THE development of the air-cooled radial engine in Great Britain is closely linked with the name of Bristol. In engine design the Bristol Aeroplane Company, Ltd, has concentrated exclusively on this type since 1920. The current series of nine-cylinder, single-bank radial engines made by the firm is a fitting testimony to the progress that has been made.
During the period 1920-37, the power output per litre of cylinder capacity has been increased by 130 per cent, weight has been reduced by 40 per cent, fuel consumption has been reduced by 25 per cent, and maximum crankshaft speeds increased from 1,625 to 2,925 revolutions a minute.
The Pegasus and Mercury engines have been developed from the famous Bristol Jupiter. The Pegasus has the same cubic capacity as the original Jupiter; the Mercury is slightly smaller. The Pegasus engines are used for civil and military aircraft, but the Mercury engines are intended for military purposes, for which their smaller overall diameter makes them more suitable.
There are four engines in the Mercury series - the Mercury VIII and IX, and the Mercury XI and XII. All four have the same cubic capacity (24·8 litres) and the same cylinder diameter (51½ in). The Mercury VIII and IX are fully supercharged engines with an international rated power of 795-825 bhp at 13,000 feet, a take-off power of 730 bhp and a maximum power of 840 bhp for an all-out level flight of five minutes at 14,000 feet. The Mercury VIII has an airscrew reduction-drive ratio of 1 to 0·572 and weighs 995 lb; on the Mercury IX the gear ratio is 1 to 0·5 and this engine weighs 990 lb.
The Mercury XI and XII engines, which in 1937 superseded the former Mercury engine, are moderately supercharged, having an international rated power of 790-820 bhp at 3,500 feet and a take-off power of 830 bhp. Their maximum power for all-out level flight (five minutes) is 890 bhp at 6,000 feet. In other ways the Mercury IX resembles the Mercury VIII, and the Mercury XII the Mercury XI.
The Pegasus series of engines is subdivided into seven types, the differences between which are mainly concerned with airscrew reduction ratios and degrees of supercharging. The moderately supercharged Pegasus XXII and XXIII replaced the Pegasus X and XI in 1937. The Pegasus XIX and XX are fully supercharged engines; the Pegasus XVII and XVIII are equipped with two-speed superchargers and were introduced in 1937. There is also the civil rated Pegasus Xc, which was adopted by Imperial Airways for the Empire flying boats put into service in the same year.
The Pegasus engines have a cubic capacity of 28·7 litres, and international rated powers varying between 785 and 855 bhp. Their weights vary between 1,025 and 1,115 lb. The international rated power of the newer Pegasus XVII and XVIII on medium supercharge is 820-855 bhp at 5,000 feet, and on full supercharge 765-800 bhp at 15,500 feet, with 980 bhp for take-off.
Full Power Almost Immediately
The rapid progress made in the design of these radial engines is largely due to the growth of metallurgical knowledge and to the development of new alloys and light metals. The new high octane fuels available have made higher power output possible by enabling the designer to increase the compression ratio of his engines. Extra power has also been derived from an increase in engine speed, and the greater stresses thus involved have constantly brought up fresh problems.
In the Bristol engines the cylinders have been developed to deal satisfactorily with the increased power outputs. The steel cylinder barrels and aluminium alloy cylinder heads have their cooling fins closely spaced and machined to give the highest possible cooling efficiency. They provide a total radiating area of 166½ square feet.
The design of the valves has been revolutionized because of the high-power outputs demanded; the exhaust valves are filled with sodium to improve the cooling, and the valve stems are hardened and have hardened buttons inserted in their ends to provide a durable wearing surface under load. The Pegasus and Mercury engines have four valves to each cylinder. The valve seats are of specially durable steel, which has a coefficient of expansion similar to that of the aluminium head into which they are threaded and shrunk; thus there is no tendency for valve seats to become loose at high temperatures.
Mounted radially round the cam chamber are nine pairs of case-hardened tappets in phosphor-bronze guides. These tappets are hollow and are slotted at their inner ends to receive the case-hardened rollers, mounted on hardened pins and floating phosphor-bronze bushes. In operating conditions all these components are completely immersed in oil spray.
The pistons are of the full-skirted type; each has one single and one double scraper ring and two gas rings. They are machined from aluminium alloy forgings - a process of manufacture which gives them the strength to withstand high temperature and pressure.
The supercharger, of the centrifugal type, has an impeller speed from seven to ten times that of the crankshaft, according to the degree of supercharging required. The impeller is machined from a solid alloy steel forging and is accurately balanced statically and dynamically.
Dual ignition is provided by two magnetos; the entire ignition system is shielded for use in radio-equipped aircraft. The sparking plugs and their leads are completely watertight. Lubrication is on the dry-sump principle, oil being drawn from the oil tank through a filter. There is also a device for providing high initial oil pressure, which enables engines to be opened up on full power almost immediately after they have started up. This feature is of great importance in military aircraft.
The exhaust manifold is of the single-outlet type and is of riveted steel construction. It is mounted by simple three-point attachments upon rubber, and its large internal volume ensures efficient flame damping and quiet operation. The engine cowlings are gilled, so that the pilot may vary the flow of cooling air to suit different conditions of flight.
In 1937 the Royal Air Force gained the world altitude record for Great Britain with a special Pegasus engine fitted in a Bristol high-altitude monoplane.