On point: the iconic Rolls-Royce jet engine styling

Jet engines - battle for the skies

Cutting-edge materials science makes modern jet engines tougher and more powerful, and also promises airlines better fuel efficiency.

In the airline business, margins are everything. Large modern jet engines make up a huge proportion of an aircraft's weight, and the fuel used is a huge part of an operator's costs. A reduction in either could be the difference between profit and loss.

Fluctuating oil prices, fierce competition and challenging carbon reduction targets set by regulators are all increasing pressure on manufacturers to improve engine efficiency.

The world's 'big three' jet engine builders - General Electric (GE), Rolls-Royce and Pratt & Whitney - have all responded, unveiling new ranges of engines packed full of innovative technology and engineering.

Speed here isn't the most important thing - in fact, the engines will operate at similar speeds to those in service today. It's all about improving efficiency, minimising the environmental impact and, most importantly, reducing operating cost.

Using lightweight but ultra-tough composite materials, advanced high-pressure compressors, new planetary gearing systems and even 3D-printed components, competition is hotting up in a worldwide commercial engine market that in 2013 was estimated to have reached around $25bn.

Power of ceramics

The modern turbofan jet engines used to power commercial aircraft are incredibly efficient, but they are heavy. Unsurprisingly, one of the biggest opportunities for improving efficiency is in reducing this weight. What is more surprising is the material proposed to achieve this: ceramics.

In its new GE9X engine, GE Aviation is using a new range of materials it calls Ceramic Matrix Composites, or CMCs.

Composites are created when two materials are combined to create a new material with more desirable properties. GE's ultra-tough CMCs are made of silicon carbide ceramic fibres and ceramic resin, which are then further enhanced with proprietary coatings.

John Blank, manager of GE's Cincinnati CMC Laboratory, explains why they are so special: 'CMCs are one-third the density of the metals they replace. We experience substantial weight savings, which translates directly into a reduction in fuel burn.'

CMCs are super-strong, but super-light, making them the perfect material for the engine's largest component: its fan. At 335cm in diameter, the GE9X's fan is wider than the fuselage of Concorde. When it enters service, it will be the biggest in the world.

'The advancements in technology enable GE engineers to design a thinner GE9X blade, which is just as strong as our current composite fan blades,' says GE Aviation's spokesman Rick Kennedy. 'Fewer, thinner blades will enhance the airflow and make for a lighter, more efficient fan that will help with the GE9X engine's overall performance and fuel burn.'

The GE9X will have just 16 fan blades; six fewer than the GE90 engine. In addition to new materials, the reduction of fan blades is possible as a result of advancements in 3D design that enable engineers to create a more swept design and large fan chord.

In the end, GE estimates that total weight savings of CMCs in its engine could improve engine efficiency by up to 10 per cent. The more efficient an engine is, the less fuel it uses - meaning much-needed savings for the operator and less in the way of emissions.

Rolls-Royce is also investing heavily in its own development of materials technology, announcing the creation of a new Composite Technology Hub in Bristol where researchers will work on developing materials for its Advance and UltraFan engines.

Due to enter service in 2020, the Rolls-Royce Advance engine will feature hollow, super plastically-formed and diffusion-bonded titanium fan blades.

The company estimates that its CTi (carbon titanium) fan system could reduce engine weight by up to 680kg per aircraft - the equivalent of carrying seven more passengers and their luggage.

It's not just the blades. The huge fan casings on both the Rolls-Royce and GE engines will be made from composite materials, potentially saving almost 700kg.

Composites are stronger, which implies they are also safer. Current engines carry a lot of weight because of all the shielding needed to protect the plane and the people inside in case something goes badly wrong, says Mark Claydon Smith, a part of the EPSRC's Manufacturing the Future team. 'In the event of a [catastrophic] failure - blocks of metal flung off a breaking engine for example - an equivalent mass of composite material is much less dangerous,' he says.

The EPSRC is investing almost £5m in composite technology research, working with Rolls-Royce, Airbus and a number of other commercial partners to increase the knowledge transfer of composite technology from the runway to the air.

Feeling the pressure

Reducing weight is one way to improve efficiency; another is increasing compression. Jet engines use multi-stage axial compressors to increase the pressure of the air entering the engine. At full power the blades of a typical commercial jet compressor rotate at 1,600km/h, taking in 1200kg of air per second. That's about the same amount of air as in a squash court.

Higher compression ratios in engines are desirable because they enable the same combustion temperature to be reached with less fuel. However, as the air in an engine is compressed, it heats up, pushing the traditional nickel alloy materials to their thermodynamic limits, which is where composites can help.

'CMC parts are vastly more heat resistant, so you don't need to cool the parts by diverting cooling air from the engine cycle,' says Blank. 'Instead, that energy can be used to contribute to the thrust of the engine.'

Around 5 per cent of an engine's energy is used to power the cooling system, energy that could potentially be saved. It may not sound much, but on single A320 this could result in an annual saving of 1,200 tonnes of CO2.

The newer engines from all manufacturers include a higher number of compression stages. The GE9X engine uses an 11-stage compressor, achieving a compression ratio of 27:1, and an overall engine compression ratio of 60:1 - the highest ever achieved in the history of aviation. It's a massive advance over its current engines with six compression stages and an overall ratio of 40:1.

In its next generation UltraFan engine, Rolls-Royce is pushing compression technology even further. It will use 'the new advanced core architecture, enhanced with further technologies and the broader application of high-temperature materials, to push the core overall pressure ratio to more than 70:1,' says Alan Newby, chief engineer of Aerospace Future Programmes & Technology. In terms of efficiency, that's almost a third more than the Rolls-Royce Trent XWB that powers the Airbus A350.

Getting into gear

In turbofans, the air travelling through the jet engine itself provides approximately a quarter of total power, with the ducted fan providing the rest. The amount of air travelling around the engine relative to the amount passing through it is called the bypass ratio. The higher the bypass ratio, the more efficient the engine is - which also entails lower CO2 emissions and a reduction in noise.

'In turbofan engines, the necessary thrust is generated most efficiently with a high mass flow at a relatively low velocity - this can be achieved by higher bypass ratios which require larger fan diameters and lower fan speed,' says Edgar Merkl of MTU Aero Engines. Merkl coordinates ENOVAL (ENgine module VALidators), MTU's four-year project funded under the European Union's 7th Framework Programme. It involves 35 European partners from industry and research and is aimed specifically at developing new turbofan technologies.

But a combination of aeroplane design and physics means that engines - and more importantly the mean radius of the fan - can't just increase in size without causing problems. In a jet engine the different parts need to work at different speeds to be most efficient. As bypass ratios increase, it's essential that the fan runs at a slower speed than the rest of the engine.

In achieving efficient high bypass ratios of up to 12:1 in its new PW1000G engine, Pratt & Whitney has introduced an innovative gearing system years ahead of its competitors.

The 113kg gearbox itself is just 46cm wide, but must handle a massive 30,000 horsepower (22MW). In addition to the weight, the engineering challenge of incorporating a gearbox and dealing with the incredible heat'produced has taken the company almost 20 years, and cost tens of billions of dollars.

It's worth it though: in doing so, Pratt & Whitney estimates the PW1000G is 20 per cent more efficient than its predecessor, potentially saving around $1.7m per plane annually. And the efficiency has been achieved without any substantial increases in engine size.

In its engines, Rolls-Royce currently uses an innovative three-shaft design, enabling the compressor and turbine to run at slightly different - and more efficient - speeds, avoiding the need for a gearbox. But to achieve the sort of bypass ratio figure demanded by the industry, it's only a matter of time.

'We will introduce a power gearbox between the fan and low-pressure turbine, to ensure the fan runs at its optimum low speed for noise, while maximising the turbine speed for efficiency. In common with our three-shaft architecture, the high-pressure compressor and turbine continue to run at their optimum speed to deliver optimum performance,' says Newby.

3D printing

Aircraft parts must be manufactured within incredibly precise specifications, with some engine components machined to a tolerance of 50μm. This is why high-precision, 3D-printed components are starting to become increasingly important in engine manufacture.

'Three-dimensional printing is certainly a way of using really difficult-to-produce materials and quite complex forms,' says Iain Todd, professor of metallurgy at the University of Sheffield. 'The sweet spot is a complexity of form, reduction in part count and complexity of material.'

In its Leap engine, GE is introducing the first flight-certified 3D-printed part, an engine nozzle. In the original design, 20 parts were machined together to create each one of the engine's 19 nozzles. Now there's just one - and it is much stronger. The GE9X engine will feature hundreds of 3D-printed components.

Ultra-high bypass

As well as precision, 3D printing offers other benefits, such as significant weight savings by reducing and removing the redundant mass of certain parts, says Todd.

The engines from GE, Rolls-Royce and Pratt & Whitney are all due to enter into service in the next decade and, while marking huge improvements, still won't achieve the efficiency rates - and related emission reductions - demanded by regulators.

So just where can the technology go?

The ENOVAL project is now looking at achieving bypass ratios of up to 20:1, with the help of cutting-edge technologies that would make fan diameter up to 35 per cent bigger and core sizes smaller. Better aircraft-engine integration matters, too, says Merkl.

But it's not that simple. More efficient, smaller and lighter core engine modules require high-stage loading and their higher operating temperatures mean they are more sensitive to leakages and clearances. 'With the technologies being developed, the expectation of optimum bypass ratios lies within 15 and 18,' adds Merkl.

Such ratios just might help manufacturers reach the stretching regulatory targets, but there will always be pressure to improve, push boundaries and save money.

In a market place known for disruptive technology - and for massive R&D budgets - Rolls-Royce is working on electric propulsion for aerospace, potentially the next disruption that may render the jet engine obsolete.

'The basic principle is that you now use electric motors to drive distributed fans to give you the propulsive force previously generated by jet engines. The ultimate question is whether you can make the electrical machines and the electrical technology as efficient and light as today's gas turbines that uses kerosene,' says Newby.

It's a whole new front in the never-ending battle for the skies.

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