7.5.4.3. Longer-Term Technology

Much of the longer term combustion research is now aimed at what is termed "ultra-low NOx technology." This technology is defined as technology that produces NOx levels that are no more than 50% of the ICAO CAEP/2 standards. All major manufacturers of aircraft engines, with the sponsorship of associated government agencies, are currently pursuing combustor technologies aimed at reducing NOx emissions to these levels, under operating conditions typical of the next generation of engines. This approach entails pursuing technology that is relevant to engines with pressure ratios from current levels (ª30 to 40) to future levels (above 50). The aim is to reduce NOx production in the vicinity of airports and at subsonic cruise conditions. Two key programs are aimed at large, subsonic, high-pressure ratio aircraft engines:

• U.S. NASA Advanced Subsonic Technology (AST) Program: Sequential NOx goals of 50 and 70% reduction below CAEP/2 standards, with comparable reductions at subsonic cruise; engine SFC reduction of 8-10% over the most recent production engines by about 2010
• EU-BRITE/EURAM Projects Low NOx I,II,III: Goal of NOx reduction more than 60% below CAEP/2; engine SFC reduction of 8-10% over the most recent production engines by about 2010.

These programs specify that there must be no compromise in performance, safety, or any other emissions parameters (smoke, CO, and HC).

Details of combustor technologies are commercially sensitive and are not openly available. However, three parallel strategies are being pursued. They vary in their NOx reduction potential and their associated increase in complexity, cost, and development challenge:

• Reductions in NOx levels close to 50% of CAEP/2 standards are being sought through the optimization of current single-stage combustor technology. This approach involves further improvements in fuel injection uniformity, better fuel/air mixing, reduction in combustor liner coolant flow (making more air available for combustion), and decreases in hot gas residence time. Such changes would have minimum impact on engine cost of ownership. The first examples of this type of technology now exist in manufacturers' product plans for the next 5 to 10 years.
• Reductions in NOx levels to 50-70% below CAEP/2 standards are being explored using multi ple burning zones in radial and axial configurations (Figure 7-23). These concepts permit local temperature and residence time in the combustor to be controlled and optimized at each engine operating condition, to minimize NOx and other emittants (Bahr, 1992; Segalman et al., 1993). In low-power operations, a single stage is fueled and optimized for stability. At high-power conditions, one or both stages,configured for lean burning, are fueled. Generally, radial staged designs are shorter and lighter but larger in diameter, making it more difficult to achieve a uniform exit temperature profile at off-design conditions. They are also more difficult to cool. Axial staged systems are longer and have a larger number of fuel injectors. In both cases, the increased complexity and weight of these combustors is expected to increase the cost of ownership, as described by DuBell (1995). A major effort is needed to minimize the cost, complexity, weight, and performance of these concepts. Reduction in NOx levels to 85-90% below CAEP/2 standards are focused on emissions from supersonic aircraft at cruise conditions. Such work presently forms part of the U.S. supersonic transport program. Combustors known as lean premixing prevaporizing (LPP) and rich burn quick quench (RBQQ) are being studied. In the former, the burning zone is fed with a lean and homogeneous fuel/air mixture. Premixing and prevaporizing takes place in a premix duct outside the combustor. The RBQQ combustor consists of three zones: A primary rich burning zone; a dilution zone, to rapidly reduce the rich mixture to a lean one without recirculating dilution air into the primary zone; and a lean reburning zone (DuBell, 1995). These combustors uniquely apply to the supersonic engine with its relatively low engine pressure ratio and its requirement for long periods at a single, high-speed cruise operating condition. Application of these concepts to future subsonic engines would pose special problems because of higher pressure ratios. For example, the LPP combustor will have to overcome the greater risk, at very high pressures, of "flashback" or upstream burning-which, if undetected, could damage the combustor. Similarly, the RBQQ combustor-with its high fuel concentration in the primary burning zone-may well result in the generation of large amounts of soot and smoke in high-pressure operations. However, some of the features found in these concepts may be suitable for higher pressure ratio subsonic engines. Partial premixing, coupled with moderately rich sector burning, represents one such concept. Multiple burning zones, together with variable geometry to control local fuel/air ratios, would be another. A concept incorporating several of these features (shown in Figure 7-23b) is being pursued under the European Union's Targeted Research Action "Efficient and Environmentally Friendly Aero-Engines." A radially staged configuration currently under "main" phase development (LOWNOxIII) combines an RBQQ candidate pilot injector with a premixing main zone injector. This concept might provide low NOx emissions with acceptable operational capabilities at LTO and cruise conditions of subsonic aircraft (Zarzalis et al., 1995).

7.5.5. Future Technology Scenarios

As part of the preliminary work associated with this report, industry was asked to consider what advances in technology might be applicable for aircraft in the year 2050. Numerous projections were made by an expert group from the aeronautical industry (engine, airframe, and aerospace manufacturers). The group provided their best judgments of fuel efficiency and NOx technology scenarios for the year 2050 (ICCAIA, 1997f) to the ICAO Committee on Aviation Environmental Protection Forecasting and Economic Support Group (FESG) for use in Chapter 9 of this report. The assumptions for the 2050 scenarios were as follows:

• Continued demand for worldwide commercial/regional/ general aviation aircraft
• Technology advancement to be addressed for all engine sizes
• Unrestricted kerosene availability
• Development time and operational life of modern aircraft = 40-50 years
• All airworthiness requirements achievable
• Economically viable
• No impact on noise
• Possible NOx reduction scenarios.

Chapter 9 of this report considers the conclusions of the group in conjunction with FESG traffic scenario projections. Together, these considerations take account not only of long-term demand but also of fuel burn and emissions (see Chapter 9). The latter two depend on three factors:

• Airframe technology developments (discussed in Sections 7.2 and 7.3)
• Trends in engine design (discussed in Section 7.4)
• Combustion system development-in particular NOx reduction scenarios, based in one case on the best near-term combustor technology and in the other on longer term NOx reduction combustor technology (discussed in Sections 7.5.3 and 7.5.4, respectively).

The benefits arising from the first item influence projections in terms of fuel efficiency alone. The other two items take into account the net effects of cycle changes and emissions reductions strategies. Two potential long-term aircraft technology scenarios emerged from these deliberations. These scenarios are summarized in Table 7-6.

The outcome of these deliberations is that a basis now exists for the development and ongoing monitoring of future research strategies as air transport and concern about its environmental impact continues to grow.

7.5.6. Summary of Key Points Relating to Combustion Technology

Several important conclusions can be drawn from the present assessment of combustion technology in relation to emissions production and control:

• Research and development programs over the past 25 years have provided a basis for vastly improved combustion systems for modern aircraft engines. Current combustion systems achieve virtually 100% energy conversion efficiency at virtually all power settings. Better fuel/air mixing processes, liner cooling techniques, and materials have contributed to this progress. The levels of unburned products, such as CO and HC, from engines are now very low, and visible smoke levels are now under control.
• Reductions in CO2 depend primarily on engine cycle, not the combustor. The role of the combustor is to ensure that the demands of the more fuel-efficient cycle (low CO2) are met without compromising engine performance.
• The more fuel-efficient engines, with their high bypass ratios, introduced in the 1970s and 1980s reduced CO2, HC, and CO emissions but increased NOx. Although technological improvements have constrained the rise in NOx, it has become clear that more substantial reductions require more radical solutions (with associated risks and penalties).
• Technology that has reduced NOx emissions at high power, near the ground, also reduces NOx at high altitude, though not necessarily by the same amount.
• Current research goals are to achieve NOx levels of 50% of current standards in 5 to 10 years. Work is also in progress to achieve NOx levels 50-70% below current NOx standards using more advanced, staged combustors.
• Although there has been a marked improvement in the measurement and assessment of trace species and aerosols emerging from engines, knowledge about their formation-hence our ability to control them-is extremely limited.

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