View source for Rolls-Royce Merlin

{{Short description|Aircraft engine family}}
{{For|the Packard-produced licensed version|Packard V-1650 Merlin}}
{{Use British English|date=July 2019}}
{{Use dmy dates|date=August 2024}}
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{{Infobox aero engine
 | name = Merlin
 | image = File:Rolls-Royce Merlin.jpg
 | caption = The Rolls-Royce Merlin|alt=A front right view of a black-painted aircraft piston engine. The words 'Rolls-Royce' appear in red text on the camshaft cover.
 | engine_type=[[Coolant#Liquids|Liquid-cooled]] [[V12 engine|V-12]] [[Four-stroke engine|four-stroke]] [[Reciprocating engine|piston]] [[Aircraft engine|aero engine]]
 | manufacturer=[[Rolls-Royce Limited]]
 | national_origin=United Kingdom
 | first_run=15 October 1933
 | major_applications= {{ubl | [[Avro Lancaster]] | [[de Havilland Mosquito]] | [[Handley Page Halifax]] | [[Hawker Hurricane]] | [[Supermarine Spitfire]] }}<!-- Please do not add the [[North American P-51 Mustang]] here. Five North American Mustang Mk X used British-built Merlins. All other P-51s used the [[Packard V-1650 Merlin]], and their use is covered on that page. Ditto for the [[Curtiss P-40 Warhawk]]. -->
 | number_built = 149,659
 | program_cost =
 | unit_cost =
 | developed_from =
 | variants_with_their_own_articles = [[Packard V-1650 Merlin]]
 | developed_into = [[Rolls-Royce Meteor]]
}}

The '''Rolls-Royce Merlin''' is a British [[Coolant#Liquids|liquid-cooled]] [[V12 engine|V-12]] [[Reciprocating engine|piston]] [[aero engine]] of 27-litre (1,650&nbsp;cu in) [[Engine displacement|capacity]]. Developed as a private venture by [[Rolls-Royce Limited|Rolls-Royce]] and first run in 1933, the engine became one of the most successful aircraft engines of World War II, most closely associated with the [[Hawker Hurricane]] and [[Supermarine Spitfire]], although the majority of the production run was for the four-engined [[Avro Lancaster]] heavy bomber.

Initially known as the '''PV-12''', it was dubbed ''[[Merlin (bird)|Merlin]]'' after the company convention of naming its four-stroke piston aero engines for [[birds of prey]]. The engine benefitted from the racing experiences of precursor engines in the 1930s. After several modifications, the first production variants of the PV-12 were completed in 1936. The first operational aircraft to enter service using the Merlin were the [[Fairey Battle]], Hurricane, and Spitfire. Experience in use led to a series of rapidly applied developments that markedly improved the engine's performance and durability. Starting at {{convert|1,000|hp}} for the first production models, most late-war versions produced just under {{convert|1,800|hp}}. The final version, used in the [[de Havilland Hornet]], had over {{convert|2,000|hp}}.

Some 50 versions of the Merlin were built by Rolls-Royce in [[Derby]], [[Crewe]], and [[Glasgow]], as well as by [[Ford of Britain]] at their [[Ford Trafford Park Factory|Trafford Park factory]] near [[Manchester]]. The [[Packard V-1650]] was a version of the Merlin built in the United States. A de-rated version was the basis of the [[Rolls-Royce Meteor|Rolls-Royce/Rover Meteor]] tank engine. After the war, the Merlin was largely superseded by the [[Rolls-Royce Griffon]] for military use, although new variants were designed and built for [[airliner]]s and [[military transport aircraft]]. Production ceased in 1950 after a total of almost 150,000 engines had been delivered. 

Merlin engines remain in [[Royal Air Force]] service today with the [[Battle of Britain Memorial Flight]], and power many restored aircraft in private ownership worldwide.

==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 RM17SM (an improved version of the RM14SM 'low altitude' Merlin 100-Series with larger diameter supercharger rotors and different valve timing) achieved {{convert|2,640|hp|kW|abbr=on}} at 36&nbsp;lb boost (103"Hg) on 150-octane fuel with water injection.<ref>Harvey-Bailey and Piggott 1993, p. 136.</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 [[Plain bearing#Bushing|bushing]].
; 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 (engine)|starters]], and the [[electric 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|14.7|psi|mbar|abbr=on}}, thus a reading of +6 means that the air/fuel mix is being compressed by a supercharger blower to 20.7&nbsp;psi before entering the engine; +25 means that the air/fuel mix is now being compressed to 39.7&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>

==Production==
Production of the Rolls-Royce Merlin was driven by the forethought and determination of [[Ernest Hives, 1st Baron Hives|Ernest Hives]], who at times was enraged by the apparent complacency and lack of urgency encountered in his frequent correspondence with the [[Air Ministry]], the [[Ministry of Aircraft Production]] and local authority officials.<ref>Pugh 2000, pp. 195–196.</ref> Hives was an advocate of [[British shadow factories|shadow factories]], and, sensing the imminent outbreak of war, pressed ahead with plans to produce the Merlin in sufficient numbers for the rapidly expanding Royal Air Force.<ref>Pugh 2000, pp. 193–194.</ref> Despite the importance of uninterrupted production, several factories were affected by [[industrial action]].<ref>McKinstry 2007, pp. 327–329.</ref> By the end of its production run in 1950, 168,176 Merlin engines had been built; over 112,000 in Britain and more than 55,000 under licence in the U.S.{{#tag:ref|Factory production numbers:
* Rolls-Royce: Derby = 32,377
* Rolls-Royce: Crewe = 26,065
* Rolls-Royce: Glasgow =23,675
* Ford Manchester= 30,428
* Packard Motor Corp = 55,523 (37,143 Merlins, 18,380 V-1650s)
* Commonwealth Aircraft Corp (CAC): NSW Australia =108 Type MK102. 1946–1952 for the CAC Avro Lincoln<ref>RAAF Museum Point Cook</ref>
* '''Overall:''' 168,176
|group=nb}}<ref name="Encyclopaedia of Aero Engines"/>{{#tag:ref|Cost: [[Pound sterling|£]]2,000 (engine), £350 (propeller)<ref>Beckles 1941, pp. 78–79. (1940 prices, unadjusted for inflation).</ref>
|group=nb}}

===Derby===
[[File:Osmaston Rolls-Royce.jpg|thumb|alt=Image of a red brick building with a central front door, the words Rolls-Royce Limited appear above the door in white letters|The [[Marble Hall, Derby|Marble Hall]] at the Rolls-Royce factory, Nightingale Road, [[Derby]] (photo 2005)]]
The existing Rolls-Royce facilities at [[Osmaston, Derby]] were not suitable for mass engine production although the [[Area|floor space]] had been increased by some 25% between 1935 and 1939; Hives planned to build the first two or three hundred engines there until engineering teething troubles had been resolved. To fund this expansion, the [[Air Ministry]] had provided a total of £1,927,000 by December 1939.<ref>[http://discovery.nationalarchives.gov.uk/details/r/C9003607 War Cabinet-Supply and Production: Fourth Report by the Air Ministry, Appendix V, sheet 3.] {{webarchive|url=https://web.archive.org/web/20160308203419/http://discovery.nationalarchives.gov.uk/details/r/C9003607 |date=8 March 2016 }} National Archives.gov.uk. Retrieved: 8 March 2016.</ref>{{#tag:ref|The Crewe works in fact had been leased to Rolls-Royce by the government.<ref>Harvey-Bailey 1995, p. 12.</ref>|group=nb}}  Having a workforce that consisted mainly of design engineers and highly skilled men, the Derby factory carried out the majority of development work on the Merlin, with flight testing carried out at nearby [[Hucknall Airfield|RAF Hucknall]]. All the Merlin-engined aircraft taking part in the [[Battle of Britain]] had their engines assembled in the Derby factory. Total Merlin production at Derby was 32,377.<ref name="Gunston2006190">Gunston 2006, p. 190.</ref> The original factory closed in March 2008, but the company maintains a presence in Derby.<ref>[http://news.bbc.co.uk/1/hi/england/derbyshire/7321327.stm Derby factory closure] {{webarchive|url=https://web.archive.org/web/20080403035424/http://news.bbc.co.uk/1/hi/england/derbyshire/7321327.stm |date=3 April 2008 }} news.bbc.co.uk. Retrieved: 24 August 2009</ref>

===Crewe===
To meet the increasing demand for Merlin engines, Rolls-Royce started building work on a [[Bentley Crewe|new factory]] at [[Crewe]] in May 1938, with engines leaving the factory in 1939. The Crewe factory had convenient road and rail links to their existing facilities at Derby. Production at Crewe was originally planned to use unskilled labour and [[sub-contractor]]s with which Hives felt there would be no particular difficulty, but the number of required sub-contracted parts such as crankshafts, camshafts and cylinder liners eventually fell short and the factory was expanded to manufacture these parts "in house".<ref>Pugh 2000, p. 193.</ref>

Initially the local authority promised to build 1,000 new houses to accommodate the workforce by the end of 1938, but by February 1939 it had only awarded a contract for 100. Hives was incensed by this complacency and threatened to move the whole operation, but timely intervention by the Air Ministry improved the situation. In 1940 a [[Strike action|strike]] took place when women replaced men on [[Turret lathe|capstan lathes]], the workers' [[Trade union|union]] insisting this was a skilled labour job; however, the men returned to work after 10 days.<ref>Pugh 2000, pp. 196–197.</ref>

Total Merlin production at Crewe was 26,065.<ref name="Gunston2006190"/>

The factory was used postwar for the production of Rolls-Royce and [[Bentley]] motor cars and military fighting vehicle power plants. In 1998 [[Volkswagen AG]] bought the Bentley marque and the factory. Today it is known as Bentley Crewe.<ref>[http://www.jackbarclayparts.co.uk/about-us/crewe-history/ Crewe factory history] {{webarchive|url=https://web.archive.org/web/20120305100743/http://www.jackbarclayparts.co.uk/about-us/crewe-history/ |date=5 March 2012 }} jackbarclayparts.co.uk. Retrieved: 24 August 2009</ref>

===Glasgow===
[[File:Rolls Royce factory -Merlin engines and female workers-1942 (original).jpg|thumb|alt=An image of workers on an engine assembly line|Workers assembling cylinder heads on the Hillington Merlin production line in 1942]]
Hives further recommended that a factory be built near [[Glasgow]] to take advantage of the abundant local work force and the supply of steel and forgings from Scottish manufacturers. In September 1939, the [[Air Ministry]] allocated £4,500,000 for a new Shadow factory.<ref>[http://discovery.nationalarchives.gov.uk/details/r/C9003498 War Cabinet-Supply and Production: First Report by the Air Ministry, Appendix XI.] {{webarchive|url=https://web.archive.org/web/20160308203426/http://discovery.nationalarchives.gov.uk/details/r/C9003498 |date=8 March 2016 }} National Archives.gov.uk. Retrieved: 8 March 2016.</ref>{{#tag:ref|This allocation had increased to £5,995,000 by December 1939.<ref>[http://discovery.nationalarchives.gov.uk/details/r/C9003607 War Cabinet-Supply and Production: Fourth Report by the Air Ministry, Appendix XI.] {{webarchive|url=https://web.archive.org/web/20160308203419/http://discovery.nationalarchives.gov.uk/details/r/C9003607 |date=8 March 2016 }} National Archives.gov.uk. Retrieved: 8 March 2016.</ref>|group=nb}} This [[Her Majesty's Government|government]]-funded and -operated factory was built at [[Hillington, Scotland|Hillington]] starting in June 1939 with workers moving into the premises in October, one month after the outbreak of war. The factory was fully occupied by September 1940. A housing crisis also occurred at Glasgow, where Hives again asked the Air Ministry to step in.<ref>Pugh 2000, p. 197.</ref>

With 16,000 employees, the Glasgow factory was one of the largest industrial operations in Scotland. Unlike the Derby and Crewe plants, which relied significantly on external [[subcontractor]]s, it produced almost all the Merlin's components itself.<ref>Lloyd and Pugh 2004, p. 61.</ref> Hillingdon required "a great deal of attention from Hives" from when it was producing its first complete engine; it had the highest proportion of unskilled workers in any Rolls-Royce-managed factory”.<ref>Robotham 1970, p. 127.</ref> Engines began to leave the production line in November 1940, and by June 1941 monthly output had reached 200, increasing to more than 400 per month by March 1942.<ref>Lloyd and Pugh 2004, p. 69.</ref> In total 23,675 engines were produced. Worker [[absenteeism]] became a problem after some months due to the physical and mental effects of wartime conditions such as the frequent occupation of [[air-raid shelter]]s. It was agreed to cut the punishing working hours slightly to 82 hours a week, with one half-Sunday per month awarded as holiday.<ref>Pugh 2000, p. 198.</ref> Record production is reported to have been 100 engines in one day.<ref name=BBCNews>[http://news.bbc.co.uk/1/low/scotland/4531580.stm End of era for Rolls-Royce plant.] news.bbc.co.uk. Retrieved: 25 August 2009</ref>

Immediately after the war the site repaired and overhauled Merlin and Griffon engines, and continued to manufacture spare parts.<ref name=BBCNews/> Finally, following the production of the [[Rolls-Royce Avon]] [[turbojet]] and others, the factory was closed in 2005.<ref>[http://www.rolls-royce.com/about/heritage/branches/scotland.jsp Hillington factory history] {{webarchive|url=https://web.archive.org/web/20090807021409/http://www.rolls-royce.com/about/heritage/branches/scotland.jsp |date=7 August 2009 }} rolls-royce.com. Retrieved: 24 August 2009</ref>

===Manchester===
{{Main|Ford Trafford Park Factory}}
The [[Ford of Britain|Ford Motor Company]] was asked to produce Merlins at [[Trafford Park]], [[Stretford]], near [[Manchester]], and building work on a new factory was started in May 1940 on a {{convert|118|acre|ha|adj=on}} site. Built with two distinct sections to minimise potential bomb damage, it was completed in May 1941 and bombed in the same month.{{#tag:ref|The new factory was bombed by the Luftwaffe in May 1941.<ref name=Nicholls103/>|group=nb}} At first, the factory had difficulty in attracting suitable labour, and large numbers of women, youths and untrained men had to be taken on. Despite this, the first Merlin engine came off the production line one month later and it was building the engine at a rate of 200 per week by 1943,<ref name=Nicholls103>Nicholls 1996, p. 103.</ref> at which point the joint factories were producing 18,000 Merlins per year.<ref name= "Lumsden p. 201"/> In his autobiography ''Not much of an Engineer'', Sir Stanley Hooker states: "...&nbsp;once the great Ford factory at Manchester started production, Merlins came out like shelling peas&nbsp;...".<ref>Hooker 1984, pp. 58–59.</ref>

Some 17,316 people worked at the Trafford Park plant, including 7,260 women and two resident doctors and nurses.<ref name=Nicholls103/> Merlin production started to run down in August 1945, and finally ceased on 23 March 1946.<ref>Nicholls 1996, p. 105.</ref>

Total Merlin production at Trafford Park was 30,428.<ref name="Gunston2006190"/>

===Detroit===
{{Main|Packard V-1650 Merlin}}
As the Merlin was considered to be so important to the war effort, negotiations were started to establish an alternative production line outside the UK. Rolls-Royce staff visited North American automobile manufacturers to select one to build the Merlin in the U.S. or Canada. [[Henry Ford]] rescinded an initial offer to build the engine in the U.S. in July 1940, and the [[Packard Motor Car Company]] was selected to take on the [[United States dollar|$]]130,000,000 Merlin order (equivalent to ${{formatprice|{{Inflation|US|130,000,000|1940}}}} in {{Inflation-year|US}} dollars{{inflation-fn|US}}).<ref name=Time>[http://www.time.com/time/magazine/article/0,9171,795076,00.html ''Time Magazine'' (8 July 1940) – Business: Ford's Rolls-Royces.] {{webarchive|url=https://web.archive.org/web/20130721023935/http://www.time.com/time/magazine/article/0,9171,795076,00.html |date=21 July 2013 }} time.com. Retrieved: 26 August 2009</ref> Agreement was reached in September 1940, and the first Packard-built engine, a version of the Merlin&nbsp;XX, designated the Merlin 28 (when supplied to the RAF) or the V-1650-1 (when supplied to the USAAF), ran in August 1941.<ref>Lumsden 2003, p. 202.</ref> Total Merlin production by Packard was 55,523.<ref name="Gunston2006190"/>

Six development engines were also made by [[Continental Motors, Inc.]]<ref name="Gunston2006190"/>

==Variants==
<!-- Units abbreviated as this is effectively a specification table -->
{{Main|List of Rolls-Royce Merlin variants}}
This is a list of representative Merlin variants, describing some of the mechanical changes made during development of the Merlin. Engines of the same power output were typically assigned different model numbers based on supercharger or propeller gear ratios, differences in cooling system or carburettors, engine block construction, or arrangement of engine controls.<ref>Bridgman 1998, p. 283.</ref> Power ratings quoted are usually maximum "military" power. All but the Merlin 131 and 135 engines were "right-hand tractor", ''i.e.'' the propeller rotated clockwise when viewed from the rear. In addition to the mark numbers, Merlin engines were allocated experimental numbers by the [[Ministry of Supply]] (MoS) – e.g.: RM 8SM for the Merlin 61 and some variants – while under development; these numbers are noted where possible.<ref>Morgan and Shacklady 2000, p. 608.</ref> Merlin engines used in [[Supermarine Spitfire|Spitfires]], apart from the Merlin 61, used a propeller reduction ratio of .477:1. Merlins used in bombers and other fighters used a ratio of .42:1.<ref>Harvey-Bailey, 1995, p. 62.</ref>

''Data from Bridgman (Jane's)<ref>Bridgman 1998, pp. 281–283.</ref> unless otherwise noted:''
{{anchor |Merlin II}}
* '''Merlin II''' ''(RM 1S)''
: {{convert|1,030|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|5500|ft|m|abbr=on}} using + 6&nbsp;[[pounds per square inch|psi]] boost (41 kPa gauge; or an absolute pressure of 144 kPa or 1.41&nbsp;atm); used 100% glycol coolant. First production Merlin II delivered 10 August 1937.<ref name="MoSh610"/> Merlin II used in the [[Boulton Paul Defiant]], [[Hawker Hurricane|Hawker Hurricane Mk.I]], [[Supermarine Spitfire (early Merlin powered variants)#Mk I (Type 300)|Supermarine Spitfire Mk.I]] fighters, and [[Fairey Battle]] light bomber.<ref name="Bri281">Bridgman 1998, p. 281.</ref>
{{anchor |Merlin III}}
* '''Merlin III''' ''(RM 1S)''
: Merlin III fitted with "universal" propeller shaft able to mount either [[de Havilland Propellers|de Havilland]] or [[Dowty Rotol|Rotol]] propellers.<ref name="Robertson 1973, p. 144.">Robertson 1973, p. 144.</ref> From late 1939, using 100-octane fuel and +12&nbsp;psi boost (83 kPa gauge; or an absolute pressure of 184 kPa or 1.82&nbsp;atm), the Merlin III developed {{convert|1,310|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|9000|ft|m|abbr=on}};<ref name="Harvey-Bailey 1995, p. 155.">Harvey-Bailey 1995, p. 155.</ref> using 87-octane fuel the power ratings were the same as the Merlin II. Used in the Defiant, Hurricane Mk.I, Spitfire Mk.I fighters, and Battle light bomber.<ref name="Bri281"/> First production Merlin III delivered 1 July 1938.<ref name="MoSh610"/>
{{anchor |Merlin X}}
* '''Merlin X''' ''(RM 1SM)''
: {{convert|1,130|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|5250|ft|m|abbr=on}}; maximum boost pressure +10&nbsp;psi; this was the first production Merlin to use a two-speed supercharger; Used in [[Handley-Page Halifax|Halifax Mk.I]], [[Vickers Wellington|Wellington Mk.II]], and [[Armstrong-Whitworth Whitley|Whitley Mk.V]] bombers. First production Merlin X, 5 December 1938.<ref name="MoSh610"/>
{{anchor |Merlin XII}}
* '''Merlin XII''' ''(RM 3S)''
: {{convert|1,150|hp|kW|abbr=on}}; fitted with [[Coffman engine starter]]; first version to use 70/30% water/glycol coolant rather than 100% glycol. Reinforced construction, able to use constant boost pressure of up to +12&nbsp;psi using 100-octane fuel; Used in [[Supermarine Spitfire (early Merlin powered variants)#Mk II (Type 329)|Spitfire Mk.II]].<ref name="Robertson 1973, p. 144."/> First production Merlin XII, 2 September 1939.<ref name="MoSh610"/>
[[File:Rolls Royce Merlin XX.jpg|thumb|alt=A left side view of a gloss grey -painted aircraft piston engine on static display|Preserved Merlin XX at the [[Royal Air Force Museum London]]]]
{{anchor |Merlin XX}}
* '''Merlin XX''' ''(RM 3SM)''
: {{convert|1,480|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|6000|ft|m|abbr=on}}; two-speed supercharger; boost pressure of up to +14&nbsp;psi; Used in Hurricane Mk.II, [[Bristol Beaufighter|Beaufighter Mk.II]], Halifax Mk.II and [[Avro Lancaster|Lancaster Mk.I]] bombers, and in the [[Supermarine Spitfire (early Merlin powered variants)#Mk III (Type 330)|Spitfire Mk.III prototypes (''N3297'' & ''W3237'')]].<ref>Morgan and Shacklady 2000, p. 129.</ref> First production Merlin XX, 4 July 1940.<ref name="MoSh610"/>{{#tag:ref|In August 1940 drawings of the Merlin XX were sent to the [[Packard Motor Car Company]] and used as the basis for the [[Packard V-1650 Merlin|Packard Merlin 28]].<ref name="MoSh610"/>|group=nb}}
{{anchor |Merlin 32}}
* '''Merlin 32''' ''(RM 5M)''
: {{convert|1,645|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|2500|ft|m|abbr=on}}; a "low altitude" version of Merlin with cropped supercharger impellers for increased power at lower altitudes and a maximum boost pressure of +18&nbsp;psi; fitted with Coffman engine starter; used mainly in [[Fleet Air Arm]] aircraft, mainly the [[Fairey Barracuda|Fairey Barracuda Mk.II]] torpedo bomber and [[Supermarine Seafire|Supermarine Seafire F. Mk.IIc]] fighters. Also [[Hawker Hurricane variants#Hurricane Mk V|Hurricane Mk.V]] and [[Supermarine Spitfire (early Merlin powered variants)#PR Mk XIII (Type 367)|Spitfire P.R. Mk.XIII]].<ref name="Robertson 1973, p. 144."/> First production Merlin 32, 17 June 1942.<ref name="MoSh610"/>
{{anchor |Merlin 45}}
* '''Merlin 45''' ''(RM 5S)''
: {{convert|1,515|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|11000|ft|m|abbr=on}}; used in [[Supermarine Spitfire (early Merlin powered variants)#Mk V (Types 331, 349 & 352)|Spitfire Mk.V]], PR.Mk.IV and PR.Mk.VII, Seafire Ib and IIc. Maximum boost pressure of +16&nbsp;psi. First production Merlin 45, 13 January 1941.<ref name="MoSh610"/>
{{anchor |Merlin 47}}
* '''Merlin 47''' ''(RM 6S)''
: {{convert|1,415|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|14000|ft|m|abbr=on}}; high-altitude version used in [[Supermarine Spitfire (early Merlin powered variants)#Mk VI (Type 350)|Spitfire H.F.Mk.VI]]. Adapted with a [[Marshall Aerospace and Defence Group|Marshall]] compressor (often called a "blower") to pressurise the cockpit. First production Merlin 47, 2 December 1941.<ref name="MoSh610"/>
{{anchor |Merlin 50}}
* '''Merlin 50.M''' ''(RM 5S)''
: 1,585&nbsp;hp (1,182&nbsp;kW) at 3,000&nbsp;rpm at {{convert|3800|ft|m|abbr=on}}; low-altitude version with supercharger impeller "cropped" to {{convert|9.5|in|mm|0|abbr=on}} in diameter. Permitted boost was +18&nbsp;psi (125 kPa gauge; or an absolute pressure of 225 kPa or 2.2&nbsp;atm) instead of +16&nbsp;psi (110 kPa gauge; or an absolute pressure of 210 kPa or 2.08&nbsp;atm) on a normal Merlin 50 engine.<ref name="Robertson 1973, p. 145.">Robertson 1973, p. 145.</ref><ref>Price 1982, p. 145.</ref> Merlin 50 series was first to use the Bendix-Stromberg "negative-g" carburettor.<ref>Matusiak 2004, p. 10.</ref>
{{anchor |Merlin 61}}
* '''Merlin 61''' ''(RM 8SM)''
: {{convert|1,565|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|12250|ft|m|abbr=on}}, {{convert|1,390|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|23500|ft|m|0|abbr=on}}; fitted with a new two-speed two-stage supercharger providing increased power at medium to high altitudes; +15&nbsp;psi boost; used in [[Supermarine Spitfire (late Merlin powered variants)#Mk IX (Type 361)|Spitfire F Mk.IX]], and [[Supermarine Spitfire (late Merlin powered variants)#PR Mk XI (Type 374) and FR Mk XI|P.R Mk.XI]].<ref>Smith 1942, pp. 655–659.</ref> First British production variant to incorporate two-piece cylinder blocks designed by Rolls-Royce for the [[Packard Merlin]].<ref name="Smith 1942, p. 656">Smith 1942, p. 656.</ref> Reduction gear ratio .42:1, with gears for pressurisation pump.<ref>Harvey-Bailey 1995, pp. 62, 169.</ref> First production Merlin 61, 2 March 1942.<ref name="MoSh610"/>
{{anchor |Merlin 63|Merlin 63A}}
* '''Merlin 63 and 63A''' ''(RM 8SM)''
: {{convert|1,710|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|8500|ft|m|abbr=on}}, {{convert|1,505|hp|kW|abbr=on}} at 3,000&nbsp;rpm at {{convert|21000|ft|m|abbr=on}}; strengthened two-speed two-stage development of Merlin 61; +18&nbsp;psi boost; Reduction gear ratio .477:1; Merlin 63A did not have extra gears for pressurisation and incorporated a strengthened supercharger drive [[quill shaft]].<ref>Harvey-Bailey 1995, p. 170</ref> Used in [[Supermarine Spitfire (late Merlin powered variants)#Mk VIII (Type 360)|Spitfire F Mk.VIII]] and F. Mk. IX.<ref name="Robertson 1973, p. 145."/>
[[File:RR Merlin 66-266.ogg|thumb|Audio of RR Merlin 66/266 starting]]
{{anchor |Merlin 66}}
* '''Merlin 66''' ''(RM 10SM)''
: {{convert|1,720|hp|kW|abbr=on}} at {{convert|5790|ft|m|abbr=on}} using +18&nbsp;psi boost (124 kPa gauge; or an absolute pressure of 225 kPa or 2.2&nbsp;atm); low-altitude version of Merlin 63A. Fitted with a Bendix-Stromberg anti-g carburettor;<ref>Air Ministry 1943, p. 6.</ref> intercooler used a separate header tank.<ref>Harvey-Bailey 195, p. 172.</ref> Used in [[Supermarine Spitfire (late Merlin powered variants)#Mk VIII (Type 360)|Spitfire L.F. Mk.VIII]] and L.F. Mk.IX.<ref name="Robertson 1973, p. 145."/>
{{anchor |Merlin 76|Merlin 77}}
* '''Merlin 76/77''' ''(RM 11SM)''
: {{convert|1,233|hp|kW|abbr=on}} at {{convert|35000|ft|m|abbr=on}};<ref name="Lo219"/> Fitted with a two-speed, two-stage supercharger and a Bendix-Stromberg carburettor. Dedicated "high altitude" version used in the [[Westland Welkin]] high-altitude fighter and some later Spitfire and [[de Havilland Mosquito]] variants. The odd-numbered mark drove a [[Marshall supercharger|Marshall]] [[Roots-type supercharger|Roots-type]] blower for [[Cabin pressurization|cockpit pressurising]].
{{anchor |Merlin 130|Merlin 131}}
* '''Merlin 130/131''' ''(RM 14SM)''
: {{convert|2,060|hp|kW|abbr=on}}; redesigned "slimline" versions for the [[de Havilland Hornet]]. Engine design modified to decrease frontal area to a minimum and was the first Merlin series to use down-draught induction systems. Coolant pump moved from the bottom of the engine to the [[starboard]] side. Two-speed, two-stage supercharger and S.U. injection carburettor. Maximum boost was {{convert|25|psi|kPa|abbr=on}} gauge; or an absolute pressure of 270 kPa or 2.7&nbsp;atm). On the Hornet the Merlin 130 was fitted in the port [[nacelle]]: the Merlin 131, fitted in the starboard nacelle, was converted to a "reverse" or left-hand tractor engine using an additional idler gear in the [[Propeller speed reduction unit|reduction gear casing]].<ref>{{cite web|url=https://www.flightglobal.com/pdfarchive/view/1946/1946%20-%200165.html|title=Flight 1946, pp. 92–94.|website=flightglobal.com|access-date=22 June 2017|url-status=live|archive-url=https://web.archive.org/web/20170729061838/https://www.flightglobal.com/pdfarchive/view/1946/1946%20-%200165.html|archive-date=29 July 2017}}</ref>
{{anchor |Merlin 134|Merlin 135}}
* '''Merlin 134/135''' ''(RM 14SM)''
: {{convert|2,030|hp|kW|abbr=on}}; derated for use at low altitude 130/131 variants used in [[de Havilland Hornet|Sea Hornet]] F. Mk. 20, N.F. Mk. 21 and P.R. Mk. 22. Maximum boost was lowered to +18&nbsp;psi gauge (230&nbsp;kPa or 2.2&nbsp;atm absolute).  [[George Henry Corliss|Corliss]] throttle.<ref>Harvey-Bailey and Piggott, 1993, pp. 145, 177, 199.</ref> The Merlin 135 was left-hand tractor.
* '''Merlin 266''' ''(RM 10SM)''
: The prefix "2" indicates engines built by Packard, otherwise as Merlin 66, optimised for low-altitude operation. Fitted to the [[Supermarine Spitfire (late Merlin powered variants)#Mk XVI (Type 361)|Spitfire Mk.XVI]].<ref name="Robertson 1973, p. 145."/>
{{anchor |Merlin 620}}
* '''Merlin 620'''
: {{convert|1,175|hp|kW|abbr=on}} continuous cruising using 2,650&nbsp;rpm at +9&nbsp;psi boost (62 kPa gauge; or an absolute pressure of 165 kPa or 1.6&nbsp;atm); capable of emergency rating of {{convert|1,795|hp|kW|abbr=on}} at 3,000&nbsp;rpm using +20&nbsp;psi boost (138 kPa gauge; or an absolute pressure of 241 kPa or 2.4&nbsp;atm); civilian engine developed from Merlin 102; two-stage supercharger optimised for medium altitudes, and used an S.U. injection carburettor. "Universal Power Plant" (UPP) standardised annular radiator installation development of that used on Lancaster VI and [[Avro Lincoln]]. The Merlin 620–621 series was designed to operate in the severe climatic conditions encountered on Canadian and long-range North Atlantic air routes. Used in [[Avro Tudor]], [[Avro York]], and the [[Canadair North Star]].<ref>Flight July 1946, p. 99.</ref>

==Applications==
In chronological order, the first operational aircraft powered by the Merlin to enter service were the Fairey Battle, Hawker Hurricane, and Supermarine Spitfire.<ref>Lumsden 2003, p. 205.</ref> Although the engine is most closely associated with the Spitfire, the four-engined Avro Lancaster was the most numerous application, followed by the twin-engined de Havilland Mosquito.<ref>Lumsden 2003, pp. 208–209.</ref>

''List from Lumsden 2003''<ref>Lumsden 2003, pp. 203–215.</ref>{{#tag:ref|Lumsden covers British aircraft only, the Merlin may not be the main powerplant for these types; for example, one or two Hawker Harts and Horsleys were used to test early versions of the Merlin.|group=nb}}
{{Div col|colwidth=22em}}
* [[Armstrong Whitworth Whitley]]
* [[Avro Athena]]
* [[Avro Lancaster]]
* [[Avro Lancastrian]]
* [[Avro Lincoln]]
* [[Avro Manchester|Avro Manchester III]]
* [[Avro Tudor]]
* [[Avro York]]
* [[Boulton Paul Balliol|Boulton Paul Balliol and Sea Balliol]]
* [[Boulton Paul Defiant]]
* [[Bristol Beaufighter|Bristol Beaufighter II]]
* [[North American P-51 Mustang|CAC CA-18 Mark 23 Mustang]]
* [[Canadair North Star]]
* [[CASA 2.111|CASA 2.111B and D]]
* [[Cierva Air Horse]]
* [[de Havilland Mosquito]]
* [[de Havilland Hornet]]
* [[Fairey Barracuda]]
* [[Fairey Battle]]
* [[Fairey Fulmar]]
* [[Fairey P.4/34]]
* [[Fiat G.55#GY|Fiat G.59]]
* [[Handley Page Halifax]]
* [[Handley Page Halifax#H.Pp Halton|Handley Page Halton]]
* [[Hawker Hart]] (Test bed)
* [[Hawker Henley]]
* [[Hawker Horsley]] (Test bed)
* [[Hawker Hotspur]]
* [[Hawker Hurricane|Hawker Hurricane and Sea Hurricane]]
* [[Hispano Aviación HA-1112]]
* [[I.Ae. 30 Ñancú]]
* [[Miles M.20]]
* [[North American Mustang Mk X]]
* [[Renard R-36|Renard R.38]]
* [[Short Sturgeon]]
* [[Supermarine Type 322]]
* [[Supermarine Seafire]]
* [[Supermarine Spitfire]]
* [[Tsunami Racer]]
* [[Vickers Type 432|Vickers F.7/41]]
* [[Vickers Wellington|Vickers Wellington Mk II and Mk VI]]
* [[Vickers Windsor]]
* [[Westland Welkin]]
{{Div col end}}
<gallery widths="180px">
File:Avro Lancaster Mk 1 ExCC.jpg|alt=An inflight image of a four-engined bomber aircraft|[[Avro Lancaster]] B I powered by four Merlin XXs
File:De Havilland Hornet F1.jpg|alt=An inflight image of a twin-engined monoplane aircraft|The "slimline" Merlin 130/131 series were designed for the [[de Havilland Hornet]]
File:Vickers Type 432.jpg|alt=An inflight image of a twin-engined monoplane aircraft. The aircraft has a camouflaged paint scheme.|The Merlin 76-powered [[Vickers Type 432|Vickers F.7/41]]
</gallery>

===Postwar===
At the end of World War II, new versions of the Merlin (the 600- and 700-series) were designed and produced for use in commercial airliners such as the [[Avro Tudor]], [[military transport aircraft]] such as the [[Avro York]], and the [[Canadair North Star]] which performed in both roles. These engines were basically military specification with some minor changes to suit the different operating environment.<ref>Lumsden 2003, pp. 214–215.</ref>

A Spanish-built version of the [[Messerschmitt Bf 109]] G-2, the 1954 [[Hispano Aviación HA-1112|Hispano Aviación HA-1112-M1L ''Buchon'']], was built in [[Hispano Aviación|Hispano's]] factory in [[Seville]] with the Rolls-Royce Merlin 500/45 engine of {{convert|1600|hp}}&nbsp;– a fitting powerplant for the last-produced version of the famous Messerschmitt fighter, as the Bf 109 V1 prototype aircraft had been powered by the Rolls-Royce Kestrel V-12 engine in 1935.<ref>Lumsden 2003, p. 214.</ref>

The [[CASA 2.111]] was another Spanish-built version of a German aircraft, the [[Heinkel He 111]], that was adapted to use the Merlin after the supply of [[Junkers Jumo 211]]F-2 engines ran out at the end of the war.<ref>Wilson, Randy. [http://rwebs.net/dispatch/output.asp?ArticleID=17 It's a Heinkel: the Luftwaffe's workhorse Heinkel 111 bomber] {{webarchive|url=https://web.archive.org/web/20060928035717/http://rwebs.net/dispatch/output.asp?ArticleID=17 |date=28 September 2006 }} rwebs.net, ''The Dispatch''. Volume 12, Number 4, Winter 1996. Retrieved: 6 September 2009</ref> A similar situation existed with the [[Fiat G.55#GY|Fiat G.59]] when available stocks of the Italian licence-built version of the [[Daimler-Benz DB 605]] engine ran short.<ref>Green and Swanborough 1994, p. 211.</ref>

The Australian built [[Avro Lincoln]] from A73-51 used Australian built [[Commonwealth Aircraft Corporation]] Merlin 102s.
A total of 108 CAC Merlins were built by the time production ended.

===Alternative applications===
{{further|Rolls-Royce Merlin alternative uses}}

A non-supercharged version of the Merlin using a larger proportion of steel and iron components was produced for use in [[tank]]s. This engine, the [[Rolls-Royce Meteor]], in turn led to the smaller [[Rolls-Royce Meteorite]].<ref>Pugh 2000, p. 254.</ref> In 1943, further Meteor development was handed over to [[Rover Company|Rover]], in exchange for Rover's [[Rover Company#Second World War and gas turbines|gas turbine]] interests.<ref>Harvey-Bailey 1995, p. 83.</ref>

In 1938, Rolls-Royce started work on modifying some Merlins which were later to be used in British [[Motor Torpedo Boat|MTBs]], [[Motor Gun Boat|MGBs]], and RAF Air-Sea Rescue Launches. For these the superchargers were modified single-stage units and the engine was re-engineered for use in a marine environment. Some 70 engines were converted before priority was given to producing aero engines.<ref>Harvey-Bailey 1995, pp. 83–84.</ref>

Experiments were carried out by the [[Irish Army]] involving replacing the Bedford engine of a [[Churchill tank]] with a Rolls-Royce Merlin engine salvaged from an [[Irish Air Corps]] [[Supermarine Seafire|Seafire]] aircraft. The experiment was not a success, although the reasons are not recorded.<ref>Martin 2002, p. 58.</ref>

==Surviving engines==
One of the most successful of the [[World War II]] era aircraft engines, the Merlin continues to be used in many restored World War II vintage aircraft all over the world. The [[Royal Air Force]] [[Battle of Britain Memorial Flight]] is a notable current operator of the Merlin. In England the [[Shuttleworth Collection]] owns and operates a Merlin-powered Hawker Sea Hurricane IB and a Supermarine Spitfire VC&nbsp;– Both can be seen flying at home displays throughout the summer months.<ref>[https://www.shuttleworth.org/collection/hawkerseahurricane1b/ The Shuttleworth Collection – Hawker Sea Hurricane IB] {{Webarchive|url=https://web.archive.org/web/20190723153152/https://www.shuttleworth.org/collection/hawkerseahurricane1b/ |date=23 July 2019 }} www.shuttleworth.org. Retrieved: 23 July 2019</ref><ref>[https://www.shuttleworth.org/collection/spitfire/ The Shuttleworth Collection – Spitfire VC]  www.shuttleworth.org. Retrieved: 23 July 2019</ref>

==Engines on display==
<!--IMPORTANT: Before adding to this list, please ensure the Museum site has ''direct'' reference(s) to Merlin engines: exclude Packard Merlins and aircraft (see talk page).-->
[[File:Merlin24 XX.ogg|thumb|Merlin24 ground demonstration]] <!-- found this video on the Griffon page feel free to move it about-->
Preserved examples of the Rolls-Royce Merlin are on display at the following [[Aviation museum|museums]]:
* [[Atlantic Canada Aviation Museum]]<ref>[http://atlanticcanadaaviationmuseum.com/engines-weapons/rolls-royce-merlin-engine/ Photo and factfile.] {{webarchive|url=https://web.archive.org/web/20160312072107/http://atlanticcanadaaviationmuseum.com/engines-weapons/rolls-royce-merlin-engine/ |date=12 March 2016 }} Retrieved: 12 March 2016.</ref>
* [[Aviation Heritage Museum (Western Australia)]]<ref>[http://www.raafawa.org.au/museum/rolls-royce-merlin#!merlin_1_227 Merlin 60 or 70 series from Spitfire VIII.] {{webarchive|url=https://web.archive.org/web/20141029132109/http://www.raafawa.org.au/museum/rolls-royce-merlin |date=29 October 2014 }} Retrieved: 12 March 2016.</ref>
* [[City of Norwich Aviation Museum]] in [[Horsham St Faith]], Norfolk.<ref>{{cite web |title=Engines List |url=http://www.cnam.org.uk/engines-list |website=City of Norwich Aviation Museum |access-date=27 August 2023}}</ref>
* [[De Havilland Aircraft Museum]]<ref>{{cite web |title=Rolls Royce Merlin Mk 25 (cutaway) |url=https://www.dehavillandmuseum.co.uk/aircraft/rolls-royce-merlin-mk-25-cutaway/ |website=de Havilland Aircraft Museum }}</ref>
* [[Montrose Air Station Museum]]<ref>[http://www.rafmontrose.org.uk/displays.html Merlins from crashed bomber.] {{webarchive|url=https://web.archive.org/web/20140421050849/http://www.rafmontrose.org.uk/displays.html |date=21 April 2014 }} Retrieved: 12 March 2016.</ref>
* [[Norwegian Museum of Science and Technology]], Oslo, Norway<ref>[https://digitaltmuseum.no/011024213743/motor Norsk Teknisk Museum] Retrieved: 28 June 2024.</ref>
* [[Polish Aviation Museum|Polish Aviation Museum, Kraków (Cracow), Poland]]<ref>[http://www.muzeumlotnictwa.pl/zbiory_sz.php?ido=177&w=a Merlin XX.] {{webarchive|url=https://web.archive.org/web/20160312063133/http://www.muzeumlotnictwa.pl/zbiory_sz.php?ido=177&w=a |date=12 March 2016 }} Retrieved: 12 March 2016.</ref>
* [[Rolls-Royce Heritage Trust|Rolls-Royce Heritage Centre]], [[Derby]]<ref>[https://www.bbc.co.uk/derby/content/panoramas/rolls_royce_heritage_centre_03_360.shtml "Introducing the Merlin" via the BBC (requires plug-in that may not work on some browsers).] {{webarchive|url=https://web.archive.org/web/20160509210140/http://www.bbc.co.uk/derby/content/panoramas/rolls_royce_heritage_centre_03_360.shtml |date=9 May 2016 }} Retrieved: 12 March 2016.</ref>
* [[Royal Air Force Museum Cosford|Royal Air Force Museum, Cosford and London]]<ref>[http://www.rafmuseum.org.uk/research/collections/engine-rolls-royce-merlin-28-12-cylinder/ Merlin 28, Cosford] {{webarchive|url=https://web.archive.org/web/20160312062511/http://www.rafmuseum.org.uk/research/collections/engine-rolls-royce-merlin-28-12-cylinder/ |date=12 March 2016 }} Retrieved: 12 March 2016.</ref>
* [[Science Museum (London)]]<ref>[http://www.sciencemuseum.org.uk/ Science Museum website] {{webarchive|url=https://web.archive.org/web/20101029103255/http://www.sciencemuseum.org.uk/ |date=29 October 2010 }} Retrieved: 12 March 2016.</ref>
* [[Shuttleworth Collection]]<ref>[http://www.shuttleworth.org/ Shuttleworth website] {{webarchive|url=https://web.archive.org/web/20130806054209/http://www.shuttleworth.org/ |date=6 August 2013 }} Retrieved: 12 March 2016.</ref>
* [[Smithsonian Air and Space Museum]], Washington, DC<ref>[https://airandspace.si.edu/collection-objects/rolls-royce-merlin-rm-14sm-mk-100-v-12-engine Smithsonian National Air and Space Museum] {{webarchive|url=https://web.archive.org/web/20160706050442/https://airandspace.si.edu/collection-objects/rolls-royce-merlin-rm-14sm-mk-100-v-12-engine |date=6 July 2016 }} Retrieved: 3 March 2017.</ref>
* [[Wings Museum]], West Sussex, England<ref>[http://www.wingsmuseum.co.uk Wings Museum] {{webarchive|url=https://web.archive.org/web/20130313191143/http://www.wingsmuseum.co.uk/ |date=13 March 2013 }} Retrieved: 12 March 2016.</ref>

==Specifications (Merlin 61)==
[[File:RR Merlin labeled.jpg|thumb|alt=A front right view of a Vee twelve aircraft piston engine centred around a large propeller shaft has components labelled by black lines to each component description. Labelled components include the propeller reduction gearbox, exhaust ports, spark plugs and the coolant pump|Rolls-Royce Merlin with components labelled]]
{{pistonspecs
|<!-- If you do not understand how to use this template, please ask at [[Wikipedia talk:WikiProject Aircraft]] -->
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|ref=''Jane's''.<ref>Bridgman 1998, pp. 280–282.</ref>
|type=12-cylinder, supercharged, liquid-cooled, 60° "Vee", SOHC, piston aircraft engine.
|bore={{convert|5.4|in|mm|1|abbr=on}}
|stroke={{convert|6.0|in|mm|1|abbr=on}}
|displacement={{convert|1649|cid|L|1|abbr=on}}
|length={{convert|88.7|in|cm|1|abbr=on}}
|diameter=
|width={{convert|30.8|in|cm|1|abbr=on}}
|height={{convert|40|in|cm|1|abbr=on}}
|weight={{convert|1640|lb|kg|1|abbr=on}}{{#tag:ref|Plus 2.5% tolerance|group=nb}}
|valvetrain=[[Overhead camshaft]], two intake and two exhaust valves per cylinder, [[sodium]]-cooled [[Poppet valve#Internal combustion engine|exhaust valve stems]].
|supercharger=Two-speed, two-stage. Boost pressure automatically linked to the throttle, coolant-air [[intercooler|aftercooler]] between the second stage and the engine.
|turbocharger=
|fuelsystem=Twin-choke updraught Rolls-Royce/[[SU carburetor|S.U.]] [[carburetor|carburettor]] with automatic mixture control. Twin independent fuel pumps.
|fueltype= 100/130 [[Octane rating|Octane]] [[Avgas|petrol]].
|oilsystem=[[Dry sump]] with one pressure pump and two scavenge pumps.
|coolingsystem=70% water and 30% [[ethylene glycol]] coolant mixture, pressurised. Supercharger intercooler system entirely separate from main cooling system.<ref name="Smith 1942, p. 656"/>
|power=<br/>
::* {{convert|1,290|hp|kW|abbr=on}} at 3,000 rpm at take-off.
::* {{convert|1,565|hp|kW|abbr=on}} at 3,000 rpm at {{convert|12,250|ft|m|abbr=on}}, MS gear){{#tag:ref|MS and FS refer to the supercharger blower speeds: Moderate/Fully Supercharged. Moderate Supercharging referred to low- to medium-altitudes operation, Full Supercharging to medium- to high-altitude operation<ref name=Flightglobal>{{cite web|title=Development of the Aircraft Supercharger|url=http://www.flightglobal.com/pdfarchive/view/1943/1943%20-%202327.html|publisher=Flightglobal Archive|url-status=live|archive-url=https://web.archive.org/web/20141029090513/http://www.flightglobal.com/pdfarchive/view/1943/1943%20-%202327.html|archive-date=29 October 2014}}</ref>|group=nb}}
::* {{convert|1,580|hp|kW|abbr=on}} at 3,000 rpm at {{convert|23,500|ft|m|abbr=on}}, FS gear)
|specpower=0.96 hp/cu&nbsp;in (43.6 kW/L)
|compression=6:1
|fuelcon=Minimum 30 [[Imperial gallon|Imp&nbsp;gal]]/h (136 L/h), maximum 130 Imp gal/h (591 L/h){{#tag:ref|Ref: ''A.P. 1565 I, P & L: Pilot's Notes for Spitfire IX, XI and XVI'' fuel consumption dependent on throttle, mixture and boost settings, plus altitude.|group=nb}}
|specfuelcon=
|oilcon=
|power/weight=0.96 hp/lb (1.58 kW/kg) at maximum power.
|designer=
|reduction_gear=0.42:1
|general_other=
|components_other=
|performance_other=
}}

==See also==
{{Aircontent
|related=
* [[Packard V-1650 Merlin]]
* [[Rolls-Royce Griffon]]
* [[Rolls-Royce Meteor]] (tank engine developed from the Merlin)
* [[Rolls-Royce Meteorite]] (cut-down Meteor)
|similar engines=
* [[Allison V-1710]]
* [[Daimler-Benz DB 601]]
* [[Hispano-Suiza 12Y]]
* [[Junkers Jumo 213]]
* [[Klimov VK-107]]
* [[Mikulin AM-35]]
|lists=
* [[List of aircraft engines]]
* [[List of Rolls-Royce Merlin variants]]
|see also=
* [[Rolls-Royce aircraft piston engines]]
* [[Rolls-Royce Merlin alternative uses]]
}}

==References==
===Footnotes===
{{reflist|35em|group=nb}}

===Citations===
{{Reflist|30em}}

===Bibliography===
* Air Ministry. ''A.P 1509B/J.2-W Merlin II and III Aero Engines (June 1940)''. London: Air Ministry, 1940.
* Air Ministry. ''A.P 1565B Spitfire IIA and IIB Aeroplanes: Merlin XII Engine, Pilot's Notes (July 1940)''. London: Air Data Publications, 1972 (reprint). {{ISBN|0-85979-043-6}}.
* Air Ministry. ''Pilot's Notes for Spitfire Mark F.VII&nbsp;– Merlin 64 or 71 engine; Mark F.VIII&nbsp;– Merlin 63,66 or 70 engine. Air Publication 1565G & H -P.N.'' London, UK: Air Ministry, December 1943.
* Beckles, Gordon. ''Birth of a Spitfire: The Story of Beaverbook's Ministry and its First £10,000,000''. London: Collins Clear-Type Press, 1941.
* Berger, Monty and Street, Brian Jeffrey. ''Invasion Without Tears''. Toronto, Canada: Random House, 1994. {{ISBN|0-394-22277-6}}.
* Bridgman, L. ''Jane's Fighting Aircraft of World War II.'' London: Crescent, 1998. {{ISBN|0-517-67964-7}}
* Fozard, John W.''Sydney Camm and the Hurricane; Perspectives on the Master Fighter Designer and his Finest Achievement''. Shrewsbury, UK: Airlife, 1991. {{ISBN|1-85310-270-9}}.
* Green, William and Swanborough, Gordon. ''The Complete Book of Fighters''. New York: Smithmark Publishers, 1994. {{ISBN|0-8317-3939-8}}.
* [[Bill Gunston|Gunston, Bill]] ''World Encyclopedia of Aero Engines (5th Edition)''. Stroud, UK: Sutton Publishing, 2006. {{ISBN|0-7509-4479-X}}
* Harvey-Bailey, A. ''The Merlin in Perspective&nbsp;– The Combat Years (4th edition)'' Derby, England: Rolls-Royce Heritage Trust, 1995. {{ISBN|1-872922-06-6}}
* Harvey-Bailey, Alec and Piggott, Dave ''The Merlin 100 Series&nbsp;– The Ultimate Military Development'' Derby, England: Rolls-Royce Heritage Trust, 1993. {{ISBN|1-872922-04-X}}
* [[Stanley Hooker|Hooker, Stanley]] ''Not Much of an Engineer'' London: Airlife, 1984. {{ISBN|1-85310-285-7}}.
* Hoyland, Graham. ''Merlin: The Power Behind the Spitfire, Mosquito and Lancaster. The Story of the Engine That Won the Battle of Britain and WWII.''  {{ISBN|978-0008359263}}
* King, H. F. [http://www.flightglobal.com/pdfarchive/view/1954/1954%20-%201286.html "The Two R's: A Commemorative History of Rolls-Royce Aero Engines. (article and images).]" ''[[Flight International|Flight]]'' No. 2363, Volume 65, 7 May 1954.
* Lloyd, Ian and Pugh, Peter. ''Hives & the Merlin.'' Cambridge, England: Icon Books, 2004. {{ISBN|1-84046-644-8}}
* Lovesey, A C. "Development of the Rolls-Royce Merlin from 1939 to 1945." ''Aircraft Engineering and Aerospace Technology'', Volume 18, Issue 7. London, MCB UP Ltd., July 1946. ISSN 0002-2667.
* Lumsden, Alec. ''British Piston Engines and Their Aircraft''. Marlborough, Wiltshire: Airlife Publishing, 2003. {{ISBN|1-85310-294-6}}.
* Martin, Karl. ''Irish Army Vehicles, Transport and Armour since 1922''. 2002. {{ISBN|0-9543413-0-9}}.
* Mason, Francis K. ''Hawker Aircraft Since 1920 (3rd revised edition)''. London: Putnam, 1991. {{ISBN|0-85177-839-9}}.
* Matusiak, Wojtek. ''Supermarine Spitfire Mk V: Mushroom Model Magazine Special, No. 6111''. Redbourn, UK: Mushroom Model Publications, 2004. {{ISBN|83-917178-3-6}}
* McKinstry, Leo. ''Spitfire&nbsp;– Portrait of a Legend''. London: John Murray, 2007. {{ISBN|0-7195-6874-9}}.
* Morgan, Eric B. and Edward Shacklady. ''Spitfire: The History''. London: Key Publishing, 2000. {{ISBN|0-946219-48-6}}.
* Nicholls, Robert. ''Trafford Park: the First Hundred Years''. Phillimore & Co. Ltd., 1996. {{ISBN|1-86077-013-4}}.
* Nijboer, Donald. ''No 126 Wing RCAF: Aviation Elite Units 35.'' Botley, UK: Osprey Publishing Limited, 2010. {{ISBN|978-1-84603-483-1}}
* Payton-Smith, D. J. ''Oil: A Study of War-time Policy and Administration.'' London: [[HMSO|Her Majesty's Stationery Office]], 1971.
* Price, Alfred. ''The Spitfire Story''. London: Jane's Publishing Company, 1982. {{ISBN|0-86720-624-1}}.
* Pugh, Peter. ''The Magic of a Name&nbsp;– The Rolls-Royce Story&nbsp;– The First 40 Years''. Cambridge, England. Icon Books, 2000. {{ISBN|1-84046-151-9}}.
* Robertson, Bruce. ''Spitfire: The Story of a Famous Fighter''. Hemel Hempstead, Hertfordshire, UK: Model & Allied Publications, 1960. Third revised edition 1973. {{ISBN|0-900435-11-9}}.
* Robotham, William Arthur. ''Silver Ghosts and Silver Dawn''. London: Constable, 1970. {{ISBN|9780094566903}}
* [[Arthur Rubbra|Rubbra, A.A.]] ''Rolls-Royce Piston Aero Engines: A Designer Remembers.'' Derby, England: Rolls-Royce Heritage Trust, 1990. {{ISBN|1-872922-00-7}}.
* Simons, Graham M. ''Mosquito: The Original Multi-Role Combat Aircraft''. Barnsley, Yorkshire UK: Pen & Sword Books, 2011. {{ISBN|978-1-84884-426-1}}
* Smallwood, Hugh. ''Spitfire in Blue''. London: Osprey Aerospace, 1996. {{ISBN|1-85532-615-9}}.
* Smith, G. Geoffrey. [http://www.flightglobal.com/pdfarchive/view/1942/1942%20-%200449.html "A British Masterpiece.  (article and images).]" ''[[Flight International|Flight]]'' No. 1731, Volume XLI, 26 February 1942.
* Smith, G. Geoffrey. [http://www.flightglobal.com/pdfarchive/view/1942/1942%20-%202609.html "Rolls-Royce Merlin 'Sixty-One' (article and images).]" ''[[Flight International|Flight]]'' No. 1773, Volume XLII, 17 December 1942.
* Tanner, John. ''The Spitfire V Manual (AP1565E reprint)''. London: Arms and Armour Press, 1981. {{ISBN|0-85368-420-0}}.
* White, Graham. ''Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the United States During World War II''. Warrendale, Pennsylvania: SAE International, 1995. {{ISBN|1-56091-655-9}}
* Wilkinson, Paul H. ''Aircraft Engines of the World 1946 (3rd ed.)''. London: Sir Isaac Pitman and Sons, 1946.
* [http://www.flightglobal.com/pdfarchive/view/1938/1938%20-%203449.html "Some Trends in engine design (article and images).]" ''[[Flight International|Flight]]'' No. 1563, Volume XXXIV, 8 December 1938.
* [http://www.flightglobal.com/pdfarchive/view/1946/1946%20-%200164.html "Rolls-Royce Merlin 130 Series (article and images).]" ''[[Flight International|Flight]]'' No. 1935, Volume XLIX, 24 January 1946.
* [http://www.flightglobal.com/pdfarchive/view/1946/1946%20-%201455.html "Two New Power Units (article and images).]" ''[[Flight International|Flight and The Aircraft Engineer]]'' No. 1961, Volume L, 25 July 1946.

==Further reading==
* Gunston, Bill. ''Development of Piston Aero Engines''. Cambridge: Patrick Stephens, 2006. {{ISBN|0-7509-4478-1}}
* [[Alex Henshaw|Henshaw, Alex]]. ''Sigh for a Merlin: Testing the Spitfire''. London: Crecy, 1999 (2nd revised edition). {{ISBN|0-947554-83-1}}.
* Jackson, Robert. ''The Encyclopedia of Military Aircraft'' Bath, UK: Parragon Books, 2006. {{ISBN|1-4054-2465-6}}.
* Price, Alfred. ''Spitfire Mark I/II Aces 1939–41''. London: Osprey Aerospace, 1996. {{ISBN|1-85532-627-2}}.
* Quill, Jeffrey. "Spitfire: A Test Pilot's Story". London: John Murray, 1983; Crecy Publishing 1996 (2nd edition) {{ISBN|978-0-947554-72-9}}

==External links==
{{Commons category}}
* [https://www.bbc.co.uk/history/ww2peopleswar/stories/67/a4628667.shtml Merlin engines in Manchester – BBC]
* [https://www.youtube.com/watch?v=-fo7SmNuUU4 Post-War Rolls-Royce film on manufacturing the Merlin – YouTube]
* [http://www.spitfireperformance.com/merlin-comparison.jpg Merlin 60 series comparison drawings – Spitfireperformance.com]
* [http://www.spitfireart.com/merlin_engines.html Rolls-Royce Merlin 61 sectioned drawing]
* [http://www.flightglobal.com/pdfarchive/view/1937/1937%20-%202642.html "Vee-Twelve Par Excellence"] a 1937 ''Flight'' article on the Merlin I and II
* [http://www.flightglobal.com/pdfarchive/view/1942/1942%20-%200449.html "A British Masterpiece"] a 1942 ''Flight'' article on the Merlin XX
* [http://www.flightglobal.com/pdfarchive/view/1947/1947%20-%200229.html "Universal Power Plants" – 1947 ''Flight'' article on postwar Merlin installations for civilian aircraft]
* [http://news.bbc.co.uk/local/leicester/hi/people_and_places/history/newsid_9005000/9005829.stm Merlin engine photo gallery from BBC Radio Leicester]
* [http://www.flightglobal.com/airspace/media/aeroenginespistoncutaways/images/5686/rolls-royce-merlin-xx-supercharger-cutaway.jpg Sectioned image of possible turbocharger installation – ''Flight International'']
* [http://www.wwiiaircraftperformance.org/Aircraft_Engines_of_the_World_Rolls-Royce_Merlin.pdf The Rolls-Royce Merlin – ''Aircraft Engines of The World'']

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