Abstract: An air-cooled internal combustion engine having a cooling air blower which is driven by a hydrodynamic coupling from the internal combustion engine. The hydrodynamic coupling is thermostatically regulated or controlled by oil flow supplied thereto, as a result of a filling thereof as a function of the temperature of a structural port, or of the exhaust gas, or of the heated cooling air. In order to adapt to the atmospheric air pressure, the oil filling of the hydraulic coupling is additionally supplied with oil via an additional filling line controlled by a barometric diaphragm and a regulator, with the hydrodynamic coupling being dimensioned larger for receiving this additional filling quantity.