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COOLING SYSTEM
The cooling system keeps the engine at its most efficient operating temperature under all driving conditions. The burning of the fuel-and-air mixture may create temperatures of 4,500° in the cylinders during the combustion stroke. The cylinder heads, cylinder walls, pistons, and valves which absorb some of this heat must be cooled. If the heat becomes excessive, the lubricating oil film will break down, lose its lubricating properties, and destroy these components. However, if an engine runs too cool, its efficiency is reduced.
There are two types of cooling systems, air-cooled and liquid-cooled, but since the car engine generally uses the liquid-cooled system, that is the only one which will be discussed in this chapter.
The cooling system consists of a water pump used to circulate the coolant throughout the system, a fan, a radiator, water jackets, a thermostat, a radiator pressure cap, and various hoses.
The water pump is belt-driven by the crankshaft and circulates the coolant through the engine block and radiator. Mounted between the radiator and the block, the pump consists of a pump body, an im-peller, a shaft and bearing, a fan pulley hub, and seals. The pump body, with an inlet and an outlet, houses the shaft, seals, and im-peller. The impeller, mounted on a shaft, has curved vanes or blades which when turned throw the coolant outward by centrifugal force. The coolant is forced through the outlet of the pump body and into the cylinder block. The pump body inlet is connected by a hose to the bottom of the radiator, and water from the radiator is drawn through the inlet to replace the water forced through the outlet by the pump. The coolant used in the cooling system is a mixture of water and antifreeze. The amount of antifreeze required depends upon the capacity of the cooling system and local weather conditions.
Water jackets cast into the engine block and cylinder heads pro-vide a path for the coolant to flow around the cylinder walls and through the cylinder heads to cool the engine.
The engine fan is mounted on the water pump shaft hub. There are several types of fans in use today;the standard fan, variable pitch blade fans, fluid-coupling fans, and thermostatically-controlled fans. Regardless of the design, the purpose of the fan is to provide a pow-erful draft of air through the radiator to cool the liquid. Some manu-facturers use a shroud surrounding the fan to improve its ability to draw air through the radiator.
The radiator, designed to hold a large quantity of coolant in close contact with a large volume of air so that the heat of the coolant will be transferred into the air, consists of tubes, cooling fins, an upper tank, and a lower tank. In some cars, the lower radiator tank houses a transmission oil cooler to aid in cooling the transmission fluid.
The thermostat, mounted in a housing between the engine block and the radiator inlet or upper hose, controls the flow of water be-tween the radiator and the block. When the engine is cold, the ther-mostat prevents flow to the radiator, allowing the coolant in the en-gine block to heat up more rapidly. As the engine heats up, the
thermostat opens, allowing the coolant to flow to the radiator. The temperature at which the thermostat opens is predetermined by the engine manufacturer, for example, 160°, 180°, or 190°.
The radiator pressure cap is designed to make the cooling system more efficient. As the cap increases the pressure in the cooling system, the boiling point of the coolant is raised. For every one pound of pressure exerted by the cap, the boiling point of the coolant is increased approximately three degrees. That is, a 15-pound pressure cap will increase the boiling point of the coolant by 45°. The coolant can thus reach a higher temperature before boiling. This increase in temperature allows the engine to operate more efficiently.
The by pass hose is either a straight or curved hose connecting the cylinder head and the block or the intake manifold and the block. The hose allows water to circulate between the cylinder head and the block when the engine is cold and the thermostat closed. This by pass system allows the engine to heat up more rapidly and evenly, preventing hot spots in the block.
In this chapter you will learn how to inspect, test, and replace all these components of the cooling system.
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