Merlin Coolant Temperature Limits

[gecko]

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

 

[Deleted member 68059]

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

Raising of temperature and water pressure was a continual progression for everyone to lower radiator sizes.

I wrote a book about it which goes into it in more detail.
8504 - THE SECRET HORSEPOWER RACE - WESTERN FRONT FIGHTER ENGINE DEVELOPMENT | Mortons Books

Raising temperature limits will require a large number of possibly complex changes to be made to each engine, the largest of which is the coolant pressure, but this
extends of course to the entire coolant system, the oil, the seals and even the engine structre/materials used in certain cases.

Since the late 30`s it was seen as highly desirable to raise the limit as much as possible, so if the manual gives a limit, there is a good reason for it. If you exceed it
the most likely outsome (in general terms, not just Merlin`s) is that you`ll boil the coolant. Its impossible to say "if you go 3 degrees over for 4minuites the engine will fail" but a total engine seizure/major mechanical failure is an entirely likely result from running almost any engine for long with coolant which is boiling.

 

[MIflyer]

From Herschel Smith's book, "A History of Aircraft Piston Engines" relative to the Curtiss Conqueror V-1570:

"Unfortunately the Army went overboard, demanding a coolant temperature of 300F (149C). At this heat the V-1570 turned from a reliable engine to a temperamental one, liable to burn valves at any time without warning."

and

"If we compare Curtiss-Wright with Rolls-Royce, we find that Curtiss was less technically authoritative and less committed to liquid cooling. Rolls could inform the RAF that the Kestrel, and later, the Merlin were to be run at 260F (127C) and the RAF would accept this as binding, but Curtiss's word did not carry anything like the same authority with the technical people at Wright Field. Rolls built only the Kestrel and Merlin; Curtiss would just as soon sell Cyclones as Conquerors."

As a result of the US Air Corps self-imposed mistake of pushing the V-1570's coolant temp too high, Curtiss, at one time the world leader in liquid cooled aircraft engine design with the D-12, abandoned that line of development.

 

[pbehn]

The coolant used was changed from glycol to a glycol-water mix sometime around 1939-40, dunno if that affected things one way or another.

 

[MIflyer]

Glycol by itself has a higher freezing temperature than a glycol-water mix.

 

[pbehn]

Glycol by itself has a higher freezing temperature than a glycol-water mix.

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 Fluid

 

[gecko]

Thanks for all the responses. So it would seem that Rolls Royce's limitations are regarded with more respect than others, though I'm still left with wondering why they increased the limits on the Merlin XII to 135 C, if 120 C was indeed a limit to be disregarded to one's peril, as well as what is going on with the dual set of limitations in the Spitfire VI pilots notes, where the higher limits seem to be only grudgingly allowed. Are we in the danger zone over 120 C or not?

And here are the relevant excerpts for the Merlin XII, taken from a 1940 edition of the Spitfire Mk.II notes, followed by those for the same engine and aircraft from a 1942 revision:

 

[pbehn]

It is a combat concession, when the pilot himself is in danger.

 

[Simon Thomas]

From "The Merlin in Perspective", Alec Harvey-Bailey, RRHT, 4th Ed, p61

 

[gecko]

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.

It is a combat concession, when the pilot himself is in danger.

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!

 

[pbehn]

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!

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.

 

[gecko]

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?

 

[Deleted member 68059]

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?

The limits basically remained pretty much static after the implementation of 70/30 water/glycol under high pressure.

For the XII, it looks like 120 is ok for up to half an hour, and 135 for 5minuites. Its very tricky working out "damage"
from these, as 120 is max climb, and climb is the worst possible situation for cooling (very high power, but very low speed),
so its a bit situationally dependant. It looks like 115 was pretty much always max continuous in the combat zone (
what this means is a bit "wooly", but is probably just the manufacturer trying to discourage pilots from abusing the
engine too much and not really a serious real limit).

If I were you I would speak to people who actually fly the things, and if you cant do that, if it were me I`d just
say to myself that anything above 115 for more than a half hour reduces the total service life of the engine
but probably doesnt cause a failure in flight, and any temps above 135 for 5minuites does begin to cause
boiling, which will probably significantly reduce the lifespan of the engine and eventually cause the coolant
to begin to boil out of the header tank pressure relief valve, which will eventually lead to a seizure.

 

[gecko]

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!

 

[pbehn]

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!

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.

 

[gecko]

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.

 

[pbehn]

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.

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"

 

[rinkol]

From "The Merlin in Perspective", Alec Harvey-Bailey, RRHT, 4th Ed, p61
View attachment 615542

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.

I recall that Sam Heron wrote that water would be more respected as a coolant if it cost $15 per gallon.

 

[MIflyer]

Glycol by itself freezes at a higher temperature than water by itself.

An item I recall from reference work I thought I knew but it seems I do not. I think it was on one of the early P-51 missions that one of Blakeslee's got hit in the radiator. They were well into occupied Europe and the pilot decided he had no choice but to bail out. But Blakeslee told the pilot not to bail out but to throttle back and start pumping the primer and let the fuel cool the engine. The pilot pumped the primer all the way back to Great Britain, by which time his glove was worn through and his hand worn down to the bone.

I wodnered why this was not more common. But then I looked in the P-51B manual and found it had a manuel primer while the later models had an electric one, whch probably did not even work when the engine was running.

 

[Deleted member 68059]

Glycol by itself freezes at a higher temperature than water by itself.