Rolls-Royce Merlin XX

Although it was famous for powering Spitfires and Hurricanes, the Merlin was also used to power many bomber aircraft. The Merlin XX, fitted to the much larger Halifaxes and Lancasters, had a two-speed supercharger which improved performance at high altitude.

High altitude performance was important for bombers, as it flying at height made them more difficult targets for German flak units to hit.