Editing Rolls-Royce Merlin (section)

==Design and development==
===Origin===
In the early 1930s, Rolls-Royce started planning its future aero-engine development programme and realised there was a need for an engine larger than their 21-litre (1,296&nbsp;cu in) [[Rolls-Royce Kestrel|Kestrel]], which was being used with great success in a number of 1930s aircraft.<ref>Rubbra 1990, p. 64.</ref> Consequently, work was started on a new {{convert|1100|hp|kW|abbr=on}}-class design known as the PV-12, with PV standing for ''Private Venture, 12-cylinder'', as the company received no government funding for work on the project. The PV-12 was first run on 15 October 1933 and first flew in a [[Hawker Hart]] biplane ([[United Kingdom military aircraft serials|serial number]] ''K3036'') on 21 February 1935.<ref name="Lumsden203">Lumsden 2003, p. 203.</ref> The engine was originally designed to use the [[Radiator (engine cooling)#Evaporative cooling|evaporative cooling]] system then in vogue. This proved unreliable and when [[ethylene glycol]] from the U.S. became available, the engine was adapted to use a conventional liquid-cooling system. The Hart, as a Merlin [[testbed]], completed over 100 hours of flying with the Merlin C and E engines.<ref>Mason 1991, p. 168.</ref>

In 1935, the [[Air Ministry]] issued a specification, [[List of Air Ministry specifications|F10/35]], for new [[fighter aircraft]] with a minimum airspeed of {{convert|310|mph|km/h|abbr=on|lk=on}}. Fortunately, two designs had been developed: the [[Supermarine Spitfire]] and the [[Hawker Hurricane]]; the latter designed in response to another specification, F36/34.<ref>McKinstry 2007, p. 53.</ref> Both were designed around the PV-12 instead of the Kestrel, and were the only contemporary British fighters to have been so developed. Production contracts for both aircraft were placed in 1936, and development of the PV-12 was given top priority as well as government funding. Following the company convention of naming its piston aero engines after birds of prey, Rolls-Royce named the engine the ''[[Merlin (bird)|Merlin]]'' after a small, Northern Hemisphere falcon (''Falco columbarius'').{{#tag:ref|The naming tradition was started by managing director, [[Claude Johnson]], in 1915 with the Eagle, Hawk and Falcon engines. There is no connection to King Arthur's [[Merlin (wizard)|legendary magician.]]|group=nb}}<ref name=Gunston137>Gunston 1989, p. 137.</ref>

Two more Rolls-Royce engines developed just prior to the war were added to the company's range. The {{convert|885|hp|kW|abbr=on}} [[Rolls-Royce Peregrine]] was an updated, [[supercharged]] development of their V-12 Kestrel design, while the {{convert|1700|hp|kW|abbr=on}} 42-litre (2,560 cu in) [[Rolls-Royce Vulture]] used four Kestrel-sized [[cylinder block]]s fitted to a single [[crankcase]] and driving a common crankshaft, forming an [[X24 engine|X-24]] layout.<ref>Rubbra 1990, p. 139.</ref> This was to be used in larger aircraft such as the [[Avro Manchester]].<ref name="Lumsden2003 p198, 200">Lumsden 2003, pp. 198–200.</ref>

Although the Peregrine appeared to be a satisfactory design, it was never allowed to mature since Rolls-Royce's priority was refining the Merlin. As a result, the Peregrine saw use in only two aircraft: the [[Westland Whirlwind (fixed wing)|Westland Whirlwind]] fighter and one of the [[Gloster F.9/37]] prototypes. The Vulture was fitted to the [[Avro Manchester]] bomber, but proved unreliable in service and the planned fighter using it – the [[Hawker Tornado]] – was cancelled as a result.<ref>Lumsden 2003, p. 200.</ref> With the Merlin itself soon pushing into the {{convert|1500|hp|kW|abbr=on}} range, the Peregrine and Vulture were both cancelled in 1943, and by mid-1943 the Merlin was supplemented in service by the larger [[Rolls-Royce Griffon|Griffon]].<ref>Rubbra 1990, p. 118.</ref> The Griffon incorporated several design improvements and ultimately superseded the Merlin.

===Development===
Initially the new engine was plagued with problems such as failure of the accessory gear trains and coolant jackets. Several different construction methods were tried before the basic design of the Merlin was set.<ref>Rubbra 1990, pp. 64–117.</ref> Early production Merlins were unreliable: common problems were cylinder head cracking, coolant leaks, and excessive wear to the [[camshafts]] and [[crankshaft]] [[main bearing]]s.<ref>Rubbra 1990, pp. 82–92.</ref>

====Early engines====
The prototype, developmental, and early production engine types were the:
* '''PV-12'''
: The initial design using an evaporative cooling system. Two built, passed [[Dynamometer|bench]] [[type certificate|type testing]] in July 1934, generating {{convert|740|hp|kW|abbr=on}} at {{convert|12000|ft|adj=on}} equivalent. First flown 21 February 1935.<ref name="Lumsden203"/>
* '''Merlin B'''
: Two built, ethylene glycol liquid cooling system introduced. "Ramp" [[cylinder head]]s ([[Intake valve|inlet valves]] were at a 45-[[Degree (angle)|degree]] angle to the cylinder). Passed Type Testing February 1935, generating {{convert|950|hp|kW|abbr=on}} at {{convert|11000|ft|adj=on}} equivalent.<ref name="Lumsden203"/>
* '''Merlin C'''
: Development of Merlin B; [[crankcase]] and [[cylinder block]]s became three separate [[Casting (metalworking)|castings]] with bolt-on cylinder heads.<ref name="Lumsden203"/> First flight in [[Hawker Horsley]] 21 December 1935, {{convert|950|hp|kW|abbr=on}} at {{convert|11000|ft|adj=on}}.<ref>Morgan and Shacklady 2000, p. 607.</ref>
* '''Merlin E'''
: Similar to '''C''' with minor design changes. Passed 50-hour civil test in December 1935 generating a constant {{convert|955|hp|kW|abbr=on}} and a maximum rating of {{convert|1,045|hp|kW|abbr=on}}. Failed military 100-hour test in March 1936. Powered the Supermarine Spitfire prototype.<ref name="Lumsden204">Lumsden 2003, p. 204.</ref>
[[File:MerlinHead.JPG|thumb|alt=A sectioned, parallel valve, aircraft engine cylinder head is shown with colour-coded internal details. Coolant passageways are painted green; the valves, valve springs, camshaft and rocker arms are also shown.|Parallel valve Merlin [[cylinder head]]]]
* '''Merlin F''' ('''Merlin I''')
: Similar to '''C''' and '''E'''. First flight in Horsley 16 July 1936.<ref name="MoSh610">Morgan and Shacklady 2000, p. 610.</ref> This became the first production engine, and was designated as the Merlin I. The Merlin continued with the "ramp" head, but this was not a success and only 172 were made. The [[Fairey Battle|Fairey Battle I]] was the first production aircraft to be powered by the Merlin I and first flew on 10 March 1936.<ref name="Lumsden204"/>
* '''Merlin G''' ('''Merlin II''')
: Replaced "ramp" cylinder heads with parallel pattern heads (valve stems parallel to the cylinder bore axis) scaled up from the Kestrel engine. 400-hour flight endurance tests carried out at [[Royal Aircraft Establishment|RAE]] July 1937; acceptance test 22 September 1937.<ref name="MoSh610"/> It was first widely delivered as the {{convert|1,030|hp|kW|abbr=on|adj=on}} Merlin II in 1938, and production was quickly stepped up for Fairey Battle II.<ref name="Lumsden204"/>
* '''Merlin III'''
: Merlin II with standardised de Havilland/Rotol [[Society of British Aerospace Companies|SBAC]] propeller shaft, and dual accessory-drive. {{convert|1,030|hp|kW|abbr=on|adj=on}} at 3,000&nbsp;rpm at {{convert|10,250|ft}} at +6.5&nbsp;lb boost.<ref name="flightglobal.com">{{cite web |first=H. F. |last=King |title=The Two Rs |url=https://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201296.html |publisher=[[Flight International|Flight]] |page=577 |date=7 May 1954 |access-date=22 August 2017  |archive-url=https://web.archive.org/web/20170211184500/https://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201296.html |archive-date=11 February 2017}}</ref> Formed basis for the [[Rolls-Royce Meteor|Rolls-Royce/Rover Meteor]] tank engine
* '''"Racing" Merlin'''
: Racing engine for 1937/38 "[[Supermarine Spitfire (early Merlin-powered variants)#Speed Spitfire (Type 323)|Speed Spitfire]]" world speed record attempt. Merlin III with strengthened pistons, connecting rods, and gudgeon-pins, running on increased octane fuel, developed {{convert|2,160|hp|kW|abbr=on|adj=on}} at 3,200&nbsp;rpm and +27&nbsp;lb boost, a power/weight ratio of 0.621&nbsp;lb per horsepower. Completed 15-hour endurance run at {{convert|1,800|hp|kW|abbr=on|adj=on}}, 3,200&nbsp;rpm at +22&nbsp;lb boost.<ref name="flightglobal.com"/>
* '''Merlin IV'''
: Merlin with pressure-water cooling for [[Armstrong Whitworth Whitley|Armstrong Whitworth Whitley IV]].
* '''Merlin V'''
: Merlin for Fairey Battle V.
* '''Merlin VIII'''
: Medium-supercharged Merlin developed for [[Fairey Fulmar|Fairey Fulmar I]], rated {{convert|1,010|hp|kW|abbr=on|adj=on}} at 2,850&nbsp;rpm at {{convert|6,750|ft}}, {{convert|1,080|hp|kW|abbr=on|adj=on}} at 3,000&nbsp;rpm for take-off using 100-octane fuel.<ref name="flightglobal.com"/>
* '''Merlin X'''
: First Merlin with two-speed supercharger, {{convert|1,145|hp|kW|abbr=on|adj=on}} in low gear at {{convert|5,250|ft}}, {{convert|1,010|hp|kW|abbr=on|adj=on}} in high gear at {{convert|17,750|ft}}. First of Rolls-Royce unitised "[[Power-egg#United Kingdom|Power Plant]]" installation designs for this engine in 1937<ref name="flightglobal.com"/> and used in [[Handley Page Halifax|Handley Page Halifax I]], [[Vickers Wellington|Vickers Wellington II]], and Armstrong Whitworth Whitley V and VII.
* '''Merlin XII'''
: Merlin fitted with 0.477:1 [[reduction gear]] installed in some Spitfire IIs with three-bladed Rotol constant-speed propeller. Rated at {{convert|1,150|hp|kW|abbr=on|adj=on}} at 3,000&nbsp;rpm at {{convert|14,000|ft}}.<ref name="flightglobal.com"/>
* '''Merlin XX'''
: Merlin X with [[Stanley Hooker]] re-designed supercharger<ref>"World Encyclopedia of Aero Engines – 5th edition" by [[Bill Gunston]], Sutton Publishing, 2006, p. 190</ref><ref>{{Cite web|url=https://www.key.aero/article/how-rolls-royce-improved-merlins-power-output|title=How Rolls-Royce improved the Merlin's power output|date=8 June 2017|website=www.key.aero}}</ref> incorporating re-designed inlet and improved guide vanes on impeller with revised blower gear ratios; 8:15:1 for low gear, 9:49:1 for high gear. New larger [[SU Carburettor|SU]] twin choke updraught carburettor. Engine interchangeable with Merlin X. Rated at {{convert|1,240|hp|kW|abbr=on|adj=on}} at 2,850&nbsp;rpm in low gear at {{convert|10,000|ft}} and +9&nbsp;lb boost; {{convert|1,175|hp|kW|abbr=on|adj=on}} at 2,850&nbsp;rpm in high gear at {{convert|17,500|ft}} at +9&nbsp;lb boost. Revised Rolls-Royce unitised "Power Plant" installation design. Engine used in [[Bristol Beaufighter|Bristol Beaufighter II]], [[Boulton Paul Defiant|Boulton Paul Defiant II]], Handley Page Halifax II and V, Hawker Hurricane II and IV, and [[Avro Lancaster|Avro Lancaster I and III]]. First Merlin produced by [[Packard|Packard Motor Car Company]] as V-1650-1 and designated by Rolls-Royce as Merlin 28.<ref name="flightglobal.com"/>

====Production engines====
The Merlin II and III series were the first main production versions of the engine. The Merlin III was the first version to incorporate a "universal" propeller shaft, allowing either [[de Havilland Propellers|de Havilland]] or [[Dowty Rotol|Rotol]] manufactured propellers to be used.<ref>Fozard 1991, p. 125.</ref>

The first major version to incorporate changes brought about through experience in operational service was the XX, which was designed to run on 100-[[Octane rating|octane]] fuel.{{#tag:ref|The Merlin II and III series were originally designed to use 87-octane fuel and later modified to allow the use of 100-octane fuel.<ref>Air Ministry 1940, pp. 6, 10.</ref>|group=nb}} This fuel allowed higher [[manifold pressure]]s, which were achieved by increasing the boost from the [[centrifugal supercharger]]. The Merlin XX also utilised the two-speed superchargers designed by Rolls-Royce, resulting in increased power at higher altitudes than previous versions. Another improvement, introduced with the Merlin X, was the use of a 70%–30% water-glycol coolant mix rather than the 100% glycol of the earlier versions. This substantially improved engine life and reliability, removed the fire hazard of the flammable [[ethylene glycol]], and reduced the oil leaks that had been a problem with the early Merlin I, II and III series.<ref name = "Fozard 1991, pp.127, 165">Fozard 1991, pp. 127, 165.</ref>

The process of improvement continued, with later versions running on higher octane ratings, delivering more power. Fundamental design changes were also made to all key components, again increasing the engine's life and reliability. By the end of the war the "little" engine was delivering over {{convert|1,600|hp|kW|abbr=on}} in common versions, and as much as {{convert|2,030|hp|kW|abbr=on}} in the Merlin 130/131 versions specifically designed for the [[de Havilland Hornet]].<ref>Flight January 1946, p. 93.</ref> Ultimately, during tests conducted by Rolls-Royce at [[Derby]], an RM.17.SM (the high altitude version of the Merlin 100-Series) achieved {{convert|2,640|hp|kW|abbr=on}} at 36&nbsp;lb boost (103"Hg) on 150-octane fuel with water injection.<ref>Lovesey 1946, p. 223.</ref>

With the end of the war, work on improving Merlin power output was halted and the development effort was concentrated on civil derivatives of the Merlin.<ref>Lovesey 1946, p. 224.</ref> Development of what became the "Transport Merlin" (TML)<ref name="ReferenceA">{{cite web |title=Quieter Argonaut |url=https://www.flightglobal.com/pdfarchive/view/1952/1952%20-%200532.html |publisher=Flight |page=242 |date=29 February 1952 |access-date=22 August 2017 |archive-url=https://web.archive.org/web/20180131072414/https://www.flightglobal.com/pdfarchive/view/1952/1952%20-%200532.html |archive-date=31 January 2018}}</ref> commenced with the Merlin 102 (the first Merlin to complete the new civil [[Type certificate|type-test]] requirements) and was aimed at improving reliability and service overhaul periods for airline operators using airliner and transport aircraft such as the [[Avro Lancastrian]], [[Avro York]] (Merlin 500-series), [[Avro Tudor]] II and IV (Merlin 621), Tudor IVB and V (Merlin 623), [[Trans-Canada Air Lines|TCA]] [[Canadair North Star]] (Merlin 724) and [[British Overseas Airways Corporation|BOAC]] [[Canadair North Star|Argonaut]] (Merlin 724-IC).<ref name="ReferenceB">{{cite web |first=H. F. |last=King |title=The Two Rs |url=https://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201300.html |publisher=Flight |page=579 |date=7 May 1954 |access-date=22 August 2017  |archive-url=https://web.archive.org/web/20170211184559/https://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201300.html |archive-date=11 February 2017}}</ref> By 1951 the [[time between overhauls]] (TBO) was typically 650–800 hours depending on use.<ref>{{cite web |title=Universal Power Plants |url=https://www.flightglobal.com/pdfarchive/view/1947/1947%20-%200238.html |publisher=Flight |page=162 |date=13 February 1947 |access-date=22 August 2017  |archive-url=https://web.archive.org/web/20180131074131/https://www.flightglobal.com/pdfarchive/view/1947/1947%20-%200238.html |archive-date=31 January 2018}}</ref><ref>{{cite web |first=H. F. |last=King |title=A Call on Canadair |url=https://www.flightglobal.com/pdfarchive/view/1949/1949%20-%200331.html |publisher=Flight |page=215 |date=24 February 1949 |access-date=22 August 2017  |archive-url=https://web.archive.org/web/20180131081513/https://www.flightglobal.com/pdfarchive/view/1949/1949%20-%200331.html |archive-date=31 January 2018}}</ref> By then single-stage engines had accumulated 2,615,000 engine hours in civil operation, and two-stage engines 1,169,000.<ref>{{cite web |title=Dart Endurance Test |url=https://www.flightglobal.com/pdfarchive/view/1951/1951%20-%201664.html |publisher=Flight |page=249 |date=31 August 1951 |access-date=22 August 2017  |archive-url=https://web.archive.org/web/20180131081922/https://www.flightglobal.com/pdfarchive/view/1951/1951%20-%201664.html |archive-date=31 January 2018}}</ref>

In addition, an exhaust system to reduce noise levels to below those from ejector exhausts was devised for the North Star/Argonaut. This "cross-over" system took the exhaust flow from the inboard bank of cylinders up-and-over the engine before discharging the exhaust stream on the outboard side of the [[Power-egg#United Kingdom|UPP]] nacelle. As a result, sound levels were reduced by between 5 and 8 [[decibel]]s. The modified exhaust also conferred an increase in horsepower over the unmodified system of {{convert|38|hp|abbr=on}}, resulting in a 5 knot improvement in true air speed. Still-air range of the aircraft was also improved by around 4 per cent.<ref name="ReferenceA"/> The modified engine was designated the "TMO" and the modified exhaust system was supplied as kit that could be installed on existing engines either by the operator or by Rolls-Royce.<ref name="ReferenceA"/>

Power ratings for the civil Merlin 600, 620, and 621-series was {{convert|1160|hp|abbr=on}} continuous cruising at {{convert|23500|ft|m}}, and {{convert|1725|hp|abbr=on}} for take-off. Merlins 622–626 were rated at {{convert|1420|hp|abbr=on}} continuous cruising at {{convert|18700|ft|m}}, and {{convert|1760|hp|abbr=on}} for take-off. Engines were available with single-stage, two-speed supercharging (500-series), two-stage, two-speed supercharging (600-series), and with full intercooling, or with half intercooling/charge heating, charge heating being employed for cold area use such as in Canada.<ref name="ReferenceB"/> Civil Merlin engines in airline service flew 7,818,000 air miles in 1946, 17,455,000 in 1947, and 24,850,000 miles in 1948.<ref>{{cite web |title=The Rolls Royce Civil Merlin Engine |url=https://www.flightglobal.com/pdfarchive/view/1949/1949%20-%201117.html |publisher=Flight |date=16 June 1949 |access-date=22 August 2017 |url-status=live |archive-url=https://web.archive.org/web/20180131083841/https://www.flightglobal.com/pdfarchive/view/1949/1949%20-%201117.html |archive-date=31 January 2018}}</ref>

====Basic component overview (Merlin 61)====
''From Jane's'':<ref>Bridgman 1998, pp. 280–281.</ref>
; Cylinders
: Twelve cylinders consisting of high-carbon steel liners set in two, two-piece cylinder blocks of cast "[[Hiduminium|R.R.50]]" [[aluminium alloy]] having separate heads and skirts. Wet liners, ie. coolant in direct contact with external face of liners. Cylinder heads fitted with cast-iron inlet valve guides, [[phosphor bronze]] exhaust valve guides, and renewable "Silchrome" steel-alloy valve seats. Two diametrically opposed [[spark plug]]s protrude into each [[combustion chamber]].
; Pistons
: Machined from "[[Hiduminium|R.R.59]]" alloy [[forging]]s. Fully floating hollow [[gudgeon pin]]s of hardened nickel-chrome steel. Three [[Internal combustion engine#Compression ignition process|compression]] and one oil-control [[Piston ring|ring]] above the gudgeon pin, and one oil-control ring below.
; Connecting rods
: H-section machined nickel-steel forgings, each pair consisting of a plain and a forked [[Connecting rod|rod]]. The forked rod carries a nickel-steel bearing block which accommodates steel-backed lead-bronze-alloy bearing shells. The "small-end" of each rod houses a floating phosphor bronze [[Bushing (bearing)|bush]].
; Crankshaft
: One-piece, machined from a [[Nitriding|nitrogen-hardened]] nickel-chrome [[molybdenum]] steel forging. [[Engine balance|Statically and dynamically balanced]]. Seven main bearings and six throws.
; Crankcase
: Two aluminium-alloy castings joined together on the horizontal centreline. The upper portion bears the wheelcase, supercharger and accessories; and carries the cylinder blocks, crankshaft main bearings (split mild-steel shells lined with lead bronze alloy), and part of the housing for the [[Propeller speed reduction unit|airscrew reduction gear]]. The lower half forms an oil sump and carries the oil pumps and filters.
; Wheelcase
: Aluminium casting fitted to rear of crankcase. Houses drives to the camshafts, [[ignition magneto|magnetos]], coolant and [[oil pump (internal combustion engine)|oil pumps]], [[supercharger]], hand and electric [[Starter motor|starters]], and the electric [[Electrical generator|generator]].
; Valve gear
: Two inlet and two exhaust [[poppet valve]]s of "K.E.965" steel per cylinder. Both the inlet and exhaust valves have hardened "[[stellite]]d" ends; while the exhaust valves also have [[sodium]]-cooled stems, and heads protected with a "[[Brightray]]" (nickel-chromium) coating. Each valve is kept closed by a pair of concentric [[Coil spring|coil-springs]]. A single, seven-bearing camshaft, located on the top of each cylinder head operates 24 individual steel [[Rocker arm|rockers]]; 12 pivoting from a rocker shaft on the inner, intake side of the head to actuate the exhaust valves, the others pivoting from a shaft on the exhaust side of the head to actuate the inlet valves.

====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>