the surrounding air is cooler than the "heat engine."
AIR-COOLING SYSTEMS
The main components of an air-cooled system are the
The simplest type of cooling is the air-cooled, or
fan, shroud, baffles, and fins. A typical air-cooled
direct, method in which the heat is drawn off by
engine is shown in figure 5-1.
moving air in direct contact with the engine. Several
Fan and Shroud
fundamental principles of cooling are embodied in this
type of engine cooling. The rate of the cooling is
All stationary air-cooled engines must have fans or
dependent upon the area exposed to the cooling
blowers of some type to circulate a large volume of
medium, the heat conductivity of the metal used, the
cooling air over and around the cylinders. The fan for
volume of the metal or its size in cross section, the
the air-cooled engine shown in figure 5-1 is built into
amount of air flowing over the heated surfaces, and the
the flywheel. Notice that the shrouding or cowling,
difference in temperature between the exposed metal
when assembled, will form a compartment around the
surfaces and the cooling air. Some heat, of course,
engine so that the cooling air is properly directed for
must be retained for efficient operation. This is done
effective cooling. Air-cooled engines, such as those
by use of thermostatic controls and mechanical
used on motorcycles, do not require the use of fans or
linkage, which open and close shutters to control the
shrouds because their movement through the air results
volume of cooling air. You will find that air-cooled
in sufficient airflow over the engine for adequate
engines generally operate at a higher temperature than
cooling.
liquid-cooled engines, whose operating temperature is
largely limited by the boiling point of the coolant used.
Baffles and Fins
Consequently, greater clearances must be provided
between the moving parts of air-cooled engines to
In addition to the fan and shroud, some engines use
allow for the increased expansion. Also, lubricating oil
baffles or deflectors to direct the cooling air from the
of a higher viscosity is generally required.
fan to those parts of the engine not in the direct path of
In air-cooled engines, the cylinders are mounted
airflow. Baffles are usually made of light metal and are
independently to the crankcase so that an adequate
semicircular with one edge in the air stream so that the
volume of air can circulate directly around each
air can be directed to the back of the cylinders. Most
cylinder, absorbing heat when passing the cylinders
air-cooled engines use fins. These are thin, raised
and maintaining cylinder head temperatures within
projections on the cylinder barrel and head. (See figure
allowable limits for satisfactory operation. In all cases,
5-1.) The fins provide more cooling area or surface and
the cooling action is based on the simple principle that
a i d i n d i r e c t i n g a i r f l ow. H e a t r e s u l t i n g f r o m
2
3
1
1
4
1
ASf05001
1. Sheet metal shrouding
3. Finned cylinder barrel
2. Finned cylinder head
4. Flywheel fan
Figure 5-1.--Air-cooled engine.
5-2
