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