Positive Crankcase Ventilation: Description and Operation

PCV System:

This system is used to eliminate emission of residual fumes and vapors from the crankcase by directing these fumes back through the combustion chamber. When the engine is running, air is drawn by manifold vacuum from the air cleaner through a hose, to the crankcase inlet air cleaner. From the crankcase inlet air cleaner, the air mixes with vapors in the rocker arm chamber and crankcase, and then is drawn up through the PCV valve in the cylinder head cover to a hose connected to the intake manifold.



There are two basic modes of operation, conditions of high engine vacuum, and conditions when crankcase pressure exceeds the flow capacity of the PCV valve.

a. When intake manifold vacuum is relatively high, such as during cruising speed, fresh air is drawn into the crankcase. After circulating through the crankcase the vapors are drawn through the PCV valve and into the intake manifold. The vapors mix with the air-fuel mixture and are burned in the combustion chamber. The PCV valve is calibrated to control the airflow at an acceptable rate.

b. When the crankcase vapor pressure exceeds the flow capacity of the PCV valve, the crankcase vapors are forced through the air cleaner element and carburetor as well as the PCV valve.