Crankcase ventilation refers to the system involved in evacuating gases from the crankcase. In small engines, the gas is allowed to leave the crankcase by means of a ventilation pipe by natural draft. In bigger engines, a pump is used to aid the removal of gases.
Positive Crankcase Ventilation
Positive crankcase ventilation is a system used in automobiles to improve emission control. In this sytem, the blowby which leaks into the crankcase is drawn and released into the intake manifold. The gases are thus sent again into the combustion chamber. This is done only when the vehicle is moving at low speeds. It is not done during higher speeds as the air/fuel mixture may become lean.