Heavy vehicle engines are usually multi-cylinder in-line types which are difficult to cool uniformly by an air cooling system. For this reason, air cooling is seldom use. In the few engines where air cooling is used however, a fan and a cowling Director air to the hotter parts of the engine where cooled air is most required, but this equipment is bulky and the fan is power absorbing.
The absence of a sound muffling coolant jacket causes noisy engine operation and the possibility of higher running temperatures when compared with liquid cooled engines, and this often lead to larger bearing clearances being used which further contributes to engine noise, in addition to lost of power. Variations in engine running temperature are difficult to control with air cooling and lead to temperature variations between different parts of the engine causing possibility of distortion. Aluminum alloys are often used for the engine components as they are better heat conductor than cast iron and steel and thin cooling fins are employed to increase the surface area of the cylinder.
The main advantages of air cooling when compared with liquid cooling are due to absence of a coolant. Periodic checking of the coolant level and topping up is eliminated. No antifreeze, corrosion inhibitors or sealing compounds are required, and the possibility of leaks is eliminated. Engine warm-up is quicker with with air cooling than with liquid cooling, and the simplicity of the system reduces maintainance and the risk of vehicle breakdowns caused by a defective cooling system.