Cooling System Key Components

     Fluid

    Cars operate in a wide variety of temperatures，from well below freezing to well over 380C．So whatever fluid is used to cool the engine has to have a very low freezing point, ahigh boiling point, and it has to have the capacity to hold a lot of heat.

    Water is one of the most effective fluids for holding heat, but water freezes at toohigh a temperature to be used in car engines. The fluid that most cars use is a mixture ofwater and ethylene glycol (CzH6Oa），also known as antifreeze. By adding ethylene glycolto water, the boiling and freezing points are improved significantly.

    The cooling system uses pressure to further raise the boiling point of the coolant. lustas the boiling temperature of water is higher in a pressure cooker, the boiling temperatureof coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to15 psi，which raises the boiling point another 25C so the coolant can withstand the hightemperatures.

    Water pump

    The water pump is a simple centrifugal pump driven by a belt connected to thecrankshaft of the engine. The pump circulates fluid whenever the engine is running.

    The water pump uses centrifugal force to send fluid to the outside while it spins，causing fluid to be drawn from the center continuously. The inlet to the pump is locatednear the center so that fluid returning from the radiator hits the pump vanes. The pumpvanes fling the fluid to the outside of the pump，where it can enter the engine.

    The fluid leaving the pump flows first through the engine block and cylinder head，then into the radiator and finally back to the pump.

    Radiator

    A radiator is a type of heat exchanger. It is designed to transfer heat from the hotcoolant that flows through it to the air blown through it by the fan. Most modern cars usealuminum radiators. These radiators are made by brazing thin aluminum fins to flattenedaluminum tubes. The coolant flows from the inlet to the outlet through many tubesmounted in a parallel arrangement. The fins conduct the heat from the tubes and transferit to the air flowing through the radiator.

    The tubes sometimes have a type of fin inserted into them called a turbulator，whichincreases the turbulence of the fluid flowing through the tubes. If the fluid flows verysmoothly through the tubes，only the fluid actually touching the tubes would be cooleddirectly. The amount of heat transferred to the tubes from the fluid running through themdepends on the difference in temperature between the tube and the fluid touching it. So ifthe fluid that is in contact with the tube cools down quickly, less heat will be transferred.By creating turbulence inside the tube, all of the fluid mixes together, keeping thetemperature of the fluid touching the tubes up so that more heat can be extracted, and allof the fluid inside the tube is used effectively.

    Radiators usually have a tank on each side，and inside the tank is a transmissioncooler. In the picture above, you can see the inlet and outlet where the oil from thetransmission enters the cooler. The transmission cooler is like a radiator within a radiator，except instead of exchanging heat with the air, the oil exchanges heat with the coolant inthe radiator.

    Pressure cap

    The radiator cap actually increases the boiling point of your coolant by about 250C．The cap is actually a pressure release valve, and on cars it is usually set to 15 psi. Theboiling point of water increases when the water is placed under pressure.

    When the fluid in the cooling system heats up，it expands，causing the pressure tobuild up. The cap is the only place where this pressure can escape, so the setting of thespring on the cap determines the maximum pressure in the cooling system. When thepressure reaches 15 psi，the pressure pushes the valve open, allowing coolant to escapefrom the cooling system. This coolant flows through the overflow tube into the bottom ofthe overflow tank. This arrangement keeps air out of the system. When the radiator coolsback down, a vacuum is created in the cooling system that pulls open another springloaded valve，sucking water back in from the bottom of the overflow tank to replacing thewater that was expelled.

    Thermostat

    The thermostat's main job is to allow the engine to heat up quickly, and then to keepthe engine at a constant temperature. It does this by regulating the amount of water thatgoes through the radiator. At low temperatures，the outlet to the radiator is completelyblocked-all of the coolant is recirculated back through the engine.