Date: Sun, 13 Dec 2009 13:23:58 -0700
Reply-To: Keith Hughes <keithahughes@Q.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@Q.COM>
Subject: Re: About coolant
In-Reply-To: <4B254514.2000703@gmail.com>
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Rocket J Squirrel wrote:
> Well, yeah it does because I started this thread to discuss on the
> generic level water v glycol+water as a coolant for high temps, but on
> another level it was to help me follow up to a discussion I had with
> my son, who drives a Jeep and off-roads in desert heat, in which he
> said that he runs water+surfactant in summer for increased cooling.
> (That he didn't flush the system before winter arrived and the coolant
> turned into slush is a sidebar -- or sideshow -- issue.)
>
> I wanted to get back to him on the relative cooling abilities of
> water/glycol+water and until the boiling point issue was raised I
> thought I (and maybe some Vanagon owners, considering the number of
> them who chimed in recommending water in summer) had it figured out.
> So I dunno whether I have an answer for my son at this time, but
> that's not what this list is about so I'll have let that go.
Well, the real question, though, is not whether straight water is a
better heat transfer medium than water/glycol. That's simple; yes it is
much better. For that single parameter. *Functionally*, however, you
have to evaluate the overall effect based on how its used.
Basically, straight water will cool better until you reach boiling, then
you have a catastrophic cascading failure; water gets ejected into the
overflow, decreasing system pressure and liquid volume, allowing more
boiling and decreasing cooling efficiency even more, creating more water
ejection, in a self reinforcing cycle. Using a water/glycol mix will
likely make the engine run a little hotter, but from the engine's
perspective, there's little difference between 210°F and 220°F as far as
what the cylinder and head temperatures get to. Whereas, once you get
pump cavitation, although the temp gauge may say 215°F, the actual
cylinder and head temps will be much higher than with water/glycol at
220°F. Once the temperature/pressure curve reaches the point where gas
drops out of solution (i.e. boiling point) the heat capacity of the
system (and thus cooling ability) drops like a rock. So, yes water
cools much better, until the point where it quits cooling much at all
and you melt down.
Also, you might point out to him that water *with a surfactant* is not
pure water either. Anything you put in it will change the freezing and
boiling points, and will also lower the cooling capacity as well. IME,
the WBX is much more in need of an increased boiling point in the desert
in summer than it is in need of a little better heat transfer medium.
The Jeep? Who knows, depends on how good the cooling system is. But if
its good enough that he doesn't need boilover protection, then he's
certainly unlikely to need the extra cooling capacity of straight water
either.
Keith Hughes
'86 Westy Tiico (Marvin)
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