Automotive Systems

Formerly Automotive Systems I

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Air-Cooled 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 (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:

  • The area exposed to the cooling medium
  • The heat conductivity of the metal used and the volume of the metal or its size in cross section
  • The amount of air flowing over the heated surfaces
  • The 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.

In air-cooled 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.

Fan and Shroud
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.

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 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.

Most air-cooled engines use thin fins that are raised projections on the cylinder barrel and head (fig. 6-3).

The fins 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.

Maintaining 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.

Paint can 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.

Replace or 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, let’s look at the most common, but also the most complex system.

Figure 6-1.—Air-cooled system.

Figure 6-2.—Air-cooled cylinder.

Figure 6-3.—Air-cooled engine.

Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

Copyright 2001-2004 SweetHaven Publishing Services
All rights reserved