Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.5.4. Reduction of Emissions
7.5.4.1. Earlier Developments
Between 1965 and 1975, low-smoke combustors were developed and incorporated into low
bypass ratio engines commonly used in early commercial jet aircraft. These changes virtually
eliminated visible smoke trails from aircraft. Introduction of higher bypass ratio engines of the
late 1960s and early 1970s-with their significantly improved SFCs-marked a new and important
step in reducing CO 2 and water vapor emissions from aircraft. These engines also emitted much
lower levels of HC and CO at low power ("idle") setting as a result of improved fuel/air mixing
and relatively high levels of pressure and temperature in the combustor at this condition.
Improved fuel/air mixing in annular combustors of the new engines also reduced take-off smoke.
The trend toward even higher bypass engines, with their improved fuel efficiency, continues
today, responding not only to the initial and continuing commercial and operational pressures but
also to increasing concern about the effect of CO 2 on the environment.
Between 1975 and 1985, new combustor design features that had better fuel atomization and
circumferential fuel staging at idle led to further reductions in HC and CO emission levels. A
typical example of such improvements in HC and CO emission is presented in Table 7-3, which
indicates the scale of some of the benefits that have already been made toward abatement of
urban air pollution burdens (Bahr, 1992).
At that time, however, there was little change in the NOx levels; although, as Table 7-3 shows,
they were well within ICAO standards, which applied to landing and take-off operations (ICAO,
1981). Emissions regulation for HC, CO, and NOx are based on the total mass of each species
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Figure 7-17: Effect of pressure ratio on NO x -specific fuel