Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
have shown that the rate at which the mass formation rate of NO x increases as the "throttle
bend" technique is used to increase thrust is similar to data measured on a wide range of engine
families over a significant range of engine operating pressure ratios (ICAO, 1995a). It is also
worth noting that, in practice, the required thrust at cruise and measured cruise SFC remain
essentially constant for the "throttle push" versions, resulting in increases in fuel efficiency levels
as payloads and number of passengers have risen.
7.5.4.2.2. Combustor design considerations
Current combustors can contribute little more to fuel efficiency because at power levels above
"idle" the energy conversion level is virtually 100% (see Tables 7-4 and 7-5). However,
superimposed on all combustor design considerations is the continuing underlying requirement
that low emissions features must not compromise basic combustion requirements or have any
significant effect on engine performance. In recent years, this requirement has imposed difficult
problems in introducing new emissions reductions features. Table 7-4 provides a list of basic
requirements, and Table 7-5 highlights options and compromises that designers have had to
face.
Figure 7-18 provides a qualitative indication of the development process as engineers work to
reconcile the combustion system constraints listed in Tables 7-4 and 7-5 and the operational
requirements of the engine.
7.5.4.2.3. Achievements
Low-emissions combustors currently fall into two categories. The first category is composed of
existing combustors, which have incorporated relatively minor changes to liner and/or fuel nozzle
designs to improve emissions. Recent examples of changes that have been quite successful in
reducing NO x emissions are shown in Figure 7-19.
The NO x emissions of state-of-the-art combustors are 20-40% lower than those of older
combustors. Figures 7-20, 7-21, and 7-22 provide evidence of some of the progress that has
been made in recent years to avoid what had earlier appeared to be an unavoidable link
between lower NO x emissions and relatively high CO and HC emissions. In these examples,
relatively minor changes to the combustor's airflow pattern and the location of fuel injection
resulted in reduced CO/HC without a NO x penalty.
Although this category of improvement entails relatively minor changes, the development and
engine recertification process remains a long one. Because safety is the overriding concern in
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Figure 7-20: Example of combustion system enhancement
for CO and HC, reduction EI basis (Mongia, 1997b).
Figure 7-21: Tradeoff between NO x and CO with a
staged combustor (Mongia, 1997b).
Figure 7-22: NO x and HC concentrations for a staged