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Positive crankcase ventilation involves; recycling these gases through a valve (called, appropriately, the PCV valve) to the intake manifold. From there they’re pumped back into the cylinders for another shot at combustion. It isn’t always desirable to have these gases in the cylinders. Because, they tend to be mostly air and can make the gas-air mixture in the cylinders a little too lean.
So the blow-by gases should only be recycled when; the car is traveling at slow speeds or idling. Fortunately, when the engine is idling the air pressure in the intake manifold is lower than the air pressure in the crankcase. And, it’s this lower pressure; that sucks the blow-by gases through the PCV valve and back into the intake. When the engine speeds up, the air pressure in the intake manifold increases. Then the suction slows down; reducing the amount of blow-by gas recycled to the cylinders.
This is good, because the blow-by gases aren’t needed when the engine speeds up. In fact, when the car is up to speed; the pressure in the intake manifold; can actually become higher than the pressure in the crankcase. Consequently, forcing the blow-by gases back into the crankcase.
Consequently, The Positive Crankcase Ventilation system does this by using; manifold vacuum to draw vapors from the crankcase into the intake manifold.
The Positive Crankcase Ventilation Valve is effective as both; a crankcase ventilation system and as a pollution control device. Positive Crankcase Ventilation systems have been standard equipment on all new cars since the early sixties.
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