Skip to main content

View Diary: How Airliners Work - Propulsion (191 comments)

Comment Preferences

  •  Can an electric motor replace the turbine? (3+ / 0-)
    Recommended by:
    twigg, Simplify, Orinoco

    I've asked around about this before and have gotten a range of answers from "Impossible" to "Unlikely" to "Maybe in the distant future" to "Hmm, interesting idea, never heard of it, sorry, I don't know", so I'll ask again.

    Given that most of the thrust in a modern high-bypass turbofan comes from the fan, not the turbine, which just rotates the fan, in theory there's no reason why an electric motor can't rotate the fan instead of a jet turbine, provided you can make one that's light, powerful and strong enough to replace the turbine, and batteries that are similarly light, powerful and long-lasting enough to take you far enough to make it worthwhile.

    Is there any reason this can't work, other than perhaps the lack of batteries that are light, powerful, safe and long-lasting enough to provide the kind of power needed for such a use right now? But if such batteries existed, and I'm guessing that someday they will, is there any reason this couldn't work?

    Why would one want to do this? Well, aside from how such engines would be quieter than even the quietest turbofan engines, they'd be mechanically simpler and thus cheaper, which combined with the much lower temperatures they'd operate at, would make them more reliable and longer-lasting, further reducing their operating costs. But most of all, you wouldn't have to carry and burn aviation fuel anymore, which is becoming scarcer and more expensive, and is a greenhouse gas pollutant (and dangerous in a crash).

    I believe that this has been done at the model level, successfully. I'm just wondering if anyone's looking at it at the full scale level, if not yet in large commercial airliners, then at least in civilian aircraft, even experimental ones. I'm sure the military is researching this, especially for drones.

    "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

    by kovie on Tue Apr 02, 2013 at 06:16:41 AM PDT

    •  Model Electric aircraft (4+ / 0-)
      Recommended by:
      elfling, Orinoco, basquebob, Bisbonian

      fly extremely well, and very fast ... but not for very long.

      As you said ... it's the energy storage that is the issue. Those batteries are very heavy.

      I hope that the quality of debate will improve,
      but I fear we will remain Democrats.

      Who is twigg?

      by twigg on Tue Apr 02, 2013 at 06:21:37 AM PDT

      [ Parent ]

      •  That's likely to change (2+ / 0-)
        Recommended by:
        twigg, Orinoco

        If and when it does, is there any other reason this won't work? It seems like the logical future of aviation for all sorts of reasons, most of them green.

        "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

        by kovie on Tue Apr 02, 2013 at 06:25:19 AM PDT

        [ Parent ]

        •  Ah, but with liquid fuel, you only have to carry (8+ / 0-)

          it halfway to where you're going.

          With the battery, you have to haul the weight all the way, so you land just as heavy as when you took off.

          So the energy density of the battery needs to be higher effectively than that of the fuel (accounting for efficiency).

          is there a mechanical engineer in the house?  

          Happy little moron, Lucky little man.
          I wish I was a moron, MY GOD, Perhaps I am!
          —Spike Milligan

          by polecat on Tue Apr 02, 2013 at 06:37:27 AM PDT

          [ Parent ]

          •  It's a differential calculation (3+ / 0-)
            Recommended by:
            polecat, TheDuckManCometh, basquebob

            and High School calculus was a long time ago.

            You have to carry some of the fuel all the way, some of it a fraction, etc ... as opposed to batteries being carried all the way, all the time.

            So the shorter the flight, the smaller the differential and that means that "long-haul" electric flying is much further off than short-haul.

            By the way, I love your sig and I have another favourite from Sopike Milligan:

            I thought I saw Jesus, on a tram.
            I said, "Are you Jesus"?
            He said, "Yes, I am".

            I hope that the quality of debate will improve,
            but I fear we will remain Democrats.

            Who is twigg?

            by twigg on Tue Apr 02, 2013 at 06:47:32 AM PDT

            [ Parent ]

            •  This diary is about simplifying math... :) /nt (1+ / 0-)
              Recommended by:
              twigg

              Happy little moron, Lucky little man.
              I wish I was a moron, MY GOD, Perhaps I am!
              —Spike Milligan

              by polecat on Tue Apr 02, 2013 at 07:28:51 AM PDT

              [ Parent ]

              •  We already hit (1+ / 0-)
                Recommended by:
                polecat

                the outer limits of my knowledge ... so I won't be complicating it further :)

                Even the "simple" math though does explain why it will be a long time before we get long haul electric aircraft :)

                I hope that the quality of debate will improve,
                but I fear we will remain Democrats.

                Who is twigg?

                by twigg on Tue Apr 02, 2013 at 07:31:01 AM PDT

                [ Parent ]

                •  Fuel-cell electric might be interesting, but (1+ / 0-)
                  Recommended by:
                  twigg

                  turbines are really efficient and reliable, not to mention cheaper than fuel-cells.  And I don't see someone putting a Stirling engine in a plane anytime soon.

                  Happy little moron, Lucky little man.
                  I wish I was a moron, MY GOD, Perhaps I am!
                  —Spike Milligan

                  by polecat on Tue Apr 02, 2013 at 07:39:41 AM PDT

                  [ Parent ]

            •  I'm not into perpetual motion, but how about (1+ / 0-)
              Recommended by:
              twigg

              an unpowered fan, a turbocharger if you will, to drive a generator to help with a top off charge to extend range, and potentially reduce battery weight?

              Republicans are like alligators. All mouth and no ears.

              by Ohiodem1 on Tue Apr 02, 2013 at 08:16:15 AM PDT

              [ Parent ]

              •  Well that is pretty much (2+ / 0-)
                Recommended by:
                Ohiodem1, polecat

                the theory behind some cars.

                I can't help but think that aircraft engineers have already thought of it.

                I imagine that a generator large enough to make a significant difference would be large, heavy and require quite a bit of fuel. They can't, for example, recover energy under braking like a car can, and overcoming the rolling resistance from a heavy car is completely different to "lift".

                So I imagine the "perpetual motion" thing applies, as does gravity and stuff :)

                I hope that the quality of debate will improve,
                but I fear we will remain Democrats.

                Who is twigg?

                by twigg on Tue Apr 02, 2013 at 08:21:45 AM PDT

                [ Parent ]

        •  About 20 minutes of flying (2+ / 0-)
          Recommended by:
          lazybum, Orinoco

          is what I could expect from an electric aircraft, compared with maybe two hours from nitro weighing about the same, so there is much progress to be made before they are comparable.

          20 minutes is quite long enough for a hobby, but it wouldn't get me from Tulsa to Dallas at full scale.

          I agree on green reasons. Even electricity generated on a large scale, from fossil fuels, is greener than trying to do it in individual engines. When we can generate power from renewables on a large scale then we will really be in business.

          I don't know quite how green cars like the Volt really are, but they are quite heavy so maybe that's a blind alley for aircraft and we need something better. Of course when we get it, cars will benefit too.

          I hope that the quality of debate will improve,
          but I fear we will remain Democrats.

          Who is twigg?

          by twigg on Tue Apr 02, 2013 at 06:42:31 AM PDT

          [ Parent ]

        •  Engine weight (1+ / 0-)
          Recommended by:
          lazybum

          you are replacing one or two turbine blades with an electric motor, which has heavy coils of wire and permanent magnets, or more heavy coils of wire, plus magnet cores.

          Electric motors might be an ideal solution for powering zeppelins, where the high heat of a turbine engine would be a problem.

          "The problems of incompetent, corrupt, corporatist government are incompetence, corruption and corporatism, not government." Jerome a Paris

          by Orinoco on Tue Apr 02, 2013 at 11:15:52 AM PDT

          [ Parent ]

    •  For Airliner Electric Propulsion ... (4+ / 0-)
      Recommended by:
      elfling, badger, basquebob, lazybum

      right now you would need better batteries or a long extension cord.

      Jet fuel has an energy content of about 43 Megajoules per kilogram or 12 kilowatt-hours per kilogram. That will not change.

      Lithium-ion batteries might contain 0.9 megajoules per kilogram or 0.25 kilowatt-hours per kilogram. That is 100 times less energy by weight. However, that number will continually change as technology advances.

      Hydrogen fuel cells are another high-tech approach, but the weight problem is even more severe. It would be very "green" since the chemical "exhaust" is only water vapor.

      On the other hand, hydrogen itself is a relatively expensive fuel. However, hydrogen can be extracted from methane, which is readily available. The problem then is that you have carbon dioxide as an "exhaust" product in addition to the water vapor.

      TANSTAAFL, I'm afraid.

      "Facts are meaningless. You could use facts to prove anything even remotely true." -- H. Simpson

      by midnight lurker on Tue Apr 02, 2013 at 07:05:01 AM PDT

      [ Parent ]

      •  No one's yet answered my core question (1+ / 0-)
        Recommended by:
        KenBee

        Of whether, obviously important batteries issues aside, there's any reason an electric motor can't replace a jet fuel-burning turbine in a ducted fan engine, in terms of power, weight, reliability, etc.?

        I.e. if the battery issue could be solved--a big if--would this work?

        "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

        by kovie on Tue Apr 02, 2013 at 07:09:34 AM PDT

        [ Parent ]

        •  An electric motor certainly can do that -- (2+ / 0-)
          Recommended by:
          kovie, elfling

          but there are efficiency issues.  There is a lot of torque with an electric motor at LOW speeds, not high speeds, so either a torque converter or gearbox would be necessary to get the higher speed airflows you want.

          There are model planes that use EDF's (Electronic Ducted Fans), and for that matter submersibles.

          Energy storage/density is THE big issue.  Sure, electric motors are more efficient than turbines, but the energy density of liquid fuels blows that advantage away.  There will have to be a lot of improvement in supercaps (capacitors) because we're pretty close to the end-of-the-line in battery energy density.  Look at the periodic table of elements to see what the best possible chemistry would be (assuming it didn't go boom): KF (potassium fluoride) would give the biggest electronegativity difference (Francium would do better, but that's radioactive).  But messing around with fluorine carries it's own risks.  Or maybe an air-breathing battery (replacing the Fl with atmospheric O2) would be viable.

          And you have to haul the battery the entire distance.  With fuel, you get to use it up on your route, reducing weight and consumption throughout the flight.

          We've got a looooong way to go to make that viable.

          Burning hydrogen has risks of its own, too.

          Happy little moron, Lucky little man.
          I wish I was a moron, MY GOD, Perhaps I am!
          —Spike Milligan

          by polecat on Tue Apr 02, 2013 at 07:25:53 AM PDT

          [ Parent ]

          •  Ok I'm definitely a layperson here (0+ / 0-)

            But are there "battery" technologies that involve the "burning" of fuel, but at relatively low temperatures and with lower emissions than with turbines? I.e. similar to the way the body burns energy during metabolism. This way you could cleanly "burn off" much of the fuel during flight.

            "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

            by kovie on Tue Apr 02, 2013 at 07:32:47 AM PDT

            [ Parent ]

            •  After we figure out how to transmit electrical (0+ / 0-)

              energy wirelessly from space-borne solar power arrays to nice, big, fixed-location earth stations, someone will figure out how to safely aim the power transmitter to a fast moving object.

              See several James Bond villains for examples.

              We must drive the special interests out of politics.… There can be no effective control of corporations while their political activity remains. To put an end to it will neither be a short not an easy task, but it can be done. -- Teddy Roosevelt

              by NoMoJoe on Tue Apr 02, 2013 at 09:03:53 AM PDT

              [ Parent ]

            •  Fuel Cells Do This, in a Sense (3+ / 0-)
              Recommended by:
              polecat, TheDuckManCometh, Simplify

              Hydrogen or reformed methane (or reformed methanol) undergoes a chemical reaction in device in which electrons are trapped and supplied to an external circuit.

              The operating temperatures can be much lower than combustion temperatures, but generally they still require cooling. Then there are still the questions of system weight and fuel storage.

              "Facts are meaningless. You could use facts to prove anything even remotely true." -- H. Simpson

              by midnight lurker on Tue Apr 02, 2013 at 09:35:00 AM PDT

              [ Parent ]

            •  Turbines are remarkably efficient (1+ / 0-)
              Recommended by:
              lazybum

              (even better for power generation because they can run at their most optimal speed) and that has a lot to do with the high operating temperatures.  In fact with actual burning methods, the Carnot cycle's efficiency is defined by the temperature difference between the coldest and hottest point in the system.  At one time it was thought that Ceramic engines would take over because they can get so much hotter than Iron.  Cost and tensile strength are bad, though.

              Turbines have problems -- typically the wear and tear (and cost!!!) of the blades.

              Fuel Cells have problems -- clogging of the ion exchange filter, wear and tear, exhaust gasses, etc.

              Happy little moron, Lucky little man.
              I wish I was a moron, MY GOD, Perhaps I am!
              —Spike Milligan

              by polecat on Tue Apr 02, 2013 at 11:16:31 AM PDT

              [ Parent ]

        •  It's "Not something you would want to do" (0+ / 0-)

          If you were going to go to the trouble of using electric motors to fly, you would drive propellers to do it. For weight, efficiency, and simplicity, props win hands down.

          There are a few 'ducted propeller' aircraft out there, but there are few advantages to the duct - noise reduction being the major one, and not much of a factor at that. And there are substantial downsides -- particularly weight, but also efficiency -- they add a lot of drag.

          For jets, the duct is necessary, because the pressure ratio of the engine -- the ratio of pressure between the engine inlet and the combustion chamber entry -- determines the maximum possible engine efficiency (it limits the maximum possible thermodynamic efficiency, exactly the same way that compression ratio determines both engine efficiency and octane requirements in cars). Large, modern turbines run on the order of 40:1, and require great care in the construction of the engine.

          "Unducted turbofans" have been built, and are in fact extremely efficient. In these engines, the outer/"low pressure" turbine blades have no duct, and are exposed to the outside world. The high-pressure compressor/gas generator contains the turbine that drives all the rest (and has a fairly conventional design).

          The downside of unducted turbofans is that the 'fan blades' run at supersonic velocities, and they are loud. Even with scimitar shaped blades and mismatched numbers of blades between fans (both of which help reduce noise), they are impracticably loud. Further, loss of a single blade will cause the engine to shake itself to pieces, so at least one such engine had explosive charges at the root of each fan blade so that if one blade was lost, its partner would be promptly jettisoned -- away from the aircraft.

      •  Erratum (1+ / 0-)
        Recommended by:
        Simplify
        Lithium-ion batteries might contain 0.9 megajoules per kilogram or 0.25 kilowatt-hours per kilogram. That is 100 times less energy by weight.
        It is reall only 50 times less energy by weight than jet fuel. Makes it much easier, then.

        "Facts are meaningless. You could use facts to prove anything even remotely true." -- H. Simpson

        by midnight lurker on Tue Apr 02, 2013 at 09:38:21 AM PDT

        [ Parent ]

        •  Even 2X is daunting (0+ / 0-)

          Every kilo of energy storage means more airframe weight to carry it, more energy expended to move it, and then a bit more energy storage yet again...

          Look at all the trouble Boeing is going through just to put a slightly lighter battery in the 787, and that's not even for powering the engines.

          A 747 or A380 churns through the power equivalent to something like a quarter of a Boston-sized city. That's chemical energy converted directly to mechanical. Think of the electrical energy storage it would take to do that.

          Of course, we could fly slower...

          Government and laws are the agreement we all make to secure everyone's freedom.

          by Simplify on Tue Apr 02, 2013 at 11:19:55 AM PDT

          [ Parent ]

          •  You just reminded me (1+ / 0-)
            Recommended by:
            Simplify

            of an article I read about the possibility of small dirigibles powered by solar and propelled by electric motors. Perhaps that's a more feasible application where speed isn't as important, like freight, weather monitoring or, er, drones.

            "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

            by kovie on Tue Apr 02, 2013 at 03:57:53 PM PDT

            [ Parent ]

    •  There are some light aircraft (4+ / 0-)
      Recommended by:
      Ice Blue, KenBee, kovie, BlackSheep1

      now flying with electric motors.

      I don't know if and when they'll be able to scale up the technology to something the size of an airliner.

      It raise some interesting problems. We currently land at a much lighter weight than at takeoff because of all the fuel we burn off.

      A battery powered plane would have the same landing weight as takeoff weight.

      If the pilot's good, see, I mean if he's reeeally sharp, he can barrel that baby in so low... oh you oughta see it sometime. It's a sight. A big plane like a '52... varrrooom! Its jet exhaust... frying chickens in the barnyard!

      by Major Kong on Tue Apr 02, 2013 at 10:43:47 AM PDT

      [ Parent ]

      •  Unless a light, affordable and efficient (0+ / 0-)

        means of generating enough electricity through the consumption of some sort of fuel could be found that changed that, perhaps working in concert with more conventional batteries and solar. I'm talking way out of my league scanning the cover of the latest Popular Mechanics speculation here.

        Incidentally, I was just in a Radio Shack and saw that they had this battery-powered RC plane for around $40 that used these small ducted fans for power. So obviously this works, only on a very, very small scale.

        I just think that with fossil fuels becoming scarcer (and eventually completely depleted) and bad for the environment no matter how clean we make them, and synthetic fuels being expensive and also bad for the environment, even if less so, we're going to need a whole new means of propulsion eventually, and likely sooner than we want to believe.

        "Liberty without virtue would be no blessing to us" - Benjamin Rush, 1777

        by kovie on Tue Apr 02, 2013 at 04:03:35 PM PDT

        [ Parent ]

      •  so in a future diary could you (1+ / 0-)
        Recommended by:
        RiveroftheWest

        compare / contrast JP-4/8 and, say, diesel? (Field expedient BUFF-grade diesel, please -- I'm pretty sure recycled french fry grease would not meet the need).

        LBJ, Lady Bird, Van Cliburn, Ike, Ann Richards, Barbara Jordan, Molly Ivins, Sully Sullenburger, Drew Brees: Texas is NO Bush League!

        by BlackSheep1 on Tue Apr 02, 2013 at 08:49:02 PM PDT

        [ Parent ]

        •  It's a lot like kerosene. (0+ / 0-)

          The relevant performance metric is 'cetane' -- the energy released by burning it. The engines are not terribly picky about what they burn, as long as it generates a lot of heat.

          What does matter a lot is that it stays liquid at all the temperatures its used at, and it gets very cold at 40,000'.

Subscribe or Donate to support Daily Kos.

Click here for the mobile view of the site