expansion tank cap - Pressure

[Fisher, Scott]

> From: Steve Kramer [mailto:skramer at mac.com]
> Sent: Friday, May 24, 2002 9:01 AM>
>
> All this talk about the coolant system is really timely with my v8.
> Replacing the expansion tank helped.   I have also replaced the upper
> radiator hoses. I also bled the system yesterday after I got
> some errant temp warnings - even though the temp was at the 100 hash
> mark.  Now I hear about the cap.  I bought one becuase I was hearing
> some hissing coming from the cap.

Don't overlook the little hose (if there is one...) from the radiator to the
expansion tank.  I'm not familiar with the V8, but on the '83 CGT there's a
small hose leading from the top of the radiator to the bottom of the
expansion tank.

> Everything is doing fine.    My question is this - I'm not a
> gearhead - why do you need the coolant under pressure ?  The
> system is moving the fluid through the radiator via the pump.
> Does the pressure keep the coolant temp down ?

No, it keeps the boiling point up.  Boiling (and this isn't a gearhead
thing, it's a lab-coat-guy thing :-) is not only temperature-dependent, it's
also pressure-dependent -- look at a cake mix and notice that they give a
different cooking time for high-altitude residents.

When a liquid (well, ANY substance, but it's easy to imagine this with water
because we've all had a hot beverage and seen steam rising from the surface)
absorbs heat, the molecules in the liquid start moving around.  When they
move around enough, they want to break out of the liquid state and move into
the gaseous state.  That's the steam rising from your morning cup --
molecules right at the surface break away into the atmosphere because
there's still enough heat in the liquid to overcome atmospheric pressure
right at the surface, at least.

Boiling take this process one step further.  The gas around the liquid (for
your coffee cup, this would be the air you're breathing) applies its own
pressure to the whole system; this pressure resists the efforts of these
high-energy molecules in their attempts to change from liquid to gas.  The
more pressure you apply to the gas, the more energy the liquid has to absorb
before it can change to a gaseous state.  The difference between a steaming
cup of coffee and a boiling pot of water is that the boiling pot has
absorbed enough heat that the molecules break out of the liquid state *in
the middle of the liquid*, not just at the surface -- the heat they absorb
gives them enough energy to overcome the pressure of the system.

It follows, then, that if you increase the pressure of the system, you also
increase the amount of heat that the liquid can absorb before it overcomes
that pressure and starts boiling.  (Conversely, if you lower the pressure,
you lower the required heat, which is why water boils at tempreatures lower
than 212 F/100 C at high altitudes, which is why Duncan Hines gives you
different instructions for baking in Colorado than in Death Valley.)

How this translates to your car: pressurizing the cooling system allows the
coolant to absorb MORE heat than if it were open to the atmosphere, which
makes the cooling system more effective.  Additionally, manipulating the
operating pressure gives the engineers one more way to manage the operating
temperature of the engine, and an engine's temperature affects its power,
economy, and emissions.

Two additional effects may interest you: first, the expansion tank is there
in part because when any fluid heats up, it expands, which increases the
pressure, which increases the boiling point, which allows it to heat up
more, which makes it expand more, which increases the pressure, which -- you
get the idea.  And second, because this closed-loop operation of temperature
and pressure in your cooling system makes the pressure rise accelerate, the
cap on your expansion tank is really a safety valve designed to "blow" at a
certain pressure to protect the other components of your cooling system or
engine.  If the pressure gets too high, you REALLY REALLY want a $5 cap to
blow before, say, a gasket deep inside your engine.

Next week: making a volcano out of dry yeast and hydrogen peroxide... :-)

--Scott "graduate of the Dan Coffey school of physics" Fisher
  Tualatin, Oregon