Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
Considering the breadth and complexity of the technology base supporting today's aircraft, this chapter cannot hope to provide more than an overview of the subject.
Emphasis has therefore been placed on the following key questions:
● What are the principal technological factors that determine the nature and scale of emissions from aircraft at altitude?
● What progress has been made to date in reducing emissions, and how may new advances in aircraft and engine technology help reduce them further in the
future?
● What data exist about actual emissions from aircraft? What is being done, and what needs to be done, to improve our understanding of and our ability to predict
the scale and nature of these emissions?
● How are emissions from aircraft currently regulated, and how do these regulations influence emissions at altitude?
● What performance might we expect from fleets operating in 2015 and 2050 and in setting the scenarios discussed in Chapter 9?
The structure and balance of the chapter have been developed to reflect the fact that advances in technology that influence the impact of aircraft on the environment
fall broadly into two categories:
● Innovations that improve fuel efficiency, thus reduce the amount of fuel burned (and mass of emissions) per passenger-km flown
● Developments that may alter the percentage concentration of a particular exhaust gas (e.g., reduce NO x for a given mass of fuel burned).
Broadly, advances that reduce the weight and drag of the aircraft fall into the first of these two categories. These advances are covered in Sections 7.2 and 7.3, which
provide background material and a review of current development themes most relevant to the fuel efficiency of modern aircraft.
Engine technology is more complex. Fuel efficiency is closely linked to engine type (e.g., high bypass ratio) and choice of thermodynamic cycles (e.g., pressure and
temperature ratios), but changes in the design of the engine's combustion system can also have a significant effect on the composition of the exhaust plume. These
two aspects of engine design are dealt with in Sections 7.4 and 7.5.
Section 7.4 introduces the principal performance and design constraints that designers of new engines face and comments on future trends. Section 7.5 takes account
of engine cycle trends on the design requirements of new low-emissions combustors. This section is a key part of the chapter because it deals with the component-the
combustor-that has the greatest potential for design changes that may reduce the concentrations of some emissions that are of concern. In particular, this section
addresses some of the issues raised in Chapter 2. The challenges are complex, and additional background material is included to describe the many fundamental
conflicting characteristics of combustion processes that must be reconciled in emissions reduction technology programs.
Trace species in engine emissions are also considered. Potentially important physical and chemical changes to these species occur in the engine as the gas travels
rapidly downstream from the combustor and through the turbine stages, where they undergo sudden changes in pressure and temperature. Section 7.6 discusses the
present state of knowledge in this field.
Section 7.7 provides background information about work to date in developing the international engine emissions database. It also reports on progress in using data
gathered from ground-based tests to predict corresponding cruise altitude emissions levels. These methods are used in the development of predicted inventories for
future scenarios in Chapter 9.
Aircraft fuel continues to be a subject of considerable interest. Kerosene-type fuels are in widespread use today and are likely to remain so in the foreseeable future.
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