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 (fig. 6-1).
Several fundamental principles of cooling are embodied in this type of engine cooling. The
rate of the cooling is dependent upon the following:
area exposed to the cooling medium
heat conductivity of the metal used and the volume of the metal or its size in cross
amount of air flowing over the heated surfaces
difference in temperature between the exposed metal surfaces and the cooling air
Some heat, of
course, must be retained for efficient operation. This is done by use of thermostatic
controls and mechanical linkage, which 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 increased expansion. Also,
lubricating oil of a higher viscosity is generally required.
engines the cylinders are mounted independently to the crankcase so an adequate volume of
air can circulate directly around each cylinder, absorbing heat and maintaining cylinder
head temperatures within allowable limits for satisfactory operation (fig. 6-2).
In all cases, the cooling action is based on the simple principle that the surrounding air
is cooler than the engine. The main components of an air-cooled system are the fan,
shroud, baffles, and fins. A typical air-cooled engine is shown in figure 6-3.
All stationary air-cooled engines must have a fan or blowers of some type to circulate
a large volume of cooling air over and around the cylinders. The fan for the air-cooled
engine shown in figure 6-3
is built into the flywheel. Notice that the shrouding, or cowling, when assembled will
form a compartment around the engine so 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 results in
sufficient airflow over the engine for adequate cooling.
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 the
airflow. Baffles are usually made of light metal and are semicircular, with one edge in
the air stream, to direct the air to the back of the cylinders.
air-cooled engines use thin fins that are raised projections on the cylinder barrel and
head (fig. 6-3).
provide more cooling area or surface and aid in directing airflow. 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.
the Air-cooled System
You may think that because the air-cooled system is so simple it requires no
maintenance. Many mechanics think this way and many air-cooled engine failures occur as a
result. Maintenance of an air-cooled system consists primarily of keeping cooling
components clean. Clean components permit rapid transfer of heat and ensure that nothing
prevents the continuous flow and circulation of air. To accomplish this, keep fans,
shrouds, baffles, and fins free of dirt, bugs, grease, and other foreign matter. The
engine may look clean from the outside, but what is under the shroud? An accumulation of
dirt and debris here can cause real problems; therefore, keep this area between the engine
and shroud clean.
cause a problem. Sometimes a mechanic will reduce the efficiency of the cooling system by
the careless use of paint. The engine may look good but most paints act as an insulator
and hold in heat. In addition to keeping the cooling components clean, you must inspect
them each time the engine is serviced.
repair any broken or bent parts. Check the fins for cracks or breaks. When cracks extend
into the combustion chamber area, the cylinder barrel must be replaced.
Now that we
have studied the simplest method of cooling, lets look at the most common, but also
the most complex system.