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