Why Did the Temperature Rise in a Sealed Automotive Cooling System?

[tommd]

TL;DR Summary

The cooling system had no pressure relief cap. Did that make the temperature rise?

Automotive cooling system
Not sure if this is an appropriate forum for this question or not, but I will try.

Many current automotive cooling systems have a system that is completely filled with coolant and no room for air or expansion, other than slight bulging of the hoses, etc. A pressure cap, typically 14 psi is on the radiator. As soon as the system starts to warm up, the coolant expands and pressure increases. When the pressure exceeds the radiator cap setting, the coolant is pushed out of the closed system and into an overflow tank. When the system cools down, the coolant is drawn back into the radiator.

I have a report of a system that, due to a service issue, ended up being completely sealed. No room for expansion of the coolant and NO pressure cap or pressure relief of any type. As the engine warmed up, we suspect the pressure went very high as there was nowhere for the expanding coolant to go, although we have no reading of the pressure. Although the operator had no way of knowing the increase in pressure, he did notice the temperature gauge went high into the “red” zone.

The engine was shut down before the pressure increased to the point of breaking a hose, radiator, or some other item due to the high pressure.
My question- I recognize with no pressure relief cap, the system would have been damaged due to extreme pressure. But why did the temperature rise?

It seems that although under higher than normal pressure, the coolant would have still been flowing throughout as normal, picking up heat from the engine and transferring the heat to the radiator where it would have been cooled. I assume the pressure would not have exceeded 100-200 psi, as the hoses or radiator would have probably blown by then. Would this type of pressure somehow have affected the coolant in a way it could not transfer the heat? Why did the temperature rise?

Under normal conditions, a pressure cap is used to increase the system pressure to raise the boiling point of the coolant. To my understanding, that is all it does. I thought it does not affect the cooling process itself. But now I am wondering, does pressure affect cooling in other ways? In this case, it may have increased the boiling point very much, but it also caused the engine temperature to go up drastically. If not a coolant issue, maybe the pressure affects the thermostat operation?

Thanks,
Tom

 

[Flyboy]

I don’t know exactly how they measure coolant temperature in the vehicle in question, but it’s possible that the pressure caused an erroneous reading in the temperature sensor.

The only other thing that comes to mind is the thermostat valve that controls the flow of coolant to the radiator. My understanding is that they’re usually a wax-filled unit which uses the melting and freezing of the wax to drive the valve to open and close, respectively. Depending on how it’s designed and installed, it’s possible that it got damaged by the pressure and wedged in the closed position. If you’re disassembling the cooling system to inspect and replace parts, I would definitely recommend taking a close look at that valve.

 

[Baluncore]

But why did the temperature rise?

A sealed system, while it is warming up and expanding, will blow water out of the system through the water pump seal and bearings, usually low down on the block. When it cools, it will suck air in through the same bearing seals. That process will continue until the compressibility of the internal air can cope with the change in pressure. The reservoir tank and pressure cap, is designed to protect the water pump seals and bearings, while preventing the top of the engine and radiator from filling with air, displacing the coolant.

When there is air in the top of the engine, the cylinder head is dry, so it overheats, and the temperature sensor reads high.
The thermal expanding wax is then blown out of the thermostat by the heat, so it fails closed, and must be replaced.
Check the temperature sensor, the thermistor calibration may have been changed by overheating, replace it if you do not know it is good.

 

[Averagesupernova]

TL;DR Summary: The cooling system had no pressure relief cap. Did that make the temperature rise?

I have a report of a system that, due to a service issue, ended up being completely sealed.

There is likely more than one issue at work here. The engine would likely run normal temp until a hose ruptured if the system was truly sealed. A stuck thermostat valve could cause overheating but even then coolant will be forced out as long as the radiator cap is working. I have never heard of a radiator cap getting sealed shut. If so it would have to be intentionally. Sounds suspicious.

 

[tommd]

Flyboy, the temperature sender would not be affected by pressure to our knowledge. The pressure did not seem to damage the thermostat as it returned to proper function when the pressure cap was restored.

 

[tommd]

Baluncore, thank you for your response. What you are saying clearly can be true, but did not happen. The pressure went up but no damage was done other some slight weeping at the frost plugs. When the pressure cap was restored all returned to operation.

 

[tommd]

The engine would likely run normal temp until a hose ruptured

Yes, that is what I would have expected. And that is the heart of the question. With no damage to any component, why did the temperature rise with only an increase in pressure.

It is not suspicious at all. This system has a remote expansion tank that is connected to the radiator, and the pressure cap is on the expansion tank. During some work, the hose that connects the expansion tank to the radiator got crimped shut, sealing the radiator off from the expansion tank and the pressure cap. Thus a totally filled cooling system with no relief valve.
Tom

 

[Juanda]

From @tommd in a different thread.

I do not know why it got posted in the other area. The responses there, although good,seem to be more oriented towards the mechanics of the cooling system, where I have little questions. My question is if this amount of pressure can affect the cooling of the water. I understand there is localized boiling in a cooling system- nucleate boiling??? Could that be affected by the pressure? As I stated above, increased pressure should only increase boiling temperature, not cooling ability???
Thanks,
Tom

What's already been proposed made sense to me but you say it doesn't explain what's going on with your vehicle.

Many current automotive cooling systems have a system that is completely filled with coolant and no room for air or expansion, other than slight bulging of the hoses, etc. A pressure cap, typically 14 psi is on the radiator. As soon as the system starts to warm up, the coolant expands and pressure increases. When the pressure exceeds the radiator cap setting, the coolant is pushed out of the closed system and into an overflow tank. When the system cools down, the coolant is drawn back into the radiator.

[...]

The engine was shut down before the pressure increased to the point of breaking a hose, radiator, or some other item due to the high pressure.
My question- I recognize with no pressure relief cap, the system would have been damaged due to extreme pressure. But why did the temperature rise?

It seems that although under higher than normal pressure, the coolant would have still been flowing throughout as normal, picking up heat from the engine and transferring the heat to the radiator where it would have been cooled. I assume the pressure would not have exceeded 100-200 psi, as the hoses or radiator would have probably blown by then. Would this type of pressure somehow have affected the coolant in a way it could not transfer the heat? Why did the temperature rise?

Your refrigerant is going through an isochoric process as it takes heat from the engine. It's getting much hotter than it would under normal operation in which it'd relieve some volume as pressure increases.
Since it's hotter, it cannot take as much heat from the engine so the new steady state is at a higher temperature so you're reading a higher temperature in your engine as well.

That's my take on it at least. I have only worked at an amateur level with engines and it's my first time seeing a problem as the one you're describing but the explanation seems plausible.

By the way, I doubt there is boiling going on as you mentioned. If the refrigerant were able to boil, it'd do so and at some point, you'd have a lot of gas in your system which would introduce several complications.

 

[Juanda]

Your refrigerant is going through an isochoric process as it takes heat from the engine. It's getting much hotter than it would under normal operation in which it'd relieve some volume as pressure increases.
Since it's hotter, it cannot take as much heat from the engine so the new steady state is at a higher temperature so you're reading a higher temperature in your engine as well.

Expanding on that, notice in the following picture how moving vertically (keeping the same volume and increasing the pressure A→B) in the liquid region (at the left of the green curve) the temperature increases significantly.
That's for water which is most likely very different to your refrigerant but the behavior is kind of like that for fluids.

Picture taken from this video. I haven't actually watched the video because I just needed the picture but I prefer to link the sources anyway.

 

[Baluncore]

I guess the bottom radiator hose expanded, but the coolant did not boil.

I would examine the thermostat and look for ways that the mechanism might fail to open when hot, if it was subjected to a high hydrostatic pressure.

 

[Averagesupernova]

Til I see an accurate diagram I won't offer anything else. A hose, somewhere was pinched. That's all we know.

 

[jack action]

During some work, the hose that connects the expansion tank to the radiator got crimped shut, sealing the radiator off from the expansion tank and the pressure cap.

Is it possible this was done while the engine was hot?

As the engine cooled down, it wasn't able to retrieve the coolant from the expansion tank. The vacuum inside the cooling system may have slowly pulled air into the system from God knows where. With air in the system, the engine now overheats.

Even if the blocking was done with the engine cold, once the pressure got high, the coolant may have leaked internally (i.e. in the cylinders, through the head or intake gasket) instead of the blocked radiator cap. Once cooled through the radiator, it sucked air from the same place since the radiator cap is still shut. You now have a vicious circle where you spit out more and more coolant and replace it with more and more air.

 

[erobz]

How about good old fashion air lock? If you have a system opened for repair air will enter. Get a slug of air across the pump and the system fails to properly circulate the coolant. Temperature (because of combustion) and pressure (because of no relief/thermal expansion tank) rise as the coolant stuck in the head continues to collect heat.

I had air lock in my hydronic system at home recently because I installed kick space heater. My air bleeders were (all of them - that's what happens when you fail to inspect things for 10 years) completely stuck closed - unknown to me. The air trapped the water in the boiler, unable to circulate it and give it off its heat the temp and pressure increased until the system shut down from over temp/pressure limits.

 

[tommd]

A hose, somewhere was pinched. That's all we know

I mentioned the crimped hose was between the radiator and the expansion tank, isolating the cooling system from the pressure cap.

 

[tommd]

Is it possible this was done while the engine was hot?

There have been several good questions, but unfortunately I do not have many specifics. Only speculation. What we know is the system was sealed off from the pressure cap. Pressure rose. No damage was done other than seeping frost plugs. I was looking for a logical explanation that may explain why the temperature in the engine rose when we typically expect pressure in the system to only raise the boiling point.
Tom

 

[erobz]

There have been several good questions, but unfortunately I do not have many specifics. Only speculation. What we know is the system was sealed off from the pressure cap. Pressure rose. No damage was done other than seeping frost plugs. I was looking for a logical explanation that may explain why the temperature in the engine rose when we typically expect pressure in the system to only raise the boiling point.
Tom

https://practical.engineering/blog/2020/2/11/what-is-air-lock

If the coolant wasn't circulating, it gets hot in the head and expands in volume. It has nowhere to expand into if the system was sealed (malfunctioning -as you say). Also, water boiling point increases with pressure so it's able to get hotter than 100 C without boiling at more than 1 atmosphere ( increasing overall thermal expansion from initial cold state) A little bit of volumetric expansion in stiff container goes a long way in a hurry.

 

[Lnewqban]

... It seems that although under higher than normal pressure, the coolant would have still been flowing throughout as normal, picking up heat from the engine and transferring the heat to the radiator where it would have been cooled. I assume the pressure would not have exceeded 100-200 psi, as the hoses or radiator would have probably blown by then. Would this type of pressure somehow have affected the coolant in a way it could not transfer the heat? Why did the temperature rise?

Welcome, Tom!

Your above statement is correct.
The increased pressure per se should not have affected the rate of heat transfer from the engine to the coolant, and from the coolant to the air.
Something stopped or drastically reduced the coolant flow and/or that transfer process in the radiator.

Please, see how pressure could affect water:
https://www.engineeringtoolbox.com/compressed-water-d_969.html

It would be interesting to know the type of thermostat that the engine had installed.
Assuming a bellows type, the increased pressure could have over-powered the expansion of the volatile fluid (alcohol, ether or acetone) inside the bellows, forcing a closing of the valve.
The vapor pressure of any of those fluids (alcohol, ether or acetone) is much lower than the estimated coolant increased pressure.

 

[tommd]

It would be interesting to know the type of thermostat that the engine had installed.

The thermostat was the standard wax type, not a bellows.
Thanks for the help.
Tom

 

[Ranger Mike]

Is it possible this was done while the engine was hot?

As the engine cooled down, it wasn't able to retrieve the coolant from the expansion tank. The vacuum inside the cooling system may have slowly pulled air into the system from God knows where. With air in the system, the engine now overheats.

Even if the blocking was done with the engine cold, once the pressure got high, the coolant may have leaked internally (i.e. in the cylinders, through the head or intake gasket) instead of the blocked radiator cap. Once cooled through the radiator, it sucked air from the same place since the radiator cap is still shut. You now have a vicious circle where you spit out more and more coolant and replace it with more and more air.

excellent, Jack!