Development of the Napier Sabre engine


D. Napier and Son Limited was a British engineering company best known for their work on engines initially for road vehicles and later for aircraft.

Napier and First World War aero engines

Napier were originally contracted to manufacture aero engines designed by other companies for what was then the Royal Flying Corps (now the Royal Air Force) during the First World War. Dissatisfied with the reliability of these engines, Napier financed and designed their own aero engine – the Napier Lion; a 12-cylinder broad arrow W configuration engine.

Napier and Frank Halford

The success of the Lion engine and a decline in car sales during the1920s led Napier to concentrate their efforts on aero engines. Napier employed Frank Halford, an aircraft designer who had designed many successful engines during the 1920s, to assist with their engine development. One of Halford’s business associates was Harry Ricardo, a pioneer in the research and development of the internal combustion engine. Along with Ricardo, Halford developed an interest in the improvements made to engine performance through the use of a sleeve valve mechanism as an alternative to the standard poppet valve.

Sleeve Valves

The premise of a sleeve valve was to control how much air and fuel were detonated with each of the engine’s compression strokes. To achieve this a machined sleeve would sit in the cylinder between the cylinder wall and the piston. The sleeve would contain holes (or, ports) and slide up and down the cylinder. The ports would line up with holes in the cylinder to allow for the expulsion of exhaust gases and the intake of fresh air.

The benefit of the sleeve valve compared to the poppet valve was its ability to perform at a higher compression ratio increasing economy and efficiency and in turn allowing for a greater horsepower.

Diagram of sleeve operation
Diagram of sleeve operations [IMechE Archive NAP/6/2/2/2/1]

The Sabre engine

Halford’s interest in the sleeve valve led to the design of a horizontally opposed 24 cylinder sleeve valved H configuration liquid cooled petrol engine which was named the Sabre. The design was based on the opposed layout of a previous Napier engine – the Dagger – but positioned the cylinder blocks horizontally to allow for the use of sleeve valves.

The first iteration of the Sabre was tested in 1938 achieving 1,350hp. Further developments continued to take place during the early 1940s and in June 1940 the engine passed the Air Ministry’s 100-hour test achieving 2,200hp.

It was the very sleeve valves that contributed to the efficiencies of the Sabre engine that were also the cause of some initial teething problems. Napier’s mass production of the sleeves had led to issues with the sleeve’s circularity which in turn had led to engine seizures. Improvements to the manufacturing methods (most notably through collaboration with rival engine manufacturer Bristol) led to improvements in the engine’s reliability.

Napier maintenance bulletin for making repairs to the sleeves [IMechE Archive NAP/6/1/420]

It took until 1944 and the development of the Sabre V engine for some of the reliability issues that had surrounded earlier versions of the engine to be resolved. The Sabre V was the last version of the Sabre engine to enter operational service and was able to consistently achieve 2,400hp.


The Typhoon (often referred to by the RAF as the “Tiffy”) was a British Second World War fighter-bomber produced by British aircraft manufacturer Hawker.

Typhoons were powered by the Napier Sabre IIA and IIB engines which were able to power them to a top speed during level flight of 412mph.

Napier Sabre II engine [IMechE Archive]

The Typhoon came into its own when the RAF were faced with the challenge of the Luftwaffe’s Fw190 aircraft. The Typhoon was the only aircraft capable of intercepting the Fw190 at low altitudes. As the nature of the war in the air progressed the Typhoon’s usage switched from a fighter to a bomber. The power of its Sabre engine allowed it to be armed with heavy bombs and rockets leading to its usage in raids against strategic German targets such as anti-aircraft positions.

Hawker Typhoon IB with Napier Sabre II engine [IMechE Archive NAP/6/4/7]


The Tempest was a British Second World War fighter produced by British aircraft manufacturer Hawker. The Tempest was originally designed to be an improved version of the Typhoon however the design deviated significantly enough from the Typhoon that it was considered a different aircraft and named the Tempest.

Several prototype Tempests were developed, two of which used Sabre engines. The mark 1 prototype used a Sabre IV engine and the mark 5 prototype used a Sabre II engine. The mark 5 prototype was chosen to be taken forward into production meaning most Tempest aircraft were powered by a single Sabre II engine. The first Tempests entered operational service in 1944.

The Tempest was originally designed to perform well at high altitudes, however like its predecessor the Typhoon, the Tempest proved its performance strength to be at low altitudes where it could be used to intercept German V-1 bombs.

Hawker Tempest V – powered by a Napier IIB engine [IMechE Archive NAP/6/4/12]