Alfa Romeo Mercurius

Bristol Mercury
Company type	Radial piston aircraft engine
Industry	Aerospace engineering
Predecessor	Bristol Jupiter
Founded	1925
Founder	Roy Fedden
Headquarters	Bristol, United Kingdom
Products	Aircraft engines
Parent	Bristol Aeroplane Company

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The Bristol Mercury is a nine-cylinder, air-cooled, single-row radial piston engine developed in Britain. Conceived by Roy Fedden at the Bristol Aeroplane Company, it powered various civil and military aircraft throughout the 1930s and 1940s. Evolving from the earlier Jupiter design, advanced versions could deliver up to 800 horsepower (600 kW) from a displacement of 1,500 cubic inches (25 L), achieved by incorporating a geared supercharger.

Approximately 21,000 units were manufactured, including licensed production in several European countries. Numerous engines remain operational today, with others preserved and displayed in aviation museums worldwide.

Design and development

The Bristol Mercury originated in 1925 when the Bristol Aeroplane Company sought a successor as the Bristol Jupiter approached obsolescence. Initial market interest was limited; however, after the Air Ministry supported three prototypes, it became another successful project under designer Roy Fedden.

With the growing adoption of supercharger technology to enhance high-altitude performance, Fedden advocated for continuous mild boost to increase output from a smaller engine. Rather than creating a completely new engine block, existing Jupiter components were adapted by shortening the piston stroke by one inch (25 mm). This reduction in displacement was compensated by supercharging to restore power to Jupiter levels while operating at higher rpm, necessitating a reduction gear for the propeller. The same approach was applied to the original Jupiter-sized engine to develop the Pegasus.

Designed with fighters in mind due to its compact size, the Mercury powered aircraft such as the Gloster Gauntlet and its successor, the Gloster Gladiator. Although initially intended that the larger Pegasus would serve bomber roles, rising power outputs led to the Mercury being employed across multiple aircraft categories. Its most renowned application was in the twin-engine light bomber, the Bristol Blenheim.

In 1938, Roy Fedden urged the Air Ministry to import American 100 octane aviation fuel. This higher-octane fuel permitted aero engines to operate at increased compression ratios and elevated supercharger boost pressures compared to the then-standard 87-octane fuel, thus enhancing power output. The Mercury XV became one of the first British aero engines type-certified for use with 100-octane fuel in 1939. It was capable of running at +9 lbs/sq.in (0.62 bar) boost pressure and was initially installed in the Blenheim Mk IV.

Additionally, the Mercury was pioneering among British aero engines as it gained approval for operation with variable-pitch propellers.

Production totaled 20,700 units manufactured by Bristol and its associated shadow factories. Beyond Britain, licensed production occurred at Państwowe Zakłady Lotnicze in Poland where it powered the PZL P.11 fighters; at NOHAB in Sweden for use in Swedish Gloster Gladiator fighters and the Saab 17 dive bomber; at Italy’s Alfa Romeo, where it was known as the Mercurius; by Walter Engines in Czechoslovakia; and by Finland’s Tampella, primarily equipping Bristol Blenheim bombers.

Variants

Note:^[1]

Mercury I: (1926) 808 hp (603 kW), direct drive configuration. Designed as a Schneider Trophy racing powerplant
Mercury II: (1928) 420 hp (310 kW), featuring a compression ratio of 5.3:1
Mercury IIA: (1928) 440 hp (330 kW)
Mercury III: (1929) 485 hp (362 kW), with a compression ratio of 4.8:1 and a 0.5:1 reduction gear

File:Bristol (NOHAB) Mercury.jpg

Mercury produced under license by NOHAB

Mercury IIIA: Minor updates applied to Mercury III
Mercury IV: (1929) 485 hp (362 kW), equipped with a 0.656:1 reduction gear
Mercury IVA: (1931) 510 hp (380 kW)
Mercury IVS.2: (1932) 510 hp (380 kW)
Mercury (Short stroke): Experimental short stroke variant (5.0 in), unsuccessful, delivering 390 hp (290 kW)
Mercury V: 546 hp (407 kW) (later evolved into the Pegasus IS.2)
Mercury VIS: (1933) 605 hp (451 kW)

File:Bristol Mercury.jpg

Side perspective highlighting valve gear components.

Mercury VISP: (1931) 605 hp (451 kW), 'P' denoting Persia.
Mercury VIS.2: (1933) 605 hp (451 kW)
Mercury VIA: (1928) 575 hp (429 kW) (precursor to the Pegasus IU.2)
Mercury VIIA: 560 hp (420 kW) (transitioned into the Pegasus IM.2)
Mercury VIII: (1935) 825 hp (615 kW), featuring a compression ratio of 6.25:1 and weight reduction measures.
Mercury VIIIA: Variant of Mercury VIII with gun synchronisation fitted for Gloster Gladiator MkII
Mercury VIIIA: Second usage of the VIIIA label, producing 535 hp, later developed into the Pegasus IU.2P
Mercury IX: (1935) 825 hp (615 kW), lightweight version.
Mercury X: (1937) 820 hp (610 kW)
Mercury XI: (1937) 820 hp (610 kW)
Mercury XII

(1937) 820 hp (610 kW)

Mercury XV: (1938) 825 hp (615 kW), derived from Mercury VIII and adapted for 100 Octane fuel instead of 87 Octane.
Mercury XVI: 830 hp (620 kW)
Mercury XX: (1940) 810 hp (600 kW)
Mercury 25: (1941) 825 hp (615 kW), Mercury XV with altered crankshaft design.
Mercury 26: 825 hp (615 kW), Mercury 25 fitted with a revised carburettor.
Mercury 30: (1941) 810 hp (600 kW), Mercury XX incorporating crankshaft modifications.
Mercury 31: (1945) 810 hp (600 kW), Mercury 30 modified with carburettor changes and fixed-pitch propeller for Hamilcar X.

Applications

File:Fokker g1.gif

Fokker G.1 powered by the Mercury engine

Note:^[2]

Operational aircraft

The Shuttleworth Collection maintains two aircraft powered by Bristol Mercury engines: a Westland Lysander III (registration G-AZWT) and a Gloster Gladiator I (registration G-AMRK). Both are regularly featured in flight demonstrations at Old Warden Aerodrome, Bedfordshire.

At Duxford Airfield, the Aircraft Restoration Company operates a Mercury-powered Westland Lysander (G-CCOM) alongside a Bristol Blenheim I (G-BPIV), a light bomber equipped with two Bristol Mercury engines. These aircraft are showcased at airshows held at IWM Duxford and various locations throughout the United Kingdom.

Also based at Duxford, The Fighter Collection flies a 1939 Gloster Gladiator II (G-GLAD) powered by a Bristol Mercury XX engine.

In Canada, both the Canadian Warplane Heritage Museum and the Vintage Wings of Canada have airworthy Lysander IIIA models in their fleets.

Exhibited engines

A Bristol Mercury VII is exhibited at the Royal Air Force Museum London.
Another Bristol Mercury VII is part of the collection at the Shuttleworth Collection.
The Fleet Air Arm Museum at RNAS Yeovilton displays a cutaway section of a Bristol Mercury engine.
The Muzeum Lotnictwa Polskiego in Kraków houses a functioning Bristol Mercury V installed on the last surviving PZL P.11 fighter.
A Tampella Mercury engine is presented alongside a Valmet Vihuri at the Finnish Air Force Museum.

Technical details (Mercury VI-S)

File:Bristol MercuryVII.JPG

Bristol Mercury VII exhibited at the Shuttleworth Collection

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External links

Audio recording of the Mercury V S2 engine installed in PZL P.11c (MP3 format)

Template:Bristol aeroengines Template:Alfa Romeo aeroengines Template:Walter aeroengines

[1] Information sourced from Lumsden 2003, pp. 104–108

[2] Based on Lumsden's list, the Mercury may not have been the primary engine for all listed aircraft

[1]

[2]

Alfa Romeo Mercurius

Design and development

Variants

Applications

Operational aircraft

Exhibited engines

Technical details (Mercury VI-S)

See also

Notes

Bibliography

Further reading

External links