Jet engines are a marvel of engineering. From precisely controlling the internal atomic
structure of metals to create turbine blades that are just one single crystal to delicate
robotic machining that humans could only have dreamed of a decade ago. The jet engines of
today are barely recognisable to those of 1940.
They’re bigger, they’re more powerful, and they are more efficient. The dreamliner’s
engines are so big, that they are the same diameter as a 737 fuselage,
and this is just part of the puzzle that has allowed the 787 dreamliner to break world
records. In March 2020, in the midst of a global pandemic,
the US government imposed travel restrictions on all European travellers. A move that cut
off France from one of it’s far flung territories in French Polynesia. Business as usual for
Air Tahiti was no more. A creative solution had to be found to replace
their regular flights between Tahiti International Airport and Charles De Gaulle Airport which,
under normal circumstances, stopped over in LAX.
Faced with disruption, Air Tahiti began operating the world’s longest flight. A 16 and a half
hour, 15,700 kilometre, non-stop flight between Tahiti and Paris. The world's longest distance
scheduled flight, and it was technically a domestic flight. [1]
The 787 made that possible, and today we are going to learn how. In our last video we explored
the incredible engineering that has gone into sculpting the airframe of the 787, but all
of that work is useless without a power system to match.
Boeing transformed the way airliners are powered to create a plane like no other.
The first step in powering up a large plane is to get the jet engines started, and that
itself is a very power hungry process. We need to get the compressor section turning
in order to achieve adequate compression for engine ignition.
We have all seen the footage of people hand cranking old piston powered planes to get
them started. This obviously is not feasible for jet engines which need to rotate at very
high speeds to start. Engineers have dreamed up many ways to complete this job.
Some engines can use explosive cartridges that look like shotgun shells that will be
fired by an electric charge, the hot gases expelled by the cartridge then drive a smaller
turbine [2], which is connected to the drive shaft through a reduction gear. Allowing the
smaller turbine to get the engines up to speed. Cartridge starters that were popular in older
military planes that may need to get into the air on very short notice with limited
ground support. Some planes, like the SR-71, had a direct
drive starting cart, which connected two massive V8 engines directly to the J-58 engine from
underneath the nacelle to get the powerful jet engines up to 4500 rpm. [3]
However, air starting is by far the most common method, where pressurised air is fed into
the turbine section directly to get the engine moving. This can be done with an external
cart, called a huffer cart [4] , which connects hoses to the engine, but most commercial airliners
are capable of generating their own pressurised air, with an APU, or auxiliary power unit.
These are smaller turbine engines located at the tail of the aircraft, that are small
enough to get started with a battery and an electric motor.
Most people aren’t even aware this mini turbine engine exists, but you can see the
exhaust here on all modern commercial airliners. The 787s APU, like other planes, is started
by a small battery, but from here the 787s system architecture is very different. The
787’s APU does not provide pressurised air to the engines. [5] It provides electric current
to two electric motors attached to each engine, which act like the start motors on your car.
However, these motors can also act as generators to provide the 787 with unparalleled electric
power. A traditional plane has 1 generator on each engine and 1 on the APU, but the 787
is anything but traditional. There are in total 6 generators on-board,
with two pairs on each engine, providing the main power, each capable of generating 250
kilowatts, with 2 more on the APU providing secondary power each capable of providing
225 kilowatts. If all 6 of these generators were running at the same time that would be
1.45 megawatts of power available to the 787 [6], four times more than a 777 is capable
of producing. [7] To put that into perspective, this is a 787
on a football field. If we needed to generate that electricity with solar panels in the
middle of the day we would need to cover about 10 football fields with solar panels. [8]
This is a lot of power, so why did the 787 need that much power and where is it all going?
The 787 uses a no-bleed air architecture. Traditionally, many aircraft systems are powered
by hot compressed air. The APUs normally provide hot air to engines to get them started, and
then once the main engines are started, hot compressed air bled from the compressor section
drives several important systems. Normally air conditioning and cabin pressurisation
is handled by the bleed air system. [9] The bleed air would be drawn from the engine at
a temperature upwards of 230 degrees celsius. This obviously would quickly turn the cabin
into an oven, so the air would first run through a complicated air-conditioning and pressurisation
system, where some of the air is cooled using a heat exchanger which uses outside air from
an surface-mounted intake called ram intakes to remove heat from the engine bleed air until
it is a suitable temperature to be distributed through the cabin.
So, we are doing work to remove energy from this valuable bleed air from the engine, this
is obviously rather inefficient, and all that ducting is heavy.
Animation D Eli And this is why the 787 does not use this
system. It’s cabin pressurisation is handled entirely electrically. We now have two inlets,
the ram air inlet and the cabin air compressor inlet.
The cabin air compressor leads to, you guessed it, the cabin air compressor, an electrically
driven device that compresses the air for the passenger cabin. [10] This compression
process heats the air up too much for direct use in the cabin, so it does require some
cooling using a heat exchanger cooled by the ram air inlet.
You can see this little door opened while the plane is on the ground, and it serves
to protect the inlet from foreign objects entering it.
Another energy intensive process that has been handed over to electric motors is the
braking system. As you can imagine the energy required to
slow down a 200 tonne plane travelling at 270 kilometres per hour is not small.
Calculating the energy required is pretty trivial. We just need to calculate the kinetic
energy of the object, which we can do with this equation, and that gives us a kinetic
energy of 562 million joules. [11] That’s a lot of energy and the vast majority
of that energy needs to be dissipated by the brakes of the plane. Reverse thrusters can
deploy that redirect air from the bypass ducts through slots that open on the side of the
engine, but this only serves to drastically increase drag by creating a large area of
disturbed air around the engines; they don’t actually provide reverse thrust.
So, the brakes need to convert a lot of kinetic energy into another form of energy. Heat energy.
You can really see this in effect during a plane's most extreme brake test. An aborted
landing test. Here the plane is flying at full load, including the weight of the fuel,
and has to abort at it’s V1 speed, the absolute maximum speed a plane can abort a landing
at, beyond that point the plane has to take off. When the plane comes to a full stop the
brakes are glowing red hot. Typically the braking mechanism of planes
is driven by hydraulics.With a hydraulic piston forcing the brake pads against the wheel to
slow down the plane. The 787 removed this bulky hydraulic system and traded it in for
brakes actuated by electric motors. Each of the 8 landing gear wheels feature one of these
units [12], and together they helped eliminate between 62 to 111 kilograms of weight from
the 787, while also being drastically easier to maintain and install. [13]
The 787 does have a hydraulic system however, but it has eliminated two large bleed air
driven hydraulic pumps that are normally used to meet peak demands during take off and landing,
where landing gear actuation and deployment of high lift devices like slats and flaps
would overload the other pumps that power the hydraulic system during cruise. The 787
had the added electric power needed to rid itself of these bleed air powered pumps, and
replace them with more efficient electrically driven pumps. [14] To boot, the 787 is able
to generate higher hydraulic pressures that allow it to use smaller hydraulic components,
which saves both weight and space. So that covers most of the system architecture,
but we haven’t even touched on the revolutionary engines themselves.
The Dreamliner is most commonly fitted with two General Electric GEnX engines. Each capable
of producing between 310kN and 360kN of thrust. About the same thrust that General Electrics
previous generation engines, the CF6 equipped to the 767, were capable of producing, but
the GEnX produces this thrust while consuming 15% less fuel. [15] That’s an incredible
leap in technology. The GEnX achieves this primarily through two
major design features that were not possible with older engines. The first is a gigantic
bypass duct that surrounds the main engine. The primary components of a modern day jet
engine are the fan, a compressor, a combustor, a turbine and a nozzle.
The ultimate goal of a jet engine is to draw in air and pressurise it as much as possible,
which increases the energy potential of the air, before mixing it with fuel and igniting
it, causing a rapid expansion and acceleration of the air.
The work done by the compressor is then recovered in the turbine section, which powers both
the compressor and the fan. Finally, the air, with its remaining kinetic
energy, travels through the nozzle where it is accelerated more and sent out the back
of the engine to produce thrust. The engine core, this part of the engine,
is what powers the engine, yet most of the air that travels through the GEnX bypasses
this section completely, only passing through the fan and through this bypass duct, and
this is one of the reasons the engine is so incredibly efficient.
Bypass ratios have continually increased over the years, which is one of the reasons engines
keep getting bigger. The GEnX engine has a bypass ratio of 9 to 1, meaning for every
kilogram of air that passes through the engine core, 9 kilograms flow through the bypass
duct. [16] This is extremely high. The engine the GEnX replaced only had a bypass ratio
of 5.7, which, during its time, was itself a very high bypass ratio. [17]
While the core takes a small volume of air and accelerates it rapidly, the fan takes
a massive volume of air and accelerates it just a little, while not requiring it to be
mixed with fuel and ignited. This drastically increases the fuel efficiency of the plane.
The fan essentially acts like a gigantic propeller. The industry has been aiming for larger and
larger bypass ratios for decades, but there are a couple of limiting factors that make
it difficult to increase the size of the fan blades.
The compressor and fan are driven by the same drive shaft, meaning they rotate at the same
speed, but because the fan is so much larger than the compressor and turbine blades, the
fan experiences much higher centrifugal forces and if the fan blades grow too long their
tips could even break the speed of sound, leading to shockwaves and a massive increase
in drag. The first problem we need to address is how
to handle those centrifugal forces. The magnitude of the centrifugal force can be found using
this equation, where m is the mass, w is the angular velocity and r is the distance from
the centre of rotation. So our force will increase with the weight of the blade, the
speed of its rotation, and it’s diameter. We want to increase our diameter to increase
our bypass ratio, so we need to figure out ways to either decrease our weight, or decrease
our rotational speed. The GEnX achieved the first requirement by
using light weight carbon fibre fan blades. Older fan blades were constructed from titanium
[18]Titanium being used for it’s excellent strength to weight ratio, but these carbon
fibre blades have an even higher strength to weight ratio, while also being much stiffer
than titanium. Allowing the carbon fibre composite blades to be longer and thinner than their
titanium counterparts. This form factor allowed General Electric to reduce the blade count
from 22 to 18. In all, this resulted in a 15% weight savings in the blades, allowing
them to spin faster without worry of damage. [19]
However, composites do have one huge drawback, their impact resistance. One of the tests
engines need to pass to reach certification is the chicken gun test, which, as you may
have guessed, involves yeeting a dead bird into the engine and seeing how it holds up.
The fan blades are the first part of the engine any foreign object will encounter and thus
need to be capable of withstanding the impact. Early development of composite fan blades
by NASA and GE showed that composite blades by themselves simply were not up to the task.
To solve the issue the leading edge of the blades are reinforced with titanium. [20]
Bypass ratios are likely to continue growing as our technologies develop. Speed reduction
gears between the main drive shaft and the fan may allow engine manufacturers to increase
bypass ratios even more. Pratt and Whitney employed this technology
in their PW1000G engine to achieved the largest bypass ratio ever seen in a commercial airline
turbofan engine. They included a planetary gear between the
main drive shaft and the turbofan drive shaft, which applied a three to one reduction in
speed, allowing the turbofan to spin at a lower 4-5 thousand RPM. This enabled the fan
to grow in diameter as the forces the larger blades had to endure were drastically reduced
with the lower speed. Giving the engine a record breaking 12.5 to 1 bypass ratio. [21]
General Electric considered the design for their new LEAP engine, currently in use by
the 737 Max, but problems with weight and maintenance of the planetary gearbox deterred
them, and that hesitance may have been justified, as the engines have had a turbulent operation
history with several groundings and engine removals, although these issues were not directly
a result of the planetary gears. However, as technology improves, we may see
more of these geared designs being used to drive even greater fuel savings.
The next design feature that helps the GEnX to reduce fuel consumption is the extremely
high compression ratio achieved through the engine's compressor section.
Compression ratio, or pressure ratio as it is more commonly referred to in aeronautical
engineering, is the ratio found by dividing the pressure at the exit of the compressor
section by the pressure at the inlet.[22] This one variable has huge implications for
a plane’s fuel efficiency, and just as we can see a clear upwards trend for bypass ratios
over time, the same can be seen for pressure ratios.
The reason for this is fairly simple: by increasing the pressure ratio we can maximise the energy
we can extract from our fuel, because there is more useful energy available for extraction
in our turbine and nozzle, than if we compressed the air less.
But it’s not quite as easy as just increasing pressure across our compressor section. In
order to achieve higher compression, we may need to increase the number of compressor
sections, which will increase the weight of the engine, which will increase fuel consumption
and reduce the advantage. Higher compression will also lead to higher
temperature, which may cause material failure in the compressor blades, and perhaps most
critically, increasing the temperature results in increased emissions of potent greenhouse
gases like nitrogen oxides. [23] Nitrogen oxide formation rates rise exponentially
with flame temperature because the additional heat provides the energy needed to break nitrogen
molecules in the air apart and allow them to combine with oxygen.[24]
To facilitate increased pressure ratios, while hitting climate change goals, this problem
first had to be addressed, and they did it through some ingenious fuel injection technology.
The compressor feeds its high pressure air into 22 fuel mixing nozzles located in a ring
around the compressor exit. Their job is fairly simple: mix the fuel into the air and ignite
it. A simple job that started development in 1995
with both General Electric and NASA. [25] The art of crafting the perfect fuel injection
nozzle is incredibly complex, a delicate, yet violent process like an orchestra whose
crescendo is a fiery explosion. With the advent of 3D printed manufacturing,
GE has been able to create a fuel injector with a complex internal labyrinth of air channels
that traditional tooling simply could not cut. The fuel injector they developed is called
the twin annular pre-swirl injector, or TAPS. So, how does it avoid those dangerous Nitrous
Oxide emissions? By carefully controlling the air fuel mixture ratios to control flame
temperatures.
If we plot flame temperatures against air fuel ratio we get something that looks like
this, with max flame temperatures occurring at stoichiometric air fuel mixtures. That
is, the mixture where air and fuel are both entirely consumed in the reaction. This is
the zone where Nitrous Oxide emissions are highest because the flame temperature is highest.
To minimize nitrous oxide emissions we can aim to be over on this side of the curve with
a rich air fuel mixture, but that would result in a lot of wasted unburnt fuel. So, instead,
we aim to be over here, on the lean side of the graph.
If we map the combustion process for GE’s previous generation injector fitted to over
22,000 CFM56 engines, we would see the air fuel mixture is initially injected in this
zone, with a rich air fuel mixture, then it goes through a dilution process to reach a
lean mixture where the fuel is fully consumed. The issue with this is that the fuel goes
through the stoichiometric region in between these two phases, creating a great deal of
nitrous oxides. [25] The TAPS fuel injector looks like this with
a central main pilot surrounded by a ring shaped main injector. Creating a two swirling
airflow that looks something like this, which uses a large amount of air in the premixing
phase to ensure thorough mixing and a lean air fuel mixture. If we map the combustion
process of TAPS it starts in the lean zone, and gets even leaner further into the combustor,
never getting close to the peak Nitrous Oxide production region. [25]
This may seem like a simple device, but it took generations of design iterations with
computer simulations gradually bringing the engineers closer to their target, and none
of their designs would have been possible without current generation metallic 3D printing
that could manufacture the complex parts with materials capable of resisting the enormous
heat within the combustor. [26] The 26 years of development paid off with 60% nitrous oxide
emission reductions and a fuel injector compatible with the record breaking 58 to 1 pressure
ratio used in the GEnX engine.[27] GE managed to increase the pressure ratio,
even though they reduced the number of compressor stages. Each of these turbine disks is 1 stage.
The GEnX contains 10 stages, 4 fewer than the CF6 engine it is replacing.
GE managed to do this through new improved blade design, which is a lot less info than
I wanted to share, but finding information on this topic was incredibly difficult. A
testament to how advanced and secretive the technology is.
The compressor does include 3 blisk stages, which are perhaps the most impressive feats
of machining I have ever seen. [28] Traditionally compressor stages are made by
attaching the blades to a separate disk with dovetail connections. This decreases manufacturing
complexity, but the dovetail connection is less durable, increases assembly and maintenance
costs and allows some air to escape through the connections.
Blisks are compressor stages that are manufactured from a single piece of metal. Making them
stronger and easier to install, while improving aerodynamic efficiency. Improving aerodynamic
efficiency was the name of the game to improve compression ratios, and ofcourse that needed
to apply to the turbine section that powers the compressor.
It too benefitted from leading edge aerodynamic modelling. The turbine also made use of new
material science breakthroughs, being the first commercial engine to make use of gamma
titanium aluminide. An advanced alloy of titanium and aluminium that combines the heat resistance
of titanium with the aluminiums weight savings, replacing the nickel alloys of the past. [29]
It took three decades to take this material from the lab to commercial use, and the dreamliner
was the first plane to make use of it, paving the way for the materials acceptance in the
industry. [30] All of these factors combined to allow the
GEnX engines of the dreamliner to reduce fuel consumption by 15% over previous generation
engines. That is an astronomical leap in fuel efficiency, and on top of all that the engine
is 60% quieter than similar sized planes. [31]
In part thanks to the GEnX’s most distinctive feature, this sawtooth pattern on the fan
casing and the engine exhaust. These features are called engine chevrons and they aren’t
just futuristic cosmetic design, but functional parts of the engine that significantly reduce
engine noise. The question is: what difference can a mere
shape change at a nozzle exit have on the overall noise pollution produced by the aircraft?
Because turbofan engines force air out their bypass chambers and turbines at speeds that
are much higher than the speed of the ambient air, the air interacts with the free-stream
air causing mixing. This mixing causes uncontrolled vortices which in turn generate excessive
noise for both the passengers and the community on the ground. [visual 11] Think of it like
water accelerating down a waterfall and striking the water below. That turbulence can be extremely
loud. The chevrons aim to control the vortices that
are shed from the engine nozzle by creating smaller vortices at each tooth of the chevron
to allow the hot, fast air being expelled from the engine to mix with the ambient air
more gently. This is said to reduce jet blast noise by as much as 30% and allowed Boeing
to use substantially less sound insulation in the fuselage walls of the aircraft, which
helped to reduce aircraft weight [32] The 787 Dreamliner truly does live up to its
name. It provides a passenger experience that is almost incomparable to that of other commercial
aircraft with advances in noise reduction, fuel efficiency and passenger comfort. This
series is usually saved for aircraft and technologies most of us will never set foot near, but the
787 could be the most advanced piece of engineering we have covered yet, and it’s a plane many
of us will have the opportunity to fly in and marvel at the decades of engineering advancement
that made it possible. This nearly half hour long video is just half
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this video detailing the complicated thermodynamics explanation of why compression ratios actually
increase efficiency, because it was just too academic for YouTube, but die hard fans of
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diagrams. The directors cut also includes explanations of the 787s nifty electronic
heads up display in the cockpit and electrochromic window shades.
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