Nearly all multicylinder engines used in automotive, construction, and material handling equipment use a liquid-cooled system, any liquid used in this type of system is called a 'Coolant'.

A simple liquid-cooled system consists of a radiator, coolant pump, piping, fan, thermostat, and a system of water jackets and passages in the cylinder head and block through which the coolant circulates. Some vehicles are equipped with a coolant distribution tube inside the cooling passages that directs additional coolant to the points where temperatures are highest. Cooling is accomplished by keeping the coolant circulating and in contact with the metal surfaces to be cooled. The operation of a liquid-cooled system is as follows:

1. The pump draws the coolant from the bottom of the radiator, forcing the coolant through the water jackets and passages, and ejects it into the upper radiator tank.
2. The coolant then passes through a set of tubes to the bottom of the radiator from which the cooling cycle begins.
3. The radiator is situated in front of a fan that is driven either by the water pump or an electric motor. The fan ensures an airflow through the radiator at times when there is no vehicle motion.
4. The downward flow of coolant through the radiator creates what is known as a thermo-siphon action. This simply means that as the coolant is heated in the jackets of the engine, it expands. As it expands, it becomes less dense and therfore lighter. This causes it to flow out of the top outlet of the engine and into the top tank of the radiator.
5. As the coolant is cooled in the radiator, it again becomes more dense and heavier. This causes the coolant to settle to the bottom tank of the radiator.
6. The heating in the engine and the cooling in the radiator therefore create a natural circulation that aids the water pump.

The amount of engine heat that must be removed by the cooling system is much greater than is generally realized. To handle this heat load, it may be necessary for the cooling system in some engines to circulate 4,000 to 10,000 gallons of coolant per hour. The water passages, the size of the pump and radiator, and other details are designed as to maintain the working parts of the engine at the most efficient temperature within the limitation imposed by the coolant.

The Radiator

In the cooling system, the radiator is a heat exchanger that removes the heat from the coolant passing through it. The radiator holds a large volume of coolant passing through it. The radiator holds a large volume of coolant in close contact with a large volume of air so heat will transfer from the coolant to the air. The components of a radiator are as follows:

• Core - The centre section of the radiator made up of tubes and cooling fins.
• Tanks - The metal or plastic ends that fit over core tube ends to provide storage for coolant and fittings for the hoses.
• Filler Neck - The opening for adding coolant. It also holds the radiator cap and overflow tube.
• Oil Cooler - The inner tank for cooling automatic transmission or transaxle fluid.
• Petcock - The fitting on the bottom tank for draining coolant.

A tube-and-fin radiator consists of a series of tubes extending from top to bottom or from side to side. The tubes run from the inlet tank to the outlet tank. Fins are placed around the outside of the tubes to improve heat transfer. Air passes between the fins. As the air passes by, it absorbs heat from the coolant. In a typical radiator, there are five fins per inch. Radiators used in vehicles with air conditioning have seven fins per inch. This design provides the additional cooling surface required to handle the added heat load imposed by the air conditioner.

Radiators are classified according to the direction the coolant flows through them. the two types of radiators are downflow and crossflow.

• The downflow radiator has the coolant tanks on the top and bottom and the core tubes run vertically. Hot coolant from the engine enters the top tank. The coolant flows downward through the core tubes. The coolant flows downward through the core tubes. After cooling, coolant flows out the bottom tank and back into the engine.
• The crossflow radiator is a design that has tanks on the sides of the core. The core tubes are arranged for horizontal coolant flow. The tank with the radiator cap is normally the outer tank. A crossflow radiator can be shorter, allowing for a lower vehicle bonnet.

The operation of the radiator is as follows:

• The upper tank collects incoming coolant and, through the use of an internal baffle, distributes it across the top of the core.
• The core is made up of numerous rows of small vertical tubes that connect the upper tank and the lower tank. Sandwiched between the rows of tubes are thin sheet metal fins. As the coolant passes through the tubes to the lower tank, the fins conduct the heat away from it and dissipate this heat into the atmosphere. The dissipation of the heat from the fins is aided by directing a constant air flow between the tube and over the fins.
• The lower tank collects the coolant from the core and discharges it to the engine through the outlet pipe.
• The overflow tube provides an opening from the radiator for escape of coolant if the pressure in the system exceeds the regulated maximum. This will prevent rupture of cooling components.

A transmission oil cooler is often placed in the radiator on vehicles with automatic transmissions. It is a small tank enclosed in one of the main radiator tanks. Since the transmission fluid is hotter than engine coolant, heat is removed from the fluid as it passes through the radiator and cooler.

In downflow radiators, the transmission oil cooler is located in the lower tank. In a crossflow radiators, it is located in the tank having the radiator cap. Both tanks are coolant outlet tanks.

Line fittings from the cooler extend through the radiator tank tot he outside. Metal lines from the automatic transmission connect to these fittings. The transmission oil pump forces the fluid through the lines and cooler.

Radiator Hoses

Radiator hoses can carry coolant between the engine water jackets and the radiators. Being flexible, hoses can withstand the vibration and rocking of the engine without breaking. The upper radiator hose normally connects to the thermostat housing on the intake manifold or cylinder head, the other end of the hose fits on the radiator. The lower hose connects the water pump inlet and the radiator. A molded hose is manufactured into a special shape with bend to clear parts. It must be purchased to fit the exact year, make and model of the vehicle. A flexible hose has an accordion shape and can be bent to different angles. The pleated construction allows the hose to bend without collaping and blocking coolant flow. It is also known as a universal type radiator hose.

A hose spring is used in the ower radiator hose to prevent it collapse. the lower radiator hose is exposed to suction from the water pump. The spring assures the innner lining of the hose does NOT tear away, close up, and stop circulation.

Radiator Pressure Cap

The radiator presure cap is used on nearly all of modern engines. The radiator cap locks onto the radiator tank filler neck. Rubber or metal seals make the cap-to-neck joint artright. The functions of the pressure cap are as follows:

1. Seals the top of the radiator tiller neck to prevent leakage.
2. Pressurises system to raise boiling point of coolant.
3. Relieves excess pressure to protect against system damage.
4. In a closed system, it allows the coolant flow into, and from the coolant resevoir.

The radiator cap pressure valve consists of a spring loaded disc that contacts the filler neck. The spring pushed the valve into the neck to form a seal. Under pressure, the boiling point of water increases. Normally water boils at 212°F. However for every pound of pressure increase, the boiling point increases by 3°F.

Typical radiator cap pressure is 12 to 16 psi. This raises the boiling point of the engine coolanr ro about 250°F to 260°F. Many surfaces inside the water jackets can be above 212°F.

If the engine overheats and the pressure exceeds cap rating, the pressure valve opens. Excess pressure forces coolant out of the overflow tube and into a resevoir or onto the ground, this prevents high pressure from rupturing the radiator, gaskets, seals, or hoses.

The radiator cap vacuum valve opens to allow reverse flow back into the radiator when the coolant temperature drop after engine operation. It is a smaller valve located in the centre, bottom of the cap. The cooling and contraction of the coolant and air in the system could decrease coolant volume and pressure. Outside atmospheric pressure could then crush inward on the hoses and radiator. Without a cap vacuum or vent valve, the radiator hose and radiator could collapse.

CAUTION - Always remove the radiator cap slowly and carefully, using a rag wrapped around the cap. Removing the radiator cap from a hot, pressurised system can cause burns from escaping steam and coolant.

The Water Pump

The water pump is and impeller or centrifugal pump that forces coolant through the engine block, cylinder head, intake manifold, hoses, and radiator. It is driven by a fan belt running off the crankshaft pulley. The major parts or a typical water pump include the following:

• Water Pump Impeller - a disc with fanlike blades that spins and produces pressure and flow .
• Water Pump Shaft - steel shaft that transfers turning force from the hub to impeller.
• Water Pump Seal - prevents coolant leakage between pump shaft and pump housing.
• Water Pump Bearing - plain or ball bearing that allows the pump shaft to spin freely in the housing.
• Water Pump Hub - provides mounting place for the belt and fan.
• Water Pump Housing - iron or aluminium casting taht forms the main body of the pump.

The water pump normally mounts on the front of the engine, with some transverse (sideways) mounted engines, it may bolt to the side of the engine and extend towards the front. A water pump gasket fits between the engine and the pump housing to prevent coolant leakage. RTV sealer may be used instead of a gasket.

Operation of a Water Pump is as follows:

1. The spinning crankshaft pulley causes the fan belt to turn the water pump pulley, pump shaft, and impeller.
2. Coolant trapped between the impeller blades is thrown outward, producing suction in the central area of the pump housing.
3. Since the pump inlet is near the centre, coolant is pulled out of the radiator, through the lower radiator hose.
4. After being thrown outward and pressurised, the coolant flows into the engine. It circulates through the block, around the cylinders, up through the cylinder heads, and back into the radiator.