Production line for the Merlin engine.
The Rolls-Royce Merlin is a V12 piston aero engine of 27 litres (1,650 cu in) capacity. Rolls-Royce designed the engine and first ran it in 1933 as a private venture. Initially known as the PV-12, it was later called Merlin following the company convention of naming its piston aero engines after birds of prey.

After several modifications, the first production variants of the PV-12 were completed in 1936. The first operational aircraft to enter service using the Merlin were the Fairey Battle, Hawker Hurricane and Supermarine Spitfire. The Merlin remains most closely associated with the Spitfire and Hurricane, although the majority of the production run was for the four engined Avro Lancaster heavy bomber. A series of rapidly applied developments, brought about by wartime needs, markedly improved the engine's performance and durability. Starting at

1,000hp for the first production models, most late war versions produced just under 1,800hp, and the very latest version as used in the de Havilland Hornet over 2,000hp. Lancasters were known to fly entire missions with their engines running at full power with no ill effects.

One of the most successful aircraft engines of the World War II era, some fifty versions of the Merlin were built by Rolls-Royce in Derby, Crewe and Glasgow, as well as by Ford of Britain at Trafford Park, Manchester.

Source: wikipedia.org