From Wikipedia, but useful....
The blow-by gases are generated when a small but continual amount of gases (air, unburned fuel, combustion gases) leak from the
combustion chamber past the
piston rings, that is,
blow by them, and the piston ring gaps to end up inside the crankcase, causing pressure to build up in there. Additional sources of blow-by that contribute to this effect are gases leaking past the turbocharger shaft, the air compressors (if present) and in some cases the valve stem seals. The blow-by gases, if not ventilated, can condense and combine with the oil vapor present in the crankcase forming sludge or cause the oil to become diluted with unburned fuel, degrading its quality and decreasing its effective life. Additionally, excessive crankcase pressure can lead to engine oil leaks past the crankshaft seals and other engine seals and gaskets. Prolonged periods of oil leaks can starve the engine of oil and damage it in a permanent way. Therefore, it becomes imperative that a crankcase ventilation system is used. This allows the blow-by gases, consisting of the combustion products and the oil vapors, to be vented through a PCV (positive crankcase ventilation) valve out of the crankcase. There are three system architectures when the blow-by gas exits the crankcase. It can either enter the air inlet manifold (closed CVS), be vented freely in the atmosphere (open CVS) or be vented in the atmosphere through a filter (filtered open CVS).
