The crankcase is, the portion of the engine block, below the cylinders, where the crankshaft is located.
For crankcase emission to work, leaked combustion gases are combined with air. Then are returned to the intake manifold, for reburning in the combustion chamber.
The device that performs this function, is known as the positive crankcase ventilation valve, or (PCV) valve.
The problem of crankcase ventilation is encountered since the beginning of the automobile because; no piston ring, new or old, can have a perfect seal between the piston and the cylinder wall. During running of the engine, the pressure of combustion forces the piston downward; and also the chamber gases past the piston rings and into the crankcase. These outlets are called crankcase vapors, or blow by.