<div class="htmlview paragraph">THE need for improvement of fuel consumption in modern aircraft is stressed. The basic economy of oil engines for airline service is briefly summarized, together with the effect of lower fuel consumption on the range and payload of airplanes.</div> <div class="htmlview paragraph">The theoretical efficiencies of the Otto and Diesel cycles are compared with the efficiency actually obtained on present aircraft engines. The data are presented in condensed graphic form for easy comparison. A prediction is made of the fuel economy that may be expected from the gasoline aircraft-engine in the near future, together with a summary of the means required to obtain it.</div> <div class="htmlview paragraph">The prospective place of the compression-ignition engine in commercial and military service is briefly outlined. An analysis of the weight possibilities of compression-ignition engines as compared to present gasoline engines is made. The two-stroke engine is selected as the type worthy of further development, and indications are that it will fall into the weight class with present gasoline engines. Both engine types will be further developed, and a specification for the future competitive Diesel is outlined.</div>