Radiators are heat exchangers used for cooling internal combustion engines, mainly in automobiles, and also in piston engine aircraft or any similar use of such an engine. The cooling is done by circulating a liquid called engine coolant around the engine block, where it is heated, then through a radiator where it loses heat to the atmosphere, and then returned to the engine. Engine coolant is usually water based, but may also be oil. It is common to employ a water pump to force the engine coolant to circulate, and also for an axial fan to force air through the radiator.
Radiators first used downward vertical flow, driven solely by a thermosyphon effect. Coolant is heated in the engine, becomes less dense, and so rises. As the radiator cools the fluid, the coolant becomes denser and falls. This effect is sufficient for low-power stationary engines, but inadequate for all but the earliest automobiles. All automobiles for many years have used centrifugal pumps to circulate the engine coolant because natural circulation has very low flow rates.
Automobile radiators are constructed of a pair of header tanks, linked by a core with many narrow passageways, giving a high surface area relative to volume. This core is usually made of stacked layers of metal sheet, pressed to form channels and soldered or brazed together. For many years radiators were made from brass or copper cores soldered to brass headers. Modern radiators have aluminum cores, and often save money and weight by using plastic headers. This construction is more prone to failure and less easily repaired than traditional materials.
A system of valves or baffles, or both, is usually incorporated to simultaneously operate a small radiator inside the vehicle. This small radiator, and the associated blower fan, is called the heater core, and serves to warm the cabin interior. Like the radiator, the heater core acts by removing heat from the engine. For this reason, automotive technicians often advise operators to turn on the heater and set it to high if the engine is overheating to assist the main radiator.
When the engine is cold, the thermostat is closed except for a small bypass flow so that the thermostat experiences changes to the coolant temperature as the engine warms up. Engine coolant is directed by the thermostat to the inlet of the circulating pump and is returned directly to the engine, bypassing the radiator. Directing water to circulate only through the engine allows the temperature to reach optimum operating temperature as quickly as possible whilst avoiding localised hot spots. Once the coolant reaches the thermostat’s activation temperature, it opens, allowing water to flow through the radiator to prevent the temperature rising higher.
This is usually a result of air in the radiator which has risen to the top and is stopping any more hot water filling the radiator. It is cured by bleeding off the air from the top of the radiator by opening the bleed nipple a small square peg at the top of the radiator, usually protected by a round cast in the radiator body and allowing the air to hiss out.
Hold a cloth close to the bleed nipple and when water starts to come out the air should have gone.
If your radiators need constant bleeding then too much air is entering the system and the problem is a bigger one requiring a central heating engineer to have a look.