Crazy idea? Unvented expansion tank

[fastar1]

So I've been looking at alternative locations for my expansion tank so I can lower my seat, and the minimum size tank I need. In the process I started wondering about making the tank itself expandable, instead of venting it to open air.

The way it would work is from cold, I would connect the expansion hose to a flexible container in the collapsed position, with no air in it. The actual size of the 'bag' would be about 10% of my coolant capacity, which I figured to be about 2x the amount of expansion I will actually need between room temperature and 210 deg F. This 'bag' will then be placed within a solid, vented enclosure to protect it and to prevent high pressures from stressing the membrane if it gets that full.

Has this been done before? Why can't it work?

I figure it will be more compact and should never need topping up.

 

[caboose56]

So I've been looking at alternative locations for my expansion tank so I can lower my seat, and the minimum size tank I need. In the process I started wondering about making the tank itself expandable, instead of venting it to open air.

The way it would work is from cold, I would connect the expansion hose to a flexible container in the collapsed position, with no air in it. The actual size of the 'bag' would be about 10% of my coolant capacity, which I figured to be about 2x the amount of expansion I will actually need between room temperature and 210 deg F. This 'bag' will then be placed within a solid, vented enclosure to protect it and to prevent high pressures from stressing the membrane if it gets that full.

Has this been done before? Why can't it work?

I figure it will be more compact and should never need topping up.

The end result is that it still ends up being whatever size the enclosure is, correct?

I also don't think that being vented to atmosphere would increase coolant loss. Even on my race bike that uses water in the cooling system the vent in my overflow is only a 1/16" hole. I would check my water level every weekend and it never needed topping up. A street bike using a water/coolant mix would have even less evaporation.

In theory sure it would work but it seems like you're fixing a problem that doesn't exist.

 

[fastar1]

The end result is that it still ends up being whatever size the enclosure is, correct?

I also don't think that being vented to atmosphere would increase coolant loss. Even on my race bike that uses water in the cooling system the vent in my overflow is only a 1/16" hole. I would check my water level every weekend and it never needed topping up. A street bike using a water/coolant mix would have even less evaporation.

Yeah you're right, it wouldn't be much smaller really. The real advantage is that it can be almost any shape, and placed in any orientation.

In theory sure it would work but it seems like you're fixing a problem that doesn't exist.

That's what I was thinking, and it would be fun to try just for it's own sake. I figure worst case, the expansion reservoirs leaks and then I have to top up the rad every time the engine cools down on the way home.

 

[NP6000]

You could also run a higher pressure rad cap - that in turn will reduce the volume of vented coolant to the expansion tank. I usually run 1.6 Bar without any trouble , but at the same time the rubber hoses are upgraded to silicon hose, bevelled + rolled hose clamps, ported water pump, ducted rad, etc.

 

[caboose56]

Yeah you're right, it wouldn't be much smaller really. The real advantage is that it can be almost any shape, and placed in any orientation.

That's what I was thinking, and it would be fun to try just for it's own sake. I figure worst case, the expansion reservoirs leaks and then I have to top up the rad every time the engine cools down on the way home.

That's another hurdle. Generally things that are flexible aren't durable.

My overflow res on my new race bike is only 250mL and i have it mounted under the ram air duct. If you haven't seen it already there's a good pic of it in my Superbike build thread.

 

[EngineerJoe]

The actual size of the 'bag' would be about 10% of my coolant capacity, which I figured to be about 2x the amount of expansion I will actually need between room temperature and 210 deg F. .

Your numbers are not far off. A 50/50 mix or water / ethylene-glycol expands at ~6% between 0F & 200F. 10% will be a little less than double the expansion. Make sure the coolant connection point on the bag is always oriented downwards and make sure the bag's working pressure is much higher than the rad cap rated pressure.

Edit: I'm not sure how well this system will work if you get air in the system (say, after a cooling system flush)

 

[fastar1]

Your numbers are not far off. A 50/50 mix or water / ethylene-glycol expands at ~6% between 0F & 200F. 10% will be a little less than double the expansion. Make sure the coolant connection point on the bag is always oriented downwards and make sure the bag's working pressure is much higher than the rad cap rated pressure.

Wouldn't that just be atmospheric pressure since the Expansion bag will be on the 'low' side of the pressure cap?

Edit: I'm not sure how well this system will work if you get air in the system (say, after a cooling system flush)

Yeah this is a concern to me too. I'll need to be very finicky about bleeding all the air out, but even then can't air pockets form on their own over time since it's H2O? I don't really have a good understanding of how that works so it worries me. But as long as there is no air, I should be able to orient the bag opening in any direction, no?

 

[RockerGuy]

So I've been looking at alternative locations for my expansion tank so I can lower my seat, and the minimum size tank I need. In the process I started wondering about making the tank itself expandable, instead of venting it to open air.

The way it would work is from cold, I would connect the expansion hose to a flexible container in the collapsed position, with no air in it. The actual size of the 'bag' would be about 10% of my coolant capacity, which I figured to be about 2x the amount of expansion I will actually need between room temperature and 210 deg F. This 'bag' will then be placed within a solid, vented enclosure to protect it and to prevent high pressures from stressing the membrane if it gets that full.

Has this been done before? Why can't it work?

I figure it will be more compact and should never need topping up.

Have u ever thought about replacing the reservoir with a 250ml nalgene bottle and connect it with a hose of whatever length? This way u can put the reservoir where-ever u like, even below the rad.

You could also run a higher pressure rad cap - that in turn will reduce the volume of vented coolant to the expansion tank. I usually run 1.6 Bar without any trouble , but at the same time the rubber hoses are upgraded to silicon hose, bevelled + rolled hose clamps, ported water pump, ducted rad, etc.

I would be wary of this. U might want to know what your bursting pressure for your rad is, lol.

In theory this is a good idea, more heat will be taken out of the engine at a higher pressure, plus u will automatically increase the boiling point of the mixture

 

[EngineerJoe]

Wouldn't that just be atmospheric pressure since the Expansion bag will be on the 'low' side of the pressure cap?

I'm not sure how much pressure drop you get across the rad cap, but a fully open rad cap will expose the bladder (overflow bag) to coolant pressure (I think). Don't forget that the hot coolant may reduce the burst pressure of the bladder. It'll be at atmospheric pressure once the system has cooled.

Yeah this is a concern to me too. I'll need to be very finicky about bleeding all the air out, but even then can't air pockets form on their own over time since it's H2O? I don't really have a good understanding of how that works so it worries me. But as long as there is no air, I should be able to orient the bag opening in any direction, no?

You'll almost always trap some air in a cooling system on initial fill and the only way to deaerate (purge) the system is to run the engine hot enough to force the thermostat to open. The continuous flow of coolant will eventually result in the air making its way to the highest point in the system (likely the rad cap). Once the engine "burps" the air to the top of the rad, it will be forced into the expansion bottle once the coolant heats up and expands.

As long as there is no air in the system, yes you should be able to orient the bag opening in any direction. If the bladder is higher than the rad and engine, it's best to enter the bladder from the bottom. If you enter the bladder from the top with this setup, your rad-to-bladder line will be higher than the bladder, and it could trap air.

I don't know how to explain the part about "pockets of air forming on their own". I've seen vapour bubbles form at operating temperatures of a cooling system that wasn't pressurized (operating w/ out rad cap), but I don't think you'd see that in a pressurized system. You could also cavitate a WP inlet (due to the low pressure at that point), but that typically happens when you lose coolant pressure and you have air in the system. I think your biggest problem will be (as you already recognize) getting the air out of the system.

 

[fastar1]

Cool cool, thanks.

 

[Brian P]

VW/Audi cars use a pressurized expansion tank that is connected to the cooling system circuit with no "radiator cap" (or any valve whatsoever) between the cooling water loop and the tank - just a hose. It still has a pressure relief valve built into the cap for the expansion tank, but it doesn't look like a normal radiator cap, and under normal circumstances it never opens (it's only there in case something goes wrong, to relieve excess pressure). The tank is always located so that it is the highest point in the system (so that any bubbles vent into the tank) and it is spherical in shape because of the pressure inside it. Doing it this way avoids gradual evaporation of the water in the expansion tank, and it allows the use of coolant additives that don't like being exposed to air on a routine basis (because air doesn't go in and out of the expansion tank every time the engine heats and cools).

 

[RockerGuy]

VW/Audi cars use a pressurized expansion tank that is connected to the cooling system circuit with no "radiator cap" (or any valve whatsoever) between the cooling water loop and the tank - just a hose. It still has a pressure relief valve built into the cap for the expansion tank, but it doesn't look like a normal radiator cap, and under normal circumstances it never opens (it's only there in case something goes wrong, to relieve excess pressure). The tank is always located so that it is the highest point in the system (so that any bubbles vent into the tank) and it is spherical in shape because of the pressure inside it. Doing it this way avoids gradual evaporation of the water in the expansion tank, and it allows the use of coolant additives that don't like being exposed to air on a routine basis (because air doesn't go in and out of the expansion tank every time the engine heats and cools).

Add BMW to that list.
My coolant system is made of plastic and if not serviced at the right interval, the plastic reservoir bottle will pop after repeated heating/cooling and high pressure. I prefer a low pressure system thank u

 

[fastar1]

Add BMW to that list.
My coolant system is made of plastic and if not serviced at the right interval, the plastic reservoir bottle will pop after repeated heating/cooling and high pressure. I prefer a low pressure system thank u

No cheap plastic tank here. I'll be using a giant size party balloon from the dollar store as a bladder. What could go wrong? LOL!

 

[RockerGuy]

No cheap plastic tank here. I'll be using a giant size party balloon from the dollar store as a bladder. What could go wrong? LOL!

I see a potential problem here.
Most latex-like rubber, especially a balloon will not withstand the heating/cooling cycles, let alone a bit of UV light from the sun during summer.

Put that balloon in your engine bay of your car for a few weeks and get back to us on that.

 

[fastar1]

It'll be fiiiiiiiiine.

 

[RockerGuy]

It'll be fiiiiiiiiine.

Please update in due time. We want to know if this works or not

 

[Brian P]

Add BMW to that list.
My coolant system is made of plastic and if not serviced at the right interval, the plastic reservoir bottle will pop after repeated heating/cooling and high pressure. I prefer a low pressure system thank u

Hasn't been an issue on the VW/Audi systems. Only problem is when some idiot doesn't pay attention to the big statement in the owners manual and the big sign on the coolant bottle itself, and ends up mixing regular green coolant with the VW G11/G12 stuff ... this results in brown sludge that clogs the system up. VW G11/G12 coolant is extremely good when used in the sealed system as designed and NOT mixed with green coolant that has an incompatible additive package. RTFM and all goes well; don't read the manual and then it all goes wrong. How many times have we seen that before ...

 

[toysareforboys]

Add BMW to that list.

Add Chevy Cobalt 2.0L Supercharged to that list as well. Now, if I could just figure out where all the coolant goes out of the "overflow" tank! It's swallowed 4L in the last two months!!?

Just had the oil changed, looked OK. Never smelt or seen any smoke out the exhaust, can't find it leaking anywhere in the engine bay, on the ground, etc.

-Jamie M.

 

[RockerGuy]

Add Chevy Cobalt 2.0L Supercharged to that list as well. Now, if I could just figure out where all the coolant goes out of the "overflow" tank! It's swallowed 4L in the last two months!!?

Just had the oil changed, looked OK. Never smelt or seen any smoke out the exhaust, can't find it leaking anywhere in the engine bay, on the ground, etc.

-Jamie M.

I have a feeling its leaked from the cap. It will open at a certain pressure. I would not overfill it above the recommended mark, especially if its plastic.

I try to put negative pressure in mines by opening the system (GENTLY) when hot, bleed the pressure out and then seal it again. So when its cool, it kinda in a semi-vacuum

 

[toysareforboys]

I try to put negative pressure in mines by opening the system (GENTLY) when hot, bleed the pressure out and then seal it again. So when its cool, it kinda in a semi-vacuum

Sounds like a plan! I'll give that a try My "overflow" tank is plastic. I overfilled it slightly last time (about 20% over the "full" line). I think the cap has some type of pressure safety thing that won't let you unscrew it when there is pressure in the tank, it just "skips" when you try and undo it. I'll try harder

-Jamie M.