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Bristol Mercury
Alfa Romeo Mercurius


The nine-cylinder, air-cooled, single-row, piston radial 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 superchargers 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 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. The same techniques were applied to the original Jupiter-sized engine to produce the Pegasus.

The Mercury's smaller size was aimed at fighter use and it powered the Gloster Gauntlet and its successor, the Gloster Gladiator. It was intended that the larger Pegasus would be for bombers but as the power ratings of both engines rose, the Mercury found itself being used in almost all roles including twin-engine light bomber, Bristol Blenheim.

In 1938 Roy Fedden pressed the Air Ministry to import supplies of 100 octane aviation spirit (gasoline) from the USA. This new fuel would allow aero engines to run at higher compression ratios 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 lbs/ and was first used in the Blenheim Mk IV.

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. Outside the United Kingdom, Mercury was licence-built in Poland and used in their 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.


Mercury I
(1926) 808 hp, direct drive. Schneider Trophy racing engine.

Mercury II
(1928) 420 hp, compression ratio 5.3:1.

Mercury IIA
(1928) 440 hp

Mercury III
(1929) 485 hp, compression ratio 4.8:1, 0.5:1 reduction gear.

Mercury IIIA
Minor modification of Mercury III.

Mercury IV
(1929) 485 hp, 0.656:1 reduction gear.

Mercury IVA
(1931) 510 hp.

Mercury IVS.2
(1932) 510 hp.

Mercury (Short stroke)
Unsuccessful experimental short stroke (5.0 in) version, 390 hp.

Mercury V
546 hp (became the Pegasus IS.2)

Mercury VIS
(1933) 605 hp, see specifications section.

Mercury VISP
(1931) 605 hp, 'P' for Persia.

Mercury VIS.2
(1933) 605 hp.

Mercury VIA
(1928) 575 hp (became the Pegasus IU.2)

Mercury VIIA
560 hp (became the Pegasus IM.2)

Mercury VIII
(1935) 825 hp, compression ratio 6.25:1, lightened engine.

Mercury VIIIA
Mercury VIII fitted with gun synchronisation gear for the Gloster Gladiator

Mercury VIIIA
535 hp, second use of VIIIA designation, (became the Pegasus IU.2P)

Mercury IX
(1935) 825 hp, lightened engine.

Mercury X
(1937) 820 hp.

Mercury XI
(1937) 820 hp.

Mercury XII
(1937) 820 hp

Mercury XV
(1938) 825 hp, developed from Mercury VIII. Converted to run on 100 Octane fuel (previously 87 Octane).

Mercury XVI
830 hp.

Mercury XX
(1940) 810 hp

Mercury 25
(1941) 825 hp. Mercury XV with crankshaft modifications.

Mercury 26
825 hp. As Mercury 25 with modified carburettor.

Mercury 30
(1941) 810 hp, Mercury XX with crankshaft modifications.

Mercury 31
(1945) 810 hp, Mercury 30 with carburettor modifications and fixed pitch propeller for Hamilcar X.

Airspeed Cambridge
Blackburn Skua
Boulton Paul P.108
Bristol Blenheim
Bristol Bolingbroke
Bristol Bulldog
Bristol Bullpup
Bristol Type 101
Bristol Type 118
Bristol Type 133
Bristol Type 142
Bristol Type 146
Bristol Type 148
Breda Ba.27
Fairey Flycatcher
Fokker D XXI
Fokker G.1
General Aircraft Hamilcar X
Gloster Gamecock
Gloster Gladiator
Gloster Gauntlet
Gloster Gnatsnapper
Gloster Goring
Hawker Audax
Hawker F.20/27
Hawker Fury
Hawker Hart
Hawker Hind
Hawker Hoopoe
Hawker F.20/27
IMAM Ro.30
Koolhoven F.K.52
Miles Martinet
Miles Master
PZL P.11
Saab 17
Short Crusader
Supermarine Sea Otter
Valmet Vihuri
Vickers Jockey
Westland Interceptor
Westland Lysander


Mercury VI-S
Type: Nine-cylinder, single-row, supercharged, air-cooled radial engine
Bore: 5.75 in (146 mm)
Stroke: 6.5 in (165 mm)
Displacement: 1,520 in³ (24.9 L)
Length: 47 in (1,194 mm)
Diameter: 51.5 in (1,307 mm)
Dry weight: 966 lb (438 kg)
Valvetrain: Overhead valve, 4 valves per cylinder – 2 intake and 2 sodium-filled exhaust
Supercharger: Single-speed centrifugal type supercharger
Fuel system: Claudel-Hobson carburettor
Fuel type: 87 Octane petrol
Cooling system: Air-cooled
Reduction gear: Farman epicyclic gearing, 0.5:1, left hand tractor
Power output:
612 hp (457 kW) at 2,750 rpm for takeoff
636 hp (474 kW) at 2,750 rpm at 15,500 ft (4,730 m)
Specific power: 0.4 hp/in³ (18.35 kW/L)
Compression ratio: 6:1
Specific fuel consumption: 0.49 lb/(hp·h) (300 g/kWh)
Power-to-weight ratio: 0.63 hp/lb (1.04 kW/kg)


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