The history of the Rolls-Royce Merlin engine

It is, quite simply, one of the most recognisable sounds of the last century. Tara Leggett delves into the history of the second most-produced engine of World War II to understand its adored status 

It produced the power that thrust so many into the airborne battles of World War II, but the world’s most famous historic aircraft engine was initially known as the somewhat forgettable PV-12. Yet Rolls-Royce’s revolutionary design, which first ran in 1933, would later follow the company tradition of naming its piston engines after birds of prey.  

The Merlin engine was born.  

Rolls Royce Merlin
1,030hp Rolls-Royce Merlin III V12 in the new Hanger 1 at the RAF Museum, Hendon, 4 December 2018. Source: WikiMedia Commons

It was 1936 before the Rolls-Royce Merlin was perfected enough to be mass produced, but it ended up being the second most produced engine during wartime, with a final production number of around 165,000. In total, the engine was used in 40 aircraft during World War II. It is primarily associated with the Supermarine Spitfire, Hawker Hurricane, Avro Lancaster bomber and the de Havilland Mosquito. While not the first operational aircraft to feature the Merlin, most of the engine’s production output accommodated the needs of the heavy bomber: the Lancaster. The aircraft needed four engines, each of which had an output of 1,480bhp, giving the aircraft a total power output of 5,920bhp. As a result, production was outsourced to America after a failure to keep up with demands in the Manchester, Derby and Glasgow factories. 

Upon its original design, the Merlin engine was a supercharged, liquid-cooled V-12 piston engine capable of up to 1,000bhp. However, as time went on the engine developed significantly. By the end of the war, most Rolls-Royce Merlin engines could produce just under 1,800bhp. Post wartime, the engine was used in the de Havilland Hornet and was capable of an impressive 2,000bhp – twice that of its original variants.  

Merlin V12 Labelled
A labelled Rolls-Royce Merlin engine. Source: WikiMedia Commons

One of the most notable features of the Rolls-Royce Merlin engine was, and still is, its sound. On one hand it could strike fear into the hearts of its enemies, and on the other it could be heard as a symbol of hope and could lift the spirits of many. The purr of a Merlin engine would eventually go down as one of the most famous wartime sounds in history, through its association with the much-adored Supermarine Spitfire. 

One of the men involved in the development of the Merlin was Sir Stanley Hooker. A mathematician, Hooker solved the early problems that saw a lack of power generated by the supercharger at low altitudes. This fix was crucial. The Spitfire and the Hurricane would be in a great deal of combat below 6,000ft in the Battle of Britain. Hooker’s improvements gave the Merlin an extra 22mph power boost, which made low-altitude dogfighting much more feasible for the aircraft. But although an important fix, this was not the end of the troubles for the early variants of the Merlin. One of its main weaknesses was that it would cut out under negative g-force during a steep dive – something which did not affect the fuel-injected Me-109 engines of the enemy. The problem was partially corrected in 1941 through the fitting of a diaphragm across the float chambers of the engine. 

Production line
Rolls-Royce factory, 1942. Female workers attach the Induction Manifolds to the Cylinder Blocks. Source: Imperial War Museum Collection

The Merlin engine’s production was stopped in 1956. Despite being gradually replaced by more modern and effective engines, such as the Rolls-Royce Griffon, the Merlin has remained in the hearts of many in the decades since its production ceased. Just hearing it at air shows and commemorative events truly reaffirms why this remarkable piece of engineering was the heart of every World War II battle.