I'd be interested in learning more about this. How does a turbofan engine increase power without increasing fuel flow?
New valve technology on the horizon
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Power output. Your previous GE115 comments give the impression you have more than a casual understanding of the topic so I dont know how much more I can add. Big motors are not my thing. But being its Sat nite and I just opened my 1st beer I'll humor you a bit.
Turboshaft engines are similar to turbojet engines in that power output is tied directly to fuel flow. The turboshaft shown by RPM/torque into a gearbox and the turbojet by thrust. In general, a turbofan is a turbojet with a ducted fan bolted on the front which will increase the amount of thrust at the same fuel flow, i.e., increases the power output without increasing fuel flow.
Bell206, thanks for indulging me. I'd buy the next round, if we happened to be co-located.
If I understood you correctly, when you said that a turbofan increased power without increasing fuel flow, you meant that the mechanism for achieving this power increase is the fan stage bolted (so to speak) to the engine core?
If I understood you correctly, when you said that a turbofan increased power without increasing fuel flow, you meant that the mechanism for achieving this power increase is the fan stage bolted (so to speak) to the engine core?
Power output.... Yes. A good example of this is to look at the history of the 737 and how its range has increased. What once was a "regional" aircraft now crosses the pond on 8+ hour legs. All due to fan technology increasing the thrust (power output) on the same or actually lower fuel flow. A 7-hour route I fly every year was usually on a 757 or 767. Now its a 737 MAX 9.
Unfortunately, I'm only in CA when under duress. Nothing personal.
It would be interesting to dig into the design and operational data of the P&W JT3C and JT3D, and the GE CJ610 and CF700 engines. Those four engines represent the evolutionary leap from straight turbojet to turbofan. While I would not attribute the range of the MAX9 entirely to engine technology, clearly the engines play a huge role in transforming a little puddle jumper to a...jumper of very, very large puddles. But what I'm interested in is how you view ceramic hot section components and their potential to increase turbine engine efficiency. I had mentioned that advanced ceramics technology could make it possible to produce small, efficient, and relatively affordable turbines. Your point, if I understood correctly, is that ceramic hot section components could not increase efficiency due to some simple fact regarding the way that turboshaft engines make power. Sorry, but the connection between thermal efficiency gains possible through the use of a lightweight turbine wheel that can withstand temperatures of over 1200C, and the fan stage example you gave, eludes me.
No offense taken--it's not for everybody.Unfortunately, I'm only in CA when under duress. Nothing personal.
That’s because I quantified my answers on power output, i.e., thrust or torque/rpm, as it relates to fuel efficiency and not thermal efficiency which is outside my wheel house. In general terms, a turbofan is more fuel efficient because 60-75% of its thrust is produced by the fan and not the exhaust of the core engine. A turbofan basically uses 50% of the fuel a turbojet requires to produce the same thrust. Hence the greater ranges on the same airframes like the 737.
With the current hot side turbine materials and coatings reaching their limits, I'm sure ceramics or some other material is getting close for the next-gen engines. I do know most turbine OEMs keep this technology quite close to the vest. However, when the discussion gets into turbine engine low/high bypass pressures & ratios and thermal efficiencies I usually go get the coffee. There is a member here who does this for a living and has in the past given a better explanation on why. But I don’t recall who that is at the moment.
Dan Thomas
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Ceramics can take more heat than the exotic alloys, but they're not as strong, yet. They're lighter, a factor in their favor. The metal rotor and stator blades in the turbine section use a lot of bleed air through and across them to keep them cool. Perhaps that bleed air could be saved with ceramics, leaving more energy for thrust. As it is now, 75% of the air passing through the turbine is used for cooling. Only a quarter is involved in combustion.
I was a foreman in a machine shop for 12 years. When we started we used a lot of carbide and some high-speed steel and cobalt steel cutting tools. There was a considerable advance in tool technology in those 12 years, and I was using coated carbides (titanium carbonitride coating) tools, and some ceramics. The ceramic stuff was super heat-resistant, with the swarf coming off red-hot some of the time. But those inserts were easy broken, and there were newer inserts showing up with microscopic tungsten whiskers in them, similar to the glass in a fiberglass/resin layup. Made them stronger. I don't know now if they have displaced the tougher coated carbides, but I note that the carbides and coated carbides are still commonly available. They haven't been obsoleted, as they would be if the ceramics had been improved much.
Those ceramics were made of finely powdered tungsten carbide and TiN and a ceramic binder and probably some other stuff. They weren't just "ceramic" as we know it. And they're expensive.
I was a foreman in a machine shop for 12 years. When we started we used a lot of carbide and some high-speed steel and cobalt steel cutting tools. There was a considerable advance in tool technology in those 12 years, and I was using coated carbides (titanium carbonitride coating) tools, and some ceramics. The ceramic stuff was super heat-resistant, with the swarf coming off red-hot some of the time. But those inserts were easy broken, and there were newer inserts showing up with microscopic tungsten whiskers in them, similar to the glass in a fiberglass/resin layup. Made them stronger. I don't know now if they have displaced the tougher coated carbides, but I note that the carbides and coated carbides are still commonly available. They haven't been obsoleted, as they would be if the ceramics had been improved much.
Those ceramics were made of finely powdered tungsten carbide and TiN and a ceramic binder and probably some other stuff. They weren't just "ceramic" as we know it. And they're expensive.
In the context of my comment about the potential for advanced ceramics to enable the production of practical small turboshaft engines, your response didn't make sense to me--especially as you'd previously mentioned small and microturbine engines in post #45. But now I understand what you meant, and it seems we were looking at it from two different angles.
In general terms, yes.
This is what I was getting at. Since ceramic hot section components have the potential to operate at much higher temps than metal alloys, a ceramic turbine may recover more energy from the gas stream while requiring less cooling air. Other metal hot section components, which must be cooled, present additional opportunities for the implementation of ceramic materials, thus further reducing the amount of excess air required to regulate component temperatures. The net result is a significant decrease in BSFC for small turboshaft engines, such that fuel consumption would be much closer to that of existing piston engines. We'll see whether a ceramic turboshaft arrives before battery-powered electric propulsion becomes a practical reality.
From a practical standpoint I think conventional metal alloy turbines burning 100% SAF are filling the void now as they are flying and some for revenue. SAF makes them "green" so everybody is happy. Micro-turbines fit in this same category. So I think as soon as they get closer to the 300hp/250kw power output mark we'll see micro-turbine applications marketed to the small airplane market.
A closer 2nd in your list from what I've seen will be hydrid ICE and fuel cell powered electric propulsion which are also flying now. IMO, until there is a major break through in battery or conductor technologies those designs will be relegated to smaller markets like eVTOL/UAM due to limited range.
And from what I do know on the ceramic side, while they certainly can handle the increased temps in a HPT environment they dont perform well with the associated higher pressures and load. I seem to recall being told even in a much smaller turbocharger application ceramic "turbine" wheels will fail when pressures and boost exceed a nominal operating speed/pressure. So I think the advancement of ceramics will be at the large motor GE and PWC levels vs the smaller side.
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Dan Thomas
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That fits with my experience with ceramic tooling. They can cut like crazy because they hold their edge, and the heat doesn't bother them much, but a bit too much cutting pressure, or interrupted cuts (cutting across keyways or splines or holes, or rounding square stock) will bust them up quickly.
Thermal shock also causes failure of any non-metal tool as well.
That might be the principal challenge faced by development programs. A primary weakness is tensile strength, obviously a crucial factor in turbine implementation. As mentioned previously, the technology has been in development for quite some time by major players, due to the potential to revolutionize large turbines for military, airline, marine, and industrial use. We can hope that a future breakthrough will trickle-down to GA, and someday give us a 150lb, 300hp turboshaft with a 5000hr tbo.
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Dan Thomas
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We've already had engines close to that. Some of the Rolls/Allison 250 engines have a 3500 hour TBO, weigh maybe 160 pounds with the PSRU, and crank out 300 to 450 HP. There are wealthy homebuilders that have used them. They're just expensive, as all turbines engines are and forever will be due to the exotic materials and their really high RPMs. Cost of a new one can pass $300K. Think 35,000 to 51,000 RPM, depending on model and HP. Balancing is absolutely crucial.
I forgot the most important criterion: $50k +/-. When CMC materials achieve widespread use in larger engines the cost may come down drastically. At some point, the gearbox will cost more than the gas generator.
Already have a couple contenders. Solar T62, 150hp, $14,000 to $21,000. Or a RR T63-A-700, 317hp, $18,000 to $25,000 (military version of the 250-C20). You'll need to put a TC aircraft under experimental exhibition though to enjoy it.
Too thirsty, though.
EDIT Here's a list of desiderata related to a new generation of light GA turbine powerplants based on ceramic hot section components: 1). Cost within 20% of current piston engines of comparable power; 2). 75% cruise BSFC of 0.50 or less; 3). TBO 4000 hrs or greater; 4). Capable of supplying sufficient bleed air for cabin pressurization and heating/cooling; 5). Capable of 50% rated torque at FL250. That's not so much to ask for, is it? It's worth noting that, with a 4000+ hr. TBO the engine could cost twice as much as a comparable piston engine and still be cost-competitive to own and operate over its entire service life.
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Dan Thomas
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Not so much to ask for? Well, I want to fly at 200 MPH with a four-seat airplane powered only by solar panels, but wishful thinking never overcomes the laws of physics. Those laws are really inconvenient, all the time. If such an engine was possible, we would have seen much better microturbines by now. The Rolls/Allison I referred to earlier has been around since 1960, more than halfway back to the Wright brothers. It has had a lifetime to get much better, but tiny turbines, like tiny propellers, simply are handicapped by their small rotor areas, and no amount of fooling with them will make them much better. Higher RPMs just blow them apart. Ceramic also has its temperature limitations, and those turbine discs could get hot enough to melt. Another inconvenient number is the power taken from the turbines to drive the compressor stages: three-quarters of it. That means that a 400-HP Rolls/Allison actually generates 1600 HP at the turbines. Is it any wonder that it burns a lot of fuel??
There have been a lot of smart people trying to improve this stuff for a very long time. Nothing we ask for is new. There is a lot of money for anyone who comes up with something like you propose, but we're nowhere near that and probably never will be. Physics.
So when can we expect run running prototype? You seem to have the necessary knowledge and there's nothing preventing you from building it or installing it on a 172. Think of the goldmine potential you could unlock... Roger Wilco the reborn Charlie Taylor!!! The notoriety you could achieve.
Dan Thomas, thank you for your perspective. I expected nothing less.
Bell206, you asked in post #64, and I responded by citing CMC technology and the potential it has to create a new generation of small turbines for GA applications. It's clear that you don't share my optimism, which is fine. The sarcasm and belittlement, however, don't help to advance an otherwise rational discussion, and seem beneath you. I expected something more.So when can we expect run running prototype? You seem to have the necessary knowledge and there's nothing preventing you from building it or installing it on a 172. Think of the goldmine potential you could unlock... Roger Wilco the reborn Charlie Taylor!!! The notoriety you could achieve.
All of us who have been involved in GA for the last half-century can be forgiven for being jaded. After seeing countless breathless press releases, ad campaigns, magazine articles, OSH demonstrations, etc. about the next big thing, only to watch it vanish into thin air taking tens--maybe hundreds--of millions in investment funding down the toilet with nothing to show for it... well, that kind of history would turn anyone into a critical, cynical skeptic.
But that 737 MAX9 that Bell206 referred to in previous posts? It's powered by CFM LEAP engines. The LEAP 1B engine makes extensive use of CMC materials including CMC airfoils in the LP turbine (and 3D-printed fuel nozzles). CMC turbine blades are a real thing, and are currently in an advanced stage of development. The only questions are whether this tech will trickle down to GA, and if so, when.
None intended. My humor can be a bit dry at times hence the smiley face. My comment was only meant to further the discussion as there is no roadblock to designing and building such an engine. People can have such interesting ideas yet few act upon them or expect others to act on them. I've never followed that path and usually take any plausible idea I have to at least the next level. Regardless, it has been an interesting discussion which I hope we can continue.
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You'd better educate yourself on the principles and limitations:
https://www.cast-safety.org/pdf/3_engine_fundamentals.pdf
https://www.smithsonianmag.com/air-space-magazine/the-little-engine-that-couldnt-6865253/
An excerpt from that second article:
Small size itself creates many design problems. Turbine blades can be made smaller, but air molecules can’t; as a result, skin friction and boundary layer effects are proportionally greater. (In engineering argot, a small engine is inherently less efficient because it operates at a low Reynolds number, an aerodynamic coefficient that relates component size to the air’s inertial and viscosity effects.) Compressor and turbine blade tip clearances are proportionally greater, resulting in greater tip losses. To maintain the most efficient turbine and compressor blade tip speeds, small engines must spin faster. Small turbine blades are also harder to cool. Oil passages become narrower, making lubrication tricky. Manufacturing tolerances shrink to watchmaker scale.
And the military wants one, too. Read the requirements, look at the math, understand the difficulties:
https://www.colorado.edu/faculty/ka...5_1243_pm_-_asen_5063_final_project_final.pdf
Proposing or demanding technology without a basic understanding of the fundamentals just makes us look foolish.
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Don’t get me wrong, I’d love for this to happen. However, economically I just can’t connect the dots. Perhaps future materials development coupled with additive manufacturing will crack the nut.
Thanks, Dan Thomas. That’s one of the great things about the internet: you can always count on encountering someone presumptuous enough to tell you what you’d better do.
The examples you cited…I did take a moment to glance through them, hoping to learn something new. Instead, the fundamentals of gas turbines paper took me back in time; back to the beginning of the Reagan administration and a Powerplants 101 class led by a teaching assistant with a strong accent and a marvelous sense of humor. A time when social interactions were conducted on a personal level and more beer could solve problems that eluded the slide rule. Good times, those.
The Williams article provides a succinct history of the EJ22, but never connects the engine’s ultimate underperformance to any of the topics under discussion here. The article concludes, “Why did the EJ22 fail? Perhaps…” and offers-up nothing more than a single sentence of unsophisticated conjecture.
The University of Colorado paper gave away the game in the introduction: “…the goal of this project was to determine how to alter parameters of a set of already-existing small-scale gas turbine engines…” [my emphasis] Clearly, the scope of the inquiry was ultimately constrained by that limitation. The paragraph continued, “On top of this, technologies were researched that could potentially make such an improvement in performance possible.” The “technologies” considered seemed limited to unremarkable methods of cooling hot section components, which were assumed to be conventional alloys. I found no mention of ceramic materials in the paper despite being relatively recent (2015-ish). But then, they did limit themselves to existing engines.
None of these examples address the use of ceramic materials in turbine engines. Maybe that’s because we’re barking up the wrong tree here, Dan Thomas. You are correct in that we need a basic understanding of the fundamentals. And to that end, I suggest that the fundamentals that we really need to educate ourselves on is materials science and engineering, with specific emphasis on ceramic composites. As one might fear, that road is long and tedious and will require considerable time and dedication, but once those concepts and their applications are understood we can begin to consider how to build a better engine using composite materials. So, to borrow some of your own words, feel free to educate yourself.