The components of most air cooling systems are quite simple. A fan or blower is installed in air cooled engine to improve air flow over the fins. Most engines that use blower fans also use shrouds or ducts to make sure the cooled air flows over the fins. There are 2 classes of fan used in air cooled engine which are
1. RADIAL-FLOW TYPE : Air is flung outwards by centrifugal force.
2.AXIAL-FLOW TYPE: Air is pushed along parallel to the axis of fan spindle.
The former fan is more compact for a given output ( but tends to be noisy ). So it is used in small engines. The latter fan is more consistent, reliable and delivers large quantities of air so it is used for heavy duty high output engines.Amount of air that is circulating between cowling and cylinders can be regulated by a throttle ring which is located on inlet side of fan. The function of this throttle ring is to vary the effective inlet passage(exposed area) of the fan to suit the operating conditions of the engine. This can be achieved automatically by incorporating a thermostat in a hot working region of the engine so that it senses the temperature change.
When the engine is cold, the thermostat activates the hydraulic servo connected to the throttle ring to resist the air flow to the fan. When the engine is hot the restriction is removed thus allowing more air to circulate.