pressurized cooling system question

if engine cooling system is up to thermostat temp will the cooling system pressure change with engine speed change such as 500 rpm vs 2000 rpm?

Engine speed will not directly affect the system pressure or the system temperature, given an adequate, clean and efficient cooling system. Reliable system prressure of a few PSI under 10 will raise the boiling point.

Complex answer to come:The thermostat temperature is a modest variable across maybe 10 to 15 degrees. If a tractor load is constant and near the working ideal wide open throttle and loaded to PTO rpm) and the outside temperature is pretty constant, the pressure cap should never come into play. The actual pressure will vary, but never reach the relief pressure. If the engine rpm is increased (while within thermostat control) the engine will create more heat from increased fuel consumption. This heat will open the thermostat more. if loaded such that the thermostat is wide open, any further load, or increase in ambient temperature will increase the coolant temperature until the radiator cap pressure is exceeded. There will be steam and coolant expelled. I hope this makes sense. Jim

In a normal system thermal expansion causes pressure to build in the system.
Once the pressure reaches the cap setting it starts releasing air from the system to hold a constant pressure. This is why you never fill a cooling system to the top with water.
The system is pressurized because it raises the boiling point of water.

The pump can add to the pressure in a restricted system.
Things like high RPM or the wrong pully combination can add to the problem.
A clogged lower hose or to small a hose along with a clogged radiator will cause a vacuum on one side of the pump and excess pressure on the other side.
You should never notice this as a working cap should release this extra pressure.

If the engine in question has a water pump pressure will increase considerably in the top radiator tank with an increase in engine speed due to the waterpump forcing flow through restriction of the radiator core.

How much would depend upon the design of the system and the condition of the radiator, for example new and clean, or at least partially restricted.

maybe tell us what engine or tractor you are referring to and why, i do know when an engine is hot the coolant will be flowing out the top then rev it up and hold it at 2000 rpm and it wont blow out the top. so when it is reved up you are cooling the coolant so u cant be increasing the psi. increase in heat makes increase in psi.

I also agree that the pressure in the radiator is directly related to the temperature the coolant is at in the system. The increase has to do with the expansion of the coolant also if the coolant reaches its boiling point that is when the pressure rises quickly but then is held in check by the pressure increase in the system pressure keepingyyy the boiling in check. The suggestion that the pressure increases in the top radiator tank or hose as the engine speed is increased is not necessarily true. As many replies mentioned that they are referring to a properly operating system. A system that pressurizes the top of the radiator probably has some plugging in the radiator tubes. You also have to remember the cooling system is a closed loop system. What pumps out the top of the engine must be simultaneously drawn back into the bottom of the cooling system for the engine. So in the scenario where the pressure in the upper radiator tank is higher from more RPM that would only last momentarily because in that system the vacuum created by the pump would likely suck the lower radiator hose closed or the pump would be overcome by the vacuum or lack of flow.

One thing to remember, is with the increase in RPM's, the water pump and fan blades also turn faster. The faster they turn, the more cooling is going on. So basically, the system is stabilizing itself when the engine RPMs increase. If the fan and water pump didn't likewise fluctuate with engine RPM's, you'd have problems. The problems you are likely wondering about.

Basically the pressure on the cap is created by the heat. If same heat, same pressure on the cap. You can argue flow restriction, and RPMs, and thermostat closed or open, and all that jazz, but it's really the heat that puts the constant pressure against the cap. The rest is really just a minor variable if anything, and if present.

Not sure why your even asking.

The radiator tubes (as many of them that are) should flow water just as fast as through a radiator hose unless cloggage.

As for water pump and a closed thermostat, the pump is pushing water against the thermostat when closed, and not the cap. Kind of like kinking off a garden hose. The pressure would be against the kink, and not the outer end of the hose. In order for a water pump to put pressure on the cap due to closed thermostat, there would have to be back pressure through lower radiator hose which there wouldn't be. But add heat to the scenerio, and there would be. Things would get hotter with closed thermostat.

Heat is the main factor on pressure.

I'm going to assume you're referring only to the pressure AT THE RADIATOR CAP, since the coolant pressure varies slightly throughout the system due to the water pump and various restrictions such as the thermostat.

The pressure at the radiator cap will be equal to the VAPOR PRESSURE of the coolant for whatever temperature the coolant is at. Since vapor pressure increases with temperature, the coolant pressure will rise with temperature up until the radiator cap pressure rating is reached. At that point the coolant will boil.

At that point the coolant will boil

I am not smart enough to figure expansion of air gap water and anti freeze but Im thinking that is not true.

A system at 15 psi and 50/50 mix will go to about 265 degrees before it will boil. Im thinking the system reaches 15 psi long before the water gets to 265.

> A system at 15 psi and 50/50 mix will go to about 265 degrees before it will boil. Im thinking the system reaches 15 psi long before the water gets to 265.

John, what keeps the cooling system from boiling over at, say room temperature? It's pressure. Water will boil at room temperature in a vacuum. In an un-pressurized cooling system, atmospheric pressure keeps the coolant from boiling. In a pressurized system, the atmospheric pressure plus the additional pressure imposed by the radiator cap prevents boiling.

Boiling occurs when a liquid's vapor pressure exceeds the ambient pressure. For straight water at sea level, that happens at 212°F. And as you point out, a mixture of antifreeze and water at 30 psia (15 psi atmospheric pressure + 15 psi radiator cap) boils at around 165°F.

Now you mention expansion. Coolant is relatively incompressible and expands with temperature. So as the engine warms up, expansion will cause the radiator cap to open and vent air and/or coolant. In an older style cooling system without coolant recovery, the effect of coolant expansion is negligible. But if you're talking about a modern system with coolant recovery, you're right, the radiator cap pressure will be reached at a below-boiling temperature because there's no place for the coolant to go other than out the radiator cap.

(quoted from post at 17:59:34 08/27/23) if engine cooling system is up to thermostat temp will the cooling system pressure change with engine speed change such as 500 rpm vs 2000 rpm?

Click to expand...

The pressure gradient across the pump will increase but the overall system pressure will remain under the Rad cap limit .
There is some difference in the rate of pressure increase during warmup with a solid system with an overflow bottle .Vs drain tube to atmosphere system that has some airspace during warmup .
A coolant pump is a high volume , low pressure design .

And the caps are designed to allow the overflow coolant to flow into the collector bottle (where the radiator is completely full when servicing and temperature forces aren't forcing some of the coolant into the bottle) to return to the radiator when the volume is reduced therein due to a lower temperature.

One problem I always had over the years, before the days of the overflow bottle/full radiator concept was that I didn't realize/know that the tank on the radiator was designed to capture the contents of the radiator. For that to happen, full was just over the top of the core, not all the way up to the cap........since I have been aware of that, my vintage tractors (not newer ones with the bottle) no longer require me to keep adding coolant because I no longer think the tank has to be full and I no longer over fill them.