A PCV (positive crankcase ventilation) valve is part of the breather system – particularly those systems with a pipe connecting the breather to the inlet manifold.
When an engine burns fuel, a tiny amount of gases slip by the piston rings and into the crankcase. Not only can these mix with oil, causing sludge inside the engine, but they can also build pressure at high engine speeds, causing the potential for blown gaskets and other damage. At low engine speeds a PCV valve will help draw any blow-by gases out of the crankcase and recycle them through the inlet – meaning no oily deposits are dumped on the road.
Better still, if the engine was to backfire, the one-way PCV valve closes to prevent the breather system being subjected to the high temperatures generated.
The PCV is also used in turbocharged engines, where a pressurised inlet manifold (under boost conditions) would mean blowing boost directly into the crankcase if a one-way valve were not fitted.