9/14/2011

GE Widebody engine SFC comparison

There is a great article at Asprire Aviation about how the B777 could evolve. There is one part which could cause misunderstandings: the fuel burn comparison between the GE90-115B on the B777-330ER/777-200ER/B777F and the GEnx that is on the B787 (GEnx-1B) and the B748-8I/F (GEnx-2B).
Daniel Tsang suggests that the GE90-115B is more efficient than the GEnx and gives SFC numbers. Now, first of all, these numbers are for Seal Level Static (MN=0) Takoff. The relevant number for long-range
widebody engines would be the SFC in cruise conditions and because of different lapse rates and worksplits between the high and the low spool it is not said that the GEnx has a higher SFC at cruise. Secondly, the GEnx number is given for the GEnx-2B, which has one stage less in both the LPC and the LPT than the GEnx-1B. That makes the -2B worse in thermal efficiency than the -1B, as the Overall Pressure Ratio is lower with the "missing" LPC stage. Suggesting that a typical LPC stage has a pressure ratio of 1.25, the overall pressure ratio is 25% lower for the -2B. The larger fan of the -1B should further contribute to a better SFC of the -1B compared with the -2B.
But let us look at two openly available information to get a better feeling about where the different widebody engines are in terms of SFC.
Here are a few sources:
  1. GEnx-1B vs. CF6-80C2B6F (B767-300ER) and GE90-94B (B777-200ER): GEnx-1B 15.4% better than CF6-80 and 6.9% better than GE90-94B
  2. GP7000 vs. CF6-80 and GE90-115B: GP7000 (A380) 11-13% better than CF6-80 (different engine versions and A/C applications) and 4% better than GE90-115B
Keep in mind that the comparisons are made for the respective Cruise Bucket SFC of the individual engines. Each engine has it's best SFC where the design point is (or close to the design point). The design point is dependent from the aircraft where the engine is to be designed for. Thus the thrust at the cruise bucket of the GE90-115B is higher than for the GEnx-1B and the absolute fuel burn is larger even with a slightly lower SFC. The SFC number therefore is just an indication of how efficiently the engine converts the energy of the fuel into propulsion, not a number to compare the absolute fuel burn (this would be SFC*Thrust for a given altitude and Mach number).
If we put the numbers together and suggest that the CF6-80C2B6F is more like 13% worse than the GP7000 (it's one of the older and lower-thrust CF6-80 versions), we get the following picture.



I am not really sure about how the GE90-94B and the -115B compare - I would have thought that the difference is larger than the 0.5% suggested here -but I did not find a direct comparison. Sources are highly welcome...
My point is: I think that the engine for the B777X will be more a derivative of the GEnx scaled up to the size of the GE90-115B.
The GEnx has a 10 stage HPC, whereas the GE90-115B has a 9 stage HPC. The smaller thrust versions of the GE90 family all have a 10 stage HPC derived from the NASA E3 core, developed in the 1970's.
The compressor of the CFM LEAP engine family (for C919, A320neo and B737 MAX) will get a further refined E3 compressor. Also the HPC of the Passport engine, to power the Bombardier Global 7000/8000 family, has the same heritage.

2 comments:

  1. Well, I did not suggest that the GE90-115B is more fuel efficient than the GEnx-1B engine.

    I stated that the "seemingly" difference is owing to the GE90-115B's bigger fan which gives more fan efficiency.

    After ruling out the "fan efficiency" difference, the GEnx-1B should be 6.9% better than GE90-94B, as you suggested.

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