Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Correlations between measured emissions at static sea level conditions and altitude operating conditions, as discussed in Section 7.7.2, are generally applicable to
small engines. Exceptions may occur for very small engines in which combustor surface to volume ratios, fuel atomization quality, combustor volumes, and so forth are
very different from those in large engines. Small engines may require additional correlation parameters to account for these differences (Rizk, 1994).
7.9.5. Combustor Technology
7.9.5.1. Historical Developments and Current Status
The ability of gas turbine manufacturers to design successful small engine combustors with
reduced emissions has improved greatly in the past 20 years. Idle efficiency data, indicative of
CO and HC levels, collected by the General Aviation Manufacturers Association (GAMA)
(Eatock, 1993) and plotted in Figure 7-39 indicate combustion inefficiencies at idle power of
between 5 and 15% in early combustors. The trend, however, shows that modern small engines
approach and even match the idle combustion efficiency of modern large turbofan engines.
Figure 7-40 shows the basic "Lipfert" correlation (ERAA, 1992; Eatock and Sampath, 1993) of EI
(NOx ) in grams of NOx per kilogram of fuel burned. The correlation relates emission indices of
NOx to compressor discharge pressure (pressure ratio) and covers most engines operating at or
near stoichiometric burning at full-power condition. Many small aircraft engines have even lower
EI(NOx ) than those anticipated for the future third generation of large turbofan engines.
7.9.5.2. Unique Combustors for Small Engines
The major challenge to reduce the emission level of NO x for small engines is overcoming size-
related constraints. NO x reduction strategies may be restricted, for example, by the smaller
Figure 7-42: Ultra-low NOx combustors for supersonic
commercial transport applications.
passage height between the inner and outer diameters of the combustor, which limits the size and number of fuel injectors that can be used for such purposes. Small
combustors are also more sensitive to minor size variations within fixed manufacturing tolerances. For these reasons, and to ensure that all such combustors will meet
a given emissions goal, "nominal design" hardware must be able to demonstrate compliance with somewhat larger emissions margins than for larger engines. Many
smaller engines use centrifugal compressors in combination with reverse-flow combustors. These combustors have a higher surface-to-volume ratio.Furthermore,
scaling down a design has the effect of rapidly increasing combustor surface area-to-volume ratio (ICCAIA, 1993). Both factors tend to result in the need for a higher
percentage of combustor airflow for cooling the combustor, leaving a smaller amount of air available for controlling emissions. Together these effects limit the direct
translation of low NO x combustor technology from large engines into smaller combustion system designs. The NASA-sponsored contract for small turbofan engines
(Bruce et al., 1977, 1978, 1981) demonstrated the problems very clearly. This work showed that even a modest 30% reduction in NO x could be achieved only using a
design that featured variable geometry and staged combustion, with a totally impractical increase in the number of fuel injectors.
7.9.5.3. Current and Future Trends
In recent years, manufacturers of small engines have continued to develop new emissions control techniques that have minimum cost and performance impact. These
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