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British Industrial History

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Bristol Engine Co: Mercury

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Bristol Mercury Mk. VIII. Exhibit at the Shuttleworth Collection.
1931. Bristol Mercury Engine.
March 1934.
November 1937.

Note: This is a sub-section of Bristol Engine Co

The name Mercury was first given to a 14-cylinder radial engine designed by Roy Fedden and Leonard Butler at Brazil, Straker and Co; it was notable for having the cylinders arranged helically instead of as two rows.

Later these designers re-used the name for another engine, better known than the first, which was made by the Bristol Engine Co. The Bristol Mercury was a 9-cylinder one-row piston radial engine used on British aircraft in the 1930s and 1940s.

The Mercury was developed by the Bristol Engine Co in 1925 because their 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 now-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. Perhaps its most famous use was in a twin-engine light bomber, the Bristol Blenheim. 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.

Each cylinder head of the engine weighs 56 lb as a forging. It emerges as a finished head weighing 17lb having passed through 106 operations.

1938 The King visited the Austin factory where controls for variable pitch airscrews and other components were being assembled for Mercury VIII engines[1].


  • Mercury VIII - 730 hp (545 kW) at 2,650 rpm for takeoff, 840 hp (625 kW) at 2,750 rpm maximum continuous power
  • Mercury XV - 840 hp (625 kW) at 2,750 rpm at 14,000 ft (4,270 m) maximum power with 87-octane fuel; 995 hp (740 kW) at 2,750 rpm at 9,250 ft (2,820 m) maximum power with 100-octane fuel
  • Mercury XX - 870 hp (650 kW) at 2,650 rpm at 4,500 ft (1,370 m) maximum power

General characteristics (Mercury XV)

  • Type: 9-cylinder supercharged air-cooled radial engine
  • Bore: 5.75 in (146 mm)
  • Stroke: 6.5 in (165 mm)
  • Displacement: 1,519 in³ (24.89 L)
  • Diameter: 51.5 in (1.307 m)
  • Dry weight: 1,065 lb (485 kg)


  • Valve-train: Four push-rod-actuated valves per cylinder, two inlet and two sodium-cooled exhaust valves
  • Supercharger: High-speed centrifugal, single-stage single-speed
  • Fuel system: Claudel-Hobson carburettor with automatic boost and mixture control
  • Fuel type: 87- or 100-octane petrol
  • Oil system: Dry sump with one combination pressure/scavenge pump


  • Power output:
    • 840 hp (625 kW) at 2,750 rpm at 14,000 ft (4,270 m) maximum power with 87-octane fuel
    • 995 hp (740 kW) at 2,750 rpm at 9,250 ft (2,820 m) maximum power with 100-octane fuel
  • Specific power: 0.65 hp/in³ (29.7 kW/L) with 100-octane fuel
  • Compression ratio: 7:1
  • Specific fuel consumption: 1.0 lb/(hp·h) (0.61 kg/(kW·h))
  • Power-to-weight ratio: 0.93 hp/lb (1.53 kW/kg)

See Also


Sources of Information

  1. The Times, 11 March 1938