Whenever you last sat on the window seat in an aircraft, especially if you had a window seat, you probably paid one good look to the engine, the little cylinder-frustum thingy on the wing.  Look up photos of the Concorde, and you won’t see such an engine at all. Whatever the case may be, some driving force seems to be pushing all aircraft to the massive speeds they need for takeoff and cruising. The machines which provide this force are amazing, and if you’ve ever wondered how they work or just have a fascination for cutting edge technology, you are in for a treat!

Jet engines

Try this – sit on a wheeled chair, making sure that your feet aren’t touching the ground, face a wall and push hard against it. What happens? Pretty obvious right? You roll away from the wall, even though you never actually applied any force directly on the chair. What about if you were to punch a door really, really hard (please don’t actually do this)? This time, your hand would get really bruised, again, even though you never directly applied a force on it.

As you would’ve probably guessed by now, both these cases illustrate Newton’s third law – that every action has an equal and opposite reaction. Now, what if I told you that you punching a door and a gigantic high performance jet engine that can deliver enough thrust to accelerate a massive metal tube with wings to nearly the speed of sound are governed by the same principle?

A GE90-115B engine, one of the most powerful in the world, with human for scale

Well let’s think about it. You need some way to provide a force to an aircraft that is in flight, contacting nothing but the air around it. You basically only have two options available to you. Either you can find some way to push against the air, which by Newton’s third law would push you in the opposite direction, or you could throw something backwards; same deal.

Jet engines (pure jet engines a.k.a. turbojets to be precise) work on the latter of these principles. Fuel is ignited in a combustion chamber, heating up and compressing the air and forcing it down a path which accelerates it out the back of the engine. This is actually quite similar to how a rocket engine works, with the main difference being that jet engines use the atmosphere as an oxidizer as well as some of the reaction mass, thus eliminating the need for a separate oxidizer and greatly increasing efficiency.

Modern jet engines use some of the generated energy to spin a large fan which in addition to supplying the combustion chamber with air also have the large majority of air bypass the combustion chamber, generating thrust by pushing back the air alone. This type of engine is called a turbofan, and is used almost exclusively on airliners due to its very high efficiency.

Diagram of a turbofan engine

Pushing the air backwards to push the plane forward is what the prime function of a jet engine is. But there’s much more to it than air just entering the engine from the front and being pushed from the rear end of the engine with a higher speed for thrusting the plane forward. When a stream of air enters the intake of the engine it has to go through the various components of the engine which are depicted in the above diagram. Inside the engine, all hell breaks loose.

The primary flow

A compressor (top) and combustion chamber (bottom) of a turbofan engine

The fan at the front of the engine sucks in massive amounts of air. Some of this air enters the core of the engine, where it is passed through a set of compressors which – you guessed it – compress the air to extremely high pressures and heat it up to a temperature more optimal for combustion as it reaches the combustion chamber. Inside, a constant stream of atomized fuel is being pumped into the chamber, which upon contact with the hot air ignites, combining exothermically with the oxygen inside the stream. This creates an incredibly high pressure mixture that escapes towards the back of the engine.

The mixture exits the combustion chamber at high speed, and is passed through a set of turbines. Here, it exchanges energy with the turbine, losing some momentum which causes the turbines to spin. Finally, the exhaust stream, which at this point is still much at a much higher than ambient pressure, enters the nozzle. The nozzle is designed to bring down the pressure of the exhaust down to that of the free stream outside the engine for maximum efficiency.

Now, where does the energy to spin the fan and compressors come from? As you may have noticed in the diagram, there are actually two sets of turbines and compressors, each linked to each other by a shaft. Each of these sets spins at a different speed, with the outer low pressure assembly (designated N1) spinning at a lower rate than the inner high pressure assembly (designated N2). As the exhaust stream passes through the turbines, it causes them to rotate, and by extension also causes their corresponding upstream compressors (as well as the fan in the case of the N1 assembly) to rotate. This pulls more air into the combustion chamber and the process continues.

This entire component of the airflow is known as the primary flow. It drives the engine, ensuring that it keeps running, and is responsible for basically all of the thrust generated by a turbojet. In a turbofan, however, things are different. The primary flow is only part of the story, with the vast majority of thrust being generated by what is known as the secondary flow.

The secondary flow

In a turbofan, a large component of the incoming airflow enters the larger part of the engine, known as the bypass duct. Here, the cool air flows through unobstructed through engine without going through the combustion chamber, being driven merely by the angled blades of the fan at the front of the engine. As only a small amount of additional fuel is required to drive the fan, this bypass duct is made as large as possible to maximize the amount of air flowing through it. This means that in turbofans, the secondary flow actually constitutes the major part of the thrust generated by the engine, which makes them much more efficient than turbojets.

A cross sectional view of a turbofan engine, showing off how little space the engine core occupies relative to the bypass duct

The precision engineering behind jet engines

Jet engines are the single most expensive component on any airliner. With even smaller engines costing in the tens of millions of dollars, these engines are made only by a few companies and sold at a premium. This isn’t for a lack of competition or price-fixing or anything like that – it’s because put simply, jet engines are incredibly complex, high tech machines. The conditions inside a jet engine are extreme to the point where simply keeping it from exploding is a challenge.

Every second, the fan sucks in up to one tonne of air, which can normally fill up a squash court, which is compressed as much as 40 times. Just the compression is enough to heat it to about 650 °C (steel loses its structural integrity at 500 °C, so there already component design issues). After this, the air moves on to the combustion chamber where it is mixed with fuel vapours, which takes things to the very extreme. Temperatures exceed 1600 °C (and remember, even the best alloys used to this date melt at 1455 °C). At the same time the turbine blades are struggling just to not melt, they are also undergoing extreme centrifugal forces (outward forces) from angular velocities amounting to as much as 15000 RPM. This might as well justify the statement that pushing air through is only a turbine blade’s secondary task; the first is merely to survive.

That being said, let’s move on to how these components are designed to be able to withstand these conditions. One of the biggest manufacturers of jet engines for aircraft is Rolls-Royce. The designing of the components in their factories takes place to an amazing level of precision, with computer guided machines shaping components to mechanical tolerances of within 10-12 cm of the intended dimensions. The most crucial components are, of course, the turbine blades, which brings us to the matter of how they are made to last inside the engine. We will look at two requirements – strength and tolerance of temperature.

For strength, first of all, a lot of research is done into materials to choose appropriate alloys. Then, is a process which is the company’s most closely guarded secret, the entire blade is grown as a single crystal of the metal using complex apparatus, giving it an exceptionally uniform and strong structure. The system to keep is from melting is even more complex. Tiny holes are drilled into the blade, using electrical discharges, and connected to a central tube. When the engine is in operation, relatively cool air is routed from the compression chamber into the blades, forming a very thin layer of the air around the blade, which keeps it cool enough.

The tiniest errors in manufacturing can have disastrous results. A Rolls-Royce Trent engine used in the Qantas flight 32 Airbus A380, had a hole in a small valve drilled half a millimeter off where it should have been, which, in time, caused a fracture, causing hot oil to gush over the turbine, resulting in a fire which destroyed the engine mid-flight, created shrapnel which rendered other systems useless, and nearly caused an explosion.

Auxiliary functions of jet engines

Though providing thrust is the primary role of jet engines, they actually serve several purposes besides just helping the plane move forward. Let’s take a look at just a couple of them.

  • Power – Nearly all the electrical power used by the aircraft when it is in flight is provided by the engines. Each engine contains something called an integrated drive generator or IDG. As you may know if you have studied electrical systems, a generator needs to spin at a constant RPM in order to generate a stable output voltage at the correct AC frequency. However, the high pressure rotor assembly (to which the generator is linked) could be spinning at different speeds depending on the throttle setting. To overcome this challenge, the IDG contains a constant speed drive or CSD that takes the input rotation from the engine core and converts it into a steady output rotation rate that remains constant despite varying input RPMs. Then, a generator is used to convert this rotational energy into electrical energy, which can be used by the aircraft’s various systems.
The IDG of a GE90 turbofan engine
  • Bleed air – Many systems on board the aircraft (and more importantly the cabin of the aircraft itself) require pressurized air in order to function. Bleed air systems take highly compressed air from the compressor stage of the engine upstream of the combustion chamber where it is ignited. Although this air is relatively cool, it is still at hundreds of degrees Celsius from the compression alone, and so it is passed through a heat exchanger that sheds some of the heat to the outside air and brings down its temperature to a point where it can be used for pressurization, air circulation in the cabin, driving actuators and hydraulic systems and many other functions.

Propeller engines

The first plane propelled by a jet engine flew on August 27, 1939. But the first powered aircraft successfully flew on December 17, 1903. And of course, planes did fly in the 36 years in between, but how? This section is not just a historical note, but something of importance today too, because many planes, drones and all helicopters still fly using that same system.

Now that you have completed last week’s part of this course, you should have a good understanding of how a wing works. To be brief, if the wing is at an appropriate angle, and wind goes against it, the wing moves up. Now, for this, you need to continuously keep moving forward for proper headwind. Now, imagine, instead, a wing mounted along the radius of a rotor. Turn the motor on. As the wing moves around, it is, essentially still getting a headwind without any linear motion. In essence, the motor and the wing will move straight up (or ahead, depending on the orientation). Of course, you need at least two such wings, or blades, for stability. And there you have it. Such propellers were used to move early aircraft forward. The propellers used to be powered by piston engines that resemble the engine of a car, but nowadays a different technology is used on modern aircraft running propeller-driven engines.

Turboprop engines

Turboprop engines resemble jet engines quite a bit, except for the fact that nearly all of the energy from the exhaust gasses going through the turbine is used to turn the propeller at the front, which moves the aircraft forward through the mechanism described above. Here, most of the air being pushed by the propeller goes around the engine instead of through it.

Diagram of a turboprop engine

Turboprops are more efficient at lower speeds because unlike the fan in a turbofan engine (which has to spin at the speed of the N1 assembly), the propeller doesn’t have to spin as fast as the turbine, and also the larger size of the propeller means that it can push more air at the same RPM. Moreover as the air doesn’t have to go through the engine cowl like on a turbofan, there is no limit to how big the prop can be made in the first place.

However, this type of engine sees limited use both due its lower thrust and much lower maximum attainable speed. Although some large aircraft do use it for some of the other utilities they provide (e.g. C-130 Hercules), they are now mostly used on smaller regional aircraft which fly routes so short that jet-powered aircraft won’t be able to take advantage of their higher speed and efficiency at altitudes due to climb and descent rate restrictions.

Assignments on the gas turbine engine

  1. How are turbofan engines on a commercial airliner started?
  2. What is the relation between turbofan performance and altitude?
  3. The Concorde used turbojets while much more efficient turbofans were available. Why?
  4. When a pilot changes the throttle setting on a turbofan engine, there is a several second delay before the engine actually reaches that thrust setting. Explain why this happens and suggest ways that the delay could be reduced without the use of better materials.

Further reading