Pratt & Whitney’s Geared Turbofan Growth Plan

By Graham Warwick
Source: Aviation Week & Space Technology

Both CFM and Pratt aim to reduce fuel burn by at least 15% from the engines now powering the A320. CFM says 50% of the Leap-1A's fuel savings come from improved propulsive efficiency, with increased fan size and bypass ratio, and 50% from improved thermal efficiency, including higher temperatures. Pratt says only 4-5% of the PW1100G's fuel savings come from the gearbox, which enables a larger, slower-turning fan to be driven by a faster, more efficient low-pressure (LP) turbine. “The other 10-11% is from the rest of the engine,” says Bob Saia, vice president next-generation engine family.

“Everyone thinks it's only the gear, but the gear just enables us to use the speed characteristics. The gear itself has no technology value,” he says, pointing out the GTF family also has a new core with an “industry-best” overall pressure ratio (OPR) of almost 50:1. This is achieved with fewer stages and a high-pressure (HP) compressor with a pressure ratio of 16:1, compared with 22:1 in the Leap-1, because of a higher pressure ratio in the faster-turning LP compressor. “Speed helps compression, so we can take parts out, and the LP turbine needs fewer airfoils to take the energy out,” Saia says.

Speed also helps when it comes to temperatures, which drive component and maintenance costs. The PW1100G runs hotter than current A320 engines, but “significantly cooler” than the Leap-1A, says Saia. “You can either move more air slower or add more temperature, and it's cheaper to make power with air than temperature,” he says. “We are not running exotic gas-path temperatures; we use the fundamental physics of speed.” More conservative temperatures combined with advanced materials and cooling schemes allow Pratt to reduce HP turbine cooling-air flow 20%, he adds.

These differences may seem academic when both rivals promise similar fuel savings on the same platform, the market sets engine prices and maintenance-cost agreements ensure the manufacturers carry the risks of their technology choices. But they could become more significant as Pratt looks to expand its GTF applications to large commercial aircraft. On paper at least, the conservative temperatures and technologies in the PW1100G give Pratt plenty of room to grow.

Hess says the company has a technology road map to achieve 20-30% fuel savings over today's engines by the mid-2020s. The plan covers the complete engine from advanced aerodynamics and lightweight rotors for the fan, through higher gearbox ratios to increase bypass ratio, cores with OPRs beyond 60, active combustor control, and new materials and cooling schemes in the turbine. The road map is aimed at averaging a 1% per year reduction in sfc for new applications and capturing half of that improvement for re-insertion into existing engines, says Saia.

Notably absent from Pratt's road map are ceramic-matrix composites. CFM is introducing CMCs to commercial aircraft engines with a static part, the HP turbine first-stage shroud. “CMCs are a third the weight of metal, twice the strength and have 20% greater temperature capability. But it is not about higher temperature, it's about not having to use cooling air, which minimizes the loss of performance,” says Gareth Richards, Leap program manager.

“We wanted to put CMCs in our fifth-generation fighter engines, but it was not cost-effective. They added a lot of product cost—they don't come for free,” Saia says. Arguing there are still issues with cost and repairability of CMCs, he says Pratt is working on materials with different chemistry but similar properties “They are at TRL 5 (Technology Readiness Level). We have run them in subsystems. But we can get to 20-30% by the mid-2020s without CMCs.”

GE and CFM, meanwhile, do not acknowledge any limitations on the conventional turbofan. GE will increase use of composites and CMCs in its GE9X for the Boeing 777X. CMC combustion-chamber liners, uncooled HP turbine nozzles, shrouds and, for the first time, blades are among technologies planned for the GE9X, which will have a 10% lower fuel burn than the GE90 powering today's 777-300ER. Others include a 27:1 pressure-ratio HP compressor. “Leap will have all the benefit of the technology from the GE9X and other development programs and we will introduce as it makes sense,” says CFM. “We expect to be 2-3% better than the GTF and this is not a position we will relinquish.”

Momentum behind GE's high-tech approach is building; more than $1 billion a year is going to develop, test and mature its commercial engine technology. Work on the GE9X began in 2010 and “the first 4-5 years are about demonstrating technology and producibility,” says Bill Millhaem, GE90 general manager. “We will run the high-pressure compressor five years before certification,” says Bill Fitzgerald, GE aviation vice president and general manager of the commercial engines operation. The first version begins test this month and two more are planned before design of the GE9X is frozen in 2015. “We will run CMC blades in 2014 in GENx demo engines.”

Power Battle—Starting Point
Fan3-D woven composite, 18 blades, 78-in.-dia.Bi-metallic, 20 blades, 81-in.-dia.
GearNone3:1 reduction
LP compressor3 stages, variable-bleed valves3 stages, higher speed, fewer airfoils
HP compressor10 stages8 stages
CombustorTAPS II low-NOx, additive fuel nozzleTalon X low-NOx
HP turbine2 stages, uncooled CMC shroud2 stages
LP turbine7 stages, titanium-aluminide blades3 stages, titanium-aluminide blades, higher speed, fewer airfoils
Sources: CFM International, Pratt & Whitney

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