Cirrus Aero-Engines Limited was a British aircraft engine manufacturer, known for their line of four-cylinder air-cooled vertical inline engines for general aviation use. The company traded between 1927 and 1931.
Cirrus engines were originally built by ADC Aircraft until Cirrus Aero Engines Limited was formed in 1927. The company became Cirrus Hermes in 1931 when it was bought by the Cirrus-Hermes Engineering Company and later became the Cirrus Engine Section of Blackburn & General Aircraft Limited in 1934, and operated as a separate division until production ended in the post-World War II era.
Cirrus's first product was the 90 hp (67 kW) Cirrus I, which passed its 50 hour type rating in 1925. It was the first air-cooled inline engine, a design that proved extremely popular for light aircraft. The basic layout was quickly copied by a number of other manufacturers. Later versions named the Cirrus II, and Cirrus III were produced each with slightly greater displacement, and power.
After purchase by Hermes the Cirrus Hermes I, II, III & IV were produced ranging in power from 105 hp to 140 hp depending on type. The later Cirrus engines were designed to run inverted.