Composites and other new and expensive materials play key roles in the engines that will power new single-aisle airliners, such as the Comac C919, Bombardier C Series and, possibly, Airbus A320 and Boeing 737 upgrades. Pratt & Whitney and CFM aim to make their future engines more efficient with material changes-some low profile, others better known-that all contribute to double-digit improvements in fuel consumption. Testing of Pratt's PW1000G and CFM's Leap-X takes place this year.
MTU Aero Engines, which holds a 15-percent stake in the PW1000G geared turbofan for the C Series, has found a new way to minimize blade-tip clearance in the high-pressure compressor. For cost reasons, blades are usually not hard-faced and carve out their tracks in a relatively soft lining in the casing. But clearance widens over time because of erosion and, as a result, engine efficiency drops and maintenance intervals shorten.
To prevent erosion, MTU has devised a process to harden the blades’ tips with cubic boron nitride (cBN) at a reasonable cost. Although the ceramic material costs about $10 per gram, the cBN layer on the blade tip needs to be only 75 microns thick. The new electroplating process uses special baskets that fit the blades and thus use a minimal amount of cBN. With their accompanying hard casing lining, the hard-faced blades will run on the first PW1000G engine to test this summer.
Meanwhile, Snecma has begun full-scale endurance tests with three-dimensional-woven composite fan blades for the Leap-X1C-the version for the Chinese airliner (Comac C919). The resin transfer molding process wraps the fibers into a resin designed for crack resistance. The development schedule calls for the new fan blades to fly in 2014. The design uses the same material for the fan case. The lighter, stronger material allows for a larger fan (and therefore a higher bypass ratio) with no weight penalty.
GE, the other partner in the CFM joint venture, uses titanium aluminide in the GEnx, which already powers the Boeing 747-8 and will soon provide the thrust for the fifth and sixth Boeing 787 prototypes. A so-called intermetallic compound, titanium aluminide features strong interatomic ties that make it resemble ceramics. It is therefore relatively brittle, a drawback the addition of other elements, such as niobium and chromium, can counter. Its main feature is its ability to withstand heat up to 800 degrees C, at half the density of more typical nickel alloys. Therefore, the GEnx's low-pressure turbine won't likely be the last engine component that draws benefits from titanium aluminide.