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. This factor inhibits the prospect of further reducing CO 2 by changing fuels. Looking further ahead, Section 7.8 also considers briefly the use of alternative fuels in the longer term beyond 2050. The later sections of the chapter concentrate on the smaller numbers of special category aircraft in the global fleet. The first of these categories is small aircraft as used in the regional sector of air transport.