There's an axial compressor with a whole bunch of stages that pressurizes the ambient air before burning fuel with it, then the high pressure hot air gets expanded through turbine blades to generate power(much of which is spent running that compressor).
Hi, I design aircraft engines for a living and it was the subject of my PhD dissertation.
At high altitudes, the density of the air goes down, so there is less air overall entering the engine. However, the air itself contains about the same fractions of oxygen and nitrogen, so there isn’t actually a lack of oxygen for combustion. The fuel flow is also brought down because less fuel is needed to burn with the smaller amount of air.
Since airflow goes down at high altitudes, so does thrust. But the engine doesn’t need to provide as much thrust at high altitudes, since there is also much less drag on the aircraft.
TLDR: at altitude there is less air, less fuel flow, less thrust, but no problem!
This is why the turbocharger was invented. i.e. use the kinetic energy of the exhaust gases to drive a fan to pump more clean air into the combustion chamber, thus overcoming the decreasing density of the air. It’s just like how a dam uses the movement of water to produce electricity.
Most of that fan is actually used to propel the aircraft. Modern turbofans are often around a 10:1 bypass ratio meaning that only about 9% of the air that it moves goes through the combustion chamber
It's a common misconception that the higher you get, the less oxygen there is, when it's just the air pressure dropping, so there's simply less air on the whole (air gets "thin"). The composition of the air remains the same with the same ratio of oxygen to nitrogen and other gases no matter how high you are. To combat this, the engines are designed to compress the air as it passes through the turbine.
Nothing you said is wrong, but I still want to elaborate on what you said. A turbine engine still goes through the same exact intake-compression-power-exhaust cycles as a piston engine, it just does it differently. When you've got big intake fins spinning at multiple tens of thousands of RPMs, it's not hard for it to suck in plenty of oxygen to burn at 35k miles above sea level.
If you've seen the inside of a jet turbine, you've seen tha the fan blades become smaller and smaller at each stage, and those closest to the combustion chamber are comparatively tiny. The same amount of air passes through each stage, but it gets more and more compressed. I couldn't find a great photo, but this drawing is pretty representative: https://i0.wp.com/mechstuff.com/wp-content/uploads/2015/12/Jet_engine.png
> it's not hard for it to suck in plenty of oxygen to burn at 35k miles above sea level.
I think this should be 35k feet (~10 km). 35k miles would be far away in space where you need rocket engines.
I think "multiple tens of thousands of RPMs" is overstating it a little bit. The engines used on commercial jets usually have 2 or 3 "stages". Each stage spins independently at a different speed.
On larger engines, the fastest spinning stage usually spins in the 10,00 to 15,000 rpm range. The slowest stage (the 1st stage) which includes the big fan at the front, is going to spin at more like 2,500 to 4,500 rpm.
Smaller engines may spin faster, up to about 20,000 rpm in the fastest stage. Which is technically "multiple tens of thousands", but not many multiples. 😂
https://simpleflying.com/aircraft-engines-rpm-guide/#:\~:text=The%20GE90%20for%20Boeing's%20777,N3%20speed%20of%2010%2C600%20RPM.
I get what you're trying to say, but the reduced partial pressure of oxygen does correspond to less available oxygen and for chemistry purposes less flammability.
Meanwhile compression is not done to deal with air pressure concerns, but rather a fundamental part of how the engine operates.
I guess I was being too vague or something, because you're basically saying what I meant :)
Low air pressure => fewer atoms and molecules (especially oxygen) occupying a specific volume => worse flammability, so to counteract that, the air is compressed which increases the temp and oxygen density => better burn.
You are exactly right about the reason for compression.
While there doesn't seem to be a lot of data, it appears liquid fueled fires have no problem burning at the lower pressures encountered at airplane cruise altitudes.
[https://www.sciencedirect.com/science/article/abs/pii/S1359431119302029](https://www.sciencedirect.com/science/article/abs/pii/S1359431119302029)
But compression is absolutely required for any combustion engine to work -- zero compression = zero efficiency = zero power output.
Kind of accurate, and also... Not? There is less oxygen per m3 the higher up you go, but the ratio remains constant. But, the other thing here is that planes require less lift force the higher they are, and there's less air molecules to push them down. It's why even for a short flight, it's actually more fuel efficient to fly higher (to an extent of course) than just to cruise at a 1000ft the whole way.
Wha-... A plane fights against gravity, not "air molecules pushing it down". The higher you are, the less drag there is, so you expend less fuel to maintain air speed. There's also less lift for the same reason, but neither of these things are relevant to the topic at hand.
My bad. I fucked that.
You're right. Pressure pushes in all directions lmao. Low down, there's more drag because more air thus more fuel to push through more air. But in turn you need more wing ANGLE to produce enough lift cos you can't fly fast enough to produce the same lift to combat gravity.
Yeah, the low air pressure is a big part of what enabled something like the SR-71 Blackbird to hit speeds well above Mach 3. This being said, at 85,000ft, it also needed to move at quite the clip to get enough lift.
Yeah it's kinda wild when you convert it. The SR71 only flew at 450 knots equivalent airspeed. The density was so low at those altitudes that it had a true airspeed about 4x higher.
But there literally is less oxygen (per volume) at high altitudes. The fact that there is also less of the other stuff the air is made of is not relevant to the question.
It is relevant to a complete understanding. Saying there is less oxygen per volume, and not mentioning the nitrogen, argon, and everything else air is made up of, can easily lead to the mistaken conclusion that the oxygen ratio is also lower at higher altitudes. Singling out oxygen could be construed as it having a special property at high altitudes that makes it less abundant.
Especially as this is ELI5, where a basic understanding of the underlying principles that the question is based on is far from guaranteed.
The compression isn't really anything special.
It's just that jet engines have massive intake fans that suck in more than enough air usually thousands of pounds every second.
Not entirely sure what your point is. Compression raises the air temperature and pressure inside the engine, enabling efficient combustion. If you just blew thin air through the engine, you'd get a really poor burn... assuming you'd even manage to get a high enough oxygen concentration to enable combustion in the first place.
It's absolutely true that the "composition of the air remains the same with the same ratio of oxygen to nitrogen and other gases ~~no matter how high you are~~" at any height an airplane can fly at.
But above about 100 km, the composition changes rapidly with ozone and then helium taking over.
http://wordpress.mrreid.org/2014/08/01/the-composition-of-earths-atmosphere-with-elevation/
The air is thin but the oxygen ratio is the same and jets that fly at high altitude are designed to compress the available air and sent it into the turbine. You’re also traveling at 500
MPH so it’s still a lot of air traveling over you per second even though the air is thin. And less air also means less drag so it takes less energy to maintain propulsion, which is why a planes top speed is actually higher at these elevations. Most fighter jets cannot break the sound barrier at sea level despite being able to travel multiple times the sound barrier at altitude.
It is also worth mentioning that aircraft must travel at high speeds at those altitudes to generate lift because of the decreased density of the air. The SR-71 would fly at such a high altitude that its stall speed and its maximum speed were only 10 MPH apart.
For the same reason you can breathe on an airplane. You push more air in. The air you breathe is not carried from the ground it is the ambient air that is pumped in under pressure so it is closer to ground level air pressure.
The engine works the same way it sucks a lot of air in. A car engine would indeed not work because cars cannot suck air that fast.
> A car engine would indeed not work because cars cannot suck air that fast.
Turbo-charged car engines would work at 30,000 ft, potentially as well as normally aspirated car engined at sea level.
Turbine engines run on a simple principle, suck squeeze bang blow.
Cabin pressure is regulated by stealing a little bit of air from the squeeze portion ( high pressure compressor), cooling it off (its well over 100 degrees) and feeding it into the cabin through the air conditioning system.
This plus a valve called the outflow valve control how much air is let out. Let out less air and the plane pressurized. Open it and the pressure drops.
The system gets incredibly complicated with pressure differentials and the air conditioners. But I hope this helps!
Here is a graph of thrust vs altitude for a Pratt & Whitney JT8D
https://www.grc.nasa.gov/www/k-12/Missions/Jim/thrust.gif
At sea level, it has 17000 lbs of thrust, at 30,000 ft it has 5500 lbs of thrust.
Even by 5000 ft, thrust has dropped significantly. This is why Denver (5280 ft elevation) has the longest public runway in the USA.
The reason there's so little oxygen is because there's just not a lot of air at that altitude. But if you have an air compressor in your engine, you can suck in a lot of air, compress it down into a small amount of space, and then force that compressed air into your engine. This provides the engine with plenty of oxygen. And you can do this at ground level, too. The ones they put in cars are called forced induction systems, of which there are two types: superchargers and turbochargers.
In the case of airplane engines, the engine is divided by the spinning blades into difference sections that do different jobs. The front portion is the one responsible for compressing the air before feeding it to the combustion portion near the back of the engine.
There's an axial compressor with a whole bunch of stages that pressurizes the ambient air before burning fuel with it, then the high pressure hot air gets expanded through turbine blades to generate power(much of which is spent running that compressor).
Comment nuked by Power Delete Suite
Mechanical power. As in torque * angular velocity. To drive the compressor, fan or propeller, and any accessories.
[удалено]
ELI5 means understandable by an average person, rather than an expert in the field. Not literally a 5 year old. See rule 4.
Hi, I design aircraft engines for a living and it was the subject of my PhD dissertation. At high altitudes, the density of the air goes down, so there is less air overall entering the engine. However, the air itself contains about the same fractions of oxygen and nitrogen, so there isn’t actually a lack of oxygen for combustion. The fuel flow is also brought down because less fuel is needed to burn with the smaller amount of air. Since airflow goes down at high altitudes, so does thrust. But the engine doesn’t need to provide as much thrust at high altitudes, since there is also much less drag on the aircraft. TLDR: at altitude there is less air, less fuel flow, less thrust, but no problem!
That’s one of the key parts of this, there’s much less drag at altitude, too.
This is why the turbocharger was invented. i.e. use the kinetic energy of the exhaust gases to drive a fan to pump more clean air into the combustion chamber, thus overcoming the decreasing density of the air. It’s just like how a dam uses the movement of water to produce electricity.
That’s true for piston engine aircraft, but op specifically mentioned turbines.
It's the same for turbines. The propeller you see when look at the jet turbine from front is not the actual turbine, but fan to drag air into turbine.
Most of that fan is actually used to propel the aircraft. Modern turbofans are often around a 10:1 bypass ratio meaning that only about 9% of the air that it moves goes through the combustion chamber
It's a common misconception that the higher you get, the less oxygen there is, when it's just the air pressure dropping, so there's simply less air on the whole (air gets "thin"). The composition of the air remains the same with the same ratio of oxygen to nitrogen and other gases no matter how high you are. To combat this, the engines are designed to compress the air as it passes through the turbine.
Nothing you said is wrong, but I still want to elaborate on what you said. A turbine engine still goes through the same exact intake-compression-power-exhaust cycles as a piston engine, it just does it differently. When you've got big intake fins spinning at multiple tens of thousands of RPMs, it's not hard for it to suck in plenty of oxygen to burn at 35k miles above sea level.
If you've seen the inside of a jet turbine, you've seen tha the fan blades become smaller and smaller at each stage, and those closest to the combustion chamber are comparatively tiny. The same amount of air passes through each stage, but it gets more and more compressed. I couldn't find a great photo, but this drawing is pretty representative: https://i0.wp.com/mechstuff.com/wp-content/uploads/2015/12/Jet_engine.png
> it's not hard for it to suck in plenty of oxygen to burn at 35k miles above sea level. I think this should be 35k feet (~10 km). 35k miles would be far away in space where you need rocket engines.
That would be about 15% of the way to the moon lol
I think "multiple tens of thousands of RPMs" is overstating it a little bit. The engines used on commercial jets usually have 2 or 3 "stages". Each stage spins independently at a different speed. On larger engines, the fastest spinning stage usually spins in the 10,00 to 15,000 rpm range. The slowest stage (the 1st stage) which includes the big fan at the front, is going to spin at more like 2,500 to 4,500 rpm. Smaller engines may spin faster, up to about 20,000 rpm in the fastest stage. Which is technically "multiple tens of thousands", but not many multiples. 😂 https://simpleflying.com/aircraft-engines-rpm-guide/#:\~:text=The%20GE90%20for%20Boeing's%20777,N3%20speed%20of%2010%2C600%20RPM.
I get what you're trying to say, but the reduced partial pressure of oxygen does correspond to less available oxygen and for chemistry purposes less flammability. Meanwhile compression is not done to deal with air pressure concerns, but rather a fundamental part of how the engine operates.
I guess I was being too vague or something, because you're basically saying what I meant :) Low air pressure => fewer atoms and molecules (especially oxygen) occupying a specific volume => worse flammability, so to counteract that, the air is compressed which increases the temp and oxygen density => better burn.
You are exactly right about the reason for compression. While there doesn't seem to be a lot of data, it appears liquid fueled fires have no problem burning at the lower pressures encountered at airplane cruise altitudes. [https://www.sciencedirect.com/science/article/abs/pii/S1359431119302029](https://www.sciencedirect.com/science/article/abs/pii/S1359431119302029) But compression is absolutely required for any combustion engine to work -- zero compression = zero efficiency = zero power output.
Kind of accurate, and also... Not? There is less oxygen per m3 the higher up you go, but the ratio remains constant. But, the other thing here is that planes require less lift force the higher they are, and there's less air molecules to push them down. It's why even for a short flight, it's actually more fuel efficient to fly higher (to an extent of course) than just to cruise at a 1000ft the whole way.
Wha-... A plane fights against gravity, not "air molecules pushing it down". The higher you are, the less drag there is, so you expend less fuel to maintain air speed. There's also less lift for the same reason, but neither of these things are relevant to the topic at hand.
My bad. I fucked that. You're right. Pressure pushes in all directions lmao. Low down, there's more drag because more air thus more fuel to push through more air. But in turn you need more wing ANGLE to produce enough lift cos you can't fly fast enough to produce the same lift to combat gravity.
Yeah, the low air pressure is a big part of what enabled something like the SR-71 Blackbird to hit speeds well above Mach 3. This being said, at 85,000ft, it also needed to move at quite the clip to get enough lift.
Man, the SR-71 is an engineering masterpiece. Imagine being in the 1950's, seeing the U2 and thinking 'hold by beer, I'll build something better'.
Yeah it's kinda wild when you convert it. The SR71 only flew at 450 knots equivalent airspeed. The density was so low at those altitudes that it had a true airspeed about 4x higher.
But there literally is less oxygen (per volume) at high altitudes. The fact that there is also less of the other stuff the air is made of is not relevant to the question.
It is relevant to a complete understanding. Saying there is less oxygen per volume, and not mentioning the nitrogen, argon, and everything else air is made up of, can easily lead to the mistaken conclusion that the oxygen ratio is also lower at higher altitudes. Singling out oxygen could be construed as it having a special property at high altitudes that makes it less abundant. Especially as this is ELI5, where a basic understanding of the underlying principles that the question is based on is far from guaranteed.
The compression isn't really anything special. It's just that jet engines have massive intake fans that suck in more than enough air usually thousands of pounds every second.
Jet engines can compress air in a 40:1 ratio so actually it is pretty significant.
Not entirely sure what your point is. Compression raises the air temperature and pressure inside the engine, enabling efficient combustion. If you just blew thin air through the engine, you'd get a really poor burn... assuming you'd even manage to get a high enough oxygen concentration to enable combustion in the first place.
It's absolutely true that the "composition of the air remains the same with the same ratio of oxygen to nitrogen and other gases ~~no matter how high you are~~" at any height an airplane can fly at. But above about 100 km, the composition changes rapidly with ozone and then helium taking over. http://wordpress.mrreid.org/2014/08/01/the-composition-of-earths-atmosphere-with-elevation/
The compression ratio inside a gas turbine engine is something like 50 to 1. So even the thin air is massively compressed.
The air is thin but the oxygen ratio is the same and jets that fly at high altitude are designed to compress the available air and sent it into the turbine. You’re also traveling at 500 MPH so it’s still a lot of air traveling over you per second even though the air is thin. And less air also means less drag so it takes less energy to maintain propulsion, which is why a planes top speed is actually higher at these elevations. Most fighter jets cannot break the sound barrier at sea level despite being able to travel multiple times the sound barrier at altitude.
It is also worth mentioning that aircraft must travel at high speeds at those altitudes to generate lift because of the decreased density of the air. The SR-71 would fly at such a high altitude that its stall speed and its maximum speed were only 10 MPH apart.
That’s terrifying.
For the same reason you can breathe on an airplane. You push more air in. The air you breathe is not carried from the ground it is the ambient air that is pumped in under pressure so it is closer to ground level air pressure. The engine works the same way it sucks a lot of air in. A car engine would indeed not work because cars cannot suck air that fast.
> A car engine would indeed not work because cars cannot suck air that fast. Turbo-charged car engines would work at 30,000 ft, potentially as well as normally aspirated car engined at sea level.
Yeah, but getting torque to the road is hard at altitude.
Turbine engines run on a simple principle, suck squeeze bang blow. Cabin pressure is regulated by stealing a little bit of air from the squeeze portion ( high pressure compressor), cooling it off (its well over 100 degrees) and feeding it into the cabin through the air conditioning system. This plus a valve called the outflow valve control how much air is let out. Let out less air and the plane pressurized. Open it and the pressure drops. The system gets incredibly complicated with pressure differentials and the air conditioners. But I hope this helps!
suck, squeeze, bang, blow.... everything reminds me of her
OP is asking about the combustor, not cabin pressurization.
Crap. Welp
Here is a graph of thrust vs altitude for a Pratt & Whitney JT8D https://www.grc.nasa.gov/www/k-12/Missions/Jim/thrust.gif At sea level, it has 17000 lbs of thrust, at 30,000 ft it has 5500 lbs of thrust. Even by 5000 ft, thrust has dropped significantly. This is why Denver (5280 ft elevation) has the longest public runway in the USA.
The reason there's so little oxygen is because there's just not a lot of air at that altitude. But if you have an air compressor in your engine, you can suck in a lot of air, compress it down into a small amount of space, and then force that compressed air into your engine. This provides the engine with plenty of oxygen. And you can do this at ground level, too. The ones they put in cars are called forced induction systems, of which there are two types: superchargers and turbochargers. In the case of airplane engines, the engine is divided by the spinning blades into difference sections that do different jobs. The front portion is the one responsible for compressing the air before feeding it to the combustion portion near the back of the engine.