The Task Ahead For China’s New Commercial Engine-Maker

Chris Kjelgaard details some of the technologies with which the newly formed Aero Engine Group of China will need to become familiar.

Chris Kjelgaard details some of the technologies with which the newly formed Aero Engine Group of China will need to become familiar in order to compete on level terms with the West’s big four commercial aero-engine manufacturers.

No-one questions that China has a genius for manufacturing high-technology items such as smart phones, miniaturized computer chips and a myriad of different kinds of consumer electronic goods in vast quantities and for very low prices.

However, the task for the newly formed Aero Engine Group of China (AEGC) in becoming competitive in international-markets with the West’s big four established commercial aero-engine manufacturers is quite different to that faced by China’s producers of mass-production items as they try to remain dominant in the global marketplace.

AEGC’s task of achieving technological parity with – and surpassing if possible – the technologies the West’s commercial-engine manufacturers are adopting in their latest engines, and are preparing to adopt in future engines, may be at least an order of magnitude greater in its degree of difficulty.

If AEGC is to become a major force in the international market outside China, it must, from the outset, be able to offer an engine which it can demonstrate can remain on wing for at least 30,000 hours without requiring a major shop visit. This is the standard which Western-manufactured engines such as the CFM56 family have achieved routinely for years.

At the same time, AEGC must put in place a robust logistical network to support the spares-delivery and MRO requirements for its engines. It must establish a network of high-quality, very reliable, licensed MRO facilities throughout the world which are capable of performing major unscheduled and scheduled MRO tasks on AEGC’s commercial-aircraft powerplants.

Even before it does so, however, AEGC’s task in designing and manufacturing its first commercial turbofan engine will involve a steep technological learning curve.

Western manufacturers have already gained a couple of decades of experience in designing, manufacturing and operating components made from advanced, high temperature-resistant ceramic matrix composites (CMCs) and installed in the engines powering high-performance military aircraft.

Through GE Aviation, manufacturing joint venture CFM International is now putting this experience to good use by incorporating CMC materials, for the first time in any commercial aero engine, in its new LEAP family.

Similarly, all the major Western manufacturers have become thoroughly adept at making use of the rapid advances in computational fluid dynamics (CFD) analysis techniques allowed by the growth in computer processing power.

They use CFD analysis to model very exactly the four-dimensional airflows (three spatial dimensions, plus the dimension of time) of the airflows passing through their engine designs, even as these designs are being refined using advanced computer-aided design capabilities.

This allows the Western manufacturers to know in great detail in advance just how the airflows in their engines will perform in operation. CFD enables them to make all sorts of 3-D adjustments to blade, vane and stator designs – at every stage in every fan, compressor and turbine module – in order to improve the propulsive and aerodynamic efficiency of their engines.

CFD analysis also allows the Western manufacturers to model the exact conditions inside the combustors and highest-temperature areas of their engines. This helps these companies model and design optimal pathways for the cooling air bled from the high-pressure compressor (HPC) stages of their engines.

At the same time it helps them develop combustor injectors which mix the fuel and air entering the combustor into a completely homogenous, very fine vapor which provides a lean mixture that burns evenly. This ensures that combustors not only are very fuel-efficient, but also that they produce very low levels of greenhouse-gas emissions.

Having gained decades of experience of very high operating temperatures from their military-engine work, the Western manufacturers are able to apply that knowledge to their commercial aero-engine designs. Today’s commercial engines have increasingly small, increasingly efficient cores but operate at ever-higher exhaust-gas temperatures (EGTs).

In a modern large commercial engine, the temperature of the exhaust gas exiting the combustor can reach more than 3,000 degrees Fahrenheit. This creates the need for manufacturers not only to develop advanced, highly temperature metal alloys with which to make their engines’ high-pressure turbine (HPT) blades, but also to acquire advanced capabilities in two other technological areas.

One is the development of advanced thermal barrier coatings, which are used to coat the surfaces of the HPT blades. Few if any metal alloys developed to date are able to withstand temperatures of 3,000 degrees-plus for long, so the blades need to be covered with special coatings which confer enough additional temperature resistance to allow the HPT blades to keep operating durably and reliably.

The other technology required is 3-D modeling of and the high-precision manufacture of cooling air pathways inside the HPT blades. Through tiny holes in the surfaces of the blades, these pathways feed relatively cool air bled from the HPC – air which actually has a temperature of several hundred degrees Fahrenheit – to cover the surfaces of the blades with a laminar air layer which has a much lower temperature than the extremely hot exhaust gas. This also acts to keep HPT blades working reliably in extreme EGTs.

New advances in cooling and engine control techniques are allowing the major Western manufacturers – CFM being the first, in the LEAP – to modulate the flows of cooling air to the HPT, to optimize engine performance during different phases of flight.

Similarly, advances in cooling-circuit designs are allowing these manufacturers to provide active tip clearance control for HPT, low-pressure turbine and HPC blades. These circuits feed cooling air into the engine casing material adjacent to the blade stages, allowing the casing to expand and contract as directed to ensure that the clearance between each stage’s blade tips and the casing is optimal for every phase of the engine’s operation.

For instance, while the aircraft is taxiing, the engine doesn’t need to be highly efficient in producing thrust, so plenty of clearance between the blade tips and the casing ensures the blade tips don’t suffer unnecessary wear. Here, another new technology developed by Pratt & Whitney for its PW1100G-JM engine also helps: coating blade tips with ultra-hard material made from cubic boron nitride so they don’t wear as fast.

Pratt & Whitney’s PW1000 family also makes good use of another capability allowed by the development of advanced CFD analysis. This is the incorporation into each engine of a reduction gearbox between the low-pressure spool and the fan drive shaft, enabling both shafts to turn at their optimal speeds to make the engine much more efficient.

The main reason P&W has been able to develop advanced, relatively lightweight reduction gearboxes for each member of its PW1000G family is that CFD analysis enabled the manufacturer to understand in great detail the 3-D flow of oil through the gearbox, lubricating and cooling its constituent gear-steel parts.

Rolls-Royce has indicated it intends to use reduction-gear architecture in its future Ultrafan large-engine designs, ending the British company’s reliance on the three-shaft architecture for its big-fan engines, whether ducted or unducted.

Another area in which AEGC must make rapid technological gains is in developing the lightweight, incredibly strong materials needed to make the fan blades of today’s commercial-turbofan engines. Whether it prefers to create its fan blades from carbon-fiber composite materials, aluminum honeycomb, a lightweight titanium alloy or a combination of all three, AEGC will need to ensure its blades have highly aerodynamic 3-D aerofoils in order to offer a high degree of propulsive efficiency.

Gaining this knowledge might sound daunting, but for AEGC the task may be made even more complex by the fact that the two largest US engine manufacturers have been making rapid advances in designing and developing adaptive-cycle fan engines.

These engines have three, not two, airstreams, the third airstream offering a variety of ways to improve engine performance throughout all phases of flight. Adaptive-cycle technology in the turbine modules also may prove highly useful in the future for improving engine performance and efficiency.

Although the US manufacturers are developing their demonstrator adaptive-cycle designs for application in the next generation of military fighter aircraft, history indicates that one day 20 or so years from now they will find ways of incorporating adaptive-cycle fan technology into commercial engines too, perhaps particularly those for supersonic commercial aircraft. Were AEGC to be unlucky, this could happen just as it was bringing to market its first large turbofan engine.

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