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  • #31
    Yes it was. Sarcasm.

    I was interested in all this so I did do a bit of further looking into it.

    There are a few very small turboprops (UAV) in development with recuperators. They were part of the original design which makes them simpler. But the science gets more complicated. The small engines are not much more than a turbocharger plus combustion changer and gearbox. Single stage centrifugal compressor, single stage turbine. The temperature of the compressor outlet will not be that high, so transfering heat into it is easier (Delta T, or difference in temperature).

    The bigger the engine, usually the compressor and turbine have many more stages, and higher pressure ratios, usually above 10. Same as the compression ratio of an ICE. The compressor outlet temperature is really high. Many hundreds of degrees. I can't remember how many degrees, and I don't think I ever flew a plane with a compressor outlet temp gauge. I want to say 500 or 600C, but that is a guess from memory.

    So a single stage engine recuperator is transferring heat from an exhaust of 800 C to a compressor inlet temp of 100C or so. A comparatively smaller recuperator works, and aids a lot in efficiency. Their claim was up to 100%

    An 8 stage engine might have the same 800 C EGT, but the compressor outlet temp is much hotter. Again, the 500C was a guess, from an old memory. It would take a much bigger recuperator to transfer heat, because the Delta T is much closer. The bigger recuperator is going to have more pumping losses, etc. And a lot more weight.

    The single stage 50 hp UAV engine was said to weigh a bit more than the ICE wankel engine it replaced, but burned very slightly less fuel.

    Big aircraft use "bleed air" for a bunch of things, including pressurization and deicing. The last engine I flew had low pressure from the 4th stage, and high pressure from the 9th stage of the compressor. The air is so hot, if you get a "duct leak" caution, or on other aircraft a "bleed air leak", It is considered a fire. It will melt aluminum. It did on airplanes I was flying. Twice.

    The larger, multi stage engines are much more efficient to start with than the single stage centrifugal designs. Extracting extra efficiency gets heavier and more expensive.

    It appears to be technology that works already in power plants, where weight is not an issue. Turbine power plant engines are basically the same as large transport jets, minus the fan. Companies that make airplane jets, sell power turbines as well.. If they thought they could adapt it to aircraft, they already have the expertise to do it.

    I did not see any large recuperated engines being advertised as in development.

    Comment


    • TurbAero
      TurbAero commented
      Editing a comment
      You’ve done some excellent research there svyolo and I can see that your understanding of the technology is better than earlier in the discussion. The larger Turboprop engines such as the PT6 can achieve a specific fuel consumption of around 0.65 lbs/hp/hr and Mr. Pratt and Whitney probably feel that this is an acceptable SFC for the engine. A recuperator would improve the fuel efficiency but would involve a huge development cost, a huge certification cost, a size increase and a weight increase. Why would they do it...

      We on the other hand have started with a clean sheet design that has a requirement for a SFC < 0.55 lbs/hp/hr to be competitive in the market (IO360 SFC is around 0.45 lbs/hp/hr at 150hp). To achieve this SFC for our turbine engine, a recuperator is a necessity and we firmly believe that we have the correct solution to achieve our targets.

  • #32
    Originally posted by Battson View Post
    Unless I am mistaken, an aero-turbine recuperator heats the inlet air stream after it's passed through the compression stages and before it reaches the combustion chamber. In which case, such a heat transfer device is bound to restrict the mass flow or cause a considerable pressure loss which reduces power output and power to weight ratio, in favour of fuel efficiency comparable to an ICE.

    You have a good understanding of the dynamics associated with a recuperator. Minimising the pressure losses both on the hot air side and the cold air side are very important and the design of a recuperator must focus closely on achieving minimal pressure losses. High pressure losses will definitely affect the overall efficiency of the engine cycle.

    Dave

    Comment


    • #33
      Dave, I flew 9 jet "types" in the last 30 years, and one turboprop for about 90 hours 30 years ago. I never designed or built one. Each one I had to memorize quite a bit. Turbines are simple in concept, massively complex in execution. A small team of very bright guys can build a decent piston engine. A small team of very bright guys can build a decent 1970's era turbine. Too much metallurgy, machining, CFD.

      A heat exchanger is a simple concept. Small jets have a couple, big ones a bunch. Fuel/hydraulic, fuel/oil, air/air. For the usual reasons. The recuperator threw me at first, because I couldn't see you could efficiently add heat to already hot pre-combustion, but compressed air. it is too hot already. So I actually thought you meant to heat the intake air.

      Looking more into it, no, a recuperator heats already hot, compressed air.

      P&W just spent billions and 20 years on a new engine called a "Geared Turbofan." With a claimed efficiency gain of 15%. So far, they have pulled it off. It, and the competing non-geared GE product have a couple of very unpleasant and uneconomic side effects. But they do burn less fuel. Neither are meeting their BSFC specs. P&W is closer on this.

      Neither do a recuperator. GE bought Walther which is an eastern European company that made turbines. They took the 601 and improved it massively. 20% improvement in efficiency. It has 3 axial compressor stages, and 1 centrifugal. Pressure ratio is around 9. No recuperator.

      These companies would sell their mothers for another 8% efficiency. A recuperator is fairly simple, although at the temperatures we are talking, the metalurgy is complex.

      I am listening. Tell me more. The pictures of your engines show an axial flow engine. How many stages?

      Comment


      • TurbAero
        TurbAero commented
        Editing a comment
        Unfortunately, for commercial reasons I cannot go into too much detail about design specifics. Once our engine is running and being demonstrated, we shall release more details, just not yet.

    • #34
      Reading more about this fascinating subject, I ran across this;



      Bill

      Comment


      • #35
        One of the things I read the last couple of days was "turbines don't scale down well". The TP100 they are running in an RV-10, and a few others, only claim .9 BSFC for their engine. A couple of sizes up, GE and RR I believe are claiming .55- .7 for their 500-1500hp turboprops. Really large turbines (777 size) have over a dozen compressor stages. The new GE ATP (improved Walther 601) has 3 axial and 1 centrifugal. The bigger engines might have 15-1 pressure ration, The GE and RR are around 8-9.

        Looking at the PBS web sight for their TP100 engine, they don't show a diagram of the internals. There are a few pictures of the outside. The first compressor stage is axial, so they probably have more than 1 stage. Probably less than the RR and GE engines.

        The PBS TP100 engine was developed over 4 years with a 45 million dollar grant from the EU. Over a dozen countries were involved. The engine is being built by a company that has made turbines for decades. They are at .9, and the RV-10 it is running in verifies that. It is burning twice what a 540 powered 10 burns at normal cruising speed. Knocking 30% off of this, recuperated, gets to .63. You would still be burning 17 gph vs 12.

        Still a hard sell for me, unless I really really couldn't get gasoline. The US military runs on JP-8, and that is what they are set up for, logistically. Transporting a small amount of gasoline is very expensive. They have a big push on for small turbines for UAV's. There is hope on the horizon. But isn't that always the case?

        Comment


        • TurbAero
          TurbAero commented
          Editing a comment
          Scaling down is very challenging. Compressor and turbine tip clearance limitations result in increased losses, the smaller those components get.

          The TP100 is definitely not an efficient engine. Even without a recuperator, its efficiency should be way better than it is.

      • #36
        This really did get my interest, but I am not sure it will work well in an airplane. Turbines engines, other than turbofans, run very hot already. I am talking about the outside of the engine. The hottest part is the combustion chamber, and turbine section. The "hot section". Those parts are relatively small. The exhaust is also pretty hot. So hot, the vast majority of exhaust pipes are placed outside the cowling in both turboprops and turboshafts. Designers and pilots hate drag. They would only do that if putting it inside was worse, or less safe. Or both.

        So a recuperator is a heat exchanger swapping heat between very hot (several hundred degrees) and the exhaust (several times higher). It will have lots of surface area. Now you have a very hot and very large surface area, under the cowl. Much larger than the exhaust pipes. There is no way you can keep that heat in, and that includes radiant heat. You would have to have a heat shield around the whole thing, and cool it with air. Now the front of your turboprop airplane starts to look like the front of a radial engined aircraft.

        The other thing is this thing needs to be made out of the same expensive and exotic stuff as the hot section, making the recuperator extremely expensive. This isn't "just an air to air heat exchanger". Cheap ducting won't work.

        This all might be easier to install in a helicopter. You could leave the whole recuperator uncowled. Installing it in an airplane might be very difficult, expensive, and heavy. I think I am back to being skeptical. It might work, but might not work very well installed in an airplane.

        Comment


        • Mark Goldberg
          Mark Goldberg commented
          Editing a comment
          John, your thoughts on this are based on a lot more information and experience that I have. But I am rooting for the manufacturer to get a good, reliable engine from all their research. I hear from a lot of people in other countries who really want alternative power plants that do not run on avgas. So it is either diesel or a turbine that would make a big improvement in how they fly and fuel their planes. I hope Dave is successful in their effort.

          But to be sure - I am not buying one until it is quite proven from thousands of hours in the field. Mark

        • TurbAero
          TurbAero commented
          Editing a comment
          The recuperator will be insulated to keep the heat in. Any heat losses to the atmosphere is heat that cannot be transferred to the compressor outlet air.

          Our engine, with recuperator will have significantly less frontal area than a LyConti. It certainly will not resemble a radial installation in size.

          In relation to materials, it could either be something such as stainless steel which would be (acceptably) heavy, (acceptably) bulky and cheap, or an "exotic" material which is actually very light, very small but expensive (acceptably so though). Our decision is whether to make it $'000s cheaper but bulky/heavy, or $'000s more expensive but light and small. Either way, the size and weight of the recuperator are acceptable. You will be able to learn our decision on which we will use in around 10 months when we plan to have the engine on the test stand.

          Mark, I'll pencil you in for delivery slot #500 shall I? :-)
          Last edited by TurbAero; 03-06-2019, 11:18 PM.

        • James
          James commented
          Editing a comment
          I posted this already above, but as usual, Mark's got his finger on the pulse of how things work outside of the US. The incentives to find alternatives really ramp up as soon as you go from a country that measures fuel in gallons to anywhere in the world that measures fuel in litres!

          James

          -----------

          Yeah Dave - I don't know much about GA aircraft, or experimental engine design, but I do know that I want to keep flying my bearhawk 10 - 20 years from now, when Avgas is no longer available in Australia. Whether it runs on diesel or JET-A1, is a piston engine or a gas-turbine, any aircraft engine that can burn a range of heavier fuels will be a winner going forward. At the very least, think about the USDF "single fuel" policy, and the future market for mass-produced light engines for drone aircraft etc, all CAD-designed and 3-D printed. These last two factors alone completely change the market dynamics and the economy of scale problems that have plagued every other engine design that has gone before.

          This is the promise - not sure if Dave's engine will be the answer or not, but at the very least I'll reserve judgement and give the benefit of the doubt to those people out there giving it a go. The deposits are in a holding account, so it's not like they're based in the Cayman Islands or something (you're not, are you?)

          Good on you mate

          James

      • #37
        All this reminds me of what a college professor told our aerodynamics class way back in the early 70's... He used the latest in aerodynamic science to prove conclusively that an object of a certain size and mass, with wings of a certain size and shape could not produce enough lift with its wings to support its weight in flight. This took nearly an entire class period, and at the very end, he explained that the measurements he had used came from a bumblebee, just as he began showing us a video of a bumblebee in sustained flight, buzzing all over the place.

        Then he turned to the class, and said "The lesson here is that we don't know everything there is to know, and that there is plenty of opportunity for improving the science to better model reality. Don't get so full of yourselves as scientists and engineers that you close the door to exploring the unchartered boundaries of the science, where true breakthroughs occur." It was a really good object lesson for us then, and one that we can apply in this case.

        My "engineering" background leads me to believe that Svyolo is almost certainly correct about the application of what we know today to the recuperator turbine engine. But in the back of my mind, I'm truly hoping that TurbAero and company are able to pull off a "bumblebee" of their own! Like Mark G, I'll be watching and hoping for their success – for a host of reasons, but I'm also not expecting for it to happen in time for me to replace the good ol' Lycosaurus engine I'm planning to use in my Patrol...

        And, for the record, as one who saved up money from my paper route for over a year to put up an unsecured deposit on a BD-5 kit (with no clue how I would ever have the money to pay for the rest of the kit...), and who never saw any product or any of that money returned, I applaud the fact that TurbAero is having potential buyers use an independent escrow company to hold the money, and that the agreement includes the ability for the depositor to withdraw their deposit at any time, for any reason without penalty. As he said, it's a good way to prove to investors that there IS a market for the product, without overly impacting the potential buyers if things go south for any reason. I've taken bigger risks with Indigogo products than these deposits would seem to represent.

        Good luck, TurbAero. I hope you prove all of us doubters wrong!

        [Edit: Just re-read the next-to last paragraph, and realized that might be the longest, most convoluted sentence ever written... Oh well, it would take way too much time to figure out how to re-word it...]
        Last edited by JimParker256; 03-06-2019, 03:52 PM.
        Jim Parker
        Farmersville, TX (NE of Dallas)
        RANS S-6ES (E-LSA) with Rotax 912ULS (100 HP)

        Comment


        • Bdflies
          Bdflies commented
          Editing a comment
          You re-read BEFORE posting? I gotta remember that! 😎

          Bill

        • TurbAero
          TurbAero commented
          Editing a comment
          Thanks for your encouragement Jim.

          We have some serious design power on our team, engineers with many years of relevant experience and a track record of delivering. It is not an easy path that we are travelling. The thing is, I am the customer as well as the manufacturer. I have my own aircraft that I want to put this engine in. My motivations aren't just in building a successful company. I want these engines for myself!

          Dave

      • #38
        There is no doubt that we need to move on from the LYContosaurus. The question is which way? A small turbine seems a good option and it has the advantage that they (or the core, at least) can be used for other applications. This solves one problem which is the potential lack of numbers to justify the R and D. A modern flat 4/6 with single lever EFII that runs at 2700rpm (and so avoids a reduction gear box) and runs on either 93 octane ethanol laced fuel or diesel would also work.

        My bet on the most successful new generation engine would be a hybrid. Electric motor, LiFePo batteries and a small diesel generator. Perhaps solar panels as well. I don't think we are far off getting the weight of the technology down to the levels needed to make it practical.

        Comment


        • JimParker256
          JimParker256 commented
          Editing a comment
          I keep hoping that a diesel-powered turbine could be optimized as a "range extender" for an electric car/plane/boat/whatever. An electric motor with sufficient battery capacity for acceleration/climb power, and a small turbine-powered generator to keep the batteries charged up to a "reasonable" level. Turning at a constant RPM with a reasonably constant load should make it pretty easy to optimize the system. (But then, if it were really this easy, we probably would already have a bunch of cars using that technology...

        • PaulSA
          PaulSA commented
          Editing a comment
          100%. There are already a couple of cars going that way and at least one aircraft under testing.

      • #39
        I hope they are successful as well. I learned a lot about small turbines the past few days of researching them. That new knowledge did not make me more hopeful for a small turbine in my future. It will take some big breakthrough to make them efficient enough for most of us. Maybe the recuperator is it.

        The little 40hp UAV engine with the recuperator was pretty slick. If I can find the link again, I will post it.

        Comment


        • TurbAero
          TurbAero commented
          Editing a comment
          Thanks svyolo. Cautious optimism/pessimism will make someone a better pilot, worker, friend or whatever. Scepticism and questioning the unknown or little known or understood is part of that process and is a healthy quality.

          This development program and the technology involved is extremely complex and is a very specialised area. It is hard to find much public information about the technology we are incorporating as some of the breakthroughs are very recent and are protected by their developers.

          The 50hp UAV engine is a proven example that this technology can work. We intend for our engine to be better than that one.

          Please give us time and we will get there.

          Dave
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