Jeff

Interesting conversation about pressure cap(s). Pressure in a coolant system seems to be crated by two phenomena, one being the vapor pressure of the coolant as the liquid heats up and the other being the expansion of the liquid, itself, due to the increased heat.

Our rotaries are much more susceptible to thermal damage than are our piston brethren in that at a given temperature (220* of so) we risk damaging some of our sealing "O" rings and resultant leakage. It would seem like the heat transfer from the coolant to the air is of prime importance to keep the temperatures in the "safe" range. Increased pressure caps, while perhaps constraining the pressure created, also allow the heat carried by the coolant to be higher before creating a gaseous state (steam).

Because of our coolant temperature restraints, the cap pressure relief should be chosen somewhat carefully. It's purpose is to release pressure and keep the system from blowing up. As the temp, and thus the pressure increases,when the cap valve opens, the remainder is sent to the overflow tank (if one is present), or blown overboard.

With an overflow tank, in the system, there is no air in the system and as the water expands the pressure cap will restrain and allow the pressure to increase until the cap setting is reached at which point it will open somewhat allowing coolant to overflow into the overflow tank. The amount overflowed will be dependant on the volumetric increase when the engine gets to its hottest temp. (again this is with a system that has no air in the coolant loop. The overflow tank is usually (at least in automobiles) made of plastic and does not have a pressure cap. As coolant is squirted into it, the air in the tank is vented to atmosphere.

Now when the engine cools down and the pressure decreases, the cap (actually 2 valves) allows some of the overflowed coolant to reenter the system so that there never is (or should be air in the loop) The overflow tank is filled from the bottom and thus when the pressure decreases, it will suck (actually the coolant is pushed by atmospheric pressure) back into it. That's why you must maintain a certain level of water in the overflow tank even when the engine is cool.

There is an alternate system which does not use an overflow tank but uses an expansion tank which itself is 1/2 or so filled with air. (the rest with coolant). This tank feeds, as a header into the pump intake and is at the same pressure as the system. As the water volume increases due to heat, it compresses the air in the expansion tank in proportion to the pressure, thus no need for the overflow tank. Even though the pressure in this tank is the same as general pressure, because the pressure downstream of the pump is greater, it is a great place to dump the fluid used to heat the aircraft. I also use a couple of small vents from the high part of the radiator and engine to vent any gas (steam) that might form or collect there. With this system, the pump always sees a head of coolant, in addition to that circulating which might have the effect of decreasing the possibility of cavitation.

The pressure cap here serves only to protect the tubing, radiator and "O" rings from over pressure.

The trick is to control the temp with the radiator. A good place to start for the pressure setting is to use what Mazda recommends for their engine. (they probably know a little more about the pressure and temperature constraints than we do. Increasing the pressure cap rating may, although treating the symptom, be counterproductive.--- Or not

2 pressure caps make no sense.

Rich