When the Great War broke out, the company was known for its motorcycle engines and particularly associated with a "pannier honeycomb" radiator design. It was already involved in aero-engine design. In 1909, the C.4 had been the only motor to complete the tests for the Patrick Alexander Competition but was not awarded the £1,000 prize, because the rules called for a engine while the C.4 only averaged . The competition was re-run the following year for more powerful engines: this time, Green gained the prize with the D.4. Up to 1912 Green was the only source of all-British aircraft engines capable of producing and so the only choice when prizes were offered for all-British aircraft. The best known case is John Moore-Brabazon's winning the £1,000 "Daily Mail" prize for a circular flight by a British pilot in an all-British aeroplane in his D.4-powered Short Biplane No. 2 in 1910.