Positive Crankcase Ventilation: Description and Operation

Positive Crankcase Ventilation (PCV) System:

The positive crankcase ventilation system is provided to prevent the blow-by gas from being emitted from the crankcase into the atmosphere. The term "blow-by gas" stands for the compressed gas that blows through the cylinder-to-piston clearance, which contains a large amount of the unburned gases CO and HC.

When the vacuum in the intake manifold is low (throttle valve open), the PCV valve is wide open due to its spring force, so a large amount of blow-by gas is drawn into the intake manifold.

When vacuum in the manifold is high, the PCV valve is limited due to the high vacuum so the amount of blow-by gas drawn into the intake manifold is small.

PCV Valve Cross Section:

The primary control is through the PCV valve which meters the flow at a rate depending on manifold vacuum. To maintain idle quality, the PCV valve restricts the flow when intake manifold vacuum is high. If abnormal operating conditions arise, (e.g. badly worn engine or sustained heavy load) the system is designed to allow excessive amounts of blow-by gases to back flow through the crankcase vent tube into the air cleaner to be consumed by normal combustion.