Fellow
Rotarians:
I've
come to the point where I need to set up the expansion / overflow
system.
My
engine guy is saying I can just have an overflow reservoir since the
pressure
cap is at the top of the engine. I've read that some 13B flyers (if
not
all) have expansion chambers which, in turn, have their own overflow
reservoirs.
My
understanding of a reservoir is a bottle that holds excess coolant which
is
expelled from the pressure cap. The coolant is then sucked back in as the
fluid
cools. An expansion chamber is a pressurized bottle, usually made of
aluminum.
I beleive it should be 1.5 qts or more.
Could
someone please correct my understanding where wrong, discuss the pros
and
cons of these systems for our purposes, and point me in the right
direction
for obtaining / constructing the parts I need?
Regards,
John
Slade
John;
You may recall we had
a discussion of this issue a few months back. My view is that there are
3-basic options discussed below. I’d suggest that in all cases, the
pressure cap be on the intake side of the pump; the lowest pressure point in
the system, otherwise you will have negative pressures at the pump inlet.
That is negative relative to the outside pressure, which at altitude is already
low. My assumption is that there is some risk of pump cavitation at high
rpm and negative pressure.
1 – Pressure cap
on system with overflow bottle. Coolant expands as the temp goes up.
Pressure increases and coolant is expelled to the overflow bottle. As the
temp goes down pressure goes slightly negative, drawing coolant back in. Disadvantage
is that there are may flight regimes where to will have ambient pressure, or
slightly less, in the system. Any time you reduce power from a prior
higher power the pressure is likely to go negative. Common approach on
older vehicles.
2 – Pressure cap
(maybe 20-25#) on expansion tank, with overflow bottle. The expansion
tank is sized such that it contains sufficient air volume over the coolant to accommodate
the expansion of the coolant, probably on the order of 1 quart of air volume (13B).
In this case, the pressure will fluctuate with power level, with the peaks at
cap pressure, but won’t go to zero until engine is relatively cool.
The overflow bottle is optional, and generally not used on vehicles that use
this setup. Might be good to have for the cases where you get into some
nucleate boiling expansion which may push some coolant out that would recover.
3 – High pressure
cap (20-25#) on the system; with overflow bottle that also has pressure cap of
lower pressure (14-16#). Overflow bottle of at least a quart. This
provides the most assurance of positive pressure on the system under all
conditions, with immediate high pressure response when you put on the power
(and rpm).
I don’t really
know if option 3 provides a necessary benefit over option 2, but I went that
way on the basis that it seemed very little more difficult, and may be better. The
attached pic shows my filler tube on the left which has a 25# cap and connects
to the return line to the pump (downstream for the radiators). It also
has the air bleed lines from my two radiators. The pressurized overflow
bottle on the left is a converted fire extinguisher (got it free from
extinguisher service place), into which I added a low level warning switch. The
coolant pressure sender is at the outlet from the pump, with an airbleed on
top.
Hope this helps,
Al
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