Editing Bristol Mercury (section)

==Design and development==
The Mercury was developed by the [[Bristol Aeroplane Company]] in 1925 as their [[Bristol Jupiter]] was reaching the end of its lifespan. Although the Mercury initially failed to attract much interest, the [[Air Ministry]] eventually funded three prototypes and it became another winner for the designer [[Roy Fedden]].

With the widespread introduction of [[supercharger]]s to the aviation industry in order to improve altitude performance, Fedden felt it was reasonable to use a small amount of boost at all times in order to improve performance of an otherwise smaller engine. Instead of designing an entirely new block, the existing Jupiter parts were re-used with the stroke reduced by one inch (25&nbsp;mm). The smaller capacity engine was then boosted back to Jupiter power levels, while running at higher rpm and thus requiring a reduction gear for the [[Propeller (aircraft)|propeller]]. The same techniques were applied to the original Jupiter-sized engine to produce the [[Bristol Pegasus|Pegasus]].

The Mercury's smaller size was aimed at [[fighter aircraft|fighter]] use and it powered the [[Gloster Gauntlet]] and its successor, the [[Gloster Gladiator]]. It was intended that the larger Pegasus would be for [[bomber aircraft|bomber]]s, but as the power ratings of both engines rose, the Mercury was used in almost all roles. Perhaps its most famous use was in a twin-engine [[light bomber]], the [[Bristol Blenheim]].

In 1938 Roy Fedden pressed the Air Ministry to import supplies of  [[Avgas#100.2F130|100 octane aviation spirit]] from the US. This new fuel would allow aero engines to run at higher [[compression ratio]]s and [[supercharger]] boost pressure than the existing 87-octane fuel, thus increasing the power. The Mercury XV was one of the first British aero engines to be type-tested and cleared to use the 100-octane fuel in 1939. This engine was capable of running with a boost pressure of +9&nbsp;lbs/sq.in (0.62 bar) and was first used in the Blenheim Mk IV.<ref>Warner 2005, pp. 100, 112, 135.</ref>

The Mercury was also the first British aero engine to be approved for use with variable-pitch propellers.

The Bristol company and its [[shadow factories]] produced 20,700 examples of the engine.<ref>Bridgman (Jane's) 1998, p. 270.</ref>
Outside the United Kingdom, Mercury was licence-built by [[Państwowe Zakłady Lotnicze]] in Poland and used in the [[PZL P.11]] fighters. It was also built by [[NOHAB]] in Sweden and used in the Swedish Gloster Gladiator fighters and in the [[Saab 17]] dive-bomber. In Italy, it was built by [[Alfa Romeo]] as the Mercurius. In [[Czechoslovakia]] it was built by [[Walter Engines]]. In Finland, it was built by [[Tampella]] and mainly used on [[Bristol Blenheim]] bombers.