Leeham News and Analysis
There's more to real news than a news release.
Leeham News and Analysis
- Bjorn’s Corner: New engine development. Part 4. Propulsive efficiency April 19, 2024
- Boeing unlikely to meet FAA’s 90-day deadline for new safety program April 18, 2024
- Focus on quality not slowing innovation, says GKN April 18, 2024
- Boeing defends 787, 777 against whistleblower charges April 17, 2024
- Dissecting Boeing CEO’s statement next new airplane will cost $50bn April 15, 2024
Bjorn’s Corner: Why e in ePlane shall stand for environment, Part 7. Efficiency of cars versus airliners.
January 31, 2020, ©. Leeham News: We now look at ways to increase the fuel efficiency of our airliner and by it, improve the CO2 situation for our environment.
Let’s start with understanding where we are with the efficiency of our present air transport system. To get a feel for where we are we will compare it to our road transport system.
Figure 1. A principal view of a two-spool airliner turbofan. Source: Wikipedia.
How an airliner turbofan is operated compared to other transport engines
The largest gains in airliner fuel efficiency was achieved on the engine side over the last decades. To understand how we can make an airliner gas turbine engine even more efficient we need to understand how it’s run today and where inefficiencies occur.
The best way to get a feel for how our airliner engines are designed and run is to compare them to other transport engines. This gives us a perspective on the task at hand.
An airliner turbofan is a rotational air pump (Figure 1) using a continuous process of compression (by the fan/compressors), combustion (in the combustor) and work extracting decompression in the turbines. The power of the turbines drives the compressors but also the fan, which is the key component for pumping the air faster out the back of the engine than its entry speed.
The efficiency of the different process stages is measured in % of achieved output power to input power. Typical transfer efficiencies on today’s best engines from e.g. axle power from the turbines to achieve compression in the compressors are just north of 90%.
But the designer only has this efficiency available in a sweet spot. The compressors/turbines work with rotating aerodynamic wings (blades/stators in engine speak), and these only have maximum efficiency in a narrow airspeed band (mass flow in engine speak). Figure 2 shows a typical compressor efficiency diagram and how the working point of the compressor varies between maximum power extraction at the takeoff/climb, then to the long cruise phase and finally to descent.
Figure 2. The efficiency diagram of a turbofan compressor. There is only one mountain top with 92% efficiency, placed at cruise mass flow rate by the designer. Source; GasTurb.
The diagram looks busy, but it’s not difficult to decode. On the Y-axis we have achieved compression work as an output pressure to input pressure ratio. On the X-axis we have the amount of air flowing through the compressor at different compressor RPMs. The lines with fractions on them like 1.02 are the engine RPM lines with 1.02 being 102% compressor (N2) RPM.
Think of the graph as a mountain map with the height rings as iso-efficiency lines. The top is at 92% efficiency and as you either climb or descend on either side you loose efficiency.
The table to the left is the legend for a mission simulation with the different simulation points for a typical airliner mission. TOC is top of climb, the most stressing point for the engine aerodynamically (the highest normalized flow), CR is cruise, TO is takeoff with its V speeds, CL climb and then Descent.
An airliner engine designer makes sure all the components in the chain align the maximum efficiency sweet spot with the phase of flight that contributes the most to the overall efficiency of the aircraft, the cruise phase. The core efficiency of the best turbofans is at 55% to 60% for this cruise sweet spot. It also means the engine has less efficiency at takeoff, climb, and descent power. The turbofan is then working outside its efficiency sweet spot.
Road car efficiency
This designing the engine to work at its efficiency sweet spot for cruise shall be contrasted with how our road transport engines are designed. In the last Corner, I wrote about our best car engines having up to 30% efficiency. This is when they are running at their efficiency sweet spot, at around 75% power.
Because our egos like overly strong engines we use a fraction of the car’s engine power in our daily cruise. Our car engines are far too big, we use about 10% to 20% of our car engine’s power when we cruise at the highway at 60mph/100kph. This is if we have a mid-size car with the smallest engine option. In our daily commute, our use of the fuel’s energy is worse. Figure 3 shows how the fuel’s inherent energy is typically used in our cars.
Figure 3. Fuel economy for a typical mid-sized automobile. Source: Wikipedia.
The useful transport work for our cars is when we overcome the Aerodynamic drag and Rolling resistance of the car. In city driving, this amounts to 7% of the energy input. In highway driving 18%. The rest is losses.
It’s no wonder one can make electric or hybrid cars which can improve on our road car’s efficiency! They are scandalously inefficient (part of which is our love of large cars with large engines)! A hybrid can run its combustion engine at the optimal 75% power when it charges the car’s batteries to extend the car’s range. Also, the braking can partly be done by reversing the electric motor to regenerate potential energy to battery energy when we coast for the stoplights.
So the added complexity/weight of a hybrid system is motivated by the outrageous inefficiency of our road car designs. For markets where large cars with large engines are popular (think SUVs) the situation is even worse than in Figure 3.
The situation for trucks is better as their engines are operating closer to their peak efficiency, as do diesel trains and ships.
I go through this so we can understand the world of difference between our road transport system, why hybrids are a good alternative there, and our air transport system where a hybrid system has a hard time to improve an already efficient system.
When we look at concepts to improve our airliner engines we must remember they are flown at their peak efficiency at 80% to 90% of the flight time and that about 80% of the developed thermal power is transferred to propulsive power through our high bypass ratio turbofans.
With these examples, we realize we must rid ourselves of the ideas from the road car industry on how to improve the efficiency further. They simply don’t apply. All ideas must be viewed in the context of the total difference in base efficiencies and operating profiles between the transport systems.
With this in our baggage, we can now look at ways to lower the fuel burn and by it the CO2 emissions for air transport.
It is also of interest to show the turbofan engine h-s diagram showing the compressors pressure ratio effect on extracting work in the turbines from the chemical energy in the fuel and the fan bypass ratio effect on transmitting shaft power to propulsion force into the engine mounts.
The effect of running the compressors much cooler thru active cooling is interesting (intercooling) and the effect of a heat pump sucking up heat from the exhaust and increasing its temperature to heat the air out of the compressor before the burner (recuperating Engine).
Still you are perfectly clear with describing the inefficiencies of regular cars operation, for turbocharged diesel Engines running at high Power it gets a bit similar effects with compression work by the turbo, intercooling, piston compression and turbo turbine with variable nozzle and compound turbines on some truck Engine models and on the Avgas DC-7 Wright cyclone R-3350
Gas turbine heat recuperation, where the exhaust air preheats the air before combustion, was used in Chrysler and Rover gas turbine cars. These being the rotating ceramic type. (Proposed by ABB & Cie for a ultra long range Me 264 turboprop in the 1940s) . It’s essential in automobiles because it greatly improves part load efficiency. The Rolls-Royce-Westinghouse WR21 Marine Gas turbine uses a stainless steal counter flow heat exchanger but it’s a little too big to make it worthwhile on an aircraft. MTU had a bit of a experimental breakthrough with a tubular counterflow heat exchanger made of Inconel ALLOY625 but great effort is required to minimise pressure loss and weight. They are intended for A321 class aircraft ie CRISP turbofan.
Modern Turbo Diesels can approach 45% efficiency at the shaft and is far less sensitive to part load operation.. Commercial HCCI homogenous charge compression ignition engine will probably operate at 50% efficiency.
The UK MoD is planning to test an EJ200 engine with a Reaction engines highly efficient heat exchanger, most likely to cool compressor air to boost the engine thrust. Using a similar boiling helium heat exchanger you could extract heat from the exhaust and thru a heat pump get up to temperatures much higher than compressor exit temperatures and thus reduce fuel burn a bit. Delta T * flow is almost constat so the higher temp the lower the flow on the hot side of the heatpump. The higher pressure you burn the fuel in the combustor the more energy you can extract from the turbines getting to a temperature much lower than the the compressor exit temperature hence limiting the energy you can harvest in the exhaust heat exchanger. Not an easy feat to get these parts fitted to a commercial turbofan engine and survive but that is what engineers are for.
Thanks for mentioning this amazing company. Reaction engines is the UK company that developed the SABRE air breathing hybrid rocket for the HOTOL single stage to orbit space plane. The breakthrough heat exchanger technology you mention they developed to precool the air prior to compressor with cryogenic fuel reduced air density by a factor of 4 greatly reducing turbo machinery size. Its amazing and will have many applications in conventional engines. Inter cooling, precooling, heat regeneration and I think in heat recovery in industry and maybe in domestic cogeneration. Their name Reaction Engines reminds me of Whittles company, Power Jets.
I believe that the optimal solution would be a 100% electric aircraft.
But the engineering challenge for a large airliner is:
How to, safely power these engines, for a 15h flight?
Nice that you want to be green but I suggest reading Bjorn’s explanations of why that simply cannot work (unless of course the government can reverse engineer the technology found in the UFOs they have stored at Area 51)!
Current batteries have over 40x lower energy density than jet fuel. So unless something drastically changes, this is just not going to happen.
That 40x for say road use batteries , but when you talk about aviation certified batteries you can currently double that.
In practice it’s about a 15;1 difference given the 90% or better efficiency of electric motors versus 33% for gas turbine and their even superior part load efficiency. Volumetric difference is narrower. Still not enough. Elon Musk thinks battery densities of 400WHr/kg is compelling for aviation. Others say 800WHr/kg. Some of the power densities (as opposed to energy densities) are becoming surprisingly high allowing full discharge in minutes. That allows conception of a sort of EVTOL aircraft which can take of and land vertically. It will save the weight needed for large wings, flops and undercarriage.
The Paris Accord requires countries such as Germany to decarbonise by 95% by 2050. Its very challenging and supposed to limit temperature rise by no more than 1,5 degrees.
The attraction of batteries is their very high efficiency, around 99% in/out and the motors at 90% to 100% (for superconducting Motors). Cryogenic Hydrogen and PtL synthetic fuels will be at most 65% efficient to create but 50% lost in the advanced gas turbine. The attraction of synthetic fuels is not only their energy density but that they can be created remotely and transported.
The industry had been relying on purchasing offsets where airlines pay for certificates that have been used to pay for projects that reduce emissions that would not have otherwise occurred:. reforestation and tree planting, capturing and burning for electricity the methane from a garbage dump or buying a efficient wood stove for a Honduran family.
These certificates are fine for now but I think they are going to become questionable and limited.
Synthetic PtL fuels, biofuels will probably have to become part of the solution at some point because 95% is a tough goal.
They will only be affordable if the per seat fuel burn is at least 50% down on current levels.
I am actually quite optimistic.
The start might be to swap out coal for natural gas, from burning coal you get mainly CO2 from natural gas you get CO2 and H2O. Germany and the US is on this track. Getting more H2O in the atmosphere will help increasing rain and snowfall. China and other large coal burners could be motivated to go the same. Some of the newest sailplanes has amazing performance and can on many days cruise straight where natural upwinds over ground is enough to keep them afloat towards their target having a glide ratio over 50. What performance would a passanger jet with glide ratio of say 70 have and what powerplant would be good enough for a 1-3hr flight?
A Stemme S10 motorised glider carries 2 people, has a MTOW of 850kg and a wing span of 23m which can be folded to 11.4. Scaling up MTOW by a factor of 100 to get a MTOW of 85000kg with a wing spane of 103m and 200 passengers. About the same MTOW and passengers as a A321neo. If battery mass fraction was 30% and energy density 250WHr/kg and propellor efficiency 85% and airframe efficiency 75% we should be able to climb to 16.5km using mgh formulae for potential energy. From there it should be possible to glide 1156km at a glide ratio of 70:1. Stemme give the range of the slightly more advanced Stemme S12
as 1759km at 55% power with 120L fuel with an 86kw rotax engine. The thing about electric powered flight is it does not need air. So long as the flutter issue can be overcome cruise might be quite good as air density at 15000m is 12%. I know it’s a slightly absurd calculation but it suggests it’s physically possible to make something useful.
Your scaling is all wrong.
There is the story of the keen plane modeller who bought a 1: 185 scale
Revell model of a B52 with a 12 inch wingspan.
He wanted it to be the right scale weight but when he divided the MTOW of the B52 of around 450,000lbs by 185 he came to 2000lb, or just under a ton.
What was the correct scale wieght of the 12 in model.
Your proposal is even more bizarre and I’ll thought out
You’re welcome to show me where I went wrong in my crude calculation. I used a cube root law to scale. To scale the MTOW and internal volume by 100 the linear dimensions would go up by 100^0.33 = 4.64. This means the surface area and wing area would go up by 4.64^2=21.52. The wing loading would also go up by 4.64 which means that speed (for take-off and cruise) would need to go up by the square root of 4.64^0.5=2.15. Due to Reynolds effects wing profile might need to be optimised. Stress at the wing root should be the same so long as similar relative skin thickness is maintained. The Stemme S10 actually performs poorly in terms of Litres/100km per passenger. A 185 scale B52 would be 1/(185^3)*450000= 0.071 lbs or 31.2 grams. About half the weight of an egg.
The Breguet equation should give a rough number. Bjorns software programs an even more exact performance number of a 70:1 glide ratio passanger aircraft. The flutter problem is worked on by NASA and others. Flying at M0.8 with a huge span wing might induce other problems but todays most advanced structure and aero codes (better than Ansys + Fluent) and materials stronger and stiffer than CFRP should get us close.
If “fugitive methane” ie leaking methane exceeds 3.2% then natural gas is worse than CO2 says the union of concerned scientists. US leakage rates are 1.2% and Australia 0.6%. The rates are sometimes as high as 9%. Hydrogen is also no answer. Fugitive Hydrogen leaks at 4 times the rate of natural gas, is about 4.2 times stronger than CO2 weight for weight as a climate warming gas, hydrogen is very hostile to the ozone layer and can be lost to the planet forever in space. I think natural gas is often not pure CH4 but diluted by CO2. If you believe the global warming concerns then batteries and PtL (power to liquids) fuels are probably the only good answer. Hydrogen should be confined to closed industrial processes.
On another note super critical carbon dioxide Rankine engines are beginning to replace steam in combined cycle power plants. Due to the high density of the CO2 the turbo machinery and micro Chanel condensers are 7-8 times smaller. We may see them on turbo props, turbo fans.
Slight correction: “If fugitive methane leakage exceeds 3.2%” then the greenhouse effects of running a combined cycle system are worse than burning coal”. That comes from the union of concerned scientists and I haven’t looked up their reference.
Your diagrams begins to show just how important ‘drag’ becomes when speeds rise.So the freeway automibe speeds rise to circa 50-60 mph.Imagine how important it becomes at 500-600 mph!drag being proportional to speed squared!
So sadly one ‘simple’ answer is to slow down a little ( the law states it would have a dramatic effect).
This (drag) was to be the prime proposal at the recent world shipping conference ( reducing steaming speeds from 22kts to 20kts).There would have been a >20% reduction in diesel consumption/CO2.Did they do it? The hell they did.This CO2 ‘human engineering experiment’ is going to play out – for the next generation of humans.
Container ships dont use diesel fuel, its a much lower grade of fuel called bunker fuel – almost tar like. Its a residue of the refinery process to produce petrol, diesel and kerosene, so isnt a prime driver of the fuel volumes like petrol for cars is.
“Slow steaming also involves adapting engines that were designed for a specific optimal speed of around 22-25 knots, implying that for that speed they run at around 80% of full power capacity. Adopting slow steaming requires the “de-rating” of the main engine to the new speed and new power level (around 70%), which involves the timing of fuel injection, adjusting exhaust valves, and exchanging other mechanical components in the engine.”
Fuel Consumption by Containership Size and Speed
https://transportgeography.org/?page_id=5955
The advantage jets have is speed let them climb to higher altitudes and thinner air and advanced engine intakes do a good static pressure recovery
Efficiency isn’t the only reason to go electric – battery power could potentially supplement takeoff/climb to downsize the engines.
You arent reading any of the technical information in these posts are you ? The current engines are sized for cruise efficiency.
Dukeofurl, your attitude in the comment sections is not constructive.
The design point is for cruise, and as you state below “the takeoff thrust has to be enough to allow one engine out” – sized for takeoff.
In fairness to Albert, the engines are optimized for cruise efficiency, but not optimally sized for cruise. Smaller engines could equally be optimized for cruise.
Albert’s basic point is that the engines could be smaller and sized closer to cruise, with electric assist. That may be true in theory, but the penalties far outweigh the advantages.
Bjorn’s basic point is that for a road vehicle, you can add electric assist and the advantages outweigh the penalties. But the reverse is true for aircraft. It’s very hard to beat the size, weight, power, and reliability combination of a turbine engine.
Cryogenic motor/generators can approach the power and weight, but not yet the size or reliability. If the electricity is turbine-generated, that triples the system weight for the same power. If battery-generated, the weight is many times greater for the same power.
So not there yet, there needs to be orders of magnitude (or more) of weight, size, and reliability improvements in the technologies, before they will be competitive with turbines.
For takeoff smaller engines can , and are used, when you have quads. The takeoff thrust has to be enough to allow one engine out. So for twins that means the takeoff continues with one engine at maximum thrust. For quads ( or triples) one engine out leaves 3 or 2 left.
Having a different type of power source just for takeoff is muddled thinking, but typical of those who advocate ‘electric’ . The extra weight – which wouldnt be used during cruise- makes the whole system use far more fuel, remembering that there is a significant drag component due to weight alone.
The current system of engines sized for cruise efficiency , and fuel use ( dont forget the drag from fuel weight) is the only one that counts. There is no such alternative ‘optimally sized for cruise’
Yes takeoff is inefficient use of fuel but that fuel isnt carried any distance and the engine materials are designed for only 10 mins or less at those high fuel use periods as the plane gains altitude to where turbofans operate more efficiently. Thats part of the high cost of modern engines, the short period of ‘hotter turbine temperatures during takeoff’ which arent needed at cruise levels.
Thats the answer to 2 different ‘optimised’ engines for takeoff and cruise, is to make the cruise sized engine burn more fuel for the 10 min take off and climb period. The extra fuel is burnt off early is very little drag penalty rather than carry heavy batteries all the way to the destination and the complications of two types of power which add even more weight.
Its clear that some people still think planes are like hybrid cars which use batteries for a short period to accelerate ( using an electric motor built into a gearbox )and then can cruise ‘efficiently’ on tiny combustion engines.
Hello . You arent listening to any of the technical points made in the previous series.
Duke, that’s a bit harsh. Design for efficiency and sizing for power are independent goals. You can have engines with greater or lesser power ratings, with the same efficiency.
It’s perfectly possible to devise a power system distribution that would be optimal at cruise and at takeoff. It’s just not practical using electrics with today’s technologies. And since the benefits would be minimal with the aircraft operating mostly at cruise, probably not worth doing anyway.
If you have this understanding, that’s great, but not everyone does. So you could try educating instead of criticizing. That’s Bjorn’s goal in writing these articles.
What do you mean ‘size for efficiency and power are independent goals’. Im surprised you are even going there. Reducing the payload used by fossil fuel is the over-riding aim.
These are not cars, where GE might say our engines are ‘more powerful than needed’ than those from Rolls. RR engineers would ROTFL if they did. GE doesnt of course. A longer runway can allow planes planes to takeoff with lesser thrust or the heaviest to do so with the thrust they have.
Theres a good reason modern planes arent like the B52 with 8 small engines .[For efficiency reasons it makes sense to re-engine a B52 with 4 modern turbofans, for technical, financial and yearly flying hours they will go for 8 business jet sized engines and have a higher yearly fossil fuel use]
A turbo prop turbine is small because it is sized to make it most efficient when driving a propeller , same goes for a business jet, a standard single aisle and so on. This is where different size matters.
Albert has said these sort of things before , that spring from his own thinking and that contradict the very article. Unfortunately too many people read it and it validates their own ideas.
Duke, as I said, you could devise a multi-optimized power system, but it wouldn’t be practical. I didn’t suggest it should be done.
That doesn’t mean the issue is settled for all time, and should never be considered again. Most people thinking about aircraft efficiency will pass through these ideas along the way. Understanding why something wouldn’t work, can be as valuable as understanding why it would. Bjorn tries to address both viewpoints in these articles. There’s value in having people thinking about them.
You’ve moved beyond some of these ideas in your own thinking and that’s great, but you don’t allow for others to be following behind you. If you want to elevate the discussion to your level, then educate. That will never happen by criticism.
Good comment, Rob. That’s the basic point I was getting at. Electric might make sense for some missions, but current technology makes it hard to buy its way on.
Actually the TSFC is better at T-O thrust than cruise, it comes from higher burner pressure in the h-s diagram. RR used to give cruise SFC and GE/PWA T-O SFC making diect comparisons hard. Bigger fan (UDF or prop) diameters help alot at T-O and initial climb and the A320neo pilots tell stories of better “bite” in the engines.
The Boeing SUGAR VOLT hybrid concept was to use a modified geared turbofan concept long considered by Rolls Royce. In this concept instead of a mechanical gearbox one has single pole turbine driven generator powering a 3 pole motor to achieve a 3:1 speed reduction for the fan. That’s my interpretation of how the speed reduction is achieved though it looks like they may even be variable speed converter in this version. The generator-motor is integrated into one housing with the gas turbine to reduce weight and conductor issues. This is likely to be superconducting and I think RR were thinking of magnetic bearings. . One aspect of the hybrid aspect on SUGAR VOLT is to use batteries to boost the takeoff power on those existing electric motors in the fan. The other is to have a fuel cell powered electric ducted fan in the tail to provide boundary layer control as well as thrust. SUGAR VOLT is thus a trijet. Given the 28:1 aspect ratio the glide ratio is going to be about 36:1 so should fly on the tail fan alone. If one adds in electric taxiing there may be considerable reductions in fuel burn.
The SUGAR concept does not need to be hybrid electric. Can be pure turbofan or turboprop.
biofuels?
Biofuels have a future but the problem is they often compete too much with land use for food production and wilderness. One poster mentioned Sweden was looking at processing waste wood from the Tarja Forrest. Investigating this we do seem to be entering an era of 3rd generation biofuels where after mechanical breakdown followed by application of bacteria and enzymes breakdown cellulosic and lignin materials to greatly increase yields of alcohols such as Butanol or Ethanol beyond that possible from using their starch and sugar content. Once you have these they can easily be converted to octane or jet fuel via ZSM5 catalyst. Membrane separation should cut distillation costs. It seems 1 hectare can provide enough fuel to fly an A320 up to 800km per year. I calculate that if the grass in and around around a typical airport were planted with sugar beet it should keep an A320 flying 1000km/day. PtL synthetic fuels seem more promising to me since they provide 20 times more fuel and can be made remotely in say the Australian dessert or of ocean platforms.
I don’t think our show have very much to do with car efficiency. Maybe a little. Car engines don’t like to be run at very high rpm whereas turbofans do. Also, rubber meeting the road is the reason for the huge gap in inefficiency. Put a propeller on a car engine and some wings 10,000 feet above the clouds and the efficiency would spike. With that said, we still have a long way to go with auto engines.
What is your day job? Standup comedian?
What is the engine efficiency “100%” baseline referenced to?
The high compression rate of 60:1 results in high thermal efficiency. How far can it be stretched, without reliability issue and high cost?
Contra rotating props/fans can give say 10-15% more in efficiency (beside what is added also in other comments). Noisy, but perhaps this can be solved by smarter design to reduce the pressure variations?
Would be interesting if you could cover in next blogs also new concepts like “flying wings”, which have benefits for the economy? (lighter, less wetted surface vs lift surface etc.), but with stability issues beside other concerns. But stability can be handled quite well with new fbw technology and also proven methods like adding canards (if they let loose the natural stability can be regained like for JAS39)
The higher pressure ratio is accompanied by higher temperatures as well, and although both lead to higher efficiencies, you begin to run into limits of materials. Even at a ratio of 60, the materials are fairly specialized.
Intercooling is an option to increase pressure ratio without exceeding temperature limits, but unless you can use the heated cooling stream in some way, it represents a loss. Also difficult to engineer in an aviation turbine.
One experimental method to increase pressure ratio is a combined compression cycle. A positive displacement compression stage is inserted before the combustor, which gives you pressure ratios of 100 or more. But obviously weight, complexity, reliability and losses will offset the higher efficiency.
The propfan idea is an established technology and the noise issues are reported to be controllable with new designs. So that will be a definite direction.
The flying wing and/or lifting body concept has been looked at, but passenger acceptance would be a factor, with stadium-type seating. Also passengers being subjected to much higher roll forces. Crash protection & evacuation also are in play.
All interesting ideas, Bjorn will no doubt present others. The most feasible design concepts right now, may be the SUGAR concept or the Aurora D8 concept, both having a more traditional fuselage.
Yes the higher compressor exit temperatures gets troublesome for very high compression ratios as you need a temperature rise to give power and you start hitting temperatures the turbine cannot stand and the cooling flow requirements gets very high. In addition with very high combustion temperatures you get new chemical reactions creating new combusion products that robs energy.
So cooling the air thru the compressor really helps the compressor efficiency, it gives you cooler turbine cooling air and allows for a massive temperature rise in the burner where the cool compressor exit air helps quenching the hot combustion zone before it hits the turbine inlet vanes.
Another way to get high combustion pressures is a pulse detonation engine aft the regular compressor, it can be done differently but circular travelling oblique shocks works as final compressors and ignites the fuel air mixture, there are other solution that produce “doughnuts-on-a-string” exhausts like seen from Aurora, still having hot high pressure combustors that generate strong pressure pulses makes for extreme materials and very short life spans.
This will be controversial, but IMO passenger acceptance should be de-emphasized if we’re trying to reduce emissions. If someone has an unfounded fear of blade out scenarios on an open rotor plane, what’s going to happen? That person won’t buy a ticket? How is that bad from an emissions standpoint?
I hope Bjorn will go in depth on the flying wing concept and specific proposals sometime. I know there’s one proposal that takes care of most of those issues.
It’s a tricky public relations issue. The more you talk about carbon neutrality, ofests and decarbonisation the more people become fixated and as you suggest anxious over a 2.0% issue. The fixation seems to be on wind turbines, solar, electric cars, electric flight, trains with people down on coal, flight. A realistic assessment shows these are relatively minor issues. It would be possible to have a cryogenic hydrogen A320 class aircraft flying in 8-10 years with hydrogen generated and liquified at the key airports themselves but the bigger problem will be to have the low carbon electricity available. PtL Power to Liquids aviation fuel will be available in large quantities this year (3000 out 0f 8000 tons year produced by from Hydro Power by Nordic Blue likely this year is ear marked for aviation). Issues such as decarbonising Iron, Cement and Metals smelting are all larger in my view, they’re 11% of global emissions as well as the big issue powering this decarbonisation.
It doesn’t help that the decarbonisation has become a polarised political issue with reason and honesty sacrificed in favour of rash emotionalism on the various hidden agendas.
I feel that the better efficiency of turbofans compared to car engines is beside the point. People are concerned with the emissions of the airline industry overall versus the automobile industry, not specifically the airplane engine makers versus the car engine manufacturers.
If we’re going to parcel out responsibility to individual parts of the industry, the engine makers should get the least blame, since beside their turbofan developments, GE and Pratt&Whitney/Allison offered propfans for the early 1990s, and Rolls Royce was maybe the most aggressive proponent of open rotors before the GTF won the last engine battle. I’d also agree that you can’t expect them to present an electric engine anytime soon.
The airframers would deserve more, but not most of the blame. Boeing gave up maybe prematurely on its 7J7 propfan proposal in 1987, and then they got rid of the remaining open-rotor friendly frames (MD90/B717) by the mid-2000s, even while oil prices were rising. McDonnell Douglas tried hardest with propfans while it was an independent company, but Airbus has never even bothered to offer a frame that could mount those engines. Even worse, none of them has made a real effort to change to a more efficient configuration than the tube-and-wing setup that’s dominated for 60-70 years.
The biggest responsibility belongs to the airlines. Other than a few scattered examples, they shied away from the B7J7/MD91/MD92 in the 1980s. If those planes had gotten launched, the first production propfan aircraft would just now be reaching the ends of their design lives. Then the airlines went with the LEAP/NEO over clean-sheet open rotor planes or blended wing body designs. The lessors probably should get the blame too, as Udvar-Hazy has not been very supportive of higher-technology concepts.
Airbus considered a propfan for what became the A320. If Boeing were to create a new jet in the B717 configuration but with propfans, composite wings, lightweight GLARE or Al-Li body, all electrical systems (pressurisation, de-icing, air-conditioning, FBW, electrical ground taxiing system) what fuel burn advantage could they expect from an advanced turbofan presumably a PW 1100G with a BPR raised from 12:1 to 15:1 or a contra rotating ducted fan?
My guess 6-7% from the new wing and maybe 10% from the prop fans?
I don’t recall Airbus mulling over a propfan for the A320. I did see lots of FUD from them about the UDF and the 7J7, though.
Pratt claimed their GTF could match the efficiency level of an open rotor by the mid-2020s. NASA said an open rotor engine would beat the GTF by 9% even after factoring in installation effects. Their 2013 O.R. model has a takeoff SFC of 0.161 lb/lbf/h and a top-of-climb SFC of 0.415 at Mach 0.78 and 35,000′ altitude. I believe NASA over PW.
Not sure what advantage the wing you outlined would bring over the A320, but I’d hope one of the OEMs would actually implement a laminar flow wing soon. Boeing tested pinholes in the wings for suction during the 1980s, and Airbus has recently tried a passive laminar flow wing on a modified A340. Both OEMs said the wings didn’t stay laminar in the M0.8x range, so a new long-range plane would seemingly need a lower designed cruise speed to get those benefits. (I’m not sure how the 777X and 787 can have vertical tails with active laminar flow control.)
“During the A320 development programme, Airbus considered propfan technology, backed by Lufthansa.[15] At the time unproven, it was essentially a fan placed outside the engine nacelle, offering speed of a turbofan at turboprops economics; eventually, Airbus stuck with turbofans. ” See https://en.wikipedia.org/wiki/Airbus_A320_family#cite_note-Aris-18 reference 14.
Decarbonisation and fuel burn per seat is going to be an imperative within the next 10 years. I personally don’t believe we are facing a rapid climate change crisis but I can see how things are going. Boeing must pull out all the stops for fuel efficiency including lowering of cruise speed. A B717 configuration with a supercritical and perhaps true laminar composite wing and the ability to handle both 2nd generation PW1100G/LEAP engines as well as UDF (in case UDF fails) is probably a good direction as going for the SUGAR concept might be too much of a stretch at this point. I envisage airlines having to pay for 100% offsets (as Lufthansa does now) or use a carbon neutral synthetic PtL fuel. ICAO/IATA are pledged working to capping emissions at 50% of 2005 levels, the Paris accord wants more: about 5%
Boeings problem is that if it starts now it will be confronted with a series of game changing technologies in 10 years.
The aviation industry is evolutionary, rather then revolutionary, except for the jet age, jumbo and supersonic, were the last failed.
Similar to nuclear industry were security demands are high. That is why I am s little skeptic about pushing enginnes cores much further, but rsther look more on the prop, wing fuselage designs to make more efficient planes, and perhaps accept a lower cruise speed as the parastitic drag has a lsrge contrbution. Add a Bar in the planes snd time fly
Lets start to use hydrogen for electric propulsion or maybe just try to study it and contact Holthausen Group in Hoogezand? They have lots of experiences about heavy engines.