British FM killing the fun of the game for allied pilots.

[Kurfürst]

Originally Posted by NZtyphoon

the lower operating temperature of the engine on this mix - also used by the Merlin XII - helped TBO and reliability. In theory the CLOD Merlin XII should not be overheating as much as it is right now. I'm not sure about the temperature difference of 70°C as quoted so I'll dig a little more.

Pure Glycol has about twice the boiling point of water but only about half the heat capacity (plus its an anti-freeze). So in effect the coolant (and engine!) temperatures are going to be much higher (since the coolant agent can carry away about half the heat), but I reckon it would boil much later. Hence why it is used in a mix with water. It would also explain why the Hurri I and Spit I overheats so fast.

Engine temperatures are a function of coolant capacity, coolant type, coolant circulation capacity and heat transfer capacity of the radiators. The amount of pressurization the cooling system also raises the boiling point.

What was the coolant capacity of the Spit and Hurri Mark Is, how many gallons/liters? Are there any cooling trials available for these aircraft?

[swift]

Originally Posted by Kurfürst

Pure Glycol has about twice the boiling point of water but only about half the heat capacity (plus its an anti-freeze). So in effect the coolant (and engine!) temperatures are going to be much higher (since the coolant agent can carry away about half the heat), but I reckon it would boil much later. Hence why it is used in a mix with water. It would also explain why the Hurri I and Spit I overheats so fast.

Engine temperatures are a function of coolant capacity, coolant type, coolant circulation capacity and heat transfer capacity of the radiators. The amount of pressurization the cooling system also raises the boiling point.

What was the coolant capacity of the Spit and Hurri Mark Is, how many gallons/liters? Are there any cooling trials available for these aircraft?

how should something that was added to increase reliability increase the risk of overheating which would strongly reduce reliability? For me this is contradictory.

[Kurfürst]

Originally Posted by swift

how should something that was added to increase reliability increase the risk of overheating which would strongly reduce reliability? For me this is contradictory.

You seem to miss a detail - the temperature of the coolant agent is rather irrelevant, it can be of very high temperature, or even near/at/above boiling point as long as the temperature of the engine components are within permissable temperature limits. The two are not mutually exclusive. Yhe coolant may boil pretty quickly, be at very high temperature, what matters is the interaction between the coolant components and the coolant, of which the coolant's temperature is an indirect indication to the pilot about the engine component's temperature.

So say with a pure glycol coolant 140 degrees celsius may indicate that the engine compontents are about (iirc) 400 celsius, while when using pure water 90 degrees may indicate the very same, since water has a much higher heat transfer capacity (its more effective at carrying away heat).

I suppose they changed the coolant agent type to water-glycol mixture on later Merlins because they realised glycol alone simply cannot transfer heat fast enough, and by adding 70% water this increased greatly. This was usually the practice anyway, an 50-50 or 70/30 mix was generally used by everyone. The DB 601A had used 47% water, 50% glycol and 3% mixture of lubrication oil and water (1:2), which may explain why the DB powered planes do not overheat so easily compared the the all-glycol cooled Spitfire Mark I / Hurricane Mark I.

Coolant circulation may be also of important - the DB 601A circulated coolant for example at 65 000 liter / hour rate.

Glycol permitted higher coolant temperatures without the mix boiling away, and was an anti freeze too for higher altitudes (where temperatures can be easily at the -40 degrees celsius range).

[NZtyphoon]

Originally Posted by swift

how should something that was added to increase reliability increase the risk of overheating which would strongly reduce reliability? For me this is contradictory.

The biggest problem with using pure ethylene glycol as a coolant is that it is less effective at conducting heat than water so, as it circulates around the engine, it is less effective at keeping engine components cool. While the high strength alloys and other metals developed by the likes of Rolls-Royce for use in aero engines were able to withstand higher temperatures than earlier metals, components such as gaskets and oil seals were more likely to fail - early Merlin engines were renowned for their oil leaks. Head gasket failure in the middle of combat ops is no fun.

A second problem is that in its pure form Ethylene glycol is flammable, thus it was a hazard to use 100% glycol in combat aircraft. The Merlin II and III series had a normal coolant temperature of 90°, and 120°C emergency maximum - close to the flash point of glycol http://www.npi.gov.au/substances/eth...col/index.html Mixing glycol with water increases the flash point while preserving the anti-freeze, anti-corrosive properties of glycol.

Thus the Merlin XII and XX series (30% Glycol, 70% water) were able to run at higher temperatures 105° normal to 125° for climb, 1 hr maximum and 135° +12 lbs boost 5 minutes while the vulnerable gaskets and oil seals were less prone to failure.

I don't know why the British adopted 100% glycol, although I remember reading about it somewhere.

[JtD]

Originally Posted by Kurfürst

What was the coolant capacity of the Spit and Hurri Mark Is, how many gallons/liters? Are there any cooling trials available for these aircraft?

Hurricane ~15 gallons, was tested in a climb with 2600rpm/6.25lb boost, result is that in English summer conditions (25°C at sea level) it can do this without overheating (oil & glycol).