Clean, Quiet, Powerful: Engines for the New Jets

When the Airbus A380 and the Boeing 787 enter service, they will have the most advanced jet engines ever fitted to airliners.

General Electric (GE), Rolls-Royce and the GE-Pratt & Whitney Engine Alliance respectively have developed the GEnx for the Boeing 787; the Trent 900 for the A380 and the Trent 1000 for the 787; and the GP7200 for the A380.

For its first public appearance on Sunday and its first flight in September, the Boeing 787 will have Rolls-Royce Trent 1000 turbofan engines installed. The GEnx will feature on 787 deliveries starting in 2008. The Trent 900 and the GP7200 are already certified for commercial service on the A380.

Since the late 1960s, all Western commercial jet engines have been turbofans. Some 90 percent of the thrust that today?s engines produce comes from cold air that isn?t burned in the combustor at the engine?s core but is driven back at high subsonic speed round the casing covering the center of the engine.

Manufacturers call this ?bypass? air. It is produced by a huge fan at the front of the engine, which also sucks in the air that does enter the engine core. The air going into the core is compressed from atmospheric pressure to a pressure of 25 to 50 atmospheres, mixed with fuel and burned to drive the turbines that drive the fan and the compressors by means of steel spools.

Today?s big turbofans have 10 to 14 turbine stages, each bearing dozens of blades. These engines generate so much thrust that every turbine blade handles as much power as a Formula 1 racing car?s engine produces, said Robert Nuttall, vice president of marketing for Rolls-Royce?s Civil Business.

The latest engines are much more environmentally friendly than earlier ones. The GEnx emits 15 percent less carbon dioxide (CO₂) than previous-generation engines for the same thrust, said Melvyn Heard, marketing manager for the GEnx program.

It also produces half as much nitrogen oxides (NOx) as its forebears. NOx pollution is a particular concern in communities near airports: High concentrations build up as jet engines idle for prolonged periods and then go to takeoff thrust.

Advances in aerodynamics and materials technologies allow new engine designs to weigh significantly less per pound of thrust than earlier engines.

GEnx fan blades and the engine?s fan casing are made from composite materials, saving 1,000 pounds of weight compared with previous-generation engines. This weight reduction improves the engine?s efficiency and fuel burn.

Rolls-Royce has used superplastic-forming and diffusion-bonding techniques to create titanium fan blades for the Trent 900 and 1000 that are hollow. These blades are even lighter than composite fan blades but are just as strong as blades made of solid titanium, said Nuttall.

Both manufacturers use advanced nickel alloys covered with proprietary ceramic coatings to make high-pressure turbine blades, since these are the hottest parts of engines. The coatings act like the ceramic tiles covering Space Shuttles, but are extremely thin.

The hotter a jet engine runs, the more efficiently it produces thrust. So Rolls-Royce and GE have designed their turbine blades to have lots of tiny holes. These holes force relatively cool air on to the blade surfaces, allowing them to withstand higher temperatures.

At 980 degrees Fahrenheit, the cooling air for the turbine blades isn?t cool at all. But since it comes directly from the engine?s high-pressure compressor stages, it is much cooler than the air that exhausts from the combustor.

Today?s computers allow manufacturers to model the vastly complex fluid dynamics of air passing through a turbofan engine and to optimize every blade?s aerodynamic design. The fan blades of the GEnx are so efficient aerodynamically that the fan only needs 18 blades. GE?s predecessor engine needed 35.

New powerplants will be 50 percent quieter than today?s engines, thanks to innovations such as the chevrons that Goodrich has designed for the 787?s engine coverings, or nacelles. The scalloped chevrons look almost like huge, regularly spaced bites out of the back of each nacelle.

Research conducted by Boeing showed that chevrons efficiently mix bypass air exiting the engine with the outside air. This prevents the two air masses from shearing against each other to cause the hissing noise associated with jet engines.

Making fan blades from stronger and lighter materials also helps reduce engine noise. Fan blades can be made bigger to slow the tips of the blades below supersonic speeds as they revolve. When the A380 and the 787 enter commercial service, almost all the noise the big airliners make will come from displacement of air by the airframes, not from the engines, said Heard.

The new powerplants represent major technological achievements. But existing turbofan engines already do an amazing job.

?The internals of the engine are at half the temperature of the surface of the sun and the pressure at the center of the engine is equivalent to being half a kilometer down in the ocean,? said Nuttall.

?Each engine sucks in one and a half tons of air every second. They are constructed of 20,000 bits,? he said. ?And every engine has to operate for five years or more without going into the garage.?

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