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stinson108
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How efficient can a jet engine get in the future ?

Mon Sep 24, 2018 5:04 am

Just wondering if anyone would know how much more efficient can a jet engine get compared to the cutting edge jet engines out there now on commercial airliners

Does anyone know what the fuel energy ratio
Efficiency is on the jet engine with the best fuel economy is ?

Does anyone know what the next leap in propulsion systems could possibly be?

Hope the questions make sense to the technos
Out there


Thanks
 
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Starlionblue
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Re: How efficient can a jet engine get in the future ?

Mon Sep 24, 2018 5:16 am

Some areas of possible improvement:
- Temperature tolerance of the turbine components. The higher the better. The hottest turbine bits are actively cooled by circulating cool air through them. Even so the temperatures are immense and producing blades that can take a higher temperature would have an impact on efficiency. BTW "cool air" is a relative term. The air in question is over 1000C, but that's still ways cooler than the blades get.
- Improved aero modeling of the various components, most particularly the fans and compressors. I don't think we've reached "peak fan" yet.
- Variable pitch blades. This is already happening, and I bet there's more to come.
- Lighter/stronger fan blade materials.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
WholaLottaLove
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Re: How efficient can a jet engine get in the future ?

Mon Sep 24, 2018 10:17 am

And of course: gearboxes for large engines.
 
mxaxai
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Re: How efficient can a jet engine get in the future ?

Mon Sep 24, 2018 10:35 am

Starlionblue wrote:
Some areas of possible improvement:
- Temperature tolerance of the turbine components. The higher the better. The hottest turbine bits are actively cooled by circulating cool air through them. Even so the temperatures are immense and producing blades that can take a higher temperature would have an impact on efficiency. BTW "cool air" is a relative term. The air in question is over 1000C, but that's still ways cooler than the blades get.
- Improved aero modeling of the various components, most particularly the fans and compressors. I don't think we've reached "peak fan" yet.
- Variable pitch blades. This is already happening, and I bet there's more to come.
- Lighter/stronger fan blade materials.

From what I've heard, compressors are almost perfect nowadays. You could become an expert and work on it for a lifetime only to achieve ~0.5 - 1 % efficiency gain. I'm sure there are some weight savings possible, though.

Some other areas with room to improve:
- Nozzle & nacelle design
- Manufacturing & maintenance

There is this study: https://docplayer.org/12283395-Fkz-um-0 ... erlin.html that tries to estimate how much more efficient engines can become. It was done for the German ministry of the environment etc. in 2008, but is still pretty up-to-date. Sadly, it's in german.
But there is an image on page 48 with various efficiencies and the conclusion that there is about 27% difference between the GEnX and an ideal (Joule) cycle (using a realistic propulsive efficiency of 75%).
 
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kitplane01
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Re: How efficient can a jet engine get in the future ?

Mon Sep 24, 2018 9:51 pm

stinson108 wrote:
Just wondering if anyone would know how much more efficient can a jet engine get compared to the cutting edge jet engines out there now on commercial airliners

Does anyone know what the fuel energy ratio
Efficiency is on the jet engine with the best fuel economy is ?

Does anyone know what the next leap in propulsion systems could possibly be?

Hope the questions make sense to the technos
Out there


Thanks


Here's two threads that might help...
viewtopic.php?f=5&t=1350975&p=19258813&hilit=turbine#p19258813
viewtopic.php?f=5&t=1341835&p=19533343&hilit=turbine#p19533343
 
CowAnon
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Joined: Fri Nov 03, 2017 12:03 am

Re: How efficient can a jet engine get in the future ?

Tue Sep 25, 2018 3:09 am

Does anyone know what the fuel energy ratio
Efficiency is on the jet engine with the best fuel economy is ?


Looks like about 71-72 percent propulsive efficiency, 39-40 percent overall efficiency for the Airbus 350 and Boeing 787.

Image

There are also some interesting graphs here:
MIT: Unified Propulsion: Efficiencies of A/C Engines
 
stinson108
Topic Author
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Re: How efficient can a jet engine get in the future ?

Tue Sep 25, 2018 7:03 am

Nice graph CowAnon
Thanks
 
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ElroyJetson
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Re: How efficient can a jet engine get in the future ?

Thu Sep 27, 2018 4:47 pm

All engines do work based on the temperature difference between the engine core and outer chamber. Thus, the hotter the core the better. There will need to be material breakthroughs to greatly increase efficiency. Cremarics are one idea being explored. A problem with cremarics is that while they tolerate high temperatures well, they are brittle and difficult to machine.

But to be clear, the greater the temperature variance between the inner and outer chambers the greater the efficiency.
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ElroyJetson
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Re: How efficient can a jet engine get in the future ?

Thu Sep 27, 2018 4:48 pm

All engines do work based on the temperature difference between the engine core and outer chamber. Thus, the hotter the core the better. There will need to be material breakthroughs to greatly increase efficiency. Cremarics are one idea being explored. A problem with cremarics is that while they tolerate high temperatures well, they are brittle and difficult to machine.

But to be clear, the greater the temperature variance between the inner and outer chambers the greater the efficiency.
707 717 727 72S 737 733 737-700 747 757 753 767-300 764 A319 A320 DC-9-10 DC-9-30 DC-9-50, MD-82 MD-88 MD-90 DC-10-10 DC-10-40 F-100
 
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lightsaber
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 12:41 am

mxaxai wrote:
Starlionblue wrote:
Some areas of possible improvement:
- Temperature tolerance of the turbine components. The higher the better. The hottest turbine bits are actively cooled by circulating cool air through them. Even so the temperatures are immense and producing blades that can take a higher temperature would have an impact on efficiency. BTW "cool air" is a relative term. The air in question is over 1000C, but that's still ways cooler than the blades get.
- Improved aero modeling of the various components, most particularly the fans and compressors. I don't think we've reached "peak fan" yet.
- Variable pitch blades. This is already happening, and I bet there's more to come.
- Lighter/stronger fan blade materials.

From what I've heard, compressors are almost perfect nowadays. You could become an expert and work on it for a lifetime only to achieve ~0.5 - 1 % efficiency gain. I'm sure there are some weight savings possible, though.

Some other areas with room to improve:
- Nozzle & nacelle design
- Manufacturing & maintenance

There is this study: https://docplayer.org/12283395-Fkz-um-0 ... erlin.html that tries to estimate how much more efficient engines can become. It was done for the German ministry of the environment etc. in 2008, but is still pretty up-to-date. Sadly, it's in german.
But there is an image on page 48 with various efficiencies and the conclusion that there is about 27% difference between the GEnX and an ideal (Joule) cycle (using a realistic propulsive efficiency of 75%).

I disagree on compressors being perfect. There is several percent to gain. In particular optimizing the output of the low compressor into the high compressor.

We will see far higher Mach number compressors that are several percent more efficient, but better bearings are required.

Same with both turbines. The gearbox will improve.

A big aspect is variable cycle technology. For example, in cruise, the LEAP cuts turbine cooling. Pratt proposed a variable fan nozzle (more optimum on longer missions). Variable pitch fan blades?

Next will be fuel cells powering ducted fans on the wings. We already have a glimpse of the technology that will obsolete the gas turbine. But wait... They will be turbines (to acheive energy density).

Lightsaber
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MatthewDB
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 4:24 am

lightsaber wrote:
I disagree on compressors being perfect. There is several percent to gain. In particular optimizing the output of the low compressor into the high compressor.


Where is the loss in the compressor transition? Is this just because they can't match optimal performance at different power settings because each spool does its own thing in finding a balanced operating point?

lightsaber wrote:
We will see far higher Mach number compressors that are several percent more efficient, but better bearings are required.


Is this due to RPM limitations? Or something else related to the bearings?

lightsaber wrote:
Same with both turbines. The gearbox will improve.


I presumed the main issue with turbines was the temperature limits, anything else? The gearbox sure would help cut the size and improve the efficiency of the LP turbine.
 
JayinKitsap
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 6:26 am

The investment to gain a % efficiency improvement keeps rising, it gets harder as it closes into the theoretical limit. Initial cost, engine weight, and maintenance cost all rise in the attempt to perfection.

I would love to see the gearbox arrive into the larger jets, but it will be for some new clean sheet design a decade or more out.
 
WIederling
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 8:30 am

JayinKitsap wrote:
The investment to gain a % efficiency improvement keeps rising, it gets harder as it closes into the theoretical limit. Initial cost, engine weight, and maintenance cost all rise in the attempt to perfection.

I would love to see the gearbox arrive into the larger jets, but it will be for some new clean sheet design a decade or more out.


Effort per relative improvement tends to be constant ( beyond inflation effects ).
That leads to exponential cost increases for absolute improvements when going near the theoretical limits.

Going from there it is always a good idea to improve on the things that are furthest away from
theoretical optimum. Best overall gain.
Murphy is an optimist
 
mxaxai
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 9:01 am

ElroyJetson wrote:
All engines do work based on the temperature difference between the engine core and outer chamber. Thus, the hotter the core the better. There will need to be material breakthroughs to greatly increase efficiency. Cremarics are one idea being explored. A problem with cremarics is that while they tolerate high temperatures well, they are brittle and difficult to machine.

But to be clear, the greater the temperature variance between the inner and outer chambers the greater the efficiency.

The core thermodynamic efficiency ("GT") increases but the overall efficiency ("ges") does not neccessarily:
Image
Your propulsive efficiency ("vor") drops because the exhaust velocity increases. So you have to extract that extra power in your low pressure turbine and transfer it to your (larger) fan. It is thus neccessary to increase your BPR simultaneously:
Image
BPR solid lines, massflow specific thrust dotted lines, overall efficiency on the y-axis, turbine entry temperature on the x-axis.
 
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ElroyJetson
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Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 4:32 pm

mxaxai wrote:
ElroyJetson wrote:
All engines do work based on the temperature difference between the engine core and outer chamber. Thus, the hotter the core the better. There will need to be material breakthroughs to greatly increase efficiency. Cremarics are one idea being explored. A problem with cremarics is that while they tolerate high temperatures well, they are brittle and difficult to machine.

But to be clear, the greater the temperature variance between the inner and outer chambers the greater the efficiency.

The core thermodynamic efficiency ("GT") increases but the overall efficiency ("ges") does not neccessarily:
Image
Your propulsive efficiency ("vor") drops because the exhaust velocity increases. So you have to extract that extra power in your low pressure turbine and transfer it to your (larger) fan. It is thus neccessary to increase your BPR simultaneously:
Image
BPR solid lines, massflow specific thrust dotted lines, overall efficiency on the y-axis, turbine entry temperature on the x-axis.


Very interesting. Thanks for your post. My statement was exactly based on thermodynamic principles. Steam engines, internal combustion, and jet turbines should all work more efficiently the greater the variance in temperature between the engine core and the bypass or outer chamber. I know physics well, but not the fine mechanical points of jet turbines. Your post makes clear the increase in thermodynamic efficiency needs to be linked to the propulsive efficiency of the actual turbines. Makes perfect sense. Thanks again.
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JayinKitsap
Posts: 2378
Joined: Sat Nov 26, 2005 9:55 am

Re: How efficient can a jet engine get in the future ?

Mon Oct 01, 2018 7:30 pm

WIederling wrote:
JayinKitsap wrote:
The investment to gain a % efficiency improvement keeps rising, it gets harder as it closes into the theoretical limit. Initial cost, engine weight, and maintenance cost all rise in the attempt to perfection.

I would love to see the gearbox arrive into the larger jets, but it will be for some new clean sheet design a decade or more out.


Effort per relative improvement tends to be constant ( beyond inflation effects ).
That leads to exponential cost increases for absolute improvements when going near the theoretical limits.

Going from there it is always a good idea to improve on the things that are furthest away from
theoretical optimum. Best overall gain.


The adage in Wastewater Treatment Plants is that the Secondary Treatment plant costs nearly 3X the cost of a primary treatment plant. This is to go from about 85% clean to 92% clean from the output. Terciary treatment costs almost 3X again to get to near 96% clean. Then it creates lots of pollution and waste handling due to the massive amount of chemicals.

Yes, it is best to go for the low hanging fruit, going after the things that are furthest from optimum.
 
CowAnon
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Re: How efficient can a jet engine get in the future ?

Thu Oct 11, 2018 6:32 am

stinson108 wrote:
Nice graph CowAnon
Thanks

You're welcome. :)

Does anyone know what the next leap in propulsion systems could possibly be?

https://aerospaceamerica.aiaa.org/features/high-gear/

    For geared turbofans, the upper limit of efficiency will probably be bypass ratios of 15:1 or 16:1, says Jean-François Brouckaert, a Clean Sky 2 project officer. Beyond that point, the fan diameter creates too much drag and the size of the gear box makes the engine too heavy. After geared turbofans, the next leap in efficiency for aircraft propulsion would come from open-rotor turbine engines and distributed propulsion concepts.

    “There’s a high level of consensus that this gearbox driving a bigger fan at a slower speed is kind of the only solution that makes sense, and then after that, there are going to be competing steps, but they’re going to look very different,” van Manen says.

A good summary of open rotor engines (a.k.a. propfans) is at http://aviationfacts.eu/uploads/thema/file_en/5a8d5c3f70726f300f000000/Fact_Sheet_Propfan.pdf:

    The removal of the nacelle brings a weight reduction of 88% to the propfan concept compared to turbofan engines. But the mass of the larger fan outweighs this weight advantage. With turbofans, the nacelle is limiting the fan size and the bypass ratio of the engine. The absence of the nacelle in propfan engines makes it possible to install fans with twice as large diameters compared to turbofans. Typically, one third of the propfan weight comes from the fans as a result (Table 1). This leads to the total mass of propfans being higher compared to turbofans of similar thrust capabilities ((Larsson et al., 2011) Hendricks & Tong, 2012).



    Propfan engines are designed to combine turboprop efficiency with turbofan airspeed. As a result, propfans can achieve low thrust specific fuel consumption (TSFC) values (Table 1). Despite the higher weight of propfans compared to turbofans, these engines do have a 36% lower fuel consumption compared to a 1990s aircraft turbofan for a specific flight. Relative to a geared turbofan with entry into service in 2020, propfans show a 15% reduction in both fuel consumption and CO2 emissions since this is directly related to the fuel flow. Part of the reduction in fuel flow is caused by the absence of the nacelle which can no longer cause drag, which increases the propulsive efficiency. This results in propfan powered aircraft requiring less fuel compared to turbofan powered aircraft for a given flight (Hendricks & Tong, 2012; Larsson et al., 2011).

A distributed propulsion aircraft might be something like the NASA LEAPTech X-Plane:

    LEAPTech’s big innovation is using 18 small engines to blow air directly across the wing, the part of the aircraft that generates the most lift. Traditionally, aircraft have a big engine or two that are used exclusively for forward propulsion, and lift is generated by the wing as a side-effect of that forward movement. As a result, wings have to be relatively large in order to provide sufficient lift at lower speeds (takeoffs and landings), and at higher cruising speeds, but all of that wing area just slows the aircraft down.

    The LEAPTech X-plane tightly integrates engines with wings, so the engines all work together to maximize the amount of air moving directly over the lift surfaces. By doing this, the wing can be optimized for cruising speeds instead of takeoff speeds, drastically improving cruise efficiency. The only reason this concept works is that with electric motors, you can just slap 18 of ‘em on there, because of how well they scale in size, weight, and power.

    Image
 
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kitplane01
Posts: 1625
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Re: How efficient can a jet engine get in the future ?

Fri Oct 12, 2018 3:18 am

CowAnon wrote:
Does anyone know what the fuel energy ratio
Efficiency is on the jet engine with the best fuel economy is ?


Looks like about 71-72 percent propulsive efficiency, 39-40 percent overall efficiency for the Airbus 350 and Boeing 787.

Image

There are also some interesting graphs here:
MIT: Unified Propulsion: Efficiencies of A/C Engines


Interesting graph (I think).

If I read the accompanying link correctly

thermal_efficiency = rate_of_production_of_kinetic_energy / energy_in_the_fuel
propulsive_efficiency = propulsive_power / rate_of_production_of_kinetic_energy

What we really want is the product of these two numbers. That's why the chart is showing the best airplanes at 40% efficient. We have a long way to go. And we can improve things even more by carrying less structural weight, having better aerodynamics, etc.

I find this really hopeful. I had wondered if we had max'ed out on progress, and had only incremental improvements left. But it seems huge gains are possible.
 
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kitplane01
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Re: How efficient can a jet engine get in the future ?

Fri Oct 12, 2018 3:26 am

Unducted fan ...

The marketing name for propellers with lots of blades.
 
CowAnon
Posts: 140
Joined: Fri Nov 03, 2017 12:03 am

Re: How efficient can a jet engine get in the future ?

Sat Oct 13, 2018 7:23 pm

kitplane01 wrote:
CowAnon wrote:
Does anyone know what the fuel energy ratio
Efficiency is on the jet engine with the best fuel economy is ?


Looks like about 71-72 percent propulsive efficiency, 39-40 percent overall efficiency for the Airbus 350 and Boeing 787.

Image

There are also some interesting graphs here:
MIT: Unified Propulsion: Efficiencies of A/C Engines


Interesting graph (I think).

If I read the accompanying link correctly

thermal_efficiency = rate_of_production_of_kinetic_energy / energy_in_the_fuel
propulsive_efficiency = propulsive_power / rate_of_production_of_kinetic_energy

What we really want is the product of these two numbers. That's why the chart is showing the best airplanes at 40% efficient. We have a long way to go. And we can improve things even more by carrying less structural weight, having better aerodynamics, etc.

I find this really hopeful. I had wondered if we had max'ed out on progress, and had only incremental improvements left. But it seems huge gains are possible.

The embedded graph is not from the MIT page, but I'm glad you mentioned that, as I didn't know the source either (I'd pulled it from a web search). The picture is from a book by the "Committee on Propulsion and Energy Systems to Reduce Commercial Aviation Carbon Emissions" in 2016, and it's worth a look. https://www.nap.edu/read/23490/chapter/6#41

    As noted above, it is not clear how close to the theoretical limits it may be possible to come with a gas turbine for commercial aircraft given aviation’s important constraints of safety, weight, reliability, and cost. Several authors have considered the question of the practical limits for simple cycle gas turbines given the potential for new materials, engine architectures, and component technologies. Their estimates of the individual limits of thermodynamic and propulsive efficiency differ somewhat (and may divide losses differently between thermodynamic and propulsive efficiency), but they agree that an improvement of 30-35 percent in overall efficiency compared with the best engines today may be achievable. As shown in Figure 3.7, motor thermodynamic efficiencies of 65-70 percent and propulsive efficiencies of 90-95 percent may be possible.

So a total achievable efficiency of 58-67 percent is implied if you just multiply those efficiency values together.

The book is strangely dismissive of turboprop/open rotor propulsion, which I might understand from a noise or speed standpoint but not coming from a CO2 reduction group. From other research I've seen, 90-95 percent propulsive efficiency can be attained by unducted fans but isn't possible from turbofan engines.

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