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Early Merlins used 100 per cent ethylene glycol colling which allowed 135ºC coolant temperature, but with the advent of the Merlin XII a major move was made to pressure water cooling using 30 per cent gycol as anti-freeze. The advantages of water as a cooling medium are considerable. With 135ºC coolant temperature the mixture at 15 lb per square inch reduces the cylinder head metal temperature by some 30ºC when compared with 100 per cent glycol.
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Harvey-Bailey, A. H. 1995. The Merlin in Perspective - the combat years. 4th Edition. Derby: Rolls-Royce Heritage Trust.
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4. The switch to pressurised water cooling reduced the maximum coolant temperature from 135ºC to 120ºC.
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The first quote from The Merlin in Perspective doesn't state that the coolant temperature had to be reduced due to the switch to water/glycol. Is there any source for the reduction from 135° C to 120° C?
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6. Therefore my best guess is that Spitfire I and Hurricane I during the Battle should have the older 135ºC limit, whilst Spitfire II should have the 120ºC limit, and then some time in late 1940 the older aeroplanes should be converted in dribs and drabs during overhauls.
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My sources don't support this theory.
Operational Notes for Pilots on Merlin II and III.
2nd Edition January 1939
Quote:
Coolant Temperatures-
The engine, which employs ethylene glycol as the cooling medium, should not be opened up to full power until the coolant outlet temperature exceeds 60°C. The maximum permissible temperatures under normal flying conditions are:-
95°C. when cruising and 120°C. on climb, and under full throttle conditions in level flight.
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Air Publication 1590B Volumne I, Merlin II and III Aero-Engines
2nd Edition October 1939, Reprinted 1939
Quote:
Leading Particulars
Coolant
Type ... Treated ethylene glycol: Specification D.T.D.344
Outlet temperatures-
Minimum for take-off ... 60° C.
Maximum for cruising ... 95° C.
Maximum for climbing and level flight. ... 120° C.
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Operational Notes for Pilots on Merlin II, III and IV.
4th Edition, April 1940
Quote:
Coolant-
Ethylene Glycol to Specification D.T.D. 344.
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Coolant Temperatures-
The engine should not be opened up until the coolant outlet temperature exceeds 60°C.
The maximum permissible outlet temperatures under normal conditions of flight are-
Cruising ... 95°C.
Climb ... 120°C.
Full throttle level fllight [sic] ... 120°C.
Recommended cruising temperature ... 85-95°C.
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Amendment List No. 4 to Air Publication 1590B Volume I (2nd Edition, October, 1938 ) November 1940
Quote:
Leading Particulars (Merlin III and V)
Coolant
Type ... Treated ethylene glycol: Specification D.T.D.344
Outlet temperatures-
Minimum for opening up ... 60° C.
Maximum for cruising ... 95° C.
Maximum for climbing and level flight (5 minutes limit) ... 120° C.
Maximum for 1/2 hour emergency* ... 120° C.
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My conclusion is that at some date after November 1940 the coolant temperature was increased from 120°C. to 135°C.
Because it might be interesting, the Amendments to Air Publication 1590B Volume I states:
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7A. The Merlin V aero-engine is basically similar to the Merlin III aero-engine, with the exception that, while its airscrew shaft is of the same general type, it is not universal in its ability to take all forms of airscrew.
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