Tuesday, November 1, 2005
GE's Simpler, Efficient GEnx
GE's Simpler, More Efficient Genx
The GE90 turbofan has spawned a number of derivative engines, the latest being the Genx. Some call it an `intelligent engine." General Electric's goal was to lower operating costs and make engine support simpler.
When Boeing's new 787 long-range twin-engine airliner started moving from the conceptual phase to the drawing boards, engine manufacturer General Electric Aircraft Engines was ready with an engine that was conceptually ideal for the new airplane. Airbus found it to be ideal, too, for its A350, the European manufacturer's newest aircraft under development.
GE didn't have to design a new engine from scratch. But it also did not have to adapt an engine designed for another application. Rather, a lot of research and previous engine development came together to produce an optimal design that would help the two new aircraft achieve lower operating costs than previous-generation airliners. This new engine also incorporates a sensor suite to make it an "intelligent engine," designed to make its support easier.
What impact does this optimal design and enhanced intelligence have on the support staffs that must eventually maintain this new engine? Given the number of units that will probably eventually be fielded, the answer is critical.
The new engine--just two months from its first run--is the GEnx, so named because, while it is the fifth generation in the GE90 family, it is also a "next-generation" turbofan. It incorporates technology that GE has worked on for many years but never implemented in a commercial engine (see sidebar, page 39).
From a maintenance perspective, the design features that make the GEnx efficient and long lasting are also going to make it need less maintenance. For example, the GEnx's fan module is nearly maintenance-free for three overhaul intervals, which could be as long as 9,000 cycles and 10 years, or longer, depending on how the airplane is operated. The 54-inch fan for the 787 engine uses composite fan blades, laid-up by hand just like those on the GE90. But the big difference is the composite fan case.
"We've taken technology from the blade," said Mike Wilking, general manager GEnx marketing, "and learned from that and taken it to another low-risk part of the engine." In addition to saving weight (350 pounds per engine), the composite fan case will suffer less corrosion. The Kevlar vest used on metal fan cases to contain debris in case the fan disintegrates from FOD or a bird strike causes corrosion because moisture gets trapped between the Kevlar and the metal.
The composite fan case, Wilking added, "is strong enough to absorb any energy from a fan-blade issue." Not that the GE90 has fan blade issues; in nine years, there have been 85 reported bird-ingestion events and no damage to the fan blades. The FAA has never issued an airworthiness directive against the GE90 fan blades (although there are nine ADs on the GE90 engine series).
The combined composite fan and case adds new benefits, too. Thermodynamically, composites are less likely to distort when operating a high rotational speeds and temperatures, according to Wilking.
One might conclude that an intelligent engine would be hooked up to a greater number of sensors. Not so, the GEnx. Each sensor presents both an opportunity and a risk, Wilking explained. The risk is that every sensor can fail, and this is not conducive to keeping the airplane in the air, making money. Operator feedback indicated that fewer sensors are preferred, he added. So the GE engineers tried a new approach to achieving the intelligent engine.
By minimizing the number of new sensors and performing more diagnostic analysis on the remaining sensors, engineers were able to extract the information needed right down to the component level. This includes the ability to look at the clearance between high-pressure turbine airfoils and shrouds.
"If there's any leakage," Wilking said, "the HP turbine can't extract that energy. When the engine tells us the blade clearances are opening up, we can close those down with an active clearance-control system." The control system itself is mechanically simple: just blow more air by the engine case, and it automatically shrinks. And a complicated mechanical contraption is not needed to achieve the same result.
The GEnx's diagnostic analysis system also will monitor each bearing in the engine because they each have their own unique frequency. If the diagnostics indicate a potential problem, the questionable bearing can be monitored as necessary until it is time to repair.
GE's diagnostic analysis system is part of the GEnx engine's full-authority digital engine control (FADEC), and Wilking anticipates being able to tap into the flow of information via the high-speed Internet systems that many airlines are installing, such as Connexion by Boeing. "For those operators," he said, "we can trend their engine real-time. If an LRU provides a fault, we can see it in flight and dispatch a maintenance team."
What all this means for the maintenance of the GEnx is a lot less maintenance over the life of the engine. Wilking views the GEnx as coming closer to a deep desire of turbine engine designers to make an engine that is not only totally on-condition in terms of maintenance but that broadcasts its maintenance needs as they develop. Why change the oil just because the engine has reached a certain number of hours of operation, if it doesn't truly need to be changed?
"Today we do preventive maintenance," he said. "Wouldn't it be nice to have it totally on-condition?"
The GEnx has another maintenance feature that mechanics should like: a built-in core water-wash system. Instead of having to run the engine at high power and spray the water through the fan and collect the leavings from the engine's other end, mechanics will be able to pull a cart up to the engine, hook up the hose via a quick-disconnect fitting, turn the water on, and flush it through the core at idle power. There is no need to collect the waste because it comes out as steam.
"It gives you your EGT margin back," Wilking said. "And the intelligent engine will tell you when it needs to be done. It could be 200 to 500 cycles [or more], and it can be done in 30 minutes." This is a significant time reduction from the four hours that it takes on current engines, he added.
The engine overhaul schedule will also be simpler. For the first run, the engine will need just cleanup and clearance restoration in the high-pressure turbine and combustor modules. After the second run, the compressor also will need attention, cleaning up the airfoils and restoring clearances. And after the third run, mechanics can take care of the low-pressure turbine and the fan modules.
GEnx History and Applications
The GEnx engine for Boeing's new 787 and Airbus's new A350 derives from General Electric's GE90 family, which was first installed in the 777 in the early 1990s. The GE90 is a 76,000- to 85,000-pound thrust giant with a feature that was unheard-of at the time: composite fan blades. The original GE90 has grown through the years, to the 94,000-pound -94B, then to the 115,000-pound -115B, which helps the Boeing 777-20LR and 300ER achieve impressively efficient long-range flights. The Engine Alliance, a joint venture of GE and Pratt & Whitney, produced the GP7000, the fourth generation in the GE90 family.
The GE90 descendent, the GEnx, is scheduled to first fly in the fourth quarter of 2006 on GE's test-bed Boeing 747.
Entry-into-service on the Boeing 787 should occur in mid-2008, followed by the Airbus A350 and, if Boeing launches it, the 747 Advanced. There will be no bleed-air system on the all-electric 787, but the GEnx on the A350 will have bleed-air.
"It's a very versatile design," said Mike Wilking, general manager GEnx marketing, "balanced [in terms of] both fuel burn for long-haul and durability for those operators who want to use a widebody on short-cycle missions.
"We're shooting for having the same cost of ownership as the traditional CF6 and 15 percent better fuel consumption on the same flight. It's a low-risk approach, advanced but proven fifth-generation architecture."
Benefits of TAPS
Efficiency is Boeing's number one goal with its new 787, and toward that goal General Electric has incorporated in the aircraft's engine, the GEnx, the twin-annual preswirlers (TAPS) combustor, designed to deliver a 15-percent improvement in specific fuel consumption over the CF6 at equivalent thrust, according to GE's Mike Wilking.
Scientists at one of GE's four global research centers, in Bangalore, India, have been working on TAPS for five years as part of the CFM56 TECH56 improvement program. The TAPS concept isn't new, but the GEnx is its first commercial application.
"This is an improved way of mixing fuel and air prior to igniting it in the combustion process," said Wilking. In traditional combustors, the process begins with too much fuel versus air, and then as combustion begins it transitions to too little fuel and too much air. "Through that transition," Wilking explained, "the fuel-air burns hot and generates nitrogen oxides [NOx pollutants]." TAPS, however, preswirls to achieve a better mixture early on when the combustion process begins. Therefore, "the transition between the rich and lean burn is significantly reduced, which reduces production of NOx by 50 percent."
The TAPS combustor runs cooler, too, Wilking said, "because it's not going through the lean-to-rich transition. Typically in traditional combustors, you get hot streaks circumferentially around the combustor. TAPS eliminates those hot streaks."
Combustion liners usually are built with thousands of tiny impingement holes to keep the liners cool as they are bathed in the burning streaks from combustion. But with TAPS, the highest heat is kept at the core of the airflow inside the combustor and thus the liners need no impingement holes to keep from being damaged. No holes mean another increment of aerodynamic efficiency and improved durability.