The freshwater circuit of an engine cooling sys-tem includes a tank that is commonly referred to as the expansion tank. Some expansion tanks are identified as the surge tank or supply tank. The freshwater tank provides a place where water may be added to the system when necessary and provides a space to accommodate changes in water volume that result from expansion and con-traction when the water is heated and cooled. The piping arrangement to and from the expansion tank provides for three basic functions: (1) per-mits excess water in the system to pass back to the tank as the water becomes warm and expands, (2) permits water from the tank to flow into the system when the water becomes cool and con-tracts, and (3) allows for the resupply of water from the expansion tank in the event of leaks in the system.

Even though the exact locations of expansion tanks vary in different engines, the tank is always located at or near the highest point in the circuit. Examples of tank location are shown in figures 7-1 and 7-3.

The manner in which venting is accomplished in the freshwater circuit of a cooling system will vary, depending on the engine. However, venting generally involves the expansion tank. In some expansion tanks, particularly in the systems of larger engines, a vent pipe from the high point of the circuit carries to the tank any steam or air bubbles that may form in the system. When steam comes in contact with the cooler water in the tank, the steam condenses back into water. The con-densation keeps the system free from steam or air pockets. The expansion tank is vented to the atmosphere. A gauge glass, located on the side of the tank, reveals the water level.

In many small engines, the freshwater circuit has no vent and operates under a slight pressure; this arrangement confines the water vapor, thus preventing the loss of water. The only escape for water vapor from a circuit that operates under pressure is through a small overflow pipe.

TEMPERATURE REGULATION

The temperature of a liquid in the cooling system of an engine must be regulated to main-tain normal operating conditions. One of the principal factors affecting the cooling of an engine is the rate of flow of water through the cooling system. The high flow rate of the water causes the heat to be carried away more quickly. As the velocity of the circulating water is reduced, the discharge temperature of the cooling water becomes higher and more heat is carried away by each gallon of cooling water circulated. As the rate of circulation is increased, each gallon of cooling water carries away less heat and the discharge temperature of the cooling water drops. Our discussion will now cover the devices that control the engine coolant direction of flow and, therefore, the temperature. These devices can be manually operated or thermostatically operated.