Under Pressure...

[guandi]

I work on engines, lots of them, from cars to trucks, Mini's to MAN's. and one thing they *all* have in common, is a pressurised cooling system.

i wont go into the basics of the internal combustion engine, but i know for a fact that they run better when they are hot (especially diesels) so from a cooling point of view its not practical to keep them cold, only just cool enough to stop them melting down or blowing up. for the size of some engines i have seen and worked on like a 32litre V12 quad turbo with a dual clutch, quadruple split gearbox and a top speed of 120mph pulling 200 tons... and a 10litre straight six, run of the mill 17tonner, they have very similar sized radiators.

sure, a BlackIceMini would work on a stock speed mid range XP, but dual overclocked 3200's? not gonna happen. this is where i started to think about other things that are noticibly different, like for instance.. the harder the engine works, the faster the pump runs, the faster air is blown through the rad by sheer forward pressure of the vehicle and the fan on front of the engine, the fact that the header tank, or reservoir, can take a very large amount of pressure, so can the hoses. some of them are rated at 200 PSI and up. (DAF Leyland 60's have TWO THOUSAND PSI rated hoses to and from the heatercore .. :shrug: )

Then i started to think on the molecular level, about densities and such. and my first few College level science lessons, and about how i had to draw the molecules of water, more densly packed the colder it was, untill it became ice, or less dense untill it became steam.

when the water in the engine gets to its max temperature, if it wasnt for the fact it was all sealed in, it would evaporate the whole lot in only a minute or two... so why doesnt it turn to steam in the pipes.... pressure!

the more pressure you put on the water, the closer the molecules become... (could you make ice this way... ) and the more heat the water is able to contain untill it gets to the rad and is cooled again... or so it seems.

can anyone clear this up, i have asked alot of experienced people, and they either dont know or have very conflicting views.

ta

[UberBlue]

Since water is nearly incompressible, It would take an ass ton (thousands of tons) of pressure to squeeze the water molecules closer together to realize even a thoretical increase in performance. The only reason to pressurize a loop is to keep the molecules from spreading any farther apart (boiling).

[Cathar]

I think all your questions can be answered by reading this interesting document:

http://www.lsbu.ac.uk/water/anmlies.html

[pdf27]

the more pressure you put on the water, the closer the molecules become... (could you make ice this way... ) and the more heat the water is able to contain untill it gets to the rad and is cooled again... or so it seems.
That's pretty much it. Boiling point at 1 Bar is 100 C, 2 Bar 120 C, 20 Bar 212 C. and above 221.2 Bar (3210 PSI) there is no such thing as liquid/gas phase water. Since you pressurise the system, it can run hotter. Since the efficiency of a radiator is proportional to the difference between the hot and cold sides, running at higher pressure will let you run hotter and so use a smaller radiator.
The other point that is so obvious it is easily missed is that hotter fluid carries more energy per unit mass/volume. This means that running hotter not only reduces the radiator area needed, but it also allows the coolant to carry more energy per unit volume - reducing the size of the cooling system needed.

[guandi]

thanks guys, very informative

Cather : link added to faves