flyrotary@lancaironline.net Mailing List Archive

Steve, nothing wrong with connecting both hoses to bottom of expansion tank as you indicate; however, it’s the connection points to the engine block that are important here. If the other end of one hose is connected to a low-point in the system the expansion tank will back-fill with coolant - that is in the static condition: engine not running, pressure cap on expansion tank removed and system not pressurized. One connection should remove steam from block; the other connection goes to top of water pump not the bottom. That is how my system is connected and I think yours is too. System pressurizes in relationship to coolant temperature; I use a 16 lb cap on my expansion tank (not lowest point either) typical cruising pressure will be 8-12 psi.

Jeff

From:	Steven W. Boese <flyrotary@lancaironline.net>
Subject:	Re: [FlyRotary] Re: swirl / expansion tank configuration
Date:	Thu, 29 Jan 2015 16:53:13 +0000
To:	Rotary motors in aircraft <flyrotary@lancaironline.net>

Charlie and Jeff,

If I am seeing things correctly, Charlie's connections to the expansion tank are both near the bottom of the tank due to the dip tube attached at 10 o-clock. This is essentially how I have my expansion tank connected. I installed a Schrader valve at the top of the thermostat housing which is the highest point in the cooling system. My entire expansion tank is below the level of the thermostat housing. When filling the system, I add coolant to the expansion tank and pressurize it through the overflow port in the filler neck while opening the Schrader valve. When coolant is expelled from the Schrader valve, the system is purged of air. A filler cap at the highest point is not needed.

The expansion tank does not back-fill with coolant unless the filler cap is released and also the Schrader valve is opened. Both conditions are necessary for the expansion tank to back-fill.

Having both the inlet and outlet lines connected below the surface of the expansion tank is a good thing in my opinion. They just need to be separated by enough distance that any bubbles can separate upward rather than be drawn into the outlet line. I keep the inlet line to the expansion tank restricted for two reasons. One is that coolant circulating through the expansion tank is bypassing the radiator and not contributing much to cooling. The other is to limit the pressure seen in the expansion tank. With a thermostat in the system that is not fully opened, I see pressures in the thermostat housing on the water pump side of the thermostat of up to 40 psi at high RPM.

With my system, I need at least a quart of air space in the top of the expansion tank when cold. If the air volume is less than this, the expansion of the coolant will fill the entire system with liquid when hot. If there is residual air space in the expansion tank, the pressure in it will vary continuously with operating temperature. If the system becomes completely filled with liquid, the pressure is constant at the relief pressure of the filler cap.

That is how my system behaves, for what it is worth.

Steve Boese

RV6A, 1986 13B NA, RD1A, EC2

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