Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Aviation and the Global Atmosphere
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7.9. Small Aircraft, Engines, and APUs
Other reports in this collection
Small aircraft engines are defined here as all turbo engines-excluding piston engines because of the very small amount of fuel burned by these engines (much less
than 5% of the world's fleet fuel burn; see Section 7.2)-that are used for regional aircraft, all turbofans with less than 89 kN thrust, all turboprop and turboshaft engines,
and all auxiliary power units (APUs) used in civil aircraft.
The small commercial and general aviation segment has been growing rapidly in recent years and is likely to continue to do so. This segment's impact on the
environment, however, is unlikely to be significant because of low NO x emissions levels associated with the generally lower pressure ratio engines they employ and
the decreasing percentage of the fleet's fuel burn they represent (see Chapter 9). Furthermore, most of aircraft in this sector fly short missions with lower cruise
altitudes and reduced potential for climatic impact. Significant improvements have been made in the idle emissions of small engines in recent years, so that CO, HC,
and NO x emissions from small regional and general aviation aircraft are often comparable, in terms of emissions per kilogram of fuel burned, to those from large
engines (Eatock and Sampath, 1993).
This section highlights the key differences between large and small engines in terms of emissions characteristics and control technologies that might apply to small
aircraft and engines. A brief overview is presented on small airframe technology, engine performance, engine emission databases, combustor technology, and unique
issues related to small engine combustors.
7.9.1. Airframe Technology
The airframe technology applicable to small aircraft parallels that of large aircraft discussed in
Section 7.3. Designers and manufacturers continue to strive to reduce drag and increase range/
payload performance with a resultant steady improvement in overall fuel-efficiency of new small
aircraft. Propeller design, of course, is of much greater importance to this sector of the world's
fleets. Detailed aerodynamics research is showing some of the significant performance benefits-
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