Odd cooling problem

[treed2 at u.washington.edu (T. Reed)]

> > 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