Positive Crankcase Ventilation (PCV) is a system to remove harmful vapours from an internal combustion engine (i.e. car engine), and specifically from the crankcase. Water, acids and unburned fuel all contribute to these vapours, which are mainly carried away by the exhaust system. They commonly get into the crankcase by being forced between the rings and cylinder walls due to combustion pressure, but also may enter as liquids in the engine's oil. The only way to remove these vapours is to vent them from the crankcase.

PCV uses manifold vacuum to suck the vapours into the intake manifold. This vapour is mixed in with the fuel/air mixture, and subsequently burned. Therefore PCV has the added benefit of reducing pollution (versus venting the vapours to the atmosphere).

The two primary types of PCV systems are open PCV and closet PCV. Open PCV draws fresh air in through a vented oil filling cap to replace the vapours that are being drawn off, while closed PCV brings in air from the air filter.

The PCV Valve is the most important component in a PCV system. It regulates the flow of vapour from the crankcase to the intake manifold. The amount of vapour that passes by the rings into the crankcase increases at increasing engine speeds, so the rate at which the vapour is drawn off must also increase at higher speeds. The PCV Valve is controlled by the engine's speed and sets this rate.

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