IN FOCUS: How to power A320neo is tough choice for airlines

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Choosing the right engine may never have been more important - or harder - than with the Airbus A320neo family. Unlike previous engine competitions, the choice between the CFM International Leap-1A and the Pratt & Whitney PurePower PW1100G is no longer a soft bet on a secondary supplier to an already chosen airframe.

The A320neo is almost identical to the previous version of the aircraft except for the choice of engine to power it. The paths taken by both engine makers to achieve the 15% rise in fuel efficiency Airbus is seeking for the A320neo, means airlines are not simply acquiring an engine but tacitly taking sides in an ongoing, furious debate about the future of gas turbine engine technology.

Gone are the comparatively subtle technical schisms which defined the differences between the International Aero Engines V2500 and the CFM International CFM56. In its place is a stark architectural, even philosophical, dispute with a reliance on new and exotic materials by the Leap-1A on one side and the introduction of a reduction gear inside the PW1100G on the other.

GEARED TURBOFAN ARCHITECTURE

Experience offers only partial assistance to airlines as they make their decisions. P&W has recently received Transport Canada certification for a smaller variant of the PW1100G which powers the Bombardier CSeries. However, the geared turbofan architecture has never been introduced into service, forcing airlines to rely on test results for key assumptions, including lifecycle maintenance cost.

Similarly, CFM joint-venture partner General Electric has managed the thermal cycle of the Leap-1A on the larger GE90 and GEnx turbofans, but is using new materials, such as ceramix matrix composites, for the first time.

Before airlines have any say, airframers cast the first vote, and they seem to be split. While Boeing rejected a competitive engine option for the 737 Max, Airbus was pleased to continue giving airlines a choice by selecting both available engines to certificate on the A320neo family.

New entrants Comac and Irkut were also split over the decision, selecting the Leap-1C and PW1400G respectively. Meanwhile, P&W's PurePower engine family has dominated the market for a new generation of large regional jets and small narrowbodies, including the CSeries, Embraer's second-generation E-Jet and the Mitsubishi Regional Jet.

Airlines differ in their engine choices as much as the airframers. In two years, Airbus has signed 54 contracts for 1,864 A320neo-family aircraft. Each deal is another opportunity for a referendum on the different engine options. So far, the orders almost evenly split between the Leap-1A (35%), the PW1100G (31%) or a selection yet to be made (33%). The CFM option enjoys a clear lead on the smaller of the two variants, including by far the most popular version with the A320neo. P&W is the strong favourite so far on the A321neo, but the number of undecided customers remains strong enough on the A320neo and A321neo to easily tip the lead towards either side.

Gas turbine engines will differ according to the manufacturer but essentially all work the same way: a gas turbine uses air to generate thrust to propel the aircraft and power to drive the engine. The air flow is ingested by the inlet fan, squeezed by the compressor section, ignited by the combustor and, finally, diffused through the turbine, which harnesses the energy of the heated gases to drive the inlet fan and compressor sections.

For three decades, airlines buying the A320 family had a choice between the CFM56 and the V2500, with significant differences between them. CFM freely acknowledges the CFM56 is usually the most expensive to buy when all other terms are equal, but that is only one factor in an airline's engine decision.

A key difference between the CFM56 and V2500 is housed in the high-pressure section of the turbine, which spins the high-pressure compressor. It is perhaps the most challenging area of any engine, as it must survive the hottest temperatures just aft of the combustor and still perform the hard work of driving the compressor.

On the V2500, IAE decided to use two rings of small airfoils called turbine stages, allowing each stage to bear only a portion of the overall load. By contrast, the CFM56 uses only one stage in the high-pressure turbine, resulting in a slight advantage for the CFM56 on lifecycle maintenance cost. One less high-pressure turbine stage means one less trip to the maintenance depot every few years.

CFM initially attempted to apply the single-stage architecture on the Leap engine family, but ultimately decided to switch to a two-stage high-pressure turbine. A likely consequence is an erosion in maintenance cost advantage, at least relative to the single-stage CFM56 versus the two-stage V2500. However, CFM believes it can offset the higher cost of maintaining two sets of turbine stages by using materials that have to be replaced less frequently.

EVOLVING MATERIALS

Materials are another matter of dispute and have been evolving as temperatures inside the gas turbine core have grown hotter. By the late 1960s, exhaust gases had grown hot enough to melt metal in the turbine stages. Engine manufacturers responded by hollowing the turbine stages and extracting cooler air from upstream of the combustor to keep the blades just cool enough to prevent melting. But CFM co-owner GE wants to eventually eliminate the cooling flow, thus preserving energy. The answer is switching to new materials that can survive hotter temperatures and, ideally, are lighter.

Since the mid-1980s, the aviation industry has been working to introduce ceramic matrix composites (CMCs). It has taken three decades to invent ways to affordably mass produce CMCs and overcome challenges such as thermal shock, in which the material shatters after exposure to extreme fluctuations in air temperature, such as an in-flight engine shutdown. However, CFM believes CMCs have finally reached the point where they can be reliably and affordably used in a non-moving component of the high-pressure turbine - the shroud which covers the blades in the first stage of the high-pressure turbine.

The Leap also features a compressor section more advanced than in any previous GE aircraft engine. The GEnx for the Boeing 787 and 747-8 introduced a combined blade and disc - or blisk - in the first of the 10-stage high-pressure compressor. CFM also uses blisks, but expands its use to the first five stages of the 10-stage compressor. The blisks, the new materials and the two-stage high-pressure turbine allow CFM to vastly improve the thermal efficiency of the Leap, yielding a double-digit improvement in fuel efficiency with a conventional architecture for a narrowbody aircraft engine.

If the Leap architecture is intended to optimise the thermal efficiency of the engine, P&W's PW1100G is mostly aimed at improving propulsive efficiency. There are generally two airflows in a turbofan engine - one that travels through the core of the engine and one which bypasses the core. The former is used mainly to drive the engine, although a small amount generates thrust. The latter, or bypass airflow, generates the majority of thrust.

A simple way to make the engine more efficient in generating thrust is to increase the amount of airflow that bypasses the engine core, or the bypass ratio. The only way to increase the bypass flow is to enlarge the diameter of the inlet fan, which is connected by a shaft to its power sources in the low-pressure turbine. In a conventional engine architecture such as the Leap, the low-pressure turbine and inlet fan rotate at the same speed. As the inlet fan diameter widens, the tips of the blades spin faster than the speed of sound, reducing efficiency and causing noise and vibration problems.

Instead, P&W introduces a reduction gear on the shaft that decouples the rotation speed of the high-pressure turbine and the inlet fan, allowing the latter to spin at one-third the speed of the former. As a result, the PW1100G has a bypass ratio of 12:1, twice the 6:1 ratio of the V2500. The reduction gear also reduces the load on the low-pressure turbine. The job of spinning the inlet fan and booster stages on the CFM Leap requires seven stages in the low-pressure turbine. The PW1100G inlet fan is 10cm (4in) wider than the Leap-1A, but uses only three stages in the low-pressure turbine.