Jet engines for commercial airliners tend to develop in evolutionary rather than revolutionary ways – a development path best appreciated during long flights over the Pacific. And in many ways, the two competing engine platforms for the Boeing 787 Dreamliner are no exception to that evolutionary rule.
Both the GEnx engine from GE Aviation and the Trent 1000 from Rolls Royce borrow their share of design features from each company’s previous engine platforms. Yet these two engine makers did not simply scale up their previous designs. In fact, they couldn’t. Boeing's performance requirements for the 787 pushed the engine makers to innovate without abandoning their proven technology frameworks.
Chief among these requirements was a drastically reduced fuel consumption compared to previous planes. Boeing claims the 787 will be 20 percent more fuel efficient than planes of a similar size. And Ron Hinderberger, propulsion team leader for the 787, says the new engines alone will produce about 8 percent of those fuel efficiency gains. “We put such an aggressive fuel consumption challenge on the engine companies that it required them to go into their development portfolios and pull forward technologies that were slated for 2010 so they would be ready for us in 2008,” says Hinderberger. Boeing likewise tightened its NOx and CO2 emissions targets with the 787. The company claims the 787 will produce up to 20 percent fewer emissions than previous planes of a similar size.
Early on in the 787’s design process, Boeing engineers decided an increased reliance on electric systems, such as new starter generators and a move to a bleedless engine architecture, would help it meet both the fuel burn and emissions targets – as well as help reduce the plane’s long-term maintenance costs. Hinderberger says both engine makers came up with designs that successfully “blend in” the plane’s large new starter generators and no-bleed architecture. But each engine company did its blending very differently.
Even for engineers not involved in the specialized field of jet engine design, these design differences offer important lessons about the broader trend toward electric systems as well as the value of design optimization when unproven technologies aren’t an option.
What Boeing calls a “more electric architecture” for the 787 primarily involves the use of much larger starter generators than were possible in years past. Each engine on the 787 sports two 250 kVA variable frequency starter generators from Hamilton Sundstrand. As their name suggests, these generators not only start the engines but also provide power for other systems during flight. A 767, by contrast, sports a single 120 kVA generator.
Boeing’s ability to pack so much power on the plane, which also has two 125 kVA generators in its auxiliary power unit, comes down to the ongoing power density improvements taking place in general industry. Hinderberger says the two 250 kVA generators on the 787 take up just a little more space than the single 120 kVA generator used on the 767 15 years ago. “Fifteen years ago, if you said ‘let's do two 250's instead of one 120,’ it would have been impossible,” he says.
Hinderberger says both engine makers accommodated the new generators without difficulty. GE’s Melvyn Heard, an engineer by training and one of the general managers responsible for the GEnx engine, describes the mechanical interface as “pretty much a straightforward gearbox.”
One key difference, though, is in how the two engine companies tap into the starter generators. On the Trent 1000 – a three-spool engine with separate shafts for the high-, low-, and intermediate compressors – Rolls Royce opted to connect the starter generator to the engine’s intermediate compressor (IP). The company did not respond to requests for an interview (read update here) but has released descriptions of this IP Power Offtake System, which includes a mechanism to couple the IP and HP compressors together to meet the relatively high start-up torque requirements. GE, by contrast, simply connects the high-pressure (HP) compressor of its two-spool GEnx engine to the starter generators, according to Heard.
Hinderberger says Boeing is agnostic about the two approaches since both meet Boeing’s requirements for engine starts and power generation. During start-ups, both engine designs can start in under 40 seconds from the two generators and under 70 seconds from one. “From our point of view, even though one engine is two-spool and one engine is three-spool, we look at them as a gearbox to provide rotating energy to generate power,” Hinderberger says. How that horsepower extraction from the engine affects its duty cycle – during idle, take-off, climb and cruise – is something that Boeing left in the hands of the engine makers. “That’s held very close to the vest by them,” Hinderberger says.
Stop the Bleed
The biggest engine-related change enabled by Boeing’s more electric architecture has to do with the elimination of bleed air. Modern jet engines normally “bleed” hot, compressed air from the engine and put it to work on useful tasks such as de-icing the wings, running pneumatic actuators and pressurizing the cabin. On the 787, which flies around with 1.5 megawatts of power, the tasks that required bleed air can now be handled by electrically driven compressors.
The benefits of no-bleed engines remain a subject of debate in aerospace circles. Hinderberger, however, believes the no-bleed architecture has some compelling advantages. One is the efficiency of the engine itself. “When you use bleed air off an engine, it does come at a fuel-economy cost,” Hinderberger says, though he’s quick to add that addition of new systems fulfilling the roles otherwise played by bleed air also affect the plane’s overall fuel burn. “It ends up becoming a really tortuous path if you try to follow it to the end,” he says. “Suffice it to say that anytime you extract less bleed air you’re doing a good thing for the engine.”
Fuel economy isn’t the only thing the bleedless architecture has going for it. The bleedless engine eliminates a long list of pneumatic components from the plane, which has both weight and maintenance implications. “What we've been able to do is eliminate large number of line replaceable units that that over time usually played a major part in recurring maintenance costs of any one airplane. Just being able to get that off airlines' books is a huge advantage,” Hinderberger says. Then consider elimination of the ducting and pipe work associated with bleed air. “What you get is an engine with an enormous amount of accessibility,” he says.
Still, not everyone is convinced bleedless engines are the only way to go. “We’re neutral on the subject of bleed air,” says Heard, who maintains that there’s no performance drop from one to the other. And unlike the bleedless Trent 1000, the GEnx platform has both bleed and bleedless configurations. As for upcoming aircraft, Airbus’s A350 will use bleed air as will an upcoming version of Boeing’s own 747.
The engine makers’ efforts on the 787 didn’t only involve accommodations for the new starter generators and bleedless architecture. While GE and Rolls Royce couldn’t run with completely unproven technologies on their engines for the 787, they still managed to achieve significant performance gains by optimizing and enhancing technologies they had used previously. GE’s engine, for example, is largely derived from its GE 90 model – though GE has made some weight and efficiency gains through materials and the combustor innovations (see sidebar).
In general, though, both engine makers “optimized and fine-tuned every element in their aerodynamic cores,” Hinderberger says. Much of that work took place in tandem with Boeing’s optimization of the airframe. “The engine companies and Boeing worked together to adjust the thermodynamic cycle of the engine. Every time the engine companies selected a new approach, we would predict the weight, drag and fuel burn and match that to the airplane,” he says. “We just kept zeroing in until we said, “We got it.’”
One thing Hinderberger finds interesting about optimization work is it produced two engines with very similar size and shape despite the big differences in the core designs. “The overall fan diameters came in very close – about an inch apart. If you look at the engines on an airplane, you wouldn’t see much difference in overall engine shape,” he says.
So does one engine performs better? “There is a difference,” Hinderberger admits, though he adds that it exists somewhere to the right-hand side of the decimal. “We feel that both engine companies, in our minds, have hit the mark in providing an engine for 787 that will result in an economic advantage for airlines,” he says.
GE Technology Saves Weight, Fuel, Emissions
The engine makers did make their new engines compatible with Boeing's more-electric, bleedless systems architecture, but that's not all they did. They also worked to optimize the aerodynamic and combustion efficiency of their engine cores. Take a look a the GEnx engine, which is based on the company's GE90 engine but has some technology “firsts” in commercial aviation.
According to Melvyn Heard, one of GE Aviation's general managers for the GEnx, the new engine is just over 15 percent more fuel efficient than the CF6 engines with similar power. Part of that fuel burn gain comes from a materials-related weight savings. Heard says GE engineers managed to cut the weight of the engines and related hardware by about 800 lb per aircraft. The company also optimized the aerodynamic design and combustion of its engine core. Here's a closer look at all three developments.
Composites Save Weight, Maintenance. GE engines have been flying around with composite fan blades for more than a decade. But with the GEnx, the company, for the first time, decided to go with a composite fan case, as well. The composite fan case is made from epoxy reinforced braided fabric from A&P Technology. Heard says fan case, which sees significant loads, required a carefully optimized braids orientation. “It's the only one in commercial aviation,” says Heard, though he adds that similar fan case construction has been proven in military applications. Heard says composite fan case is responsible for the bulk of the engine-related weight savings – about 350 lb/engine. According to Heard, some of the weight savings comes from the fact that these composite fan cases are strong enough to contain a suddenly-detached fan blade without the need for a kevlar wrapping that has been standard practice with aluminum fan cases. But weight savings aren't the only thing composites bring to the table. They also have maintenance implications. “Composites get rid of the corrosion issue,” Heard says.
Better Combustion. GE also implemented a combustor technology it has been maturing for the past decade. This Twin Annular Pre-Mixing Swirler (TAPS) system uses two swirlers adjacent to the fuel nozzles to pre-mix fuel and air prior to burning. According to Heard, this swirl creates a more homogeneous and leaner mix of fuel and air that burns at lower temperatures than in previous designs. One result is a significant reduction in NOx emissions. Heard estimates the GEnx engine has 50 percent lower NOx emissions than a comparable CF6 engine. Other types of emissions and particulate levels fall, too. Heard says TAPS may have a maintenance benefit, as well. “Its creates a more uniform temperature profile, which is a friendlier environment for the engine components,” he says.
Aerodynamic and Other Improvements. The GEnx also features aerodynamic optimizations throughout. Heard says GEnx has half the number of fan blades, at 18, than a CF6. This reduction, which helps improve noise levels, was made possible through optimization of the blade shape. In the high-pressure compressor, GE makes use of three blisks, which integrate the fan blade and disks into a single part. The integration saves both weight and maintenance costs, Heard says. Finally, the GEnx engine is the company's first commercial engine to use counter-rotating spools as a way to minimize parts count and weight. Heard says that this design decision resulted in a 10 percent parts count reduction. Put differently, GE went from a six- stage low pressure turbine to a seven-stage design on the GEnx “without any weight increase,” says Heard.
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