Date: 09 Dec 97 03:54:24 From: Chuanga@cris.com (H Andrew Chuang) Organization: Concentric Internet Services References: 1 2 3 4
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In article <airliners.1997.2834@ohare.Chicago.COM>, Matthew Willshee <firstname.lastname@example.org> wrote: >On 29 Nov 1997, H Andrew Chuang wrote: >> The bypass ratio of the GE90 is around 9 to 10, much higher than its >> competitors (the PW4084/90/98 and Trent 800 [bypass ratio of around 6]). >> (Various "superfan"/UHB designs have bypass ratio of around 12 to 15.) >> The GE90, even though a heavier engine, does have a slightly better fuel >> efficiency. > >Be careful here. Engine weight doesn't come into fuel efficiency >calculations. Specific Fuel Consumption is measured in pounds (of fuel) >per hour per pound force (of thrust). > >You can't really compare engines of different types with this because the >fuel efficiency of the engine aircraft combination also depends on the >engine's weight, the nacelle and the integration with the airframe. SFC >is important, but is not the only factor. > >If you improve SFC by putting a huge fan on the front of the engine you >will make the nacelle bigger and heavier. You will then have to burn more >fuel to cope with the extra weight and nacelle drag. The end result might >be better or worse aircraft fuel burn depending on how everything balanced >out. I agree with your general statements here. I would also like to add that it may also depend on the stage length of a trip. A heavy engine with low thrust specific fuel consumption (TSFC) may use more fuel on a short trip than a light engine with high TSFC, but less fuel on a long trip. I believe that's a reason why most of the GE90 orders are for the B777-200IGW. Nevertheless, based on various data I have seen, I believe the GE90, some 3,000 lbs heavier than the Trent 800 per engine, is slightly more fuel efficient than both the PW4000 and the Trent 800 (not just the TSFC).