In 1914 Otto Hieronimus manufactured the six-cylinder Hiero E, also known as the Hiero 6 engine which was derived from earlier 4-cylinder engines. The Hiero engine like the Austro-Daimler powered many of Austria's World War I aircraft. Hiero aircraft engines were designed by Otto Hieronimus, a famous Austrian auto racer of the early 1900s. His initial designs were liquid-cooled inline engines built by the Laurin & Klement Automobile Works of Austria.
The Hiero E had a 135 mm x 180 mm bore/stroke (15.46L / 943.4cuin) and delivered 200-230 hp. The engine had the typical features of an inline vertical 6-cylinder: aluminum bed, cast iron cylinders, one inlet and one exhaust valve per cylinder controlled by bars and rockers, with the cam-shaft in the bed, and dual ignition by two Boschmagnetos. During World War I, the highly regarded Hiero engines were built under license by Essler, Warschalowski & Company of Vienna and the Breitfeld-Daněk company of Czechoslovakia. A point of recognition of Hiero engines compared with other Central Powers OHC inline's is that the intake was on the right and the exhaust on the left. After World War I, production was continued by the Avia company and powered a number of their early aircraft.