Editing Rolls-Royce Merlin (section)

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

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

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

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

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

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