Editing D. Napier & Son (section)

===Second World War===
[[File:Napier Sabre01.jpg|thumb|[[Napier Sabre]] engine]]
Starting from scratch, Napier decided to use the new [[sleeve valve]] design in a much larger [[H engine|H-block]] 24-cylinder engine, soon to be known as the [[Napier Sabre|Sabre]]. Designed under [[Frank Halford]], the engine was very advanced and proved to be difficult to adapt to assembly line production. Therefore, although the engine was ready by 1940, it was not until 1944 that production versions were considered reliable. At that point efforts were made to improve it, leading eventually to the Sabre VII delivering {{convert|3,500|hp|kW|abbr=on}}, making it the most powerful engine in the world, from an engine much smaller than its competition.

Napier also worked on [[Aircraft diesel engine|diesel aircraft engines]]. In the 1930s it licensed the [[Junkers Jumo 204]] for production in England, which it called the [[Napier Culverin|Culverin]]. It also planned to produce a smaller version of the same basic design as the [[Napier Cutlass|Cutlass]], but work on both was cancelled at the outbreak of war in 1939.

Napier developed a marine engine from the Lion aero engine, the petrol-driven Sea Lion, which could deliver {{convert|500|hp|abbr=on}} and was used in the [[RAF Rescue Launch|"Whaleback" Air Sea Rescue]] Launches.
[[File:Napier Deltic Engine.jpg|thumb|[[Napier Deltic]] engine, cut away for display]]
During the war (1944) Napier was asked by the [[Royal Navy]] to supply a diesel engine for use in its [[patrol boat]]s, but the Culverin's {{convert|720|hp|kW|abbr=on}} was not nearly enough for its needs. Napier then designed under the leadership of Ernest Edward Chatterton, Chief Engineer, the [[Napier Deltic|Deltic]], essentially three Culverins arranged in a large triangle. Considered one of the most complex engine designs of its day, the Deltic was nevertheless very reliable, and was taken into service after the war as a [[locomotive]] powerplant (in [[British Rail]]'s [[British Rail Class 55|Class 55]]) in addition to the [[torpedo boat]]s, [[minesweeper (ship)|minesweeper]]s and other small naval vessels for which it was designed.

Also during the Second World War a six-cylinder 300 cubic inch road-vehicle engine was commissioned by the government, but this design was sold to [[Leyland Motors]] by 1945.