Detail view of a Mercury VII engine and Rotol
propeller
The Bristol Mercury was an evolution of the earlier Jupiter engine, retaining many of its predecessor’s features but with notable differences. The primary distinction was a shorter stroke of 6.5 inches, which resulted in a smaller overall diameter of 51.5 inches and a displacement of 1520 cubic inches (24.9 liters). This relatively small displacement limited its potential as a frontline aircraft engine during World War II.
Introduced in 1927, the Mercury engine was used in the Short Crusader for a Schneider Trophy attempt, which ultimately failed due to non-engine-related issues, specifically crossed aileron controls that led to a crash. Despite this setback, the experience helped Bristol in developing the Mercury to operate at higher boost pressures and rotational speeds.
By the time of World War II, the Mercury was capable of producing 965 horsepower at 2650 rpm, with an overspeed capability of 2750 rpm. The engine featured typical Bristol construction, including a two-piece, vertically split aluminum crankcase held together by nine through bolts between the cylinders. The crankcase supported a two-piece crankshaft running on two roller bearings, allowing the use of a one-piece master connecting rod. Power transmission was managed by Farman epicyclic bevel reduction gears with a 0.57:1 ratio.
The Mercury was equipped with single-stage, single-speed supercharging. Bristol did not develop two-stage supercharging for their engines, although they did investigate turbosupercharging as early as 1923. The Mercury’s cylinders were made from nitrided chrome-molybdenum steel alloy, with forged aluminum heads shrunk and screwed onto the barrels. It featured a four-valve per cylinder arrangement in a pent-roof combustion chamber, with sodium-cooled exhaust valves.
A notable characteristic of the Mercury, which harked back to its Jupiter lineage, was the semi-exposed valve gear. The pushrods were enclosed in streamlined tubes, while the rocker pivots had a cover. However, the valve stems, valve springs, and the extremities of the valve gear were left exposed. This design choice was somewhat antiquated even at the time but was a distinctive feature of the Mercury’s design lineage.