Positive Crankcase Ventilation: Description and Operation

The term "blow-by gas" stands for the compressed gas and exploded gas which blow through cylinder-to-piston clearance, which contain a large amount of unburned gases such as CO and HC. The PCV (Positive Crankcase Ventilation) system is provided to prevent the blow-by gas from being emitted into atmosphere and it operates as follows.

When the vacuum in the intake manifold (1) is low (throttle valve open), the PCV valve (3) is wide open due to its spring force.

Thus a large amount of the blow-by gas (5) is drawn into the intake manifold from cylinder hard cover (2). On the other hand, when the vacuum in the manifold is high, the PCV valve opening is limited due to the high vacuum. Thus the amount of the blow-by gas drawn into the intake manifold is small.