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FUELS for use in aircraft engines are discussed with reference to their antiknock value, volatility, vapor-locking and engine-starting properties, gum content and availability, and to antiknock agents. The usefulness of a fuel for spark-ignition engines is stated to be limited by its tendency to heat the cylinder and the piston unit. Definite evidence is available that the tendency of fuels to heat the cylinder unit is not always in accord with their tendency to cause audible knocking. The fuel required depends upon the compression ratio of the engine, its volumetric efficiency, the design, size and temperature of the cylinder unit, and the rate of revolution. Mid-Continent Domestic Aviation gasoline having an approximate antiknock value of 50 octane-50 heptane gives excellent results if the engine output is kept within the limitations of this fuel but is not suitable for many modern aircraft engines if flown wide open at sea level.

THE great increase during the last 20 years in the knowledge of fuel behavior in piston engines the author attributes largely to the use of full-scale engines for fuels testing and engine development work, the use of pure compounds as reference standards, and the standardizing of laboratory knock-test methods. He sees preignition, fuel-air mixture distribution, and stability of stored fuels as engine-fuel problems which may have to be tackled in the next few years if development of the piston aircraft engine continues. Mr. Heron presented this lecture after receiving the Horning Memorial Award for 1945.