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Thread: Jet Engine Operation Confusion

  1. #1
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    Jet Engine Operation Confusion

    I'm having difficulty understanding the operation of a jet engine.
    I've seen multiple explanations, but one thing still confuses me, and I suspect I'm missing an important point.

    For a moment, let's ignore the turbine and imagine the compressor is powered by an external source, such as an electric motor.
    So my simplified engine consists of a compressor, a chamber behind it, and an exhaust.

    The compressor sucks air into the engine and pushes it into the chamber behind it.
    Pressure in the chamber increases from all the air being pushed in by the compressor. I imagine it would reach equilibrium as the amount "leaking" out the exhaust matches that being pushed in by the compressor.

    Now, we add some fuel to the chamber and ignite it. This heats the air, expanding it, and increasing its pressure.
    The pressure in the chamber is now higher than that which the compressor created. Why doesn't this cause the air to "back up" through the compressor, stopping the incoming air?
    I'm imagining the engine sputtering as over-pressure events repeatedly halt the incoming air, rather than the nice smooth operation we see in reality.
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

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    I’m not sure, but I would guess intuitively that the compressor is powerful enough to stop that from happening.
    As above, so below

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    Quote Originally Posted by Jens View Post
    I’m not sure, but I would guess intuitively that the compressor is powerful enough to stop that from happening.
    Hmmm... I had assumed that the pressure created by the compressor was the max it could generate, and any additional pressure would cause a backup, but maybe that isn't the case.

    Perhaps it is something like the following:
    If I plug the exhaust of my simplified engine, the compressor could pressurize the chamber to a maximum pressure, say twice the outside pressure.
    But with the "leak" of the exhaust, maybe the pressure only can get to 1.5 times the outside pressure.
    After combustion, the pressure increases, but perhaps only to 1.75 times the outside pressure. The compressor can handle it, and you still get a jet exhaust?
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

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    That's a failure mode of jet engines, called compressor surge. I think it was a common problem with early jet engines, but I haven't checked.
    In a properly working jet engine, the gas behind the compressor is allowed to expand and slow as it passes into the burner, which allows efficient combustion, and that means the pressure just behind the compressor is the highest pressure anywhere in the engine, while the flow rate at the entrance to the burner is the lowest flow rate in the engine.
    Have a look at the diagram here, in particular the gentle downward slope in the pressure curve between the back of the compressor and the front of the turbine.

    Grant Hutchison

  5. #5
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    Yes even modern engines can surge at high load in, for example, air outside the acceptable working temperature and the compresser temporarily stalls, stopping thrust. My tutor from Rolls Royce many years ago was a terrible passenger because he always thought he could hear the engines about to surge during take off. A rare anecdote about the Concorde test pilot Brian Trubshaw, : I was listening to his commentary as he flew the Olympus engine test airplane , in an always calm manner, “left engine surging”, pause “now right engine surging” and so on. There was no instrumentation to indicate surge so we asked him how he knew. “ Oh I get 7 g sideways” he explained.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    And to answer the OP the compressor is of course a set of aerofoils on a disc, they can stall if the flow is not compatible with the blade speed. That reduces the torque to drive the compressor, to near zero. Then the turbine speeds rapidly up as it loses that torsion load, and the engine is out of balance. It can recover, but it can burst with the extra spin rate. So surge is not good.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    Thanks for the replies, folks. The chart that Grant linked is particularly interesting.
    I may have many faults, but being wrong ain't one of them. - Jimmy Hoffa

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    I'm no expert but I think there's a minimum air velocity through the engine before combustion is allowed to occur, which would direct the combustion pressure rearward out through the turbine and exhaust rather than forward toward the compressor.

    If you read or watch Youtube videos on how to start a jet, they use compressed air from the plane's APU, an already running engine or an external air compressor cart. This air gets the turbine shaft (and compressor) spinning and once it's at a certain minimum speed, the ignition sequence occurs (igniters fire, and once start RPM is reached, fuel is introduced). Once it lights, the RPM increases to idle speed.

  9. #9
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    Yes that is right but the incoming air is first directed by static blades, after a bird cutting set or fan, then enters a rotating set of stages, increasing the pressure but each stage has to do work powered by the shaft from the turbine. That increases as the engine accelerates and the air flow increases but if the compressor blades stall in any stage, usually the first stage, that work collapses so for a very short time the turbine runs faster, making the compressor go faster and probably deepening the stalled condition so it runs away while there is combustion pressure. Soon that collapses allowing the whole rotor to slow down but it could have exceeded the max rate. So, surge ( the rate increase) can cause a blade to break off.
    sicut vis videre esto
    When we realize that patterns don't exist in the universe, they are a template that we hold to the universe to make sense of it, it all makes a lot more sense.
    Originally Posted by Ken G

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    The pressure in the combustion chamber is NOT higher than than what comes out of the compressor. That's where you're going astray.
    Combustion adds energy, but it's in the form of heat and velocity. It then expands out the back, producing thrust. In the case of a compressor stall, the pressure drops and fire blows out the front, which is not a good thing.
    Similarly, the highest pressure in a liquid rocket engine is at the exit of the turbopumps.
    Cum catapultae proscriptae erunt tum soli proscript catapultas habebunt.

  11. #11
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    Expanding gas, like electricity, goes where there's the least resistance. An explosion/expansion in an environment which already has higher pressure on one side and lower pressure on the other side will expand not spherically but in a lopsided shape toward the low-pressure area. A simpler way to visualize it might be in terms of the momentum of individual particles. Before participating in the explosion/expansion, they already had a net momentum toward the back, and that net momentum is conserved after participating in the explosion/expansion, which adds up in one direction but partially cancels out in the other; the whole fireball tends to be moving from the front of the engine to the back just as much as the air was doing so before.

    * * *

    There is a type of engine in which a version of this is not a failure but the normal way it operates. It's called a "pulsejet". There's a physical barrier between the compression area and the combustion area, which cyclically opens to let air in and closes for explosions. That way, not only do the explosions and the compression area not interfere with each other, but the closed front wall of the combustion area also gives the explosion something to push forward against, while the same closed wall, being at the back of the compression area, gives the pressure on that side of it a moment to build up higher than it would otherwise. (In fact I'd expect that the higher pressure in front of the barrier is probably used, in at least some models, to push the barrier mechanism back open again for each cycle.) The principle can be used with air pressurized either by a compressor or by the ram air pressure effect, so a pulsejet can also be a type of ramjet if it's just missing the compressor, but the word "ramjet" is normally used for non-pulsejets with a smooth continuous action. You could also say that, for the point in each cycle when the combustion area is not taking in air, it's briefly working as a rocket (which runs out of oxidizer in a fraction of a second and thus needs to be refueled over & over again), but nobody ever does describe it that way. Similarly, you could also say that the back section of a pulsejet, from the valved barrier back, is essentially a single cylinder from a piston engine, without its piston, but nobody ever does that either. Wow, our laughing smiley here looks terrible.

    I've read that this engine type can still be found out there today, in small cheap things like remote-control toys or disposable drones, but in WWII it was used in the V-1 and gave the V-1 its signature sound because of its ongoing series of separate explosions. Here is a recording. Here are a couple of video samples shot from behind, at which angles you can see the light flashing on & off in the exhaust. (But beware trying to get the engine's cycle rate from that; the apparent flash rate in the video seems to be a result of Moiré interference with the frame rates in the WWII-era camera and our modern playback method. I'm just showing the video to demonstrate that there is an on-off cycle, not necessarily how fast or slow it really works.)

    The pitch in the first recording seems lower to me than most propellers, but also seems to have a bit of a metallic effect to the sound, like sheet metal being warped... or that could be an artifact of the recording. I wouldn't know that it's not one or more propellers myself if I weren't being told that, but, back then, more people were more used to hearing propellers and identifying which kind of engine/plane made which kind of propeller sound. And in their contemporary experience, this thing sounded distinct enough to be easy to casually identify, resulting in nicknames like "buzzbomb". I once read somewhere that the rough up-&-down sound of Subulba's engine pods in the pod race scene in the Star Wars prequel when Anakin is a kid was based on or inspired by the V-1 sound in some way, although its cycle is audibly much slower and the type of sound it makes is less explosiony and more grindy & mechanical, more like an idling piston engine.

    * * *

    A new discovery for me in my own second video link: When it cuts away from the rear view of the V-1 and shows a couple of people with binoculars, the color of their clothing in the belly area is so light that I thought at first their bellies were bare! Then one turned around and showed that the whole back was clothed in a dark color, so I figure there was a large light-colored patch built in on the front of the uniform, which would have told co-workers something about the wearer's job/role from a greater distance than shoulder & collar insignias could, like the color-coding of deck crew uniforms on modern ships (especially aircraft carriers).
    Last edited by Delvo; 2021-Feb-28 at 05:24 PM.

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    The ram-rocket interests me...(air augmented rocket)

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