Automotive Cooling System
The internal combustion engine generates a large amount of heat that is not transformed into useful energy. To keep engine temperatures within a range in which the fuel will not preignite spontaneously, and to prevent any destructive buildup of engine heat, some type of cooling system must be used to dissipate excess heat.
The overwhelming majority of American automobile engines are water cooled. In this system relatively cool water is drawn from the bottom of the radiator and circulated by a small centrifugal pump through jacketed spaces in the cylinder head and block. Absorbing heat as it is pumped through the engine, the water is returned to the top of the radiator, where, as it passes down through the many small copper tubes of the radiator core, it transfers waste heat to the airstream passing through the radiator.
Mounted directly behind the radiator, the typical cooling fan has four to six blades (unevenly spaced to reduce noise) and is driven by a V-belt from the crankshaft. Its main function is to pull air through the radiator when the car is motionless or moving slowly. At road speeds above 45 miles per hour, the forward motion of the car forces enough air through the radiator to cool the engine adequately. Ideally, then, the cooling fan should absorb no engine horsepower at high road speeds. In one fan design a viscous fluid clutch inside the fan hub automatically regulates fan speed in response to temperature changes in the air passing through the radiator core.
Optimum engine operating temperatures (160° -180° F or 71°-82°C) are maintained by a thermostat. This device responds to temperature changes in the engine water jackets, restricting the flow of water discharged into the radiator until the engine is up to normal operating temperature.
A pressure cap on the radiator increases cooling-system capacity by raising the temperature point at which the water boils. In this sealed system the expansion of water as it absorbs heat from the engine puts the entire cooling system under pressure. Because the boiling point of water is raised about 3° F for every pound per square inch (1.7°C for every kg/cm2) of pressure above atmospheric, pressurizing the system increases its heat capacity. Pressure caps are variously calibrated to maintain a pressure of 12 to 17 pounds per square inch.
Water expands as it freezes; therefore, an antifreeze solution must be added to the cooling system to prevent cracking the cylinder block, head, or other components at temperatures below 32° F (0° C). Nonevaporative ethylene glycol, as well as less expensive but faster-evaporating solutions of alcohol or methanol, afford good protection against damage from freezing. Chemical inhibitors usually are added to retard corrosion.
In the air-cooled system, waste heat is rejected to the atmosphere by circulating a large volume of air over the engine. Built-in cooling fins on the exterior surfaces of the cylinder block and head increase the surface area exposed to the cooling air. Heat transfer is further improved by making the cylinder block and head of aluminum, an excellent conductor of heat.
A circulating fan, driven by a V-belt from the crankshaft, directs air to the engine through sheet-metal ducts. Engine temperature is controlled by a thermostat-operated damper that regulates the volume of cooling air blown over the engine fins. In addition, the engine-lubricating oil is cooled by circulating it through a small air-cooled heat exchanger.