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I have been looking at the operating limitations for various early Spitfire variants, referencing the Air Ministry Pilot's Notes. I am curious about the increase in maximum coolant temperature, rising from 120 C to 135 C.
Merlin II/III is consistently 120 C on every aircraft I have Pilot notes for that used it (Battle, Hurricane, Spitfire).
Merlin XII has it as 120 C in the June 1940 edition of the Spitfire Mk.II notes, but 135 C in a 1942 edition.
Merlin 45/46 and 50 series has it at 135 C in a Feb 1944 edition of the Mk.V notes.
Merlin 47 shows it as 120 C with a combat concession in brackets of 135 C in a Jan 1942 edition of the Spitfire Mk.VI notes.
So my question is, what caused the increase to 135 C as the max coolant temperature? Was it simply a concession to the realities of the demands of combat as suggested by the Spitfire Mk.VI notes, or was there a modification (sometime in 1941 assumedly) that made the Merlin able to handle higher temperatures? The late edition Spitfire Mk.II notes may suggest this, as they allow for 135 C, but make no mention of it being a combat concession, and indeed by that point the Mk.IIs were all in O.T.U.s, so if 135 C was only granted out of combat needs, one would think the Mk.IIs at O.T.U.s would be held to the original 120 C. Would the Merlin 45 have initially been only cleared for 120 C, since all I have are much later limitations?
I have been under the impression that the Merlin I/II/III series really didn't handle higher coolant temps well at all, so I assume 120 C is a hard limit for that engine, though I could be wrong. But could a Merlin XII (or possibly 45 as well) handle 135 C just fine from the beginning even though only cleared for 120 C initially? Or was 120 C really the danger limit at first until a modification was introduced that allowed it to be raised?
For context purposes, I'm working on a simulation of early Spitfire variants and need to know where the risk of damage begins on the different Merlin variants regardless of what published limitations are. Thanks for any help.
Dan
I know, depending on the mix, the viscosity, latent heat and other stuff changes. This affects the performance of a cooling system but I don't know exactly how, it is a science in itself. Ethylene Glycol Heat-Transfer FluidGlycol by itself has a higher freezing temperature than a glycol-water mix.
It is a combat concession, when the pilot himself is in danger.
As per my post #6 and Simons post #9 its complicated. All I know is how little I know apart from basic principles. I worked in an engineering office in south England and the guys next to me were all HVAC (heating ventilation and air conditioning) specialists. Heat transfer build-up and conduction is very complicated. You would think the coolant temperature was directly indicative of the engine temperature, which it is, but when you change the coolant, or many other things, then it is different. As per the post from Simon with Glycol Water mix, a coolant temperature of 135C was already safer than just with Glycol in terms of temperature. a 30C difference on the external temperature is a lot (based on what I heard from the guys at work). It may be that the higher temperature increased wear, but if you go much over such limits you can get into a cycle that makes things go south very quickly, I would say it would be a good idea for the pilot to consider ALL options.So this would suggest that a Merlin XII or later running with coolant at 135 C may have lower CHTs than a Merlin III running at 120 C. Seems to bode well for reliability up to 135 C for these later engines, though I'm sure at a cost in TBO.
As that timeless piece of aviation wisdom goes - the limitations only apply if you plan on flying the aircraft again; if subsequent flights do not appear likely, there are no limitations!
And all I know is that I'm in over my head in this particular area! I appreciate the info. Though obviously there are no hard and fast "x number of seconds at y temp and it breaks" rules in real life but I'll have to make some of one sort or other for the simulation to be able to determine when and when not to cause damage to the engine. What I am currently thinking is to slightly increase the chance of damage (above the existing chance of a random failure) when the coolant temp is above 120 C. Above 135 C the chance will go up significantly and the coolant temp will tend to climb rapidly once the coolant boils. Does this seem reasonable?
Water temperature is only an indicator, in addition to changes to the Merlin in the Spitfire and others there were also changes to cooling capacity in both water/glycol and oil. An engineer who knows this "stuff" can tell you that when the water or oil temperature is at 120 or 135C what the temperature of the exhaust valve area and combustion chamber is. Things happen like valves stretching and breaking or oil breaking down and con rods being thrown. Things are not always as they seem at first sight, if you have massive cooling capacity it is a great situation to be in, but if that massive cooling capacity still doesnt keep you below 120C then you are going to fly past 135C and beyond very quickly. I burned holes in pistons on my GT 185 Suzuki three times, despite everything being correct in carburation and timing , it happens in seconds. When I raced motorcycles I ran them on aviation fuel (Avgas), not for more power but for some security, a friend of mine had a seizure on the Isle of Man, he was in hospital there for months and never the same person again.That all seems to make sense to me, and I definitely agree about engine life being reduced over time, and hopefully I can find a way to model that too.
I think I have to disagree about the limits remaining static - at least the published limits anyways:
June 1940, Merlin XII - 120 C
Jan. 1942, Merlin 47 - 120 C with combat concession of 135 C
July 1942 and on, all 70/30 variants - 135 C.
Perhaps this is more a reflection of the powers that be deciding to accept shorter engine life as the supply of spare engines became more plentiful, rather than the threshold for an in-flight failure risk changing due to design modifications.
I have had a couple opportunities to talk with actual warbird pilots, though I get the impression that they do their very best to avoid gaining any firsthand experience in the matter of what happens when you run too hot!
Detonation something that kills an engine. It is easy to forget that all the heat is produced in the combustion chamber and is then spread about by various means, a temperature "in the green" of 120C or 135C is indicative of combustion chamber and exhaust valve temperatures where the heat is actually produced. When the Merlin was changed from a single to two piece block there were differences in the water flow. Basically, what I am saying is "its complicated"Some of that sounds more like detonation than simple overheating (not that those things are mutually exclusive). Yes, of course there is a wide range of temperatures for different parts of the engine for any given coolant temp, but lacking that level of intimate knowledge of the engine, one would have to assume that under most circumstances, so long as coolant temps remain in the green, the temperatures of the individual parts remain at levels they can withstand. Eventually of course nothing lasts forever and if you make a habit of abusing it, things can come to a rather abrupt halt.
Water has a higher heat capacity (better heat transfer) than glycol, but glycol has a higher boiling point. The use of a mix of glycol and water under pressure (this further raises the boiling point) is a good overall solution, though it does require some care in the design of the engine cooling system to operate under pressure.From "The Merlin in Perspective", Alec Harvey-Bailey, RRHT, 4th Ed, p61
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Glycol by itself freezes at a higher temperature than water by itself.