The minimum jacket water inlet pres

The minimum jacket water inlet pressure requirement is 6.5 psi for AT engines. It is met by ensuring the proper pump pressure inlet pressure (Net Positive Suction Head, NPSH). The pump inlet pressure is a result of both the expansion tank pressure (C) and the vertical water column head pressure from the height of the expansion tank (B). Valve (A) does not establish jacket pressure. Rather it is used to regulate flow and engine delta T.Some systems use a gas-pressurized expansion tank, or flexible diaphragm in an expansion tank. (A pressure cap or pressure relief valve is still required) Such systems have a number of advantages: 1) Proper pressure is maintained on the entire system at all times (even during cold engine start-up), via an air compressor and pressure sensing unit. 2) Since pressure can be "artificially" created even at cold start, the expansion tank mounting height (water column static head) can be much lower than required for a standard system. 3) A diaphragm would allow expansion of the coolant, while maintaining separation of the coolant and air (glycol becomes corrosive when in contact with oxygen). To maintain coolant quality, it is preferable not to introduce air into the cooling system upon cool down. Therefore, some system use nitrogen, an inert gas, to pressurize the tank.