Cooling System Operation

  The cooling system，shown in Fig. 6. 1，is made up of the passages inside the engineblock and heads, a water pump to circulate the coolant，a thermostat to control thetemperature of the coolant，a radiator to cool the coolant，a radiator cap to control thepressure in the system，and some plumbing consisting of interconnecting hoses to transferthe coolant from the engine to radiator and also to the car's heater system where hotcoolant is used to warm up the vehicle's interior on a cold day.

    A cooling system works by sending a liquid coolant through passages in the engineblock and heads. As the coolant flows through these passages，it picks up heat from theengine. The heated fluid then makes its way through a rubber hose to the radiator in thefront of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooledby the air stream entering the engine compartment from the grill in front of the car. Oncethe fluid is cooled, it returns to the engine to absorb more heat. The water pump has thejob of keeping the fluid moving through this system of plumbing and hidden passages.

    A thermostat is placed between the engine and the radiator to make sure that thecoolant stays above a certain preset temperature. If the coolant temperature falls belowthis temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluidinstead through a bypass directly back to the engine. The coolant will continue to circulatelike this until it reaches the design temperature at which point the thermostat will open avalve and allow the coolant back through the radiator.

    In order to prevent the coolant from boiling, the cooling system is designed to bepressurized. Under pressure，the boiling point of the coolant is raised considerably.However, too much pressure will cause hoses and other parts to burst，so a system isneeded to relieve pressure if it exceeds a certain point. The job of maintaining the pressurein the cooling system belongs to the radiator cap. The cap is designed to release pressure ifit reaches the specified upper limit that the system was designed to handle. Prior to the1970s，the cap would release this extra pressure to the pavement. Since then, a systemwas added to capture any released fluid and store it temporarily in a reserve tank. Thisfluid would then return to the cooling system after the engine cools down. This is what iscalled a closed cooling system.