Details about Positive Crankcase ventilation And Its System

In most of the engines, a crankcase ventilation is produced by the positive crank case ventilating system. In this system, filtered air from the carburetor air cleaner is introduced in the crank case. The air carries away the blow-by gases and petrol through a special P.C.V. Valve. Thus, the blow by gases and vapors enter the engine combustion chamber along with fresh charge and are burnt there.

The consists of a spring-loaded tapered valve for flow-control. The crank case pressure and the manifold vacuum act together with the aid of spring to close or open the valve. At idle and low speed crank case emissions are very less due to lower cylinder pressure. Moreover, manifold vacuum is high. It permits the valve to open slightly and allows small flow through PCV valve. This flow would be sufficient to keep the crank case clean.

At normal speeds, blow by increases and manifold vacuum decreases due to which the valve moves slightly and increases the flow. The engine may backfire during cranking now a high pressure will be produced in the intake manifold which would cause the valve to break seal, thereby ceiling the inlet. Thus, the crank case is protected from the  back fire.

At high speeds or heavy loads, when the throttle is held wide open there is practically no manifold vacuum acting on the valve due to which the valve opens maximum. It is increasing the flow to maximum capacity of  the PCV valve. When the crank case emissions exceed the flow rating of  the PCV valve, the excess emissions are forced by the crank case pressure to bypass the PCV valve into the air cleaner.