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?

 

[notnats]

I think Patprimmer might have spotted the problem when he suggested pump cavitation.

At 3 bar gauge, the boiling point is 144°C but at 2 bar this is 133°. If the low pressure side of the pump impeller is experiencing a pressure drop of 1 bar or more at high rpm, then local cavitation might reduce the pump capacity enough to keep the coolant temperature high after the airflow returns to normal.

The coolant returning from the radiators to the jacket may actually be cooler than normal, but reduced flow rates through the jacket would result in a higher temp after passing through the engine.

Maybe a slower speed for the pump would help, or a redesigned impeller, or a higher system pressure.

Regards,

Jeff

 

[ccw]

Hi Pats,
I have not heard back so you must have replaced the old thermostat by now ;-). A couple of other posts on this threads prompts me to offer the following:

1. This appears to be a tricked out racing engine so standard cooling system design criteria may not apply: BUT standard auto thermostats work like this: A return spring holds the thermostat closed (this is the cold engine position). An expanding magic elixir** in the "power pill" hydraulically forces the main poppet open (this is the hot engine position) working against the return spring at a preset rated temperature usually around 180F. The power pill action is very non-linear. So, the thermostat can fail open if the return spring breaks or weakens, OR closed if the magic elixir** in the "power pill" goes liquid and escapes or partially escapes around the piston "o"-ring into the coolant fluid stream, never to return, OR somewhere in between if crud gets in the main poppet seat. Those who like this subject may want to start on AutoStat 101 at:

http://www.ehow.com/how_7683_tell-cars-thermostat.html.

2. We have not discussed the oil lube system. You say it is a dry sump and measure the oil temp in the tank. When you get the "slip stream" spike you are losing viscosity with increasing oil temp. This can have two deleterious effects: 1. the lubrication efficacy drops which can reduce film thickness in such places as the main bearing and push rod journals, driving oil temps even higher, 2. oil pump mass flow output can simultaneously drop also. There may be a hysteresis effect in oil viscosity vs. rising temp. compared to falling temp. This would also lead to two operating points for the same external conditions. Do you have accurate delivery oil pressure measurement for this phenomenon to go along with your temperature maps?

** Back in my Robertshaw days this was a closely guarded company secret concoction.

 

[Paulista]

If you all will allow me to butt in ....

Can anyone explain what the logic is behind :

1. 2 to 3 bar pressure maintained in the coolant on the output side of the water-pump

coupled with:

2. A radiator cap with a relief valve rated considerably lower, in the 0.6 to 1.1 bar range ?

Thanks

 

[patprimmer]

Where does he say he has a 0.6 to 1.1 bar radiator cap. That one slipped by me.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

The increased pressure raises the boiling point of the water.

Additional waterpump pressure only occurs within the engine castings, there being a deliberate restriction located at the cylinder head water exit point. This allows the water in contact with the combustion chamber and exhaust ports within the head to resist flash boiling into steam bubbles.

These high temperatures are very localized, so the superheated water quickly mixes to a lower average temperature with the bulk water flow through the rest of the cylinder head.

Average cylinder head water discharge temperature is therefore no real indication of the peak temperatures reached in various critical hot spots within the head.

The rest of the cooling system external to the actual engine is also pressurised, but only to typically 12psi. But here the water is well mixed and the conditions for boiling much less extreme.

Over the years many racers have discovered that removing the thermostat totally, can lead to overheating. The theory being that the water travels too fast through the radiator to cool properly. That is simply not possible.

The real reason is flash boiling within the cylinder head which can very quickly lead to pre ignition, cracked heads or force a lot of water out past the radiator cap.

Once steam bubbles form they grow rapidly and can self sustain. Only backing off the throttle will save it.

If it has to survive more than a couple of seconds of flat out full throttle load at high power, the higher the internal water pressure within the head, the safer it is going to be.

 

[Paulista]

Warpspeed,

Thanks for the explanation - I really appreciate it.
The 12 psi pressure rating you state for the rest of the cooling system blends in with the radiator cap rating of standard production cars.

Patprimmer,

True, he made no mention made of radiator cap rating.
The reason I did is that I am building header tanks for the twin-diesels on my boat and the mfgr´s recommended rating is 0.6 Bar, whereas other diesels use 1.1-1.4 bar caps.

 

[Warpspeed]

Paulista, the only explanation of this effect that I have so far seen in print is in, "Four Stroke Performance Tuning" by A. Graham Bell, ISBN 1 85960 4358 (on page 298).

 

[Paulista]

Warpspeed,
The book - only if I order it directly from Amazon etc.

As for the formation of steam bubbles and the thermal runaway that ensues (positive feedback), I have witnessed it happen on my port diesel.

Quite dramatic, specially if you are out on the water, to see the coolant being spewed out vigorously via the relief valve in the radiator cap, once you are up and running and the engine comes on load....

Took some painstaking investigation to finally nail it down to retarded injection timing (massive heat rejection, the 2nd Law at it´s most impressive rendering !)