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,’