Radiator fluid, commonly called coolant, is responsible for several tasks in a vehicle. Cooling the vehicle's engine is only one function of radiator fluid. The automatic transmission in a vehicle has its hydraulic fluid cooled by the radiator fluid, and the engine's water pump is also lubricated by the flow of coolant through the pump. In the wintertime in cold-weather areas, the coolant also provides heat for the vehicle by flowing through a smaller radiator known as a heater core. As the engine heats up the fluid in the cooling system, a small fan blows through the heater core and carries the warm air into the passenger compartment of the vehicle.
While water can be used as radiator fluid, it is not recommended for typical use since it does not work as efficiently as manufactured coolant, commonly referred to as anti-freeze. Anti-freeze is manufactured to not only prevent the radiator fluid from freezing during cold winter weather, but it also absorbs heat. In doing so, it cools the engine much better than water alone.
In diesel engines, a heater is commonly installed in the engine block or the lower radiator hose to allow the radiator fluid to be kept warm while the engine is not running. This helps the engine to start easier in the cold weather by warming the oil and keeping it thin and runny. It also warms the combustion chambers to increase the starting efficiency of the engine.
In the hot weather of summer, the radiator fluid keeps the engine cool by passing through a radiator. The radiator is a series of cooling tubes or passages with a weave of thin metal, called fins, between the tubes. The coolant flows through the tubes while the heat in the coolant is dissipated into the fins.
The cooling fan pulls air from outside of the engine compartment through the radiator fins to cool the fluid. The radiator also has reservoirs on each side or the bottom, depending on the type of radiator. The lines from the automatic transmission send the transmission fluid to be cooled by the radiator fluid.
As the hot transmission fluid enters the radiator, the radiator coolant absorbs the heat and allows the transmission fluid to return to the transmission a great deal cooler than it left. The transmission fluid actually flows through a hard steel line running through the radiator tank in a U fashion and exits the radiator to be coupled with the fluid lines from the transmission. In some turbo- and supercharged engines, the air from the turbo or supercharger is passed through an inter-cooler, a type of radiator that uses radiator fluid to cool the incoming air charge and create more horsepower.