The impact of potential aviation gas turbine fuels available in the near to midterm, is reviewed with particular reference to the small aviation gas turbine engine. The future course of gas turbine combustion R&D, and the probable need for compromise in fuels and engine technology, is also discussed. Operating experience to date on Pratt & Whitney Aircraft of Canada PT6 engines, with fuels not currently considered of aviation quality, is reported.

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Current concern with reduction of pollution from combustion systems resulted in a research program being undertaken to evaluate the potential of variable combustor geometry for reducing emissions from an annular reverse flow gas turbine combustor. A device which simulated a variable baffle in the annulus surrounding the combustor was investigated. Results indicated a good potential for this approach, if the combustor was specially developed for low emissions. The fluid dynamics of this concept were investigated in a model using flow visualization techniques. The hot tests under this program were carried out at atmospheric pressure with full simulation of combustor flows. Significant reductions in emissions and corresponding improvements in combustion efficiency were observed when a baffle was incorporated in the combustion system. It is recognized that all important pressure effects are absent in this work, but a subsequent program is planned to demonstrate the variable geometry concept on an engine over a full range of operation.

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The EPA aircraft emission regulations were promulgated in 1973 and resulted in urgent investigations of many approaches aimed at reducing gas turbine emissions with minimum penalties to normal combustion performance. The impact of this work on small aircraft gas turbine engines is discussed, and emission reduction techniques and data are presented.Unique problems experienced with smaller gas turbine combustion systems are reviewed as are the potential difficulties of developing higher performance small combustors in the future, without the benefit of the complex and costly mechanical approaches which are applicable to the larger engines. The impact of relaxed fuel specifications and alternate-source gas turbine fuels is discussed in terms of altered fuel properties and development of fuel injection technology.