Aircraft engines pollute less than ever, yet air transport emissions are growing. Regulators are preparing new measures

Julian Moxon/PARIS

Gradually, the aviation industry is waking up to the fact that it faces fundamental changes to its operations as a result of the atmospheric pollution it causes.

Air transport has so far not been subject to the kind of intense environmental attention that has been directed at other transport systems. Although this is largely because it still contributes only a small proportion - about 3% - of total global warming emissions, it is also true that it is only around airports that the public is aware of the pollution caused by air travel, whereas by far the largest proportion takes place in the upper atmosphere. And yet, while engine emissions around airports are regulated, those in the upper atmosphere are not.

Major uncertainties still surround the effect of aircraft engine emissions on the upper atmosphere. What seems certain, however, is that not only is there an effect, mainly from the production of carbon dioxide, oxides of nitrogen, water and other chemicals, but that the impact on global atmospheric composition may be more serious than previously thought.

The "bible" on aircraft pollution is the landmark 1999 report by the Intergovernmental Panel on Climate Change (IPCC) called "Aviation and the Global Atmosphere"(Flight International, 20-26 October 1999). This went into unprecedented detail about what - and, more significantly, what is not - known about the effects of aircraft engine pollution.

The report made no attempt to make policy recommendations or to suggest preferences as to how aviation and its associated rulemaking bodies should approach the emissions problem. Instead, it provided a comprehensive study into all of the gases and particles emitted by aircraft into the upper atmosphere and the roles they may play in modifying its chemical properties and in initiating the production of contrails and cirrus clouds. It looked at how the properties of the atmosphere might be modified in terms of climate change and how the ozone layer could be affected, changing the amount of ultraviolet radiation arriving at the Earth's surface.

The report is not sensational and therefore has become a trusted document upon which regulatory and industry bodies are basing their work. It admits that a huge amount of work remains to be done in the area of atmospheric aircraft emissions, not least to understand the trade-offs that are thought to occur between exhaust chemicals that on the one hand may increase the global warming effect, and others that might actually reduce it.

In summary, the IPCC report makes the following points:

· Global passenger traffic is expected to grow by about 5% to 2015, whereas total aviation fuel use will grow by 3%, the difference being because of improvements in engine fuel-efficiency.

· In 1992, aviation contributed about 3.5% of the total man-made pollution that adds to the greenhouse effect. The report points out, however, that it is not yet possible to separate the influence of aviation on the effect from that caused by other man-made sources.

· Aircraft produce about 2% of total carbon dioxide (CO2) emissions per year, or about 13% of CO2 emissions from all transport sources. (CO2 is the principal global warming pollutant.) This is projected to increase to 3% by 2050. The report notes, however, that CO2 remains in the atmosphere for at least 100 years, so the effects are cumulative.

· Emissions of nitrogen oxides (NOx) from subsonic aircraft are estimated to have increased ozone concentrations at cruise altitudes by 6% since 1992. This is projected to increase to about 13% by 2050. Aircraft are more effective at producing NOx in the upper atmosphere (troposphere) than at the surface. But NOx is known to destroy another greenhouse gas - methane - so the effect may be mitigated. Ozone increases caused by NOx are also thought to be offset partially by aircraft sulphur and water emissions. The report says this is an area in which more research is badly needed.

· Aircraft contrails, produced by water vapour from the exhaust, covered about 0.1%of the Earth's surface in 1992. This is expected to increase to at least 0.5% by 2050 - faster than the rate of increase in fuel consumption because the number of jet-powered aircraft that fly at contrail-producing altitudes is increasing. Contrails are of increasing concern because they can remain in the atmosphere for a long time and directly influence the greenhouse effect. They are also known to contribute to the development of cirrus clouds, which cover about 30% of the Earth's surface. The report asks for further research into the correlation between the two.

· Technology advances have significantly reduced emissions per passenger kilometre. Aircraft now in production are about 70% more fuel-efficient than they were 40 years ago. A further 20% reduction is expected by 2015 and a further 40-50% by 2050 - although this is taken into account in estimates of emissions in mid-century.

Environmental groups have not yet reached the level of sophistication achieved by the IPCC in their assessment of aircraft pollution - but it is only a matter of time. Indeed, the UK-based Friends of the Earth released its own study earlier this year, "The Myths of Flying", which pointed to a number of areas, such as the lack of any tax on aviation kerosene, in which it considers aviation to be unfairly privileged. It has also launched a campaign, "The right price for air travel", bringing together 300 environmental and citizens' organisations in 20 countries to "raise public awareness of the environmental effects of air travel and urging national and international governments to take action".

Environmental concerns are being taken increasingly seriously by regulatory bodies and international organisations. The European Commission has already made clear that it intends to develop a regulatory approach on atmospheric emissions which, if the ICAO assembly in 2001 fails to reach what it regards as satisfactory progress on noise and emissions regulation, it may pursue unilaterally. The Commission has also increased dramatically funding for research into cleaner, quieter aero-engines in its Fifth Framework research programme, which aims at "breakthrough achievements in the environmental performance of aircraft and their engines and the understanding and assessment of the atmospheric effect of aircraft exhaust gas emissions".

Funding will be channelled mainly through the Key Action on Global Change, Climate and Biodiversity under Thematic Programme 4 on Energy, Environment and Sustainable Development. In addition, research and development on aircraft and engine aspects of engine and aircraft exhaust gases and noise emissions will be part of the New Perspectives in Aeronautics programme. This includes funding for ultra-low emissions combustor concepts.

The Commission's directorate-general for transport and energy agrees that there are "a lot of uncertainties, but it is clear we are confronted with a problem". One senior official says that while it is clear the overall contribution of aviation to man-made greenhouse gases is small, "we must prepare for the need to address improvements in environmental performance".

There is little dissension from the accepted faith that advances in technology have enabled aviation to make impressive reductions in pollution over the last 20 years. It is also clear, however, that the rate of change has slowed and is now at a plateau, there having been no quantum reductions in fuel efficiency since the introduction of high-bypass ratio turbofans in the early 1970s. New turbine materials enabling higher combustion temperatures and improvements in aerodynamics have come along, fan efficiencies have been improved through better design, and three-dimensional aerodynamics is now featured in all stages of some engines. All these have led to reductions in fuel consumption, and hence pollution, enabling aviation emissions to be reduced despite its growth. However, taken over the last 10 years, the EC says the average annual rate of fuel consumption reduction has slowed to less than 2%. This is not likely to change for the foreseeable future, even though the engine manufacturers in particular, but also the airframe companies, are working on new technologies all the time. In today's environment, however, in which any modification or new design is subject to severe cost/benefit and safety analyses, not all of the technologies are guaranteed to make it into the air.

Another factor is that aircraft take much longer to develop than, say, cars, and so the rate of introduction of new technologies is slower - typically 10 or 15 years. The technologies also have to be suitable for the tough environment in which they will work. Laminar flow, for example, while promising useful improvements in aerodynamic efficiency - and a resulting 1-2% lower fuel burn - has been through extensive development and test, but has not yet proved cost-effective and reliable enough to buy its way on to the aircraft.

The engine industry

By definition, aircraft engines are at the very heart of the two environmental issues facing aviation - noise and pollution. In the days of pure turbojets and low-bypass ratio turbofans, aircraft were extremely noisy and often produced significant quantities of smoke at take-off. However, this was tolerated by a public still flirting with the tremendous advances brought by jet power, and which had yet to suffer the consequences of the growth that was to come.

The introduction of high bypass-ratio engines in the late 1960s and early 1970s dramatically cut fuel burn - and hence emissions of most pollutants, but this was offset rapidly by the industry's sheer rate of expansion. Although engine manufacturers are recognised as having contributed two-thirds of the 50%reduction in fuel burn per passenger achieved by aviation over the last 30 years, they now face a tougher regulatory environment than ever before.

The quest for fuel efficiency has always concentrated on increasing core temperatures to improve the thermodynamic efficiency of the engine. While the resulting improved fuel burn means that overall emissions are cut, it has a disadvantage in terms of one important greenhouse gas pollutant - nitrous oxides.



"Reducing NOx is the main challenge," says Hamish Low, engineering director of combustion systems at Rolls-Royce. Other pollutants, particularly carbon dioxide, are also being targeted - but Low points out that the amount of CO2 produced by an engine is directly related to the amount of fuel it burns and is therefore cut automatically as specific fuel consumption is reduced. "NOx is produced as a result of high flame temperatures in the combustion chamber, so our design philosophy aims to reduce those temperatures," he says. High combustion temperatures and pressures are fundamental to improved fuel efficiency, and so engine cycle developments make the job of controlling NOx more difficult.

At present, R-R's combustion technology has enabled levels of the pollutant to be maintained at just 70% of the current NOx regulatory standard (CAEP 2/1997) set against a background of higher core temperatures and pressures.

For the time being, R-R is sticking firmly to single-annular combustor designs for all of its powerplants. It rejects the double-annular approach adopted by General Electric in the GE90 engine (see p125) because it believes that the cost and reliability advantages of its own proven technology outweigh GE's claimed performance and pollution reduction improvements. The UK company is therefore concentrating on improving the aerodynamics and materials of its single-annular combustor.

"We have programmes in place to meet any future planned NOx standards," says Low. "We have a two-track approach, but we intend to stay with single-annular designs as long as we can." He adds that R-R has double-annular - or staged - designs ready "as soon as we're confident it is the right way to go".

Double-annular combustors are "very complex and their potential to reduce NOx is diminished at higher temperatures", says Low. NOx production is sensitive to the amount of time the mixture spends at high temperatures and pressures. The requirement is for a double-annular design comprising a rich, high-residence time "ignition" zone and a weak, low-residence time main zone, with maximum flame stability in the pilot zone and minimal NOx production in the main zone. The volume of the combustor must also be minimised to reduce the dwell time of the fuel/air mixture.

Low says that Rolls-Royce, in common with other engine manufacturers, is studying pre-mixing the fuel and air before injection. This holds the promise of the next "step change" in NOx reduction, he adds, because it results in a more uniform mixture that does not have the hot spots that give rise to NOx production. "We believe this technology will be consistent with the NOx levels that will be set for 2012/2015," says Low.

General Electric/Snecma

CFM International produces the only engine in the world with a dual-annular combustor (DAC)available as an option for airlines and is seeing sales of DAC-equipped engines increase steadily as pollution-related financial penalties become more popular in Europe. Sales engineering director Francis Couillard says, however, that while combustor technology is central to pollution reduction, "people must realise you can't treat this problem in a piecemeal way". He says reoptimisation of the engine cycle is also a "vital component" to ensuring that the best results accrue from improvements in efficiencies brought by improved three-dimensional compressor and turbine blade design.

The DAC design originated in the NASA/ GE Energy Efficient Engine research programme in 1978-83. Its two annular combustion chambers work either alone or together to optimise the burning of kerosene throughout the aircraft's operating range. The outer combustor is optimised for the engine running at idle conditions, when CO2 and unburned hydrocarbons are prevalent, calling for high flame residence time and a larger chamber to ensure complete combustion. The inner combustor is optimised for full throttle running, when NOx reduction is the main consideration, achieved through the lower flame residence time and reduced flame temperatures possible with the smaller combustor. The fuel flow to both chambers is regulated by a full authority digital engine control - an essential development for this type of combustor.


The first engine to be equipped with a DAC was the CFM56-5B/2, which entered service on a Swissair Airbus A320 in January 1995. Since then, DAC engines have built up more than one million flight hours on 180 Airbus and Boeing 737 New Generation aircraft.

The most recent DAC customers are Air Europe Italy and Volare. Interestingly, all DAC engines have to date been ordered by European airlines, reflecting the increasing emphasis on emissions taxes at airports such as Zurich, which started taxing emissions in 1997. This is based on a scale of landing fee penalties which goes from 0% to 40% of the basic fee and is calculated on a complex measurement of NOx and unburned hydrocarbons produced by an engine, taking deterioration into account. CFM points out that its DAC-engined aircraft attract no surcharge. In Sweden, for example, it claims a 5% reduction in landing fees for DAC-equipped aircraft, amounting to a $70,000 saving per year. CFM also claims that DAC engines produce NOx emissions 37-46% lower than those provided for in the ICAO2004 standard. "Only the DAC engine is capable of supporting that standard," it says.

GE's main emissions reduction effort for future engines is based on a pre-mix combustor concept derived from the DAC, known as the twin-annular pre-swirl (TAPS) combustor.

Developed in conjunction with NASA under the Advanced Subsonic Technology (AST) programme, TAPS is being tested for potential application in the GE/Snecma TECH56 initiative to develop technologies for new and current CFM56s as well as other GE powerplants, including the CF6 and GE90 families.

"TAPS is not yet a proven concept , but it has shown a lot of promise in scale testing," says GE Aircraft Engines combustor technology manager Willard Dodds. He says that the combustor has the potential to reduce emissions of carbon monoxide, hydrocarbons and even nitrous oxides by up to 50%. Verification of these claims will begin soon, when GE starts full annular rig tests of the combustor.

The TAPS design takes the DAC principle further by pre-swirling the fuel/air mixture to achieve a higher degree of mixing before it enters the main combustion chamber. This reduces the length of chamber necessary to achieve full combustion and meets one of the main anti-NOx criteria - reduced flame residence time and lower flame temperature. A further advantage is that the ceramic centrebody separating the two DAC combustion chambers is eliminated. This has been a source of maintenance problems in the original chamber.

Although the TAPS combustor is aimed at new designs, CFMI's Couillard says it "could be retrofitted to existing engines if regulations forced it".

Pratt & Whitney

Pratt & Whitney is also working with NASA on the Ultra-Efficient Engine Technology (UEET) and Turbomachinery and Combustion Technology (TCT) programmes and is funnelling new combustor technology into its PW6000, planned PW8000 and joint venture GP7000 under its "Green Engine Programme". The leader of the project, Robert Tierney, says the programme's efforts "reflect the goals of our parent company, United Technologies, to reduce waste generation and toxic air emissions by 60% and energy and water use by 25% by 2007".

The starting point for much of the work is the Technologically Affordable Low NOx (TALON) combustor design, which P&W developed for the PW4000 models and is introducing to the 2.54m-diameter fan version of the engine. The second-generation TALON II reduces NOx emissions by 23% compared with the first-generation TALON I unit on earlier PW4000s. Together with the company's "floatwall" combustor liner, P&W says the latest burner helps cut CO2 emissions by 10% and hydocarbon emissions by 63%. Key to the improvement is redesign of the fuel injectors which speed the "quench" period, or the time when fuel burn is at its peak temperature in the combustor. P&W says: "This reduces the total amount of time the fuel-air mix is in the combustor. Since the engine's quench is what creates NOx, the reduced quench cuts down on NOx emissions."


NASA has identified CO2 and NOx as targets of the UEET programme - the recently launched successor to the Advanced Subsonic Transport and parallel High Speed Research projects. UEET goals will "address long-term aviation growth potential, without impact on climate, by providing technology for dramatic increases in efficiency to enable reductions in CO2 based on an overall fuel savings goal of up to 15%". The NOx reduction target is 70% for take-off and landing, with substantial cuts to allow aircraft to cruise without affecting the ozone layer.

Within UEET, the focus for combustor research is the TCT project, which is investigating active combustion control as a means of developing lean-burning devices. This novel concept counteracts the instabilities normally associated with lean-burning combustors by modulating fuel injection in the combustor itself. TCT initiatives include the use of micro-electromechanical systems (MEMS) to manage and monitor the burn pattern and allow "smart" pulsing of fuel within the combustor. NASA believes the "smart" engine will ultimately be capable of teaching itself to control the spikes and instabilities that currently make such lean combustors impractical.

Another concept under study for NOx reduction is lean direct injection. This involves building a complex series of fuel mixers in a laminated construction. Although the mixer appears to be exceptionally complex, NASA says it is relatively simple to build because it only requires the vertical stacking of several layers, each of which is etched with different patterns of channels and drilled with holes to allow the fuel to mix with air. Early flame tube tests are showing NOx reductions of up to 80%. In tandem with computational analysis, NASA is also developing the National Combustor Code, which will provide engineers with the key to future combustor design work.

Source: Flight International