Experience has shown that some compression and firing stroke, some combustion gases inevitably escape past the pistons into the crankcase. This is known as piston blow by. The fumes consist mostly of unburned fuel (hydrocarbons). As an engine wears, piston blow-by increases. Because unburned hydrocarbons form an explosive mixture, dilute the sump oil and form sludge, car makers ventilate the crankcase to let them out. Until emission control regulations took effect the ‘road draught’ system was used, in which forward motion of the car created a vacuum at the outer end of a ventilation tube from the crankcase. Fresh air was drawn in usually through the oil filler to replace the vented crankcase fumes.
Unfortunately ‘road draught’ ventilation was ineffective below 25mph and oil contamination was high in engines used mostly in town traffic. Positive crankcase ventilation was introduced to control emissions because unburned hydrocarbons are poisonous, it was also introduced to provide ventilation regardless of the road speed. A positive crankcase uses the vacuum in the inlet manifold to suck fumes from the crankcase and pass them to the combustion chamber where they are burned.
The system contains a positive crankcase valve that acts as a fire precaution and adjusts the flow of fumes back-fire occurs in the manifold, the back-flow pushes the valve down, blocking the route to the crankcase. When the manifold vacuum is high, at
Idling speed or small throttle openings, the valve lifts against spring pressure, inducing the flow of fumes into the manifold.