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 engine had the typical features of an inline vertical 6-cylinder: aluminum crankcase, cast iron cylinders, one inlet and one exhaust valve per cylinder controlled by bars and rockers, as part of a "SOHC" (single overhead camshaft) valvetrain like the Mercedes D-series aviation engines of the German Empire, and dual ignition with two Bosch magnetos.

A point of recognition of Hiero engines compared with other Central Powers OHC inlines 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.