Internal combustion engine

A gaseous fuel internal combustion engine (100 Fig. 1) has a crankcase and an associated ventilation system 110. The ventilation system has a gas pressure source 114 and a ventilation path 116 to transport gases expelled from the crankcase. The expelled gas includes blow-by gas 105 that has gaseous fuel, and the ventilation system has a monitoring system 167 to monitor an actual parameter indicative of a flowrate of the expelled gas. The monitoring system provides one or more outputs, for example pressure and/or flow and/or an audio alert and/or a visual alert, based on the monitored actual parameter. Alternatively, a cyclonic oil separator (130 Fig. 18) forms a vortex 402 in a gas 400 travelling between an inlet 160 and an outlet 162. The separator induces a pressure drop between the inlet portion and the outlet portion, and the separator reduces the concentration of oil entrained in the gas flow presented to the inlet. The separator also includes a pressure take-off 164 at the inlet and a pressure take-off 166 at the outlet for a pressure transducer so that a pressure differential may be determined. The arrangement may ensure a sufficient purge of the crankcase so that crankcase gases are maintained outside of flammability limits.