Ok-disaster2022 t1_irnxgcc wrote
Yeah but it needs the entire system to achieve that. Thermodynamics are weird.
Aviator506 t1_irozpeh wrote
Yup, but the concept is still very simple at its core. Follows the same 4 stages of a traditional internal combustion engine like what's in most cars. Suck, squeeze, bang, blow.
The fan and the outer stages of the compressor is driven by the turbine and sucks the air in
The inner stages of the compressor squeezes the air.
Fuel is mixed in and goes bang in the combustion chamber
The burnt exhaust gas blows out the back and spins the turbine
Turbine spins the fan and compressor and the cycle continues.
brainblasttt t1_irpho3i wrote
It’s funny cause even in the military when they teach you about jet engines they still teach you “suck, squeeze, bang, blow” instead of the proper “intake, compression, combustion, exhaust”
WavingWookiee t1_irqnolx wrote
I was very disappointed when I was doing an engineering seminar with some school kids and I put a big picture up of "Suck, Squeeze, Bang, Blow" related to an engine and I didn't even get even a stiffled snigger, I worry about the next generation! When I was in school, had someone come into class and put that up, I'd have been howling and quite possibly asked to leave the classroom!
julie78787 t1_irqud4l wrote
That's been going on for a very long time.
zutonofgoth t1_irpbsnh wrote
It's important to note there is more accelerate than squeeze. It's almost an order of magnitude less than a car engine. The back end of the jet is open with only some blades blocking the exit of the heated gas.
Some people get the idea the pressure is massive but it's more like bike tyre pressure, so jet engine shrouds don't need to be very strong.
straighttoplaid t1_irpgao3 wrote
Modern engines can run pretty high compression ratios. The GE9X is advertised as having a 61:1 overall pressure ratio. I don't think you're going to see ~900 psi in a bike tire.
zutonofgoth t1_irpquv6 wrote
The compression ratio is the difference between the static air pressure at the front of the engine and the dynamic pressure at the rear of the engine. The dynamic pressure includes velocity in the calculation. It's not really comparing apples with apples. So you can not just multiply the values out.
900psi is more like rocket engines?
julie78787 t1_irqujss wrote
A lot of rocket engines have maximum pressures in the combustion chamber of 200-300 bar, so 3,000 to 4,500 PSI.
Aviator506 t1_irpfbmr wrote
Yeah, most of the squeeze is done by the forward momentum of the engine through the air. And at a certain speed (~mach 3) you don't even need compressors, it will act as a ram jet, which is what the SR-71 would do as well as the fictional DarkStar in TopGun Maverick. It's certainly MUCH more complicated than how I explained, I just did the basic principles of it.
straighttoplaid t1_irpggj7 wrote
> Yeah, most of the squeeze is done by the forward momentum of the engine through the air.
That is not true for commercial engines.
Aviator506 t1_irpx2ly wrote
High bypass turbo fans like airliners do get a significant bit of the compression from the forward momentum. They will never be able to function like ram jets, but the momentum makes a big difference especially at high altitude where the air is less dense and there isn't as much to suck in from the fans/compressors alone. The high bypass engines of the airliners will function much more efficiently at high altitude because of this than that of a low bypass military fighter, however it won't be able to achieve as high of a top speed as that same fighter under the same conditions. Basically the low bypass gets the advantage of speed while the high bypass gets the advantage of fuel efficiency, but both greatly benefit from momentum based compression.
KGandtheVividGirls t1_iruxjqu wrote
Think what you’re saying here is engine pressure ratio, or EPR. Ratio across the entire engine, not at a particular location, or station. The speed of the air doesn’t change that much after hitting the inlet and first stage of the compressor. A series of blades do accelerate the air then the air passes through a set of stators which are divergent ducts which slow the air converting that energy into pressure. This happens across multiple stages. At the end of the compressor is a diffuser which acts like an extra special stator. Air is at the highest pressure here. On a large modern engine it could be 800psig and over 1000F. This is simplified, there are things that need done with a compressor to get it working across the operating range.
rafaeldiasms t1_irpr3cz wrote
Suck, squeeze, bang, blow. So, just a regular Saturday then
Wasabi_Guacamole t1_irqekiy wrote
You get it regularly? Lucky lad.
julie78787 t1_irquege wrote
I drive an electric car. Explains why I'm not having any fun :(
Awellplanned t1_irqwo3c wrote
You are just soaking. That’s the Mormon version.
mcampo84 t1_irrd46f wrote
Guy’s gotta work, amirite?
ViolaPurpurea t1_irqjetz wrote
> the concept is still very simple at its core.
I see what you did there!
ChuckFerrera t1_irr84da wrote
Mostly correct. The compressor is directly attached to the high pressure turbine (HPT) via the high pressure shaft. The low pressure turbine (the 4 stages aft of the HPT), are directly connected forward to the high bypass fan through the center of the engine via the low pressure shaft. That is, the compressor and the HPT spins independent of the LPT and fan.
Clay_Statue t1_irp3ab5 wrote
I'm impressed that they could figure this out 80 years ago with slide rules and data tables.
RJFerret t1_irp3q8h wrote
Easier as slide rules don't auto-update in the midst of needing them.
kled7 t1_irp75wu wrote
It’s not so much the math. It’s testing. The math is pretty straightforward, but it was the prototyping and testing that got the engines we have today.
HEBushido t1_irp93tc wrote
You need the components to actually fit the math
Stealth_NotABomber t1_irpbq2s wrote
And the material sciences/metallurgy to make parts that can withstand the heat and pressure.
gnusmas5441 t1_irpczq6 wrote
If I understand correctly, another challenge is finding and shaping materials that can withstand the forces generated by the fan blades spinning very fast and having to withstand, among other things, the forces generated by their tips moving much faster than their base at the center if the engine.
evanc3 t1_irpejoo wrote
Plenty of materials that can do this, the trick is to get one that doesn't creep over time because the blade tips are remarkably close to the engine wall (tip clearance)
slater_just_slater t1_irq2fgg wrote
Creep is why most 1st stage turbine blades are single crystal. Fun fact, single crystal blades are "grown" not cast.
They are also the most boring thing ever to look at under a metallograph.
evanc3 t1_irq2u47 wrote
Still blows my mind!
But yeah I can imagine lol
slater_just_slater t1_irq4gst wrote
What is interesting is that it's the same principle to how single crystal silicone is grown for microchip substrates. However the blades are hollow.
Since they have no grain structure, nothing responds to an acid etc, on a metallograph it's just a white blob.
They do have a thermal barrier coating (TBC)
One of my jobs as a process engineer at Rolls Royce was the TBC coating. If you are bored here is a link about it.
Sands43 t1_irpaqbm wrote
Also metallurgy and lubricant technology
NativeMasshole t1_irp6gcs wrote
Math took a hell of a lot more manpower than it does now. It's crazy to think how much labor supercomputers save us compared to calculating by pen and paper.
[deleted] t1_irpcdn1 wrote
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Archmagnance1 t1_irpkcq4 wrote
That's saving labor. You need less man hours to do the same amount of calculations with the same amount of accuracy. If you do more with the same amount of labor, you're still saving labor compared to if you didn't have supercomputers.
Imagine how many man hours it would take to do a complex weather simulation to track the probability path of a hurricane? How many teams would you need working on the problem to give updates every 2 or 3 hours? Using information that is old and probably no longer valuable when they started?
[deleted] t1_irpp8uo wrote
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Archmagnance1 t1_irpu9s2 wrote
That's still saving labor though. It wouldn't be done because it takes too much labor to do then verify.
[deleted] t1_irpuew6 wrote
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Kumqwatwhat t1_irpnu64 wrote
> Thermodynamics are weird.
You just summed up like my whole third year of university.
SatanLifeProTips t1_irpjcrb wrote
Turboprops work the same way. It is more efficient to take a shaft drive output out a jet engine than it is to shoot all the air through the jet engine. Running a prop or a ducted fan is the most efficient way to do it
The prop IS slightly more efficient than the ducted fan at lower speeds/altitude, that’s why you see them on lower speed aircraft that do regional flights like the (miserable to be inside) Dash 8.
But get up to cruising altitude and the ducted fan is king.
Apotropaic_Sphinx t1_irppu1t wrote
Also you can reverse thrust on a turboprop. The C-130 can literary do a 3-point turn while taxiing.
High-bypass jets do have thrust reversers, but they're more for slowing down while landing.
slater_just_slater t1_irq2mxh wrote
Another big issue is blade tips going supersonic. It makes it really really loud. The Tu-95 is ungodly loud because of this.
InitechSecurity t1_irpfhr5 wrote
How Jet Engines Work https://youtu.be/L24Wf0VlTE0
Bananenweizen t1_irqhsll wrote
In principle, it is not different than your car engine. Burn fuel to get energy available, then use this energy to spin parts of your vehicle designed to push the vehicle forward when spinning. Only instead of ICE and wheels pushing against the road (with a lot of gears in-between) you have a turbine and a propeller pushing against the air (while mounted in the same axle).
pzerr t1_irrv9jy wrote
The fan at the front is really just a form of a turbojet with normal propellers. It is just propellers with a shroud around it.
Fun fact. The shroud makes these engines slightly less efficient but people dislike open propellers.
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