The development of internal combustion engines for reduced exhaust emissions has resulted in significant increases in the amount of heat dissipation into engine cooling systems. Traditionally, increases in the required amount of heat dissipation has been accomplished by improving the radiator cooling capacity through increasing the core size of the radiator. In addition, increased coolant and cooling air flow has been used to deal with the increase in required heat dissipation.
Packaging space for larger radiator cores and high energy consumption due to increased coolant and cooling air flow limit the amount of heat dissipation capacity increase that can be accomplished with these traditional approaches.
It is possible to improve cooling capacity by elevating the maximum permissible coolant temperature above traditional levels. The adoption of pressurized cooling systems which permitted operation with coolants at 100° C./212° F. was a step in this direction. The addition of expansion tanks assisted in maintaining such temperature levels. However, it has become desirable to elevate coolant temperatures to even higher levels.
Utilization of elevated coolant temperatures requires proper pressurization under all operating, stand-still and ambient conditions in order to control cooling characteristics, secure coolant flow, prevent cavitation and cavitation erosion and to prevent unwanted boiling and overflow.
Temperature and pressure increase becomes more critical as the heat dissipation from the engine approaches the cooling capacity of the cooling system. A now traditional approach for pressurizing cooling systems is to rely on closed expansion or pressure tanks which depend on temperature increases of coolant and air to create and maintain desired pressures. Such a system communicates with ambient air by opening two way pressure valves to thereby communicating the system with ambient air to entrain new air into the pressure tank when entrapped air and the coolant cool to create a vacuum in the system. Such systems are passive and vulnerable to leaks. Moreover, if such a system is depressurized for any reason, such as maintenance or top-off, pressure is reduced to ambient and operating time and cycles are needed to increase the pressure in the system.