Pratt & Whitney remains doubtful about the maturity of ceramic matrix composites (CMCs) for aircraft engines even as a competitor prepares to mass produce the high-temperature material for commercial engines.

"I struggle at [the question of] whether or not CMCs will pay off in the short or medium term," says Paul Adams, P&W's senior vice president of operations and engineering. "Right now I don't see a path forward for large-scale integration of CMCs."

P&W's stance sharply contrasts with General Electric's bullish embrace of the exotic material. A version of the CFM International Leap-1 engine will enter service in 2016 relying on GE-designed CMCs.

CMCs will form a stationary ring that encircles the moving blades on the second-stage of the high-pressure turbine. The ring acts as a shroud to prevent the leakage of heated exhaust air around the tips of the turbine blades. GE also is studying CMCs to replace advanced metals in the moving blades of the same turbine stage, but has not set a date for the transition.

"We're 'all-in' on [CMCs]," says Dale Carlson, GE's general manager for Leap engine technology strategy.

Those diverging philosophies highlight the growing technology split between P&W's geared turbofan and GE's Leap engines that have already been ordered to power thousands of narrowbody aircraft, including the Airbus A320neo, Boeing 737 Max, Bombardier CSeries, COMAC C919, Irkut MS-21 and Mitsubishi Regional Jet.

The most visible difference between the two engines has been P&W's adoption of a reduction gear. But P&W and GE also disagree on how to approach thermal management problems. Making engines more fuel efficient means raising pressures, which increases temperatures inside the core by hundreds of degrees Celsius and beyond the melting point of today's metal alloys.

That means engine designers have to adopt even more advanced cooling systems or shift to materials that can tolerate higher temperatures.

GE's engines designs use both approaches, but are weighted to the latter. As a composite of silicon-carbide ceramic fibers and ceramic resin, CMCs have one-third of the density of metal, but survive in temperatures up to 1,480°C (2,700°F), or 200°C to 240°C hotter than conventional metals.

P&W's approach on the geared turbofan introduces no new materials, but adds an all-new "super cooling" system to keep the geared turbofan's metallic turbine blades below melting temperatures.

"Cooling air is still the best way to increase turbine inlet temperature," P&W's Adams says.

Adams concedes that rising core temperatures will eventually force cooling systems to become more sophisticated in the absence of advanced materials. Instead of simply piping the air straight into the turbine, the coolant itself will have to be cooled. This means the extracted air first must be routed outside of the engine core and through heat exchangers, then returned to the core and down into the hottest section of the turbine.

For GE, such an approach seems too risky.

"For a commercial application such as the Leap we don't need to use the additional complexity of active cooling when we have a superior material solution at hand," Carlson says.

But the readiness for CMCs for all but the most benign environments in the engine core is still debated. CMCs have been studied since the mid-1980s in the US by NASA, and finally introduced in stator vanes of the third-stage low pressure turbine of GE's now-cancelled F136 fighter engine.

But CMCs have been slow to enter commercial production because of the high cost of manufacturing and questions about the reliability of the material in operational service. P&W's Adams estimates that CMCs still cost between 10 to 100 times more to manufacture than conventional materials. Moreover, Adams says, GE may be forced to use CMCs because the company lacks the advanced cooling technology developed within P&W.

Carlson responds that GE has resolved the problems that prevent P&W from immediately applying the new material.

"What Pratt & Whitney doesn't have is access to our trade secrets and proprietary information in how we design and build CMC parts," Carlson says. "It's quite a unique process and we've figured out how to do this."

As a long-term goal, P&W thinks CMCs have the potential of allowing gear-based turbofans to become more fuel efficient. Indeed, the company clarifies that its long-term aim is "to solve issues with cost and reliability for CMCs".

Source: Air Transport Intelligence news