Date: Tue, 10 May 2016 20:15:25 -0400
Reply-To: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dennis Haynes <d23haynes57@HOTMAIL.COM>
Subject: Re: Filling the coolant reservoir...
In-Reply-To: <CA+az7_4QRMY=hivQnB0FC5+NdHnE=D44GVES40i3vb9LbFXthg@mail.gmail.com>
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The thermodynamics of an automotive cooling system are actually somewhat complicated. There are a number of different designs but for the most part there 2 distinctly different methods of operating them when it comes to pressure control, fluid level and make up recovery. The difference is weather the system is completely filled (no air or head space in the pressurized system, or with the head space in the pressurized (under the pressure cap) system.
In the early days of pressurized cooling systems you filed the system and left some space at the top of the radiator. If overfilled or if something went wrong some coolant would be lost increasing the space available for the next time. In this type of a system the pressure in the system will be function of the vapor pressure of the hottest coolant mass in the system. The complicated part to understand is that you can have hot coolant in parts of the engine and the radiator can still be at ambient. The hot coolant in the engine and the antifreeze mix will determine the vapor pressure. In the system the pressure cap acts as relief valve.
In systems such as the Vanagon there should no head space in the pressurized part of the cooling system. Thus not only does the pressure cap deal with the vapor pressure but the expansion of the coolant medium itself. So here the cap is actually a pressure "control" device.
As for how much volume change the system can have you need to consider going from a fully cold system, engine and maybe heater cores hot, and then the entire system hot. All of this continuously changes while driving. So you can go from almost 5 gallons of coolant at some winter temperature all the way to all of being at 220F. With the pressure control cap always keeping the pressure at some 12-14psi when everything cools down the system will actually go into a vacuum. When caps don’t work properly and let air or coolant back in the symptoms include collapsed hoses and even air getting pulled in through small leaks. Back in the old days it was common to fit hoses with inner springs to keep them from collapsing, especially for the water pump inlet. Oh so that brings up another point. As much as the system will operate at some overall pressure there are pressure differences throughout the system. No pressure difference, no flow. So there is more than one reason to keep a system under pressure. Besides the obvious to prevent boiling you want to make sure the pump inlet never goes into a negative pressure mode. Ten you get aeration and cavitation and make steam flashes. This is why the low coolant system is set up the way it is. When that light goes on do not rely on the gauge to determine that it is OK to keep driving. Lose enough flow and gauge won't tell you anything until after you make steam. Keeping the system pressurized also keeps the coolant in contact with parts way above the bulk temperature of the coolant. Think of a pot on the stove. As long as there is water on the bottom all is good. Cook the pot dry and as the metal reaches the flame temperature the pot gets ruined. Same with your cylinder liners and heads. The pressure also reduces cavitation erosion. This is especially important for the head studs and cylinder sleeves.
Anyway back to you question, In Vanagon, with everything working properly the coolant system pressure is "controlled" by the pressure cap.
Dennis
-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of John Rodgers
Sent: Tuesday, May 10, 2016 6:57 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Filling the coolant reservoir...
Anyone know the actual range of coolant system pressure our vanagons run at. The coolant absolutely expands and contracts, as the engine heats and cools, thus the need for the overflow tank. The overflow tank is marked maximum and minimum when hot or cold, and the overflow tank is vented on the top backside with three little holes to prevent it from blowing out of something goes wrong in the system. But it would be interesting to know what the actual system pressure range is, especially since sea level atmospheric pressure is 14.7 psi. That weight of air is enough to push the coolant back in to the system as the coolant cools and shrinks. So what is actual system pressure at both ends of the spectrum - hot and cold?
John
On May 10, 2016 17:30, "Scott" <SCOTTDANIEL@turbovans.com> wrote:
perhaps have the guy google 'coolant recovery pressure cap for automotive cooling systems,
then maybe he'll believe coolant expands, such a cap accomodates expansion by putting the 'extra' in overflow tank , then sucks it back during cool-down.
cars have used this basic system in one form or another since the 70's , even a little earlier.
( rather oddly, my 1985 Mercedes car still uses the old fashioned system ..the air space at the top of the rad is the expasnion zone for coolant..no overflow / recovery tank. )
re last comment below ..
all cars have some form of a pressure relief cap.
Haven't heard of a totally closed one yet, though modern car cooling system pressures can be quite high.
here's a whole google search page the guy could read ...help him a lot I imagine.
in one of those they mention 24psi for a more modern car.
Our vans run in the 13 or 14psi range max I believe.
I have not found the blue caps to be extra reliable. I always carry a spare.
for example...
saw one Subaru engine conversion with a head gasket issue ..
after problems with the first cap and pressure tank, they put on a new blue cap and pressure bottle.
pressure got so high...and the new blue cap did not release pressure, that the pressure bottle puffed out like a football.
I still have it, it's only slightly warped where the level sensor screws in ...
so new I hate to just throw it in landfill.
scott
On 04/26/2016 08:20 AM, Dan N wrote:
> Neil,
>
> he said that he gives room for the coolant to expand... I told him
> when the coolant expands the blue cap will open and the coolant goes
> to the expansion tank behind the license plate...
>
> he didn't buy it...
>
> I am sure he's thinking as modern car with the radiator and a filling
> tank with no pressure cap and no expansion tank..
>
> haha...
>
> :-)
>
>
