Further … for our previous flights, the expansion tank was holding from 10-15psi before engine start … on Saturday, the tank was only 2-3psi before engine start. I think what happened was the engine heat increased the pressure from 3psi to 11psi during climb-out but after we leveled off and reduced throttle the combined reduction in power and increased cooling air flow simply cooled the engine significantly. The reduction in coolant temperature resulted in decreased pressure and the oil temperature followed for the same reasons. I don't know how low the oil temp went as I was fixated on the coolant pressure.
It appears that adding the second air purge line from the rear iron to the top of the expansion tank has changed the cooling system pressures
dramatically. Previously there was one purge line from the high point in the water pump outlet to the bottom of the expansion tank … with this setup, the system would pressurize to the cap pressure and stay there no matter what.
Am I getting fooled here because the system was not pre-pressurized?
Or are the temps just so low that I'm in unknown territory?
Jeff
Nothing stated so far is cause for alarm.
I saw the same thing with my system. Once the engine heat cycled there would be far less than the relief pressure showing and at rest would sit at just a few pounds. I could determin nothing of value from watching the pressure gage. The water temperature is the only information needed in the long run. The pressure gage could alert you of a sudden loss of coolant, and that is about all.
I gave up on it. I installed a Shrader valve in the pressure tank and just charge the system to relief pressure with the engine cold. Then as the system warms up, it sits at 18 pounds. Once under way with air cooling the radiator, the pressure falls off of max and drops to 10 to 12 pounds. On cool down the pressure drops to below zero and coolant is recovered from the catch tank back flowing through the pressure cap. So then it sits static with no pressure. This is a variation on what should happen. If I put too much coolant in the pressure bottle, then there is less air to act as a spring against the pressure cap. This give you a profound change in pressure as the cap valves off any pressure above cap value.
Also if you have coolant sitting in the catch tank, the pressure bottle will slug with coolant after shut down, and the system will perform in a very different manor. In my case the output of the cap is fed to the bottom of the catch tank. This may be poor thinking on my part, but I can look into the catch tank before each cycle. If coolant remains in that tank then there is a problem. First, none should have been expelled at all.
And on cool down the slugged pressure bottle and coolant in the catch tank shows you that there is air in the engine, or more likely combustion gasses leaking past an "O" ring.
If the pressure tank is full or nearly full there will be good sized swings in pressure in response to engine temperature. Since the heating of coolant will cause the reliease of coolant when at cap pressure, then at slightly reduced temperature, you will get a bigger reduction in pressure.
In the car, the pressure bottle is not filled full but is kept at about 25% depth. This allows the bottle to act as an accumulator, like airplane accumulators. Like a spring if you like. So, should cap rating be exceeded, only air will be expended, and not coolant. So, a temperature increase expands the coolant, and air pressure in the bottle will be at coolant pressure, and when the pressure exceeds the cap value only air is expelled.
When the engine cools again, coolant pressure will return to nearly zero, or even below zero depending on bottle volume.. Just as you would expect.
The ratio of coolant to accumulator volume determins the cold pressure remaining after a cycle. You would also see a different number for each cap value.
In old american cars, the bottle was just a make up tank and there was no pressure cap on it. The pressure cap was on the radiator, and the system was slugged during all operation. Usually the system operated at cap pressure, and would expell coolant up to maximum operating temperature. Then on cool down, the reverse flow valve in the cap would open, when pressure in the engine dropped below local air pressure, and expelled coolant would return to the radiator from the recovery bottle.
In more modern cars you see no pressure cap at all. A very large pressure bottle is used. The cap will not allow any leakage at all. Coolant expands pushing some coolant into the bottle that is kept about 10% full and no attention is payed to how much pressure is being generated. So on cool down, the system returns
to zero pressure as coolant contracts and some returns to the engine.
A dutyfull gas pumper filled my wifes bottle up full, and a few blocks away the slugged system pushed the end off of the radiator.
Water temps above 190 should be suspect. Oil temps above 160 costs power (rotor face temps) but should be kept below 200 degrees in any case.
Lynn E. Hanover
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