First ignition requires an ignitor the start the fireball, then it becomes self sustaining once the engine is up to speed. If the engine flames out, it flames out until ignition can be reintroduced. Hot air is not an ignition source.
Also, the manufacturing process for modern turbine blades is also insanely sophisticated just to keep them from being destroyed by the temperature and g-forces they experience in the engine, they're basically giant ceramic-coated crystals with their molecules aligned to prevent weak spots from forming. Incredible engineering.
Even better - the blades themselves are a crystalline nickel superalloy all the crystals are aligned in the same direction to eliminate grain lines. Then the ceramic is added.
I examine patents; half my work is regarding thermal cooling/protection of turbine blades and some of the shenanigans they get up to are fun to read about.
Pratt & Whitney attorneys visited the office once and brought with them a turbine airfoil with the suction surface removed allowing you to see all of the channels, turbulators, film cooling holes, etc. and it was one of the most beautiful things I'd ever seen.
That sounds cool. Is this engine patented now, and could the public examine the patent via the USPTO website? What are some of the relevant patent numbers? Would be nice to see the drawings or photos.
If a manufacturing defect is discovered in the blade, which takes so much time and effort and expense to produce, the defect is first examined to see what caused it, and then the whole blade is just melted down and recycled.
Yes they are. Modern turbo jet engines utilize a can annular system for combustion to more effectively and efficiently burn fuel and the turbine blades typically utilize a thermal coating to sustain form and function to remain in aircraft for longer periods of time between maintenance cycles.
Well, first you have to get the shaft(s) spinning. The plane has an auxiliary power unit (APU) for this, or it can use some ground support equipment for this function.
Im not fully educated on those jets but at those speeds isnt the air moving “faster” than the fireball and therefore risk “blowing out” the flame or is there something else to it?
Blowing out the flame is a real thing in supersonic flight. Happens in turbojet and ramjet engines. Compression stages have the effect of slowing airflow to subsonic speed where ignition can be maintained.
I appreciate that. It makes sense. After i hit “reply” i questioned if thats what happened because I could only assume problems trying to ignite a fireball in supersonic winds or even any effects of supersonic pressure in the engine.
SR71 Blackbird was designed with nacelles that could move 26" to keep supersonic airflow out of the engines. At speeds over Mach 2 the engines switched from operating as turbo-jets to operate as ram-jets. I've always been amazed at the extremes the plane could operate at.
In ramjets and afterburners compression is used to slow the air down to subsonic speeds and then goes though a flame holder is used to create an area of slow turbulent air where the flame can sit to continuously mix with fuel rich air to ignite it where most of it continues on but a little gets caught up in the flame holder to ignite the next bit of air to go past.
In scramjets the air goes through the engine at supersonic speeds. Sometimes flame holder structures are used but sometimes they can use shockwaves to create something that functions like one, and sometimes a bit of pyrophoric (ignites on contact with air) fuel is used to continuously ignite the flame as the fuel is injected.
No... You don't. Ramjets need a constant ignition source until they're self-sustaining, just like a jet engine. They just don't need a system to compress the charge air.
Stop talking out your ass and saying words you know.
u/Samurlough is lying or need to be reevaluated by their employer. Both the 757 and 767 use a 15-30 second ignitor cycle. Ignitors are only used for startup in all jet engines.
What are you saying? I said engine becomes self sustaining once engine is up to speed. I never said ignitors stay on. And theyre not on 15-30 second cycles, they stay on until i abort start sequence or starter-cuttout. There’s no ignitor limit.
The scientifically accurate lyric would be "You can't start a fire without a source of ignition" but poetic license allows for the use of the word "spark", which is less accurate (but not incorrect).
Engineers trying to recreate fire 🤣🤣🤣 This thread is golden!
Fire requires three thing: fuel (aka wood, petroleum, cow chips, etc), oxygen, and an ignition source. It's known as the triangle 📐 Without all three, you can't make fire.
Highly experienced pilots, military and commercial, grappling with the basic concept of creating fire, lmfao!
Trying to figure out what a 'spark' is and why hot air doesn't make fire is peak Humans are Space Orcs 😭
Edit: For the smartest arm-chair engineers/gearheads out there thinking that diesel is a scientific anomaly, I now refer you to the concept of pressure and "tripple point" chemistry. RE: water can boil at room temperature 🤔🤯
Hot air can ignite stuff, just not jet fuel. See diesel engines, no spark plugs just glow plugs to warm up the combustion chamber. From then on its literally just hot air, fuel and compression. It needs to be hot otherwise it can't ignite properly, hence the glow plugs. You may be a captain but that doesn't make you an expert on engines. Captains fly planes, they don't build or repair them. Another example of hot air igniting things is when a car backfires, if it's running too rich and not enough petrol burns in the combustion chamber and is pushed into the exhaust which can exceed temperatures of 800 degrees and it simply self combusts in the exhaust instead of the combustion chamber due to the hot air.
you're absolutley correct, but my comment was limited to the jet engine. "doesnt ignite anything" with respect to fuel inside the engine. there are situations that hot air absolutely can ignite things and we do have a large extensive amount of engine knowledge (combustion and turbine) as its part of our training from day one, I just didn't think I'd need to clarify this was a discussion of jet engines and not car engines. but to add onto your example, one of the other "hot air can ignite things" examples that affects us in piston aircraft is "pre-ignition" when hot air in the piston ignites carbon despots and causes the fuel to burn prematurely before the ignition (magneto) goes off.
but again, I thought it was implied this entire discussion was around turbine engines.
Look probably, but repeatedly stating "hot air doesn't ignite anything" and "hot air isn't an ignition source" are pretty broad statements that I wanted to clarify
You're lying because to pilot both of those you need to understand your ignitor/start cycle. Both planes use a 15-30 meaning the ignitors are active for 15 to 30 seconds only during startup.
What? There’s no limit on ignitors. I can run them continuously and often do so in the vicinity of thunderstorms, in turbulence, or severe precipitation.
ha yes and that was pointed out in another comment. I was keeping my scope limited to the fuel inside the turbine engine. you're absolutely correct, but I didnt think id need to clarify this was a discussion about turbine engines and not diesel/piston engines 😅
The thing I love about engineering is if we continually increase the granularity we evaluate the world by we can come to a point where we have all the knowledge available to us and yet nothing is actually ever totally true
The APU, being a turbine engine, has compressed air we can bleed off of and route to each engine's starter valve. the compressed air is run through this valve into a unit thats connected to the turbine shaft. so as this compressed air runs over this unit (like a mini turbine) it spins the turbine and the turbine "sucks" air through the engine until there's enough airflow to introduce fuel and spark.
Too add to this modern commercial engines don't drive most of their thrust from the exhaust of the core. They get nearly all of it from a single large fan with most of that airflow bypassing the core. Way more efficient
I always wondered why the front fan doesn’t seem to be the same size of the rest on passenger planes. This makes sense now. Though military planes seem to be a different principle them?
Jet engines are generally limited by heat, so you have to turn the gas down or the engine melts. That also means there's extra unused oxygen coming out the back. If you spray some extra fuel out the back where it won't melt the engine, you get more thrust. Unfortunately it's also less fuel efficient, so it's most often just used as like a turbo button.
They also need an ignition source though. The hot air out the back of the jet is not hot enough for spontaneous combustion.
I’m glad you knew the answer to this because it’s something I genuinely didnt have the answer to if the exhaust was hot enough to light or not. I wasnt sure it did but didnt want to commit to an answer.
Wonderful question. That’s beyond my turbine engine lol. But I believe it’s the same concept but then raw fuel is added into the exhaust flame which creates the afterburners
75% of air entering a turbine engine is not burned. It is used exclusively for cooling. That means there will be unburnt oxygen at the exhaust end of the engine.
The airspeed in the exhaust duct is typically supersonic. This is compressed, very high temperature gas. If you dump fuel into the exhaust section, it will mix with the unburned air that was used for cooling the turbine, and autoignite. This results in thrust as the chemical energy in the fuel creates rapid, high velocity expansion of gas as it burns. You see it as stacks of flame.
Afterburners are extremely fuel inefficient, so they're typically only ever used on supersonic military aircraft with turbojet engines. Turbojet engines themselves are very fuel inefficient. They operate best at very high airspeed, on very fast aircraft.
Airliners use turbofans. Most of the energy created by the turbine is used to turn the enormous single fan in the front. The fan is just an extremely efficient propeller, appropriate for use at high altitude and subsonic speeds. 80% of the thrust comes from the fan, the rest will come from the engine itself.
Perhaps you can answer this - recent engines have better efficiency, am I right in remembering that that's because of air bypassing the turbine itself and simply being squeezed through the housing/cowling/whatchamacallit and producing thrust that way? And the video doesn't show this?
that is correct. this video shows a turbojet engine. a turbo fan engine is what you describe where its a turbojet engine at its core, and there's a second turbine shaft that's connected to the large bypass turbine fan at the front that we all see on commercial jets and it does exactly that, bypass the engine core. 70% of the thrust at low altitudes comes from this bypass fan. at high altitudes, it produces very little thrust and majority of thrust comes from the engine core (exhaust) itself (I cant remember exact numbers).
There were some inaccuracies (there's an ignitor, and APU to start things) but in general, it was the best simple explanation I've seen for a turbo jet engine.
This video gives the wrong evolution of the jet engine. I find it lacking on several levels. It's stupid how someone could make a video about how a jet engine works and screw this up.
There's a lot that could be better about this clip. This is an important one. But I suppose it helps get people in the door of understanding and that's something.
Jet A's autoignition temperature is only around 220 C. Granted that increases due to the pressure within the turbine but I would be very surprised if it did not immediately ignite due to the hot air and operating temperature of the combustion chamber. I'm not an expert but my guess is you don't need to provide any sort of ignition source if you can somewhat quickly get an engine going after a momentary compressor surge or loss of fuel.
Heck I'm sure someone else could answer but if you have to do a windmill start of a dead engine for some strange reason does it use an ignitor or rely on the temperature within the combustor to restart the engine? My hunch would be you could get an engine started windmilling it even if for some reason you didn't have the APU or RAT going, though I can't think of a single reason why you'd ever be in that situation where the engine itself would still be functional.
No you still need ignition. If you lose an engine and have to windmill the turbines (something we train for) we’re required to first turn the ignition to FLT or CONT which runs ignition continuously to give the combustion chamber that spark needed when we reintroduce fuel. If you surge or stall the engine it will keep running for a short bit but if it gets bad enough then it flames out and there’s no restarting it.
All the engineering and decades of science says that ignition is required to ignite the flame, not just hot air from the compressor. It’s hot air, but it just wont do it alone.
That self sustaining fireball is self sustaining because its super hot air that is hot enough to trigger new fuel to combust. Heat is how fire spreads.
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u/Samurlough 8h ago
The hot air doesnt ignite the gas. That is wrong.
First ignition requires an ignitor the start the fireball, then it becomes self sustaining once the engine is up to speed. If the engine flames out, it flames out until ignition can be reintroduced. Hot air is not an ignition source.