Addressing Environmental Concerns

Although the industry contributes only two per cent of the carbon dioxide emissions from fossil fuel use, the aviation industry per se is beset with environmental issues. It is reported that nitrogen oxide (NO_X) emissions from burning jet fuel cause acid rain and smog. Also, the emission levels are likely to double by 2020. To counter this, in 2001, the European aviation industry set a target to reduce fuel consumption by 50 per cent per passenger-kilometre by 2020 and NO_X emissions from commercial aircraft by 80 per cent during the same year.

Hence, the onus lies on every segment of the aviation sector, from the aircraft manufacturer to the engine maker to an operator to a maintenance team to reduce the environmental impact. The engine maker comes into much more play as efficient engines make a substantial difference on not just the performance of the aircraft but also in monetary terms, effecting savings to the operator. Engine manufacturers understand this reality and are constantly working towards improvement in engines.

The urgency to have engines, which make a difference environmentally as well as financially, is seen by all. Spurred by environmental regulations and ever rising fuel costs, jet engine manufacturers are developing products that reduce emissions and noise pollution. Two well-known engine manufacturers, Honeywell Aerospace in Phoenix, USA, and Rolls-Royce in Derby, UK, are carrying on enormous research work that is going on to make engines that address the concerns of not only operators but also of those who are championing the cause of the environment. Researchers in Europe and America are focusing on next-generation concepts such as open rotor and embedded engines to increase efficiency and reduce noise.

For instance, Rolls-Royce at its Derby facility is working on the revolutionary Trent 1000. To maintain its leadership position, the company has made a commitment to invest in research in areas of advanced technologies such as:

• High efficiency compressors and turbines with fifth-generation 3D aerodynamics.
• Advanced lean burn low emission combustors.
• Advanced light-weight, heat resistant materials such as ceramic matrix composites or CMCs which allow the engine to operate at temperatures equivalent to half the temperature of the surface of the sun while maintaining component lives.

Rolls-Royce’s Advance

Earlier this year Rolls-Royce announced not one but two new engine designs. The first called, “Advance” offers some 20 per cent better fuel burn than the first Trent engines and will be ready by the end of this decade. The second, “UltraFan”, a geared design that could provide at least 25 per cent improvement in fuel burn and emissions, is expected to be ready for 2025.

Rolls-Royce also deploys, where beneficial, new technologies to its in-service engine programmes. New technologies have helped produce the Trent XWB, said to be the world’s most efficient aero engine. The Trent XWB will be entering service this year on the Airbus A-350 XWB with Qatar Airways.

Rolls-Royce has signed up to goals set by the Advisory Council for Aeronautics Research in Europe (ACARE) an organisation with about 40 members including government agencies and private companies. ACARE calls for the following achievements by 2020, compared with the baseline year of 2000: 50 per cent reduction in fuel burn and carbon dioxide (CO₂) emissions per passenger-kilometre; 80 per cent reduction in NO_X and 50 per cent reduction in perceived external noise levels.

Of the 50 per cent reduction in fuel burn however, 20 per cent is expected from engines, 20 per cent from airframes and ten per cent from better efficiency in air traffic management such as flying more direct routes. European engine maker MTU, by contrast, has proposed its own Clean Air Engine (CLAIRE) strategy, targeting a cumulative 30 per cent reduction in fuel burn from the power plant alone by 2035.

Geared Turbofan Advantages

Similarly, General Electric GEnx and the Pratt & Whitney Geared Turbofan (GTF) have shown significant advances in environment-friendly technologies. GE, like most engine manufacturers, has started using lightweight materials. New in the GEnx is the use of a composite fan case which reduces weight and improves corrosion control. The composite fan case alone trims off 350 pounds, compared with a metal version.

Pratt & Whitney aims to drive engine efficiency higher and pollutant emissions and noise levels lower with its Geared Turbofan, a system that differs significantly from the Trent 1000 and the GEnx. Pratt & Whitney Canada is a leader in the green evolution in aerospace with a new-generation of technologies and environmental stewardship across all facets of its business, outperforming the most stringent ICAO standards. Today it is working on some 600 ‘Green’ projects in collaboration with 20 universities across the country.

Pratt & Whitney’s Low Emission Technologies

Today, Pratt & Whitney Canada is a leader in developing low emission technologies. This achievement is the result of a revolutionary Technology for Advance LOw NOx (TALON) combustion technology that reduces emissions while delivering outstanding performance, durability and economy in operations.

Pratt & Whitney Canada’s PW307 is the greenest engine in its market and shows an improvement of more than 30 per cent in emissions relative to ICAO standards. Its TALON 2 combustor technology also meets Zurich 5 requirements for no surcharges.

Another great example is Pratt & Whitney Canada’s new PurePower PW800. This engine will show an improvement of up to 50 per cent in NO_X emissions relative to ICAO standards, as well as an improvement of 35 per cent in carbon monoxide emissions. In the longer term, new technologies are being developed by Pratt & Whitney Canada to reduce NO_X down to the 80 per cent reduction level, benefiting from further combustion and engine performance improvements.

“A gear system, inserted between the fan and the low-pressure turbine allows us to run the fan and the low turbine at different speeds to optimise the fan speed independently from the low-pressure turbine speed,” Paul Adams, Senior Vice President (Engineering) has said. The company aims at bypass ratios of up to 16 to 18 and a noise level of 20 dB below the prescribed ICAO standards.

Key partners in GTF development include MTU Aero Engines for the low-pressure turbine, Avio for the fan drive gear system, Volvo Aero for the turbine exhaust case and Goodrich for the nacelle. The technologies currently under development that are being hailed as plausible solutions in the medium term include the intercooled engine, the intercooled recuperated engine and the open rotor.

CFM LEAP Cuts Fuel Burn

GE is collaborating with partners Snecma, Airbus, Boeing, COMAC and Nexcelle to develop CFM LEAP (TM) aircraft engines for the Airbus A320neo, Boeing 737 Max and COMAC C-919 families of aircraft due to enter service in 2016. By utilising an extensive suite of advanced engine technologies, the CFM LEAP (TM) aircraft engines are being designed to provide significant reductions in fuel burn, noise and NO_X emissions compared to the current CFM engine models offered in this aircraft class, the CFM56-TI, at equivalent levels of maintenance cost and reliability.

Sustainable and Green Engine ITD

The Sustainable and Green Engine (SAGE) ITD of Clean Sky demonstrates five engine technologies contributing towards the environmental targets set by ACARE. There are six engine projects contained in the programme, each targeting specific technologies and market sectors, led by a leading member of the European engines industry.

Open rotor technologies offer the potential for significant reduction in fuel burn and CO₂ emissions relative to turbofan engines of equivalent thrust. Higher propulsive efficiencies are achieved for turbofans by increasing the bypass ratio through increases in fan diameter but there is a diminishing return to this improvement as nacelle diameters and consequently weight and drag increase. Open rotor engines remove this limitation by operating the propeller blades without a surrounding nacelle, thus enabling ultrahigh bypass ratios to be achieved.

Further improvements in propulsive efficiency can be gained for open rotor engines by using a second row of propeller blades rotating in opposition to the front row to remove the spin from the column of air to give a more direct thrust. The technical challenges of counter rotating open rotor engines are many, but are principally reduction of the noise created by the propeller blades to counter the loss of attenuation provided by a turbofan nacelle.

Rolls-Royce has developed an open rotor propeller design to minimise noise and has demonstrated the effectiveness of these designs through scaled rig testing in the FP7 DREAM programme. The SAGE1 project is planned to acquire technology for the propulsion system, increasing the Technology Readiness Level (TRL) to TRL 5.

The various R&D efforts of all the engine manufacturers are going to impact the future of aviation as fuel and environment concerns are going to remain on top of the agenda.