Engines engaged – powering the A320neo and Boeing’s 737 MAX

Efficiency and performance are key, as fuel-burn reductions of around 15% become standard requirements – with a further 10% widely tipped to be the expected norm by the middle of the next decade.

The choice of new engine options for the Airbus A320neo and Boeing’s 737 MAX, among others, highlights the range of innovative technologies that have been developed to help reduce overall environmental impacts and deliver all-important cuts in fuel consumption.

Leading edge aviation propulsion

LEAP-X is a new family of twin-spool, high bypass turbofans, developed as successors to CFM56.

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CFM International, the equal-shares joint company of France’s Snecma and the USA’s GE Aviation, for instance, are fielding a new generation engine family – LEAP (leading edge aviation propulsion). A twin-spool, high bypass turbofan, it incorporates a range of innovative features which arose out of the company’s LEAP56 research and technology programme, which was set up in 2004 to develop a successor to the CFM56 – said to be the best selling engine family in commercial aviation history.

The inclusion of unique technologies and extensive use of composite materials is intended to achieve an impressive performance – increased propulsive and thermal efficiency allowing for reduced fuel-burn and lower emissions, while allowing the reliability and maintenance costs of the CFM56 to be matched.

LEAP’s performance targets are ambitious: 15% improvement in fuel efficiency compared with the best current CFM56 engines, 75% lowering of noise footprint and a 50% reduction in NOx emissions against the Committee on Aviation Environmental Protection CAEP/6 standards.

A number of factors contribute towards these goals. CFM has, for instance, opted for a unique fan blade technology – a 3-D woven composite, using resin transfer moulding – which imparts high durability and a significant weight reduction to the design. The more complex blade shapes this makes possible means fewer actual blades need to be used. There will be just 18 in the LEAP fan – half the number in the earlier CFM56-5B.

The weight benefit continues with the use of composites on the casings too, leaving each LEAP-powered aircraft weighing, according to CFM, ‘much less’ than a plane using similar engines with conventional metal fan blades and casings.

The engine core too employs novel and advanced features, including an ultra-high pressure ratio ten-stage compressor driven by a two-stage high-pressure turbine, and the second-generation, twin annular pre-swirl (TAPS II) lean-burn combustor – that is primarily responsible for the huge planned cut in emissions.

Aiming at certification in 2015, LEAP has already been selected to power three new single-aisle airliners. When they come into commercial service in 2016, the Airbus A320neo will offer the LEAP-1A as an option, and the -1C will be the sole Western engine for China’s Comac C919. One year later, the LEAP-1B is to be the exclusive power-plant aboard Boeing’s 737 MAX.

PW1000G – a game-changing geared turbofan engine

Pratt & Whitney’s new PW1000G utilises ‘game-changing’ geared turbofan architecture.

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The alternative option on the Airbus A320neo, as well as being the exclusive selection for the Bombardier C-Series, Irkut MS-21 and the Mitsubishi regional jet, Pratt & Whitney’s new PW1000G also addresses these same goals of efficiency, emissions and noise – to an essentially identical effect.

It too aims to slash NOx exhaust gases to 50% below CAEP/6, cut noise to a quarter and burn 16% less fuel than today’s best engines – with the promise of even greater savings in the future, as new airplanes come into service.

Achieving this, however, has taken the engine’s design along a different route.

The first of the ‘PurePower’ family, the PW1000G arose out of the earlier geared turbofan (GTF) programme, and it is this revolutionary GTF architecture that the company claim make it "game-changing".

Said to be the most efficient in its class, the large, light-weight fan is a ‘hybrid-metallic’ construction – a high-strength, proprietary design with a black erosion coating and titanium leading edge, which extensive testing has shown offers both the high intrinsic impact-resistance of metals and the low-weight advantages of composites.

Separated from the other engine modules by P&W’s innovative gear system, this fan can rotate relatively slowly, while the low-pressure compressor and turbine are able to operate at their higher, optimum speeds, thus increasing engine efficiency and achieving headline reductions in fuel consumption, emissions and noise.

Add to this the NOx-slashing Talon-X combustor, together with deliberate moves towards compact design, fewer parts and inbuilt high-cycle durability and, according to Pratt & Whitney, the era of geared architecture is only beginning.

Integrated propulsion systems

Nexcelle are working with GE on the integrated propulsion system (IPS) for the Passport 20 engine.

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Other new and developing technologies are being harnessed in the quest for greater efficiency. CFM, for example, are exploring the possibility of an open rotor version of LEAP, which could mean a 26% reduction in fuel burn – but that is unlikely to be a prospect before 2030.

More immediately, however, Nexcelle are working in close cooperation with GE on the integrated propulsion system (IPS) for the latter’s new Passport 20 (formerly TechX) engine, which is destined to power the Bombardier 7000 and 8000 long-range, heavy business jets when they enter service in early 2016.

The new nacelles being developed employ high-temperature composite materials, making them 15-20% lighter, and use directed-flow, pneumatic nozzle technology for de-icing, a high-efficiency system first developed for the GE/Honda HF120 engine and which requires less engine bleed air. The IPS will also feature retractable ‘kicker plates’ on the thrust reversers to improve in-flight aerodynamics. According to Nexcelle, by retracting when the engines are put into reverse, they should contribute around an additional quarter of a percent saving in fuel consumption.

IPS concepts are increasingly featuring in new designs as the need grows across the industry for extra fuel reduction, improved performance and improved maintenance requirements. Although the engines themselves obviously account for most of this, the extra boost achievable by an optimal IPS is not something to be ignored, and Nexcelle’s exhibition of its functional IPS half-scale model at September’s Aviation Expo/China 2011 drew much interest as a result.

It seems likely that many of the state-of-the art concepts on display – such as low-drag front ends with one-piece air inlets, integrated fan cowls, translating O-duct thrust reversers to improve improved fan flow-path and performance-enhancing mounts – will soon be standard features on all new designs.

Looking further to the future, however, if engine manufacturers really want to go all the way on efficiency, emissions and noise, then they could do worse than emulate the approach of Pipistrel’s Taurus Electro which won the Lindbergh Electric Aircraft Prize at last year’s AERO 2011 air-show in Friedrichshafen, Germany.

Scaling up the 54-horsepower electric motor – and the integrated ‘solar trailer’ that powers it – for use as a commercial aero-engine, however, might prove to be a very long term project indeed.