PCV System Description - Overview
Some of the unburned gases in the combustion chamber will leak past the gap between the pistons and cylinders into the crankcase. The blow-by gases that include the unburned gases are returned through the positive crankcase ventilation or PCV valve and are burned. This prevents the blow-by gases from flowing into the atmosphere.
In addition, the breather chamber of the cylinder block separates oil from blow-by gas, which prevents the oil from entering the combustion chamber.