Jeff,
Your sketch of my system is correct. When the engine is not running, the pressure in the two lines to the expansion tank are both referenced to the level of the coolant in the expansion tank. This means there will be no flow. If the expansion tank were
to fill up, where would the coolant come from? It can't come from the block since it is sealed. I have not observed any change in the coolant level in the expansion tank when the filler cap has been left off for several days. I don't think weeks or months
would make any difference. Even when the filler cap is attached, the pressure in the expansion tank is slightly less than atmospheric due to the coolant recovery action of the relief valve acting like a check valve. There is no pressure in the expansion
tank to stop backfilling when the system is cold and I have never seen backfilling occur.
You are correct that the level in the expansion tank will rise when the Schrader valve is opened and air is allowed to enter the block allowing coolant to flow out and downhill.
Filling the system from bone dry has not been an issue. I just fill as much as I can into the expansion tank with the Schrader valve open (it's easiest to just take the valve core out for this). The coolant fills the block to the level of the coolant
in the expansion tank. Coolant flows into the block through the line connected to the expansion tank bottom. Then the Schrader valve is sealed and the expansion tank pressurized. Air is then bled out of the Schrader valve. The volume of air that is bled
out is usually less than the capacity of the expansion tank so the tank normally does not have to be refilled and repressurized, although that can be done if necessary. There normally is some air in the hoses to the radiators that is not purged by this method.
Running the engine up to a temperature such that the thermostat opens results in this air being transferred to the expansion tank. This transfer hasn't caused the expansion tank to go dry, for me at least. Running the engine up to temperature once has been
enough to purge the air from the system.
I have also flown the plane with the filler cap loose. This caused no ill effect such as coolant loss or overheating. The pressure reading of the expansion tank just stayed at zero. I wish I could claim that this was intentionally done as a data gathering
experiment. That was not the case, however.
Steve
From: Rotary motors in aircraft <flyrotary@lancaironline.net> on behalf of Jeff Whaley <flyrotary@lancaironline.net>
Sent: Thursday, January 29, 2015 2:37 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: swirl / expansion tank configuration
Hi Steve, I’ve attached a sketch of our two systems as I understand them. Your system is equivalent to a gas can turned upside down with a very small outlet hose and the air cap sealed tight – no, or at least very little flow out the bottom.
I’m pretty sure if you de-pressurize your system, remove the expansion tank cap and come back 2-3 weeks later, the level in the expansion tank will have increased. You won’t see any change during a quick top-off in a couple minutes. If you open your Schrader
valve it will overflow in a few seconds. I understand your systems’ equilibrium once it is topped-up and you are in maintenance-mode – but how do you fill your engine when bone-dry? I assume you close/pinch the lower hose and fill the block from the top as
complete as possible; at this point you can seal the Schrader valve, add coolant to expansion tank, open lower hose, pressurize it and force it into bottom of engine while slowly opening Schrader valve.
Jeff