closed vs. open cooling system

[JDanno]

I'm at the stage where I need to make a decision on whether to use a closed or open cooling system. By closed I mean where the overflow bottle has a pressure cap and open means a "loose" cap like most autos have now days. What are the pros and cons of either system? Thanks!! Dan

[TSwezey]

Ours is closed. I believe the boiling point is much higher under pressurized systems.

[Mike S]

If I understand the OP question, it is not whether to pressurize or not, it is whether to have the overflow bottle see the pressure, or just ambient.

Far as I know, most cars run the pressure cap on the radiator, and the overflow bottle is at ambient, with a cap that does not hold pressure.

My truck has a pressurized overflow.......

Personally, I would go for the pressure cap on the radiator, just a little less to fail under the heat/pressure of operation.

[JDanno]

I bought used a overflow tank that is pressurized but I had doubts about it. This is what I needed to know. Thanks guys. Dan

[DanH]

Originally Posted by TSwezey

Ours is closed. I believe the boiling point is much higher under pressurized systems.

In particular as you gain altitude

An unpressurized overflow tank can be located almost anywhere. The system pressure cap needs to be at the high point with the airplane at rest. Try to design with a small debubbler tank under the pressure cap if possible; it's the only place you want to trap air, which is then pushed out to the overflow tank with the next pressure rise.

A pressurized header needs both coolant and air volume, so it tends to be relatively large. The practical problem is how to locate it at the high point in the system, which can be hard to do inside the upper cowl.

Not direct to your question, but a tip; design a quick drain into your system at a low point.

[kgood]

My WAM diesel uses a VW spherical pressurized plastic tank. It looks exactly like the tank in my VW Jetta. It is mounted high in the cowling. It has two outlets, one 3/4" or 5/8" in the bottom, with a hose that tee's into the lower radiator hose, and one 5/16" with a hose leading to the high point in the engine case.

There is no overflow tank, or even a provision for one. The tank is has high-low markings to keep the tank about half-full. The rest is for air/expansion. I keep the level about half way between the two marks. After a flight and the engine is hot, the level is 1/2" or so over the full mark. I never lose any coolant.

The system works great. One very cool thing is that it has sensors in it, that are wired to my engine instrumentation / data logger that gives me a "low coolant" warning with a flashing red light (haven't had it come on yet, but it worked when the tank was empty).

Just letting you know what works for me, since this is what the AE forum is for!
Kurt

[Rotary10-RV]

Group,
Most automotive engines are now going to a pressurized expansion tank. These tanks are not a "overflow" tank in the convensional sense. They are designed to keep the heat exchangers full, which is the best for heat transfer. These tanks are also plumbed to allow the engine and radiator to bleed air into the tank. While I don't like the car that much, I saw a great example of the use of the pressurized tank on the Pontiac Solstice GTP. It is a small turbo engine and cooling is at a premium. There are bleeds on both ends of the engine and the high point of the radiator. There is no conventional unpressurized overflow tank. The advantage is that the tank is always "active" with the bleed lines constantly open. There is no need for a valve on the cap either. The amount of air in the tank is calculated to limit the pressure rise to under the safety release pressure on the cap.
As a reference the P-51 Merlin used very high water pressure. IIRC they used as much as 50 PSI. The high pressure prevented boiling at high altitude.
Bill Jepson

[SHIPCHIEF]

In engineering class they taught us that the jacket water system must exclude all air to prevent corrosion. Kgood, your WAM system has continuous agitation with air in the circulating tank. Maybe it's not a problem with proper addatives in the cooling water?
So I built my first cooling system with a circulating tank to vent the high points on the engine, and return from the bottom of the tank to the water pump suction. The expansion tank was conventional, and close beside the circulating tank. The idea being to have the continuous venting, plus the expansion tank to assure all air is removed from the system.
When redesign was required (cooling system didn't cool) I removed the circulating tank, it was too heavy. Now I take extra effort to purge the air from the system before starting the engine,(lift tail, add more water etc) and plan to have expansion push the trapped air out of the highest point; the thermostat housing. After a few start-stop cycles, the air should be all gone.
Disadvantage? not as much reserve cooling water in case of a small leak.
I'm just in early ground running, so I wouldn't say my system is proven yet, but so far-so-good. trying to keep the weight down is a continuing challenge. Still, it's cause for review...