Figure 1-48.-Thermostat operation.from the block to the pump when the passage to theradiator is blocked by the closed thermostat (fig. 1-48).Some stationary engines and large trucks areequipped with shutters that supplement the action of thethermostat in providing a faster warmup and inmaintaining proper operating temperatures. When theengine coolant is below a predetermined temperature,between 185°F to 195°F, the shutters, located in front ofthe radiator, remain closed and restrict the flow of airthrough the radiator. Then, as the coolant reaches propertemperature, the shutters start to open.AIR-COOLING SYSTEMThe simplest type of cooling is the air-cooled, ordirect method, in which the heat is drawn off by movingair in direct contact with the engine. The rate of thecooling is dependent upon the area exposed to thecooling air, the heat conductivity of the metal used, thevolume of the metal or its size in cross section, theamount of air flowing over the heated surfaces, and thedifference in temperature between the exposed metalOverflow TankThe overflow tank serves as a receptacle for coolantforced out of the radiator overflow pipe and provides forits return to the system. As the engine cools, thebalancing of pressures causes the coolant to siphon backinto the radiator.Cooling systems using an overflow tank are knownas closed cooling systems (fig. 1-49). Coolant is usuallyadded to this system through the overflow tank that ismarked for proper coolant level. NEVER remove theradiator cap located on the radiator unless you arepositive the system is cold. If there is any pressure in theradiator, it will spray you with hot steam and coolant.Use extreme caution when performing operator’smaintenance on a closed cooling system.Expansion TankSome engines use an expansion tank in their coolingsystem (fig. 1-50). The tank is mounted in series withthe upper radiator hose and is used to supply extra roomfor coolant expansion and generally takes the place ofthe upper radiator tank. The pressure cap and theoverflow line are also mounted on the expansion tank.Figure 1-49.-Closed cooling system.1-36
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