Not a question... just a massive thank you for what you do. I'm just a regular guy with a passing interest, and wanted to know how a jet engine works. I've now watched loads of your videos and I find them absolutely fascinating. Even though I'm a novice, I think that you explain the very technical terms as clearly, concisely and as understandable as possible. Thank you sir!
You know what's interesting, while some of us have a higher level of engineering experience or just mechanical knowledge and what he says lines up with established knowledge, if you didn't know any of that base material you'd just have to take his word that he knows/knew what he's talking about and not showboating as a guy who pushes a broom around a jet engine shop. =D
Funny you should know I grew up in rural Manitoba... my first jobs were all farm related. When I was a teen we flew Cessnas and Cubs like motorcycles and snowmobiles. Later working with the turbine crop dusters they always had trouble with bleed valves and shut downs. I've never flown an actual jet but the few times I flew those planes I was told if you can't land it you'll die and if you hurt it, the owner will kill you instead.
We used bleed air on our ships for a lot of various things. Masker air was used to make the ship sound like a rainstorm to submarines. Prairie air to come out of the leading edge of the prop to change the signature of the ship. Anti-icing air for our intakes when it gets below icing conditions. We could also do cross bleed starts to start other turbines.
Ive always had the understanding that "bleed air" had 3 types (as you said). Starting purge, running balance/cooling (the air sent to the center bearing is also needed for air flow stability along with cooling), and service air (used for running services outside the engine.
On the R.R. Alison turbine engines Ive worked on, for helicopters, the bleed air port has 2 functions. When the engine is at 60% or idle, air blows out of it back into the atmosphere so that the compressor doesn't blow the flame out in the combustion chamber. at 100% the airflow reverses to help feed the compressor and keep the engine cool. It all happens with no moving parts, its just part of the design of the compressor fan (13:1) in this case.
Bleed air valves are for air start purposes. Bleed air from the 13 stage or whatever the manufacturer deems is for deciding and air conditioning packs. On the Lockheed C 130 I worked on, you start #1 engine then open bleed air valve so the manifold is filled with bleed air to start #2,3,4 engines. Great video! (You must start the APU to utilize it's bleed air to start #1!)
In my previous experience bleed valves were called "blow off valves" in industrial centrifugal compressors. Those would be open until the compressor reached operating rpm and would slowly close after the VIGV reached maximum opening perecentage. Then the VIGV would close slowly and modulate as per requirement. That was done to prevent the compressor to go from idle to load all of a sudden which can cause surging.
this guy and his videos are awesome. the bleed valves are like a compression release in older, bigger motorcycle engines that you couldn't kick over unless you released some combustion pressure
thank you for the information regarding the bleed air. I'm working as a process control tech, one day we had a problem in one of the power generators. They were GE gas turbines with GE redundant control system mark VI. Operations couldn't start the generator because of a problem in the bleeding valves, there were only two bleed valves. After tracing back inside the turbine compartment, we have noticed that the bleed valve actuators are operated by the turbine's compressor output and this compressed air was controlled by a solenoid valve located in the accessory compartment where you will find the lube oil system ad the started motor with. After testing the solenoid alone by forced signal through the control system, we have found it functional and healthy. We asked the operation to try the generator for another start " cool down between every start was set at 30 minutes". Again another failure in the bleed valves. At the end we had managed to test the valves actuators alone together at the same time by using an external source of air supply and found a big deviation in time, about a 22 seconds difference. It was confirmed that this wasn't normal, and the main issue was in the second valve actuator leakage. It has been replaced and the deviation was set to be less than 3 seconds after the second startup of the gas turbine. The limit switches on every valve actuator was the main reference to us, because it was unsafe to start the turbine while the doors of the compartment were open. My questions: Are aircraft engines equipped with the same setup? Is this a real issue in aircraft engines? Are the bleed valves in the aircraft engines designed to also be used as a surge protection when startup? sorry for the long story, thank you again for your explanation.
Cool, I was about to ask what exactly it was. When he said that it was a single-spool turboshaft engine I figured it had to be a mechanical load. Neat idea to use what you already have and where you have a good idea about its power consumption. But sucking/blowing air through these sharp-edged variable holes is certainly going to make a huge noise. I was aware of all the different kinds of "bleed air", but never thought about the possible confusion. Another major use for "customer bleed air" is wing de-ice/anti-ice in most jet airliners.
I want to thank you for this and all your videos, this is good infromation about the air bleed valves. When the engines starts, it makes easier to turn the engine and other reason is to cool down other componets of the engine. There is on point, the turbo prop entgine, this one doesn;'t use the power turbine. Thanks a lot. Please send some snow to here.
JORGE E Thomas they are there to prevent destructive compressor surge in axial flow compressors...they require minimum flow .....all other uses of bleed or recycling valves is sympathetic....
Thank you so much for doing these videos and answering questions. I am learning so many things about jet engines that have been on my mind for a number of years. I Have always been interested in this subject but being a big dummy do not know the correct question to ask or things get real complicated real fast and other commentators seem to be all over the place. You have a wonderful, natural gift. Teaching seems to be in your genes. It is also very obvious how much joy you feel when the word turbine is mentioned. You absolutely love your job.
I am confused. My prof told us that bleed valves, or what he called them, bleed hatches are used to bleed off the excess air in order to avoid surge of the compressor. Though, maybe he was right partially, since the only showpieces we have in our uni are the vintage soviet engines, and their age vary from 40 to 60 years or even more, and the older the engine, the more unorthodox it's construction is.
Hi Jay, Thanks for the full explanation of 'bleed air'; it seem all kinds of systems like to suck air off of the compressor! Please note that I placed 'air' in between 'suck' and 'off'!! That homemade load machine is interesting & sounds like it would work to me, however most likely very noisy! I'll be watching for that video... Darren
What I would like to know is the history of the use of compressor bleed for customer pneumatic power. What was the first engine to be designed with a system specifically to tap air off the compressor for airframe pneumatic power? What was the first aircraft, military or civil to use compressor bleed for pneumatic power? I'd also like to know what the first aircraft was to use an air cycle machine or pack for environmental control.
Jay, several military helicopters use turbine engines that use a bleed air system you may not be aware of. These engines include the T53 (all models), L-712 (to include the L-11 or L-7-11and all following models). There are many more gas turbine engines that we use in the military that use the same bleed air system. However, none of these same type systems use the air... for ANYTHING. These engines produce an excess amount of compressed air by the time it reaches the centrifugal compressor. So, to control over compression in the outer combustion chamber there is a system of bleed air that is comprised of a large vacuum operated bleed valve that in turn opens a bleed band that surrounds the case halfs usually just after the second last axial compressor stage. The band is a stainless steel band that when tightened it prevents bleed air. Whan the actuator is in open position the band allows a specific amount of air to escape right to the atmosphere as directed by several other system.... such as the fuel control, the torque sensor and for some a few other sensors. This bleed air system was the real first coined 'bleed air' because it is what it is, just bleed air, not 'service air'. But service air didn't sound right so any system using the name calls it bleed air. A truly well engineered turbine engine shouldn't need that kind of bleed air system... but it does. For ninety percent of all other engines... that's wasted service air. PS: Also, these same engines use a separate bleed air system that actually is used for service air. Most are just used for 'anti-icing' around the intake cowling. This air is of the highest pressure as it comes from what we call the 'hot air gallery'... and yes, after being squashed as it does in a turbine engine... it really does get hot. This gallery sits right over the blades of the centrifugal compressor. The point at which the air is at it's hottest. As we techs know... hot is bad, even for intake air. The hotter the intake air is... the less expansion it is capable of during combustion.
Great video. You opened up a can of worms with this subject. There are so many different uses for bleed/discharge air. You have everything you talked about plus more depending on the engine. There is labyrinth seal pressurization, engine anti-icing, aircraft inlet anti-icing, fuel system purge, emergency oil pressurization, balance piston seal pressurization and much more.
I know that you are somewhat sensitive about answering questions that have already been answered.....completely understandable. However I have been trying to find the answer to this question in your videos and I have not yet had any luck. So....I will ask: Is there a quantifiable limit as to how much bleed air an engine can divert away from the combusters without effecting the performance of the burning of the fuel? A large commercial jet obviously uses vast quantities of compressed air to run various systems....is there a practical limit as to how much can be used without effecting thrust characteristics? Thanks much!
Bleed air is bled from the compressor to be used for a lot of different purposes. If air was bled from the combustors, there would be a lot of extremely hot and high pressure air and burning fuel that would ruin various engine and airframe accessories and components. Hope that helps.
AgentJayZ I've been Netflix style binge watching your videos. My question is about the bleed air valves. Is the purpose of the valves to unload the compressor to allow for it to reach the target rpm unimpeded by compression resistance? Thanks
One part you may or may not have mentioned on here what bleed air is used for besides a/c, cabin pressurization. It's is also used for de-ice on the leading edges of the wings, vertical & horizontal stabs, windscreen and engine inlets. Also engine starting either from an APU or ground start cart like you use to start your engines at the test cell and to help pressurize hydraulic systems. I work on KC-135's and seen the different ways bleed air is used. There are great videos on RU-vid that go over the bleed air systems of many different aircraft types like from 737 to a 747.
Jay, at 3:53 you talk about cooling the sump. Is there also a rotary pump that draws the air and oil out of the sump? Also, tell your developer to build a heat sink and install it around the ad on compressor. Have the working engine draw air through the heat sink. (naturally the heat sink needs to be configured right.) When the pressure drops around the heat sink, heat will be forced into the air. You will yield more psi and the process will be much more efficient.
The scavenge pumps on this engine are gear pumps, and they are designed to handle a mixture of oil and air. Most of the cooling air exits out of the sump vent, but some of it is in the form of bubbles in the oil drawn out by the scavenge pump. The scavenge pump has a greater capacity than the amount of oil being pumped in.
... and for the heat sink idea... I think I'll leave the design just the way it is, the way the hundreds of professional engineers working for years on this project intended it to be. No matter what anyone thinks about their "innovative" or "game changing" idea... they have to remember the ability and talent they are attempting to improve upon. I do, and so do all the rest of us. Cheers !
I am wondering why a compressor section would be used for the load, as opposed to something already fully developed like a water brake. Although I guess this "air brake" load is self-cooling and can take the shaft rpm directly without reduction. Makes me chuckle a bit, visit a wood shop and all the jigs and fixtures are made of wood, in the metal fab shop the same bits are made of metal, I guess in the turbine shop everything is made of turbines!
The only things developed for loading the 501/T56 are reduction gearboxes and electrical generators ( which need to power something in order to be a load ), or the prop reduction gearbox and propeller. Both of those options are more complex and expensive than just adding another compressor, which is designed to turn at 13,800 rpm ( I incorrectly say 13,600 in the video). A water brake would require a reduction gearbox rated at 10,000hp, and a supply of water, plus we would have to buy the thing... We have lots of T56 compressors...
This is very resourceful, how the available t-56 compressor could be used to load the massive power coming from the Allison 501 turboshaft engine... It eliminates the need for an expensive and complex gearbox. I wonder what the torque & horsepower ratings will be, coming through that shaft...
Of the three types of "Bleed Air" discussed; which is responsible for Engine Surge and how does clearing a surge (throttling back) prevent an engine from destroying itself?
The bleed valves are the ones you mean. They help offload the diffuser at lo-rpms because the engine compressor is so efficient. With regards stall/surge conditions. once established, the bleed valves have no control over them (as far as I know...certainly not from the ones I've worked on, anyway). That is purely a function of offloading the engine, either thru decreasing the RPM on a variable speed engine or thru offloading engine pressure with a constant speeder such as the 501. However, sometimes this just isn't possible, especially if the engine has suffered a mechanical failure. In which case, it's time to reach for the yellow and black painted handles and hope for the best.
I have been studying the dynamics of internal shockwaves ( Mach+). Do you know of any good material on this as it pertains to turbine engines? Can you explain any of it in layman's terms? Thanks JayZ for all the knowledge you share!
Most airflow within a standard jet engine is subsonic, remembering that the speed of sound is not constant, rather a function of fluid temperature. So throughout the core of the engine, the speed of sound varies due to temp differential. Once outside, however, it's a function of the ambient temp. This also applies to the inlet. That's why variable geometry intakes are required for high performance aircraft. As AgentJayZ mentions, down below, have a gander at the Rolls Royce book "The Jet Engine". It's basically the sumpy's bible.
The big disadvantage I see with the single shaft systems is the power needed for starting. You've mentioned the LM1500 and LM2500 are started with about 100hp, can you give an indication of how much hp this engine needs to start? I imagine it would be quite a bit higher than engines producing thrust or powering a free power turbine. It also looks like this load system wouldn't produce much load until the engine got to running RPM. I imagine that would make starting easier than normal, right?
I realize it been almost 5 years since this video was posted but Back in the late 60's when in the USAF I ran the P&W J52 P3 in combined systems test cell of the AGM28B Hound Dog Missile and it had a series of bleed Valves around the compressor case at about the No 5 or 6 compressor wheel. I remember I had to check them when activated from the control room during the engine check portion of the combined systems check. What was the function of these bleed valves on the J52 P3? Any Ideas?
Jay you have gone over several bleed air, on different engines, but can you find or create a video that shows how the two different systems work (Bleed Band System, Bleed Valve System). I have scoured FAA.gov, 8083, and jeppessen books and the best i can find are some poor drawing diagrams that don't explain or show their functions well at all. thank you.
Holy balls, that thing is going to be putting out a lot of air. I love it! I assume the goal is to get the company more involved with rebuilding and testing those -501 engines?
We share our test cell with Maddex Turbines, and they already do a lot of 501 work. This new setup will allow us to measure torque on the output shaft under full power. It is a specific request made by an important customer that uses these engines to produce electricity. And yes, we will have a 3,500 Hp air compressor on our hands...
I never thought about how the engineers would have had to make so many provisions for air...like the blanket of air that surrounds the flame to cool it... No wonder jet engines can handle such an enormous flame within!!! Prior...I just thought it could handle those temps magically...and i was correct to always question that in my mind... AWESOME!!! Not understanding something mechanical, and knowing that I don't, bothers me, seriously.
I have a little CAE-140 engine from a MA-1 GPU and I see a compressor bleed air path for a couple more functions. One assists in the actuation of the butterfly valve for the main compressor bleed (from the start cart), and the other leads, via a filter, to the hydromechanical fuel control unit. But maybe these aren't considered 'bleed air' because there is very little actual flow of air?
They will, most likely, be servo or sensing pressures as opposed to bleed air. Though I am unfamiliar with the type of donk you're referring to, this is pretty standard for most turbines.
I noticed you said that the bleed valves open on the 501 because the high pressure section can't handle the air from the low pressure section on start. That makes sense but I guess I am confused because you later said the 501 was a single shaft engine. Why are the bleed valves needed on a single shaft engine lacking an HP system? Thanks!
Dan8432 I meant the lower pressure stages at the front of the single shaft compressor are more efficient at low rpms than are the later, higher pressure stages on the same rotor. It does get confusing sometimes.
MrShobar No. Deicing is done on the ground by a crew with glycol fluid sprays, and in the air with inflatable "boots" on the leading edge of the wing.The only wing anti-icing I have seen is with electrical heating strips on the leading edge of the wings. Engine inlet anti-icing does use compressor bleed air, and when it's on, it reduces engine power by a few percent.
I was an engineer with the Boeing Company some years ago. We used bleed air for wing deicing on the wing leading edges on all of the airplanes since the 707 (the B-47 and B-52 too). Regards.
I imagine that this single shaft engine is the reason the engines on older hercs are shut down with the props locked in fine pitch, so they will be easier to start, where as the newer ones (J model etc) shut down with them feathered. I imagine the J model must have received a different engine with a free turbine.
You are thinking exactly right. Both types of engines have their own type of response to pilot input. If you can find a pilot who's flown aircraft with a single-shaft engine (Garrett TPE331), and a power-turbine engine (P&W PT6), ask him / her about it.
Yes, the C130J uses Allison (now RR) 2100, which have a free power turbine. A good comparison could be Do-228 (TPE331) vs. Twin Otter (PT6a), which have very similar mission profiles and are also the two most common types at Lukla in the Himalayas. Look it up.
I can answer half of that question myself as I have a couple of hundred hours on Twin Otters. As my first experience of a turboprop I found it to be a lot more responsive than I expected, especially at high torque. I've never flown a single shaft engine so can't comment on how it feels to fly but I'd be interested why Garret went for that design when the PT-6 configuration has so many benefits. I can't imagine it saving weight or being easier to maintain, maybe a lot of efficiency is lost in the twists and turns of the PT-6's intake. I can't be sure that is the reason though without doing some research. Hopefully one day Jay will feature them in one of his videos!
Calum MacFarlane My **guess** is that since (in the case of a single spool/transmission setup) there is near zero lag when the power levers are advanced since the engines/props/transmissions are already spinning at 100% RPM.... In the case of a two shaft arrangement, particularly at lower power settings, there could be a bit of lag when power is advanced... maybe it takes time for the turbines/props to fight inertia and spin up.(?)
In both cases the prop should be governing at the speed set by the pilot. I would guess response is dictated by how quickly a demand for power can be fed to the power turbine. In a twin spool arrangement the gas generator has a lot more freedom to change the mass flow than when it is bolted to the prop reduction gear.
I saw your video while I was searching for an explanation about active clearance control(ACC) but the video was about bleed valve and bleed air .it would be helpful if u could explain about ACC in modern tubofan engine such as ge90/genx.
Excellent description of a complex subject. Those two side by side bleed valves...can I think of them as a sort of "Compression Release" for startup purposes ? They let air bleed out during startup, to allow things to get up to a self sustaining speed more easily, without so much resistance? The other types I understand from your description...
+SquillyMon The bleed valves prevent aerodynamic stall of the compressor blades at lower rpm, allowing the compressor to function. It would be more accurate to say that they increase compression. It helps to ignore everything you know about piston engines.
It's more complicated than that. The bleed valves are not there to unload the compressor at design speed. They are there to increase air speed through the compressor at lower rpm to prevent stalling. When a compressor stalls, it stops working, and it requires less power to turn it. So... at operating speeds, opening the bleed valves would unload the compressor, but they are not used that way. At lower rpm, the bleed valves must be open to allow the compressor to work; closing them would cause a stall, which is not good for it, but also technically unloads it. I suggest you watch my video about compressor stall.
Regarding cabin bleed air (air passing from the compressor directly into the cabin) is it safe to assume that the smells of fuel inside the cabin, are from worn bearing seals, or from jet fuel that somehow flows in that direction? I travel a lot and some planes smell more that others. I love your efforts here and actually bought the gas turbine book that you showed us. Keep up the good work!!!
Fuel is introduced well downstream of any compressor bleed. A leaking front bearing seal may allow oil into the bleed air, but usually bleed air does not directly pressurize the cabin. It is used to run a small compressor, much like an automotive turbocharger, so that there is no way engine air can get into the cabin. That's as far as I can go, because I'm not a plane guy.
Agent Jay z... I have a question but it's not about the bleed valves/ air. where do carbon deposits on the main oil filter come from specifically for PT6A-27?
If this is one of those questions that's been answered a million times, just say so and I'll hunt for it on my own (or you can point me in the right direction). I'm just curious; as busy as you must be, what motivates you to take the time to make these videos?
Cool. Will look forward to it. You're very knowledgeable and apparently enjoy sharing/teaching, which makes your videos the preferred source of learning (at least for me). You seem like someone I could really enjoy working for; that is, if I could handle the Canadian climate. ;)
I have a question for you ? I am a AD in the Navy i work on the T-56A-14 or 501. Why is it that that you guys are taking the air on the old compressor section from the bleed valves on the 5th and 10th stages vs the 14 stage on the diffuser section, wouldn't you rather have 14 stage bleed air?
There is also a bleed air exit (I guess from the 4th stage of the Compressor for this case) that is used for anti-icing of the inlet guide vanes, am I wrong?
Regarding the bleed air uses, what kind of pressure(psi) and air movement(cfm) run through the bleed air systems?(as in the Orenda engine I figured that the different uses of it require different amounts of air and use "restrictions" in some fashion. Love the channel Jay!
For the internal engine air bleeds for cooling... I have no idea; the rates of flow are not listed in the engine information manuals, and the pressures will depend on engine rpm. The customer bleed air is variable on demand, and it's pressure will depend on what stage of the compressor it is sourced from. Different designs for different engines. The airflow through the bleed valves depends on when they close, which is different in different engines, but it is very large.
The air, coming out from that 'mecanical load' cmpressor, should have quite a high temperature, like 300+ celsius. This air will be sucked to the main engine intake as they are close. So, isnt it bad for the engine to be feeded by such a hot air?
Yes, almost always. I remember seeing some early J79s where the anti-icing air was taken from the combustor outer case. The design was later changed to use CDP taken from the back of the compressor. Pressure and temperature is about the same at both locations, so probably to use shorter tubes and maybe to disturb the pattern of cooling air flow around the liners less.
so there is customer "bleed air" for aircraft usage, and then there is internal cooling air, that's the air ducted thru the engine thru pipelines and valves used to cool certain parts of the engine most specifically, the combustion section and the turbine section. But what about the external cooling you were talking about? like I know that it mixes with the gas path downstream of the exhaust, but what does it specifically do?
@@AgentJayZ I've just recently found out your channel, kinda nervous about an upcoming exam so yeah :) what books would you suggest? I'm really sorry for these urgent questions.
I've always wondered what compressor stall is, ive read descriptions but they are all a bit abstract so hard to get my head around. Especially when the air reverses flow and flames come out the front, i really don't get how that happens.
Stall/surge is primarily an axial flow problem (tho not exclusively so, I should add). It's because an axial compressor works thru a series of convergent (accellerating) and divergent (pressure recovery) ducts. A divergent duct works closely to its designed airflow limits. Outside of this, the airflow becomes turbulent and stagnates. However, the compressors in front of the stagnated section are still compressing so all of a sudden that air can no longer pass thru the engine and must go out the only way it can...thru which it came. And BOY do you need a change of undies if you hear one up close! I should mention this is a rather simplistic explanation, but I hope it helps.
why don't they call the bleed valves compression release? also, on the engine "dyno" don't you need a valve on the exhaust side of the compressor more than the inlet or is there enough restriction in the end plate that the bleed air system will do? i assume the intake restriction is for unloading the engine at startup. if you were to shut the intake valve quickly while the "dyno/load" was at high load, would it attempt to stall the compressor?
+jon “tweakit” spon 1) because that's not a good description of what they do 2) restricting the inlet is more effective than restricting the outlet. Where is the throttle plate for a piston engine? 3) that is the basic idea, although we may need to restrict both the inlet and discharge...
hi sir.. thank u for ur nice efforts. I have a question about ,is there a difference between compressor bleed valve which is used to avoid compressor stall and surge and a bleed valve from which bleed air is usually taken for the purpose of de-iceing,cabin pressurisation,etc
Is the cooling air that cools the bearing also what pressurizes around the bearing inclosure to keep oil that gets past the seal out of the engine? what would the range of air pressure and what temps would you see when engine is at operating power? Is the air that is used by the plane systems closed circuit like when a car uses vacuum servos where there is no leak of pressure or vacuum in cars and just the pressure itself is doing the work? Thanks for the hard work you have a first rate site.
Yeah the internal cooling air is also used for seal pressurization, in both labryinth seals and pressurizing some carbon seals. The pressure depends on the engine type, engine compressor ratio and where in the compressor the air is taken from. The bleed air used in aircraft systems like air conditioning, anti-ice, cabin pressurization is exhausted overboard once it's been used, so it's not a closed circuit.
They do. Bleed valves are a less precise, but adequate solution to the problem of axial compressor stall. Variable bleed valves are better, and almost as good a solution as are variable stators.
Could it be that the term bleed valves derive from their function as the engine starts? Let's take a nowadays airliner. You use bleedair from ground, APU or one engine (cross bleed) delivered via the bleedvalves to the hp stage. That's how I would interconnect these two terms.
John Doe That's exactly what I try to explain as not the way to describe it. The valves you are mentioning could be called accessory air valves, starting air valves, or even bleed air valves. But what they are not, is bleed valves. I know it seems like nit picking, but that's because there are so many components that do nearly the same job. Bleed valves release air only during starting and transition to flight idle. They are not suitable for use as an "on demand" source of air or power, so the air they release is not put to any use. It's called bleed valve discharge air, and it would be slightly misleading to call it "bleed air". You can call it whatever you want, but you might get a lot of questions about what exactly you are tallking about.
AgentJayZ Good that you have clarified that. It's quite easy to get confused with these terms. Another thing that helped me to understand is in the comments below as you wrote: "bleed valves allow air to escape in order to increase velocity of air through the forward stages of the compressor, reducing the angle of attack of the rotating blades, preventing them from stalling." For me it didn't become clear in the video since I thought bleed valves would also inject air into the stage. Correct me if I'm wrong tho (:
John Doe The compressor compresses air, so everywhere after the first stage it contains compressed air. It increases that compression at each stage as the air travels through it. By definition, compressed air is at a higher pressure than uncompressed ( i.e. ambient) air. One of the basic properties of matter in the universe we live in is that gases will always flow from an area of high pressure to one of low pressure if there is a pathway between the two. So opening a hole in the case of an axial compressor which leads to the outside will allow the compressed air inside to escape.
another great video thank you for the effort just to educate us very selfless on your part and I appreciate it. question for you the bleed air you called customer bleed air for systems on plane is this the same one planes use to start other engines with or is that a different bleed air outlet? looks small for the size of hose starters use but I don't know. Thanks again Jay
I was wondering if you could explain how blow off valves (or BOVs) function? It is my understanding they are operated not just during startup and are activated somehow by a pitot tube measuring airflow velocity and are hydraulically opened/ closed. Is there some sort of control unit in between? Could you clear this up?
Jay, On the Spey bleed valves, are they not also used beyond startup? I thought they were operated through the intermediate percent range and closing up at high power? Fully/partially open at idle? Do they not kinda mirror the function of VSV's? Thanks as always, hope we all get to see the Allison run! That should be a noisy and breezy test cell!
The Spey has bleed valves for both the LPC and the HPC. By the time the engine gets to what might be considered the bottom of its working range of rpms or power settings, all bleed valves are closed. Bleed valves are there to help get the engine started, and also to get the engine from idle to significant power... You are right: there are two ways to avoid stall in a high-performance compressor - bleed valves and variable stators. Variable pitch compressor blades would also work, but nobody has done that yet.
AJZ, Good video about bleed air. In an airliner does the cabin ventilation and pressure come from bleed air? If so, what is the proximate cost in loss of engine efficiency ? The reason I ask, in the early days of jet airliners there seemed to be never a lack of cool air in the cabin but now you get baked until you have reached 10,000 feet or so. I think cabin air is being recirculated to cut down on the use of bleed air off the engines to save fuel. Thanks for you time, John Valos
In airliners, it all comes from the bleed air. As for the A/C, the bleed air will cause parasitic power loss. It all depends on the many factors to determine the amount of loss. One method to help ease the use of bleed air for cabin cooling/heating is the use of the "Recirculation Fan". Larger engines don't experience as much power loss as would smaller engines. If you look at the King Air 350, when you apply prop and anti/deice heating, you get a significant loss of power. The more equipment/systems that demand for bleed air, the more the engine must work to compensate for the loss of air flowing through the engine. If you want to get more in-depth, I'd suggest checking this site out to get all the technical info: www.b737.org.uk/index.htm
Also, in regards to the loss of engine efficiency question, it all depends on the ITT (Inlet Turbine Temp). When you use bleed air, your taking away cooling air for the turbine. The more or higher demand for bleed air, the less cooling air you will have. This will cause higher ITT's. So, to prevent this pilots will lower thrust settings to compensate for the increase in ITT. Most of today's engines have an updated FADEC that can do this automatically. Some, however, must be monitored manually (like the CRJ). So the "loss of power" that an engine will have with the use of bleed air is based off how much cooling air is lost and how much less the engine must operate to not exceed ITT limits.
Here is a forum discussion about bleed air and power loss in turbofan engines. forums.jetcareers.com/threads/turbofan-power-vs-bleed-air-extraction-the-mechanics.86319/
I think you are right, John. You can feel nowadays that the A/C doesn't come on until takeoff, and the engines are really working. As with any business these days, penny pinching is the whole strategy, and any bleeding of air from the compressor of a turbine engine reduces the power output, so for a given power, it costs more fuel. Aircraft designers and engine engineers are fine with that, but it really bothers the accountants, and they control the world, don'tchya know...
I perused the comments to see if this question had already been answered and it appears not to have been. The compressor attached via the shaft from the engine is, I take it, experimental. What is its intended purpose for this project?
We were/are developing an air dyno, or a way to load our shaft output engines. The compressor uses a huge amount of power, the ratio of which you know, because you've watched my Your Questions Answered videos... In order to test our shaft output engines to full power, we need to connect their output shafts to a mechanical load. The compressor shown in this video is being used to load a 4500 Hp engine.
Hi... Not sure if this is a 'dumb question' as you say... but here goes anyway... Is this not a super complicated way of putting a load on a turbine under power. Maybe I'm oversimplifying things here... but could you not just strap a variable pitch prop (via reduction gearbox) to provide the load then increase or decrease the pitch to vary the load?
+Mac Anix That's a lot of machinery, and the prop would be mounted high up in the air. The mount for the prop would need to handle the torque produced by the prop, and the thrust of it. It would not be simpler, and require a lot of setup time.
so your load cell will be exhausting it's hot air directly in front of the test engine... ? the inlet air temperature greatly affects the engine performance of a turbine engine... maybe you are planning some ducting?
A few things to consider, the air exiting the load cell will return to atmospheric pressure, cooling down as it does so. The exhaust air from the load cell will also mix with air from the atmosphere which should keep inlet temperatures barely above ambient, with a slight cooling from the inlet decompression maybe even below ambient.
Terminology is important, or things get confusing. There is no load cell here. The load compressor discharges its air through radial vent ports, so the effect described by x65535x is even more pronounced. But the heating of the inlet air for the working engine is something we will be keeping an eye on.
Upon quick e-search I concur, my usage could use some of my "pedantism." You did mention in the video that there is a load cell measuring the torque on the shaft going into the load compressor. Using wikipedia as a reference to the definition of a load cell.
Amazing video! So that is basically the same concept as an engine dynamometer used to produce a variable and measurable load to test the performance? How is the output tq being measured by the driven compressor? Is this as simple as psi air to percent aperture will equal tq required to produce and how are these numbers determined or is there some type of load sensor directly attached to measure output? Thank you for the great videos!
Thx for this once again! 3500 HP compressor? Hook this up to your engine-lift-thing :-) (Maybe open the roof first...) I would like to wish you a powerfull pre-christmas time! Can't wait for the next vid!
High performance aircraft gas turbine engines run the turbine at temperatures above the melting point of the turbine blade materials. To deal with this air from the compressor is used to cool the turbine blades. Where exactly is this “cool” air taken from, why, how is it pumped through the system? What are the problems with this method?
That happens in the combustion chamber and the air coming in the compressor has high pressure but is not 'hot' so that air can be used to pressurize and for cooling purpose
+Rama Moorthi The cooling air is not taken from anywhere. It's better to describe it by saying of the total air flow coming out of the compressor, only 20% or so is used for combustion of fuel. The way this works is described in several videos I have made. The Combustor Liners series, and also in many of the videos in the Your Questions Answered playlist. There's a sort of index there. Have a look at Your Questions 6 and Jet Questions 51.
The starting of rotation from rest is cause by the starter. It brings the rotor up to a speed where it can supply enough air flow to support combustion and allow the turbine to function as the source of torque to turn the rotor at operational speeds.
Agent... are these "bleed valves" similar to a surge valve on a JT8D? Id like to see a surge valve and how it actuates/commonly fails to modulate closed on a JT8D
ajz - understand the air systems, but im curious what type of alignment is used between the engine and your test/load cell - obviously the turbine will heat up as load is increased so was just curious what alignment considerations were taken while bolting these two units together
The coupling shaft is splined on both ends, allowing for axial movement. The forward compressor is rigidly mounted. The engine has one fixed mount to secure its location, and three floating mounts to allow for axial and radial changes in size.
