Powering the Dreamliner: How the 787’s GEnx Engines Work
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A jet airliner is nothing without its engines. Giving them the power to climb up to 43,000 feet and the range to fly half way around the world, the engines are what give the aircraft life. Larger and more powerful, yet quieter than ever before, the modern jet engine really is a marvel of human engineering achievement.
When an aircraft is designed, it’s normally done with a couple of engine options from different manufacturers. This gives the airline customers the choice, depending on their commercial needs. The Boeing 787 Dreamliner, which I fly, comes with the option of either the General Electric GEnx or the Rolls-Royce Trent 1000.
With 53% of all 787 orders, the GEnx is the most popular choice with the Trent 1000 accounting for 33%. The remaining 14% of orders remain undecided. The GEnx now has more than 1,600 units on order, making it the fastest-selling high-thrust engine in GE’s history. The GEnx provides 15% more fuel efficiency and 15% less CO2 the the CF6 engine, which powered the 747-400.
It’s also an airline favourite for its maintenance performance, using 30% fewer parts and going 20% longer before needing major engineering work. These improvements on older engines are due to increased use of lightweight composite materials and specialised protective coatings. I’ll focus on the GEnx for this explainer.
The GEnx is a massive bit of kit. The front rotating fan measures 2.82 metres across. The whole engine weighs 6 tonnes and can produce 76,100 pounds of thrust.
“A pound of thrust is equal to a force able to accelerate 1 pound of material 32 feet per second per second”.
Let’s break that down: 32 feet per second is the same as the force of gravity — 9.8m/s². Therefore, if an engine could produce 2.2 pounds of thrust, it could accelerate a 1 kilogramme block at 9.8m/s². Taking the 76,000 pounds of thrust, which the GEnx can generate, means it can accelerate a 34.5-tonne block at the same rate. Pretty powerful indeed.
And how much does all this cost? Just a casual $28 million.
This all sounds really impressive, but how exactly is this massive force created? It comes down to four distinct parts: suck, squeeze, bang and blow.
The first stage of the process requires a massive amount of air to be drawn into the engine. During the takeoff roll, the 2.82-metre front section fan can be spinning at nearly 2,400rpm, drawing in over 1.2 tonnes of air every second. However, of this 1.2 tonnes entering the engine every second, only 120 kilogrammes actually enter the engine core.
The GEnx is a high-bypass engine, meaning that most of the air drawn in isn’t actually used in combustion to generate thrust. Instead, the engine design creates a duct whereby 90% of the air flows around the outside of the engine core. This is called the ‘bypass ratio’ and at 9:1, the GEnx has one of the highest ratios in the industry.
As the air is drawn into the engine intake, the bypass air is accelerated by the front stage fan. By accelerating such a large volume of air, some thrust is created. The remaining 120 kilogrammes of air per second is channeled towards the core of the engine where the next stage of the process takes place.
Once the air has passed through the front stage fan, things start to get serious. From here, the air enters the first stage of the engine core, the compressor. Here, a series of 14 spinning fans squeeze and compress the air, massively increasing its energy potential. As the pressure increases, so does the temperature.
The air first comes into contact with the Low Pressure Compressor (LPC), or ‘Booster’. At this stage, four fans take the ambient air and start to compress it. Entering this stage at around 11°C (on takeoff), it leaves at around 80°C as it passes into the second stage of compression, the High Pressure Compressor (HPC).
In the HPC, 10 more fans take this air and compress it further still. Entering at 80°C, it leaves at over 600°C and 40x the pressure it was when first entering the engine. Roaring hot and bursting with energy, it’s now ready for the spectacular part.
The high pressure, high temperature air is then forced into the combustion chamber. Here, fuel is sprayed into the fast-moving air by a ring of fuel nozzles. The fuel flow is controlled by an Electronic Engine Control (EEC).
Each engine has its own EEC, which has full control over the engine, using thrust lever inputs from the pilots to control the output thrust. Amongst many other tasks, the EECs regulate the flow of fuel in the combustion chamber, depending on the thrust required. If the pilots are demanding more thrust, the EECs instruct the fuel nozzles to spray more fuel.
This high energy air is ignited, creating hot, expanding gases reaching temperatures of up to 2,700°C. As a result, this section of the engine is made of ceramic materials. The hot gases are then sent into the rear section of the engine.
The high energy air exiting the combustion chamber flows into the turbine stage of the engine. Here, the energy is used to create thrust and also to keep the various stages of the engine in front of it running.
Entering the High Pressure Turbine (HPT) stage, the hot air turns two rotating fans before going into the Low Pressure Turbine (LPT) stage where it turns a further seven fans. These turbine fans are connected to the front stage fan and compressors by two rotors, which drive the forward stages. These stages also power the engine gearbox, which provides power to other aircraft systems.
Bit confused? Let’s explain it. The front stage of the engine, the front fan and compressors prepare the air for ignition, yet to be able to turn, they need power. The rear part, the turbine stage, is where the power comes from. The engine uses this power to drive the the front stages by use of a rotor. The rear stages physically drive the front stage.
As this hot air exits out the back of the turbine stage, it reaches the final stage of the engine, the nozzle. This is the part that actually produces the thrust for the aircraft to move forwards. When the energy-depleted air passes over the nozzle, in addition to the bypass air from the start, an exhaust force is created. This is the thrust.
The final stage of this process is where innovative technology has reduced the noice of the 787 engines compared to other types. The edges of the engine casing seem to have had pieces cut out of them, creating a circle of rounded teeth. This is called scalloping.
As the cooler bypass air passes over these points, it is directed towards the hot air stream, where it mixes slightly. This mixing reduces the noise generated as the hot air comes into contact with the atmosphere.
“But how do you turn the front stage to get power before you have power to drive the front stage? It’s like the chicken and the egg!” If you’re asking the question, you’re understanding how the engine works.
On engine start, it uses electrical power from the small engine in the tail (the APU) or ground power to turn the turbines. Once there is rotation in the turbines, the forward stages start to move. As soon as the forward stages are moving, the process explained above can start to happen. Finally, when the compressor stages are providing air at the right conditions, igniters in the combustion chamber start sparking. Then, when the conditions are just right, the fuel is injected into the air mixture where it ignites.
The hot gases from the ignition of the fuel pass into the compressor stage, in turn speeding up the front stage, in turn creating even more compressed air, which then ignites to create even more high-energy air.
Once the process has reached a critical point, the engine is providing enough power to sustain itself and the use of the extra electrical power is no longer needed.
Have a look at the video below. Whilst not a GEnx on a 787, the start up principle is the same. You can see the front fan stage start to turn using power from the turbines. As the rotation reaches a critical speed, you can hear the moment when the fuel ignites and the rotation speed increases further.
Once the engine speed has reached its idle running speed, the sound becomes constant and the engine is now running by itself.
The massive amount of thrust generated by the engine is also beneficial on landing. After touch down, with the engine power at idle, we pull up a second set of levers to engage the reverse thrust. This causes a vent to open on the side of the engine and barriers to extend in the area where the bypass air flows.
These barriers block the bypass air from passing out the back of the engine and direct it forwards through the vents. Whilst the reverse system doesn’t directly increase the braking effectiveness, it does help the braking system by reducing the energy that the brakes have to endure.
The 787 is the first of a new generation of aircraft to have more systems powered by electricity than previous types. The brakes, the flaps and the engine start system are all electrically driven. This means that a massive amount of weight can be saved by reducing the components needed to power these systems.
Each engine has two Variable Frequency Starter Generators (VFSG), powered from the engine gearbox. These act as starter motors during engine start and then generators once the engines are running. These four generators then power the entire electrical system on the aircraft.
Should one of the engines be shut down, the APU in the tail also has two VFSGs that can take over from the two that were lost. Even if the APU was to fail, also, the aircraft can still fly safely on the two remaining VFSGs.
Fuel and Oil
During a long flight, fuel can get cold sitting in the wings flying through air that can be minus 60°C. At the same time, the oil circulating the engines gets quite hot. In order to warm the fuel to the temperature required when entering the combustion chamber, the engine takes advantage of the hot oil temperature.
Passing through a heat exchange system, the two fluids pass by each other, exchanging their heat. Essentially, the hot oil warms the cool fuel, cooling the oil.
From here, the fuel passes through a filter to ensure that any impurities are removed before it reaches the combustion chamber. It then goes through a second pressurisation stage before entering the Fuel Metering Unit (FMU).
Here, the EEC controls how much fuel is directed into the engine, depending on the need demanded by the pilots.
The engine on any airliner is a complex piece of kit. It has to run reliably for hours on end, getting a rest for only a couple of hours before going again. The new GEnx on the 787 take this reliability to new levels. Using 30% fewer components, it’s 15% more efficient than previous types. New technology also means that it’s quieter, providing a more comfortable flight for passengers and less disturbance for those living around the airports into which it flies.
Featured photo by Ryan Patterson.
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