Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.5.3. Production of Engine Emissions
Other reports in this collection
Pollutant formation in combustion, regulated aircraft emissions during landing/take-off, and currently unregulated cruise emissions are covered in some depth by
Brasseur et al. (1998). For convenience, we briefly describe the principal points here, with somewhat greater detail given to more recent findings about soot and
particulate emissions.
Under ideal conditions, combustion of kerosene-type fuels produces carbon dioxide CO2 ) and water vapor (H2O), the proportions of which depend on the specific fuel
carbon to hydrogen ratio. Figure 7-14 shows the ideal and "real" combustion processes. Figure 7-14 also illustrates the scale of the combustion products by showing
that at cruise conditions they constitute only about 8.5% of total mass flow emerging from the engine. Of these combustion products, only a very small volume (about
0.4%) of residual products arise from non-ideal combustion processes (soot, HC, and CO) and the oxidation of nitrogen (NOx ).
Table 7-2 gives typical emission levels for various operating regimes. The emission values are quoted as an emission index (EI) in units of grams of emittant species
per kilogram of fuel burned.
Table 7-2 clearly illustrates the constancy of emissions indices for CO2 , H2O, and SOxO throughout the flight cycle. These emissions are directly related to the fuel
consumption of the engine in its various flight phases. In contrast, emissions such as NOx , CO, HC, and soot are strongly influenced by a wide range of variables but
particularly engine power setting and ambient engine inlet conditions. CO and HC are products of incomplete combustion. They are highest at low power settings,
when the temperature of the air is relatively low and fuel atomization and mixing processes least efficient. This problem area is proving responsive to improvements
linked to detailed studies of basic fuel/air mixing processes. NOx and soot (not shown in the table), on the other hand, are highest at high power settings.
The majority of NO x emissions are generated in the highest temperature regions of the combustor-usually in the primary combustion zone, before the products are
diluted. The fundamental processes of NO x formation are well known and documented (reviewed and described in detail by Bowman, 1992). They are best expressed
as a function of local combustion temperature, pressure, and time. Combustion zone temperature depends on combustor inlet air temperatures and pressure, as well
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