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Mercedes, Volvo and others use a single expansion reservoirs with pressure
cap that are filled about half way when cold. As the engine heats up the
air is compressed - expands again as things cool down. With no compressed
air, the cooling system pressure goes to zero (or very slightly negative)
any time you reduce the power from a higher level, even at high rpm.
I run a 23# cap on the filer neck (on left in photo, connected to the pump
inlet side, no air under it), and a 15# cap on the overflow bottle (on right
with cap off) - which only has a couple inches of coolant in it when cold.
Al
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"With no compressed air, the cooling system pressure goes to zero
(or very slightly negative) any time you reduce the power from a higher
level, even at high rpm."
Are we talking absolute pressure here or zero pressure relative to
atmospheric?
What value pressure cap does Mercedes and Volvo use? I suspect if the engine
get hot enough it could blow up the tank or the rad if no pressure relief
were used.
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Zero psig (gauge) meaning relative to atmospheric. I think they both use a
standard 15 psig pressure cap.
The coolant is incompressible, so as the engine heats up during high power,
say during climb out, the expanding coolant is expelled to the over flow
bottle after the cap pressure is reached. Now throttle back and the temp
drops just a few degrees, and the coolant contacts a bit the pressure goes
away because there is nothing 'expandable' to maintain it. Talk to Tracy
and others flying with coolant pressure readout, and they wil verify this
occurs. It is not necessarily an issue to be concerned about.
But that's why I (and Dave) have a pressurized overflow bottle that has air
in it. As the coolant is expelled into the overflow bottle, it builds
pressure (up to cap value), and that pressure is pushed back through the
primary cap by the expanding air to keep some pressure on the system when
the temp drops a bit.
With 2 pressure caps in series as I have, the max pressure is the sum of the
two. At high temp I will see coolant pressure of close to 40 psig, 23 plus
15.
------
Ken has a pressure gage on his cooling system that indicates pressure
relative to
atmospheric. As the engine warms up the pressure goes up and stays up
regardless of power or RPM I think.
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If it does it means there is air trapped in the system somewhere.
Al
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