The General Electric GE90-115B engine is an advanced ultra high bypass turbofan engine that was developed especially for the twin engine Boeing 777 airliner. The GE90 is the ultimate turbofan engine and holds the Guiness World Record for the most powerful engine in aviation for thrust production. The engine is rated for 115,000 lbs. of thrust, though on the test stand it has been run up to 127,900 lbs. of thrust to demonstrate the incredible peformance/reliability margin and the potential for future growth. The GE90 also holds the record for the highest pressure ratio of any aviation engine in the world at 45:1. That means that the engine's compressors compress atmospheric air to approximately 45 bar, or to 661 psi before feeding that high pressure air into the burner. Along with tremendous power, the GE90 is the most efficient engine in the sky, with specific fuel consumption rivaling that of a typical large Diesel engine. High efficiency occurs due to the high compression ratio, high turbine inlet temperature, and high bypass ratio, which converts most of the high energy combustion gas into shaft horsepower to drive the massive ducted fan at low rpm to provide the major portion of the engine's total thrust.
Air entering the engine nacelle is drawn up by the gigantic, 128 inch diameter all composite fan. It is of interest to note that the diameter of the fan and nacelle are essentially the same diameter as the whole fuselage on a Boeing 737. The single stage fan features all carbon composite construction with specially contoured blades for high efficiency. The fan turns at a maximum speed of only 2,550 rpm, but flows enough air in doing so to make tremendous thrust. Air from the fan can follow one of two paths. A vast majority of it enters the bypass duct, and travels around the core of the engine where it is exhausted out of the rear to provide thrust. The engine features a 9:1 bypass ratio, so 9 times more air travels through the bypass duct than actually enters the core of the engine. The air that does enter the core is compressed further by the low pressure compressor or booster, which is a four stage axial compressor made of titanium blisks, and which turns on the same shaft as the fan. After the air is compressed by the low pressure compressor, it is then fed to the 9 stage axial high pressure compressor, which turns at a much faster 10,850 rpm and brings the airflow up to the design pressure ratio of approximately 42:1 and higher. The high pressure compressor is a 9 stage axial design with a high pressure rise per stage concept. The blisks are made of titanium and nickel based alloyed stainless steels in the latter stages due to the ultra high temperatures of compression. Variable geometry compressor stators as well as compressor air bleed at various stages ensure surge-free operation throughout the flight envelope.
Compressed air is fed to the through flow double annular burner, with duplex fuel nozzles and high temeperature exotic alloy construction. The double annular combustor (DAC) is a new concept developed by GE with the intention of improving fuel burn and reducing harmful emissions. The double annular combustor features an additional inner combustion ring, into which the fuel nozzles extend. At lower power settings, the outer ring is being fed fuel only, but as power increases, the inner ring is introduced and the fuel is burned in stages for more complete combustion. Hot combustion gases are cooled to a temperature upwards of 2,700 degrees fahrenheit by dilution air in the combustor, before being fed through the high pressure nozzle to the cooled, two stage high pressure axial turbine.
The axial turbine is made through proprietary construction techniques, but it can be safely assumed that General Electric has utilized all of the latest in high tech, high temperature construction, with transpiration cooled blades made of single crystal formed high temperature surface treated alloys. Unfortunately, there is little information available on the composition of the hot section components, due to the ultra competitive nature of the business. The high pressure turbine drives the high pressure compressor and the accessory gearbox, which drives all of the ancillary systems that keep the engines running and the aircraft in the air. Accessories include low and high pressure fuel pumps, multiple oil pressure and scavenge pumps, an AC generator for electricity, hydraulic pumps to actuate the aircraft's control surfaces, and a pneumatic turbine starter. After the two stage high pressure turbine, the combustion gas is expanded through the low pressure turbine, which is a six stage axial design, featuring special high temperature titanium alloys and transpiration cooling. Both the high and low pressure turbines feature Active Clearance Control, an electronically controlled system which uses compressor bleed air to heat and cool the turbine blade roots, allowing them to expand and contract slightly to control turbine blade tip clearance over the whole power range, allowing for better sealing at high and low power settings.
After the combustion gases leave the low pressure turbine, there is still ample energy left to provide a very healthy kick of thrust out of the core exhaust tailpipe, although a vast majority of engine thrust is developed by the huge, slow moving fan at the front of the engine. The fan air that enters the bypass duct around the core engine is exhausted through the bypass duct nozzle, concentric with the core exhaust. The bypass exhaust wraps around the core exhaust and actually helps to quiet the exhaust roar significantly. In a turbofan engine such as this, most of the engine noise is actually produced by the fan, and the contoured, swept back fan blades even make the fan less noisy.
The GE90 is the current benchmark for large, commercial aircraft turbofan engines with its incredibly high efficiency, reliability, and tremendous thrust capability.
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