George (
down under)
George;
My system has two
pressure caps. The filler neck on the left in the photo connects to the
pump inlet side of the loop (lowest pressure point). That neck has a 23 #
cap; and when the system is full and purged, there is never any air at that
cap. Overflow from the main filler is plumbed to the bottom of the
overflow bottle. The overflow bottle normally is maybe half to
two-thirds air as
required to accommodate the expansion from cold to hot. It has a 15#
cap.
The idea is to
always have positive pressure on the inlet side of the pump to inhibit
cavitation. Without a pressure cap on the overflow bottle, expansion of
the coolant in the engine pressurizes the system as it heats up. But with no air
in the system, after the peak is reached, the pressure goes back to zero (or
slightly negative to draw coolant back from the overflow bottle) any time the
temperature drops a bit; as when you reduce power after
climb-out.
There are no caps on
the radiators (2) as they are mounted at lower levels. There are air bleed
lines from the highest point of the tank on each rad that go back to the filler
neck; which is always at the lowest pressure in the loop so the air will go
there.
With both caps fully
latched, it is possible for the pressure to peak at about 38 psi (23 plus 15),
at sea level, if the overflow bottle is full (coolant fully expanded).
This give more margin (higher boiling temp) and less likely cavitation at
extreme conditions. Generally there is always residual air in the bottle that is
compressing, so the pressure doesn’t reach peak. Caps are ‘differential
pressure’ so at higher altitude the absolute pressure in the system is
lower.
A similar, and
simpler approach is to have an expansion tank that has air in it connected
directly to the system, say as a filler tank with a pressure cap, that is only
half full when cold. Pressure builds as things heat up, up to the max cap
pressure. The difference in my approach is that the pressure builds very
quickly to the system cap pressure because the coolant is incompressible (not
counting some expansion of hose connectors). So even if the engine is not
fully warmed up and I give it full throttle on takeoff, the system is at least
23 psi at the pump inlet. Same is true any time later when power (and RPM)
is increased.
Hope this all makes
sense,
Al