Can there be two solutions to steady state cooling 1

[PatS]

Has anyone come across a phenomenon whereby an engine can achieve a stabilised steady state running temperature (for both oil and water)then, due to a change in conditions run at higher temperatures, and finally not be able to return to the original steady state conditions even though all external factors and operating loads have returned to the orginal conditions?

 

[ivymike]

sounds like it may have accumulated gas pockets within the cooling system, or created sludge on the inside of coolant passages due to boiling of the coolant...

 

[PatS]

I don't think this is what is happening. It is on a race engine and the radiators are thourougly cleaned every 500 kms (air side and fluid side) The water system also operates at 3.75 bar gauge and great attention is paid to local boiling during the engine development. It also occured simultaneously on both the oil and water side (although of course one can drive the other).

 

[patprimmer]

Does this happen every time the engine is run under these conditions.

If the engine is stopped and restarted, does it return to normal, or must it be given sufficient time to cool down while not running.

If it requires for it to be stopped and cooled down, does anything on the engine get serviced, adjusted, topped up (coolant level for instance)etc.

What are the changes in conditions that cause the temperature increase.

Regards

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[ivymike]

Also, what is meant by "running hotter?" Higher coolant temps at some location?

 

[PatS]

OK, the basics are that the car can run a number of different cooling configurations. We will always run the configuration that achieves the required heat rejection with the minimum loss of aerodynamic efficiency. Typically rad air inlet velocity is around 10 to 15% of free stream. Temperatures are always measured in the same place. For water this is pump inlet and for oil it is in the bottom of the oil tank for the dry sump system.
Operating conditions in terms of avergae revs, average throttle opening and average speed for a race car are very similar lap after lap but rad air inlet velocity can be reduced considerably when following another car. Generally, when you get out of the slipstream of another car temperatures will reduce to "normal". In this case, even when you get out of the slipstream and rad air inlet velocity returns to normal the temperatures are staying high.
If the engine is stopped and then re-started a few minutes later the temperatures will run normally. It does not cool significantly in this period. (water maybe 10 degrees and oil 1 or 2)
The increase in temperature during this abnormal running is of the order of 7 degrees on oil and 4 degrees on water.
Hope this helps!

 

[patprimmer]

It sounds like pump cavitation or steam pockets, but like you say, unlikely at over 3 bar.

Is something expanding and causing an obstruction.

Is a suction hose collapsing.

Is the pump impeller slipping at the higher temperature.

What are the temperatures.

What is the coolant.

What is it's boiling point at the lowest pressure seen at the suction side of the pump inlet.

Regards

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[richdubbya]

this can happen when the total cooling systems abilty to absorb heat is regulated to create a certain temperature. Your system is working at maximum to retain that temp. When behind a car and temp rises, there is no reserve cooling capacity to cool the engine back to the original temp. to avoid this you have to have some kind of temp regulation(thermostat) to achieve your target temp with some extra cooling capacity to bring the temp down when needed.

 

[richdubbya]

sorry i left this out,

another way is to increase the cooling capacity temporarily when needed. an example would be a driver controled air inlet that could be opened for a short time while losing downforce/adding drag but then returned to its original position. maybe just changing for one long straight would be enough to cool it back.

 

[patprimmer]

It could be operated with a solenoid and a temperature switch, but this still does not explain why the temperature at equilibrium changes.

So long as it works I guess it's OK.

Regards

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[richdubbya]

I believe what he is saying is the equilibrium point is adjusted by air inlet volume, so the system is working at maximum to keep it say at 180 F, when it goes up because of loss of air volume (behind a car), say to 190 F the system only has the capacity to absorb engine heat that is being created with no reserve to cool it down after the pass. so the excess heat generated remains there until the engine generated heat is lessened (shut off) or the cooling capacity is increased.

 

[patprimmer]

richdubbya

At constant power, constant airflow, constant coolant circulation and constant heat exchanger surface, the temperature will change until there is a balance between heat input and heat rejection.

The greater the temperature difference between the air and the coolant, the greater the cooling efficiency of the system.

If everything else remains constant, but the airflow drops, the temperature will go up to obtain a heat input, heat rejection balance at the new airflow.

If the airflow is restored to the original condition, but all else still remains constant, the temperature should return to original to maintain equilibrium or balance in the system.

As I understand it is that in this case, this is not happening, and the question is why.

Regards

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[PatS]

Patprimmer.
I will try and answer those questions.
1. It is a very rigid system. It has some silicon hose (very short lengths) but these are aircraft spec and changed very regularly.
2. Ditto
3. The impeller shaft is gear driven and the impeller keyed to the shaft. Bench test results do not indicate slippage
4. The system is designed to run at 130 deg C and has a "do not exceed" temp of 140 degrees C. Typically it will actually run at 120 degrees C and peak at 130 degrees C.
5. The coolant is deionised water with about 5% by volume commercial inhibitor.
6. The pump inlet pressure ranges from around 20 mbar below system pressure at minimum RPM to 110 mbar below system pressure at maximum revs. The system pressure (in these particular test conditions) was around 2.4 bar (absolute) at minimum revs and 2.2 bar (absolute) at maximum revs.
It has still got me scratching my head!
The best reference I have found on heat exchanger design is the classic "Compact Heat Exchangers" by Kays and London. They use the approach of an equilibrium line and an operating line for heat exchangers (on a plot of Th against Tc)
In theory (as always) these are nice straight lines with no hysterisis and providing the operating line is above the equilibrium line you have a cooling system.
However they do not have the same slope and therefore there is an intersection. If we imagined that we were operating near this intersection and had hysterisis then maybe this is an answer?

 

[strokersix]

Sorry to ask the obvious but are you using a mechanical gauge with a sticky movement to register temperature?

 

[PatS]

No. It's a PT100 thermistor going into a 10 bit AtoD

 

[Warpspeed]

What is the actual measured water pressure within the cylinder head at full power ? There may be steam pockets forming around the combustion chamber and exhaust ports. Once these form, they can be self sustaining, driving the cooling water away from the hot surface.

The system may then run in two different cooling modes fully wetted, or with steam pockets. I am just guessing here, but it may be worth thinking about.

If the waterpump can pressurize the engine against a flow restrictor in the outlet sufficiently, the increased water pressure within the engine can go a long way to preventing steam pockets from forming. This function is usually achieved by the thermostat having a deliberately reduced flow area. A good waterpump should be able to achieve perhaps 50 psi at flat out full power Rpm.

 

[ccw]

Hi Pats,
This non-linear effect happens to me all the time when my engine block cooling jacket thermostat goes bad. It all looks very strange on the gages (both water and oil) until one realizes the thermostat is sticking at various operating points. Run without it, see if you get the same effect.

 

[wr180561]

I think running the engine without thermostat may overheat the engine since in this case it would achieve only partial cooling. I would say, replace the thermostat.

However, want to know if this is a phenomenon that appeared sometime down the road of the engine life or it is there from day one, i.e. a design feature ?

Regards

 

[patprimmer]

A sticky thermostat would tend to stick in the wider open position rather than partly closed. Why would it partly close when the engine was hotter.

Regards

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[patprimmer]

Does the engine start to detonate at 130 deg.

A slight detonation might put more heat and move the equilibrium point to a level were the detonation is maintained, and you get a self supporting closed loop.

Steam pockets will actually cause the system to absorb less heat from the motor, so the chamber will be hotter, but the coolant cooler.

Could the extra expansion discharge some water at 130 deg, then as it starts to cool the coolant level is reduced and the radiator efficiency is reduced until the water is topped up.


Regards

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