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