Radiator:
We all know what the
radiator does. But dissipating heat is not the easiest of jobs, particularly here in Australia, and ensuring the temperature is kept a little above boiling point has proved problematic for many foreign makes that have travelled our roads. The science or art of getting the temperature of an engine correct is set partly by the size of the
radiator and partly by the pressure valve in the filler cap. Together, these two components have to control the engine temperature over a wide range of working conditions, in conjunction with the thermostat.
Water enters the radiator having taken the heat away from the engine. The water becomes extremely hot in the process, so the radiator has to take the heat out of the water. It does this by channeling it through a series of fine tubes. Air flows past these tubes as the car moves along, thus cooling the water inside.
The
radiator has three main components: two tanks and the central 'core'. All three are made mainly of brass to improve heat dissipation. The tanks are mounted above and beneath the core, or on either side of the core in the case of cross-flow radiators. The upper, or 'header', tank is the larger of the two, and so allows room for the hot water to expand as it is pumped in from the engine. The radiator filler cap is fitted to the upper tank, and the bottom of the tank is pierced at the points where the tubes of the radiator core enter. The lower tank is similarly pierced, and the radiator drain tap is generally fitted to this tank.
There are two types of radiator cores in common use. The 'tubular' design consists of a number of round or oval tubes soldered into the tanks. The hot water runs down into these tubes from the header tank and air circulates between them. The tubes are made of thin brass sheet, often as fine as 0.125 mm (0.005in.) to encourage heat dissipation. This is also helped by wide copper fins that run at right angles to the tubes. Much of the heat from the water is conducted through the pipes into the fins. These spread the heat over a large area and expose it to a greater flow of air. The copper fins also protect the delicate pipes from minor knocks that might cause a leak.
The second, 'cellular', type of radiator core is constructed of pressed metal sections, soldered together to form a series of narrow passages. These also open into the radiator tanks. The main difference between this and the tubular type is that the spaces between the water passages are used to carry air. Cross-flow radiators operate on the same principle as the conventional radiator, but are simply tipped on one side, so that the water flow is across the radiator rather than down it. This design has come into greater use as car profiles have become lower. A low bonnet obviously demands a low radiator, which in turn means that the pipes in the core are short. The water inside is therefore exposed to the air-flow for a brief period. If the pipes run across the radiator rather than down it, the pipes will be longer and the cooling made more efficient.
The radiator pressure cap is an important part of the
cooling system, as it raises the boiling point of the water and therefore allows the
cooling system to absorb far more heat. At sea level, at atmospheric pressure, water boils at 100�C (212�F), but if it is confined under pressure the boiling point can be increased by 1.6�C (3�F) for every psi of pressure over atmospheric. This is the effect the radiator pressure cap has on the water. For example, if a 12psi pressure cap is fitted, the coolant will not boil until it has reached a temperature of 120�C (248�F). The danger of boiling during a long climb on a hot day is thus reduced, and the temperature at which the coolant will boil while driving at high altitudes is increased.
The filler cap has two important valves. The first holds the water under pressure. As the water in the radiator header tank becomes hot, it expands and presses against this valve. The valve keeps the water in the header tank until the pressure of its expansion overcomes the spring controlling the valve. Once the water pressure has reached the point where it can raise the spring and open the valve, water surges out of the radiator and through the overflow pipe. On cars with a sealed
cooling system this pipe leads to the expansion tank, but on more conventional cars it simply directs the excess water on to the road. The pressure in the radiator then falls and the spring pushes the valve shut.
This spring therefore controls the extent to which the radiator is pressurized. If the spring is weak the water will boil at close to the usual temperature. If the spring is strong the boiling point will be much higher. These springs are carefully manufactured, and the pressure they produce in the radiator is usually marked on the filler cap. There is one complication to this otherwise simple system. When the water cools it contracts and creates a vacuum in the radiator. This vacuum could cause the radiator core to collapse under the pressure of the external air, and this is where the second valve in the cap comes in. Air enters the radiator through the valve in the filler cap, which is opened by the vacuum in the core. The air displaces the vacuum and raises the pressure in the radiator.
Engine baffles are fitted in the engine compartment of many cars. These baffles are simple deflectors designed to direct the air flow through the radiator and round the engine. On some engines, a shroud on the back of the radiator, usually surrounding the fan, causes the fan to draw the air through the total area of the radiator core. Some cars have baffles between the grille and radiator to ensure that the car's movement forces air past the radiator. Water pump
The cooled water leaves the lower radiator tank and runs through a hose to the water pump, which assists the flow of water through the
cooling system. The water will flow naturally on the thermosyphon principle, but this flow is not sufficient to cope with the heat generated by modern car engines. The pump is generally mounted on the front of the cylinder block. On some rear-engined water-cooled cars, however, the radiator, water pump and fan are separately mounted to the side of the engine. The Chrysler Imp is an example of this system.
Regardless of their location, most water pumps work in a similar way. The pump generally consists of a cast-iron or aluminium housing with ball-bearing races in which a shaft revolves. These bearings are sealed at each end. A carbon ring is fitted to the front of the bearing housing to prevent water seeping along the shaft. Another seal at the opposite end holds in the lubricating grease and keeps out the dirt and grit that accumulates on the engine. A metal or plastic impeller is fitted to the front of the shaft. It is a small disc fitted with a number of vanes which may be either curved or straight. The impeller revolves at high speed, and water is trapped between the vanes and forced through the channel in the engine block. A pulley is fitted to the other end of the shaft and is driven, through the fan belt, by the crankshaft.
The driving speed of the water pump can cause problems in high performance engines. If the pump is driven, for example, at half the speed of the crankshaft the water circulation will probably be inadequate when the car is pulling hard in top gear. On the other hand if the ratio is stepped up to equal engine speed very serious problems can develop at peak revolutions. The suction at the inlet to the pump can become so great as to cause cavitations. This means that a pocket of steam is created in the suction area, which interrupts the flow of water through the pump. A large and expensive engine seizure is the usual consequence of cavitations, so the driving speed of the water pump has to be calculated very carefully.
