The LEAP-X engine program from CFM International is well positioned to be the engine of choice should Boeing choose to re-engine the 737NG series. While Boeing has a “triple option” available – minor upgrades to the existing product, a re-engining to match the expected Airbus NEO, and an all new airplane. CFM International has an exclusive relationship with Boeing, and the LEAP-X-1C engine planned for the COMAC C-919 will be available in the 2016 time frame.
While Pratt & Whitney has achieved more publicity for its innovative Geared Turbofan, CFM International has also been hard at work on its new LEAP-X engine, which also promises double-digit improvements in fuel efficiency. CFM International is a partnership between General Electric in the United States and SNECMA in France that was recently extended through 2040.
GE and SNECMA, via CFM, are jointly pursuing two technology architectures for the next generation of aircraft engines — the LEAP-X advanced turbofan, and a future open rotor design. The engine core currently under design could be utilized in either configuration in the future. Of course, the key advantage of the advanced turbofan is that it can fit under the wing of today’s aircraft, making it a candidate for re-engining programs for both Airbus and Boeing, as well as for new aircraft designs like the COMAC C-919, for which it has been selected.
The LEAP-X advanced turbofan program is being designed to replace the venerable CFM-56 series that has more than 16,500 engines in service on more than 8,000 aircraft, including the DC-8-70 series, the A320 family, 737NG family and the A340-200/300 aircraft.
The LEAP-X will be well positioned for a potential re-engining programs. The fan size of the LEAP-X will be only slightly larger than today’s CFM-56-7, and will fit under wing of the 737NG and A320 families, making it an easier installation than the larger Pratt & Whitney 1000G. We expect Airbus to offer both engines on the A320 NEO family, and Boeing to offer only the LEAP-X should it decide to re-engine the 737NG, given its exclusive engine relationship with CFM for those models, and tighter ground clearance.
It is expected that the LEAP-X advanced turbofan will offer a 16% lower fuel burn than today’s CFM-56-5B/7B engines, while cutting NOx emissions by 55% and beating ICAO Stage 4 noise requirements by 10-15 decibels. These significant increases in performance and efficiency will be available in the 2016 time frame, in time for the C-919 and projected re-engining programs for Airbus, and perhaps Boeing.
From the fan through the exhaust nozzles, the LEAP-X introduces a number of improvements over the CFM-56 series. The fan for the LEAP-X will introduce composite blades to reduce weight, using a new 3D woven resin transfer moulding technology. This process takes the current state of the art two dimensional blades used on the GE90 and GEnx engines one step further. Carbon fibers are three dimensionally woven together on a loom to provide additional structural strength when impregnated with resin and cured in an autoclave. This process enables the use of weight savings materials while maintaining the strength to meet bird strike certification requirements.
The number of blades on the LEAP-X will be reduced to 18, from the 24 on the CFM-56-7B, using advanced aerodynamics to improve air flow. The net result will be an increase in bypass ratio from about 6:1 today to 10:1 on the LEAP-X.
The engine core also benefits from technologies developed for the GEnx engines designed for the Boeing 787 and 747-8 programs. The LEAP-X will incorporate twin annular premixing swirlers (TAPS) that produce a more efficient combustion than the single annular combustors used in today’s engines, resulting in better fuel efficiency. This improved mixing of air and fuel also results in a significant reduction in NOx emissions, which result from inefficient combustion.
The LEAP-X can utilize either single or dual stage high pressure turbines. The tradeoff is complexity and maintenance cost versus fuel efficiency, as higher overall pressure ratios result in a lower fuel burns. A final configuration decision will be made, likely influenced by fuel prices, before the first full engine demonstrator, scheduled for 2012.
The Long-Term Future – Open Rotor
CFM believes it could achieve an additional 10% fuel burn improvement with an open-rotor concept for the next generation of aircraft (making it a 26% improvement). With fan diameters about the same size as the fuselage of existing narrow-body jets (168 inches), new designs would be required to enable these large open rotor engines to be fit to large pylons or innovative tail systems for ground clearance.
Besides size, open rotor designs present additional challenges. With two rows of counter-rotating blades, open rotors present issues with respect to the potential failure of an open fan blade. Additionally, open rotors will be slower than today’s aircraft, with cruise speeds of M.75 rather than the M.80 to M.85 achieved today, impacting transcontinental flights. Noise, with no duct shrouding of the fans, provides an additional challenge for engineers. Both GE and Rolls Royce are pursuing open rotor concepts, while Pratt & Whitney believes it can achieve similar levels of fuel savings using its smaller, shrouded Geared Turbofan.
If Open Rotor solutions prove feasible, the design of future narrow body aircraft may look quite different from today’s models.
The Challenge for CFM
Today, CFM is perhaps best known for its reliability and ease of maintenance. With a history of strong on-wing performance, the greatest challenge in introducing new technologies is to maintain the levels of reliability associated with their current product. Successfully transitioning technologies developed for lower-cycle wide body engines to the high cycle, high stress narrow-body operating environment will be a key success factor for CFM. If past is prologue, they will be up to the challenge, and deliver a reliable, fuel efficient engine with significant savings over today’s models.