Editing Rolls-Royce Merlin (section)

====Technical improvements====
Most of the Merlin's technical improvements resulted from more efficient [[supercharger]]s, designed by [[Stanley Hooker]], and the introduction of aviation fuel with increased [[octane rating]]s. Numerous detail changes were made internally and externally to the engine to withstand increased power ratings and to incorporate advances in engineering practices.<ref>Lovesey 1946, pp. 224–226.</ref>

=====Ejector exhausts=====
[[File:SpitEjectors.JPG|thumb|alt=The right side of an uncowled, installed aircraft engine, with details of the exhaust system|Merlin 55 ejector exhaust detail, Spitfire LF.VB, ''EP120'']]
The Merlin consumed an enormous volume of air at full power (equivalent to the volume of a [[single-decker bus]] per minute), and with the exhaust gases exiting at {{convert|1,300|mph|km/h|abbr=on}} it was realised that useful [[thrust]] could be gained simply by angling the gases backwards instead of venting sideways.

During tests, 70 [[pounds-force]] (310 [[newton (unit)|N]]; 32&nbsp;[[kilogram-force|kgf]]) thrust at {{convert|300|mph|kph|abbr=on}}, or roughly {{convert|70|hp|kW|abbr=on}} was obtained, which increased the level maximum speed of the Spitfire by {{convert|10|mph|kph|abbr=on}} to {{convert|360|mph|kph|abbr=on}}.<ref>Price 1982, p. 51.</ref> The first versions of the ejector exhausts featured round outlets, while subsequent versions of the system used "fishtail" style outlets, which marginally increased thrust and reduced exhaust glare for night flying.

In September 1937 the Spitfire prototype, ''[[Supermarine Spitfire (early Merlin powered variants)#Prototype K5054 (Supermarine Type 300)|K5054]],'' was fitted with ejector type exhausts. Later marks of the Spitfire used a variation of this exhaust system fitted with forward-facing intake ducts to distribute hot air out to the wing-mounted guns to prevent freezing and stoppages at high altitudes, replacing an earlier system that used heated air from the engine coolant radiator. The latter system had become ineffective due to improvements to the Merlin itself which allowed higher operating altitudes where air [[Lapse rate|temperatures are lower]].<ref>Tanner 1981, A.P.1565E, Vol.1, Section II.</ref> Ejector exhausts were also fitted to other Merlin-powered aircraft.

=====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>

=====Carburettor developments=====
[[File:Cavanaugh Flight Museum-2008-10-29-027 (4270566340).jpg|thumb|Preserved Merlin 63 showing [[intercooler]] radiator, [[supercharger]] and [[carburettor]]]]
The use of [[Carburetor|carburettors]] was calculated to give a higher [[Power density|specific power]] output, due to the lower temperature, hence greater density, of the fuel/air mixture compared to injected systems.<ref>Hooker 1984, p. 62.</ref> Initially Merlins were fitted with float controlled carburettors. However, during the Battle of Britain it was found that if [[Supermarine Spitfire|Spitfires]] or [[Hawker Hurricane|Hurricanes]] were to [[Flight dynamics (aircraft)|pitch]] nose down into a steep dive, negative [[g-force|''g''-force]] (''g'') produced temporary fuel starvation causing the engine to cut-out momentarily. By comparison, the contemporary [[Messerschmitt Bf 109|Bf 109E]], which had [[Gasoline direct injection#Early systems|direct fuel injection]], could "bunt" straight into a high-power dive to escape attack. RAF fighter pilots soon learned to avoid this with a "half-roll" of their aircraft before diving in pursuit.<ref>McKinstry 2007, p. 205.</ref> A restrictor in the fuel supply line together with a diaphragm fitted in the float chamber, jocularly nicknamed "[[Miss Shilling's orifice]]",{{#tag:ref|Invented in March 1941 by [[Beatrice Shilling]], an engineer at the [[Royal Aircraft Establishment]], Farnborough.|group=nb}} after its inventor, went some way towards curing fuel starvation in a dive by containing fuel under negative G; however, at less than maximum power a fuel-rich mixture still resulted. Another improvement was made by moving the fuel outlet from the bottom of the [[SU carburetor|S.U. carburettor]] to exactly halfway up the side, which allowed the fuel to flow equally well under negative or positive g.<ref>Smallwood 1996, p. 135.</ref>

Further improvements were introduced throughout the Merlin range: 1943 saw the introduction of a [[Bendix Corporation|Bendix-Stromberg]] [[pressure carburetor|pressure carburettor]] that injected fuel at 5 [[pounds per square inch]] (34&nbsp;[[kilopascal|kPa]]; 0.34 [[Bar (unit)|bar]]) through a nozzle directly into the supercharger, and was fitted to Merlin 66, 70, 76, 77 and 85 variants. The final development, which was fitted to the 100-series Merlins, was an S.U. [[Fuel injection#Throttle body injection|injection carburettor]] that injected fuel into the supercharger using a fuel pump driven as a function of crankshaft speed and engine pressures.<ref>Lumsden 2003, p. 212.</ref>

=====Improved fuels=====
[[File:AP1590B AL4 361B.jpg|thumb|100 px|Page from Pilot's Notes Merlin II, III and V (A.P.1590B), explaining the use of +12lbs boost and 100 Octane fuel.]]
At the start of the war, the Merlin I, II and III ran on the then standard 87-octane [[avgas|aviation spirit]] and could generate just over {{convert|1,000|hp|kW|abbr=on}} from its 27-litre (1,650-cu&nbsp;in) displacement: the maximum [[Turbocharger#Pressure increase (or boost)|boost]] pressure at which the engine could be run using 87-octane fuel was +6 pounds per square inch (141&nbsp;kPa; 1.44&nbsp;[[Atmosphere (unit)|atm]]).{{#tag:ref|The British measured boost pressure as lbf/sq&nbsp;in (or psi), and commonly referred to it as "pounds" of boost. The normal atmospheric pressure at sea level is {{convert|14.5|psi|mbar|abbr=on}}, thus a reading of +6 means that the air/fuel mix is being compressed by a supercharger blower to 20.5&nbsp;psi before entering the engine; +25 means that the air/fuel mix is now being compressed to 39.5&nbsp;psi.|group=nb}} However, as early as 1938, at the 16th [[Paris Air Show]], Rolls-Royce displayed two versions of the Merlin rated to use 100-octane fuel. The Merlin R.M.2M was capable of {{convert|1,265|hp|kW|abbr=on}} at {{convert|7,870|ft|m}}, {{convert|1,285|hp|kW|abbr=on}} at {{convert|9,180|ft|m}} and {{convert|1,320|hp|kW|abbr=on}} on take-off; while a Merlin X with a two-speed supercharger in high gear generated {{convert|1,150|hp|kW|abbr=on}} at {{convert|15,400|ft|m}} and {{convert|1,160|hp|kW|abbr=on}} at {{convert|16,730|ft|m}}.<ref>Flight 1938, p. 528.</ref>

From late 1939, 100-octane fuel became available from the U.S., [[Aruba|West Indies]], [[Abadan Refinery|Persia]], and, in smaller quantities, domestically,<ref>Payton-Smith 1971, pp. 259–260.</ref> consequently, "...&nbsp;in the first half of 1940 the RAF transferred all Hurricane and Spitfire squadrons to 100 octane fuel."<ref>Lloyd, p.&nbsp;139</ref> Small modifications were made to Merlin II and III series engines, allowing an increased (emergency) boost pressure of +12 pounds per square inch (183&nbsp;kPa; 1.85&nbsp;atm). At this power setting these engines were able to produce {{convert|1,310|hp|kW|abbr=on}} at {{convert|9000|ft|m|abbr=on}} while running at 3,000 revolutions per minute.<ref name="Harvey-Bailey 1995, p. 155."/><ref name="Encyclopaedia of Aero Engines">Gunston, p. 144.</ref> Increased boost could be used indefinitely as there was no mechanical time limit mechanism, but pilots were advised not to use increased boost for more than a maximum of five minutes, and it was considered a "definite overload condition on the engine"; if the pilot resorted to emergency boost he had to report this on landing, when it was noted in the engine log book, while the engineering officer was required to examine the engine and reset the throttle gate.<ref>Air Ministry 1940.</ref> Later versions of the Merlin ran only on 100-octane fuel, and the five-minute combat limitation was raised to +18 pounds per square inch (224&nbsp;kPa; 2.3&nbsp;atm).<ref>Air Ministry 1943, p. 25.</ref>

In late 1943 trials were run of a new "100/150" grade (150-octane) fuel, recognised by its bright-green colour and "awful smell".<ref>McKinstry 2007, p. 356.</ref> Initial tests were conducted using {{convert|6.5|cc|impfloz|lk=on}} of [[tetraethyllead]] (T.E.L.) for every one [[imperial gallon]] of 100-octane fuel (or 1.43&nbsp;cc/L or 0.18&nbsp;U.S.&nbsp;fl&nbsp;oz/U.S.&nbsp;gal), but this mixture resulted in a build-up of lead in the combustion chambers, causing excessive fouling of the [[spark plug]]s. Better results were achieved by adding 2.5% [[N-Methylaniline|mono methyl aniline]] (M.M.A.) to 100-octane fuel.<ref>Lovesey 1946, pp. 222–223.</ref> The new fuel allowed the five-minute boost rating of the Merlin 66 to be raised to +25 pounds per square inch (272&nbsp;kPa; 2.7&nbsp;atm).<ref name=Price170>Price 1982. p. 170.</ref> With this boost rating the Merlin 66 generated {{convert|2,000|hp|kW|abbr=on}} at sea level and {{convert|1,860|hp|kW|abbr=on}} at {{convert|10500|ft|m|abbr=on}}.<ref>Wilkinson 1946, p. 195.</ref>

Starting in March 1944, the Merlin 66-powered Spitfire IXs of two [[Air Defence of Great Britain]] (ADGB) squadrons were cleared to use the new fuel for operational trials, and it was put to good use in the summer of 1944 when it enabled Spitfire L.F. Mk. IXs to intercept [[V-1 flying bomb]]s coming in at low altitudes.<ref name=Price170/> 100/150 grade fuel was also used by [[de Havilland Mosquito|Mosquito]] night fighters of the ADGB to intercept V-1s.<ref>Simons 2011, pp. 126–127.</ref> In early February 1945, Spitfires of the [[RAF Second Tactical Air Force|Second Tactical Air Force]] (2TAF) also began using 100/150 grade fuel.<ref name="Ber1994199.">Berger and Street 1994. p. 199.</ref>{{#tag:ref|Monty Berger, Senior Intelligence Officer of 126(RCAF) Spitfire Wing, 2 TAF, alleged that there were still problems being experienced with the new fuel on his wing, which was mistrusted by many pilots in the Wing.<ref name="Ber1994199."/> However, another source states that the transition to 150 Grade went without problems.<ref>Nijboer 2010, p. 100.</ref>|group=nb}} This fuel was also offered to the USAAF where it was designated "PPF 44-1" and informally known as "Pep".<ref>{{cite web|url=http://napoleon130.tripod.com/id860.html|title=Fuel|website=napoleon130.tripod.com|access-date=22 June 2017|url-status=live|archive-url=https://web.archive.org/web/20170211184204/http://napoleon130.tripod.com/id860.html|archive-date=11 February 2017}}</ref>