Internal combustion engine

A gaseous fuel internal combustion engine (100 Fig. 1) has a crankcase 104 and an associated ventilation system 110. The ventilation system has a gas pressure source 114 that in use transports ventilation gas from a gas inlet 140a to the crankcase via a ventilation path 116. The internal combustion engine can have an engine structure that includes a cylinder head 106 coupled to one or more cylinders 112. The ventilation gas is transported along the ventilation path to a separate outlet 140b from the gas inlet to the cylinder head and then to the crankcase, and transports to the outlet expelled gas which is a combination of the ventilation gas and a blow-by gas 105 that includes the gaseous fuel from the crankcase. In use, the gas pressure source can induce a below-atmospheric pressure in the crankcase. The gas pressure source may be a form of pump or a blower, or a turbocharger or a compressor, for example. The gas pressure source may be downstream of the crankcase and the ventilation system may include an oil pre-separator 128 upstream of the gas pressure source to reduce a concentration of oil in the expelled gas from the crankcase. The arrangement allows purging of gases in the crankcase and/or engine.