Powering the future: Designing tomorrow's aircraft engines

Nowhere has this been more apparent in recent years than in the design and development of combustion engines. Energy and power generation throughout the world is built primarily around the effective combustion of fossil fuels to produce energy. But, scarcity and environmental concerns have given a clear imperative to combustion engines: to get more with less. Despite being perhaps the most energy efficient and reliable combustion engines on the planet, aircraft engine manufacturers have been outlining plans and technology road maps that aim to help slash aircraft emissions by 50% over the next 10 years. While an aircraft engine development programme can cost more than $2 billion, it is money well spent in a market worth an expected $3 trillion between now and 2030. Aircraft engines are some of the highest value engineered products on the planet. To understand what that means, and to put it in context, a Ford Fiesta, weight for weight, has a value roughly equivalent to a Big Mac. An aircraft engine on the same measure is, by contrast, worth the same as a Rolex. The need to change To fly an airliner at transonic speeds requires an extraordinary amount of energy. On average, a mid-sized airliner will need more potential energy onboard than a Royal Navy frigate. It means the fuel of the future, for airliners at least, is likely to remain Jet-A or Kerosene, widely regarded as the highest energy density, room temperature and fuel available. Efficiency – specifically fuel burn improvement – is something the three largest aircraft engine OEMs have been developing since the beginning of commercial air travel. Pratt and Whitney, General Electric, and Rolls-Royce have all broadly been following the same technology development trend up until now: ie improving the thermal efficiency of the engine core (turning fuel into shaft power) and improving propulsive efficiency (turning shaft power into forward thrust). Multiplied together, this gives an overall efficiency estimated to currently be around 30-38%, depending on the aircraft and engine. Broadly, this has been done by running the engine cores hotter and by improving the control of the combustion process through the engine. In addition, ever smaller cores have been used to assist in thermal efficiency improvements. The other area of improvement is in propulsive efficiency, which has seen front engine fans become much larger to increase the bypass ratio (i.e. the ducted air around the core of the engine). This trend has seen low bypass turbofan engines with a bypass ratio of 2:1 being replaced by high bypass engines of 5:1, with a new generation of ultra-high bypass engines beginning to be used, typically aiming to be around 10:1 or more. However, it is clear the status quo of smaller cores and bigger fans is reaching a practical limit. Engineering the next generation of aircraft engines is now pushing materials, mechanical knowledge, packaging, and reliability into new and uncharted territory. "You want to get the core of the engine to spin very quickly for high power density, so you can make it smaller," says Dr Alan Epstein, vice president of technology and environment at Pratt & Whitney. "But, you also want to make the fan very large and allow it to rotate slowly. So, clearly, it's a good idea to change the speed ratio between the core of the engine, which makes the shaft power, and the fan that propels the aeroplane." Up until now, shaft and fan speed have been the same. And, while it sounds obvious, engineering a gear capable of transmitting the horsepower at the necessary efficiency has been the Holy Grail for aircraft engine manufacturers. Indeed, Pratt and Whitney's commitment to the geared turbofan is seen as risky and has at times been mocked by competitors. "But, we have such a gear now," says Dr Epstein. "The gear is 0.5m in diameter, weighs around 130kg, transmits 30,000hp at 99.5% efficiency and it will run for 20 years before it needs maintenance. That was the kind of gear we needed to make this work and be able to present it as a practical option. But it has taken a dedicated team of engineers 25 years to develop." The geared turbofan offers much-needed longevity to Pratt and Whitney. Indeed it is believed it opens up areas for further optimisation and efficiency improvement, both of which have been getting harder to find on existing turbofan platforms and engine configurations. However, the expected 15% or more improvement in overall engine efficiency of Pratt and Whitney's geared turbofan is also reliant on a number of other technical innovations. The fan blades will be much larger, but need to be ultra-lightweight. In addition, they need to be stronger, more reliable, and able to take multiple bird strikes. Fan blade technology development has seen increased consensus, with carbon fibre composite fan blades generally considered favourable. Indeed, General Electric has been the first to developed and certify composite fan blades for its GEnx series of ultra high bypass engines. "And we also thought fan blades would have to be a 3D woven composite," says Dr Epstein. "We developed them for the geared turbofan, but it turns out they're not. "Actually the best option is a hybrid metallic. It is a little lighter and can be made on any machine tool and is therefore half the cost. Also, we can make them thinner out of metal than we can out of woven composite, giving about a 1% improvement in efficiency." Rolls-Royce, too, has chosen to stick with metal, at least for the time being, selecting titanium for the fan blades of its upcoming Trent XWB. It is, however, preparing to make the switch to lighter composite fan blades as the manufacturing technology matures. To assist in this development, it has set up a joint venture with GKN Aerospace to develop and test composite fan blades and the associated production systems. It coincides with another GKN Aerospace-led project called G5Demo, part of the Swedish Green Aerospace Demonstrator programme, which aims to contribute to reduction of aircraft CO2 emissions by 50% by 2020. GKN Aerospace's contribution to the G5Demo project will include introducing innovative manufacturing technologies and advanced materials such as lightweight state of the art engine structural parts, which include turbine structures that are 15% lighter and able to operate at temperatures 200°C higher than today, as well as a metal composite hybrid fan-frame that is 30% lighter. "The pressure to reduce weight and operate at higher temperatures has become really strong," says Robert Lundberg, director of European research and technology programmes at GKN Aerospace Sweden. "We are studying the replacement of the fan-frame, and the large fan structure with carbon fibre composite materials instead of titanium as it is today. "The problem with designing these parts is actually one of manufacture. The target is to be able to produce composite parts with the complicated geometry, with the right technical requirements, and at a high rate." Rolls Royce has also set out more clearly its plans for future developments, and it looks to be following Pratt and Whitney's lead of using a reduction gear between the shaft and fan. The expectation is that developments are some 10 years off. Dubbed the UltraFan, the geared design will also use a variable pitch fan system as well as having an advanced engine core to improve fuel burn efficiency and lower emissions. The aim is to offer a 25% improvement in fuel burn and emissions. While Pratt and Whitney and Rolls Royce have committed to using a gear, General Electric is yet to announce such an undertaking and has instead already brought to market its advanced ultra-high bypass engine, the GEnx. The engine is highly optimised, but perhaps the most revolutionary component is its combustor. The Twin Annular Pre-mixed Swirlers (TAPS) allow an extremely uniform fuel and air mixture that provides uniform flame temperature through the chamber. Systemic improvement "Something unusual happens during integrations," says Dr Epstein. "If I know I have a gear, a large fan, and a lightweight nacelle, each offers a 2% improvement if I optimise the system architecture. But, instead of 6% improved fuel burn improvement, I get 16%. It works because of system integration and that is a key part of future engine design. "The bad news is, if the engineers do not deliver one of these technologies, then we don't get that extra 10%, we end up with 4% fuel burn improvement, and that difference will be an utter disaster." Not just composites Though composites have become synonymous with lightweight and performance, engine components operating at higher temperatures have not been able to benefit quite as much due to higher operating temperatures. A turbine structure is typically made using a nickel based super alloy at the exhaust of the engine. This is commonly produced using an advanced casting process. However, GKN Aerospace is exploring the option of replacing the casting process with a welded structure. Essentially it's the same material but in wrought and sheet metal form. "It is a lot stronger as you use sheet or forged material, so you can go down to a thinner cross-section and get a 15% weight saving," says Robert Lundberg, director of European research and technology programmes at GKN Aerospace Sweden. "This is not a new material, but a new method. "With such a welded structure you can add high temperature material where it is needed. This is a way of saving weight, having a bi-metallic or combination of different metals, welded together."