Abstract: An internal combustion engine includes at least one combustion chamber and an inlet conduit whereby air for combustion of fuel is introduced into the combustion chamber. The inlet conduit has a main inlet and an auxiliary inlet for introducing main and auxiliary air flows respectively into the inlet conduit. Filtered and heated air is passed through a humidifier and a preheater to an air suction control valve connected between the preheater and the auxiliary inlet to provide the auxiliary air flow. The air suction control valve operates in response to increase in pressure in the combustion chamber to reduce the flow of air from the preheater to the auxiliary inlet.