Air cooling is usually employed for small capacity engine like scooters, motor cycles or in aircraft applications where the weight of the system is main criteria or in some cases of industrial applications.

The air cooling is achieved by forcing the air over the cylinder and cylinder head.

The heat transfer rates from the cylinder to air depend upon the velocity of air relatives to metal surfaces and the heat transfer coefficient between metal and air.

Since the heat, transfer coefficients are relatively low, the cylinder wall temperatures achieved are higher for air cooled engines compared to water cooled engines.

Since the wall temperatures are needed to be kept upto a limit of 250C for efficient operation of the engines, it is necessary to increase the heat transfer rates either by increasing the relative the relative velocity of air or the surface area of metal exposed to surroundings or by both methods.

In case of two wheelers and aircrafts, the high air velocities resulting from their motion it taken to advantages while in case of multi cylinder air cooled engines used for cars the air velocities are increased. With the help of a fan driven by the engine crank-shaft.

Cooling Fins:

In order to increase the surface area of the fins are provided on metal surfaces exposed to cooling are. Cooling fins are either cast integral with cylinder and cylinder head or they may be fixed separately with the cylinder block.

The various types of cooling fins are shown in fig.

The length, spacing and thickness of the fins are represented by l, s, and t respectively.

The heat transfer capacity of the system depends upon the temperature difference between cylinder and the cross-section of the fin and its length.

The heat is gradually dissipated from the root of the fin upto its tip and to the surroundings air, due to this is a temperature gradient exists along its length from the root to the tip.

At the tip of the fin the heat transfer capacity becomes minimum due to least temperature difference and hence it is less efficient. Therefore, the thickness at the tip can be reduced due to decrease quantity of heat flowing at this point.

It is found that the rectangular fins are less efficient and heavier compared to trapezoidal or triangular fins.

The heat transfer rates can be increased by increasing the surface area i.e., by increasing the number of finds. However, the increased number of finds will reduce the spacing of between them which results into decreased heat transfer rates due to interference of boundary layer.

A spacing of 2.5 to 5mm is usually kept for the fins of length 25 to 50 mm.

Baffles:

The Heat transfer rates from cylinder walls can also be increased considerably by providing baffles which the air through the spacing between fins at high velocity.

The best heat transfer rates are obtained with negligible kinetic energy at entrance and exit of air.

The entry of air to baffles is rounded to keep the energy loss at entrance to minimum.

Various types of baffles are shown in Fig.

The normal baffles are used for S.I. engines. In this types of baffles the kinetic energy of air leaving the baffles is high.

In order to reduce the exit K.E., the exit baffles the used in which the kinetic energy of air is converted into pressure energy during the flow of air in exit pipe. This also reduces the pressure losses. Short baffles are used for C.I. engine shown in Fig.

Advantages of Air Cooling System:

1. It is light in weight and simple in design due to absence of water jackets, radiator, Coolant and piping connections.

2. It is almost maintenance free.

3. No antifreeze solution is required and can operate at low temperatures.

4. No problem of coolant leakage, corrosion and clogging of radiators etc.

5. The system is cheap.

6. Installation is easy.

Disadvantages and Limitations of Air Cooling System:

1. System is noisy.

2. It limits the ratio of compression ratio.

3. Specific fuel consumption is high.

4. Cooling is not uniform. It may cause the distortion of cylinder.

5. Cylinder wall temperature are high.

6. Its use is limited to scooters and motor cycles due to exposure of cylinders to air stream.