Editing Rolls-Royce Merlin (section)

=====Supercharger=====
Central to the success of the Merlin was the supercharger. [[Cyril Lovesey|A.C. Lovesey]], an engineer who was a key figure in the design of the Merlin, delivered a lecture on the development of the Merlin in 1946; in this extract he explained the importance of the supercharger:
{{blockquote|The impression still prevails that the static capacity known as the swept volume is the basis of comparison of the possible power output for different types of engine, but this is not the case because the output of the engine depends solely on the mass of air it can be made to consume efficiently, and in this respect the supercharger plays the most important role&nbsp;... the engine has to be capable of dealing with the greater mass flows with respect to cooling, freedom from detonation and capable of withstanding high gas and inertia loads&nbsp;... During the course of research and development on superchargers it became apparent to us that any further increase in the altitude performance of the Merlin engine necessitated the employment of a two-stage supercharger.<ref>Lovesey 1946, p. 218.</ref>}}

As the Merlin evolved so too did the supercharger; the latter fitting into three broad categories:<ref name="Lumsden p. 201">Lumsden 2003, p. 201.</ref>
# Single-stage, single-speed gearbox: Merlin I to III, XII, 30, 40, and 50 series (1937–1942).{{#tag:ref|Because of an accelerated design process the timelines of Merlin development overlapped; for example, the two-stage supercharger was being designed before there was a need to introduce the modified Merlin 45M and 55Ms to counteract the threat of the [[Focke-Wulf Fw 190]].|group=nb}}
# Single-stage, two-speed gearbox: experimental Merlin X (1938), production Merlin XX (1940–1945).
# Two-stage, two-speed gearbox with [[intercooler]]: mainly Merlin 60, 70, and 80 series (1942–1946).

The Merlin supercharger was originally designed to allow the engine to generate maximum power at an altitude of about {{convert|16000|ft|m|abbr=on}}. In 1938 Stanley Hooker, an [[University of Oxford|Oxford]] graduate in applied mathematics, explained "...&nbsp;I soon became very familiar with the construction of the Merlin supercharger and carburettor&nbsp;... Since the supercharger was at the rear of the engine it had come in for pretty severe design treatment, and the air intake duct to the impeller looked very squashed&nbsp;..." Tests conducted by Hooker showed the original intake design was inefficient, limiting the performance of the supercharger.<ref>Hooker 1984, p. 45.</ref>{{#tag:ref|The function of the supercharger is to compress the fuel/air mixture entering the engine cylinders; any pressure loss to the [[Centrifugal compressor|impeller]] (also called the rotor) would impair the supercharger's efficiency.|group=nb}} Hooker subsequently designed a new air intake duct with improved flow characteristics, which increased maximum power at a higher altitude of over {{convert|19000|ft|m|abbr=on}}; and also improved the design of both the impeller, and the diffuser which controlled the airflow to it. These modifications led to the development of the single-stage Merlin XX and 45 series.<ref>Hooker 1984, pp. 46–50, 52, 235–247.</ref>

A significant advance in supercharger design was the incorporation in 1938 of a two-speed drive (designed by the French company [[Farman Aviation Works|Farman]]) to the impeller of the Merlin X.<ref>Lumsden 2003, p. 206.</ref>{{#tag:ref|Rolls-Royce took out a licence in 1938 to build the two-speed drive.<ref>Rubbra 1990, p. 71.</ref>|group=nb}} The later Merlin XX incorporated the two-speed drive as well as several improvements that enabled the production rate of Merlins to be increased.<ref>Smith February 1942 p. b.</ref> The low-ratio gear, which operated from takeoff to an altitude of {{convert|10000|ft|m|abbr=on}}, drove the impeller at 21,597&nbsp;rpm and developed {{convert|1,240|hp|kW|abbr=on}} at that height; while the high gear's (25,148&nbsp;rpm) power rating was {{convert|1,175|hp|kW|abbr=on}} at {{convert|18000|ft|m|abbr=on}}. These figures were achieved at 2,850&nbsp;rpm engine speed using +9 [[pounds per square inch]] (1.66&nbsp;[[Atmosphere (unit)|atm]]) (48") boost.<ref>Smith February 1942 p. d.</ref>

In 1940, after receiving a request in March of that year from the [[Ministry of Aircraft Production]] for a high-rated ({{convert|40000|ft|m|abbr=on}}) Merlin for use as an alternative engine to the turbocharged [[Bristol Hercules|Hercules VIII]] used in the prototype high-altitude [[Vickers Wellington|Vickers Wellington V]] bomber, Rolls-Royce started experiments on the design of a two-stage supercharger and an engine fitted with this was bench-tested in April 1941, eventually becoming the Merlin 60.<ref>King 1954, p. 578.</ref> The basic design used a modified Vulture supercharger for the first stage while a Merlin 46 supercharger was used for the second.<ref name="Lovesey 1946, p. 220.">Lovesey 1946, p. 220.</ref> A liquid-cooled [[intercooler]] on top of the supercharger casing was used to prevent the compressed air/fuel mixture from becoming too hot.{{#tag:ref| A hot mixture could either pre-ignite before reaching the engine's cylinders or [[Engine knocking|detonate]] in the engine.|group=nb}} Also considered was an exhaust-driven [[turbocharger]], but although a lower fuel consumption was an advantage, the added weight and the need to add extra ducting for the exhaust flow and waste-gates meant that this option was rejected in favour of the two-stage supercharger.<ref name="Lo219">Lovesey 1946, p. 219.</ref> Fitted with the two-stage two-speed supercharger, the Merlin 60 series gained {{convert|300|hp|kW|abbr=on}} at {{convert|30000|ft|m|abbr=on}} over the Merlin 45 series,<ref name="Lovesey 1946, p. 220."/> at which altitude a Spitfire IX was nearly {{convert|70|mph|kph|abbr=on}} faster than a Spitfire V.<ref>Price 1982, pp. 142, 167.</ref>

The two-stage Merlin family was extended in 1943 with the Merlin 66, which had its supercharger geared for increased power ratings at low altitudes, and the Merlin 70 series that were designed to deliver increased power at high altitudes.<ref>Price 1982, pp. 153–154, 170.</ref>

While the design of the two-stage supercharger forged ahead, Rolls-Royce also continued to develop the single-stage supercharger, resulting in 1942 in the development of a smaller "cropped" impeller for the Merlin 45M and 55M; both of these engines developed greater power at low altitudes.<ref>Lumsden 2003, p. 210.</ref> In squadron service the LF.V variant of the Spitfire fitted with these engines became known as the "clipped, clapped, and cropped Spitty" to indicate the shortened [[wingspan]], the less-than-perfect condition of the used [[airframe]]s, and the cropped supercharger impeller.<ref>Price 1982, p. 135.</ref>