Pressurized liquid cooling system for an internal combustion engine

A liquid cooling system for an internal combustion engine includes a conventional radiator and circulating system and pressure cap, the latter having pressure and vacuum relief valve components for limiting the maximum operating pressure of the system and for limiting negative pressures to avoid damage during cooling after engine shutdown. A second, temperature responsive, pressure relief valve provides a lower system relief pressure as long as the temperature of the coolant in the radiator top tank remains below a certain level. When that level is exceeded, a thermally actuated valve excludes the second pressure relief valve from the system and maximum operating pressure is then limited, at a higher level by the valving of the pressure cap. In an alternative embodiment, an infinitely variable pressure relief valve responsive to and controlled by changes in coolant temperature replaces the fixed second pressure relief valve and its thermal actuator. As the temperature of the coolant increases, the set point of the variable pressure relief valve also increases and the system relief pressure varies automatically with the coolant temperature over a range of pressures whose upper limit is significantly greater than the set point of the relief valve in the pressure cap so that maximum system operating pressure is still limited by the pressure cap.