Editing Rolls-Royce Merlin (section)

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

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

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

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