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�� blades was even more important because of another change in CFM’s propulsion architecture for Leap. One quick way to improve fuel efficiency is to increase the ratio of air passing through the inlet fan that bypasses the combustion process in the engine core. So, the 5:1 bypass ratio of the CFM56 expanded to an 11:1 bypass ratio for the Leap engine. To increase the mass flow of air bypassing the engine core, it is necessary to widen the diameter of the inlet fan. For example, the 173cm (68.1in) fan diameter of the A320’s CFM56-5B has been widened by 14% to 198cm on the Leap-1A. Of course, as the inlet diameter increases, the low-pressure turbine (LPT) that drives the inlet fan has to do more work. CFM adopted a seven-stage LPT for the Leap, compared to a four-stage LPT for the CFM56-5B. BOOSTED RATIO Another critical area of innovation for the Leap is the compressor system. CFM elected to concede to P&W an advantage with bypass ratio. By inserting a reduction gear between the LPT and the inlet fan, P&W was able to increase the bypass ratio of the GTF to from 5:1 to 12:1. Instead, CFM also focused on radically improving the efficiency of the compression process in a narrowbody engine. The Leap engine is designed with 3D aerodynamic blades from the inlet to the back of the turbine section. These feature scimitarshaped tips with blades curved from the inner to the outer sections. “That technology of that 3D aerodynamics is carried all the way through the engine at every stage, and its impact is probably greatest in the compressor,” Richards says. It allows the Leap engine to more than double the compression ratio compared to the CFM56 to 22:1, roughly matching the compression levels achieved by the GE90. Increasing the efficiency of the compressor was also the goal that drove CFM to install CMCs in the shroud of the high-pressure tur- BREAKDOWN OF A320NEO FAMILY ENGINE DECISIONS flightglobal.com No of engine decisions No of aircraft orders % of orders A319neo CFM 1 20 57% Pratt & Whitney 1 6 17% Undecided 1 9 26% A320neo CFM 11 628 40% Pratt & Whitney 9 494 32% Undecided 18 432 28% A321neo CFM 1 12 4% Pratt & Whitney 5 74 27% Undecided 9 189 69% SOURCE: Ascend Online The Leap has a monopoly position on the 737 Max bine, which is the mechanism that extracts energy from exhaust gases to power the compressor system. Metal shrouds in the same location would have to be actively cooled to keep from melting. The cooling air is siphoned out of the compressor section, reducing the airflow used for combustion and decreasing efficiency. “By using CMCs in that shroud we no longer need that cooling air that is extracted from the compressor, and that’s a debit to the efficiency of the compressor,” Richards says. Another result of the higher compression ratio saddles CFM with a difficult decision early in the design process. A key feature of the sales pitch for the CFM56 over the now- P&W-owned V2500 engine was lower maintenance cost. That advantage was driven mainly by a key difference between the rival engines in the HPT. While the V2500 employs a two-stage HPT, the CFM56 is designed with only one. With fewer moving parts in one of the hottest sections of the engine, that small difference made the CFM56 inherently cheaper to operate and in some cases easier to sell. The CFM56 is the undisputed champion in the narrowbody engine wars. In direct competition with the NARROWBODIES LEAP VS PW1000: THE ORDERS TOTALS No of aircraft orders Aircraft types Leap PW1000 Airbus A320neo 660 574 Boeing 737 Max 1,164 N/A Comac C919 275 N/A Bombardier CSeries N/A 173 Mitsubishi MRJ N/A 165 Irkut MS-21 N/A 163 Total 2,099 1,075 SOURCE: Ascend Online V2500 on the A320, the CFM56 has a lead of almost 10 percentage poins. The new bypass ratio requirements of the Leap engine, however, rendered CFM’s singlestage HPT obsolete. CFM instead designed the Leap with a two-stage HPT, matching the number of stages designed into the competing engine. It was a necessary price to pay, however, to achieve the higher efficiency levels possible with a 22:1 compression ratio. The last piece of the Leap technology puzzle is an advanced combustor. Besides reduced fuel burn, the airlines also want the new single-aisle aircraft to generate significantly less harmful emissions, such as nitrous oxide (NOx) and unburned hydrocarbons. The latter is produced when the combustor burns the fuel-air mixture too hot, and the former is caused by “cold spots” inside the combustor. For the GEnx engines powering the Boeing 747-8 and 787, GE introduced the twin-annular premixing swirler (TAPS) combustor, which is designed to reduced NOx emissions by 50% compared to the CAEP/6 standard. For the Leap engine, CFM is introducing the TAPS II combustor with improved mixers in a smaller geometry, yielding a 60% improvement in NOx emissions. � Peek inside the Leap engine with a video shot at the 2012 Farnborough air show: flightglobal.com/leapvideo 19-25 February 2013 | Flight International | 31 Boeing
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