Coolant pressure at M30 engines

[deQuincey]

knowing that there is a cap in the expansion tank rated at 1 Bar we can assume that coolant pressure must be under 1Bar

but have you found any specifications on this topic ?

what is the correct cooland pressure ?

does it change with rpms ?

i will appreciate your experience on this topic

regards

 

[Mike Goble]

There is no single coolant pressure. It varies around the system, with the highest point being the outlet of the pump, and the lowest being the inlet of the pump. If the pressure exceeds the relief valve in the cap, it vents. One drawback of older-style down flow radiator systems was that the pressure relief was on the high-pressure side of the radiator, resulting in coolant being vented at high pump speeds.

 

[Stevehose]

There is no single coolant pressure. It varies around the system, with the highest point being the outlet of the pump, and the lowest being the inlet of the pump. If the pressure exceeds the relief valve in the cap, it vents. One drawback of older-style down flow radiator systems was that the pressure relief was on the high-pressure side of the radiator, resulting in coolant being vented at high pump speeds.

What about when the engine is not on, and the pressure is built up in the system so caution is needed when taking the cap off, isn't this the pressure DeQ is referring to or is it the same thing?

 

[Mike Goble]

What about when the engine is not on, and the pressure is built up in the system so caution is needed when taking the cap off, isn't this the pressure DeQ is referring to or is it the same thing?

You should always be cautious when removing the radiator cap on a warm engine. With no flow, the pressure is the same throughout the system. Even though the radiator may be just warm, there can be hot spots in the coolant that will boil if you release the pressure quickly.
When the pressure cap is on the coolant temperature can exceed the atmospheric boiling point. Once the pressure cap removed, the coolant will boil if the temperature is above the atmospheric point. It can boil all at once, resulting in a dangerous geyser erupting from the system.

 

[deQuincey]

What about when the engine is not on, and the pressure is built up in the system so caution is needed when taking the cap off, isn't this the pressure DeQ is referring to or is it the same thing?

thank you
i refer to the pressure in the system during engine performance

If we assume that pressure varies through the system, and if we consider that the expansion tank is sort of control point

but my question is still open

which is the pressure that a healty engine shows ? maybe to focus in one point, say in the expansion tank

between 0 until 1 bar for sure, but should it be 0,5 0,8 ?
does it vary with rpms ?

thank you

 

[Mike Goble]

thank you
i refer to the pressure in the system during engine performance

If we assume that pressure varies through the system, and if we consider that the expansion tank is sort of control point

but my question is still open

which is the pressure that a healty engine shows ? maybe to focus in one point, say in the expansion tank

between 0 until 1 bar for sure, but should it be 0,5 0,8 ?
does it vary with rpms ?

thank you

You can fix the expansion tank pressure at 0 bar by loosening the cap. It doesn't really affect anything until the temperature exceeds the boiling point of the coolant. I had a leak in my expansion tank right at the solder line and it would blow the coolant out until the level of the coolant was lower than the leak, and then it would operate just fine. I would check the lever every week or so and it stayed consistently at that level.
I will install a pressure gauge on my new expansion tank today and report back on the readings under various conditions.

 

[deQuincey]

Elaborating my ideas: TO CHECK HERE FOR MISTAKES....

The cooling system is a closed system that has an expansion tank where a quantity of air accumulates.
This tank has a cap calibrated to 1 bar of pressure
the meassured pressure is AIR pressure.
The objective is to have a system in which the internal pressure is higher than the atmospheric pressure and thus ensure that the boiling temperature of the coolant increases above 100ºC and in this way the liquid does not boil when passing through the walls of the cylinders (which in fact are the hottest point in the engine for the coolant) and can dissipate heat from them.

If the boiling temperature increases with the pressure, it seems reasonable to think that the system will try to get as close as possible to the maximum pressure of 1 Bar, since at that pressure the boiling temperature will be higher.

It is understood that the pump provides a pressure to the coolant, but that this is negligible compared to the pressure due to temperature. I myself have verified that by increasing the rpms the general pressure only rises 0.1 or 0.2 bar.

all of this is right ?
any mistakes on my side ?

 

[Stevehose]

I would replace the word "refrigerant" with "cooling" and "coolant" to avoid confusion with the air conditioning "refrigerant" namely R134, R12 etc.

 

[Arde]

I do not think the expansion tank is to regulate the pressure to a constant value. If it were, one would consistently lose coolant that escapes when open or lose air when air escapes. If so one would need to add coolant over time, or the cap would need to open to let air in when negative pressure in the expansion tank is excessive as it cools off.

I view the cap as a mechanism to avoid catastrophically high pressure that can blow up hoses, and thus cascade into faster overheating through coolant loss. The cap valve does not prevent overheating, for that we have temp gauge, light, and shutting down the engine.

If the above is correct, the engine is meant to operate at pressures well below the 2 Bar so that it never has to open in the absence of a fault.

My E10 had no cap, which means that pressure was indeed limited to 1 Bar, but I had to keep adding coolant every couple of weeks...

 

[eriknetherlands]

Interesting question @deQuincey ; I do not have the answer.

I do like to brawl about things , and dump my thoughts anywhere however

(Note that the 1.0 bar purge pressure is actually 2.0 bar atmosferic pressure; it is 1.0 bar OVER atmosferic pressure. )

I would think the system would operate during normal driving condition at a near zero state of pressure for the following reason: I would consider the system to be closed; closed as there is no addition of air or coolant during driving. There are thus only two factors that can create pressure: 1 thermal expansion of the media: coolant and the air in the tank, and 2. gas formation.
- The coolant expansion is a linear thing, and even from freezing point (0) to 99 degree Celsius the volume chance effect is small: (99*the factor of 0.00021 = 0,02 thus 5 liters will become 5,1 Liters (gallons) volume increase (Wikipedia: for every 1 degree Celsius increase in temperature, water will expand by 0.00021 times its original volume)
If the pressure in the air volume in the tank is ambient (1bar) at the begin, and the airvolume was 1 liter, then the 0.1 liter of fluid expension will increase the pressure by + 10% going to 1.1 bar.
- To the second point; Gas formation. IF it occurs, it increases the volume by a factor of ~1600, so that would indeed seriously build pressure. But I doubt that gas formation occurs during normal operation, as the system tries to limit the max temp to 95 or 97 Celsius, doesn't it? (correct me if i'm wrong, isn't that what the temperature sender does; open the cooling circuit to push the temp down? ) If that is correct, then gas will not form throughout the system. This leaves gas formation close to the cylinder wall: for this I think the speed at which the coolant passes through the cylinder head is too high for the coolant to heat up beyond the boiling point.

But for this last thought i have little argumentation to defend it, it's just a thought.

 

[mulberryworks]

Plain water under 2 Bar pressure (atmospheric +1) will boil at 120°C. Adding coolant will raise that temperature some depending on the mix and the product. A 90° thermostat should open at the design temperature, but the capacity of the cooling system and load on the engine will determine what the ultimate temperature will be. Certainly, it will be hottest next to the combustion chambers and boiling there would be bad as the cooling ability will fall off quickly and damage could occur.
When overheating happens the coolant cap should release excess pressure so that damage to hoses and radiators doesn't happen. The engine could be in danger of damage then and should be shut down. I warped the head in my Volvo when I lost coolant because of a failed head gasket and the engine got quite hot.
I agree with the posts above that state engine coolant would normally be under more or less atmospheric pressure, ignoring the pressure of the water pump pushing it around and through the engine and radiator.

 

[eriknetherlands]

A further consideration: We always talk about that the pump builds pressure, but remind that the pump also actively pulls in the fluid on the the other side.
I see no reason to not call it a "hot-water-removal-suction-device".

it's another argument to think that the pump actually does not build up much pressure during normal operation; it's there to just overcoming the friction created for the needed relocation of the hot water. Directly after the pump is the highest pressure. Just in front of the pump we should see the lowest pressure of the system.
Would the pressure in the expansion tank be roughly halfway between those 2 levels?

but, I am violating my own adage here: measure first, then comment.

 

[Arde]

but, I am violating my own adage here: measure first, then comment.

Here are my measurements:

1) E10 had a water leak on an engine block plug. The temperature needle would start going above normal, if I stopped and waited idling it would not drop very fast, if I kept driving it would go up faster, if I kept driving but downshifted the temperature would drop.I did about 90km of highway in a lower gear just to get it safely home and repaired.

That means that when the water volume was not enough to cool the engine, moving the same volume faster was enough to cool it. This is unrelated to water pressure and fully related to water speed, and shows that speed is proportional to revs. That must mean that pressure after the pump is proportional to revs because pressure delta is what provides motion speed.

If the water path was obstructed somewhere then most of the pressure delta would accumulate around the obstruction. I think Erik is right that the pressure delta would be an increase on one side and a decrease on the other as the pump sucks...

2) Again driving 1977 E24 for 5 highway hours low coolant level caused higher temperature and the auxiliary fan kicked loudly in resulting in lower temperature immediately. In this case it means that the air speed at the radiator matters a lot even for a lower water volume. That car had auxiliary fan and oil cooler. It never overheated. The main conclusion is that a 77 BMW dies from rust before overheating can kill it.

3) Volvo 240 with 1/4 Million miles. I think the expansion tank cap was not working, in retrospect. The expansion tank would visibly expand and contract probably due to head gasket leaking engine pressure into coolant. The water pump leaked. I replaced the water pump to fix the leak and the car blew head gasket...The bad water pump was acting as pressure valve...