mafi29
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Scalability Of Engine Performance

Wed Dec 08, 2010 9:08 pm

I've read several time here that twin engine aircraft are more fuel efficient that similar quads. I searched in this forum and the web but I've not found a satisfactory physical/technical explanation yet, why one big engine should be more efficient than two smaller engines with half the thrust each. BTW I'm not talking about maintenance cost and alike, but only fuel consumption.

For example, where does the advantage of the 77W over the 346 in SFC come from? The GE90-115B of the 77W was certified 2003 and the Trent 500 2000 (according to wikipedia), so not a different generation, I would assume.
Or is the GE90 just the better design?

If the SFC really increases with smaller engines, what about regional jets? Must be terrible then...

Any information is appreciated.
 
PITingres
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RE: Scalability Of Engine Performance

Wed Dec 08, 2010 9:33 pm

I gather that blade (turbine and/or compressor) tip clearance is one of the issues. The relative leakage past a blade tip/casing gap is a larger percent of total in the smaller engine. Other scaling issues probably kick in too. Any volume (mass) : surface area effect will scale favorably with larger engine size.

Lightsaber is the guy to answer this definitively, though.

[Edited 2010-12-08 13:36:41]
Fly, you fools! Fly!
 
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SEPilot
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RE: Scalability Of Engine Performance

Wed Dec 08, 2010 11:06 pm

As I replied on the Civil forum where this topic came up, I believe that a lot of it comes from thermodynamics. The smaller an engine, the more surface area compared to the gas volume the gases encounter; and hence the more heat is lost. Power comes from heat, and hence the more heat is lost, the lower the efficiency.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
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jambrain
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RE: Scalability Of Engine Performance

Thu Dec 09, 2010 12:20 am

Quoting mafi29 (Thread starter):
For example, where does the advantage of the 77W over the 346 in SFC come from? The GE90-115B of the 77W was certified 2003 and the Trent 500 2000 (according to wikipedia), so not a different generation, I would assume.
Or is the GE90 just the better design?

If the SFC really increases with smaller engines, what about regional jets? Must be terrible then...

Some real data from http://www.caa.co.uk/default.aspx?catid=702

For some engines the TSFC in g/kNs-1*

V2533-A5____________ 10.1451
CFM56-5C4/P_________ 9.3062
Trent 556-61________ 8.4881
GE90-115B___________ 9.1263

Doesn't look to me like GE90 has any TSFC advantage at all, the explanation must lie somewhere other than the simple GE90 is bigger and more efficient!

Could it have rather more to do with the fact that the dry operating weight of the A340-600 is 579 kg per seat and the 777-300ER is 530 kg per seat!
http://theaviationspecialist.com/350-550_mission_table.gif

I don't know where the cruise TSFC numbers in WidebodyPhotos database come from but they always seem to favour Boeing! I am going off the (Take off thrust not cruise) published data from CAA.

*http://en.wikipedia.org/wiki/Thrust_specific_fuel_consumption
Jambrain
 
KELPkid
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RE: Scalability Of Engine Performance

Thu Dec 09, 2010 12:31 am

Quoting mafi29 (Thread starter):
If the SFC really increases with smaller engines, what about regional jets? Must be terrible then...

   one of the items that came up time and time again when fuel prices were becoming astronomically high in 2008   One of the reasons that QX is retiring their CRJ fleet in favor of an all Dash 8 fleet.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
mafi29
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RE: Scalability Of Engine Performance

Fri Dec 10, 2010 12:34 pm

Thanks for the data, Jambrain. What confuses me is that the GE90-115B has 7% higher TSFC at static thrust and 7% lower at cruise (compared to the Trent 556-61). Are these numbers consistent? I would expect the ratio TSFC_static / TSFC_cruise to be more or less constant, at least for same generation engines.

For the Trent 556 the TSFC_cruise is 16.089 g/kN/s (0.568 lbs/hr/lbf in SI units) resulting in a ratio TSFC_cruise / TSFC_static of 1.8955. This ratio for the GE90-115B is 1.645. That is a difference of over 15%.
Any ideas? Am I missing something?   
 
SchorschNG
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RE: Scalability Of Engine Performance

Sat Dec 11, 2010 10:44 am

TSFC in cruise and at sea-level static do not have to have a fixed relationship.
Additionally, the published data is notoriously incorrect.
Some publish max thrust SFC, other use uncorrected test stand data. So, never trust any data from the internet on this.

Twin jets are not by definition more efficient. It is a question of technology.
A quad will have less installed thrust, hence saves some weight. It will have some more weight due to system requirements (additional piping, four generators, etc).

Aerodynamically, 4 engines feature more wetted surface than a twin with similar thrust.

Finally, from the structual point of view, 4 engines can be lighter. The engines distribute the weight better over the wing. They additionally allow lower landing gear and you can use a smaller vertical tail. Usually four engines make it easier to damp flutter, a huge problem for large elastic structures like wings.

So, in conclusion, it is not primarily the number of engines but the specific aircraft: the B777-300 simply is a damn efficient airframe placed in the very sweet-spot of fuselage length and diameter. The A340-600 is far away from that, probably a few frames stretched too far.
In the end it always depends on technology: a modern wing may be of better technology, allow better margins against flutter despite having only two engines. There are advances in aircraft design, but sometimes they are hidden. One advance is to build a wing with two engines at equal weight of that with 4 engines.

An A380 with 2 engines would be way heavier. Likewise an A320 with 4 engines.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
SchorschNG
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RE: Scalability Of Engine Performance

Sat Dec 11, 2010 10:53 am

Quoting Jambrain (Reply 3):
Could it have rather more to do with the fact that the dry operating weight of the A340-600 is 579 kg per seat and the 777-300ER is 530 kg per seat!

At how many seats?
Another problem of comparison: Airline A wants this pitch and that seat width and these number of monuments. Turns out, they can seat 20 guys more in a B777-300ER than in a A340-600. The relative weight of the aircraft sinks.
For example, they place a 10-abreast economy on the B773, whil a 9-abreast on the A346 is even less desirable.

Airline B is a premium-fixated airline, wants is differently. 10-abreast seating in B773 is not wanted, but for example wants only window and aisle business class seats. Turns out, the A346 may accept this layout better and use space more efficiently. Also, they may be differences in the route it flies, probably can translate the four engines into a specific advantage at some airports or routes.

I agree: the B773 is overall the more efficient aircraft, but some airlines still bought some A346s.
And they did that for a reason.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
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SEPilot
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RE: Scalability Of Engine Performance

Sat Dec 11, 2010 3:46 pm

Quoting SchorschNG (Reply 7):
I agree: the B773 is overall the more efficient aircraft, but some airlines still bought some A346s.
And they did that for a reason.

Most of them bought them before the 77W was available, and all of them bought them initially before the 77W proved how good it was. The specs Boeing gave before the 77W flew were not much better than the A346; during flight test they found out that it was actually significantly better, and has been improved even more since. I do not think anyone ordered the A346 after the 77W entered service that was not already flying it. Can anyone contradict this?
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
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jambrain
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RE: Scalability Of Engine Performance

Sat Dec 11, 2010 8:17 pm

Quoting SchorschNG (Reply 6):
Additionally, the published data is notoriously incorrect.

  

Quoting SchorschNG (Reply 6):
Aerodynamically, 4 engines feature more wetted surface than a twin with similar thrust.

I don't see why you would say that.

For the same fan pressure ratio and bye-pass ratio thrust would scale with Inlet area, 2 x thrust = 2 x area, Wetted surface would scale with inlet area.

if anything the extra spare thrust carried by a twin would make the twins engines larger then a quad.

The GE90-94B is about 60% larger Area Fan Diameter (in) (123 / 97.4)^2 =1.6 and 67% more thrust 93,700 / 56,000

The GE90-115 is a much higher fan pressure ratio engine and so has a much better ratio of wetted surface to thrust.
Jambrain
 
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SEPilot
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 12:14 am

Quoting Jambrain (Reply 9):
Quoting SchorschNG (Reply 6):
Aerodynamically, 4 engines feature more wetted surface than a twin with similar thrust.

I don't see why you would say that.

For the same fan pressure ratio and bye-pass ratio thrust would scale with Inlet area, 2 x thrust = 2 x area, Wetted surface would scale with inlet area.

This is not true. The wetted area of the nacelle is a function of the diameter (or circumference) of the engine; the inlet area is a function of the square of the radius. Twice the inlet area does not require twice the diameter; it requires 1.414x the diameter. On top of that, there is other wetted area in the pylons. And apparently the efficiency gains of a larger engine outweigh the disadvantage of having to have more reserve thrust to deal with engine out.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
tdscanuck
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 1:43 am

Quoting mafi29 (Reply 5):
What confuses me is that the GE90-115B has 7% higher TSFC at static thrust and 7% lower at cruise (compared to the Trent 556-61). Are these numbers consistent? I would expect the ratio TSFC_static / TSFC_cruise to be more or less constant, at least for same generation engines.

I suspect that's mostly two-spool vs. three-spool. Three-spool will flatten the TSFC vs. thrust curve because it's capable of getting closer to optimal over a wider range of operating conditions.

Tom.
 
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jambrain
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 9:13 am

Quoting SEPilot (Reply 10):
The wetted area of the nacelle is a function of the diameter

Sorry? It may be a function of diameter / circumference but then multiplied by length, larger diameter engines are longer!

the wetted area of a tube will be circumference * length

length of a T556 is 155in GE90 length is 287in i.e. 85% longer and 123 / 97.4 i.e. 27% larger circumference.
Jambrain
 
airbazar
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 2:48 pm

Quoting SEPilot (Reply 8):
I do not think anyone ordered the A346 after the 77W entered service that was not already flying it. Can anyone contradict this?

QR, EY, I think. A few more ordered A346's after the 777ER launch but before it's EIS (SA, CX). But your point is still valid, not many  
 
cobra27
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 6:59 pm

Yes those TSFC number from Jambrain can't be right. I read in a book published in the year 2002 that GE90 had the best SFC, now GP7200 (around 4% better than GE90) or the trent 900 (not sure on number, the one that is used on A380) has he highest in the future it will be GENX or Trent XWB.
Maybe the numbers are taken in the non flying testbead at sea level conditions at various thrust levels (you can find that info on ICAO engine emmision databank it clearly says in the file that info is only for sea level like conditions). It just woulnd't make sense, GE90 has much more higher bypass ratio, the core is more loaded (turbine temperatures and pressures)
 
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jambrain
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 7:47 pm

Quoting cobra27 (Reply 14):
higher bypass ratio

It is an often repeated and factually incorrect statement that GE90-115 has higher bypass ratio

GE90-115B
bypass_____ _7.08
pressure ratio_42.24

Trent 556-61
bypass_______7.5
pressure ratio__36.63

the GE90-94B had a high Pressure ratio of 8.33 but not the 115B

These http://www.caa.co.uk/default.aspx?catid=702 data are at sea level at engine OEM test beds and not identical conditions.

If you can link to a better actual data source I would be interested.

I am not claiming Trent 556 has better SFC at FL30 in cruise (I don't have the data it may be) I am saying it isn't an open and shut case!
Jambrain
 
cobra27
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RE: Scalability Of Engine Performance

Sun Dec 12, 2010 10:24 pm

Yes the 115 doesn't have the highest bypass ratio, somehow interesting.
I was comparing more to CFM56 then Trent 500.

I wasn't able to find anything except your link. You can get various numbers from books. Cruise SFC data is hard to find.
And is most important for long range aircraft. IIRC BA RR powered 777 were better for shorter routes and GE powered for longer.
 
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SEPilot
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RE: Scalability Of Engine Performance

Mon Dec 13, 2010 2:38 pm

Quoting Jambrain (Reply 12):
Sorry? It may be a function of diameter / circumference but then multiplied by length, larger diameter engines are longer!

the wetted area of a tube will be circumference * length

length of a T556 is 155in GE90 length is 287in i.e. 85% longer and 123 / 97.4 i.e. 27% larger circumference.

Yes, a bigger engine is likely longer, but that is not you stated in your post. And all wetted area is not created equal; I suspect a longer, smaller diameter cowling will have less drag than a larger diameter shorter cowling with the same wetted area. The added length creates less added drag than would increased diameter adding the same amount of area (assuming equal levels of aerodynamic design.)
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
mafi29
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RE: Scalability Of Engine Performance

Mon Dec 13, 2010 9:00 pm

Ok, I'll try to summarize what I've learned so far.

The ratio TSFC_static/TSFC_cruise can vary quite a lot and could be due to the three-spool design of the Trent and the two-spool design of the GE90. So both the TSFC_static and the TSFC_cruise for both mentioned engines can be plausible.

Data published by WidbodyPhotos in

Quoting Jambrain (Reply 3):
http://theaviationspecialist.com/350-550_mission_table.gif

is one of the few internet sources of cruise TSFC but its reliability is subject to discussion.
I'd tend to trust the data published by the UK Civil Aviation Authority. (No contradiction though, both could be correct)

The TSFC is presumably increasing with the engines size. Of course, it has to do with thermodynamics and surface effects.


The question remaining is, how exactly the TSFC scales with the size of an engine, and why. I'd really like to understand that. Maybe I should just get a textbook of high bypass turbofan design and figure it out by myself   , but I would be happy if someone who has done that already could just tell me. 

Thanks a lot for all the contribution so far!
 
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Starlionblue
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RE: Scalability Of Engine Performance

Mon Dec 13, 2010 11:37 pm

Two other factor in aircraft selection:
- Purchase/lease price. An aircraft that is more expensive to operate can "saved" by a lower price. Total cost of ownership is what counts.
- Availability of aircraft. If you can get the more expensive one today and the cheaper one in five years...

Quoting airbazar (Reply 13):
A few more ordered A346's after the 777ER launch but before it's EIS (SA, CX). But your point is still valid, not many

AFAIK CX leased the 346s and have given them all back.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
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jambrain
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RE: Scalability Of Engine Performance

Tue Dec 14, 2010 12:23 am

Quoting mafi29 (Reply 18):
Maybe I should just get a textbook of high bypass turbofan design and figure it out by myself

I would recommend Cumpsty:-
http://www.amazon.co.uk/Jet-Propulsi...nce/dp/0521541441/ref=pd_rhf_p_t_1

(I don't have any vested interest) just a good read!
Jambrain
 
tdscanuck
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RE: Scalability Of Engine Performance

Tue Dec 14, 2010 5:28 am

Quoting mafi29 (Reply 18):
The TSFC is presumably increasing with the engines size. Of course, it has to do with thermodynamics and surface effects.

*Decreasing* with size. At equal technology levels, large engines are more efficient and have lower TSFC (they burn less fuel to provide the same specific thrust).

Quoting mafi29 (Reply 18):
The question remaining is, how exactly the TSFC scales with the size of an engine, and why.

For a big engine:
-Tip clearances are relatively tighter, so less losses
-Loss of heat to the structure drops (volume to surface ratio is increasing), so less losses
-Physically larger engines make more complex designs easier to implement, like variable stator vanes or three spools, so you can get better performance

Tom.
 
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jambrain
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RE: Scalability Of Engine Performance

Tue Dec 14, 2010 8:16 am

Quoting tdscanuck (Reply 21):
For a big engine:
-Tip clearances are relatively tighter, so less losses
-Loss of heat to the structure drops (volume to surface ratio is increasing), so less losses
-Physically larger engines make more complex designs easier to implement, like variable stator vanes or three spools, so you can get better performance

As I have posted before, the smaller losses from tip-clearance ratios have a law of diminishing returns once you get over a certain point. Looking at the industrial power market you don't see 3000MW Gas turbines do you? (the Siemens SCC5-8000H at 530MW and 444 tons is pretty large !! and >60% efficient)

Also it's interesting to note that a Trent 60 gas turbine from RR at 58MW scale has a better simple cycle operation efficiency of 42% then the Siemens SCC5-8000H at 342MW and 39%

http://goliath.ecnext.com/coms2/gi_0...7058610/SGT5-8000H-on-its-way.html
www.rolls-royce.com/Images/trent60_brochure_tcm92-10920.pdf
Jambrain
 
mafi29
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RE: Scalability Of Engine Performance

Tue Dec 14, 2010 8:31 am

Quoting tdscanuck (Reply 21):
*Decreasing* with size.

   Yes.
That's what you get when you reverse a sentence, stop half way, and click post message.   

Quoting tdscanuck (Reply 21):
For a big engine:
-Tip clearances are relatively tighter, so less losses
-Loss of heat to the structure drops (volume to surface ratio is increasing), so less losses
-Physically larger engines make more complex designs easier to implement, like variable stator vanes or three spools, so you can get better performance

Thanks!
 
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SEPilot
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RE: Scalability Of Engine Performance

Wed Dec 15, 2010 6:51 pm

Quoting airbazar (Reply 13):
Quoting SEPilot (Reply 8):
I do not think anyone ordered the A346 after the 77W entered service that was not already flying it. Can anyone contradict this?

QR, EY, I think. A few more ordered A346's after the 777ER launch but before it's EIS (SA, CX). But your point is still valid, not many

Remember, it was not until the 77W actually entered service with the airlines that they realized how good it really was, and it has improved since.
The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
 
cobra27
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RE: Scalability Of Engine Performance

Thu Dec 16, 2010 7:48 pm

Industrial turbine above 100MW are mostly single shaft and are used for cogeneration mostly. I think they are not made from such expensive metals as aviation engine, I think the turbines are made from high quality steel
Aeroderiavite turbine are 2 or 3-shaft designs, sometime free turbine is used. they are better suited for frequent start stop operation
 
dakota123
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RE: Scalability Of Engine Performance

Thu Dec 16, 2010 9:13 pm

Quoting Jambrain (Reply 22):
As I have posted before, the smaller losses from tip-clearance ratios have a law of diminishing returns once you get over a certain point. Looking at the industrial power market you don't see 3000MW Gas turbines do you? (the Siemens SCC5-8000H at 530MW and 444 tons is pretty large !! and >60% efficient)

530 MW is for a combined-cycle power plant, with steam turbine. The CTG itself is 375 gross MW and 40% efficient (about the same as the CF6-80C2). Still quite large!

So-called frame units are designed such that exhaust temps remain high (over 1,100 °F) for good steam production for subsequent use in a steam turbine. it is certainly possible to extract more work and lower the waste exhaust heat, but more can be gained in the steam turbine since if one is going to spend the capital on a heat recovery steam generator, steam turbine, auxilliaries, you might as well get something really worthwhile out of it. A modern aero unit will have an exhaust temp of 875 °F or less (assuming full power).

Mike

[Edited 2010-12-16 13:14:26]
 
dakota123
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RE: Scalability Of Engine Performance

Thu Dec 16, 2010 9:25 pm

Quoting cobra27 (Reply 25):
Industrial turbine above 100MW are mostly single shaft and are used for cogeneration mostly. I think they are not made from such expensive metals as aviation engine, I think the turbines are made from high quality steel
Aeroderiavite turbine are 2 or 3-shaft designs, sometime free turbine is used. they are better suited for frequent start stop operation

Generally they use materials and, while you didn't mention it, cooling technologies every bit as exotic as in use in an aero -- in some cases much more so such as steam-cooling of combustors (G-class machines) and even turbine blades (H-class machines). Because they only rotate at 3,600 (or 3,000 RPM) the components must be very large to deliver the desired power levels. Therefore the blade materials need to be fairly exotic to adequately deal with the temps and pressures involved (nearly as high as in an aero) and not rupture or creep over time.


Mike

(edit was to rewrite the last couple of sentences for clarity)

[Edited 2010-12-16 14:09:16]
 
PolymerPlane
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RE: Scalability Of Engine Performance

Thu Dec 16, 2010 11:26 pm

Quoting Jambrain (Reply 3):
Some real data from http://www.caa.co.uk/default.aspx?catid=702

For some engines the TSFC in g/kNs-1*

V2533-A5____________ 10.1451
CFM56-5C4/P_________ 9.3062
Trent 556-61________ 8.4881
GE90-115B___________ 9.1263

Doesn't look to me like GE90 has any TSFC advantage at all, the explanation must lie somewhere other than the simple GE90 is bigger and more efficient!

Could it have rather more to do with the fact that the dry operating weight of the A340-600 is 579 kg per seat and the 777-300ER is 530 kg per seat!
http://theaviationspecialist.com/350-550_mission_table.gif

I don't know where the cruise TSFC numbers in WidebodyPhotos database come from but they always seem to favour Boeing! I am going off the (Take off thrust not cruise) published data from CAA.

*http://en.wikipedia.org/wiki/Thrust_specific_fuel_consumption

You are just comparing the take-off SFC. if you use the total fuel consumption for the scenario in your source, you'll see that the GE90 uses less fuel per output thrust.

Though I really doubt the accuracy of the data. If you take the comparison between the GE90-94B and the trent 895, same class, similar thrust. The difference in fuel burn data is really really big. it's not even close. If the data are accurate, I don't see why anybody would want to buy a trent 895 powered 777. It has slightly lower-rated thrust with around 20% higher fuel consumption on segments tested.

The often cited 15% figure is really big enough to kill A340NG vs. 777LR, I don't see why trent 895 doesn't see the same fate, unless the CAA figures are just plain wrong.
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