Odd cooling problem

[MK1Scirocco16v at comcast.net (Dave Ewing)]

Your right on here, Toby, except for one small calculation.  If the cap is
not working correctly the system will overheat faster due to the lack of
pressure.  If the system is under the right amount of pressure it boils at a
higher temperature since it doesn't have access to the atmosphere.  This is
why some vehicles' optimal coolant temp is around 230 F.

Your are right about the motor still being hot and there is actually a term
or phrase for this but I can't think of it right now.  Something like Heat
Absorption or something.

Dave
----- Original Message ----- 
From: "T. Reed" <treed2@u.washington.edu>
To: "Mark" <mardak@cogeco.ca>
Cc: "Calimus" <calimus@techography.net>; <Captnbr@aol.com>;
<Scirocco-L@scirocco.org>; "L F" <rocco16v@netzero.net>
Sent: Thursday, September 11, 2003 2:23 AM
Subject: RE: Odd cooling problem


> > > Once the engine is off, the source of heat is gone and
> > > temperatures start to drop, not rise.
> > > Think about it.
> >
> > Larry, you're DEAD wrong!  And I'm putting my money where my mouth is.
Try
> > this for yourself.  Hook up a coolant system pressure tester but don't
pump
> > it up. Run the car until it's hot enough that the rad fan comes on -
shut
> > the car off, and watch the gauge...  There's ABSOLUTELY NO WAY TO
DISPUTE
> > THIS, the pressure goes up, every time...  Some of the other posts
explained
> > exactly why this happens...
> >
> > Mark.
> > 80 S
> > 81 S ABA/JH/4K
>
> Yeah, I'm with Mark here.
>
> Cooling is a function of the speed with which coolant flows through
> the radiator. It goes in the radiator hot and comes out "cold". There is
> of course a limit to this but a correctly designed cooling system should
> be operating within the limit.
>
> The engine block gets very hot (hundreds of degrees) and has a lot of
> heat-conductive mass since it's made of metal.
>
> Now, in order for the engine to maintain a steady temperature you have to
> move heat away from it as fast as the chemical reactions inside it
> (combustion) are creating heat.
>
> The amount of heat adsorbed by a given segment of coolant depends on the
> temperature of the engine and the time the segment spends inside the
> coolant passages of the block. When the engine gets very hot, the
> thermostat opens completely (rather than just part way) and the flow
> increases, and when the rpms are high the flow increases because the water
> pump spins faster. But even if the coolant is moving slow all of the time
> it will still cool effectively (but only "slow" to a point.. it will
> eventually reach it's capacity and boil) because it is moving slowly
> through the radiator as well, giving it additional time to dissipate heat.
>
> Let's say the heat energy contained in the block is H + h, where h
> is the amount of energy added per second of combustion taking place. The
> cooling system must remove 'h' per second to maintain a constant
> temperature. When you shut off the engine, it stops removing heat
> completely. and even though the 'h' is no longer being contributed by new
> combustion, the base heat H is still there (the engine is still hot) so in
> several seconds the now motionless coolant "maxes out" and boils/expands.
>
> Anyway, the point of all this is that when you shut off the engine, the
> block may not be adding any heat but the temperature of the coolant is
> still rising. That's why if you're close to overheating and you shut the
> engine off you will frequently hear the coolant reservoir cap blow a
> second or two later, on its own.
>
> If you've ever parked and turned off your Scirocco and heard the fan turn
> on as you were walking away.. that's another example of what I'm talking
> about.
>
> -Toby
>
>
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