This paper first traces the early development of aviation engines in various countries. The six-cylinder Mercedes, V-type twelve-cylinder Renault, and six-cylinder Benz engines are then described in detail and illustrated. Various types of Sunbeam, Curtiss, and Austro-Daimler are also described. The effect of offset crankshafts, as employed on the Benz and Austro-Daimler engines, is illustrated by pressure and inertia diagrams and by textual description. The paper concludes with a section on the requirements as to size of aviation engines, four curves showing the changing conditions which affect the engine size requirements. These curves relate to variations of temperature, air density, engine speed, airplane speed and compression ratio required to compensate for decrease in air density, all as related to varying altitude.