A Giant Step in Jetliner Propulsion
In November 1995, the General Electric GE90 turbofan engine
made its flight debut aboard a Boeing 777 jetliner operated by
British Airways. Developed and manufactured by GE Aircraft Engines
(GEAE), Cincinnati, Ohio, in concert with program participants
in France, Italy and Japan, the GE90 is one of the most powerful
air breathing engines ever flown. It is also one of the most
efficient, one of a trio of advanced technology, very high thrust
jetliner engines that offer new levels of operating economy and
environmental acceptability.
The GE90's history underlines the lengthy time and large investment-$1.5
billion in this instance-required to bring a major aerospace
system from concept to service entry. By the time it was certified
in 1995, the engine had been several years in development. Actually,
though, some of the key technologies incorporated in the GE90
trace their roots to joint GE/NASA research in the 1970s.
A GE Aircraft Engines technician is dwarfed by the mammoth
GE90 jetliner engine, which incorporates multiple technologies
developed in joint GE/NASA research programs. The large sphere
is a test structure.
The GE90 was initially certified at 84,700 pounds thrust,
but it is capable of thrust levels well beyond that. Only two
engines are required to power the big widebody 777, which approaches
in size the four-engine Boeing 747 jumbo jet.
It is a type of engine known as a high bypass turbofan. Used
in most modern airliners, the turbofan is a propulsion system
in which some of the air taken in is compressed, burned in a
combustor and expelled to generate power for driving the fan
and compressor. A greater amount of the air bypasses the combustion
process. In the GE90, the relatively cool bypass air is pushed
rearward by a huge (123-inch-diameter) multibladed fan to mix
with the hot exhaust gas; the result is a very large gain in
overall thrust with minimal fuel expenditure.
Propulsion engineers use the term "bypass ratio"
to indicate how much of the ingested air bypasses the combustion
chamber; generally speaking, the higher the ratio, the more efficient
the engine. The GE90's very high 9:1 ratio makes a big contribution
to noise and fuel burn reductions. Specifically, the GE90 features
a fuel consumption rate 10 percent better than the large commercial
turbofans of the pre-1995 generation; a noise level two decibels
lower; and emission reductions of oxides of nitrogen (35 percent),
carbon monoxide (25 percent) and unburned hydrocarbons (60 percent).
These features have great attraction for airline operators.
Fuel is one of the major elements of total operating costs, and
fuel efficiency of the order the GE90 offers is vitally important
to the world's airlines, who are just beginning to recover financial
equilibrium after years of heavy losses. The engine's environmental
characteristics provide a valuable bonus, not only in community
good will but in a further contribution to improved airline earnings
through avoidance of noise and emissions taxes now being levied
by foreign nations.
Although GEAE spent years refining them, the basic fuel consumption
and environmental improvement technologies that provided the
springboard for what eventually became the GE90 stemmed from
the company's participation in two NASA research programs.
The first, initiated in 1969 and continuing through the 1970s,
was the Quiet Clean Short-haul Experimental Engine (QCSEE) program
conducted by Lewis Research Center. QCSEE focused on then-advanced
technologies to lower engine noise and address the most troublesome
aircraft-emitted contaminants. The program was eminently successful;
ground tests of research engines in the 40,000-pound-thrust class
demonstrated noise reductions 8-12 decibels (60-75 percent) below
the quietest engines in civil transport service. They also demonstrated
new technologies to effect sharp reductions in emissions of carbon
monoxide and unburned hydrocarbons.
Also in the 1970s, GEAE joined with NASA in a joint Energy
Efficient Engine (E3) program managed by Lewis Research Center.
Like QCSEE, E3 targeted emission reductions, but emphasized new
design techniques for minimizing fuel burn. Highlighting that
program was development of a new type of compressor core and
an advanced combustor. The GE90's compressor and dual-dome combustor
are direct descendants of technology developed in the E3 program
and thoroughly proven in extensive ground tests; they are the
principal factors in the engine's economical fuel burn, reduced
emissions and low maintenance cost features.
In developing the GE90, the company drew upon technology gained
from multiple sources, including the NASA experience; expertise
acquired in building high thrust systems for military aircraft;
and GEAE's development and manufacture-in cooperation with SNECMA
of France-of the CF6 and CFM56 families of engines, which have
years of service with many of the world's airlines.
The development effort began in the late 1980s and progressed
to hardware test in 1992. The ground and flight test program
was the most exhaustive ever undertaken by the company, involving
some 7,600 hours and 19,000 cycles of endurance testing to simulate
more than two years of actual airline experience and maintenance.
Thrust levels of the GE90 series continue to increase. The
first growth model-the GE90-92B-achieved its rated thrust of
92,000 pounds in the spring of 1996. Research engines have topped
that; the GE90 has operated for more than 150 hours at thrust
levels above 100,000 pounds, and has demonstrated a thrust capability
of 110,000 pounds. The engine is designed to power all models
of the 777 in development or planned, plus other subsonic commercial
widebodies contemplated for introduction over the next 20 years.
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