Quote:
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.
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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).