In reciprocating internal combustion engines, the gas that leaks at the piston, rings and liner system is usually called blow-by. The blow-by is a complex mixture of air, burned and unburned gases and oil mist. In order to avoid external pollution, the blow-by is recycled in the air intake system. This is called Closed Crankcase Ventilation (CCV). The CCV is the cause of major issues as air intake system fouling, oil consumption and participation to the exhaust catalyst poisoning phenomena. During recent decades a quite simple oil separation system based on baffles was acceptable. Emissions regulations are now tougher and a more efficient blow-by separation system is required.A suitable and efficient means to achieve a good separation is to use the coalescence principle through a specific media. The coalescing separator is a good compromise between efficiency and pressure drop.To respect the majority of car manufacturers' specifications, the most interesting technical solution is to use a rotating coalescing separator. Thanks to this solution, the crankcase pressure is negative for all engine's working conditions and the oil mist separation efficiency is very high, even for a thin oil mist generated at max torque and max power. This solution requires a device to rotate the coalescing separator (electrical engine, a link with the camshaft or with the crankshaft…).