GRAHAM WARWICK / WASHINGTON DC
Three manufacturers are in the race to supply engines for Boeing's 7E7
Three manufacturers are competing to power Boeing's planned 7E7 in the certain knowledge that one, and possibly two, of them will not make the final cut later this year. Only one is offering an all-new engine, and all three are tempering their technology ambitions to ensure their engines are affordable, as well as efficient.
Given Boeing's requirement for significant risk-sharing on the 7E7 programme, speculation surrounds whether the company will follow the precedent set with the 777-300ER and select a sole-source engine supplier. "We will select one or two, and which one or two, later this year," says Boeing senior vice-president, 7E7 programme.
Pratt & Whitney is arguably the engine manufacturer with the most at stake, as its share of the widebody market is in decline. It is the only one offering a clean-sheet engine. "We recognise what we have today will not meet Boeing's goals," says March Young, general manager commercial engine marketing. "We have to look at a new centreline engine, maybe with some scaling from existing engines."
The requirement is for a 60,000-70,000lb (267-311kN) thrust-class turbofan with significantly lower fuel consumption, noise and emissions than engines powering today's Airbus A330s and Boeing 767s. General Electric and Rolls-Royce are basing their proposed 7E7 engines on the GE90 and Trent 900, respectively. P&W's offering, which has been dubbed the PW-EXX, combines an all-new high-pressure (HP) core with a low-pressure (LP) system derived from the PW4000.
P&W believes a clean-sheet engine provides greater scope for reducing parts to simplify manufacturing and maintenance. "With a scaled engine there is a limit to how much you can reduce parts count. And the equation on the 7E7 is affordability, not just efficiency," says Young.
The thrust requirement results in an engine with the same 2.84m (112in) fan diameter as the PW4000 powering the Boeing 777, "but we have to shrink the core to get the efficiency", says Young. "The bypass ratio on the 112in PW4000 is around 6; the PW-EXX will be around 10."
The new core will have a 10-stage HP compressor and two-stage HP turbine. Overall pressure ratio will be increased to around 50, from just under 43 for the PW4000 on the 777, to improve specific fuel consumption (sfc).
GE is aiming for a 13-15% fuel burn reduction versus the CF6-80C2/E1 powering the 767 and A330 respectively, and is looking at a pressure ratio close to 50 for its 7E7 engine, compared with up to 42 for the GE90 on the 777. The engine is based on a 60%- to 70%-scale version of the GE90 core, incorporating improvements from the company's Gen-X technology platform. "The core is evolving," says Mike Benzakein, general manager advanced engineering. "Every year we run a new version with improved components."
Bypass ratio will be higher than the GE90's 8-9, "and probably around 11:1", says Benzakein. "We are evaluating noise versus performance. They do not go together any more, as a higher bypass ratio means higher drag." Boeing's goal for the 7E7 is to meet London Heathrow's QC1 noise limit for departure and QC0.5 for approach. "That's much tougher than Chapter 4," he says.
R-R's proposed 7E7 engine is a scaled and improved version of the Trent 900, the 70,000-80,000lb-thrust engine under development for the Airbus A380. Technology from the company's Vision10 programme, which encompasses the affordable near-term low emissions (ANTLE) demonstrator engine, will be incorporated "where it makes sense", says Charles Cuddington, managing director - airlines.
The Trent 900 introduces a swept fan and contra-rotation - the high-pressure core rotates in the opposite direction to the LP and intermediate pressure (IP) systems, improving performance and simplifying the three-spool engine. The Trent 7E7 will have a higher bypass ratio for lower noise and sfc, "and we will go further with contra-rotation", says Mike Howse, director, engine technology, with the IP spool, not the HP, rotating in the opposite direction.
Noise reduction technologies being considered include a scarfed inlet and serrated nozzles, although the latter have an impact on fuel burn and their benefit depends on bypass ratio "so they are not always applicable," says Howse. Boeing's emphasis on low ownership cost is influencing technology decisions. "We are plotting noise versus maintenance cost for the first time," says Benzakein.
Boeing is looking for at least a 50% reduction in NOx emissions from today's levels. "Emissions are critical, as increasing the pressure ratio to reduce fuel burn hurts on NOx," says Benzakein. GE expects to achieve a 65% reduction, using the TAPS (twin annular premixing swirler) combustor tested under the Tech56 demonstrator programme. "We have run TAPS at a pressure ratio of 55 and got 70% NOx reduction, so we feel we can beat the target without compromising performance," he says.
R-R has tested a single annular combustor providing a 30% NOx reduction, and a double annular design achieving a 45% reduction, but favours a direct injection, lean burn, single annular combustor which has demonstrated a 60% reduction in NOx emissions.
"Fuel staging with a single [concentric] injector offers lower cost and weight and higher reliability than double annular, with better emissions," says Howse. "It's a good candidate for the 7E7."
P&W is looking at the next iterations of its technology for advanced low NOx (Talon) combustor. The Talon II, now available in the PW4000, reduces NOx by 30% and testing of the Talon III lean direct ignition, single annular combustor is under way. "Boeing is giving us challenging targets. We think we can get there with the technology we are looking at," says Young. "The next step is demonstrating them. We learned with the PW6000 that we need to do a full gas-generator demonstrator to get to the required technology readiness level."
All three companies are cautious about pushing the state of the art. "Technology from demonstrator programmes has to buy its way on," says Cuddington.
The Trent 500-based ANTLE demonstrator will run in February next year, a timescale that is "well placed for the 7E7", he says. "But on the 7E7 it's not just about the best sfc for the sake of it; it's what makes the best economic sense. ANTLE combuster and turbine technologies will have to buy their way on, and we have not finished the debate."
Assuming Boeing sticks to its plan to launch the 7E7 by mid-2004, the selected manufacturers, or manufacturer, will have to make their technology decisions by mid-2005, to allow engine certification by mid-2007 and 7E7 entry into service in mid-2008. "We will offer as many technologies as appropriate when we have to make the decision," says Howse.
Source: Flight International
