Editing Rolls-Royce Merlin (section)

====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"/>