Now that you are flying, how is
your series connection of coolant bottles with pressure caps working? I have
constructed something similiar, and there was some discussion that claimed
that it wouldn't work. I would be interested in what you found
out.
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