Internal combustion engines must match several requirements such as good efficiency and low fuel consumption rate; when they are applied on scooter they are subject to some other restrictions. Nowadays, both low pollutant emissions and low noise level are requested for this engine since scooters are usually city vehicles. To match these requirements several aspects must be investigated: one of these may be the cooling system. There are usually three cooling methods, i.e. free stream air cooling, liquid cooling and forced air cooling. The first one is usually not employed in scooter engines because of its arrangement inside the scooter body (due to functionality and aestheticism). The second one may present some plant complications caused by the heat exchanger and ducts. A forced air cooling system presents usually lower complication, lower weight and greater reliability. Nevertheless, in order to keep engine temperatures below lubricant and structural limit, high mass flow rate may be necessary since air has smaller coolant efficiency compared to liquids. Moreover cooling air, supplied by a fan, requires high pumping power which may be excessive at high rotational speed; the fan itself may produce excessive noise reducing comfort. Sometimes, it may be hard to define the air flow demands in order to properly cool the critical parts (i.e. cylinder head); poor design may result in an excessive air mass flow demand and high pressure losses. Consequently the fan requires an excessive power and emits high noise level. Proper coolant distribution around the cylinder and the engine head reduces the overall air mass flow demand, rising indirectly engine efficiency. Usually the geometry of a forced air cooled engine is quite complex because of fins and other internal passages. To study coolant distribution and heat transfer a three-dimensional approach is then required. Computational fluid dynamic calculations, provided by commercial codes, can give useful suggestions about flow distribution around a finned cylinder. This paper will show an analysis of a typical air cooled scooter engine. Air mass flows and cooling efficiency are shown at several engine rotational speeds.

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