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Cooling system

Home Motoring About my car Car components Cooling system

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Cooling system - functions

• As the engine runs and the combustion process begins.
• The heat in the combustion chamber during combustion is approximately 1900 degrees Celsius.
• This heat is transferred to the cylinder head, cylinder walls, valves and the pistons - these parts heat up very quickly and expand.
• Without some form of cooling, this expansion becomes critical as the pistons begin to touch the cylinder walls and eventually seize in the cylinder bores.
• The result will be catastrophic damage to the engine.
• When the engine runs cold it is inefficient, pollution is at its maximum because of the need for rich fuel mixtures and engine wear is high due to friction.
• The engine is designed to be most efficient when running at its optimum temperature.
• The job of the cooling system is to accommodate the transfer of heat from the engine and maintain the optimum operating temperature.

Components

• Engine block and cylinder head:
  • The engine block and cylinder head are designed with passageways to allow coolant to pass through.
  • The passageways direct the coolant around the cylinder walls and the cylinder head near the valves where the temperatures are highest.
• Coolant:
  • The heat from the engine transfers to the coolant, which eventually goes to the radiator where it is cooled.
  • The coolant is a mixture of water, anti-freeze (usually ethylene glycol) and various additives to prevent corrosion.
  • The presence of ethylene glycol increases the boiling point of the fluid beyond that of water (100 Celsius).
  • With a 50/50 mixture of ethylene glycol and water the boiling point is increased by about six degrees Celsius.
  • Because of the extreme combustion temperatures this is not enough and putting the coolant under pressure increases the boiling point even further.
• Hoses:
  • The hoses simply allow the transfer of coolant between components.
• Radiator:
  • Most modern radiators are made with an aluminium centre sandwiched between two plastic tanks.
  • The centre or core contains a series of tubes, which allow the movement of coolant between the tanks.
  • The heated coolant flows through an inlet in the top tank and passes through the tubes, where it is cooled by air.
  • Between the tubes are aluminium fins, which help with the dissipation of heat.
  • The action of air blowing across the fins keeps them cool.
  • As the temperature decreases the coolant moves to the bottom of the radiator and back into the engine through the outlet in the bottom plastic tank.
  • In cars with automatic transmissions the bottom tank in the radiator houses a separate compartment for cooling the transmission fluid. 
• Radiator pressure cap:
  • As the coolant heats it expands and pressure builds up in the system. The radiator cap controls this pressure.
  • The cap is set to operate at a predetermined pressure.
  • When this is reached a valve in the cap opens and releases coolant through the overflow tube into the expansion tank reducing the pressure.
  • As the engine cools the coolant contracts in the radiator and creates a vacuum in the top tank.
  • This operates another valve in the cap and allows coolant to be drawn from the expansion bottle back into the radiator maintaining a constant coolant level.
• Expansion bottle:
  • This is a plastic bottle, which is connected to the radiator by an overflow tube.
  • The coolant level in the bottle fluctuates with changes in temperature.
  • The level rises as the coolant heats up and expands.
  • The level is best checked when the engine is cold and the coolant should be set on the low or cold level mark.
• Water pump:
  • The coolant is circulated through the engine by means of a belt driven water pump.
  • The pump is usually mounted in the engine block and connected to the radiator by a hose.
  • The coolant enters the pump through the hose and is then forced into the block by the vanes inside the pump.
• Cooling fan:
  • In order to keep the engine cool, air must pass through the radiator core.
  • When the vehicle is moving this is not a problem, but when the car is stationary assistance is needed.
  • The cooling fan provides this.
  • There are a number of types of fans; fixed, viscous and the most common type the electric fan.
    • The fixed fan has a set of blades attached to the water pump drive pulley, which rotate at the same speed as the pump.
    • The viscous or fluid coupling type spins at less than the water pump speed until the engine reaches a certain temperature and then the fluid in the coupling, which gets thicker when heated, causes the fan to rotate at the same speed as the pump. The advantage of this over the previous type is that when the engine is cold the fan is turning slowly and this allows the engine to heat up quickly.
    • The electric fan is a motor, mounted either in front or behind the radiator with a fan attached.
    • A thermostatic switch or the vehicle's computer switches on the electric fan, only when the engine reaches a predetermined temperature. This, like the previous type allows the engine to heat up quickly but it also controls the engine, more accurately, within a given temperature range. 
• Thermostat:
  • The most efficient engine from a fuel economy and pollution point of view is the one that reaches its normal operating temperature quickly and maintains a constant temperature.
  • The thermostat achieves this by blocking the flow of coolant from the engine to the radiator until the correct operating temperature has been reached.
  • Think of the thermostat as a tap.
    • The tap is closed when the vehicle starts from cold and coolant only circulates through the block and cylinder head, this allows the engine to heat up quickly.
    • When the coolant temperature rises above 83 degrees Celsius approximately, the tap begins to open and when the temperature reaches 93 degrees Celsius approximately, the tap opens completely allowing an unrestricted flow of coolant to the radiator.
• Heater:
  • The heating system is like a miniature version of the engine's cooling system.
  • It contains a small radiator, an electric fan and a heater tap, which regulates the flow of coolant through the heater.
  • When the heater tap is switched on (the hot position) hot coolant from the engine flows into the heater radiator, the electric fan or heater motor then blows air through the radiator.
  • This air is heated as it passes through and is blown into the cabin of the vehicle via the heater vents.