The Rolls-Royce Merlin is a British liquid-cooled V12 piston aero engine of 27-litre capacity. Developed as a private venture by Rolls-Royce and first run in 1933, the engine became one of the most successful aircraft engines of World War II, most closely associated with the Hawker Hurricane and Supermarine Spitfire, although the majority of the production run was for the four-engined Avro Lancaster heavy bomber.