kilogram per cubic meter. Dr. Attia did some quick math for our benefit: a typical UPS truck is 23 cubic meters, and accordingly, a jet engine pulls in the volume of about 53 UPS trucks worth of air — per second.
"It's the mass flow of air that is the most important part of the thrust equation," Attia said. Speich concurred, noting that Pratt & Whitney has been focused for 20 years on propulsive efficiency: "pumping a lot of air," as he put it.
The Fan Blades
The energy created by the fan blades is stunning. And every engine manufacturer seems to have a colorful way to explain the energy captured in one single blade. One manufacturer said the energy in a single fan blade in operation could launch a small car over a seven-story building. Another: it's enough to hoist nine double-decker buses (or 13 bull elephants.)
The fan blades for the Pratt engines are made with high-strength aluminum alloy with a titanium leading edge. Other jet-engine makers use hollow titanium blades or blades wrapped with carbon fiber. Fun fact: the fan blades themselves are mini wings, generating lift.
One thing you notice when you get near the engine is just how close the fan tips are to the engine casing. In fact, P&W built them to with such precision that they rub against the rubber inner casing just a bit, millimeters, which creates a small groove in the rubber. The tolerances must be incredibly small.
Supersonic Fan Tips and The Geared Turbofan Solution
In flight, the fan blades spin at around 3,000 RPM. Any higher and the fan tips start to run supersonically, making a huge amount of noise in the form of a piercing drone. In contrast, the low pressure shaft spins at 12,000 RPM and the high-pressure shaft at around 20,000 RPM. So, how do you slow down this rotation — going from a high RPM at the back of the engine to a lower RPM at the front?
Back to engine design.
Passing right through the middle of the core is a "shaft within the shaft". One shaft turns the low-pressure turbine, low-pressure compressor and the fan, which you can see on the diagram above. Another shaft turns the high-pressure turbine and the high-pressure compressor. Each component needs to rotate at different speeds for each stage.
To get the fan at the front slowed down, "[y]ou need more stages of lower pressure to run the fan at a slower speed than the high-pressure shaft," Speich said, referring to the conventional two-spool engine design. These additional stages add weight and negatively affect fuel efficiency.
And that's where the geared turbofan, or GTF, comes in. It's the most significant development in engine technology in 20 years.
First, over time P&W figured out how to make a lightweight gear box. The current gear box is around 250 pounds; the first attempts were closer to 600 pounds. The gear reduces the rotation speed three to one. If the low-pressure shaft is running at 10,000 RPM, the gearbox will act to reduce the fan itself to 3,000 rpm but — critically — without adding more lower-pressure stages. Pratt has been working on it since Speich joined the company, and actively for 20 years of testing.
"With the gear, you can turn the fan slower but let the rest of the components rotate at the speed that's most efficient for them," Speich explained. In turn, you need fewer stages of low pressure — and less component weight — to run the fan at that slower speed.
"The gear bought its way into the engine," Speich said. "All those learnings...and finally today the technology has caught up."
Efficiency Gains Over Time
Speich has been at P&W since the mid-1970s, and joined just after P&W launched the JT9D, which powered the first Boeing 747. "Those first engines had a bypass ratio of around 4.5 to 1," Speich said. They were also made with steel fan cases and forged steel components, which was quite heavy.
Compare that to the GTF engine, which boasts a bypass ratio of 12 to 1. The engine is reported to offer 15% gains in fuel efficiency. "That's huge in this space," Attia said emphatically.
Speich noted that his company is seeing better than 15% efficiency gains. "I remember when hitting a one to two percent increase in fuel efficiency was hitting a gold mine," he said, looking back on his career at the company. The GTF is currently flying on five platforms: the Airbus A320Neo series, the Airbus A220, Embraer's E-2 jets, the Russian-made Irkut MC-21 and the Mitsubishi MRJ. (The latter two aren't in commercial service yet.) You'll fly them in the US with Hawaiian, Delta and Spirit among others.
"When it comes to aerodynamics, materials, structures, physics...everything — all of these are pushed to their limits," Attia said. “I think they are the most fascinating machines ever made by man.”
And in case you were wondering, the little swirl in the middle of the nose of the engine is to let anyone know — visually — whether the fan is spinning or not.
Mike Arnot is the founder of Boarding Pass NYC, a New York-based travel brand, and a private pilot.