In an internal combustion engine, a crankcase ventilation system (CVS) is a one way, pressuresensitive passage which allows the natural build up of gases to escape from the crankcase in a controlled manner.. Blowby, as it is often called, is the result of combustion material from the combustion chamber "blowing" past the piston rings and into the rotating assembly's housing.
Crankcase ventilation system Wikipedia
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