AIR-COOLING SYSTEM

The simplest type of cooling is the air-cooled, or direct method, in which the heat is drawn off by moving air in direct contact with the engine. The rate of the cooling is dependent upon the area exposed to the cooling air, the heat conductivity of the metal used, the volume of the metal or its size in cross section, the amount of air flowing over the heated surfaces, and the difference in temperature between the exposed metal

Figure 1-49.-Closed cooling system.

Figure 1-50.-Expansion tank.

Figure 1-51.-Air-cooled engine.

surfaces and the cooling air. Some heat must be retained for efficient operation. This is accomplished by the use of thermostatic controls and mechanical linkage that open and close shutters to control the volume of cooling air. You will find that air-cooled engines generally operate at a higher temperature than liquid-cooled engines, whose operating temperature is largely limited by the boiling point of the coolant used. Consequently, greater clearances must be provided between the moving parts of air-cooled engines to allow for the increased expansion.

In air-cooled engines, the cylinders are mounted independently to the crankcase so that an adequate volume of air can circulate directly around each cylinder. The circulating air absorbs excessive amounts of heat from the cylinders and maintains enough cylinder head temperatures for satisfactory operation. The cooling action is based on the simple principle that the surrounding air is cooler than the engine heat. The primary components of an air-cooled system are the fan and shroud and the baffles and fins. A typical air-cooled engine is shown in figure 1-51.

Fan and Shroud

All stationary air-cooled engines must have fans or blowers of some type to circulate a large volume of cooling air over and around the cylinder. The fan for the air-cooled engine shown in figure 1-51 is built into the flywheel. When the engine is assembled, the shrouding, or cowling, forms a compartment around the engine so that the cooling air is properly directed for effective cooling. Air-cooled engines, such as those used on motorcycles and outboard engines, do not require the use of fans or shrouds, because their movement through the air creates a sufficient air flow over the engine for adequate cooling.

Baffles and Fins

In addition to the fan and shroud, some engines use

baffles or deflectors to direct the cooling air from the fan to those parts of the engine not in the direct path of air flow. Most baffles are made of light metal and are semicircular with one edge in the stream of air. Most air-cooled engines use fins. These are thin, raised

projections on the cylinder barrel and head. The fins provide more cooling area or surface and aid in directing air flow. Heat, resulting from combustion, passes by conduction from the cylinder walls and cylinder head to the fins and is carried away by the passing air.