1. Necessity
Blow-by gas includes a large amount of unburned gas, which leaks out from the gap between the piston ring and cylinder wall into the crankcase. The Positive Crankcase Ventilation (PCV) system forces the blowby gas into the air intake system and re-burns it. Using the vacuum of the intake manifold, blow-by gas is drawn in. Therefore, the PCV valve is installed between the intake manifold and cylinder head cover. Generally the generated volume of the blow-by gas becomes greater when the engine load is large (the manifold vacuum is small). On the other hand, it becomes less when the engine load is small (the manifold vacuum is large).
2. Operation
The valve passage becomes narrow because the generated volume of the blow-by gas is less when the manifold vacuum is large.
(1) Engine stopped:
The valve closes by the spring force.
(2) Idling or decelerating:
The valve is drawn in further because of the strength of the vacuum.
The vacuum passage is narrow and the amount of blow-by gas flowing is small.
(3) Normal operation:
The vacuum passage is wider than at the time of idling or decelerating because of the normal vacuum.
(4) Acceleration or high load:
The valve is opened even with low vacuum to open the passage to full width.
Some gas is drawn from the cylinder head cover into the front of the throttle valve (air cleaner side) when the actual amount of generated gas is larger than that of the gas, which passes through the PCV valve.